Potato Blight Epidemics

POTATO BLIGHT EPIDEMICS

Throughout the World

Agriculture Handbook No.174 UNITED STATES DEPARTMENT OF AGRICULTURE POTATO BLIGHT EPIDEMICS

Throughout the World

By A. E. COX and E. C. LARGE Plant Pathology Laboratory Ministry of Agriculture, Fisheries, and Food Harpenden, England

Agriculture Handbook No. 174

Published by Agricultural Research Service UNITED STATES DEPARTMENT OF AGRICULTURE

Washington,^ ' D. C:^" JanuaryT^ I960ir»/n

For sale by the Superintendent of Documents, Government Printing Office, Washington 25, D. C. Price $ FOREWORD Among the important diseases of miajor food crops the late blight of potato is one of the most difficult for us to understand and combat. Despite the intensive studies that have been devoted to it, the conditions for blight occurrence are still not thoroughly known. Frequently, a severe unexplained outbreak of the disease, or its appearance in a region where it was previously unknown, revives problems that had been considered already solved or raises new questions. Observations wherever the disease appears throughout the world contribute knowledge for better understanding and solution of these problems. This worldwide account of recent epidemics of potato late blight provides a useful background of epidemiological information. This Handbook is the first review of conditions associated with late blight outbreaks throughout the world. Its length is due to the inclusion of the detailed information that is necessary to permit comparison between regions and correlation with climatic data. The detail cannot be omitted without lessening the precision and therefore the usefulness of the data. We gladly acknowledge our indebtedness to the authors who undertook this arduous task, and to others of the Plant Pathology Laboratory, Ministry of Agri- culture, Fisheries, and Food, Harpenden, England, who made this study possible. The inception of this collaborative effort was a joint consideration between the Crop Plant Disease Forecasting Project of the Crops Research Division, Agri- cultural Research Service, United States Department of Agriculture, and the De- partment of Plant Pathology, University of Wisconsin, Madison. The program was administered by the Wisconsin Alunrini Research Foundation with funds for conduct of the work provided by the United States Government. The results of this extensive study should be useful to plant pathologists throughout the world and especially to the many plant pathologists of the Crop Plant Disease Forecasting Project. It is hopedthatthe review will provide a basis for additional studies on the impact that plant diseases have on crops and the subsequent economic conditions in various parts of the world.

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n PREFACE This study was made possible by the Wisconsin Alumni Research Fovindation, U.S.A., which administered funds for the purpose. Through the efforts of A. J. Riker, University of Wisconsin, who was especially interested in assembling world data about important epidemic plant diseases, the authors were able to undertake a comprehensive review of recent epidemics of potato blight throughout the world. A. E. Cox, agriculturist, and E. C. Large, plant pathologist. Plant Pathology Laboratory, Ministry of Agriculture, Fisheries, and Food, Harpenden, England, under the direction of W. C. Moore, of that Laboratory, worked jointly in making contacts with specialists, in assembling the data, and in preparing the report. The maps and diagrams were prepared by V. J. Pearson, of the above-mentioned Laboratory. The material on which this world study of potato blight has been based was derived in part from published literature, but to a far greater extent it has been based upon unpublished information generously provided by leading workers in different countries and from England and Wales, verbally or in correspondence with the author s. The help sought by the authors has always been most readily given and has varied greatly in kind. Special acknowledgments are nnade to the library of the U. S. Department of Agriculture and to the following specialists:

A. Angus, Northern Rhodesia W. G. Hoyman, U. S. A. G. R. Bates, Southern Rhodesia H. Johannes, Germany F. Beran, Austria P. Journet, France W. Black, Scotland M. A. Keay, Uganda R. Bonde, U. S. A. W. E. Kerr, Southern Rhodesia P. M. A. Bourke, Irish Republic D. Lihnell, Sweden C. J. Briejer, Netherlands M. M. de Lint, Netherlands L. C. Callbeck, Canada T. P. Mclntosh, Scotland L. Caudri, European Plant R. McKay, Irish Republic Protection Organization C. J. Magee, Australia H. T. Cook, U. S. A. P. R. Miller, U. S. A. J. C. CuUen, England A. E. Muskett, Northern Ireland H. M. Darling. U. S. A. R. M. Nattrass, Kenya H. Darpoux, France J. S. Niederhauser, Mexico D. Delaney, Irish Republic L. C. Peterson, U. S. A. R. W. Dickey, U. S. A. J. E. van der Plank, South Africa CM. Driver, New Zealand I. Polykov, U. S. S. R. H. Dubniak, Poland H. N. Racicot, Canada P. Dumas, France H. Richter, Germany A. H. Eddins, U. S. A. E. A. Riley, Tanganyika C. J. Eide, U. S. A. K. Scheibe, Germany S. Fish, Australia C. Bazan de Segura, Peru C. E. Foister, Scotland J. H. Simmonds, Australia E. F^rsund, Norway P. W. Smallfield, New Zealand S, Fridriksson, Iceland J. D. Tallantire, Cameroons G. P. Georghiou, Cyprus H. J. Toxopeus, Netherlands J. Grainger, Scotland J. Ullrich, Germany P. Grainger, Canada R. W. Vasudeva, India E. Gram, Denmark E. Ventura, France E. G. Gray, Scotland G. C. Wade, Australia M. Guntz, France R. W. Wallace, Northern Ireland J. M. Hirst, England J. R. Wallin, U. S. A. D. Hoctor, Irish Republic J. M. Waterston, Cameroons J. C. F. Hopkins, England W. Welvaert, Belgium J. G. ten Houten, Netherlands V. E. Wilkins, European Plant J. L. Howatt, Canada Protection Organization

m CONTENTS

Page Page INTRODUCTION 1 ASIA- - - 181

Potatoes as a World Crop 3 India - 181 Some Factors Affecting Potato Yields 5 Life History of the Fungus on Po- AUSTRALASIA 185 tatoes 7 **Blight Weather'' -- 9 New Zealand - -- 185 Australia - 191 THE BRITISH ISLES 10

England and Wales - 10 CONCLUSIONS - 196 Scotland 36 Northern Ireland 4-3 The Definition of Climatic Conditions 196 Republic of Ireland 48 Effect of Mean Temperature on Po- tato Cropping 196 CONTINENTAL EUROPE 60 Effect of Rainfall on Potato Growing - I96 Definition of Blight Epidemics I96 France 60 Determination of the Effects of Blight Belgium 67 Epidemics on Yield I97 The Netherlands 71 Frequency of **Blight Years" in Dif- Denmark 85 ferent Regions I97 Iceland 89 Incidence of Blight and Weather Con- Norway 92 ditions 198 Sweden 95 The Duality of the Disease 199 Western Germany 99 The Disease in the Tubers 199 Austria 107 Compensatory Effects on Yield of Poland 109 Blight Epidemics and Rainfall 199 Union of Soviet Socialist Republics - - m Economic Effects of Blight Losses: Cyprus 115 Tropical and Subtropical Regions - - 200 Economic Effects of Blight: Main NORTH AMERICA 118 Potato-Growing Countries 200 Course of Establishment of Blight in United States of America 118 New Areas 201 Canada 140 Carryover of Blight From Season to Mexico 151 Season 201 Effect of Varietal Resistance in Modi- SOUTH AMERICA - 155 fying Blight Epidemics 202 Potato Blight Forecasting 202 Peru -- 155 Protective Spraying: Materials 203 Chile 160 Protective Spraying: Extent of Use- - 203 Protective Spraying: Effect on Control AFRICA 166 of Blight in Tubers 204 Haulm Destruction 204 Union of South Africa 166 Control by *'Disease-Escape'* Meth- Southern Rhodesia 170 ods 205 East Africa 171 Indications of Subjects for Further Kenya - 172 Inquiry 205 Uganda 175 Tanganyika 177 LITERATURE CITED 207 Northern Rhodesia 177 West Africa 178 INDEX 220

IV INTRODUCTION

Over a century has now passed since the It would be idle ^:o attempt to de s cribe potato potato late blight fungus, Phytophthora in- blight epidemics except against the background festan s (Mont.) de Bary, found its way from of the distribution and climatic requirements the New World to the Old, and first ravaged of the potato cultivations in which they occur. the potato crops of Europe. Since 1845 the To do this for all countries, and for all the fungus has been the subject of innumerable differing regions in countries extending across studies: The commonly occurring stages of its whole continents, could not be profitably at- life history and the phases of its parasitism tennpted in a couple of years. This study is upon its solanaceous hosts are now well known; therefore confined to a limited number of but, like the true cryptogam that it is, it still countries, and regions within those countries, contrives to conceal much from human scru- which among them probably experience most tiny. After more than a century of search its of the types of epidemic that occur in the world. sexual reproduction by means of oo spore s For each country studied a fairly full section has only recently been found occurring on po- or case study was compiled, after first setting tato plants in the field--in Mexico. The means forth some very brief general observations on by which it succeeds in producing new races the potato as a world crop, and on the life or strains elsewhere in tíie apparent absence history of the fungus, for the assistance of in- of a sexual phase remains a subject of dis- terested readers who maybe neither profes- putation. The relations between sporulation sional agronomists nor plant pathologists. and spread of the fungus and the microclimatic An indication of the world distribution of conditions within potato crops are now under potato growing is given in figure 1. In general, active investigation but are still very im- principal areas of production were chosen for perfectly known; real progress in blight fore- the closest study. However, in view of the casting has been made in recent years, but recent introduction of potato blight intoanunn- forecasting is still upon an empirical basis. ber of countries where it was previously un- A vast expense of money and labor goes each known--a second wave of world dispersal year upon control measures, but our debt to related to the speeding-up of communications, the partial blight resistance of some varieties expansion of agriculture and commerce, and cannot be realized and the rationale of spray- troop movements of this twentieth century-- ing in relation to the frequency and nature of some countries where relatively few potatoes the epidemics is still imperfectly understood. are grown but where interesting or alarming Of all the mysteries of potato blight that epidemics have occurred were included. Some will surely be engaging agriculturalists, plant countries are also included to exemplify limit- pathologists, and plant breeders for genera- ing conditions both for the crop and for the tions yet to come, perhaps the greatest, and occurrence of the disease. A map showing the certainly the most economically important, world distribution of Phytophthora infestans is the real extent and nnagnitude of the losses on potatoes is already available (Commmon- that the disease causes among the potato wealth Mycological Institute (C.M.I.), 1954;^ cultivations of the world. There is an intrinsic the distribution of the fungus is now for all difficulty in measuring the incidence and the practical purposes coincident with that of the effects of any plant disease over extensive potato itself. Serious losses from the disease areas, and the branch of plant pathology con- on outdoor tomatoes have occurred in recent cerned with such work is still in its infancy. years iñ many countries, but the world dis- It is only in very few countries of the world tribution and present status of tomato blight . that there is sufficient field survey informa- has already been investigated (Miller and tion to define the courses of the blight epi- O'Brien, 1955), and the disease on tomatoes demics or to support even approximately is referred to only where it is of particular accurate estimates of the losses they cause. importance as a link in the chain of infection At present, very little precise information is of potato crops. [ available. In the section on each country the extent The objective of this study is (1) to explore and distribution of potato growing, the im- as far as possible the findings in those coun- portance of the potato in the country's food tries where special surveys have been nnade; supplies, the principal varieties grown, the (2) to draw attention to the methods used; and nature of the cultivations, and the times of ¡3) then to apply the results in principle for planting and harvesting are given first. Then , "he interpretation of such scattered informa- the cropping periods are related to the long- ion as is available in other countries, to term average rainfall and average daily mean Provide the beginnings of a mid-twentieth cen- temperature for each month of the year at ury conspectus for the world. It cannot be put ligher than that; a little has been done, more emains to be done, and it will have to be done ^ References to Literature Cited (p. 207) are herein indicated by the n the field. name of the author (or authors), followed by the year of publication. o o ■I ll> |x g 1

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2 representative weather stations. For this pur- other countries, as in England and Wales, pose a number of small **climagrams" were during recent years systematic surveys on constructed to show up as clearly as possible hundreds and sometimes thousands of crops similarities and differences in widely sep- were nnade each year, and these surveys were arated parts of the world. The explanatory condensed and sumnnarized. In general, epi- notes on the climagrams given in figure Z, demics for the period 1947 to 1956, inclusive, are important not only in helping to define were reviewed; but where important series of cropping conditions, but also, later, in re- trials were made over sequences of years vealing the mean temperatures and rainfall before that, the earlier trials were sonnetimes associated with the several types of blight included. epidemic. These climagrams show only the Where possible the unrestricted epidemics mean temperature line. The diurnal range occurring on unsprayed crops--usually prin- of temperature also affects cropping and the cipal maincrop varieties--are described. Then incidence of blight, but this and other climatic the modifying effects upon the epidemics of factors are discussed in the text. Fuller any spraying or other control measure s adopted relevant information on climate is available in the country, with practical particulars of inKendrew (1937). those measures and the extent to which they No attempt is made in the climagrams to are used, are shown. Under control measures portray humidity conditions, although low tem- not only protective and destructive spraying perature s in relation to latitude often reflect are considered but also the far more subtle the modifying effects of cloud and coastal or and often almost unconsciously adopted meas- mountain fog. Very detailed humidity obser- ures that become more apparent from a study vations at frequent intervals during the day of the blight epidemics themselves, such as are needed to reveal the actual incidence of measures of disease escape, retardation of **blight weather." For those few countries infection by the avoidance of overlapping where such observations have been made, the cropping, and the like. findings, and the methods used, have been The work of the past quarter of a century described in detail; but only very cautious on attempts to breed blight-immune varieties inferences can be drawn from humidity data by taking advantage of the genes for resist- available for the rest of the world. Average ance present in Solanum demis sum and other humidities are among the nnost unreliable of wild potato species; the consequent discovery climatic parameters. of new biotypes or races of the blight fungus; The incidence of potato blight in each coun- and the further stages of attempting to defeat try or region is determined by the frequency these by search for "field resistance** and with which **blight years** occur and by an other means, are considered briefly in the account of what constitutes a *'blight year** sections for countries in which the work has in that particular locality, in terms of the been done. The part played by partially re- stage in the growing period at which the potato sistant varieties, old or new, in modifying plants are defoliated by the disease, and also the incidence of blight and reducing losses, in terms of the consequent effect on total yield is a matter most germane to any study of and on blight infection of the tubers. Some blight epidemics. countries had considerable data; in other Finally, the main findings for each country countries statements were obtained that in are summarized briefly at the end of each such and such years the disease was **severe** section. In a final chapter, without attempting on »'moderate" or ''slight,** with little to de- a full interpretation of the body of the work, fine such terms, except perhaps the results the writers have set out some factors that of a few spraying trials and some clues from appear common to blight epidemics in the scattered observations on the foliage. In some world as a whole.

POTATOES AS A WORLD CROP

In such a study as this, which by its terms Potatoes do best under moist, temperate of reference concerns only a single disease, conditions, where the growing season is long. there is a danger of conveying the impression The greatest concentration of potatoes occurs that late blight is the major cause of low in northern and central Europe (see fig. 1), yields in potato crops wherever blight occurs and the European continent as a whole contrib- and low yields prevail. This is entirely fal- utes about 75 percent of the world's produc- lacious. Lack of rain, inadequate nutrition, tion. The maritime climate of northwestern poor cultivation, and virus diseases from Europe is almost ideal for potato growing, 3oor seed are often far more potent factors. and it is from this area that the highest aver- To put matters into perspective, it is neces- age yields (10 tons per acre in the Netherlands îary to say something of the requirements of and Belgium) are recorded. In many European he potato crop and its part in world agricul- countries potatoes form a large part of the ural economy. people's diet. In North Ameri-.a the crop is CLIMACRAMS

construction line to indicate mean temperature line when temf)erature line ri Mean doily Max-f Min rises above 50°F

70-,.ü$ual upper limit tor potato aoppinq 60 RAINFALL Inches 50-^'ower limit

M J J A -Month of the year

MAINCKOPS 0 = December etc.

- length occupied by -rainfall histograms shaded — alternative way of indicating these words indicates to emphasise growing period growing period spread of planting and rainfall during that and lifting period

NOTES

The mean air temperature line is shown. The mean maximum êmpeâtuê line usually runs 5°-7^F above the mean line, and the mean minimum 5^~ y'F below it, in maritime areas. In areas with a continental climate the mean diurnal variation is greater(±9° to ± 12*^F).

The'hump'of the mean temperature line above 50°F characterises the potato qrowinq season. Its lenqth indicates the period during which there is potato qrowth above qround. Its heiqht indicates favourability or otherwise of main part of the qrowinq season for tuber production Cupper limit about 70°F).

In the southern hemisphere December is made to occupy the place of June in the northern hemisphere, so that'summer'is always in the centre of the diaqram.

Lonq-term averaqe fiqures for each month of the year are qenerally showa Portions of the diaqram for the croppinq season only in individual years of special interest are sometimes shown below.

'Rainfair includes other forms of precipitation: snow,haiLetc. Figure 2. —Explanatory notes on the climagrams used in this study. relatively less important, and the short grow- industrial purposes. Peru is one of the few ing season and high summer temperatures over countries with a high human consumption of much of the continent affect yield adversely. potatoes that has no important alternative out- Potatoes are a major crop in some South Amer- let for the crop. Countries with a medium ican countries, notably in Peru, parts of Chile, consumption rate (200 lb. per person) include Argentina, and Brazil. In countries on or near Great Britain, Holland, and Sweden. Most of the Equator they are grown at high altitudes, as these have alternative uses for potatoes other temperatures at low altitudes are too high. than the ware market, but in Great Britain Although the distribution of potato growing surplus potatoes can prove an embarrassment is in close accordance with the distribution of and are only reluctantly taken for livestock population in Europe, North America, and feeding. The U.S.A., Canada, and Australia South America, this is clearly not so in the represent countries with a low rate of human countries of Asia, in many of which rice is consumption (100 lb.). The ware and seed the staple food. The largest acreages of markets are the only outlets for potatoes in potatoes grown in the Far East are in China, such countries, and at the present time the India, and Japan, but here it is only in Japan crop is barely holding its ground in the face of that the crop makes an important contribution competition from other vegetables. Lastly, to the diet of the people. Potatoes are only a there are a number of countries where potato minor crop in Africa, where they are grown consumption is very low (generally less than principally at high altitudes. The acreage is 20 lb. per head per year). These include India, small in Australasia, but this is chiefly be- Pakistan, and Mexico, where potatoes become cause of a low demand for potatoes due to the in effect a luxury crop and their contribution small population. to the national diet is negligible. Apart from their use for human food, pota- The present planting of potatoes throughout toes are fed to livestock, mainly pigs, and they the world is about 57 million acres (F. A. O., are used in industry for the production of 1956). This represents only about 12j percent potato flour (farina) and alcohol. The use of of the land area devoted to wheat, which is potatoes for livestock is largely confined to the most important grain crop. There is little those countries where there is a large agri- international trade in potatoes. This is ex- cultural labor force. According to the impor- plained by the bulkiness of the crop, its tance of potatoes in the diet of the population, liability to deterioration during transit, and to the countries of the world can be divided stringent important regulations. Nonetheless, roughly into four groups. Those with a high some important trade goes on in seed pota- human consumption of potatoes (300-400 lb. toes, notably from Holland, Canada, Ireland, per head per annum) include a number of and Scotland. There is also some trade be- European countries--European Russia, Ger- tween countries on a snaall scale in out-of- many, Poland, and France. In each of these season potatoes; as, for example, the export countries a large proportion of the total of early potatoes to Great Britain from a potato crop is fed to livestock and used for number of Mediterranean countries.

SOME FACTORS AFFECTING POTATO YIELDS

Climatic conditions, daylength, soil type, slightly widened, say to 580-65°, this range cultural treatment, and the incidence of dis- represents the usual condition occurring in eases and pests all affect the yield of pota- practice during the latter part of the potato- toes. A few of the principal factors have now growing season in those parts of the world, to be considered individually, as a part of the notably northwestern Europe and northeastern background of our study, but their interrela- North America, where the largest potato tionships must also be borne in mind. The op- yields are produced. The many climagrams timum water requirement of a potato crop, for given in following sections bring out this example, will depend on temperature and soil point. The temperatures during the last 2 conditions, and climate profoundly affects dis- months before lifting are most important, as ease and pest attacks. it is then that the bulk of the crop is formed. TEMPERATURE. The work of Bushneil Potatoes are intolerant both of high tem- (1925) is usually referred to when considering peratures and of frost. Bushnell (1925) found the optimum temperature conditions for tuber that tuber production ceased at 84oF., and formation in the potato crop (Van der Plank, Bald (1941) has said that in areas of Australia 1946; Burton, 1948). Bushnell found that maxi- where mean monthly temperatures exceeded mum tuber production occurred over the tem- 750 potatoes did not produce a profitable crop. perature range 59°-64o F., but his experi- Thus, potatoes are not grown on any large ments were conducted with plants grown at scale in any part of the world where the mean constant temperature, a state of affairs very temperatures during the hottest part of the different from that in nature. However, it growing season is much in excess of 70°. would seem that if Bushnell's limits are Again some allowance needs to be made for amerences between continental and maritime conditions, often characteristic of high alti- climates. To some extent, too, there is an tudes, yields are generally very low. Rainfall interaction between temperature and daylength is generally adequate in maritime areas or (Driver and Hawkes, 1943). Thus, short days, localities under maritime influence, as in of less than 12 hours, modify the effect of northwestern Europe and the northeastern high temperatures. areas of North America, where the highest In most of the principal potato areas of the potato yields are obtained, and fluctuations world frosts mark the beginning and end of the in yield from year to year are not usually very growing season. The mean temperature at great. Rainfall is often insufficient under which frosts are likely to occur varies ac- continental conditions, as over a large part cording to situation, but in practice planting of European Russia and the midwestern states usually takes place in the month that m.ean of the U.S.A, The crops then often suffer from air temperatures approach 50° F,, and lifting drought, and yields fluctuate considerably from begins at about the same temperature, (See year to year. fig. 2 and other climagrams.) In certain Potatoes may also be grown wholly or partly frost-free areas, as in the Ayrshire early under irrigation, as, for example, in the Medi- potato-growing district of Scotland, potatoes terranean countries, western U.S.A., and parts are planted, and a considerable amount of top of South America and India. In these areas it growth is made before mean temperatures is channel irrigation that is almost universally reach 50°; and in the Sierra of Peru the employed. The use of spray irrigation is con- whole growing season occurs at about this fined to supplementing rainfall under more temperature; but these areas are exceptional. temperate conditions. The importance of temperature is thus seen DAYLENGTH. Work on the effect of photo- to lie not only in its effect on tuber produc- periodism on yield and maturity of the potato tion but also in the effect on the length of the has been fully reviewed by Driver (Driver and growing season, and the length of the growing Hawkes, 1943). Most of the world's potato season is often too short to allow maximum crop is grown at middle to high latitudes potato production, particularly under conti- under long-day conditions, and these favor nental conditions. Very roughly the potato- maximum vegetative growth. Under short-day growing areas of the world may be divided conditions in equatorial regions, top growth is into two main types: Those such as Aroostock smaller, maturity is speeded up, and yield is County in Maine, U.S.A., northern Sweden, decreased. This latter statement has, how- and the Leningrad district of the U.S.S.R,, ever, been challenged by Mor eau (1944a), where the growing season is short (about 120 working in East Africa. Particular potato days) and there is insufficient time for late varieties have a definite photoperiodic re- or maincrop varieties to produce their maxi- sponse. Thus, a short-day variety is suited mum crop; and those, such as Great Britain, only to short-day conditions, and when grown Holland, Australia, and New Zealand, where under long-day conditions it will fail to produce the growing season is long (160 days or more) tubers. But many varieties are day-neutral. and late varieties die down naturally some- An example is the American variety Kennebec, time before lifting. which outyielded all others in trials in Florida RAINFALL. Although an adequate supply of and Iceland. In Florida a winter crop is grown water is the most important factor affecting and the daylength may be as short as 9 hours; yield in the potato, it is impossible to gen- in Iceland in June the daylength reaches 21 eralize about the plant's actual rainfall re- hours. quirement. The distribution of the rainfall SOIL CONDITIONS. Most of the potato crop over the growing season is important. A given is grown on sandy or closely allied soils, amount of rain evenly distributed over the life which can be easily worked and which dry of the plant may be just what is required, but out quickly after rain. This is particularly the same amount coming in a few heavy rain- important where the growing season is short, falls may lead to waterlogging of the soil and as under continental conditions, because plant- actual mechanical damage to the potatoes. ing and lifting have then to be done quickly The rate of water loss fronn the soil both and delays due to waterlogged soils cannot be by evaporation and drainage is also tolerated. Light soils, however, are generally relevant. short of organic matter and their moisture- The greater part of the world's potato crop holding capacity may be so low that the yield is grown under rainfall without the aid of ir- suffers. Where the growing season is long, rigation. The rainfall may be excessive, as soils of heavier body (medium and heavy silt in the high valleys of Mexico, parts of Africa, loams) often make the best potato soils, as in and the hill crop potatoes in India; under such the Netherlands and New Zealand. UFE HISTORY OF THE FUNGUS ON POTATOES

HOW THE DISEASE SPREADS small, but a primary focus of the disease has been established in the potato field. It is not within the scope of this study to From this focus of infection more spores review the immense world literature on the are disseminated; another crop of lesions cov- biology of Phytophthora infestans. All that is ers a greater radius; these lesions in turn necessary is to explain a few technical terms sporulate with successive spells of favorable frequently used in the following pages and the weather, and after two or three such genera- way in which the fungus lives as a parasite on tions of the fungus, blight spots may be found its potato host, with the requirements for its here and there, generally but lightly dis- survival and spread. It is in the light of this tributed over an entire field, and beginning knowledge that all accounts of late blight in other fields in the vicinity. Epidemic spread epidemics, wherever they may occur in the may then be said to have begun. world, must be seen and interpreted. At first the damage is negligible, for the The fungus survives the winter as living crop loses only a few leaves here and there, myceliunn in potato tubers. When these in- but if the attack is not checked by protective fected tubers, planted or remaining in the soil spraying or dry weather, there will be pro- as groundkeepers, produce new growth in the gressive decay of the foliage as the estab- spring, the fungus, still in the form of liv- lished lesions increase in area and new ones ing mycelium, succeeds in invading a few of develop. The course of the epidemic may be the shoots--a very few of them--and comes rapid or slow, always according to weather aboveground within their tissues. When certain conditions, and the crop may be affected in necessary temperature and humidity conditions two distinct ways. (1) If the foliage is killed occur, the mycelium of the fungus produces prennaturely the yield will be reduced by what hyphae, known as sporangiophores, . which is in effect a reduction of the growing period; thread out, usually through the stomata of the the earlier the haulnn is killed the greater the leaves and stem, and project into the air. On loss of crop. (2) Andif living sporangia from the these sporangiophores the microscopic spor- diseased foliage reach the tubers themselves, angia of the fungus are formed. They become these also may be infected, causing a further detached and drift off when ripe, and they are loss, either in the ground or in storage. also dispersed by rain. Some reach healthy In wet weather, sporangia are washed down potato leaves and stems nearby and if these from the foliage onto the soil. Normally the remain wet from rain or dew for a sufficient soil cover protects the tubers to a great ex- time, the sporangia germinate. At rather high tent, but where the soil cover is inadequate temperatures they may germinate directly, or the soil texture or moisture content render each sporangium putting forth a single germ it permeable by the sporangia, some tubers tube, but generally and under cooier conditions will be infected before lifting. If, at lifting, they gernninate indirectly, the protoplasm the tubers are exposed while the fungus is within the sporangium first breaking up into a still sporulating on partially diseased foliage number of smaller swarm-spores, or zoo- or if living sporangia are still present in the spores, which swim about in the water on the surface layers of the soil, further infection leaf, and may each produce a small gernn tube of the tubers will occur. This infection will of its own. After the spore has germinated not be apparent at the time, but it will develop it may infect the potato leaf or stem on which in storage. it is resting, and in this infection process the The infection of the tubers completes the germ tube of the spore penetrates the leaf life cycle of the fungus. The vast majority of cuticle, or, occasionally, enters the leaf by the blighted tubers rot in the ground or during way of a stoma, and then initiates a growth storage, at least to such extent that there is of the fungus mycelium in and among the leaf no living mycelium left in them by the following cells. The invading mycelium debilitates the spring. As far as possible, blighted tubers cells on which it feeds, and as these begin to are removed from seed stocks, and of the decay it spreads peripherally into fresh tissue. few that pass undetected not one in a hundred The period from infection (successful pene- gives rise to affected shoots aboveground. The tration by the gernn tube) to the appearance of first infected shoots among the crops are rare , a visible blight spot, or lesion, on the leaf, is and hard to find, but these, together with oc- known as the period of incubation. casional infected shoots arising from ground- . Given a recurrence of the same favorable keepers, and fromi discards by clamp sites , temperature and humidity conditions that in- or cull piles, are sufficient to start the blight duced sporulation on the first infected shoots, epidemics each year. the new lesions will in turn produce spore- HOW HUMIDITY AND TEMPERATURE bearing hyphae, and the first generation of CONDITIONS AFFECT THE FUNGUS the fungus is completed. At this stage, the number of blight spots on the originally in- The temperature and humidity conditions fected plant and those nearby is generally quite known or believed to be critical for each phase of the survival and spread of the fungus The ripe sporangia may, however, be dislodged may be stated briefly as follows: and dispersed by rain at any time of the day or SURVIVAL OFMYCELIUM IN THE TUBERS. night. It may be expected that where seed tubers GERMINATION AND INFECTION. Crosier are subjected to temperatures of 86^F. or found that free water or dew on the leaves is over for any considerable time in storage, necessary for the germination of the sporangia. transit, or in the soil, the fungus will not Direct germination, without the production of survive in them (Jensen, 1887), There appears zoospores, can occur over the temperature to be no lower limit for the survival of the range 48° to 79° F., but has a definite optimum mycelium; chilling or frosting that does not at about 75°. Little direct germination occurs kill the tubers does not kill the fungus. Van in 8 hours, and about 32 hours is necessary der Zaag (1956) found that the mycelium was for nnaximum germination in this way. Direct killed when seed tubers were immersed for germination therefore probably plays little 1 hour in water at 110°. This treatment dam- part in the spread of blight in the field. aged the tubers themselves when carried out Indirect germination, by zoospores, requires in April, but not in December. free moisture, and a rather low temperature SPORULATION. In experiments under rather (50° to 59°F., optimum 55°) for a period of artificial constant te nape rature conditions, i.e., only 1/2 to 2 hours. It may also take place at without the diurnal variation of temperature temperatures as low as 37° or as high as 70°, occurring in nature, Crosier (1934) found that but the time required is considerably longer. sporangia were never formed in an atmosphere After the zoospores have germinated a fur- at less than 91 percent relative humidity, ther period of at least 2 to Zj hours at 50° and that humidity approaching 100 percent was to 77°F. is required for the infection process, required for their abundant formation. In a or penetration of the term tubes into the host saturated atmosphere, temperatures of 62° to tissue. The whole process of indirect germi- 72°F. were optimum both for rapidity and nation and infection thus requires a period of intensity of sporulation. Sporangia commonly Zj to 4j hours with free moisture on the leaves appeared within 8 hours and were abundant and cool conditions over at leasthalf this time, in 14 hours. At 46° to 59° sporangia were INCUBATION. After penetration of the germ fairly numerous in 24 hours and plentiful in tubes into the host tissue the fungus develops 58 hours. At 37° only a few sporangia were most rapidly at 64° to 70°F., i.e., at about formed in 6 days. Sporangia produced at tem- the same temperature as that which is also peratures over 7 0° did not gernainate well. optimunn for sporulation. Under such condi- The sporangia were found to lose their viabil- tions Crosier found that new lesions appeared ity in 1 to 2 hours in very dry air (20 to 40 in 3 to 5 days. Thran (cited by Uhlig, 1955) percent relative humidity) and in 3 to 6 hours found that the incubation period could be as in air at 50 to 80 percent relative humidity. much as 13 days at 53°, 6 days at 60°, and Although the main sporulation conditions as 2 to 3 days at 68°. Very varied figures have defined by Crosier are supported by the find- been given by other workers, but in general ings of the majority of workers, it must be the period is 3 to 5 days. noted that the capacity of the fungus for some SURVIVAL OF THE FUNGUS IN LEAVES restricted sporulation in the field under ap- AND STEMS. Crosier found that continuous parently adverse high temperature and low temperatures of 86° F. checked the growth of humidity conditions was reported by Wallin the fungus in the field, but that intermittent (1953). temperatures of 68° to 86° during the day LIBERATION OF SPORANGIA. In explora- were not particularly unfavorable to it, and tions of the spore content of the air at 6 feet that in potato stems it would tolerate inter- and at 18 inches above the ground in potato mittent high temperatures up to 94°. In one fields during the course of potato blight trial, one stem lesion out of 197 developed epidemics in England, Hirst (1953) has found sporangia on plants exposed to bright sunshine that rapid liberation of sporangia starts at and maximum temperatures of 110°to 115°for about 8 o'clock in the morning (0800 hours) 6 days. This capacity of the fungus to remain and reaches its peak at about 10 o'clock viable in stem lesions during hot dry weather (1000 hours). This is consistent with De Bary*s and to start sporulating again when more (1887) suggestion that spore discharge occurs favorable conditions recur has been noted in with the hygroscopic twisting movement of the England by Clayson and Robertson (1955), and sporangiophores exposed to rapidly changing extensively investigated in the U.S.A. by Hoy- humidity, and indicates that liberation coin- man and Wallin (1957), who reported survival cides with rising temperatures and the disap- of the fungus after exposure for short periods pearance of dew and raindrops from the leaves. to temperatures as high as 114°. ^BLIGHT WEATHERS

As it is a part of the rhythm of nature for conditions for sporulation and germination air temperatures to rise during the day and of the fungus as determined in the laboratory. to fall at night, it might be expected that the Very detailed micro-climatic investigations, blight fungus, preferring rather warm con- such as those of Johannes (1953a) in Ger- ditions for sporulation and rather cool con- many and of Hirst at Rothamsted, are neces- ditions for swarm-spore germination, would sary to reveal what constitutes *'blight produce its sporangia during the day (given weather" in nature and how it is, in fact, the necessary high humidity) and release related to any given combination of climatic them in the evening to germinate during the parameters. cooler night and in the night dews. Although ' *Blight weather** is a convenient term there may be some sporulation during the day. for weather that permits the increase and Hirst has shown, at any rate under English spread of the fungus in the field. The prob- conditions, that the peak liberation of sporangia lem of what constitutes a "blight weather takes place in the morning. This renders the spell,** or "critical period,** has arisen in parasite less efficient than it might otherwise attempts to forecast outbreaks. Progress in be, and imposes a natural check on distant this art has conne from making intelligent spread of the disease by airborne sporangia. guesses in the formulation of empirical rules, There is no such restriction, however, on and then trying them out. With successive local spread by splash and drip dispersal nnodifi cation s and limitations, these rules, of the sporangia by rain. set out in this study in the sections on the Although high humidity plays an extremely Netherlands, England and Wales, the Irish important part in the development of a blight Republic, the U.S.A., and Western Germany, attack, so also does rain. Rain provides now embody all we have of real knowledge water necessary for the germination of the of blight weather; as a result, "fundamental sporangia on the leaves, it raises the humidity biological and meteorological .research'* is within the crop by wetting the soil, and it is laboring to catch up with the practical find- responsible for much of the local dispersion ings. of infection. Hänni (1949) and Uhlig (1955) In the consideration of blight in relation to put wet weather very high in their lists of weather, there are successive levels on which conditions favoring the mass increase of attention may be focused. First, the micro- blight, and this is in accord with conamion scopic level that is concerned with the be- observation. havior of spores in damp channbers; then The interplay of diurnal changes of atmos- the close-up of blight lesion distribution in pheric humidity and temperature and the dif- small plots with meteorological instruments fering requirements of the fungus for sporula- among the plants; then the midview of epi- tion, spore discharge, and germination, demics in limited regions and attemipts to together with all the divers effects of wind, forecast them; and finally the long-shot view rain, and varying density of crop growth, of the world as a whole. This study is pri- render it unwise to attempt to postulate marily concerned with the world view of what will or will not constitute "blight weather" blight weather and the broad climatic cir- from a mere consideration of the optimum cumstances in which such weather occurs. THE BRITISH ISLES

ENGLAND AND WALES England and Wales, with a population ac- near Birmingham; and in Durham, supplying cording to the 1951 Census of about44n:iillions, the coal mining districts of the north. now produces 5 to 6 million tons of potatoes The principal maincrop varieties are Ma- each year, most of which are for hunnan con- jestic and King Edward. Majestic accounts sumption within the country. A small tonnage, for about 45 percent of the maincrop acreage, notably of surpluses, is fed to stock, but and King Edward (including Red King), which very few potatoes are used for industrial is regarded as the quality potato, for about purposes. The potato acreage, which reached 35 percent. Other popular varieties are Arran over a million during the 1939-45 war, stood Banner, Arran Peak, Dr. Mclntosh, and Red- at about 650,000 acres in 1954, of which skin, each contributing 3^ percent to the 530,000 acres were maincrops (including total maincrop acreage. Very few really late second-early potatoes), the rest being first- varieties are grown. early potatoes (M.A.F.F. [Gr, Brit. Ministry The planting of maincrops starts at the end of Agriculture, Fisheries, and Food], 1956). of March and continues into early May, ac- The consumption of potatoes per head is about cording to the season. Whole seed tubers 200 lb. per annum. are planted with the sets about 18 in. apart Maincrop production is concentrated in a few in the rows, and the rows 27 to 30 in. apart districts, with an even and relatively sparse (28 in. is the standard width). Lifting usually distribution elsewhere, as shown in the dis- begins about mid-September, but most of it tribution map (for 1954) (fig. 3). The principal is done in October. The growing season is districts for maincrops are the Fens and the therefore rather long--about 160 days. The area round the Wash, where the crop is grown average yield is about 9 tons per acre. Chitting on deep organic loams or on silt soils pro- (sprouting before planting) is practiced fairly ducing very heavy yields; the Vale of York, extensively in early districts and in the Fens, where the chief soils are sandy loams; Lan- and this gives the crops a start of about 2 cashire and the Cheshire plains, mostly on weeks extra growth at the beginning of the sandy soils of glacial origin. Important potato season. A newly revised bulletin on **Potatoes'* areas are also in Bedfordshire, Essex, and (M.A.F.F. [Gr. Brit.], 1957) provides full in- Kent, with London as their nearest market; formation on potato cultivation in England and Wales. Early varieties are grown in most of the principal maincrop districts; but the first supplies for the early summer market come ENGLAND ft WALES from Cornwall, Pembrokeshire, the Scillies, and the Channel Islands. The principal first-

Each Dol rcprcscnU lOOO Acres early varieties are Arran Pilot and Home Guard. In 1954, Arran Pilot accounted for over 50 percent of the total first-early acreage, and Home Guard for 21 percent. Other popular early varieties include Ulster Chieftain, Ulster Prince, Duke of York, Eclipse, Ninety- fold, and Sharpe's Express. Epicure and May Queen have diminished in popularity since 1945. In Cornwall and Pembrokeshire the planting of early potatoes starts in early February and continues into March, with lifting toward the end of May or early June. The seed tubers are planted 12 to 15 in. apart in rows 24 to 28 in. apart. Often they are grown on the flat, without earthing up. Yields are from 3 to 5 tons per acre. Early varieties in the other districts, notably parts of Lincoln- shire, the Cheshire Plain, Bedfordshire, Essex, and Kent, are planted later and provide the continuation of supplies of '*new potatoes in June and July. Seed-potato growing in England and Wales is chiefly confined to the production of good FIGURE 3.--Distribution of maincrop potatoes in England and Wales. •* once-grown'* stocks--for which there is a

10 certification scheme. Most of the **new*' seed among the early varieties contribute to the Is imported from Scotland and Ireland, al- sources of infection of the maincrop in the though the upland or coastal areas in Devon, vicinity each year. Cornwall, Cun:iberland, and parts of Wales, where climatic conditions are unfavorable DELIMITATION OF '»BLIGHT ZONES" for the spread of virus diseases, produce small quantities of the higher grades of During the second World War the need was certified seed. About 60 percent of the main- found for some precise but practical field crop potato acreage is planted with certified method of defining the course of blight on seed, almost all of which is imported from potato crops in different seasons and in Scotland or Ireland (Boyd and Lessells, 1954). different parts of the country. As a result of this, an assessment key (as shown in col- Key for the assessment of potato blight on the umn one) was worked out by a Disease Mea- haiilm surement Committee of the British Mycologi- cal Society (B.M.S., 1947). Blight This key was successfully employed in Percent local surveys (Moore, 1948) and for the study 0.0 Not seen on field. of foliage protection in spraying trials (Large, 0.1 Only a few plants affected here 1945). In 1950, when a new section for plant and there; up to 1 or 2 spots in a disease measurement and quantitative survey 12-yard radius. work was started at the Plant Pathology 1.0 Up to 10 spots per plant, or gen- Laboratory (Large, 1953b, 1955a), the key eral light spotting. was put to use in national surveys. These 5.0 About 50 spots per plant, or up to surveys, using a uniform plan, continued 1 leêiflet in 10 attacked. <'¡ for 7 years, are reported by Large (1956). 25.0 Nearly every leaflet with lesions, Thousands of records have been obtained plants still retaining normal from all parts of the country; for England form; field may smell of blight, and Wales, it is at last becoming possible but looks green although every to define, measure, and compare the blight plant ciffected. epidemics occurring over a sequence of years 50.0 Every plant affected and about and to put an evaluation of the losses the half of leaf area destroyed by epidemics have caused upon a factual basis. blight; field looks green flecked This is a realization, in some part, of plans with brown. long laid by W. C. Moore, whose address 75.0 About three-quarters of leaf area to the Association of Applied Biologists in destroyed by blight; field looks 1949 on the significance of plant diseases in neither predominantly brown nor Great Britain (Moore, 1949) provides the green. In some varieties the background for all that follows. youngest leaves escape infection, Two points on the curves obtained by plotting so that green is more conspicuous progress of blight on a time scale are of than in varieties like King particular importance (Large, 1952a). The first Edward, which commonly shows is the 0.1-percent stage, defining the beginning severe shoot infection. of epidemic spread, or ''outbreak,'* for pur- 95.0 Only a few leaves left green, but poses of forecasting and the timing of spray- stems green. ing. The second is the 75-percent stage of In the earlier stages of a blight epidemic, defoliation, at which time further tuber devel- parts of the field sometimes show more opment is arrested (see fig. 8 and the section advanced decay than the rest, and this on computation of losses). This latter stage is often associated with the primary is discussed first, as the 75 percent maps foci of the disease. Records may than (75 to 90 percent in practice) provide the best be made as, 1 + pf 25, where pf 25 single picture of the severity and frequency means 25 percent in the area of the of the epidemics in . the several^ regions of primary foci. the country (fig. 4). Losses by potato blight in England and Wales Of the 6 years illustrated, 1950, 1953, and occur chiefly on the maincrop and second- 1954 were *'blight years,'* with losses of all early varieties. Although the first outbreaks the September growth over most of the south of blight each year are usually on the early of the country, and losses of some of the varieties, these are lifted before sufficient August growth also in the southwest and the defoliation occurs to affect the yield, and Fen district round the Wash, but with little usually before the tubers are infected. The or no loss in the north. In the other years following study of late blight epidemics in losses of growth occurred in the west, but England and Wales therefore refers essen- only here and there in the other zones. From tially to the disease on maincrop potatoes a study of the maps (Large, 1958) it becomes (taken throughout as including second-early possible to divide the country into four main varieties), although, undoubtedly, blight foci **blight zones":

11 sey; with an earlier sub- ENGLAND 6 WALES zone containing Corn- wall and Pembroke. Fen zone An area round the Wash, roughly equivalent in acreage to that of the Isle of Ely and the Holland division of Lincoln- shire, but extending somewhat into Hunts, and West Norfolk. Southern zone All the rest of the country south of Denbigh, Cheshire, Derby, and Notts., but including the Ke Steven and Lindsey Divisions of Lincoln- shire, and also the northern county, Cum- berland, where the epidemics are similar to those in the south. Northern zone All remaining northern part of the country. These zones are indicated in figure 5, and it will be seen how closely they represent the mean pattern of shading--for time of 7 5-90 percent defoliation--in both the blight years and the years with little blight in the east. Naturally, these zones are not in fact limited by county boundaries, but their areas are here reckoned in equivalent county potato acreages in order to arrive at the approximate percentage of the country's total maincrop acreage that is contributed by each zone. This, on the basis of the 1954 distribution (fig. 3), is given in table 1.

FREQUENCY AND SEVERITY OF THE EPIDEMICS ACCORDING TO ZONE

With the above zoning, and taking in the years 1947 and 1949, which were dry years, and 1948, a '*blight-year" rather similar to 1950, the frequency of epidemics of differing degrees of severity, in the 4 zones, over the 10 years, 1947-56, was as shown table 1. The mean dates at which growth was stopped by blight are for unsprayed maincrops, and by reference to figure 8 and the section of computation of losses of which it is part, it is FIGURE 4.--Zoning of England and Wales according to the date at possible to proceed directly to estimates of which the 75 to 90 percent blight stage was reached on unsprayed the mean loss in yield due to defoliation on maincrop potatoes in the years 1950-55. the unsprayed crops in each zone and for England and Wales as a whole over this Southwest zone- - -A southwestern and period. These estimates are entered in the west-coastal belt in- table--probably the first of their kind ever cluding the potato grow- to be presented for an entire country. ing areas in Cornwall, This analysis indicates that if all the main- Devon, Somerset, Glou- crop potatoes in the country had been left cester, Monmouth, unsprayed during the 10 years, the mean Glamorgan, Car- annual loss due to premature defoliation by marthen, Pembroke, blight would have amounted to about 8 per- Cardigan, Merioneth, cent; that is to say, 8 percent of the total Caernarvon, and Angle- yield that would have been obtained had blight

12 ENGLANDS WALES

FIGURE 5.--Key map, showing blight zones in England and Wales. been entirely absent. But in the southwestern 10, about 14 percent; and in the large southern zone, where every year was a ' *blight-year,*' zone, with only five moderately severe blight the mean loss would have been about 20 per- years in 10, about 7 percent--less than the cent; in the Fen zone, with 6 blight years in average for the country as a whole.

13 TABLE 1. --Frequency and severity of potato blight epidemics in England and Wales, 1947-56 Mean loss per Number of years in 10 in which growth was annum the 10 stopped by blight at— Maincrop years to— Zone acreage End Aug., Mid-Sept., End Sept., Mid-Aug. Whole 13 percent 4 percent no Zone 28 percent country- loss loss loss loss

Percent Percent Percent Southwest 8 5 3 0 0 20 1.7 Fens 13 2 1 3 14 1.8 Southern 51 0 5 2 3 7 3.7 Northern 28 0 0 5 5 2 0.6

England and Wales.... -- — ------7.8

In the worst of the individual blight years, for a **blight year" (1954). There is no con- such as 1950 or 1953, the national loss, on stant relation between the outbreak date and the same basis, would have been about 16 the date at which tuber development is arrested. percent; and in the years with least blight, The progress curves may be steep, or they such as 1951, 1952, or 1955, a little over 2 may be long-drawn-out, according to weather percent. conditions. Even in severe epidemics it is rare for the attack to progress from the COURSES OF THE EPIDEMICS IN THE FOUR 0.1 to the 75 to 90 percent stage in less than ZONES 5 weeks, and in a lingering attack the period may be 2 months or more. Having established the blight zones of the Blight is not found at the 0.1 percent stage country by reference to the 75 to 90 percent in all the unsprayed crops in a region on the maps, we may now consider the development same date; there is commonly a range in of the epidemics within each zone. Figure 7, time of about a fortnight, between the first given in the section on forecasting (p. 18), and the last appearance of this small amount shows the outbreak (0.1 percent) map, and of blight. Table 2 gives the range of outbreak the regional blight progress curves obtained dates for each zone for the 7 years, 1950-56. TABLE 2. --Times of outbreak, and weeks of growth lost at the end of the season, maincrop potatoes, 1950-56

Southwest zone Fens zone Southern zone Northern zone Year^ Growth Growth Growth Growth Outbreak Outbreak Outbreak Outbreak lost lost lost lost

Weeks Weeks Weeks Weeks 1950-B... Early July 7 Early July 7 Late July 5 Esirly Aug.- 3-1 mid-Aug. 1931 Late July- 3-5 End of Aug. 2-3 Early Aug.- 1-3 Late Aug.- 1 early Aug. mld-Sept. mid-Sept. 1952 Early July- ^-^ Mid-Aug.- 1-2 Early Aug.- 1-3 Early Aug.- 1-3 early Aug. end Sept. end of mid-Sept. Sept. 1953-B... Early July- 6-8 Early July 5-6 Late July- 5 Late July- 1-3 late July end July late Aug. 195^-B... Early July- 7-6 End of July 5 Late July- 5 End of July- 1-3 late July end July late Aug. 1955 Late June- 5-7 End of July- 3-5 Late Aug.- 1-3 Mid Sept.- 0-1 mid-Aug. late Aug. end Sept. end Sept. 1956-B... Late July- 5-7 End of July 6-7 Early Aug. 2-5 Early Aug.- 2-3 early Aug. late Aug.

■'• B = blight-year in the Fen and southern zones.

14 "Early July'' signifies the first 2 weeks of that month; ** mid-July'' the middle 2 weeks; ENGLAND 6 WALES '*late July" the last 2 weeks; and **end-July" . Northern Zone the last week of July and the first week of DURHAM RAINFALL August. In this table the 7 5 to 90 percent dates inches for the several zones and years are also given, but they are expressed in terms of the number of weeks before the end of September at which this stage was reached. The resulting numbers are in fact good code figures, of real biological significance, for express- J FMAMJ JASOND ing the severity of a blight epidemic, where, as in England and Wales, tuber development . South-western Zone in crops free from blight normally ceases PENZANCE by that date. By reference to the histograms at the bottom of figure 8, or the growth curve above them, these code figures may be translated directly into probable losses in tons per acre, or in percentage of potential crop. In the southwest zone outbreaks occur in

July every year, with frequent outbreaks in JFMAMJJASOND June, and even in May on the early varieties in Cornwall and Pembroke. In the Fen zone . Southern Zone they may occur early or quite late in July even in the blight years; while in the southern RAINFALL 4 zone generally outbreaks are in late July in inches the blight years, with a tendency to be earliest in the coastal regions and latest in the central, inland area. In the northern zone, August outbreaks are most common, with many fields

remaining unaffected until late August or FMAMJ JASOND

September. Outbreaks tend to be somewhat BLIGHT YEARS OTHER YEARS earlier in Lancashire and the southern part of the Vale of York than elsewhere in the -60°F r^^^'^'^ ^""^-ôO'F

zone. RAINFALL 50"F 4( 50° F inches WEATHER CONDITIONS

Climagrams showing the long-term average rainfall and temperature conditions in the

four zones are given in figure 6. The upper J J A S J J A 5 diagram, for Durham, in the northern zone, represents a district where the epidemics are FIGURE 6.--Long-term average monthly rainfall and mean tempera- latest and least severe; while the second mres in relation to potato cropping for three zones in England and diagram, for Penzance in the extreme south- Wales, with July to September means for five blight years and five western subzone, represents conditions where other years in the southern zone. they are earliest and most severe. It will be seen that even between these two extremes are at altitudes below 200 ft., the average there is no very great difference in mean annual rainfall does not exceed 30 inches. rainfall over the growing period. The prin- Rainfall and mean temperature figures for cipal difference is in the higher winter and the months of June, July, August, and Septem- spring temperatures at Penzance, which, with ber over the 10 years, 1947-56, are given in freedomfrom frosts in the sheltered coves, per- table 3. Data for Cambridge have been chosen, mit the spring cropping of first-early varieties. as the deviations from average at this station However, very great local variations in are fairly representative of a very large both annual and summer rainfall occur in part of the important southern blight zone England and Wales. The country is moun- and also of Fens. The years are grouped tainous to the west and north. On parts of the into **blight years** and the **other years,** Pennine s, the Welsh mountains, and the high in which the blight attack was late and caused moors of Devon and Cornwall, the annual rain- little loss. fall exceeds 80 inches and it reaches 150 To bring out a fact that disturbs all pre- inches in a few places. Over most of the dis- conceptions about reduction of potato yields tricts in which potatoes are grown, however, in blight years, the mean potato yields for and certainly in the principal districts, which England and Wales, for each of the years.

15 TABLE 3.--Rainfall and deviations from average temperature at Cambridge for June to September, 1947-56, and mean national potato yields in '^blight years^» and other years

Rainfall Mean temperature Mean Year, average, Potato and mean June July Aug. Sept. June July Aug. Sept. yield

Inches Inches Inches Inches °F. °F. Tons/acre Long-term average... 2.1 2.2 2.4 1.6 ' 50 62 61 ' 57

"Blight years": 1948 2.4 1.5 ^.5 1.6 +2 0 +1 +3 7.6 1950 1.5 7.7 2.6 2.5 +5 0 0 -1 7.7 1953 1.9 2.0 2.5 1.3 -1 -1 0 0 8.7 1954 , 1.7 2.7 3.6 1.7 -1 -4 -2 -2 7.9 1956 1.9 3.5 4.9 1.4 -3 0 -4 +1 8.3 Mean 1.9 3.3 3.9 1.7 0 -1 -1 0 8.0

"Other years": 1947 3.3 1.5 .2 1.0 +4 +3 +5 +3 5.S 1949 3.3 1.2 .2 +2 +4 +3 +7 6.6 1951 1.2 .8 3.3 2.2 0 +1 -1 +2 7.9 1952 1.7 .5 2.6 2.3 +2 +2 +2 -4 7.9 1955 2.1 .5 1.6 1.6 0 +2 +4 +1 7.3

M9aii 1.9 1.3 1.9 1.4 +1 +3 +3 +2 7.1

are added to table 3. The figures are taken HUMIDITY CONDITIONS IN THE FOUR ZONES from the last published Agricultural Statistics (M.A.F.F., 1956) with comparable estimates The southwest zone, in which severe or fairly from other sources for the years 1955 and severe epidemics occur almost every year, 1956. The figures are for all varieties (early is exposed to southwesterly winds bringing as well as maincrop), but they serve well as in- in moist maritime tropical air. In this zone dex figures for the level of yield in each year. from June to September, there are 1,000 to The blight years are characterized by high 1,250 hours during which the relative humidity rainfall and average or slightly low tempera- is 90 percent or more. These are mean dura- tures in the months of July and August; while tions as found by Smith (1954, 1956a) in his the other years, with little or no blight, were humidity surveys of the country over the associated with low rainfall and temperatures years 1950-54. Cumberland is also exposed decidedly above the long-term average to this moist air stream; but most of the ( + 3°F.) in these months. These differences southern blight zone is in the lee of the are shown in figure 6. mountains and high land to the southwest and The first and overriding economic effect has a seasonal mean of only 750 to 1,000 hours of the abundant rain and moderate tempera- of such high humidity. In the northern zone tures in the blight years was to favor the particularly, dry areas occur in Lancashire growth of the potato plants, thus raising the and Cheshire in the immediate shelter of yields above average despite whatever losses the Welsh mountains; and also in the lee there may have been from potato blight. of the Pennine s in Yorkshire, where the That abundant rainfall and moderate tem- count of high humidity hours is less than peratures in July and August also favor the 750. This goes far to explain the low level development of severe blight epidemics is of blight incidence in the northern zone. In well brought out in table 3, but this also shows the Fen zone, near the Wash, there is a high that a severe epidemic, such as that of 1953, water table in low-lying land, with exposed can occur with only about average rainfall water in many drainage channels. Undoubtedly (Z\ in.) and normal temperature (about 61° F.) the Fen zone differs from the rest of the in August. Such conditions can occur any- south in high humidity due to these causes, where in the country, and they may or may and, in comnnon with some other east coastal not accompany severe blight attacks. The areas, it is occasionally affected by fogs main differences responsible for the differing comiing in from the North Sea. general levels of incidence of blight in the Even such regional humidity differences, four blight zones are to be sought, not in however, do not entirely account for the mean temperature and rainfall, but in humidity. differences in frequency and severity of blight

16 in the four zones, and in particular districts the crop, and the next spell of critical weather within the zones. These differences have been occurs, that distant or epidemic spread begins. studied over the past 7 years in terms of the Epidemic spread occurs when blight at the 0.1 frequency and times of occurrence of Beau- percent stage by the B.M.S. key (see p. 11) is mont periods;2 that is to say, of humidity found over a considerable area in a field crop, periods of such duration and at such a tem- and this stage is taken as constituting an perature as to permit sporulation and spread ^'outbreak'* for forecasting purposes in Eng- of the blight fungus itself. The recording of land and Wales. these periods all over the country has been Important studies have been made by Hirst primarily for forecasting purposes; but it (1955) on the very early stages of the blight has also revealed the weather spells after epidemics in England and Wales. However, the outbreak, during which further spread is there is still a cryptic and only partially ex- favored and the epidemics can progress most plored phase in the development of blight rapidly. (See fig. 7.) epidemics, which includes the whole of the FORECASTING happenings in the fields between the inadvertent planting of blighted tubers in April and ,the Since 1950, potato blight forecasting by the first appearances of new infections at the'0.1 Beaumont humidity-tempe rature rule, with percent stage on the resulting foliage in July, instruments in screens 4 ft. above ground level August, or even September. Whatever the true at standard weather stations, has been investi- story of this phase may be, forecasts based gated and developed throughout England and on the occurrence of the first flushes of Beau- Wales. Some 60 weather stations have taken mont periods in screens after the last week in part in the work, and the validity of the fore- June are now enabling correct regional spray casts (based on hourly records of humidity and warnings to be given year after year in Eng- temperature) has been tested by hundreds of land and Wales. Thus, forecasting in this observations of blight outbreak dates in the country is now moving out of its investigatory potato fields in all parts of the country each stage into firm establishment in practice. year (Large, 1953a, 1956). From this work it The maps (fig. 4) show plainly that it is chiefly has been found that with observations in in the southern and Fen zones that forecasting standard screens at weather stations and not services are required, and it is here that the actually in the crops, individual Beaumont scheme as now developed is giving its best periods are often at fault, but that **flushes** performances. of such periods, occurring at a number of Figure 7 shows the operations chart for a stations within a blight zone, as revealed by an blight year (1954). There was the usual flush '^operations-chart'* (fig. 7), provide a reliable in June, too early to herald the outbreaks. In basis for regional forecasting when an 1954 it was the flushes about July 10 and July 17 additional working rule is introduced. This that gave warning throughout the southern and additional rule is that in the southern. Fen, and Fen zones of first outbreaks occurring over the northern blight zones, **flushes" occurring be- last week of July and the first week of August. fore the last week in June are disregarded, and In the following year, 1955, there was no flush that only **flushes" at or after that time are of warnings after the end of June until about to be taken as warnings. August 13 in these zones, correctly indicating This is anadaptationof the Beaumont scheme the absence of outbreaks until the end of to the particular circumstances within the August, too late to justify protective spraying. blight zones in England and Wales (other than In England and Wales the possibility of in the southwestern zone, where June flushes forecasting by 90 percent humidity criteria has are valid). This adapation reflects the fact that also been investigated (Smith, 1956b). By although June flushes occur every year, and analyses over the 5 years 1950-54, it was they do undoubtedly permit the sporulation of found that the substitution of 11 hours at 90 the blight fungus on the very few and widely percent relative humidity for the Beaumont scattered infected shoots arising from blighted humidity requirement would give some slight tubers planted each year, they do not initiate improvement in the performance of the scheme epidemic spread. The June flushes give rise as now operated, but that the disadvantages of to small primary foci involving a few plants a changeover would, in practice, outweigh the only, and it is not until the potato plants begin advantages. to meet the drills, reducing ventilation within AMOUNT OF PROTECTIVE SPRAYING DONE *By the Beaumont rule (Beaumont, 1947), spells of not less than 48 consecutive hours during which the relative humidity does not fall The percentage of the crops in England and below 75 percent and the temperature is not less than 5(P F, are Wales in which protective spraying or dusting expected to be followed by blight outbreaks within 14 to 21 days. Near-critical periods are those in which these conditions are not is done is provided by records from a repre- broken for more than 1 hour in the 48. sentative sample of some 2,000 crops each

17 '. 1 s OD "> 11 1 i:_, 1 J JJ LJ---I 1: tt T , 1 1 ii^H i - 1" 1 i 00 ^r-l^^ J X -1 i-( ' -i-i -1 -- --1 ^-1 ^ -1J' ÏJ'^^' 1 1 [ 1 1 1 1 1 1 1 1 ^ r(iiiiir-"tl"""""" . Ë.2 3:!^_jj_.j__j__!_.^ i_v ÎÎ-HÎ------'-^^^- d s _|, J -^^,]H!J ^J^. T ' ^ •' 1 1 M. 1 ^ ' JJ j JJ j_ 1 uJ U J- o ¿ .HHjj tj-i-ijjjjî -"HHHHiUi -ijjH;!l H"J i" H «"« «*. Q ,ir,,4||JMM 1^^^ il 111Ij 1 1 ^^ 1 ~*0 - - JJ -4J 111 *J- ♦.-- T ♦>" ♦* ». n •• 1i tf> 1 Í ..|0> *• T 1 ^^ î'i ^ 9 "ii ~ 11111" 1 M]"i";r^ n^ c rH 0 1 1 ^ -1 ^—r--ii|1"^""t" ] .s ^ i1 HIII H -" " « JC 2i c 0> 3 jQ Wl -"M"!"'! - 3 Je: ><: t O 5

CO LU tf —5 - s c r^ < N (/) _i gS n* Sf ^ ¿-Si <(9 II

18 year on which crop-check weighings are car- losses of crop due to defoliation, from blight ried out for the purpose of estimating seasonal progress curves for the haulm read in conjunc- supplies.3 Table 4 shows the percentages of tion with bulking curves for the tubers, has the maincrops in the four blight zones which been given by Large (1945, 1952). The practical received any protective treatment in each of use of this method is here briefly explained by the years 1953-56. reference to fig. 8.

TABLE 4. --Percentage of maincrop potatoes ENGLAND 6 WALES that received at least one protective treatment in England and Wales, 1953-56

Zone 1933 1954 1955 1956 PERCENTAGE :i::::::—=^— -.-<--A OF CROP , ^^

^^ ■

, • ,"■" 1 Per- Per- Per- Per- 'A- ^' r—1 cent cent cent cent ^î ' ~i" " ^ ^ X-T 1 i J^~^'^~^ ' Southwestern.••• 16 16 19 32 /> -t1^ 1 ^ Fens 55 49 -à' * i 51 62 Z .: 1 ^ ■

Southern • , 4-0 44 38 50 ^ ■■ Northern. t 4 8 4 7 ^ y Average^ England ~^ '■■ -r- and Wales 28 32 28 39 Z ih "^' _i_ , On the showing of these figures the proportion of the crops protectively treated was about the same in the year with very little blight lO 20 in a lo r>o (1955) as in the blight years 1953 and 1954, but AUGUST SEPTEMBER ^ I I there was an increase in the blight year 1956, 72% 96% IOO% and the number of applications, not ascertained in these surveys, was undoubtedly greater in the BLIGHT ON blight years. The figures indicate that in recent HAULM years only some 30 percent of the crops (miain- per cent crop) in England and Wales were protectively treated; the proportion reached 50 to 60 percent in the Fens and some parts of the southern zone, notably Kent and Essex; 20 percent in the southwestern zone, and only about 5 percent in the northern zone. As regards variety, in 1956 TONS PER about 60 percent of all the King Edward crops WEEK in the survey were sprayed; 30 percent of the (lO-Ton crop) I^Ib Majestic; and also 30 percent of the other 8 7 6 5 4 3 2 varieties. WEEKS FROM END OF SEPTEMBER Thus, it will be seen that in England and FIGURE 8«--Mean growth curve for maincrop tubers in England and Wales, where more than half the crops are left Wales, showing percentage of crop obtained according to the date at unsprayed, it is possible first to estimate the which blight reaches 75 percent on the haulm. Also the mean tuber loss caused by unrestricted blight epidemics production per week in a 10-ton crop free from blight is shown. on unsprayed crops, as has been done in table Dotted growth curve shows deviation in late-planted crops. 1, and then to proceed to examine the extent to The upper graph in this figure gives the which this loss is, or could be, reduced by growth curve for maincrop tubers provisionally protective spraying, in each of the now-defined taken as normal for England and Wales when blight zones. the haulms are unaffected by blight and die down naturally at the end of September. The COMPUTATION OF BLIGHT LOSSES AND curve is based on the results of fortnightly GAIN FROM SPRAYING liftings on 60 crops in the 6 years 1940-45 The derivation of the method, now employed (Doncaster and Gregory, 1948), and is there- in England and Wales, for estimating average fore a mean curve. In some years the course of tuber development was relatively early and This work of crop check weighing is done by the Potato Marketing in others relatively late. The curve is for Board, with whose kind permission these and other supplementary Majestic, the principal English variety, but figures were obtained for the Plant Pathology Laboratory in the course of the **crop-check surveys" included in this study. A brief experience so far has shown that within prac- account of the organization and scope of the surveys is given in a tical limits, it may also be used for the other paper by Large and Honey (1955) on the incidence of common scab early maincrop varieties grown in England and of potatoes in England and Wales. Wales, notably King Edward (Rosser, 1957).

19 Increase in the crop of tubers is taken to 8.3 tons. On a 15-ton potential crop all these cease when blight on the haulm reaches the 75 figures are one-and-a half times as much. percent stage; the slight increase, if any, after It must be remembered that the provisional that stage is offset by the slowing down of normal growth curve is derived from the mean growth before it, which begins at the 50 percent for a number of crops over a number of years. stage. Its valid use is therefore for estimating mean Hence, when the blight progress curve on the losses oVer a number of years. Individual crops unsprayed haulm is as **A,*' in the middle in individual seasons miay vary considerably diagram, and reaches the 75 percent stage in in rate and timing of tuber production, and on mid-August, only 72 percent of the potential them the losses from blight and gains from crop is produced and the loss (of potential spraying must be expected to vary about the crop) is 28 percent. long-term norm. If spraying retards the destruction of the One of the principal causes of deviation is haulm by a little over a fortnight, giving a late planting (Dyke, 1956), which tends not only progress curve as **B,*' with the 75 percent to reduce the potential yield but also to in- point at the end of August, then 87 percent of crease the weekly increments of growth in the the potential crop is produced and the loss is latter part of the growing period. As a guide to 13 percent. The gain from the fortnight's the type of growth curve that may be expected prolongation of growth over the second half of where the crop is planted late (or has suffered August is therefore 28 minus 13, or 15 percent retardation in its early stages), a dotted curve of the potential crop. is added to figure 8, representing the mean Except in those very rare cases in which the course of tuber development in 16 late-planted haulm is kept completely protected from blight crops unaffected by blight in the southern zone until the end of the growing period, the poten- in the years 1955 and 1956. Where the growth tial crop, were there no blight, is not known. curve follows the normal course, about half the This can be estimated, however, from the potential crop is laid down by the end of July; growth curve. If the yield from a sprayed crop, but on the dotted curve only about a third of the with 75 percent point at the end of August is, potential crop is then present. for example, 12 tons per acre, then 12 tons is The prolongation of growth from spraying, 87 percent of the potential crop. And the poten- i.e., the mean separation in time between the tial crop is therefore 13.8 tons per acre. two more or less parallel blight progress Similarly, but with less accuracy, the potential curves for sprayed and unsprayed, is, strictly, crop can be estimated from the yield of the best measured between the 50 percent, or unsprayed areas. **half-decay,** points on the 2 curves (Large, The gain from a given number of days pro- 1945). As, however, the difference in time longation of growth over a given part of the between the 50 and 75 percent stages is com- growing period will vary according to the level monly less than a week and the rate of tuber of fertility of the field and the vigor of the bulking on the normal growth curve does not potato stock, in short, according to the potential vary greatly over so short a time, the simplic- yield. Thus, to obtain a loss from blight, or ity of estimating prolongation of growth be- gain from spraying, in terms of tons per acre, tween 75 percent points on the curves for the the percentages read off from the growth curve sprayed and unsprayed crops outweighs any are multiplied by the potential yield in tons per possible slight gain in accuracy that would acre. In the example given above, where the result from stepping back to the 50 percent blight progress curve on the unsprayed is as points for this purpose. "A,'* and that for the sprayed as'*B,*'the gain from spraying, 15 percent, amounts to 1.5 CHECKS OF LOSS ESTIMATES BY SPRAY- tons per acre where the potential yield is 10 ING TRIALS tons per acre, and to Z^ tons, where the potential yield is 15 tons per acre. Since 1941, several series of spraying trials, The histograms at the bottom of figure 8 show each over a number of years, have been car- the normal increment of yield produced in each ried out in various parts of England and Wales. week, numbered backwards from the end of In these trials yields determined by weighing September, for a potential crop of 10 tons. This the crop from replicated plots were related is perhaps the most general level of yield in to the blight progress curves for the sprayed England and Wales, and the diagram is there- and unsprayed haulm. The **actual'* gains from fore useful for making quick approximate spraying were computed with the *'expected*' estimates of gains and losses with little calcu- gains, calculated from the 75 percent dates on lation. If, for example, in a bad blight year, the haulm in accordance with the method and spray deposits are washed off rapidly by rain example given in the previous section, using and cannot be renewed in time, only one week's the provisional normal curve for tuber develop- growth (say week No. 6) is gained by spraying ment (fig. 8). on a potential 10-ton crop, and this gainamiounts The first series was carried out by Beau- to only 0.7 ton. The unrecovered loss (weeks mont and Large (1944) in Devon and Cornwall 1 to 5) is 1.7 tons, and the yield obtained is (southwestern blight zone) during the yearr

20 1941-46. In the 4 years 1943-46, the effect of 5 the mean results (some hitherto unpublished) two >yell-tiined applications of freshly prepared for each year are given for two groups: Corn- bordeaux mixture (4.5.50), applied at about 100 wall and western districts of Devon where gallons per acre, by 5- or 7-row horse-drawn blight is exceptionally severe (extreme western or tractor sprayers with drop lances, was subzone), and the rest of Devon where condi- tested on uniform stocks of certified Majestic tions are more representative of the south- at a dozen centers or more each year. In table western zone as a whole. TABLE 5.--Results of spraying trials with 2 well-timed applications of bordeaux mixtures on Majestic potatoes in Cornwall and Devon, 1943-46

Date when blight Gain from spraying on unsprayed Prolonga- Total yield haulm reached-- tion of of Expected Year and area Crops growth by Actual 0.1 75 unsprayed from 75 spraying percent percent plots percent stage stage lifting dates

1943: Number Days Tons/acre Tons/acre Tons/acre Cornwall and western Devon 6 June 26 8 17 2.7 3,5 Eastern Devon 5 July 21 Aug. 23 21 6.6 2.9 1.5

1944: Cornwall and western Devon 10 July 25 Aug. 20 18 13.7 2.8 2.8 Eastern Devon 5 Aig. 8 Sept. 9 12 12.5 1.1 .7

1945: Cornwall and western Devon June 25 July 23 18 6.7 2.4 2.9 Eastern Devon July 17 Sept. 19 21 8.9 1.6 2.0

1946: Cornwall and western Devon July 7 Aug. 4 15 9.3 2.3 2.6 Eastern Devon July 18 Aug. 23 18 11.5 3.2 2.2

Here the **expected** gains, especially when 1947-51, at Goole on the Humber (fig. 5). This averaged over the run of years, are in good center is on the border of the northern and general agreement with the *'actual'* gains southern blight zones, where blight conditions as determined by lifting, which for all the labor more closely resemble those of Hertfordshire involved, were still subject to a standard error in the southern zone than those in the greater of at least half a ton per acre. In the more part of the north. The spraying was again done easterly part of the area (lower figures) the 75 with freshly prepared bordeaux mixture percent stage of blight on the haulm was (4.5.40), but with a knapsack sprayer. The two reached about a week after mid-August in 3 well-timed applications were put on at the rate years out 4, and in early September in 1 year of 200 gallons per acre on Latin-square plots only (1944). This is in good accordance with the of certified Majestic each year. The relevant frequency for the southwestern zone over the results are shown in table 6. later period of 10 years (1947-56) given in In the two blight years, 1948 and 1950, the table 1. The computed mean total loss of expected gain from 3 weeks prolongation of potential crop by blight over the 4 years was growth with blight at the 75 percent stage at 18 percent as against the mean of 20 percent the beginning of September was again in close entered in table 1, and the gain from spraying, agreement with the ** actual'* increase found by averaging 2.2 tons per acre over the 4 years, lifting, with yields subject to a standard error was about 12 percent. In these trials, therefore, of about ±0.3 ton per acre. In the two dry years, two carefully timed applications of one of the 1949 and 1951, when blight caused no loss, there most rain-re sis tant spraying materials was an indication of a loss caused by spraying. (freshly made bordeaux mixture) saved about This could not be attributed to wheel damage, two-thirds of the defoliation loss by blight. as the plots were sprayed by hand. It was The next series of trials was carried out by probably due to copper injury following the Beaumont, Bant, and Storey (1953) during insect damage common in dry years. Some

21 TABLE 6.--Results of spraying trials with 2 well-timed applications of bordeaux mixture on Majestic potatoes at Goole in Yorkshire, 1947-51

Gain from spraying Date when blight on Prolongation Total yield unsprayed hauljn re ached-- of of Expected Year growth by Actual 0.1 75 unsprayed from 75 spraying percent percent plots by lifting percent stage stage dates

Days Tons/acre Tons/acre Tons/acre 1947 Aug. 5 Sept. 28 14 14.9 +0.1 0 1948 Aug. 13 Sept. 6 21 10.3 +1.0 1.2 1949 Sept. 9 Sept. 28 10 15.5 -.9 0 1950 Aug. 4 Sept. 2 20 13.6 +2.1 1.6 1951 Aug. 28 Sept. 28 7 14.4 -.9 0

yellowing and scorching was observed on the knapsack sprayer, and copper oxychloride sprayed plots. (50 percent Cu) was used at 5 lb. per 100 The third series of trials was that conducted gallons per acre. There were always 2 and by Large, Storey, Taylor, and Yule (1954) over sometimes 3 applications, timed as far as the 6 years, 1948-53, at Terrington, south of possible in relation to Beaumont periods. The the Wash, and on silt land at the border of the trials were made each year in each of the 5 Fen blight zone (fig. 5). The variety was King counties, Cheshire, Shropshire, Staffordshire, Edward, with good certified stocks in all years; Warwickshire, and Worcestershire. In general the replicated plots were large--up to an acre the epidemics were earliest in Worcestershire, in area and running the whole length of the and table 8 shows the leading results in this field; and the spraying was done on contract. county. A 13-row tractor-hauled sprayer was used to Here again there was very good agreement apply 6 lb. per acre of copper oxychloride between the expected and the actual gains, and (50 percent Cu) at 100 gallons per acre, except in all cases the expected gains were within in 1949 when a proprietary burgundy mixture the standard error of those determined by was used. This high-volume spraying was lifting. compared with low-volume spraying, using an Occasionally, in individual trials, the esti- Agro air-blast machine with droplegs, and also nnated gain has varied significantly from that with dry dusting. The number and timing of the determined by lifting, owing sometimes to a respective applications were as shown in figure greater effective prolongation of growth than 9, which gives the blight progress curves for that indicated between 75 percent points on the the several treatments and years. In table 7 the curves for sprayed and unsprayed where these leading figures for the spraying treatments are have not shown the usual parallelism; and abstracted. sometimes to exceptionally late bulking, to late In the course of these trials separate planting, or to other causes. weighings were made from rows damaged and In a comparable group of 32 crops of Majes- not damaged by the passage of the sprayer tic and King Edward, all planted about the wheels. With 4 rows in 13 so damaged, the same time--the first half of April--and all in mean loss due to this cause was 4 percent of the southern blight zone, on which fortnightly the potential yield. A deduction of 4 percent liftings were made in supplementary work on from the expected gain has therefore been made the crop check surveys in 1956, 15 reached in the table. With this correction the *'ex- the 75 percent stage, or were burned off, pected" gains are once again in close agree- shortly before the end of August, and 17 did not ment with the **actual" gains as found by lifting reach this stage until mid-September. The the crop (yields subject to standard error of mean yields, in tons per acre, for the two to.5 ton per acre). In these trials there was a groups are shown in table 9. statistically significant loss from spraying in The mean potential yield for both groups of the two hot and dry **no-blight'* years, 1949 crops was about 12 tons per acre. The percent- and 1952. This loss considerably exceeded the ages of the potential crop present on the loss due to wheel damage, and it was probably several dates were, therefore, as shown in caused by copper injury, as at Goole, but in this table 10. case following both insect and wheel damage The percentages in table 10 corresponded in the hot dry season. very closely to the normal growth curve in A fourth series of trials was carried out in figure 8, and the mean gain from prolongation the west Midlands over the years 1950-54 of growth from the end of August to mid- (Rosser, 1957). Here the trial variety was September, 1.3 tons per acre, was in exact King Edward, the spraying was done with a accordance with expectation. 22 ENGLAND 6 WALES

Jun€ July August September October lOO -jd—fr 7^ ^^-i:^— BLIGHT 1948 ON d = dusting HAULM Q- atomising p«r cent SO w- wet spraying t= top spraying Gain from spraying I TON per acre

lOO fd—75" TT Tw&a 1949 NO BLIGHT 50

Loss from spraying 2 TONS per acre

lOO 75 ^ id dTTd"^ T I950 Twaa waa ^&Q^X I

50 ■^-^-^^ i Burnt-

Gain from_ spraying 3 TONS peracre O

lOO Td fd I'd jrd 1951 TWêC TWiO&a TwaaTW

1952

1953

HGURE 9.-Progress charts of blight on unsprayed. sprayed, and dusted King Edward potatoes in trials at Terrington. near the Wash. 1943-53. showing dates of applications and approximate gains and losses from spraying.

23 TABLE 7. --Gain or loss from spraying King Edward potatoes at Terrington, 1948-53

Date when blight on Gain from spraying Prolongation Total reached— of growth yield of Year Actual Expected 0.1 75 by Ijnsprayed spraying plots by from 75 percentage percent lifting percent dates stage stage

Days Tons/acre Tons/acre Tons/acre 1948 July 30 Aug. 13 12 16.3 +0.9 +1.9 1939 (") 0 14.8 -2.0 -.6 1950 July 24 Aug. 17 25 14.0 +3.0 +3.0 1951 Mg. 30 Sept. 22 10 16.2 -.1 -.6 1952 (") (') 0 18.9 -1.8 -.7 1953 July 7 Aug. 13 13 16.6 +2.6 +2.3

No blight.

TABLE 8.--Gains from spraying King Edward potatoes in Worcestershire, 1950-54

Date when blight on Gain from spraying Prolongation Total yield reached of of Year Actual Expected growth by unsprayed from 75 by 0.1 percent 75 percent spraying plots percent lifting stage stage dates

Days Tons/acre Tons/acre Tons/acre 1950 July 24 Sept. 4 22 12.9 2.3 + 0.9 1.5 1951 July 31 Sept. 20 10 11.8 .5 + .7 .3 1952 Aug. 7 Sept. 16 14 10.4 1.0 + .6 .5 1953 July 24 Aug. 24 3 10.5 -.4 + .7 .3 1954 July 20 Sept. 4 16 14.4 1.6 + .3 1.3

TABLE 9. --Meanyields of 32 crops of Majestic and King Edward potatoes in southern blight zones as related to date blight reached 75 percent stage, 1956

75 percent Total yield blight stage Crops End of lYiid- reached End of Mid- End of July Aug. Aug. Sept. Sept.

Number Tons/acre Tons/acre Tons/acre Tons/acre Tons/acre End of Aug 15 6.1 8.4 9.9 10.3 10.3 Llid-Sept 17 5.6 7.6 10.3 11.5 11.6

24 TABLE 10. --Yields in table 9 expressed as percentages of mean potential yield (12 tons per acre)

Percentage of potential yield L, 75 percent stage reached— Crops End of Mid- End of Mid- End of July Aug. Aug. Sept. Sept.

Number Vnñ of Au? 15 51 70 S3 86 86 Ml ñ ->Sp"nt 17 47 63 86 96 97 Expected yield (see fig. 8) 50 72 87 96 100

Further work is required on checking the about as good results as high-volume machines normal growth curve and in adapting the applying the same materials. But owing to method of estimating blight losses for use on the weight, complexity, and high cost of the individual crops. Enough has already been done, Agro sprayers, they have since been replaced however, to show that the present curve can by low- or medium-volume machines of the now reasonably be employed without further type introduced for the application of 2, 4-D delay for the computation of average blight and MCPA weed killers on cereals. These losses over a number of seasons, in the several machines apply 20 to 40 gallons of spray blight zones of England and Wales. fluid per acre at pressures of 50 to 100 lb. per sq. in. through fine nozzles. Such ma- PROTECTIVE SPRAYING: MACHINES AND chines, which are light, relatively inexpen- MATERIALS sive, and often already on the farm, make it easy for farmers to do more potato spray- Freshly prepared bordeaux and burgundy ing themselves. The old difficulty of water mixtures, although unsurpassed in point of cartage is greatly reduced. But it is doubtful efficiency, are now very little used for potato if spraying the plants from above only, as is spraying in England and Wales. Their place usual with these machines, gives as good has been taken by copper oxychloride and protection of the foliage as spraying with the cuprous oxide preparations, usually applied at traditional droplances (Large and Taylor, the same copper dosage (2.5 lb. Cu per acre). 1953). As yet the machines are often not These preparations are much less resistant to sufficiently adapted mechanically for the washing off by rain than the bordeaux and particular and rather exacting job of potato burgundy mixtures (Large, Beer, and Patter- spraying; infinite trouble, as well as very son, 1946), and they save the need for a certain poor spraying, will result from blockage of amount of skill and labor in the preparation the fine nozzles unless the filters are kept in of the spray fluid on the field. Unlike bordeaux good order. With copper oxychloride (50 per- and burgundy mixtures , copper oxychloride and cent Cu) at 400s. per cwt., the cost to the cuprous oxide preparations can be used for farmer for materials only, is 18s. per low-volume application. Generally, they give application. nearly as good protection of the haulm as the A considerable amount of dusting is done in bordeaux and burgundy mixtures, except where the Fens. At one time 6 to 7 dustings were high retention is really needed to resist heavy given in the course of the season, but the rain, as in the Fens in the years 1954 and more usual number of dustings is now 3 or at 1956, when the performance of the newer most 4. Dusting is now by no means cheap in preparations was very disappointing. comparison with spraying. Copper dusts con- Much of the more efficient spraying is now taining 20 percent Cu are the most popular, done on contract with high-volume miachines and these are applied at 16 to 20 lb. per acre. (applying 80 to 100 gallons per acre), and with Such dusts cost about 170s. per cwt. in 1956, wide booms (11- to 15-row widths) to keep so that the cost of materials per application down wheel-damage as much as possible. The was about 30s. It is generally reckoned that contract charge for such spraying, with 5 lb. 2 dustings are equivalent to one thorough per acre of copper oxychloride (50 percentCu) spraying in protective effect, but dustings may was about 45s. per acre per application in 1956. do rather better than this if the applications Two sprayings are usually applied, three are are well timed in relation to the blight attack. Since 1950 a considerable acreage of sometimes given. Air-blast low-volume sprayers (Agro potatoes is now sprayed from the air in eastern sprayers) applying copper oxychloride or and southern England. Both helicopters and cuprous oxide in 10 to 20 gallons of water fixed-wing aircraft are used. The standard per acre came into fairly extensive use after amount of copper oxychloride is generally 1945; These machines, with nozzles set well applied in 3 to 6 gallons of water per acre. down between the rows on droplegs, gave The contract charge for this air spraying has

25 been about the same as for spraying with must not be exaggerated. In order that the ground machines. Although many farmers present estimates may be correct, at least have been well satisfied with the results and in principle, a small loss from copper injury the wheel damage from the passage of ground is assumed in the Fen and southern zones machines is avoided, there is still a lack of only, in the years (3 in 10) when the 75 per- critical trial data for assessing the efficiency cent blight stage is not reached until the end of this form of application. of September, if at all. The figure is put as Dithiocarbamates were not launched on the equal to that by wheel damage, thus, in these market for potato spraying in England and zones and years, the total for loss by spraying Wales until 1957, and little experience has so damage is estimated at 6 percent of the far been gained of their use under English potential crop. conditions. If the need for frequent renewal As against this, the figures taken for gain of the spray deposits (Large and Beer, 1946) from spraying in the blight years are put at can be overcome, they may have a useful their very best, and are such as can be field of application (Cox, 1957), especially in achieved only by efficient spraying, with two the eastern parts of the country where some well-timed applications in general, and three copper injury (following insect attack) is caused in seasons with exceptionally heavy rain. It by routine spraying in the warmer and drier is assumed, for simplicity of estimation, that seasons. In the wetter seasons when nnore the net gain frona spraying (after deducting blight develops, copper could be substituted the loss by wheel damage) is equivalent to for the later applications. two clear weeks' prolongation of growth, at whatever time in August or early September THE GAIN FROM SPRAYING it may occur, and that in effect the actual prolongation is some days more than this to In estimating the net gain from spraying, compensate for the wheel damage. On this one difficulty is in making fair allowance for basis the gains and losses, according to the the damage caused by the passage of the time at which blight reaches 75 percent on sprayer through the crop. In the trials at the unsprayed field, for maincrops of 10-ton Terrington this danciage was found to amount potential yield are as shown in table 11. to 4 percent of the potential crop when a 13- For a 15-ton potential crop, which is the row sprayer was used. The corresponding most common level of crops in trials, and on damage with an 11-row machine would be 5 the better commercial crops on which most percent, and with a 7-row machine as muchas spraying tends to be done, all the above figures 7 1/2 percent. But the crops at Terrington would be increased by a half. This increase were exceptionally dense and heavy; in lighter would give a net gain from spraying in the and more upright crops the damage is obviously worst blight years of about 2 tons per acre less, but much spraying is done, especially by and for the other blight years (75 percent farmers, with machines of quite narrow row- stage at end of August) about 11/4 tons per width, and care is not always taken to drive acre, despite the losses by wheel damage. the sprayer over old wheel tracks when re- Quite often, in individual cases, higher gains peating the applications. For the purpose of from spraying, up to 3 tons per acre, are the present estimates a mean figure of 3 obtained; but growers in the Fens, in such percent is taken for loss of potential yield years as 1954 and 1956, wo\ild have been by wheel damage. very content with gains of about 2 tons. There is also the matter of the copper If the data from tables 1 and 11 are com- injury to the haulm in hot dry seasons. This bined, the gains from spraying on maincrops, effect is certainly seen occasionally, but it over the 10 years, 1947-56--assuming the TABLE 11. --Net gain or loss from spraying according to the date when blight reaches the 75 percent stage on a potential crop yield of 10 tons potatoes per acre

Yield of Unrecovered loss 75 percent blight on unsprayed Net gain Potential unsprayed from crops by— By wheel crop crops spraying By damage blight

Tons/acre Tons/iacre Tons/acre Tons/acre Tons/acre Mid-August , 7.2 r 1.4 0.3 1.1 10 End of August..•. 8.7 .8 .3 .2 10 Mid-September..., 9.6 .1 .3 0 10 End of September< 10.0 -.6 .3 0 10

26 TABLE 12. Gain from routine spraying over 10 years (1947-56) for maincrop potatoes in Enerland anri Walec

Percentage Mean net Percentage Contribution of Mean gain Zone gain from of to total acreage for spraying zone national acreage sprayed gain

Percent Percent Percent Southwes t Ö 11.0 20 2.20 0.18 Fens 13 55 3.02 .39 Southern 51 2.4 40 .96 .50 Northern 28 .3 5 .01 ~~ England and Wales — — 30 — 1.07

TABLE 13. --Gain from spraying only where needed over a 10-year period in England and Wales, 1947-56

Percentage Percentage Contribution Mean net of Mean gain of to national Zone gain from acreage for zone total gain acreage spraying sprayed

Percent Percent Percent Southwest 8 11.0 20 (10) 2.20 0.18 Fens •• 13 7.2 55 (6) 3.95 .52 Southern...... 51 4.0 40 (5) 1.60 .80 Northern • 28 0 5 (0) 0 0

England and Wales -- — 30 — 1.50

Number in parentheses refers to years in 10 in which spraying is needed and assumed done.

amount of spraying done had been at its recent contract at 45s. per acre on 30 percent of the level over the whole period, that all of it had acreage each year, this gain would be obtained been efficient spraying, and that it had been at a cost to the growers of about £675,000. done every year--would have been as shown If the spraying were done on the same in table 12. percentage of the acreage in each zone as that Thus, if routine spraying were done every shown in table 12, but only in blight years, year on the percentage of the acreage now or, to be more precise, only in those zones receiving any protective treatment and all and seasons where the 75 percent stage is this spraying gave the best results obtainable reached by the end of August, then the analysis in practice, the mean gain over a 10-year of gains and the saving in spraying would be period would be only about 1 percent of the as shown in table 13. Spraying on this basis potential maincrop yield from the country asa would increase the long-term gain to about whole. This would represent a saving of about 0.15 ton per acre, and through the avoidance of one-eighth of the total defoliation loss due to spray damage in the **no-blight*' years it blight (table 1). With the actual mean yield would save nearly half (46 percent) of the at about 7.5 tons per acre, the nnean potential spraying over a 10-year period. Thus, there yield (were there no blight) would be about would be a mean annual gain of 75,000 tons 8.2 tons per acre. The gain would therefore of potatoes worth about £825,000 at a cost for be about 0.09 ton per acre, or a difference of spraying of about £310,000. The justification one point (0.1 ton) on the average yield for for forecasting is that it can help toward the the country. attainment of this better result. If the price of potatoes is £11 per ton, this In fact, protective spraying in England and gain, on a half million acres of maincrops, Wales is not done either wholly by routine would represent a gain of some 50,000 tons (table 12) or wholly according to need (table of potatoes worth about £550,000. With the 13). Also, if the inefficiency of some of the mean cost of spraying of 2 applications by spraying is considered, the mean national

27 gain is probably nearer the lower figure of 0.1 picture and give the impression that the over- ton per acre on the long-term average. all loss from blight in tubers is much greater In a bad blight year, such as 1953, however, than is in fact the case; while the sporadic the gain fronn spraying may amount to 0.35 and extremely variable incidence of the loss ton per acre in the mean yield for the country. renders it extremely difficult to assess by Such a gain is very substantial, reckoned on survey methods. the whole acreage, but its value is lessened by Losses due to blight in the tubers may be the fact (brought out in table 3) that in the divided into two fairly distinct parts, namely: blight years the national yields tend to be 1. Losses caused by infection of the tubers above average, as the rain that favors the in the ground before lifting and apparent development of blight is even more favorable at lifting. for the growth of the crop. In 1953, the actual 2. Losses caused by infection at or shortly yield was 8,7 tons, as against the average of before lifting, and developing in store. 7.5 tons for the 10-year period, and more In some countries the second of these losses potatoes than were gained by spraying must may be by far the greater, for if the potatoes have gone for stock feed at v:4 per ton. The are lifted when the haulm is still partly green year 1953 was one in which the yield was and with the blight fungus actively sporulating exceptionally high, but the mean yield for on it and the lifting is done in wet weather, the 5 blight years in the period 1947-56 was very extensive infection of the exposed tubers 8 tons per acre, as against a mean of 7.1 tons is to be expected and considerable losses for the other 5 years. The years giving rise will occur in store. It is also possible that to shortage of supplies (and high prices) are some infection at lifting may occur from not usually the blight years in England and spores surviving in the soil for 10 days after Wales--they are the drought years in which the haulm is dead, as reported by Murphy there is little or no blight. (1921). In England and Wales, however, most Despite these national considerations, the of the maincrops are not lifted until October, gain from spraying achieved by individual and by or before that time much of the haulm growers, or groups of growers in particular has either been killed off by blight or by districts, is often very considerable. At present normal autumnal dying down. It is generally prices a net gain of only about 8 cwt. per acre only in the late-maturing crops (notably in the will pay for two sprayings, and in a blight northern blight zone), in some crops kept year in some districts there is a likelihood green unprofitably long by protective spray- of obtaining 20 or even 40 cwt. increase for ing, and in crops lifted early to supply an this outlay. immediate market, that there is much real Routine spraying (spraying every year) is opportunity for infection at lifting to occur. profitable in the southwest zone, and just Even so, if the lifting is done in a dry spell profitable in the Fen zone. In the large or the blight lesions on the haulms have southern zone, protective spraying is only dried up, very little infection may occur. profitable if it is done according to need. Opportunity for the tubers to be infected This implies a nice discrimination in selecting in the ground before lifting is not restricted to the crops to be sprayed--those of the higher those comparatively few crops in which the potential yield and those of the more suscep- haulm is still partly green near lifting time. tible varieties, such as King Edward, which Infection occurs wherever the soil permits gives the best return from spraying--and an access to the tubers of spores washed down equally nice judgment of seasonal conditions. by rain, and it persists over the entire period The forecasting work in England and Wales during which blight is sporing on the haulm-- is designed to help in this latter connection, from the beginning to the end of the foliage by indicating the time at which the first attack, and perhaps for 10 days afterward. spraying can be given to the best advantage in Moreover immature tubers are considerably blight years and by discouraging spraying in more susceptible to infection than mature years when little blight is likely to develop. tubers at lifting (Boyd and Henderson, 1953). Thus, infection in the ground before lifting is BLIGHT IN THE TUBERS responsible for the most widespread occurrence of blight in the tubers and possibly for the Losses due to blight infection of the tubers greater part of all the loss from this cause in vary greatly from crop to crop in England England and Wales. and Wales. Even in bad years for tuber infection more than half the crops suffer no loss, Losses Due to Infection Before Lifting or a very trivial loss, from this cause. But, against this, a small proportion of the crops Weighings of blighted tubers carried out in always siiffer a heavy loss, even up to 2 tons the crop-check surveys since 1953 now proper acre or more, and now and then the loss vide good data on the waste due to blight of an entire clamp is reported, owing to heat- infection before lifting in a representative ing and secondary rots following blight. The sanaple of some 2,000 crops in England and occasional severe losses tend to color the Wales each year. Table 14 shows the mean

28 TABLE 14. --Mean losses of potatoes caused by blight infection before lifting, England for blight in tubers. This diagram brings out and Wales, 1953-56 the salient fact about the distribution of losses due to blight in tubers: The large number of crops that scarcely suffer at all; and the few Blighted tubers 1953 1954 1955 1956 crops m which the loss is high. In 1954 57 percent of the 2,109 crops in the survey Hundredweight per acre had no blighted tubers (or less than 1/2 cwt. 2.5 3,4 0.2 2.9 per acre) before lifting; about 25 percent Percentage of crop..,. 1.3 2.0 .1 1.6 had 1 to 5 cwt. per acre; about 8 percent had 5 to 10 cwt.; and there were sharply decreasing percentages at higher rates. Although the loases in all these crops over the country as mean national loss was only 3 1/2 cwt. per a whole in the years 1953-56. acre, more than 100 crops in the 2,109 had Thus, in the three blight years, 1953, 1954, a loss of over a ton per acre, and two had and 1956, the mean loss was 2 percent; and in over 3 tons. the dry year with very little blight on the No firm association can yet be established haulm, the loss was negligible. The distribution between the level of tuber infection and the varied greatly according to region, as shown type of soil; heavy losses may occur by the maps (fig. 10) for 1953 and 1954. The occasionally, and negligible losses very fre- variation according to region was from less quently on both heavy and very light soils. than 1 cwt. to lOto 15 cwt. per acre. In general, But the level of loss certainly varies according the losses were highest where the blight to the potato variety. In the 3 years, 1953, attack on the haulm had been earliest and 1954, and 1956, the breakdown according to most severe, with the important exception of variety of the mean losses in the crop-check the relatively heavy losses shown in parts of surveys is shown in table 15, the northern zone in 1953. This was in accord- The mean loss was over twice as great ance with past experience (Moore, 1946, 1948) in King Edward as in Majestic. In Arran that late attacks of blight in the north may, in Banner and Gladstone, the loss was over some years but not in all, cause heavy losses six times as great as in Majestic. Indeed, by tuber blight. in the histogram for 1954 (fig, 11 ) of the 88 The frequency of occurrence of different crops shown in which the loss was over 1 levels of loss among the crops is shown in ton per acre, 23 were Arran Banner, 38 King figure 11 for 1954, the worst of the 4 years Edward, 7 Gladstone, and only 12 Majestic,

FIGURE 10.--Regional distribution of losses due to blight in tubers, 1953 and 1954.

29 the amount of blight in the tubers. The op- ENGLAND 6 WALES portunity for tuber infection is greatest when 1 —57 PER CENT OF CROPS the attack on the haulm is long-drawn-out, least when it is rapid.

Losses Due to Infection at Lifting -25 PER CENT OF CROPS The proportion of the maincrops in England and Wales in which the tubers may be infected from blighted haulm at lifting can be judged to some extent from table 16, which is based on observations made in the course of crop- check surveys. It is evident that in more than two-thirds of the crops over this period of years, the haulmi was dead well before lifting began, and that in all except the 2 years 1951 and 1952, in which the 75 percent blight stage was in mid-September, there was little growth left in the rest of the crops. The only direct survey evidence bearing Each dot represenfs one crop upon the annount of infection occurring at in rhe survey lifting and developing subsequently in the Mean loss : 3¿cwt per acre clamps, is that from a systematic sampling of 700 clamps in all parts of the country carried out by advisory mycologists in the — 6 PER CENT OF CROPS 1942-43 storage season (Moore, 1948), In this survey the average loss from rots in March " was about 6 percent--half of which was attributed to blight. In only 38 (5.4 percent)" of the clamps did the loss from blight exceed, — 4 PER CENT OF CROPS 10 percent, although in several it reached 301 percent, and in one 43 percent. These figures [ indicate the same kind of distribution oi\ losses in clamps as that shown in figure 11,,' — I PER CENT OF CROPS ifUMi for blighted tubers in the ground before lifting. ^ I -5 -lO -15 -20 -25 -30 -35 -40 -45 -50 -55 -60 -65 The year 1942 was one with much blight in' • BLIGHTED TUBERS CWT PER ACRE MEAN September (75 percent stage about the end of August in the worst-affected parts of the FIGURE 11.--Frequency of différent levels of blight in tubers before south, and a week or two later elsewhere) lifting in a survey of 2,109 crops in England and Wales, 1954. and the progress of the attack was fairly' rapid. It was recorded at the time of the , despite the preponderance of crops planted in survey that the losses from tuber blight had ' Majestic. been somewhat greater in the previous year, The estimation of blighted tubers in the 1941, when the attack on the haulm was more ' crop-check surveys has not yet been done in drawn-out. In some years, therefore, the losses all types of season in which tuber infection in clamps may be higher than in 1942, but in ^ may be prevalent. The reports on the diseases some other years, such as 1955, the losses^ of the crops in England and Wales (Moore, are undoubtedly less. It may not be un-^| 1946, 1948, and earlier bulletins) indicate reasonable, therefore, to take the mean figure^ that only the very dry years (like 1955) in of 3 percent, carefully determined by accurate ■ which there is little or no blight on the sampling in 1942-43, as a provisional estimate ^ haulm at the end of September can be regarded of the mean annual loss caused by blight in ^^ as ''no-blight" years for the disease in the clamps in England and Wales. tubers. Years in which the 75 percent stage on This loss is made up of the tubers infected ^| the haulm is not reached until mid-September at lifting together with a fairly high proportion í may be among the worst for blight in the of the tubers infected in the soil before lifting. |^ tubers; and years in which this stage is Many of these latter, where they are obviously J reached at the end of August (blight-years and severely blighted, are rejected when the for the disease on the haulm) are most crop is picked by hand, and tiiey are not put ^ frequently also bad years for blight in the into the clamp. The proportion is quite in- ^ tubers, although sometimes they are not. It determinate, and must depend upon the weather j is the slope of the blight progress curve, at lifting, the type of labor employed, and rather than the 75 percent date, that affects other factors. ^

30 TABLE 15.—Mean losses of potatoes from blight infection of tubers before lifting, according to variety, 1953, 1954, and 1956

before Crops in survey Blighted tubers Variety lifting 1953 1954 1956 1953 1954 1956

Number Number Number Percent Percent Percent Majestic 1,019 954 1,005 1 1 1 KT TiP" Edward 379 566 591 2 3 2 Ä-rran Banner. ••••.•• 120 92 62 4 9 11 Gladstone 41 28 16 8 10 17

All varieties 2,070 2,109 2,057 1.5 1.7 1.7

TABLE 16. —Percentage of potatoes lifted in relation to state of haulm before lifting, England and Wales, 1951-56

State of haulm some 1951 days before lifting 1952 1953 1954 1955 1956

Percent Percent Percent Percent Percent Percent Dead 66 57 84 S5 56 94 Little growth left 25 34 16 15 AA 6 Much growth left 9 9 0 0 0 0

From a review of all the available evidence of protecting the tubers by keeping blight it would appear that the mean total loss from off the haulm altogether. This would often blight infection of the tubers for all varieties, require far more applications than the two and for England and Wales as a whole, is or at most three that are generally given, and about 3 percent of the crop, and that this loss it would scarcely be economic. As it is, is made up of about Z percent apparent at protective spraying quite often prolongs the lifting and 1 percent developing in store. growth of partially blighted haulm beyond This estimated mean loss due to blight in the stage at which useful increase of yield the tubers is less than half of the estimated can result, and thereby increases the amount mean loss caused by premature defoliation of tuber infection. (table 1); but it is a more constant loss, and Cutting and removing the blighted haulm, it probably occurs in all except the very dry as a measure for decreasing the amount of ••no-blight*' seasons; that is to say in 8 or 9 infection of the tubers, was advocated in the years out of 10. 19th century; indeed, alnaost as soon as the disease appeared in Europe. However, the CONTROL MEASURES measure was expensive and difficult to carry The principal measures for the reduction out on any large acreage. In the early 1930*s of losses due to blight infection of the tubers one of the chemical manure and sulfuric acid in England and Wales are (1) efficient earthing companies in Norfolk carried out experiments up of the rows to insure good soil cover over on the spraying of the haulm with diluted the tubers; (2) the postponement of lifting sulfuric acid as a substitute for mechanical until the haulm has been dead for 10 days; destruction. The kill of the haulm was quite and, (3) the avoidance, as far as is practicable, spectacular, and by 1934 several large growers of varieties like Gladstone, known to be very in Norfolk and Lincolnshire who had been susceptible to tuber infection. persuaded to give the new process a trial Protective spraying of the haulm has some were hailing it with great enthusiasm. They effect in reducing tuber infection in the ground sprayed 3,500 acres in that year and claimed before lifting, by keeping blight off the haulm that crops from unsprayed areas, on being for about a fortnight in the early stages of taken from the clamps, frequently contained the attack and thereby reducing the period up to 50 percent of blighted tubers, wherea^s during which washing down of spores can those potatoes from the areas "burned-off** occur. But in England and Wales protective with sulfuric acid were almost completely spraying is rarely employed for the purpose free from the disease.

31 The first critical trial was carried out by In 1951, when there was a world shortage Bates and Martin (1935) under what can now of sulfur, soditun and potassium arsenite be seen to have been optimum conditions, on mixtures were introduced for haulm destruc- a crop of King Edward at Terrington, near the tion in England and Wales. Their use has Wash. A 10 percent solution of B.O.V. (Brown increased very rapidly since that time, despite Oil of Vitriol, 77 percent H2SO4) was applied the availability of acid. In 1956 about half of to the green haulm at 100 gallons per acre on all the destructive spraying was being done September 16, 1934, when only a few plants with arsenites and half with acid. The were yet affected by blight, and, as the yields arsenites, usually in the form of liquid prep- showed, almost the whole obtainable crop was arations containing 10 lb. of As 2P3 per gallon, already under the plants. (See fig. 8 for ex- are applied both by high- and low-volume pectation of increase of crop after machines, at the rate of 1 gallon in 80 to September 16.) After the date of spraying 100 or in 20 to 30 gallons of water per acre, blight spread all over the unsprayed plots, respectively. The action of the arsenites is giving opportunity for washing down of spores slower than that of acid—the leaves take up for some 3 weeks, until the potatoes were to a week to go brown, while those sprayed lifted on October 6, when much of the blighted with acid are blanched and limp on the day haulm was still green. The total produce from of application; but there is little to choose in the sprayed and unsprayed plots was stored point of eventual completeness of destruction separately in the same clamp, and examined of the haulm. The contract charge for spray- for blighted tubers at the end of January 1935, ing with arsenites in 1956 was considerably when the clcimp was opened. Table 17 shows less than that for acid, being about £2 IBs. the results. Od. per acre. Arsenites have the advantage that they can be applied by farmers themselves, TABLE 17. - -Percentage of King Edward tubers with sprayers available on the farm that are with blight in clamp from plots sprayed with often unsuitable for the application of acid. sulfuric acid, Terrington, January 1935 The first systematic trials of haulm destruction at a nvunber of centers and over Total several years, in England and Wales, were Treatment Blighted tubers yield those carried out by the National Agricultural Advisory Service in 1951-53, for the purpose of testing substitutes for sulfuric acid (Large, Tons/ Cwt./ Percent 1952b, 1952c, 1954). The destructive spraying acre acre of crop was done at each center at a time in accord- Unsprayed 11.9 9.1 3.9 ance with practice in the district, and the Burned off with 10 percentage of blighted tubers in the sprayed percent B.O.V. .. 12.0 1.6 .7 and unsprayed parts of the crops was deter- Standard error. + .03 +1.4 mined both at lifting and after a period of 6 weeks in storage. The mean results, with acid, for 9 trials in 1951, 11 in 1952, and 11 These results were obtained under about in 1953, were as shown in table 18. optimum conditions. The total percentage of Despite the fact that many of the untreated blighted tubers in the unsprayed crop was plots in these 31 trials were lifted while there about 4 percent, and the acid spraying, done was still a little growth left in the haulm, when there was still very little blight on the blighted tubers from the unsprayed plots haulm, reduced the figure to about 1 percent. exceeded 3 percent after storage in only 9 Since 1935, •'burning-off•• the haulm has trials and were less than 1 percent in 17 trials. become firmly established in practice, and In the crops where heavier tuber infection indeed it is extremely popular among potato occurred, the destructive spraying showed growers. This is not only because it can, advantages varying according to the date on in some circumstances, save a good deal of which it was done, the progress of blight on blight in the tubers, but because it undoubtedly the unsprayed crop, the lifting date, and the facilitates lifting and kills weeds. It "cuts rainfall at lifting. All these variable factors losses," saves further concern about the are illustrated for 1953 in figure 12, which progress of blight in the crop, and, apart also shows the use made of blight progress from the lifting, it puts an end to the job for curves in the study of haulm destruction. As the season. a result of the trials as a whole it was con- Most of the acid spraying at the present cluded that destructive spraying, whether with time is done by contractors, who apply 20 acid or with arsenites, as usually carried out gallons of undiluted Brown Oil of Vitriol per in practice, can at best save only about half acre with special acid-resistant machines. The of the infection occurring in the ground before contract price for this treatment in 1956 lifting. Here and there, it may save con- was about £4 per acre, the value of about siderable infection at lifting if the weather 7 cwt. of ware potatoes. happens to be wet and the haulm would not

32 TABLE 18.—Percentage of blighted tubers at lifting and after storage in spraying trials with sulfuric acid, 1951-53

Blighted tubers

Treatment 1951 1952 1953

At ACter At After At After lifting storage lifting storage lifting storage

Percent Percent Percent Percent Percent Percent An"!d SDraved «.•••.•..••.•>••••• 2.2 2.6 0.4 0.9 1.7 2.5 Untreated • •..••....•••.•• 2.2 2.8 1.3 2.4 3.5 3.7

otherwise have been dead in time* Similar in England and Wales; and also by spraying resiilts were obtained with mechanical haulm- with certain tar oil fractions, which have not cutters or piilverizers, which are now avail- come into any extensive use in England and able and are used on a relatively small acreage Wales but are nevertheless excellent non- poisonous and noncorrosive haulm killers ENGLAND 0 WALES (Large, 1952b, 1952c, 1954).

GAINS AND LOSSES FROM HAULM

CUMBERLAND (Mojcstic) DURHAM CArranPeaO DESTRUCTION July August Septcmb« 11* The amount of haulm destruction now done II 3% in England and Wales may be judged from rtijf 9% the percentages of the crops in the crop check i Bl 1 1 J 1 1 surveys on which farmers carried out this YORKSHIRE (Maicst ■c) treatment: 17 percent in 1953; 22 in 1954; 10 in 1955; and 30 in 1956. II In 1956 haulm destruction was carried out 4 on 47 percent of the 591 King Edward crops;

nTTrrTTTilml on 21 percent of the 1,005 crops of Majestic; 1 1 L_^^J 1 1 1 I and on 26 percent of the 461 crops of other LINCOLN (Glodttonc") varieties. The mean weights of blighted tubers per acre, before lifting, in the untreated crops, and in the crops on which haulm destruction was done, with or without protective spraying, are shown in table 19. CAMBRIDGE (King Edword) Apart from the indication that , the mean T" weight of blighted tubers was increased in m 11 1954 by protective spraying without subsequent 8% haulm destruction, these mean figures from 20% some 8,000 crops provide no evidence of any .A 1 11 1 U^ J 1 UJ U U I general reduction of blight in tubers as a KENT CMojestic) result of haulm destruction. However, haulm 1 destruction was probably done where it was ^ Â most needed, so that the figures are not in- T consistent with the probabüity that there were ^^^rttílilÍ 1 % considerable gains on some crops. ;t JU Against this, reduction of blight in tubers by destructive spraying of the haulm can KDL % Haul m killed br blight lOO Dote of A generally only be achieved at the expense Date of haulm lifting of some loss of the healthy crop. The loss destruction f 50- Blightcd tubers 4c % caused by stopping the growth of the haulin Beaumont period % too soon in destructive spraying can be judged r Total roinfoll per week from the diagrams in figure 12 and the mean worth of each week's growth in August and FIGURE 12.—Percentage of bli^ited tubers in relation to progress of September as shown in figure 8. If the haulm blight on haulm, dates of haulm destruction and lifting, and rainfall is destroyed when blight reaches the 75 per- at 11 trials in England and Wales in 1953. Upper figures indicate cent blight stage, then the loss of yiclf /^« percentage of blighted tubers with haulm destruction; lower figures to shortening of the growing period will be Indicate percentage of bUghted mbers without haulm destruction.

33 TABLE 19. —Mean weights of blighted tubers negligible. However, there will then be little per acre before lifting after various reduction of the period during which the spores treatments, England and Wales, 1953-56 can wash down into the soil, and therefore little hope of much reduction in the amount of blight in the tubers before lifting. If, on the Mean weight of blighted other hand, the destruction is done at the tuhers before lifting Treatment 25 percent blight stage or earlier, there can be considerable reduction of the period during 1953 1954 1955 1956 which the spores can wash down, but there will be a curtailment of useful growth. Haulm destruction Cwt./ Cwt./ Cwt./ Cwt./ This question of the crop sacrifice at- tendant upon haulm destruction was investi- after protective acre acre acre acre gated extensively in the spraying trials carried spraying 3.2 4.2 0.02 2.7 out on King Edward in all the counties of the Haulm destruction West Midlands over the years 1950-54 (Rosser, 1957). In each trial there were plots that only 3.4 3.4 .01 3.1 received no protective spraying but which were burned off in early August (mean date, Protective spraying only... 2.8 5.6 0 3.1 August 8); at the end of August (mean date, September 1); and in mid-September (mean No protective or date, September 18). The mean total yields destructive and weights of blighted tubers in the 30 trials spraying 2.3 2.8 .40 2.4 were as shown in table 20. The expected reduction in yield from the premature killing TABLE 20. —Effect of haulm destruction at various dates on total yield and on weight of blighted tubers in trials on King Edward potatoes, England and Wales, 1950-54

Mean date of haulm destruction Item -Aug. 8 Sept. 1 Sept. 18 Sept. 30

Blighted tubers tons/acre 0.25 0.40 0.60 0.65 Total crop tons/acre 9.50 12.60 13.40 13.80 Percentage of potential yield 68 91 97 100 Expected percentage (fig. 8) 62 SB 97 100

of the haulm, as read from figure 8, is added grower it nnay be very profitable now and then. for comparison. But as the reduction of blighted tubers has to These trials provide confirmiation that within be 10 cwt. per acre to pay for the treatment practical limits the growth curve for Majestic, and for some few cwt. of sound crop lost, the given in figure 8, is also valid for King Edward chances of its being profitable are not more in England and Wales. They also left no doubt than 1 in 10 on King Edward and 1 in ZO on about the loss of crop involved when premature Majestic (see frequency diagram, figure 11 haulm destruction is employed in attempts et seq.). It is perhaps best to regard haulm to reduce tuber infection before lifting.Haulm destruction in England and Wales as a farm destruction in early August reduced the weight operation worth its cost in preparing the field of blighted tubers from 13 to 5 cwt. per acre for lifting, especially needed when the crop at the expense of a loss of 4 tons in the total has to be lifted early, and sometimes paying crop. Haulm destruction at the end of August a bonus by reducing blight in tubers in other reduced the blighted tubers from 13 to 8 cwt. circumstances. per acre at the expense of 1 ton of the total crop; and haulm destruction in mid-September RESISTANT VARIETIES caused no significant reduction either in yield or in the weight of blighted tubers. Potato breeding for blight resistance In view of the loss of some healthy yield throughout the world owes much to the pioneer- that is almost inseparable from haulm destruc- ing work done in England by the late Radcliffe tion as normally practiced, and the low mean N. Sal aman on the employment of Solanum level of blight in tubers in England and Wales, demis sum as a source of blight resistance, as revealed by the trials and surveys, it would and the recognition by him and his co worker s appear that the mean national gain from reduc- at Cambridge of the existence of biotypes of tion of blight in tubers by haulm destruction the blight fungus in the early 1930's (Salaman, may well be almost nil. For the individual 1949). The progress made up to 1939, and

34 the prospects of success in breeding for three-quarters of a century. Such resistance resistance from R-genes, and for what is now has always been one, among the many desirable called "field resistance," as seen by those qualities that the breeders have striven to two great exponents of the rival schools of obtain. A number of excellent new potato vari- thought, Salaman and Pethybridge, was pre- eties, other than those few with R-genes, have sented in their lifetime and with their aid come into commercial cultivation in England by Large (1940). The influence of Salaman and Wales in recent years. and Hudson, and the exploration of Hawkes in The testing of potato varieties is the re- South America (Hawkes 1941, 1947) led to sponsibüity of the N.I.A.B., and Culien, in the establishment of the Commonwealth Potato charge of this work, has recorded the per- Collection at Cambridge,^ to provide potato formance of these new varieties in trials breeders with new sources of R-resistant and throughout the country (Culien, 1953, 1955, other genetically valuable material. Much work 1956). Home Guard (1943) and Ulster Premier on the fundamental problems of breeding for (1945) are among the best in the first-early ^ blight resistance has been done, not only group; Craig's Alliance (1948), Craig's Royal by Black in Scotland (q.v.) but also by Howard (1948), and Ulster Dale (1950) in the second- at the Plant Breeding Station, Cambridge early varieties; and Dr. Mclntosh (1944), (Howard, 1953), and by Müller and others at the Arran Viking (1945), and Ulster Supreme National Institute of Agricultural Botany, also (1946) among the maincrop potatoes. Of these at Cambridge (Müller, 1953; Müller, Culien, last. Dr. Mclntosh and Ulster Supreme are and Kos trow icka, 1955). about equal to Majestic in point of susceptibility Since 1945, six named R-gene résistant to blight, while Arran Viking (as yet) is dis- ' varieties, raised in Scotland and Northern tinctly more resistant. Ireland, have been submitted to trial at numer- Apart from their long period of testing ous centers in England and Wales by the before recommendation, potato varieties in National Institute of Agricultural Botany England and Wales, as elsewhere, undergo an (N.I.A.B.). All six of these varieties are inexorable process of screening, in practice, attacked by blight on thé haulm before the as a result of which the more blight-suscep- end of their growing period, and all six are tible varieties fail to establish themselves or susceptible to blight in the tubers. The one pass out of cultivation, unless, like King variety of the R3 genotype--Pentland Ace Edward, they have some other compensating (I95I), a second-early potato--has been found quality that renders them profitable to grow. to pulp badly on cooking. Of the five named The very susceptible varieties, Up-to-Date and varieties of the Rj genotype, Craig's Snow British Queen, despite their unrivaled quality, White (1948), a maincrop (comparable with have almost disappeared, and Gladstone, very Arran Peak in point of maturity), has failed susceptible to blight in the tubers, is going because of its extreme tolerance of virus Y, the same way. On the other hand, it is un- its late maturity, and its indifferent cooking doubtedly largely because of its low sus- quality. Craig's Bounty (1943), also a maincrop, ceptibility to blight in the tubers that Majestic has failed because of its poor quality and long has reached and still maintains its place as stolons. Ulster Torch (1955), another main- the predominant maincrop variety. The adop- crop, is still promising, but has badly shaped tion of Majestic probab' represents the most tubers. Pentland Beauty (1955), a first-early economically effective measure of blight con- variety, is all right so far. trol yet achieved in England and Wales. Of the six varieties, only one, Orion (1947), The importance of this can be seen fromi an early maincrop (comparable with Majestic table 15. If an '*improved King Edward" could for maturity), has survived the long series of be produced by the breeders, t combine the registration and N.I.A.B. trials (which take shape, pink particolored skin, and accepted 10 years from the production of the first quality of King Edward with the tuber resist- seedling by the breeder) to reach the stage of ance of Majestic, that achievement alone recommendation by the N.I.A.B. in 1956. That would save the country at least 1 percent of honor was shared with only four varieties for the present King Edward production, or about lifting as maincrops: Majestic, King Edward, 14,000 tons of potatoes almost every year. Ulster Supreme, and Arran Viking; but it was Another little realized, but nonetheless sig- a restricted recommendation for special pur- nificant measure, adopted in England and Wales poses, as Orion has yellow flesh and in England for the reduction of blight losses, is repre- and Wales it is suitable for chipping or process- sented by the local preference for Arran ing only. Banner as the principal nnaincrop variety in Potato breeding, specifically for **field re- Cornwall and other parts of the southwest sistance" to blight as defined by Müller (1953), where the earliest blight epidemics occur. has scarcely yet started in England and Wale£;, Arran Banner, with its high susceptibility to but breeding for as much practical resistance blight in the tubers, can scarcely be said to as possible has been going on for at least have miuch resistance to the disease, but because of its heavy yield and early bulking * Now at the John Innes Horticultural Institution, Bayfordbury, Herts. it can produce a good crop before blight

35 defoliation losses begin. This is an example spraying. In fact, only about 30 percent of the of control, not by disease resistance, but by crops arc sprayed, and the actual gain from **disease escape/' spraying does not amount to more than about 1 percent of the total potato production on a long- SUMMARY: ENGLAND AND WALES term average. The mean loss due to blight in the Work on the definition and quantitative tubers is about 3 percent on the same basis. measurement of blight epidemics, by their This loss, on which destructive spraying, effects both on the foliage and the tubers, practiced on a third to a half of the crops, began in England in 1941. Since 1950 systematic makes little or no impression, probably affects surveys designed to provide the necessary the national economy more than the greater information for the realistic evaluation of losses by defoliation, which are in general national losses have been carried out over the more than offset by high yields due to the whole country each year. On the foundation of abundant rain in the blight years. The tuber this work it now becomes possible, for the loss occurs in all except the few outstandingly first time, to attempt apreliminary but closely dry years, that is to say, in 8 or 9 years out documented estimate of the present economic of 10, and 3 percent of the national production significance of potato blight in England and represents some 120,000 tons of potatoes, Wales. The attempt has here been miade, and or the produce of 15,00.0 acres. the many facets of the work, as well as the Although the mean national gain from pro- methods used, have been described in some tective spraying is slight, the gain to individual detail. Briefly, in England and Wales, *'blight- growers in some districts and in some seasons years,** characterized by 75 percent defolia- may be very appreciable--typically 2 tons per tion of the unsprayed maincrop potatoes be- acre where the 75 percent blight stage is tween mid-August and the end of August, occur reached on the unsprayed haulm by mid- in about 5 years out of 10 in the southern half of August, and about 1 ton where this stage is the country, with later epidemics in the other reached at the end of August. Individual gains years. The mean loss by premature defoliation from destructive spraying may also be con- on unsprayed crops over a 10-year period is siderable here and there, especially when about 8 percent of the potential crop for the the crop has to be lifted early, but the chief country as a whole. Of this, less than one-half gain from this process is a saving of time could be recovered by efficient protective and trouble at lifting. SCOTLAND Scotland is noted for its production of high- grade seed potatoes, most of which are sold to plant the ware acreage in England and Wales. Seed potatoes are exported to a number of other European countries, notably Spain, and also to South Africa. In 1955, of the 135,000 acres of potatoes grown in Scotland, approximately 60 percent (80,000 acres) were for seed production. Seed potatoes are grown throughout the country. Consumption of ware potatoes is on a level similar to that in England and Wales-- approximately 200 lb. per head per annum. Average yields are around 8 tons per acre.

POTATO DISTRIBUTION AND VARIETIES

A large part of Scotland consists of mountainous areas unsuitable for arable cultivation. Cropping is largely confined to the lower ground, in particular the coastal and inland districts running up the east of the country. The largest concentrations of potatoes are found in the three adjoining eastern counties: Angus, Perth, and Fife. Elsewhere in the east, Aberdeen, Kincardine, and East Lothian are Each dot = 1000 acres important potato areas. In the southwest in Ayrshire there is an early potato district confined to a narrow coastal strip with its main center around Girvan. Figure 13 shows the distribution of potato growing in Scotland. FIGURE 13.--Distribution of potato growing in Scotland,

36 Although well over 50 different named varieties are grown, 7 varieties account for 80 Kincardineshire to just percent of the total potato acreage. Three north of the Moray Firth. of these, Ar ran Pilot (early), Majestic, and Elsewhere, scattered King Edward (maincrop), are grown almost crops are grown in the entirely for export as seed. The remaining valleys, with some pro- four varieties, Epicure (early), Kerr's Pink, duction on the islands, Redskin, and Golden Wonder (maincrop) are notably the Orkneys. grown principally for home ware consumption. East region.--62 percent of the potato A description of the varieties grown is given acreage. A very important in a publication on seed potatoes (Scotland seed-growing area lies to Department of Agriculture, 1?52). the north and south of the Firth of Tay. Ware and SOIL AND CLIMATIC CONDITIONS seed potatoes are produced in the Lothians and Ber- wickshire. Most of the crop is grown on well-drained West region.--18 percent of the potato sandy or silty loams. These are usually in a state of high fertility, due to the system of acreage. Potatoes are farming practiced, with its emphasis on a grown throughout the area, principally on low ground, balance between crops and livestock. Summer with concentrations near temperatures are low: Mean monthly figures Dumfries (seed), south- at this time of the year do not usually exceed west Wigtownshire (early 60 F. Rainfall is usually adequate, although in ware), and Girvan (early some potato areas, notably parts of East ware). Lothian, conditions in the summer may sometimes be too dry for the crop. A marked feature of climatic conditions in the potato growing areas is the variation, particularly in SCOTLAND rainfall and humidity, within short distances. This results from the hilly or mountainous SiS'Ô ORKNEYS nature of the countryside and the varying distance of the crops from the sea. Early potatoes are planted at the end of February or early March, and lifted in June; the maincrop season is from the end of April to October.

BLIGHT EPIDEMICS: GENERAL It would be unrealistic to attempt to review blight epidemics for Scotland as a whole, owing to variations in the severity of the disease from place to place and the differences in climatic conditions. Some division of the country is necessary before the problem can be approached. Unfortunately, it is not possible to divide the country up into a small number of climatic regions. The position is further complicated by the growing of early and maincrop varieties side by side in the seed- growing areas, with variations in the incidence of blight on the 2 types of potatoes. For the purposes of this study, a simple division into 3 main regions has been made, based on the 3 advisory regions of the Scottish Agri- cultural Advisory Service. These three regions--the north, the east, and the west-- are shown in figure 14, and they can be de- FIGURE 14.--Advisory regions and bcalities of Scotland referred to scribed briefly as follows: in text. North region.--Includes 20 percent of the C. E. Foister has provided a summarized total Scottish potato account of potato blight in Scotland for each acreage. Most of the po- of the years from 1946 to 1955. These accounts tatoes are grown on rela- have been built up from records supplied by tively low ground near the the three regional plant pathologists of the coast, from the south of advisory service and by the potato seed

37 inspectors of the Department of Agriculture actions of the Royal Highland and Agricultural for Scotland. This material is not published. Society of Scotland. Some additional information has been incor- Table 21, which has been condensed from porated by Foister from annually published Dr. Foister's account, shows the incidence accounts on the productivity of the principal of blight on a regioncd basis over the 10-year Scottish arable crops, given in the Trans- period. TABLE 21. —Incidence of potato blight in Scotland, 1946-55

Mount of blight North Region East Region West Region

Moderate•••••••••••.•••. 1949, 1953 1948, 1949, 1950, 1954 1950, 1953 Slight 1948, 1954, 1955 1946, 1951, 1953 1947, 1952, 1955 Negligible 1946, 1947, 1950, 1947, 1952, 1955 1946, 1948, 1949, .41, 1951, 1952 1954

It is not possible to define exactly what is Blight occurred every year, with outbreaks meant by •'negligible," *'slight," and "mod- at the end of July or in early August in 6 out erate" attacks of the disease, but in the analy- of the 9 years. It seems very probable that sis given in table 21, the terms used refer to the first appearance of the disease on the the combined effect of the blight attack on the plots at Auchincruive will precede generalized haulm and on the tubers. field outbreaks on maincrop varieties in the The most noticeable feature of table 21 is west of Scotland. Similar data are not available the number of years in which the disease is for the east region, but, in general, field described as "negligible" or **slight." In the outbreaks there are most unlikely to occur north region, the disease was "negligible" before outbreaks in the west. or ''slight" in 8 out of 10 years; in the east The most detailed information on the effect region in 6 out of 10 years; and in the west of blight on the haulm comes from the west region in 8 out of 10 years. The east region region and has been provided by Grainger, is the worst affected by blight during the 10 again from data obtained on the Auchincruive years under review, and this is the most im- plots. Table 22 sets out the dates on which portant potato-growing area. Within the east 100 percent blight destruction of the tops region blight is most serious in its effects occurred, for 2 potato varieties, over the in the seed-growing areas near the Firth of period 1949-54. Tay, but the principal losses here appear to be caused by tuber infection. TABLE 22,—Date on Which 100 percent blight of haulm occurred at Auchincruive, Scotland, BLIGHT ON HAULM on Kerr's Pink and King Edward potatoes, 1949-54 Although reports of isolated primary foci may be made very early in the summer, 100 percent blight on generalized field outbreaks of the disease haulm of— in particular areas very seldom occur before Year the end of July or the beginning of August. Kerr's Pinic King Edward According to Foister, blight usually appears first in the southwest of Scotland, Gray, 1949 September 12 September 16 Clouston, and Biggar (1952) have analyzed 1950 2S 10 first reports of blight for the north region 1951 27 28 from 1943 to 1952. During this lO-year period, 1952 16 10 these reports came between late June and the 1953 10 third week in July, but they did not signify 17 1954- 20 3 general outbreaks. For the west region, Grainger (1955) has given the times of the Average date first appearance of blight on his disease (6 years) September 20 September 13 phenology plots at Auchincruive over the period, 1944-52. These were as follows: Mid- End Early Mid- End The 100 percent stage was generally reached July July Aug. Aug. Aug. around the middle to the end of September. The 75 percent blight stage will have occurred 1948 1944 1946 1945 1949 some days before this, but, even so, the 1947 1950 1951 evidence from table 22 is that, at this center, 1952 annual yield losses through premature defoliation of the plants are very small.

38 For the north region, Elizabeth G. Gray has informed the writers that seed crops are usu- tion losses by blight in seed crops are likely ally burned off before blight reaches the 75 to be very small indeed, as the majority of these crops are burned off before the 75 percent percent stage, and blight on the haulm is not blight stage is reached. often severe enough to affect yield, except in coastal districts. In the east region, A. E. W. If the acreage of early ware crops is added Boyd has reviewed his observations* made to the acreage of seed crops, these two are found to account for three-quarters of the between 1949-56, from which he concludes that total Scottish potato acreage. It is therefore the 75 percent date for Majestic in the east and southeast of Scotland is usually during the on only a quarter of the total acreage (the maincrop ware) that defoliation losses of any first week in September; but he stresses the importance are likely to occur, and even here variations in blight incidence throughout the region* they have been shown to be small. In Scotland, it is unusual for the 75 percent BLIGHT IN TUBERS blight defoliation stage to be reached much before early- to mid-September in any year. There can be no doubt that blight in tubers If this is so, yield losses through defoliation is the problem which is of most concern in are likely to be small. The data for England Scotland, because of the importance of the seed and Wales (fig. 8) indicate that when blight crop. Since 1953, figures have been obtained reaches the 75 percent stage by mid-Sep- in the course of crop-check weighings by the tember, there is an average loss of only 4 Ministry of Food and Potato Marketing Board, percent of the potential potato yield. The in Scotland as in England and Wales, which Scottish potato-growing season is somewhat give the weights of blighted tubers present at later than in England, so that the 75 percent lifting in a representative sample of 600 to 800 stage by mid-September probably indicates a Scottish potato crops each year (principally slightly greater loss than 4 percent, but this maincrop varieties). Table 23 shows the re- does not happen every year. The lack of precise sults for 1953-56. In 1953, 1954, and 1956 a information on these various points shows the fair amount of blight was present on the necessity for more detailed survey work, potato haulm, at any rate in parts of Scotland. although the variable topography and weather Over the 4-year period, the average amount conditions may render this rather difficult. of tuber infection causedby blight before lifting An appraisal of defoliation los se s in Scotland was under 2 cwt. per acre, which, expressed as must take account of the predominant part a percentage of the average potato crop, equals played by the seed potato crop. The practice about 1 percent of the actual yield. This puts among growers is to destroy the tops of their the problem into perspective. It means that in seed crops at the time, usually during the first most of the crops, even in blight years, tuber half of Septennber, when they consider that infection in the soil is either absent or very they have obtained the maximum yield of seed- slight. The few crops in which really heavy sized tubers (2^ in. x l|in.). Thus, the defolia- infection occurs, with a loss of up to 3 tons

TABLE 23. —Mean weights of blighted tubers recorded at lifting, Scotland, 1953-56

Region and county 1953 1954 1955 1956

North region: Cwt./acre Cwt ./acre Cwt ./acre Cwt ./acre Ross and Cromarty, Inverness, Caithness 0.4 0.7 0.2 Aberdeen, Kincardine ..7 .3 .3 East region: Perth, Angus 1.6 2.8 0 2.2 Fife : 3.0 1.4 0 4.9 Lothians, Berwick 4.4 1.6 0 3.6

West region: Ayr, Lanark, Argyll < 3.7 2.4 2.3 2.7 Dumfries, Kirkcudhright , 1.0 1.0 1.0

Scotland (all regions) 2.8 1.6 .2 1.9

Number of potato crops in survey 600 810 782 732

39 of potatoes per acre, tend to get an undue SCOTLAND amount of publicity with a consequent exag- North Reqion geration of the problem. CRAIBSTONE It would appear from table 23 that tuber RAINFALL Inches blight infection is least important in the north region, where in 3 out of the 4 years only very small amounts of the disease were recorded, but it is too early to draw any firm conclusions about regional differences. Varie- tal susceptibility to blight infection of the tubers, and soil conditions are important and need further analysis. Blight infection of the potatoes at lifting, by f, East Region contact with diseased haulm, is potentially a NORTH BERWICK greater source of loss in Scotland than in RAINFALL England and Wales. This follows from the Inches later growing season in Scotland combined with a shorter harvest period. Particularly in seed crops, however, this problem has been tackled by chemical and mechanical haulm destruction. In 1956, for example, haulm destruction was carried out in at least 50 percent of all Scottish potato crops. Recent field and laboratory experiments carried out in the east of Scotland and reported by Boyd and Henderson . West Region (1953) have indicated that as potato tubers AUCHINCRUIVE RAINFALL mature, their resistance to blight infection Inches increases. From this, the authors conclude that '*very early lifting for seed or for the early ware market, with blight present on the haulm, will have much more serious consequences than later lifting, when the haulm i s only slightly green."

BLIGHT AND WEATHER DATA

NORTH REGION: Long-term average tem- FIGURE 15.—Long-term average monthly rainfall and mean temperatures in relation to growing periods for three centers in perature and rainfall ñgures for Craibstone, Scotland. the North of Scotland College farm near Aber- deen, are shown in figure 15, together with for Perth, at the head of the Tay estuary, show planting and lifting times. The low summer that mean temperature conditions there arc temperature, which in the two hottest months, very similar to those at North Berwick (I.e., July and August, does not exceed a mean of BS^to 60° F. In July and August), but average 56® F., is one of the principal factors in the rainfall Is higher (2.9 Inches In July and 3.4 generally small importance of blight at this Inches In August). center, despite a relatively high rainfall. The WEST REGION: Figure 15 shows long-term temperature figures for Craibstone are fairly weather data for Au chine rul vein relation to the typical for potato growing throughout the re- growing season for early and malncrop potatoes gion, but rainfall and humidity conditions vary In the southwest of Scotland. The most Im- widely. Blight Is usually worst near the coast, portant early-growing district Is centered where humidities are Increased by coastal around Glrvan, which Is about 20 miles south fogs. of Auchlncrulve. Average temperature and EAST REGION: Conditions here, although rainfall conditions at Glrvan are similar to very variable, are generally drier than In the those shown In the diagram. That potatoes can other two regions. At North Berwick (fig. 15), be produced during such a cold growing season for exannple, average annual rainfall Is only (end of February to Jime) Is due to two main 24 Inches, and the normally low summer rain- factors: (l)Only a Umlted number of fields, fall, here and throughout East Lothian, Is one very near to the seashore and practically frost of the factors associated with the low Incidence free, are used for these early crops; and (2) of blight. In the east region, blight Is most the potato variety grown Is Epicure, whlchhas serious along the north and south banks of the a remarkable capacity to recover after being river Tay, and this may be partly ascribed to damaged by frost. This early ware crop the frequency of sea mists (**haar") leading to virtually escapes any damage from blight, a high humidity conditions during the potato- fact that Is, no doubt, closed linked with the growing season. Long-term weather records low-tempe rature conditions.

40 Despite the relatively high siimmer rain- end is served. In seed crops, it is to stop fall at Auchincruive, blight on the haulm is growth when the maximum quantity of seed- of only minor importance in maincrop potato sized tubers have been formed. The practice varieties. This may be, in part, a temperature is not compulsory, as in other countries (e.g., effect. Holland and France), where the haulms of seed crops have to be destroyed by a certain date CONTROL MEASURES to prevent the late spread of virus diseases. Little or no critical experimental work has Copper compounds, of which copper 03cy- been done in Scotland on the economics of this chloride is the most important, are used in premature killing in the seed crop. Whether protective spraying against blight. A high it pays or not will depend very much on the proportion of this work is carried out by variety and on the relative value of seed and contractors, who normally spray at high- ware potatoes. Up till now, the practice seems volume rates. Low-volume spraying is not to have been accepted as a good one without practiced on any scale. The close spacing of question, but experimental evidence is required the seed crops (about 12 in. x 26in.), combined against which to test it. with the generally moist growing conditions leads to a luxuriant growth of the potato haulm, BLIGHT FORECASTING and considerable damage may be done by the passage of spraying equipment through the Grainger (1950) has discussed the results crop. This problem has been referred to by of forecasting potato blight in the west of Gray, Clouston, and Biggar (1952), in writing Scotland during 1944-48, using Beaumont about the north region. They say that it is periods as the criteria on which warnings noteworthy that there is probably more en- are based. The method was found to be success- thusiasm for spraying in the island counties ful, provided that the conception of a '*zero than anywhere else in the north. The reason date" (in this case, July 1) before which for this is that on the island crofts spraying warnings were considered invalid, was em- is done by hand ( chiefly with knapsack sprayers ) ployed. Further evidence that this method is without accompanying mechanical damage to a success in the west of Scotland has been the potato tops. provided by Grainger (1955), and in this later The amount of protective spraying carried paper, he describes an apparatus for auto- out varies in different parts of the country. matically recording the occurrence of Beau- Most is done in the important seed-growing mont periods. A limited test of the same areas near the Firth of T ay. There can be little method of forecasting blight in the north of doubt that the object of some of this spraying Scotland during 1943-46 has also been re- combined with later haulm destruction, is to ported as successful by Gray, Clouston, and keep the tubers free from blight infection, Biggar (1952). rather than to prolong the growth of the potato Since 1949, the Department of Agriculture crop. In some of the counties of southwest for Scotland, in conjunction with the Meteoro- Scotland, notably Ayr, Wigtown, Kirkcudbright, logical Service, has beeninvestigating the pos- and Lanark, there is little or no protective sibilities of a blight forecasting service for spraying. the whole country. Each year, an attempt has Data from the "crop-check surveys'* (table been made to correlate Beaumont periods at 24) show the approsdmate percentages of the a niunber of weather stations with observed maincrop s on which protective spraying and outbreaks of blight in the field: these latter chemical or mechanical haulm destruction reports to be sent in chiefly by the Depart- have been practiced during recent years for ment's potato seed inspectors. The method the whole of Scotland. has not operated so successfully in the east Boyd (1952) has reviewed the various of Scotland, where most of the potatoes are methods of haulm destruction used in Scotland. grown, as it has in the other areas. At the present time, spraying with sulfuric The rapid changes in micro-climatic condi- acid is the nciethod most commonly employed. tions over short distances make it very diffi- The main aim of haulm killing is not to protect cult to apply a forecasting system to the whole the tubers from blight infection^ although this of Scotland. Fortunately, with the present TABLE 24. —Number of potato crops in Scotland survey and percentage of crops sprayed and haulm destroyed, 1953-56

Item 1953 195^ 1955 1956

Potato crops in survey number. • 600 810 782 732 Protective spraying I)ercent. • 27 32 25 30 Haulm destinlction percent. • 52 49 47 52

41 tophthora infestans, which is controlled by a emphasis on seed-potato production, coupled ¿umber of single major genes, either alone or with a generally late attack of blight, the need in combination. The presence of four such for such a service is not so great as in many genes has been demonstrated. Recently, Black, other countries. Mastenbroek, Mills, and Peterson (1953) have proposed that an international nomenclature POTATO BREEDING should be adopted in designating these genes, and for this purpose have reconamended the adoption of Black's system. In this, the four W. Black of the Scottish Society for Re- genes have been named Ri, R2» R3» ^^^ ^4« search in Plant Breeding, now at Pentland- The 15 possible genotypes of potatoes contain- field, Roslin, Midlothian, has bred a number ing these genes, together with the recessive (r) of blight-resistant potato varieties, which are shown in figure 16 alongside the equivalent have come on to the market since 1948. In genotype designations previously used by Mills all cases, the resistance to blight was de- and Peterson in the U.S. A. and by Mastenbroek rived from Solanum demi s sum. The best known in Holland. of these varieties has been Pentland Ace, con- The same workers have also suggested the taining the R3 gene; but none of the varieties have been accepted commercially on a large adoption of an international system for naming scale. All have been attacked by specialized blight races, to be based on the reaction of races of the blight fungus soon after their particular races to the previously named introduction. (See also England and Wales: genotypes. Races are named according to the genotypes that they attack. Thus, race 0 Resistant Varieties.) Black (1952), in parallel with workers in attacks none of the resistant genotypes; race 1 other countries, has been responsible for much attacks the recessive and genotype Ri; race of the pioneer research on the genetics of 1,2 (pronounced; "one, two" and not ** twelve*') blight resistance. The genetics of so-called attacks the recessive, and genotypes Ri, R2, '•major gene" resistance has been most fully and Rl R2f but all other genotypes are resistant explored. In this type of resistance, which is to it. No genotype is resistant to blight race shown by S. demi s sum derivatives, the plants 1,2,3,4. This interrelationship between host exhibit a hyper sensitivity reaction to Phy- and parasite are shown in figure 16 with the

GENOTYPE RACES OF Phytophthora infestans

_ - F - Scotland A El H J D G E B2 - c 1 - U.S.A. A D C B _ BD BC - - - - - Holland NI N2 N5 N4 _ _ N7 N6 _ _ N8 - N9 - 2,3,4 International 0 1 2 3 4 1,2 1,3 1,4 2,3 2,4 3,4 1,2,3 1,2,4 1,3,4 r r r r 5 S S s s S S S S S S s S S S s s Rj D R9 R| - S - - S S S - - S s s -

S s R2 C R? R2 - - s - S S S - s s -

R3 - RQ R3 - - - s S S - s S - S s s

s S s c s S s s R4 ^ »^2,5 h - - - - -

R|R2 CD - R|R2 - - - - S - - - s s - - s

RjRg - - R,R3 - - - - S - - - s - S - s

R1R4 BD - R,R4 - - - - - S - - - s S - s

R2R3 - - R2R3 - - - - - S - - s - - s s

R2R4 BC - R2R4 ------s - - s - s s

R3R4 - - R3R4 ------s - - S s s

R1R2R3 - - R1R2R3 ------s - - - s

RjRjl^ BCD - R,R2R4 ------s - - s s '^l'^3'^4 " " ^\^3^A ------s ------s s - - - s

- = Resistant S = Susceptible

FIGURE 16.--International system of designating interrelationships of genes and races of Phytophthora infestans.

42 names previously given to the blight races by SUMMARY: SCOTLAND the different workers. It is claimed that *'The advantages in designating genes and races in A predominant partis played by seed produc- this manner are (a) it probably allows for tion in the potato industry, and defoliation any extension that may be necessary In the losses caused by blight are not usually con- future; and (b) it illustrates immediately the sidered important in Scottish seed crops, inter-relationships between races and between owing to the almost universal practice of genes and races without referring to diagrams prematurely destroying the haulm in these or tables." crops chemically or mechanically. Present This system of naming genes and races indications are that blight does not normally has now been adopted by all the leading workers cause serious yield losses through premature throughout the world. Black's work, which has defoliation in any of the major potato-growing included the identification of blight collections areas. More detailed survey information, par- from many countries, has brought to light the ticularly for maincrop ware varieties, is fact that the common race of blight throughout needed to be sure of this, but climatic dif- the world is now race 4. ferences throughout the country make it dif- The chief emphasis in Black's search for ficult to carry out systematic surveys. Data blight-resistant potato varieties is now placed from surveys of tuber blight Infection suggest on ''field resistance," as "major gene "re- that there has probably been a tendency to sistance in a variety is of short duration, overestimate the losses from this cause. Dr. owing to specialization of the blight fungus. Black of the Scottish Society for Research In A recent review of blight resistance work Plant Breeding has carried out fundamental in Scotland has been given by Black work of world w^de importance on the genetics (1954). of blight resistance.

NORTHERN IRELAND

Northern Ireland, wjth a population accord- acreages of Gladstone and Arran Peak (5 to 6 ing to the 1951 census of about 1.4 millions, percent), and 1 to 2 percent each of Majestic, has a total area of 2j million acres under King Edward, Dunbar Standard, and Arran crops and pasture. In 1951 (Northern Ireland, Consul. Ballydoon Is extensively grown as a Ministry of Agriculture, 1956), about 144,000 first-early variety for the home trade, and acres were under potatoes and the total pro- British Queen as a second-early. In Northern duction was 1,197,000 tons, of which 265,000 Ireland, the predominant Arran Victory is tons were used for human consumption. In- among the varieties least susceptible to blight. dicating a consumption per head of about 400 lb. per annum. In Northern Ireland, as in INCIDENCE OF BLIGHT Scotland, a high proportion of the potato crops are grown for seed. In 1951 the shipments of As would be expected from the distribution seed amounted to 84,000 tons, and 171,000tons of the rainfall in the months of July, August, were used for planting in Northern Ireland. and September (fig. 18), blight conditions in Also, 126,000 tons of ware were shipped, and Northern Ireland are generally severe, and In that year, in which the yield was high (8.3 more so in the north and west than in the south tons per acre), 324,000 tons were used for and east. The cllmagram for Garvagh In stock feed. Londonderry (fig. 19) shows that the average Potato growing is fairly equally distributed rainfall there Is higher than that for ^'blight among the six counties (fig. 17). The most years" only In the southern zone of England Important seed-growing areas are in the and Wales (fig. 6). The rainfall figures for northern parts of Londonderry, Antrim, and Armagh, perhaps more typical of the ware- mid-Down; the malncrops for ware are chiefly growing area, for the months of July and in a middle belt; and most of the early and August in the years 1947-56 are given in second-early varieties for ware are in County table 25 as against the potato yields for Down and parts of County Antrim in the Lagan Northern Ireland taken from the official sta- Valley. tistics. It will be seen that, as in England The principal maincrop varieties are Ar ran and Wales, 1947 and 1955 were dry years (and Victory, Kerr's Pink, and Arran Banner. Of also years in which the yield was lowest), but thèse, the purple -skinned late maincrop variety even at Armagh, where the rainfall is lower Arran Victory is by far the most popular for than In Londonderry, 8 years out of the 10 had ware, and it accounts for some 38 percent of over 5^ Inches of rain In the 2 months of July the acreage. Kerr's Pink follows with about and August. 17 percent, and Arran Banner, also much in The necessity for routine spraying Is not In demand for seed for export, occupies about question In Northern Ireland. Professor 15 percent. The whole range of British varie- Muskett of Queen's University, who, with R.W. ties Is grown for seed, with fairly large Wallace of the Ministry of Agriculture, has

43 RELAND

Each dot = IOOO acres

FIGURE 17.--Approximate distribution of potato growing in Northern Ireland and the Irish Republic.

44 GARVACH

SEPTEMBER

FIGURE 18.-Average monthly rainfall and teniperamres in Northern Ireland and Irish Republic, showing weather stations referred to in text.

kindly provided most of the information on SPRAYING MATERIALS AND SPRAYING which this section of the review is based, re- TRIALS calls that in his 35 years' experience there have been very few years with less than a ton Burgundy mixture is the material still gain from spraying in the maincrops grown chiefly used for protective spraying; the pre- for ware in Northern Ireland, and the usual ferred program is to use the 1 percent mixture gain in Arran Victory is l| to 2 tons per acre. for the first application (to avoid injury to the Outbreaks of the disease on the haulm occur young haulm by the stronger mixture) and to by mid-July; the attack is almost always under use the 2 percent mixture for the second and way on unsprayed maincrops in the second third applications to give as much protection half of July, and thereafter its progress may as possible from mid-August into September. be very rapid, with complete killing of the The leading results of a series of trials of haulm by early August in the north. Such dusting versus spraying with burgundy mix- very rapid attacks are exceptional, however; ture, carried out in County Down over the and attacks from mid-August to late August years 1924-30 (Muskett, 1929; and Muskett are more usual for Northern Ireland as a and Cairns, 1931) were as shown in table 26. whole. In the trials in 1924 and 1925 two applica- The first spraying, which should perhaps tions of 1 percent burgundy mixture were ideally be given at the end of June or beginning compared with two of copper dust (15 percent of July, is in practice generally given in mid- Cu) at 20 lb. per acre. In 1927 two applica- July, with great regularity. The second spray- tions of similar dust at 30 lb. per acre were ing is given toward the end of July. About 98 compared with two applications of 2 percent percent of the farmers put on the first spray, burgundy; and in 1930 three applications of dust and about 85 percent the second. The keenest (16 percent Cu) were compared with three growers give a third spraying about mid- applications of burgundy, the first two at 1 August. A mean gain of 1 j to 2 tons per acre percent and the third at 2 percent. It was con- is consistent with an average of about a fort- cluded that dusting was neither so efficient nor night's prolongation of growth, due to spraying. so economical as spraying with burgundy In seed crops the gain inyieldis much less, as mixture. even the maincrop varieties are burned off In the three consecutive years 1924-26, a before the end of August, but the protective series of spraying trials was carried out at spraying has the very important function 4 or 5 centers in each of the 6 counties, of minimizing the risk of infection of the covering the whole range of maincrop varieties. tubers. The mean results of these trials (hitherto un-

TABLE 25. --Rainfall In County Armagh, and potato yields for Northern Ireland, 1947-56

Rainfall and potato yields 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956

Rainfall in— July inches.. 3.4 2.5 2.7 4.Ö 3.3 1.7 4.1 4.6 1.8 3.8 August do.. .6 3.5 3.0 5.3 4.5 3.9 3.7 2.3 1.1 6.3 Potato tons per acre •. 5.6 7.9 7.6 7.5 8.3 7.9 8.1 7.0 6.5 7.0

45 published), for two sprayings with 1 and 2 per- IRELAND cent burgundy mixture, respectively, are shown in table 27. Garvaqh LONDONDERRY The results of such trials carried out at RAINFALL 5 many centers over consecutive years con- Inches stitute a survey and leave no doubt about the level of blight losses. On the basis of the bulking curve for Majestic in England (fig. 8) the mean yield for the unsprayed (9.3 tons, or 74 percent of a potential yield of about 12^ tons) would indicate a 75 percent date for the unsprayed haulm about mid-August; with cor- J FMAMJ JASOND responding dates for the haulm twice sprayed with 1 ajid 2 percent burgundy mixture, about ^Markree Castle September 7 and September 15, respectively. SLICO RAINFALL 5 This agrees very well with general observa- Inches tions on the course of the disease on sprayed and unsprayed haulm in Northern Ireland. Numerous other spraying trials were carried out over the years 1936-40 at Hillsborough Agricultural Research Institute in County Down; at Greenmount Agricultural College, JFMAMJJASOND Muckamore, in County Antrim; and again at 20 to 30 centers throughout Northern Ireland, in connection with pioneer work on the testing of the then-new copper oxy chloride and cuprous RAINFALL 5 _ Inches oxide sprays. The main indication from these many trials was that 2 percent burgundy mixture was perhaps unnecessarily strong, at any rate for the first application, and that both cuprous oxide and copper oxychloride preparations, when used at the sanne copper dosage as 1 percent burgundy mixture (0.25 percent JFMAMJJASOND Cu) would have about the same protective effect. A series of trials, again hitherto unpublished, was made at Hillsborough in each of RAINFALL 5 _ Inches the 5 years, 1936-40 on the second-early variety British Queen. The trial crops were generally planted rather late (in May) éuidthey ripened off aboutthe second week in September. The leading results for the controls and plots twice sprayed with 2 percent burgundy mixture (8.10.40) are given in table 28. It has

JFMAMJJASOND been possible to estimate the 0.1 and 75 percent dates for the unsprayed haulm very closely from the notes made on the condition of the haulm at various dates, although no blight RAINFALL key was then used. The 75 percent dates on Inches the sprayed haulm were indeterminate as blight and ripening-off effects were con- current in September. The blight attack was evidently sonncwhat later than usual in 1940, but on the average over the 5 years the spraying appears to have given a useful prolongation of growth J FMAMJ JASOND from about mid-August until the end of August FIGURE 19.--Long-term average nxjnthly rainfall and mean tem- or just into September each year, with an peratures in relation to growing periods for five centers in Ire- average gain in total crop of 2^ tons per land. acre.

46 TABLE 26. "Results of dusting versus spraying trials on potatoes in County Down, Ireland, 1924-25, 1927, 1930

Mean total yield Year Trials Potato variety Unsprayed Dusted Sprayed

Number Tons/acre Tons/acre Tons, acre 1924. 2 Lochar 12.6 I2T3 ^ 3 1925, 3 ..do 15.4 15.9 17.2 1927. 1 Up to date.... 9.3 10.7 1930. 1 British Queen. 12.4 12.9 14.4

TABLE 27.--Mean results of spraying with burgundy mixture on maincrop varieties throughout Northern Ireland, 1924-26

Mean total crop Blighted tubers

Twice sprayed Twice sprayed Year Trials 'Unsprayed 1 percent 2 percent Unsprayed 1 percent 2 percent mixture mixture mixture Tn1 xtiire

Number Tons/acre Tons/acre Tons/acre Cwt./acre Cwt./acre Cwt./acre 1924. 27 9.0 10.7 11.1 15 12 12 1925 28 9.1 11.4 11.9 10 5 5 1926 21 9.8 12.3 13.2 5 5 3

Mean...... — 9.3 11.5 12.1 10 7 7

TABLE 28.--Results of spraying trials on British Queen potato in County Down, Ireland, 1936-40

Date on which blight on unsprayed haulm Total yield Blighted tubers reached— Year 0.1 75 Unsprayed Sprayed Unsprayed Sprayed percent percent

Tons/acre Tons/acre Cwt./acre Cwt./acre 2 1936 July 14 Aug. 10 6.Ö 0.3 7 1 1937 July 22 Aug. 15 9.1 11.8 4 1930 July 22 Aug. 10 11.3 15.7 11 6 20 1939 Aug. 3 Aug. 21 12.3 15.0 8 3 2 1940 Aug. 5 Aug. 24 12.7 13.6 12.9 6 6 Mp Q-n ...... •>■>••• Aug. 16 10.4

47 HAULM DESTRUCTION the famous Arran series raised by Donald Mackelvie on the island of Arran in Scotland Haulm destruction to prevent both virus and earlier in this century. blight infection of the tubers is obligatory in Much work is done by the Ministry's In- all seed crops in Northern Ireland entered for spectors and by Professor Muskett's staff Stock Seed Certificates. The early varieties at Queen's University to assist potato breed- are burned off about the last week in July, ing in connection with the important Northern and the maincrops about the last week in Ireland seed trade; and the National Institute August--a little earlier or later in each case, of Agricultural Botany have potato centers according to the conditions of the season. for raising virus-tested stocks in North Sodium chlorate is chiefly used, the rate of Antrim, and for multiplying new stocks near application being about 28 lb. per 100 gallons Coleraine for trials in England. Blight-re- of water per acre. Sodium chlorate is pre- sistant seedlings and new blight-resistant ferred to sulfuric acid, as it does not require varieties are tested at these centers under the acidproof machines, and it acts more slowly, prevailing severe blight conditions; but the giving opportunity for the tubers to ripen off. commercial cultivation of R-gene resistant Little burning off is done on the ware crops. varieties in Northern Ireland is so far neg- A good indication of the losses due to ligible. The choice of varieties for ware crops blighted tubers in ware crops protectively is dictated by the consumers' demand for sprayed in the usual way is given in a report quality rather than by any consideration of on variety trials (Northern Ireland, Ministry blight susceptibility. of Agriculture, 1954). In trials at 13 centers Doling (1957)has recently surveyed Northern in 1954 and 99 centers in previous years the Ireland for races of Phytophthora infestans. mean percentage of blighted tubers in Arran In the commercial varieties he found only race Béinner was 4.2 percent of the total crop (14 4 to be widely distributed. In 73 isolates, 70 cwt. per acre) and in Majestic 2.3 percent were race 4 and only 3 were race 0. (7 cwt. per acre). These figures are of the same order as those shown in the spraying SUMMARY: NORTHERN IRELAND trials, and indicate a general loss of 2 to 4 percent by infection before lifting. Tuber Potato blight is severe almost every year infection tends to be less on the close soils in Northern Ireland. Routine protective spray- of North Derry than on the poorer open soils ing is imiversal, and the need for this has of mid-Antrim. been well established by a great number of spraying trials carried out systematically POTATO BREEDING throughout the country. Any unsprayed haulm is killed by blight between the end of July Many of the principal new British potato and the end of August, according to locality, varieties in recent years have been raised variety, and season, with mid-August as the in Northern Ireland by John Clarke on his mean 75 percent date. The mean gain from 2, small farm at Mosside, near Ballycastle in or rarely 3 sprayings is l| to 2 tons per the north of Antrim. His introductions include acre or about two-thirds of the total defoliation Ulster Chieftain, Ulster Premier, and Ulster loss caused by blight. There is a constant loss Prince as flrst-early varieties; Ulster Ensign of 2 to 4 percent in ware crops due to blight and Ulster Emblem as second-early potatoes; in tubers. Northern Ireland has an important Ulster Torch, Ulster Beacon, Ulster Tarn, seed potato trade. In the seed crops, some and Ulster Knight as maincrop varieties, and of the yield of ware-sized tubers is lost by Ulster Supreme, a late maincrop. This wealth burning off the haulm in August, but the blight of new varieties can be compared only with losses are slight.

REPUBLIC OF IRELAND The Republic of Ireland (Eire), with a population, according to the 1951 census, of about The acreage under potatoes, apart from 3 millions, has a total land area of about 17.4 temporary rises to 500,000 and 430,000acres, nülUon acres. In 1954, some 292,000 acres respectively, in the first and second World were under potatoes, 1,516,000 under corn Wars, has fallen steadily from about 458,000 and other crops, and 7,929,000 under pasture. acres at the beginning of the century to 292,000 The potato production in that year was about acres In 1954. But, against this, the yield 24 million tons, or about 7.7 tons per acre per acre has risen from 4.2 tons in 1900- (Ireland (Eire) Central Statistics Office, 1955). 1909 to 7.8 in 1940-49, and it was over 8Í The consumption of potatoes per head In 1947- tons per acre In the years 1950-53. 48 (F.A.O. 1948) was 420 lb. per annum, The rise In yield during this century reflects or about twice that in England and Wales. the work done by the Department of Agriculture In the promotion of good cultivation and

48 manuring, varietal improvement, the sup- to blight. Epicure is still one of the most pression of virus diseases, and campaigning popular flr St-early potatoes. for spraying against blight. Small acreages of Alpha and Voran are Sugar beet has played an important part grown here and there, but the production of in the recent history of the root crop in potatoes for industrial purposes is very small. Ireland. This crop, introduced in 1926, was However, the production of potatoes for in- occupying 74,000 acres in 1954, with a pro- dustrial purposes (alcohol, starch, etc.) in duction of 674,000 tons. Thus: In 1900-1909, Ireland has been carefully investigated by 458,000 acres produced 1,853,000 tons of Drew (1941) and Drew and Deasy (1939), who potatoes for a population of 3.2 million; in concluded that continental varieties had little 1954, 366,000 acres produced 2,248,000 tons superiority in point of starch production over of potatoes plus 674,000 tons sugar beet for Arran Banner ajid other high-yielding varieties a population of 3.0 million. already grown in Ireland. Also, to the account for 1954, there must be added an important export trade in seed CLIMATIC CONDITIONS potatoes of the highest quality and freedom from virus diseases, built up since the 1920's Ireland has a temperate climate with abun- through the devoted labors of Davidson, dant rainfall, particularly well suited to the Delaney, and their coworkers in the Potato potato crop, of which it is the traditional Inspectorate, aided by the pioneering inves- European home. Conditions are not so cold tigations of Pethybridge, Murphy, and McKay. in winter or so warm in sumnner as in eastern Davidson (1937) describes the foundation of England or in continental Europe. In configura- the industry, and McKay (1955) provides a tion the country is a **saucer," or large practical account of all the work on potato central plain, comprising the drainage area of diseases that has contributed to the present the river Shannon with much bog land, sur- well-being of the potato in Ireland. rounded by broken mountainous country to the north and west and by hills to the south DISTRIBUTION AND VARIETIES and east. The country is swept by maritime air from the southwest, and humidity condi- As the map (fig. 17) clearly shows, potato tions tend to be fairly uniform over most of growing is fairly evenly distributed over the the country as the mountains arc too broken whole of Eire. The chief seed-producing dis- to afford much shelter in their lee. In the tricts are in Donegal, Mayo, and around absence of surveys for the 75 to 90 percent Athlone; early varieties are grown most ex- stages of blight, such as have been made in tensively in the south--in Cork, Waterford, England and Wales in recent years, it is not and Wexford--and also in the Dublin area; possible to zone Ireland for severity of blight but maincrops for war« are also grown in all as has been done for England and Wales in districts. The cultivation is mostly on small figures 4 and 5. On the whole the worst farms. districts for blight are those with the highest The principal maincrop varieties are now rainfall in the months of July, August, and Kerr's Pink and Arran Banner. Kerr*s Pink, September; and the long-term average rainfall a late maincrop variety, popular for its good distribution maps, given in figure 18, serve yield and quality, is grown in all parts of the to indicate the regional differences. The blight country, and probably accounts for about half epidemics tend to be severe in the west, and the present acreage. Arran Banner, the other perhaps most severe in Galway, Roscommon, most extensively grown variety, is notably and parts of Offaly and Westmeath. They earlier in maturity. It is grown not only for tend to be least severe in the east, notably ware but also for seed--large quantities of near Dublin, which is in the driest part of which are produced for export, principally the country. to Mediterranean countries. Majestic, King The climagrams (fig. 19) enable the mean Edward, and the whole range of British rainfall and temperature conditions in vari- varieties are also produced for seed. Culinary ous parts of Ireland to be compared with quality in potato varieties is traditionally those in England and Wale s--and other parts appreciated in Ireland, and small acreages of the world. At Markree Castle, County of the superb Golden Wonder, and also some Sligo, which, with Garvagh in Londonderry of the best varieties of the past, even Champion, in Northern Ireland, fairly well represents arc still grown. Up-to-Date, despite its present the mountainous and wind-swept seed-growing high susceptibility to blight, is still in demand areas of Mayo, Sligo, and Donegal, the mean for its quality in many parts of the world, temperatures in July and August are rela- notably South Africa, and it is still grown tively low, reaching only about 58° F., and the in Ireland both for seed and ware. rainfall in the months of July to September The second-early variety, British Queen, is very high. At Cork, in the south, given as is also widely grown, again because of its fairly typical of the southern coastal zone quality and despite its high susceptibility from Cork to Wexford where many early

49 varieties are grown, the diagram is very ford, and Wexford; and at about the same similar to that for Pcnzance (fig. 6). At time, or at most a little later, in Galway Birr, Offaly, in the central area, the long- and Mayo. In only 9 years out of the 50 was term average rainfall and temperatures over the appearance of blight delayed beyond mid- all years are similar to those of the **blight June, and in most years it became more or years" only in the southern aind Fen zones less prevalent throughout the country from of England. At Dublin, although this repre- the end of June onward. sents the driest part of Ireland, the average These early outbreaks were by no means rainfall follows the same pattern as at Birr, always followed by severe epidemics, as the and it is not very much less. Even in this progress of the disease was frequently checked area blight epideniics tend to be more fre- by subsequent warm dry weather--rarely until quent and severe than in the southern zone the end of the season but quite often until of England. Although there are regional dif- late August or early September. ferences in the incidence of blight in Ireland, The general courses of the blight epidemics these differences are much less than in after the outbreaks each year, for nnany England and Wales. For the purpose of esti- years past, have been described by McKay mating average blight losses it nnay not be in appendices to the Annual Reports of the greatly misleading to treat Ireland as a Minister for Agriculture. The accounts of whole--as a single blight zone--with a higher the epidemics for the 10 years from. 1947 to frequency of severe epidemics than any other 1956 (McKay, 1948-57) enable these years such zone in the British Isles. to be grouped into **blight years,*' when the As the climagrams would suggest, planting disease was more or less severe, and "other and lifting dates for the potato crops in years," when its incidence was relatively Ireland (Eire and Northern Ireland) are simi- slight. What constitutes a '*blight year" in lar to those in the south of England and Eire nnay be seen from the following brief Wales. Early varieties in the southern coastal extracts from McKay's notes, which refer districts are planted in February and March to the general incidence of blight. As pro- for lifting at the end of May and in June. The tective spraying is the rule in Eire, the nnaincrops are generally planted in April, progress of the disease, as described, must and lifted in October (Kerr's Pink) or during be taken as referring in the main to sprayed September (Arran Banner). crops and not to the unrestricted epidemic were the crops left unsprayed. RECENT BLIGHT EPIDEMICS In the famine year 1846, Father Matthew Blight years Blight conditions recorded that the potatoes from Dublin to Cork were blooming in all the luxuriance of 1948 Appeared early, but not general an abundant harvest on July 27, while on until near nnid-September, then August 3 there was one wide waste of putrefying rapid. Tuber infection low. vegetation. In terms of our present (B.M.S.) 1950 General from beginning of August, scale for blight assessment on the foliage but not viriilent until mid- this would suggest that the attack, which had September. Tuber infection low. probably started in early July, was still incon- 1951 Prevalent from beginning of spicuous from the roadside, though perhaps August, then a severe epidemic, up to 25 percent by July 27, but that it was at during which the superiority of 50 to 75 percent a week later. Such very early burgundy mixture was shown up. defoliation, which would now be expected to Tuber infection high in some result in a loss of half the crop, still occurs in parts. some years in unsprayed crops of susceptible 1952 ComjTLon from, mid-July onward, varieties in county Galway, but 75 to 90 percent rapid kill of tops in September. dates before mid-August appear to be excep- Tuber infection low. tional, and in many years the bulk of the 1953 General from mid-July onward, defoliation loss occurs in September. long-drawn-out attack, reaching The dates of flrst outbreaks of blight have its peak in September. Some been well determined in Ireland, where the heavy tuber infection from lifting Department's instructors and potato inspectors while tops not completely dead. have been systematically reporting on first 1954 Widespread from mid-June on- occurrences since 1907. A retrospect of the ward, but development some- results of this work for 50 years (1907-56) what erratic up to harvest. has now been published by McKay (1957). 1956 General by the end of June, little This unique record shows clearly that the first severe killing of the foliage until outbreaks (often on clamp sites or waste the end of August, then rapid. heaps) usually occur in May or early June Long-drawn-out attack with some in the early-potato districts of Cork, Water- heavy tuber infection.

50 other years Blight conditions ^^ The distinction between •*blight years" and **other years" cannot be made very precisely 1947 -Prevalent from end of June, but for Eire, as indeed there was only 1 of the spraying and very warm weather 10 years (1949) in which the blight attack was in August stopped spread on main- negligible, but the data in table 29, read in crops. Became active again in conjunction with McKay's notes, would sug- September. Tuber infection neg- gest that the frequency of '»blight years'* is ligible in late varieties. 7 or 8 in 10, and that in these "blight years," 1949 Only of minor importance despite all the spraying that is done, the throughout the season, owing to growth of such maincrops as Kerr*s Pink is the exceptionally dry, warm generally stopped by blight in September-- weather. probably by about mid-September; with the 1955 Warm weather in July halted the unsprayed crops going down some weeks attack, but sporing continued on earlier. lower leaves and revived on un- Table 29 shows clearly the checking effect sprayed crops in September. of low rainfall and higher mean tenriperatures Some serious tuber infection from during July and August on the blight epidemics. lifting when tops not completely It also shows that, as in England and Wales, dead. the years of low average national yield are Table 29 shows the mean rainfall, for Eire not usually the "blight years," but the years as a whole, in the months of June, July, with relatively warm dry summers in which August, and September in each of the years blight is least severe. This would not be so 1947-56, and also the mean temperatures in in Ireland but for the protective spraying, each month in terms of degrees Fahrenheit which enables the crop to reap the benefit deviation from the long-term averages shown of the good growing conditions in the blight in figure 18. It woiild have been correct to years. express these temperature deviations as a range; e.g., as "-1 to + l" for July 1950 in- PROTECTIVE SPRAYING stead of **0," but for simplicity the mean of the two extrennes of the range is given, and After half a century of use and many this serves well enough to differentiate be- challenges from other materials, burgundy tween the warm and the cool seasons. The mixture is still the material officially rec- mean potato yields for Eire, each year, as ommended and most generally used for potato given by the official statistics, are also entered spraying in Ireland. The main reason for this in the table. lies in the high resistance of the deposits

TABLE 29,--Mean rainfall, mean temperature deviation from long-term averages, mean potato yields for Eire, 1947-56

Rainfall Mean temperature deviations-*- Mean Year and average potato Jime July i^. Sept. June July Aug. Sept. yield

In. In. In. In. Tons/acre Long-term average 2.8 3.7 4.2 3.2

Blight years: 1948 3.9 3.1 3.9 3.9 -1 -1 0 -1 ^.5 1950 2.6 5.0 6.3 6.4 -1 0 +1 -1 S.5 1951 2.4 3.2 5.2 5.^ +1 +1 -1 +1 8.6 1952 2.5 1.4 3.6 2.4 0 +1 +1 -3 8.6 1953 1.3 4.3 4.7 0 0 +1 +2 8.8 1954 2.7 4.2 3.2 5.7 -1 -2 -1 -2 7.7 1956 3.1 5.3 3.6 4.9 0 0 -2 +2 9.2

Other years: 1947 4.7 4.0 1.2 3.7 +1 -1 +5 +2 6.8 1949 1.2 3.2 4.0 2.6 +3 +2 +2 -f4 7.7 1944 5.1 .9 2.1 3.5 0 +3 +4 -1 7.4

^ See fig\ire 18 for average temperatures.

51 to washing off by rain and the consequent the rapid destruction of the haulm by blight relatively long period of protection given, in September commonly renders the treat- which, as may be seen from McKay's notes, ment unnecessary. Where haulm, destruction is important not only for prolonging the is done, the material chiefly used is sodium growth of the haulm to increase yield, but chlorate. No arsenites are employed. also for keeping blight off the haulm as long SPRAYING TRIALS as possible to reduce the opportunity for the infection of the tubers. A series of spraying trials on the variety One percent burgundy mixture (freshly made Up-to-Date was carried out to a unifornn up with 4 lb. copper sulfate and 5 lb. washing plan over the years 1927-32 at Glasnevin, soda per 40 gallons of water) is now the Dublin, by Murphy and McKay (1933). These standard spray, but 2 percent burgundy is trials were made over a run of years in which still preferred in the west, and is resorted slight, average, and severe attacks appear to elsewhere in wet seasons. The mixture is to have been very fairly represented, in com- applied at the rate of 120 gallons per acre, parison with the general accounts of the still often by knapsack sprayers, but by horse- epidemics in more recent years. The spray- or tractor-drawn machines on the larger ing was done by knapsack sprayer, using farms. The first application is generally generous amounts of 2 percent burgundy mix- recommended toward the end of June, and ture, with 2 applications in 1927 and 1930, the second about 3 weeks later, but the and with 3 applications in the other years. The application dates vary according to the preva- leading results are given in table 30. lence of blight, forwardness of the crop, and Fromi these results it will be seen that even weather conditions. A third application is at Dublin the haulm was completely killed by occasionally given; hence, again the need for blight at the end of August or in early Sep- a spray material that, applied for the last tember 5 years in 6. The mean gain from time towards the end of July, will give pro- spraying over the 6 years was 2.5 tons per tection for the longest possible time in August. acre. Additional trials with 3 sprayings were Copper oxychloride and cuprous oxide sprays, also made on British Queen and Arran Vic- at the same copper dosage as 1 percent tory in 1931, and on Golden Wonder in 1932, burgundy mixture, are used to some extent and the mean increase of marketable crop in Eire, but despite their convenience in use, from all 7 trials in which 3 sprayings were they have not made the same headway as in given was 3^ tons per acre. England and Wales or other parts of Europe. In discussing the losses due to blight and The predominant variety, Kerr's Pink, would the econonnics of spraying Murphy and McKay normally grow on to the end of September in pointed out that in most of the many field the absence of blight or with very efficient experiments on spraying in Ireland the in- spraying. In Galway blight can devastate Kerr*s crease of sound yield was from. l|^ to 2 tons Pink during July, and in the worst years the per acre. Even with this gain, the sprayed unsprayed crops would be dead by the first plots always suffered to a greater or less week in August, with a loss of about half extent despite the spraying. The authors con- the crop. Although this is rare, even with sidered that 3-2 tons per acre, the gain achieved three sprayings most of the haulm is usually with fairly complete protection by the 3 spray- killed by mid-September. In Monaghan and ings in their trials on all varieties would not the north midlands Kerr's Pink is sometimes be an overestimate of the total average loss sprayed only once in the drier years. In from, blight in Ireland as a whole. In other Wexford and Waterford, where the unsprayed words, the normal practical spraying in Ire- haulm is sometimes killed by mid-August, land saves about half the total loss from and almost always by the end of August, two blight; an estimiate in close accordance with sprayings are usual, with a third in wet the findings, in this study, on the efficiency seasons. The variety Up-to-Date, where it is of spraying in blight years in England and still grown, siiffers rather more severe attack Wales. ^ than Kerr's Pink, but when sprayed as for Some phytotoxic checking of the growth of Kerr's Pink the differences in loss of yield the haulm by the very strong 2 percent are not great. Arran Banner, which occupies burgundy mixture was noted by Murphy and a considerable part of the potato acreage in McKay in their trials. In the very severe most districts, often escapes much reduction blight year 1932, in which 1 percent burgundy of yield by blight, as it reaches maturity about was included for comparison, it gave as good the end of August. It is, however, very sus- results as the 2 percent mixture. A wealth ceptible to blight in the tubers. Where grown of trial data on the comparison of 1 percent for seed, as in Donegal, it is often sprayed and 2 percent burgundy mixtures is to be only once, about the first week in July, and found in the historic reports by Pcthybridgc then the haulm is burned off with chlorate (1910-19) on potato diseases in Ireland. The about mid-August. average results of a series of trials carried Relatively little haulm destruction is done out in Galway over the years 1911-18 were in Eire, except on the seed crops. No doubt as follows:

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53 Tons per acre from Fortnightly liftings on this protected crop Crop: crops sprayed with showed the proportion of the ñnal or potential burgundy mixture of-- crop that was present on various dates from July 5 onward. These figures are given in table 1 percent 2 percent 31, and to this table are added the corresponding figures later determined for Healthy- 10.8 10.8 Majestic in England and Wales (see fig. Blighted .5 .4 8). There is remarkably close agreement Total- 11.3 11.2 between Murphy's first figures for Up- to-Date and the mean figures obtained for Thus, it would appear that the expensive Majestic in England; which indicates that, and very strong 2 percent burgundy mixture in 1937 at any rate, the late maincrop has little advantage over 1 percent burgundy, Up-to-Date was bulking in Ireland at about except where very heavy deposits are required the same average rate as Majestic in to persist as long as possible from the last England. spraying. From periodic weighings of the leaves Copper dusting was compared with spraying and total haulm, which he also made in in the trials of Murphy and McKay, and even the course of his trials. Murphy concluded at very heavy rates of application dusting that when the weight of the haulm had been was inferior to the burgundy mixture. B oui sol, reduced to about 55 percent of its the first of the colloidal copper fungicides, maximum and the weight of the leaves to was included in the trials in 1931 and 1932; between 30 and 40 percent of their niaxi- not until its rate of application (in terms of mum, tuber production ceased. At this Cu dosage) was brought up to half that of point of stoppage of tuber formation he 1 percent burgundy mixture (namely to 1,251b. described the field as looking gray Cu per 100 gallons) did the results approach green at a distance, with stalks still green, those of burgundy mixture. but with all the leaves dead except for tufts of 6 or more at the apices of the SPRAYING IN RELATION TO THE DEVELOP- shoots. This stage, corresponding to 75 MENT OF THE POTATO PLANT to 90 percent on the later evolved B.M.S. scale (B.M.S., 1947) occurred about Au- Following up clues that became apparent gust 28 on the unsprayed plots at Glas- in the course of the trials made with nevin in 1937. In effect, as Murphy pointed McKay, Murphy (1939) went on in the years out, the final yield from the unsprayed 1934-37 to evolve a new kind of potato plots should be approximately the same spraying trial that was to reveal the as the yield reached by August 28 on essential relationship between increase of the plots kept entirely free from blight yield and protection of foliage, and pro- by frequent spraying. vide the yardstick by which blight epidemics could be measured, not only in TABLE 31.--Progress of tuber development of Ireland, but throughout the world. No longer Up-to-Date potato in Glasnevin Ireland, 1937, content with determining yields from the and of Majestic in England sprayed and unsprayed crops, he studied the whole course of growth and decline Up-to-Date of the haulm, and of increase in weight Majestic, of the tubers (1) in plots kept completely Date Proportion proportion free from blight by frequent applica- lifted tions of a weak copper fungicide having Yield of total of total no phytotoxic effect, (2) in plots commer- yield mean yield cially sprayed, and (3) In plots left Tons/ unsprayed* acre Percent Percent It was not until 1937 that the method July 5 1.74 12 was perfected, and in this year, which July 19 was one in which the blight attack (start- 3.90 27 34 Aug. 2 47 ing on July 24) was average at Glasnevin 6.87 32 (with 2.0 inches of rain in July, 2.7 in Aug. 16 9.90 68 73 Aug. 30 12.24 84 86 August, and 1.4 in September), Murphy Sept. 13 at last had plots of Up-to-Date in which 13.93 96 96 the haulm was kept completely free from Sept. 27 1A.64 100 100 blight, with no phytotoxic effects and no premature dying off owing to dry weather-- From a plotting of the tuber growth curve a more or less perfect crop growing on from the data in table 31, the total yield under to the end of its growing period. the perfectly protected plants on August 28

54 was 11.9 tons per acre. The actual yield theory. The total yields present by various as determined by lifting was 11.7 tons per dates in the plots tiiat received each of the 3 acre, in almost exact accordance with the treatments are shown in table 32. TABLE 32.—Total yields of Up-to-Date potatoes at Glasnevin, Ireland, with 2 treatments and no treatment, 1937

Yield Treatment July 5 July 19 Aug. 2 -Aug. 16 Aug. 30 Sept. 13 Sept. 27

Tons/acre Tons/acre Tons/acre Tons/acre Tons/acre Tbns/acre Tons/acre Coinplete control 1.7 3.9 6.9 9.9 12.2 13.9 14.6 2 percent burgundy (3 sprayings) 2.1 4.3 7.3 9.6 11.3 12.9 13.6 TInsDraved 1.9 4.6 7.5 10.0 11.5 11.7 11.7

The date of stoppage of growth in the plots (earlier in Galway, later in Dublin, but August sprayed with 2 percent burgundy mixture (on 23 on average). Spraying with 3 applications June 30, July 16, and August 9), as estimated sometimes defers the stoppage of growth by by Murphy's weighing method, was September blight until mid-September or even until the 24, very little different from that in the plots end of the season, but it is probable that with with complete control (Sept. 26). But the the usual 2 sprayings, and sometimes only 1, curves given in his paper for the progress the average gain from spraying can be put at of decline of the foliage show a wider sepa- about half a month's full growth at the end of ration in late August and early September, August and beginning of September, equivalent so that the 75 percent point by the English to stoppage of growth on September 8. The method would have been perhaps a week earlier, total defoliation loss from blight and the gain in better accordance with the difference of from spraying, on a 127-ton potential total yield. However, there was a lower maximum crop, on the basis of Murphy's figures would weight of haulm in the plots sprayed 3 times then be as shown in table 33. with the strong 2 percent burgundy mixture, It is difficult to form any estimate of the which indicates some checking effect on growth mean loss due to blighted tubers in Ireland. that may also have affected the yield. As in England and Wales, the losses in individual crops may range from negligible ESTIMATION OF BLIGHT LOSSES IN IRELAND am^ounts to several tons per acre. Perhaps the loss of 4 or 5 percent as found by Pethy- The method that Murphy's work suggested bridge over his long series of trials is fairly of estimating blight losses, from a mean representative for sprayed ware crops, and growth curve for the tubers and survey infor- the loss could be expected to be twice as great mation on the dates at which growth is stopped in the unsprayed. Estimates of 4 and 8 per- by blight on the sprayed and unsprayed crops, cent, respectively, for the losses due to blighted has not yet been applied and developed in tubers are entered in table 33. The mean total loss on a 12|-ton potential crop due both to Ireland as it has been in England and Wales. defoliation and to blight in tubers then become s There are not the necessary data for any close 3.8 tons per acre; of this loss, 2.3 tons per analysis of the blight losses in Ireland region acre is recovered by the usual commercial by region or year by year, and it may well be spraying. that Kerr's Pink often puts on rather more of These figures are in reasonable agreement its tuber weight in September than is indicated with the estimate of 3j tons per acre total loss by Murphy's data for Up-to-Date in the single made by Murphy and McKay on the basis of season, 1937. It is of interest, however, to take their trials; and they serve to illustrate that Murphy's figures as a basis and then see what the method of estimation based on Murphy s measure of defoliation loss they would indicate growth curve gives results of at least the right when some near estimates are made of mean order. It should be borne in mind, however, dates of stoppage of growth by blight. that table 33 is for ware maincrops and not Perhaps it would be fair to say that in 7 or for the seed crops that are burned off pre- 8 years out of 10 in Eire the average date at maturely, and in which the percentage of which blight would stop growth on the lïiain- blighted tubers is much less. It is also based crops, if they were left unsprayed, would be on the growth curve for a late maincrop, and about the end of the third week in August

55 TABLE 33.—Loss estimations for maincrops in Eire on basis of Murphy's findings

Potential crop Sprayed Item Unsprayed (no blight)

Date of stoppage of growth Aug. 23 Sept. 8 Sept. 27

Percentage of potential crop (from Murphy*s growth curve) 76 92 100

Total crop tons per acre.. 9.5 11.5 12.5 Gain from spraying tons per acre.. 2.0 Total defoliation loss tons per acre.. 3.0

Percentage of blighted tubers B 4 0

Healthy crop tons per acre.. 8.7 11.0 12.5 Gain from spraying tons per acre.. 2.3 Total loss tons per acre.. 3.^

it may not hold for Arran Banner. In the sprayings can be applied knowledgeably and to occasional dry, warm seasons, such as 1949 the best advantage. The very early spells of and 1955, the blightlosses are undoubtedly less, blight weather, corresponding more or less but the lack of rain reduces the crop. to the first few outbreaks, which, as McKay (1957) has so clearly shown, often occur in May FORECASTING or even in April, are not made the subject of press notices, as this would confuse the growers and might lead to premature spraying. With severe or moderately severe blight It is not until the early blight weather spells epidemics occurring in 7 or 8 years of 10, have been such as to permit 3 to 5 generations routine spraying every year is well justified of the fungus that an initial warning of the onset in Ireland. The requirements of a forecasting of the disease is issued. This is almost system for Ireland are different from those in invariably in the latter half of June, when it has England and Wales, where over much of the the effect of strongly reinforcing the Depart- country spraying is rarely needed at all, and ment's long-standing advice that the first spray even in the Fens only about every other year should be put on about that time, according to is a blight year. In England one of the main weather conditions. objects of forecasting is to discourage spray- A minimum humid period favoring the spread ing in the ••no-blight'* years when it can do of potato blight consists, according to Bourke's positive harm. In Ireland spraying is needed "Irish Rules," of almost every year, but the fact remains that Twelve consecutive hours with relative the Irish farmers are attempting to combat humidity not below 90 per cent and tem- severe epidemics with a very small number perature not below 50° F, followed im- of spray applications--gene rally only two, and mediately by a further four hours with the at most a reluctant three. The highly retentive plants wet, either due to rain, drizzle or burgundy mixture is used, and this helps, but high hximidity (fog, mist or dew). still the mean gain from spraying is only about Such a minimum **blight weather spell" half the total loss from blight, and there is means one having an "effective duration" of every inducement for trying to raise the over- 1 hour; any longer spell has an effective dura- all efficiency of the spraying by the best tion of 1 hour plus the excess over the re- possible timing of the spray applications in quired minimum. relation to the weather conditions. This is a tempe rature-humidity rule, similar In 1952, a new approach to forecasting was to the Beaumont rule as employed in England made by Austin Bourke of the Irish Meteoro- and Wales in that it has the same minimum logical Service. In this system, as it has since temperature requirement and that it demands been developed and put into use in Ireland a continuous spell of high humidity; but the (Bourke, 1953a, 1953b, 1955), the stress is on rule differs from the Beaumont rule in that it detecting every progressive spell of weather was derived from a theoretical consideration favorable for the development of blight and on of the findings of Crosier (1934) on the condi- advising farmers whenever the more important tions required for the sporulation and infection of such spells occur, from June through August, processes of the fungus and in that a shorter so that not only their first sprayings but also minimum period at higher relative humidity is their second and, where necessary, their third taken as critical.

56 In the operation of the system, routine hourly weather reports from a network of 9 or A part of the sequence of these maps for 10 weather stations are used, and the "effec- 1953, from Bourke (1953b) is here reproduced tive durations'* of all **blight weather spells" in figure 20 to illustrate the method. After the spell shown on June 21-22, a first spray (humidity conditions measured in standard screens 4 ft. above ground level) are entered warning was broadcast on June 22 as follows: on maps from day to day at the Central Fore- Weather favorable for the spread of potato blight occurred over most of Ireland cast Office in Dublin. The probable geographi- during the past week-end, and is likely cal limits of each spell are then determined by to recur in the west during the next few examination of the synoptic weather charts for the period in question. days. Blight may be noticeable aboutmid- week in many areas. Weather will be suitable for spraying during most of the week. IRELAND Other warnings were issued following the CHART IJ CHART 14 spell shown for July 5-6, and later spells on Jiüy 19-20, August 4-9, and August 29 (for burning off). In effect, these warnings gave very good advice for the season, which was one in which blight became more or less general from mid-July onward. It has not been possible to test the Bourke system of forecasting in Ireland so thoroughly as the Beaumiont system (with operations charts) has been tested in England and Wales, owing to the lack in Ireland of any systematic blight surveying and assessment by standard key. The forecasts have nevertheless been in reasonably good accordance with the general

June 19th June 2/st -22nd observations on blight made by the Depart- CHART IS CHART 16 ment's inspectors who have participated in the work, and there is no doubt that Bourke's method is among the most sensitive yet proposed for the measurement of "blight weather." The application of the method in Chile is illustrated in figure 79 of this study. The advance made by Bourke does not lie so much in the choice of the humidity criteria, for Smith's (1956b) work on forecasting by 90 percent humidity criteria in England and Wales has indicated that there is usually a fairly close correspondence between Bourke and Beaumont periods when the principal spells of blight weatíier occur. Both methods give good results in the hands of those experienced June 26th .2dth in their use. The important step forwardisthe CHART 17 CHART 18 correlation of critical humidity periods (however defined) with specific weather situations as revealed by synoptic maps and with air ma s s analysis (Bourke, 1957). This is a procedure that naay find an application in many parts of the world, especially where forecasting has to be attempted for a wide area with no very close network of weather stations. To illustrate the possibilities, Bourke's chart for mid-July 1956 is here reproduced (fig. 21). Working in Dublin and from a study of the synoptic maps alone, he was able to see that a severe blight weather spell which occurred in Ireland over the period July 14-17 must adso have occurred June 30th -July 2nd July 5th-6th in England and Wales, and indeed over much of western Europe. This was fully substantiated for England and Wales, and as far as could FIGURE 20."Part of a series of charts showing areas of Ireland later be ascertained, it was also true for the affected by successive spells of blight weather in 1953. Effective areas indicated in other countries--from the durations of the spells at the several stations are given in hours. (Bourke, 1953b.) Baltic countries to northern France.

57 IRELAND

HIQH SURFACe WEATHER MAP 0«00 GMT. JULY 14. 1956.

FIGUEŒ 21.—Blight weather in Ireland and parts of western Europe, July 13-18, 1956 (Bourke, 1957): a, Surface weather map for July 14, showing continental slow-moving, or "puddle," low; b, effective duration of blight weather in Ireland, July 15-18; and c, approximate limits of blight weather arising from the synoptic situation (cross hatching shows the area most seriously affected).

58 SUMMARY: REPUBLIC OF IRELAND premature defoliation and blight in tubers of 3f to 4 tons per acre. Of this loss about Zf Potato spraying is now so universal in Eire tons per acre can be saved by 2 or 3 applica- that it is difficult to estimate what the loss tions of burgundy mixture, which prolongs the from blight would be in its absence. No blight useful growth of the haulm by 2 or 3 weeks and surveys or critical series of spraying trials also substantially reduces tuber infection. Eire have been carried out recently. But from con- has an important seed-potato trade. Burning off sideration of the weather conditions, general is practiced on the seed crops, and blight accounts of blight in each year, and the results losses in them are small. Many fundamental of trials xinder similar conditions in the past, contributions to knowledge of potato blight, it is estimated that in Ireland the haulm of including its relation to the growth rhythm of unsprayed maincrops would be killed by blight the potato plant and more recently to fore- between mid-August and the end of August in casting methods, have come from Ireland--the 7 or 8 years out of 10, with a miean loss through traditional home of the potato in Europe.

59 CONTINENTAL EUROPE

FRANC E

France grows annually about 2,500,000 acres TABLE 34.--Acreagetí of leading potato of potatoes. Half the crop is used for human varieties grown for seed in France, 1955 consumption, about 284 lb. per head each year (Russell (E. J.), 1954). Large quantities are Certified Variety fed to livestock, principally pigs; small seed amounts are manufactured into starch; and the rest is used for seed. Certified seed production is on a sznall scale. In 1954 it accounted Acres for slightly under 3 percent of the total potato Bint je ' 43,171 acreage. But the seed growers have been Ackersegen - 5,920 responsible for much of the technical progress Institut de Beauvais. 2,438 made in French potato production during recent Etoile du Léon 2,354 years. An account of the seed-potato industry Saskia ■ 2,240 and its future trends has beengivenby Demes- ArxEjn. BaJiner ■ 2,191 may (1954), The output of French certified B. F. 15 2,065 seed plus imports of such seed from other Ker Pondy 1,529 countries, mainly Holland, is sufficient to plant Krasava 1,294 only about one-fifth of the total acreage. This Urgenta 1,065 may, in part, account for the rather low average yield of potatoes in France, which for the period 1948-50 was 5 tons per acre The outstanding point is the preponderance (F.A.O., 1955). of Bintje, which is widely grown throughout France. It is very susceptible to blight, but, POTATO DISTRIBUTION AND VARIETIES as it is relatively early maturing, Bintje often The distribution map (fig. 22) shows a escapes the full severity of epidemics. Acker- scatter of potatoes throughout the whole coun- se gen is the most popular late variety, and try. There is a marked concentration in Etoile du Leon and Saskia the most common Brittany (19 percent of the total potato acreage early varieties. was in this province in 1954), and smaller concentrations in north-central France (the SOIL, CLIMATE, AND CULTIVATION Paris basin through to Nord), eastern France, and across the center of the country (northern In Brittany and the Central Massif (fig. 23), part of the Central Massif). The main early the potato soils are generally light, and are potato-growing areas are in Brittany and in detived from underlying granite and schist the departments surrounding the mouth of the rock formations, in northern France, including river Rhone in the southof France. In the latter the Paris basin, and in eastern France, the area, 56 percent of the potato acreage is soils are heavier, but usually overlie a sub- planted witii early varieties. Early varieties soil that is predominantly sandy. are also grown in the vicinity of the large France exhibits the three main European towns; e.g., in the Paris basin and near Bor- ciin:iatic types--maritime, continental, and deaux. Brittany is also the most important Mediterranean. In the potato-growing areas seed-growing area, accounting for approxi- of Brittany, the climate is predominantly mately 70 percent of the total seed acreage« maritime, although it is nriodified In the depart- The only other important seed district is in ments farthest from the coast. This maritime northern France, concentrated in the five de- climate is characterized by a marked autumn partments, Nord, Pas de Calais, Somme, Oise, and winter rainfall peak, relatively mild win- and Aisne. ters, and cool summers. Total annual rainfall Some idea of the relative innportance of the is around 30 inches. In northern F ranee (Nord, different potc.to varieties grown in France can Pas de Calais), there is a modified maritime be gaged from figures published by the Seed climate, whereas farther south in the Paris Growers* Association: They have also given area continental influences are nciore impor- a description of these varieties (Federation tant. The potato-g rowing districts of eastern Nationale, 1955a, 1955b). Table 34 shows the France are under the influence of a f\illy con- acreages of the 10 leading French potato tinental type of climate. Sumimer and winter varieties grown for seed in 1955. temperatures are more extreme than in the

60 ^—^ . •,•• .A. :•.v. • . : • •

• • • ; • .•.•••••••. •• •••.?. ■■■. ..•••.•••.••■•.••'•/ • • '. • r^ • ....• • •

Each dor = IOOOacres

FIGURE 22.--Distribution of Dotato growing in France, 1954. (After Klatzmann, 1955.) west, and the rainfall peak occurs in the sum- carried out at the end of February to early mer months. Total annual rainfall, approxi- March. Seed crops in Brittany are burned off mately 24 in., however, is lower than in the compulsorily in mid-July. The maturity of west. The true Mediterranean climate with its mid-season and late varieties varies from hot summers and characteristic periods of approximately mid-August to mid-September. drought is confined to the coastal areas of One-third of the total manpower in France southern France. Generally speaking, this is is still engaged directly in agriculture, and not an important potato-growing area, apart apart from the north-central area of the from the early crops grown around the mouth country, mechanization is not very far ad- of the river Rhone. vanced. Potatoes are still largely planted by The main planting takes place in mid-April. hand. The seed is not usually cut. Potato rows Planting of early varieties in Brittany is are about 28 in. wide, and mid-season and late

61 FRANCE

V. N O RD;

•.■■;A I S NE. ÁRDENNES;.':.^ t'Af-tigny.; -, Metz.••....-.^^ Ar^ Reims ": .MOSELLE-'-" ■■ " :.. '•'•.BAS BMTTANY jr^Ppris ■;•■ 'Slrasbourq Ploudalmézeau« _ _ ... ■••.Versoi ¡les ••'•RHIN '/'^ /Í ■•CÔTEs DU NORD Brest^ f^v^ i"' »Rennes •HAUT.« Choteoulin .Orleans .' '•..•'.RHIN'.

•.CREUSE •.; ■•.•••.., ■_ 'HAUTE.'.- .PUYDEDÔME VIENNE ..■:••••.,•;•; Clemont'-.. '•: ." -Fernand

C ENTRA L >.M A S'S I F":

AVEYRON

.'• Vjóulouse 'Montpellier»^

FIGURE 23."Key map of France.

varieties are planted from 18 to 22 in, apart valley areas (e.g., valleys of the Garonne and in the rows. The potatoes are almost always Durance) where some potatoes are grown under ridged up# Storage is in farm buildings or, irrigation, heavy losses fromblightmay occur. where the winters are mild, in field clamps. In the north of France, the four main pota to- A description of ware potato production growing districts differ from each other in methods has been given by Malmont^ (1954). climatic and soil conditions and blight Inci- dence. (1) In Brittany the disease occurs every FREQUENCY OF BLIGHT EPIDEMICS year. (2) In north-central France, which In- cludes the Paris basin, blight Incidence appears The importance and incidence of blight to be fairly uniform, although there are cli- vary considerably between the different areas. matic differences within this area. Severe Broadly speaking. If a line is drawn across attacks occur about 1 year In 3. (3) In eastern France from Strasbourg to Bordeaux, the France blight Is somewhat less Important than disease is important north of the line. South In north-central France. (4) In the northern of the line, blight is considerably less impor- part of the Central Massif the disease Is only tant, although in the intensively cultivated river occasionally severe.

62 Very little published «urvey data on the in- Since 1955, systematic observations on the cidence of potato blight on a y ear-by-year basis exist for France. Oucomet and Foex (1939) development of blight on haulm in relation to weather conditions have been made at a num- have simimarized verybriefly the attacks of the ber of centers throughout France by the Plant disease from 1919 to 1937. In 1953, shortnotes were given anonymously in the Annales dea Protection Service in collaboration with M. Epiphyties on attacks in 1951, and Barbotin Guntz. The aim is to use these data to improve the existing blight warning services. Progress (1953) has given an account of the progress of curves supplied by M. Guntz for selected blight in 1952. Information on other recent centers in the main growing regions of northern years has been given to the authors by H. Dar- France during 1955 and 1956 are shown in poux and M. Guntz of the SUtion Centrale de figure 24. Pathologie Végétale at VersaUles, and by P. Dumas of the Service de la Protection des Végétaux. From this, it was possible to divide FRANCE the 10 years 1947-56 into 3 categories; i.e., years in which the incidence of the disease was slight, moderate, or severe. Table 35 lOO BLIGHT 1955 shows the results of this division: ON HAULM TABLE 35.~Incidence of potato blight in per cent 50 - France, 1947-56 I North Centrol O Slight loo 1956 1947 1949 - 1952 7 JyJ June July Auqusf Sept

FIGURE 24.—Progress of blight on unsprayed haulm in principal It is not very satisfactory to have to treat potato growing regions of northern France in 1955 and 1956; France as a single unit in this way, but it is variety. Bint je. not possible to give concise data for the separate growing regions, as the information The year 1955 was exceptional for blight. It is too fragmentary. In broad terms, the three occurred first for a distance of about 50 km. categories used in table 35 can be defined as round the coast of Brittany, but made slow follows: progress in dry weather except on the coast. Slight: Blight attacks are either ab- In north-central France, dry weather present or very late throughout vented the development of the disease, but northern France. there were heavy rains in eastern France, Moderate: Blight attacks are important where blight is not usually bad, and these rains in only some parts of northern resulted in severe blight attacks. In 1956, France, or the disease is late blight appeared late, but made very rapid in appearance throughout the progress on the tops of potato plants through- whole area. out northern France. Severe: Blight attacks occur early and are severe throughout north- LOSSES DUE TO PREMATURE DEFOLIATION ern France. BRITTANY: Although the first generalized BLIGHT ON HAULM field outbreaks of blight in this region do not The incidence of blight is usually considered normally occur before mid-June, first reports to follow a fairly regular pattern from year to in the area usually come in much earlier, year. The disease is present every year in sometimes by mid-April. The evidence so far Brittany, where it starts about the middle of available suggests that there are considerable June; the tops of unsprayed potatoes are variations in the incidence of blight between usually dead by mid-July. It is normally first the coastal regions and inland Brittany. This reported in the Paris area about 2 weeks after is shown up in figure 25, which presents blight its appearance in Brittany, but the course of progress curves for unsprayed Bintje at three the disease both there and in eastern France is centers. Ploudalmézau is on the coast; much less certain than in Brittany, and its Chateaulin is about 15 km. from the coast; and incidence is not so severe. In the Central Rennes, which is a good deal farther east than Massif, blight is usually late and is rarely the other centers, is at least 60 km. from the severe. sea.

63 loo 1954 tivc purposes arc lacking. The disease is less BLICHT ON importent in these areas than It is in Brittany, HAULM and as a result there may be a tendency to per cent 50 minimize it. The only information on which tentetiveloss estimates can be made for northern France, O outside Brittany, comes from a series of loo y^-^ 1955 spraying trials against blight carried out at '0/ Attigny (Ardennes), during the period 1948-55. \/0/ >

Qj / effect on the haulm, and to compare sprayed 5Q 0/ 0/ 0/ / plots to an unsprayed control. In only 2 years or/ Q. / U/ out of 8 were the spray plot s lifted and weighed. J Table 36 shows the dates on which the 75 June July August" Sept percent blight stege was reached on the unsprayed controls each year, and the increase in FIGURE 25.—Progress of blight on ünsprayed haulm at centers in yield from spraying in the 2 years that the Brittany, France, 1954-56; variety, Bintje. crop was weighed.

There are insufficient data on which to base TABLE 36.--Date when 75 percentblight yield loss estimates for this area» but some occurred on unsprayed Bintje potatoes and general statements can be made. The 75 per- increase in yield 2 years when crop was cent blight stage on unsprayed crops is reached sprayed, northern France, 1948-55 very early along the coast. Figure 25 shows .. . that at Ploudéilmézau, this stage was reached Date 75 percent Increase in at or before mid-July during 1954-56. However, Year "blight occurred on yield from this early defoliation is unlikely to have a unsprayed plots spraying serious effect on potato yields, as the coastal strip is the early growing region and immature potato crops will be dug there from the Percent end of May onward. Brittany is the principal 1.948 Aug. 6 seed-growing area, but blight is unlikely to 1949 9 affect the yields of these crops seriously, as, 1950 11 in any case, they have to be burned off in mid- 1951 16 July under compulsory regulations designed 1952 (^) to prevent the spread of virus diseases. The 1953 July 17 33 seed crop, also, receives the greatest amount 1954 Aug. 14 of protective spraying. 1955 19 20 Despite the importance of early varieties and of seed-potato growing in the region,most of the potato acreage is planted with mid- ^ No blight. season varieties (in particular, Bintje) for the ware market. It seems likely that relatively The variety grown was Bintje. Planting took heavy yield losses due to prem.ature defolia- place around the first week of May, which is tion following blight attacks will occur very rather late, except in 1953, when it was at the frequently in crops of these ware varieties if end of March. In only 1 year was blight absent. they are unsprayed. In 1956, blight attacks Of the 6 years when the 75 percent blight stage were late, but figure 25 shows that at Chateau- was reached in August, the latest date (Aug. 19) lin the 75 percent stage on unsprayed crops occurred in 1955, when there was a 20percent was reached before the end of July, and at net gain from spraying. Rennes the same stage was reached before the At this stage, the results presented in table middle of August. The time of naaturity of 36 can only be used as an indication that blight Bintje is roughly the middle to end of August, losses due to premature defoliation in this part so that unsprayed crops of Bintje will have lost of France m.ay be fairly consistently heavy. up to 4 weeks of growth. Weather conditions at Attigny are very similar NORTHERN FRANCE, outside Brittany: Dif- to those in the potato-growing districts of ferences in the incidence of blight between the eastern France, which is not looked upon as a remaining potato-growing areas of northern bad blight area. The inconsistencies between France occur, but precise data for compara- the Attigny results and the general opinion about 64 blight in the area can only be cleared up by more detailed trial and survey work aimed at FRANCE estimating losses. 6 r-Brittany BREST RAINFALL 5 BLIGHT IN TUBERS Inches

H. Darpoux has told ud that '*the damage caused by blight is nearly always the result of premature destruction of the potato foliage, leading to a fall in crop yield. Tuber infection is usually of little importance except in sum-

mers with a particularly heavy rainfall." Both J F M Jk M J s o N D in 1954 and 1956, however, the rainfall in the latter part of the summer was high, and heavy 6 r- Eastern France losses from infection, while the tubers were METZ still in the ground, were reported. Tuber in- RAINFALL 5 fection is in general regarded as of small im- Inches portance in view of the widespread growing of Bintje, which is a very susceptible variety. In 14 years of potato experiments carried out at high altitudes in the Central Pyrénées, Chouard (1949) reported that in no year did blight affect the keeping quality of the crop, despite the fact that no control measures were taken against the J FMAMJ JASOND disease. . Southern France It is generally considered that the light soils AVIGNON in Brittany and the Central Massif are more favorable to tuber infection with blight than the more loamy soils in the north-central and eastern growing regions of northern France. RAINFALL 5 The problem of tubers being infected at lifting Inches time, by coming into contact with diseased haulm, does not usually arise in France, owing to the concentration on early-maturing varieties.

BLIGHT AND WEATHER DATA The three main types of climate--maritime (Brittany); continental (easternFrance); Medi- FIGURE 26.—Long-term average monthly rainfall and mean temper- terranean (southern F ranee)--under which atures in relation to growing periods for three regions in France. potatoes are grown are illustrated in figure 26. The climagrams give long-term average tem- westerly wind reach the higher ground that perature and rainfall figures for three repre- the bulk of the rain is precipitated. Nontheles s, sentative centers, together with planting and the almost invariable story is for blight to be lifting times. worse on the coast« Brittany, which is the worst blight area in The high summer temperatures and gen- France, has a relatively low summer rainfall. erally dry conditions account for the minor Compared with eastern France the average importance of blight in southern France, and rainfall is lower in Brittany in May, June, at the same time point the reason for the smaill July, and August, although annual rainfall is interest in midseason and late varieties in the considerably higher, due to the high autunrm area. and winter precipitation. This points the fact that humidity and not rainfall is the more CONTROL MEASURES important factor affecting blight attacks. The importance of humidity is borne out from a consideration of the two blight curves for Most of the protective spraying against Brittany in 1955 shown in figure 25 above. blight carried out in France goes on in the In that year, rainfall during the growing season Brittany peninsula, and there, spraying is was greater at ChateaulinthanatPloudalmézau concentrated on the seed crop. Little or no on the coast. But blight progress was very »praying is done on the early crop. In the much quicker at Ploudalmézau, due to higher potato areas of north-central France, where humidity conditions. Rainfall in the center of the farms are large and highly mechanized, Brittany is higher than on the coast, since it there is somie protective spraying. In other is not until the clouds driven by a prevailing areas, where much of the crop is grown on

65 small farms by peasant proprietors, little described some of the different methods of action is taken against the disease. haulm destruction practiced on seed crops, Spraying is most usually carried out at and at the same timie he has compared the high-volume rates (about 1,000 liters per efficiency of a number of different chemicals hectare, which is equivalent to 89 gallons used for this purpose. per acre). Generally, 2 or 3 spray treatments One method that the French hope to use in are given to mid-season and late varieties. order to reduce blight losses is to grow quickly In Brittany and other areas where spraying maturing potato varieties that will escape most against the Colorado beetle is compulsory, if not all the effects of blight attacks. To a combined treatment against blight and the some extent, they already look upon the wide- beetle is often applied. DDT is the material spread growing of Bintje as a control method. most commonly used against the Colorado Although very susceptible to blight, Bintje beetle, and the usual practice is to combine matures early (mid-August to end of August, the second treatment against this pest with when planted about the middle of April), the first antiblight spray. Experiments with and, except in Brittany, it usually escapes low-volume spraying against blight have been serious yield losses as a result of premature reported by Jouin, Hascoet, and Ventura (1954). defoliation even when unsprayed. The Dutch E. Ventura has told us that in 1951, when blight variety Sirtema, which was put on the market was severe, application rates below 300 liters in 1952, meets this need for very rapid per hectare (27 gallons per acre) did not give maturity, but, unfortunately, it is a poor an adequate control of the disease. quality potato and is unlikely to have any Sprays are more efficient and economical commercial future. Work at La Station than dusts, and are far more widely used. Génétique in Brittany aims to produce a good Most protective spraying is still carried out quality, quick maturing variety based on breed- with copper compounds, the most important ing with the old German variety. Erste von of these being bordeaux mixture, copper oxy- Nassengrunde. Some work on blight races is chloride, and copper oxide. Experiments have also being done at this station, but hope of been carried out by Raucourt, Lansade, and producing blight-resistant varieties from Ventura (1949) to compare the relative effi- Solanum demis sum hybrids has been given up, ciency of bordeaux mixture containing 1 per- owing to the ability of the fungus to produce cent and 2 percent copper sulfate. In a bad new races. blight year (1948), the 2 percent bordeaux mixture was found to be more efficient under POTATO BLIGHT FORECASTING conditions where only 2 or 3 sprayings were given. In another series of trials in northern The whole of France is covered by a network France in 1954 reported by Guntz, Hascoet, of 14 regional Stations d'Avertissements (Agri- and Ventura (1954), a depressive effect of cop- cultural Warning Stations) under the adminis- per on potato yields was noted when blight tration of the Plant Protection Service. A was absent or attacked the crop late in the recent account of the working of these stations growing season. Reduction in yield from the has been given by Journet (1955). In brief, use of 2 percent bordeaux mixture in these advice on the correct timing of spray and dust trials was about 10 percent. treatments against a wide range of crop pests The organic fungicides are not used on any and diseases is sent out at regular intervals scale. It has been found that they do not reduce from the stations to cooperating growers in the crop in dry seasons, but when blight has their regions. been bad the organic materials have had to be For potato blight, growers are advised applied more frequently than the copper com- individually by post card when it is time to pounds to give a comparable protection to the spray. The warnings are based on field ob- potato foliage* This finding does not accord servations of blight sent in by observers well with one of the French farmer's main (agricultural advisers, farmers, etc.) plus a aims, which is to keep his number of sprays general consideration of past and present down to a minimum. Some recent spraying weather conditions in the region. No specific trials in France have compared the relative weather rules are used, and the stations do efficiency of copper and organic fungicides not provide a forecasting service in the strict (Guntz and Ponchet, 1952; Ventura and sense of the term. Advice is also broadcast Raucourt, 1953). over the radio and put in the local newspapers. Haulm killing is not practiced to any extent Table 37, based on information given to the outside the seed-growing districts, in par- authors by P. Dumas, shows the diates on ticular Brittany. The tops of certified seed which advice was given to farmers to spray crops have to be destroyed within a certain against blight by the stations during 1955. period, as laid down by the seed inspectors, France is thus well ahead of most other to prevent the late spread of virus diseases, countries in offering a blight warning si^rvicc and this practice also serves to prevent tuber to all growers. Full advantage of this service blight infection. Materials used are sulfuric is not being taken at the present time, but acid and sodium chlorate. Monot (1956) has interest in this type of advice is shown by

66 TABLE 37.--Dates of spray warnings in France, 1955

Station May June July August

Angers — 9, 21 20 Avignon ^ 27 6, 13, 2Ö Beaime — 10, 2Ö 2 18 Bordeaux — B, 22 6 Clermont-Ferrand. — 11 21 I^on ^ 24 7, 21 1, 2 26 17 Montpellier 23 6, 20 4, 19 2 2 Orleans — 11, 29 19 Paris — 1Á, 30 21 Reims — 15 4, 27 Rennes ^ 18 3, 16 6 Strasbourg — 20 9, 2 21 Toulouse -- 8, 22 18

^ Spray warning for early crops. ^ Advice to kill the haulm of severely infected crops.

the progress of the scheme as a whole. Ac- though surveys of blight on the haulm are now cording to Journet, membership is increasing being made and there have been numerous by 25 to 30 percent each year. spraying trials. Chemical control is concentrated on the valuable seed crop. The quick- nciaturing Bintje is the most widely grown SUMMARY: FRANCE variety and largely escapes the worst effects of blight epidennics. French breeders arc try- Blight is most serious in northern France, ing to produce an even more rapidly maturing particularly in the Brittany area, where the ware potato suitable for winter storage. There climate is maritime • Information on defoliation is a well organized plant disease and pest and tuber blight losses is generally lacking, warning service.

BELGIUM

In Belgium potato yields are ajnong the estim^ated annual consumption of potatoes per highest in the World. Some 250,000 acres of head in 1947-48 was 315 lb. (F.A.O., 1948). potatoes are grown each year, with an average The breakup of the potato acreage by province yield of about 10 tons per acre (F.A.O., 1956). and also into early, nmidseason, and late vari- The greater part of the crop isusedfor human eties, together with the acreage grown for seed consumption, and only a small proportion is fed is shown in table 38. The figures refer to the to livestock or used for manufacture. The 1950 growing season (Belgium, Ministère des TABLE 38.—Acreages of early, midseason, late, and seed potatoes in each province, Belgium, 1950

Early Midseason Late Seed Total Province potatoes potatoes potatoes potatoes

1.000 acres 1^000^ acres 1,000 acres 1,000 acres 1,000 acres Antwerp 9.3 7.3 13.7 0.1 30.4 brabant 3.2 14.3 12.8 30.3 61.4 West Flanders.... 2.7 48.4 9.7 .6 2.0 41.6 19.2 .7 63.5 East Flanders.... 18.8 Hainaut .9 10.3 7.6 .2 1.1 5.3 6.6 Liège 11.2 Ilmbourg .3 3.0 7.9 .2 3.0 11.6 15.5 Luxembourg 7.5 Namur .4 1.7 3.4 245.2 Belgium (total) 19.2 130.7 93.2 2.1

67 Affaires Economiques, 1953). Belgium prov- is the most popular early potato. Of the mid- inces and places named in the text are shown season varieties, after Bintje, come Record, in the map (fig. 27), Eigenheimer, and IJsselster. The principal The principal midseason and late ware- late varieties are Alpha and Voran. producing areas are in the belt of sandy Most of the potatoes are grown on light, soils running across the north of Belgium, sandy soils. These soils are inherently very with the heaviest concentration in West poor, but with the favorable rainfall and a Flanders and East Flanders. Early potatoes high level of management and manuring, they are grown in the region of Duffel and Mechlin, produce excellent crops. In the main potato- which lie between Antwerp and Brus sels^-There growing areas of northern Belgium, the annual is an early potato district near the coast, rainfall is about 28 to 32 inches per annum, around Lombardsijde in West Flanders; else- with a slight peak in the summer and autumn where some early potatoes arc grown in the months. Winters are relatively mild and the region of large towns. Home seed production sumnner is cool. (See climagram, figure 28.) is concentrated in the polder region of West The growing period for Bintje and other Flanders and East Flanders, and in the higher midseason varieties is from the middle or land in the province of Luxembourg. The end of April to early September; late varieties varieties grown in Belgium are the same as planted in the middle of April are mature at those in the Netherlands. Andries (1953) has the end of September. reviewed the importation and home production Potatoes are grown chiefly on small farms. of seed potatoes. He shows that Bintje is In Flanders, farms are very small, about 5 the variety most widely grown and Eersteling hectare (12^ acres) in size, and they suffer

BELGIUM s t A

*. . . • * ^« Antwerp '•Lombardsijde ANTWERP • Duffel SI'N • Ghent '..... ^Mechlin. srN ST FLANDERS : LIMBURC

• Brussels

BRABANT

H A 1 N A U T ■'': Cembloux ; LIEGE

' . • • :* N A M U R

LUXEM B OURC

Li brament '.

50 miles

FIGURE 27.--Key map of Belgium.

68 further from the disadvantage of being carved 1954: Blight moderate. "Climatic condi- up into small strips, very often widely scat- tions did not favour the appearance tered, which seriously hinders the efficiency of the disease until late in the grow- of working. Despite their small size, they are ing season. The result was that farmed on intensive Unes; i.e., the only way early varieties, which are very sus- the farmers can get a living is by producing ceptible to the disease, and fre- big crops. Their main crops are rye, oats, quently attacked, escaped infection. barley, and potatoes. Most of the work is done The disease appeared on mid- by hand and horse labor. season and late varieties in an irregular fashion, according to BLIGHT EPIDEMICS: FREQUENCY AND area. In the south of the country, DISTRIBUTION losses were small. In Flanders, Brabant and Antwerp, the potato Since 1941, annual accounts of the blight fields were more heavily attacked. epidemics on potatoes in Belgium have been Crop yields were reduced, and given in the bulletins of the Agricultural losses were increased by storage Institute at Gembloux by R. V ander walle. These rots, which in some cases reached accounts, which are in very general terms, 20-40 per cent of the crop." have been included in reports on the annual 1948: Blight "slight. "In general, the dis- incidence of a large number of plant diseases ease appeared rather late, and as (see Vanderwalle (1942)). From them, it is a result, it caused little damage, possible to group the different years into at any rate to the potato haulm. three categories: severe, moderate, or slight, There was a certain amount of tuber according to the incidence of blight (table 39). blight in susceptible varieties." In no year was the disease absent. From table 39 it would appear that the frequency of severe blight years is about 1 TABLE 39. —Incidence of blight in Belgium, year in 3. Belgium is a small country (11,755 1941-55 sq. miles), with fairly uniform weather conditions. Nevertheless, there is a marked tendency for blight to start earlier and be more severe Severe Moderate Slight in the northwest, which is the principal pot ato- growing area, than in the south and east. 1941 1942 1944 It seems likely that in the Flanders area the 1946 1943 1947 frequency ot severe blight years is somewhat 1951 1945 1948 greater than 1 year in 3. 1953 1950 1949 1954 1952 1955 LOSSES FROM BLIGHT There is no existing survey material in No report was made for 1951, but indica- Belgium on which estinnates of blight losses tions are that this was a *'severe* ' year. can be based. Rigot (1953) has suggested that The reports for 1954 and 1955 have not yet the total of losses caused by potato blight is been published, but W. Welvaert has provided somewhere between 150 to 200 million francs information from which the authors have com- per annum. This is equivsilent to a loss of pleted the table. about half a ton of potatoes per acre over the The relevance oí the terms ''severe," "mod- whole potato acreage each year, and repre- erate,•' and "slight" is best illustrated by sents approximately 4 percent of the potential quoting from Vanderwalle's accounts for three crop. typical years. These are 1953, a "severe" It is evident from the high average yield of year; 1945, a "moderate year"; and 1948, a potatoes that blight losses are to a large extent "slight" year. controlled. Even so, severe losses do occur. 1953: Blight severe. "The disease caused This is illustrated by what happened in Flanders heavy losses to potato growers, in 1954, which was a bad blight year. The tops notably in the variety Bintje. It of Bintje, despite the fact that practically all appeared early (July 1) and a large- crops of this variety are protectively sprayed scale outbreak developed during the in the region, were dead by mid-August. This next 8 days. On late varieties, the means that from 2 to 3 weeks of growth were disease was less important and lost by these crops, representing between less damaging. Protective treat- 10 to 15 percent loss of crop due to premature ments were carried out frequently defoliation. in July to counter the effect of the According to W. Welvaert, tuber infection rains. Where this was done a good is not a serious source of loss in the main control of blight was obtained. potato-growing areas of northwestern Belgium. There were also heavy tuber rot Iiifection before lifting is generally only slight, losses in this year." mainly because of the sandy soils. Infection

69 at harvest is avoided by delaying lifting until their income in this way. Outside Flanders, the potato tops are dead. This is made easier less spraying is done. by the concentration on growing midseason The majority of the farmers use copper varieties in the area and by the relatively great preparations, chiefly copper oxychloride but labor force (due to the small size of the sometimes cuprous oxide or bordeaux mixture. holdings) available to deal with the harvest. A few farmers (not more than 5 percent) Vanderwalle in his annual accounts of blight are using organic fungicides (zineb). To some reported a number of years in which cases extent, the antiblight treatments are combined of severe tuber blight infection of Bint je with DDT or lead ar senate against the Colorado occurred. It is not possible to tell from these beetle. The contractors' charge for a course accounts how widespread the trouble was, but of 3 protective sprayings in 1956 was 1,200 it seems fair to infer that severe cases are to 1,500 francs per hectare--equivalent to relatively isolated. Tuber blight infection i s not £3 10s. to £4 10s. per acre. Haulm destruction a serious problem for the country as a whole. is practiced only in seed crops. Bintje re- BLIGHT AND WEATHER DATA mains the most popiilar variety, despite its extreme susceptibility to blight. In 1941, Van- Figure 28 shows the long-term average tem- derwalle wrote: ''Undoubtedly, Bintje pos- perature and rainfetll data, together with the sesses very great assets, both from a culinary main planting and lifting times for potatoes in point of view and also on account of its out- Flanders. Tcmperatiire and rainfall conditions standing yield, but because of its great sus- in 1954, when blight was severe, and in 1955, ceptibility to blight, it shoiildbediscouraged.*' when it was slight, are compared. In 1954, Up to the present, however, no other variety the first field outbreaks of the disease occurred has been capable of dislodging Bintje. about naid-June, and the tops of midseason Some work on breeding blight resistant potato varieties were dead by mid-August. In 1955, varieties has been carried out at the Station blight appeared only at the extreme end of pour l'Amélioration de la Culture delà Pomme the summer. de Terre, at Libramont in southern Belgium The high July rainfall in 1954 appears to (Rigot, 1953). So far, no commercial varieties have been one of the main causes of the severe have come from this station. Roland (1946) attack; August and September rainfalls were examined the foliage susceptibility to blight also above average, the mean temperatures of a number of commercial varieties, which were generally below average in thatsiimmer. he classified as resistant (e.g., Aquila), slightly By contrast, 1955 was a hot dry svimmer. susceptible (Ackersegen), susceptible (Indus- June, July, and August rainfalls were all trie), and very susceptible (Bintje). below average; July being a particularly dry Little or no research or experimental work month (0.9 inches of rain). July mean tem- is being done in Belgium at the present time peratures were 2° F. above average, and on potato blight control. The last paper on August temperatures 3^ above average. this subject was that published by Vanderwalle Vandenplas (1955) has explained the coop- and Roland (1948), which dealt mainly with a eration that exists between the meteorological series of experiments covering the influence service in Belgium and the agriciütural ad- of certain cultural treatments. Including visory and research services. This has not so far included any detailed work on potato ^Flanders blight. Jaivenois and Beck (1956), however, have described a pilot experiment carried out RAINFALL in 1955 in Hainaut, in which protective spray inches applications were made to potatoes following Beaumont periods. CONTROL MEASURES In Flanders all the farnners apply a number of protective spray treatments each year. These are given to the midseason and late varieties; it is mostunusualfor early varieties to be sprayed. Normally 2 applications are given: The first, just before the crop meets RAINFALL in the rows, and the second 2 to4 weeks later. Inches Welvaert has said that many of the farm.ers do not spray sufficiently often in bad blight years, when amininaumof 3 applications should be given. Most of the spraying is done by the farmers themselves, very often with knapsack sprayers. There are also a number of spraying contractors at work, many of whom are local farmers who possess a tractor and FIGURE 28.—Montbly rainfaU and mean tempcraiure for Flandew, Bclglurn: Above, bng-tei-m average; below, ••blight'' and ••no sprayer and are only too glad to supplement blight" years. 70 manuring and date of planting, on the incidence and severity of the disease. but in Flanders severe blight conditions appear to occur somewhat more frequently than 1 year SUMMARY: BELGIUM in 3. The small size and awkward layout of the holdings has not prevented growers from Most of the potatoes are grown on small following an annual spray program against farms, which produce excellent crops. Little blight. The system of farming is achieved precise data are available on the importance of only at the expense of a low standard of living blight or the losses that the disease causes, to the growers. THE NETHERLANDS During the period 1951-55, approximately and silty loams. The land is low lying; a 400,000 acres of potatoes were grown each large proportion of it is below sea level, and year in the Netherlands. The average yield of the soil water table is controlled. The ample 10 tons per acre is among the highest of any soil moisture and favorable temperatures country in the world (F.A.O., 1955). Only 30 induce rapid growth of the potato plants; the percent of the crop is used for home ware variety Bintje planted in mid-April is meeting consumption, which in Holland is about 240 lb. in the drills by mid-June. The principal ware per head per annum. Of the rest of the crop, and seed potato areas of the Netherlands are 25 percent is used for manufacture, principally on the sea clay soils. De Streek, the most into farina; 20 percent is fed to livestock; and important early district, is a sea clay area in the remainder is exported as ware potatoes or the northeast part of the province of North as seed, or used for seed at home. The export Holland. Seed-potato production comes under of seed toFrance, Belgium, and other European the control of the N.A.K. (Nederlandse Al- countries, as well as to South America and gemene Keuringsdienst) with headquarters at North Africa, is a most important branch of Wageningen. The spread of virus diseases in the potato industry (Netherlands Ministry of the seed crops is controlled by the compulsory Agriculture, 1953). destruction of the potato haulm on a certain date each year, the so-called **E'* date. This varies slightly from year to year and accord- POTATO DISTRIBUTION AND SOIL CONDI- ing to the grade of the seed, but destruction is TIONS about the middle of July. RIVER CLAY soils are much less extensive Potatoes are grown in all parts of the country, than the sea clay soils and are generally less with a particularly heavy concentration in the fertile. The Betuwe region in the south of *'peat colonies" of Groningen and Drenthe in Gelderland is the most important river clay the northeast (fig. 29). The acreages of potatoes area, and is a ware-producing district. in the several provinces in 1955, as given in SAND soils are the principal soils of the Nederlandse Rassenlijst (1956), were: south and east of the Netherlands. Rainfall is very similar to that of the sea clay areas, Thousand but late frosts during the last weeks of April Province: acres and first part of May are more likely to occur, so that planting is delayed. Growth, Groningen 48.4 too, is slower as the sands are not so well Friesland 22.0 supplied with soil moisture. The sand soil Drenthe 70.9 regions are areas of mixed farms, and the Overijssel 51.4 potatoes grown are chiefly for fodder, but Gelderland -- 30.6 are also for ware. The demand for ware Utrecht - 3.0 potatoes from the sand soils increases in North Holland 28.7 years of low potato production, so that dual- South Holland 29.4 purpose varieties suitable for both ware and Zeeland 26.2 fodder are widely grown. North Brabant 45.9 RECLAIMED PEAT ('•DALGRONDEN")soils Limburg 20.0 are in the northeastern part of the country. The top layer of the soil in the **peat colonies" Total (Netherlands) 376.5 is manmade. Instead of leaving a sterile subsoil after the peat had been dug in these areas, The soils of Holland can conveniently be the top layer of peat was used to cover the divided into four main classes, each with its subsoil and this in turn was covered with sand own well-defined type of potato production. excavated from canals and ditches and brought SEA CLAY soils occur in the north and west to the site by barge. These soils are devoted of the country. These soils are derived from very largely to the production of potatoes for the sea, and in texture they are fine sandy manufacture.

71 NETHERLANDS

^-■^^ ** Rottumerooq

GRONINGEN • Groningen \ à^' •• < ' • . .* ' '• / FRIESLAND \ / •. / • / D R E N T H E Ï >•••...••■■•• 1 • Emmeloord 1 -E.Polder . -/ NORTH '. • • ' \ HOLLANDS r^ /^^ u y'y^ OVE R YSS E L \ / V /- • De But •• .^ ^ C E L D E R L A N D / SOUTH ;' UTRECHT \ sa-N HOLLAND..* •Waqeninqen • Doetinchem y

.-^ ->^.s-''^

•. )

NORTH BRABANT • Gemert * • y^

^ ^. 1 Flushin \ y { \ \ '—>^^Terneuzen/ "^X, * • ^»- LIMBURG ^

FLANDERS ,'rv/ lOO miles r /

HGURE 29.-Key map of the Netherlands.

72 TABLE 40. --Potato varieties listed for resistance to blight in the Netherlands, 1955

Percentage of Blight resistance ■*• Variety Dutch potato Maturity group acreage Foliage Tuber

Percent Voran , 30 Late 7 6.5 Bint je , 16 Midseason. 3 3 Eigenheimer...., 12 . .do 5 3 Eersteling , 5.5 Early 3 3 Noordeling 5 Late 7. 8 IJsselster 5 Midseason, 6 8 Other varieties 26.5

■"■ Higher numbers indicate greater resistance to blight.

The distribution of the 1955 potato crop ac- WEATHER AND CLIMATIC CONDITIONS cording to soil region was-- Soil region: Thousand acres Although the types of soil and amounts of p. . standing water in canals and ditches are very pt^ rM lía different in the sea clay areas to the west and Kiver vîay ib.ö ^^^ ^^^^ ^^^^^ ^^ ^^^ ^^^^ ^^ ^^^^ Netherlands, öand ------oi*f there is no part of the country more than about Reclaimed Peat _84^ ^QQ ^^^^ ^^^^ ^^^^ ^^^^^^ ^^^ ^^ ^^^^ ^^^^ Total 376.5 to break the sweep of the prevailing south- POTATO VARTFTTFS westerly and northwesterly winds coming in IÛIAIU VAKliLlliLb ^^^^ ^^^ North Sea. Even the highest parts A description of Dutch potato varieties is of the sand areas are not more than 300 ft. given by Hogen Esch, Nijdam, and Siebeneick above sea level. The summer rainfall and (1955) and in the Nederlandse Rassenlijst temperature conditions over the whole country (1956). This variety list, a new edition of which are very uniform. This point is demonstrated is published each year, covers all agricultural clearly by the climagrams given in figure 30 crops and includes much statistical informa- for Groningen in the north; De Bilt on the tion. Potato varieties are divided into four border of the sand and clay areas in the middle; main groups--for ware, fodder, manufacture, Flushing in the extreme south of the sea clay and export. Many of the varieties fall into two lands in Zeeland; and Gemert on the sand lands or more of these groups. Thus Voran, which of eastern North Brabant, near the German occupies the greatest acreage of any single border. At all these stations the long-term variety, is grown for manufacture, fodder, average temperatures are about 63^ F. in and export as seed, although its chief use is jîy and August, and the corresponding mean for manufacture. Bintje, the next most im- rainfall for these months is between 2 and 3^ portant târiety, is grown principally for ex- inches. port as seed and ware. The most popular Although the winter temperatures in the variety for home ware consumption is Eigen- Netherlands are muchlower, the mean summer heimer, and the most important early potato temperature and rainfall figures are very is Eersteling (synonymous with Duke of York). similar to those for Penzance in the southwest The Rassenlijst includes a table in which all of England (compare fig. 6). With a macro- potato varieties are given a value on a scale climate like that of Penzance and a topog- from 0 to 10 for a number of economic raphy like that of the English Fens, blight factors, including resistance to diseases. In conditions in the low-lying parts of the Nether- table 40 the most important Dutch varieties lands would certainly be expected to be severe, are listed, together with their foliage and ^n important consideration affecting the tuber resistance to blight, as assessed by this yields of potatoes and the incidence of blight scale. The higher numbers represent the ^n the Netherlands is that in the low-lying greater degree of resistance. regions the crops are grown with a controlled Early varieties are normally planted toward water table, so that except in a few areas, the end of March, and lifting starts at the notably on some of the sand lands to the east, end of May. The growing period of midseason the effects of drought do not so greatly affect potatoes (Bintje) is from mid-April to the end the influence the annual level of potato yields of August. Late varieties (Voran) are planted as in England and Wales. Table 41 shows the at the end of April or beginning of May for mean monthly rainfall and tempe rature figures lifting at the end of September. at De Bilt for each of the years 1950-55. The

^3 TABLE 41.—Potato yields, with rainfall and temperatu res at De Bilt , Netherlands , 1950-56

Mean teniperature and Mean 75 Rainfall déviation percent Year Yield blight date June July Aug. Sept. June July Aug. Sept. (Bintje)

Tons/ In. In. In. In. 2F. OF. acre 1911-50 2.4 2.9 3.2 1.7 60 63 63 58 1950 2.6 4.2 7.6 4.8 +3 0 0 -2 9.6 Aug. 1 1951 1.8 2.0 4.7 2.2 0 -1 -1 +2 9.7 Aug. 8 1952 2.6 3.9 3.6 2.2 -1 0 0 -5 10.9 Aug. 18 1953 3.2 3.0 4.5 1.5 0 0 -1 -1 9.8 July 25 1954 3.6 5.7 4.4 3.5 -1 -5 -2 -1 9.4 Aug. 8 1955 2.0 1.4 2.4 2.6 -2 0 +2 0 10.7 Aug. 18 1956 2.7 5.7 4.9 1.9 -5 -1 -5 +1 9.5 Aug. 5

average potato yields for the Netherlands are At least 4 hours of dew during the night also given, together with the estimated mean was assumed if the mininnum night tempera- 75 percent date for unsprayed Bintje* ture was at least l^C (1.4^ F.) lower than the In the Netherlands, the years of highest dewpoint at 1900 hours on the preceding eve- average national yield (1952 and 1955) were ning or 0800 hours on the following morning. not the years in which the rainfall was highest. With the aid of these *'Van Everdingen In 1954 and 1950, when the rainfall was Rules" and readings from instruments placed nnarkedly high, the yield was below average. 16 inches above ground level in potato fields, Also, the high yields in 1952 and 1955 were the issuing of blight warning reports was in the years in which the blight attack was started in the Netherlands in 1927. The latest* Thus, it would appear that, with the validity of the Van Everdingen rules under relatively constant supplies of the water needed English conditions was studied by Wiltshire by the crops, blight has a far more direct (1931) and Beaumont (1947). As a resxilt of effect on the annual level of potato yields in the his studies year after year under the severe Netherlands than in England and Wales, and blight conditions of southwest England, Beau- is of greater importance in the national mont was able to propose a simplification of economy. Van Everdingen rules. Beaumont's rule re- Although mean rainfall and temperature con- quires two meteorological measurements only, ditions are very similar throughout the Nether- i.e. minimum temperature and relative humid- lands in the summer months, the humidity ity. The Beaumont rule (see section on England conditions, which chiefly affect the incidence and Wales) was then very extensively tested in of blight, are variable from area to area. the Netherlands, as described by Post and A study of the criticad weather periods shown Richel (1951), and it has been adopted since in figures 31 and 32 reveals that the variation 1948 with the following modifications: even between low-lying clay areas and the For a critical period there must be two obviously drier sand areas is not so great as consecutive days on which the mean rela- might be expected. tive humidity does not fall below 82 per cent on one day and 79 per cent on the POTATO BLIGHT FORECASTING other. The mean relative humidity is taken as the average of readings made in stand- The Netherland workers have been the pio- ard screens at 2.2m (about 7 ft.) above neers in potato blight forecastine. Continuing ground level, at 0800, 1400 and 19'00 hours. the investigations of Li^hnis (1924), Van Ever- The minimum temperature on one of the dingen (1926) found that outbreaks of blight two days must be at least 50^ F. occurred after a period in which the weather Observations from a network of weather satisfied the following conditions: stations over the whole country are employed 1. At least 4 hours of dew during the by the K.N.M.I. (Royal Netherlands Meteor- night. ological Institute) at De Bilt for determining the 2. Minimum temperature during the night: critical periods, and notifications of their 50^ F. occurrence in the several regions are issued 3. Mean cloudiness of the following day to growers over the wireless. The use of at least 0.8. synoptic maps sometimes enables the critical 4. Precipitation on the same following day at least 0.1 mm. periods to be forecast aday or two before their occurrence, and this earlier notice gives the

74 NETHERLANDS of the disease. This agreement is reached ¿ _ Groninqen despite the fact that the Netherlands criteria, with humidity measurements at only 3 times during the day, arc far less exacting than those RAINFALL 4 employed for the forecasting in England and Inches Wales, where the measurements are made every hour during both day and night and a break of only 2 hours at any time breaks the period. With outbreaks commonly occurring in the second half of June, critical periods in June, and indeed in late May, are certainly J FMAMJ J ASOND significant in the Netherlands, and, as in the southwest of England and Wales, there is no need for the postulation of a **zero date." Phenological observations on the state of forwardness of the crops are, however, taken RAINFALL 4 into account in issuing spray warnings. Inches

INITIATION OF BLIGHT EPIDEMICS IN THE SPRING

In his classical paper on the overwintering and epidemiology of phytophthora infe stans, Van der Zaag (1956) describes a very thorough search for the first appearance of infected plants in the spring. The district chosen for this work was De Streek, a region in the north RAINFALL 4 Inches of the province of North Holland where potatoes, chiefly of the early Variety Eersteling, occupy about 80 percent of the total acreage and where potato blight is usually found earlier than anywhere else in the Netherlands. In 1953, when the most extensive search was made, 27 foci were found to be established in the Eersteling crops in De Streek between May 26 and June 18. . North Braban!- Of these, 15 were scattered over an area of CEMERT about 16 sq. km. (4,000 acres) and could be recognized with reasonable certainty as having

RAINFALL arisen from diseased tubers planted. In one Inches case, in which careful measurements of spread were made, it was found that where only about 40 plants in the immediate vicinity of the focal plant were infected by June 4; by June 23 ten percent of the plants at a distance of 100 yards to the northeast and 1 percent of those at a distance of 300 yards, were infected. From this and many other observations it was con- FIGURE 30.--Long-term average monthly rainfall and mean tempera- cluded that a single infected plant arising from ^tures in relation to potato cropping for four centers in the Nether- an infected tuber planted could infect an area lands. of at least 1 sq. km. (240 acres) within a growers a little extra time between the warning month under weather conditions normal in and the outbreak in which to do their spraying. De Streek, and that the attacks each year In figures 31 and 32 the principal critical almost certainly originate in this way. Infec- periods given for the years 1951-55 are shown tion from volunteer plants on refuse heaps in relation to the blight progress curves for play little part in the story. Van der Zaag two representative potato varieties. In general, estimated that about 100 diseased tubers would a very good correspondence is shown between have to be planted to obtain a single focus, the first critical periods and the dates of and that these would be contained in about blight outbreaks and between subsequent pe- 1,600 kg. (li tons) of the Eersteling seed riods of blight weather and the rapid progress potatoe s.

75 NETHERLANDS

August September 1951

o • • • ■ • •■• • • ••• whole country lOO 1952

/ / 50 - / areas •

o ^ • • ••••••••••■•• whole country lOO 1953 <^'°V_^/.-^ clay_^y ./y' sand r / , / ^*— c /<•—sprayed 50 -

sand areas clay areas

lOO 1954 :nd / /clay- sand/ clay S Y /NtS/^-S ANeS 50 -

■ ■ sand areas ■ ••■ clay areas 100 unsprayed—^ J^^^ 1955 clay // /

50 - - sandsy-/ / / / '1 ^ /// / / / 0 ' -//.^^^--XclayNeS • • ••• •• sand areas • • ■ ■ •• ■ • clay areas lOO 1956

• • • sand areas • • • clay areas

■ Critical weather period for whole area. • Critical weather period for parts of area.

HGURE 31."Blifliit progress curves for Bintje variety in the Netherlands in 1951-56, with the critical weather periods for each year.

76 NETHERLANDS

June Auqusr September 1953

1954

1955

1956

• • •

^ Critical weather period for whole area. • Critical weather period for parts of area.

FIGURE 32.—Mean hUglit progress curves for Voran variety in the sand areas of the Netherlands, 1953-66, with the critical weather periods for each year.

In Van der Zaá.g*s view, the development of of the Netherlands would be about 1 per square an epidemic in any given district must depend kilometer in De Streek (as found by observa- upon (1) the number of primary foci, (2) the tion), 1 per 20 sq. km. in North Friesland percentage of the land area occupied by and Zeeland, 1 per 70 sq. km. in North Brabant, potatoes. (3) the susceptibility of the varieties and as few as 1 per 600 sq. km. in Utrecht-- grown, and (4) the macro- and micro-climate. with an average of 1 per 60 sq. km. (about The effect of the degree of tuber suscepti- 15,000 acres) in the Netherlands as a whole. bility of the variety on the number of primary An extremely interesting observation was foci could be expected to be very great. In an made on the island of Rottumeroog, 11 km. experiment in which 200 to 300 inoculated north of the Groningen coast, in 1954. Here tubers of several varieties were planted in an area of a quarter of an acre was planted soil at a mean temperature of 50® F. the per- with tubers that were entirely healthy, and centage of emerged plants that produced shoots there were no groundkeepers. On a close infected from the tuber was 10 in Eersteling, inspection of the island on July 25 no diseased 4 in Eigenheimer, and none in Voran. Taking plants were found. When blight had spread into account the factor of varietal suscepti- throughout the province of Groningen, blight bility and the percentage of land occupied by also appeared on the test crop on the island, potatoes. Van der Zaag surmised that the indicating that the sporangia were carried by probable number of primary foci operative in the wind over a distance of at least 11 km. starting the blight epidemics in various parts fnearly 7 miles) without losing their viability.

77 RIVER CLAY Certain districts NETHERLAND BLIGHT SURVEYS SINCE 1951 along the Maas, Since 1951 a countrywide survey of the inci- Waal, Ryn, and dence of blight has been conducted each year Ijssel. in the Netherlands by the Plant Protection South North Brabant and Service in collaboration with advisory officers Llmburg. and farmers. The accounts of this work are SAND Middle --Gcldcrland and given in the Yearbooks of the Service (Nether- Oveiijssel. lands Plantenziektenkundige Dienst, Wagenin- North Frlesland, Gron- gen, 1953-55; Lint and Meijers, 1956 and 1957 { ingen, and Drenthe, (in press)). Some 400 to 600 observation fields including the vil- have been included in the surveys each year, lages on reclaimed and a wealth of infornnation has been obtained peat land« on the extent to which spraying is practiced, the dates of outbreak and progress of blight BLIGHT ON THE HAULM on the haulm, and the percentages of blighted tubers at lifting. Some information has also In the review of thé very detailed results been obtained on the effect of the blight attack of the Netherlands surveys over the 6years, it on yield, but the figures derived in various is first necessary to consider how many ways from sampling small areas have been crops of each variety were included in the inconmplete and not always very reliable.How- survey for each region and what percentage ever a limited number of spraying trials have of these crops were sprayed. This informaticm, been conducted in several districts each year for 1953, is given in table 42. for which good yield figures are available. In the sea clay areas the number of un- Both the surveys and the trials have been sprayed crops has been too small for the carried out in relation to the warning serv- extraction of mean figures for the unsprayed, ice operated by the K.N.M.I. at De Bilt. so that for these important areas the survey Throughout the work a standard key has provides information about the incidence of been used for the assessment of blight on the blight on the sprayed crops only. In the sand haulm. Most of the definitions are the same areas, where less than half the crops were as in the B.M.S, key (see section on England sprayed, comparative figures are available and Wales), but categories intermediate be- for both sprayed and unsprayed potatoes.. On tween 0.1 to 1.0 percent have been added, average the sprayed Bintje and Eigenheimer and the stages are designated by figures on an crops received 4 to 5 spray applications each arbitrary 0 to 10 scale. Progress curves ob- year in the sea clay areas, and 2 to 3 applica- tained by plotting according to this s cade are tions in the sand land area. The Voran crop, reversed: The scale figures being in order, not on the sand lands, received two applications. to attack, but of freedom from attack (10 = no This matter of number of applications and attack, 0 = completely dead). Atypical curve is of materials used is considered later in the shown in figure 33, which also gives the B.M.S. section on spraying practice. equivalents of the scale. In figures 31 and 32 The information provided by the surveys progress curves for the Netherlands are plotted concerning the variety Bintje is here chosen according to the English method, using these for the fullest study because of the importance equivalents, to facilitate comparison of the of this variety, not only in the Netherlands curves with those for other countries. For the purposes of the survey, observations have been confined to the three principal maincrop varieties, Bintje, Eigenheimer, and IÜ o Voran, and the results analyzed for these Durch Ol B.M.S. 9 - ^ 03 varieties. Seed crops, which are burned off Scale Scale Ö - 05 prematurely, have been excluded. In 1951 and for Per cent 1952 mean results were given for the Nether- Bliqhr 7 - l-O on lands as a whole, but in 1953 and subse- 6 50 Haulm quent years the means have been broken 5 25 down according to reg ions--de fine das follows: 4 50

''South --Zeeland,western parts 3 75 of North Brabant, and 2 : 95 the South Holland Is- 1 lands. 99 1 1 1 1 \,.- lOO SEA CLAY ^ Middle-The rest of South Hol- 8 15 22 2<) 6 13 20 27 3 lO 17 24 3 1 land, North Holland, June July Auqusr and the North East Polder. North--Friesland and Gron- FIGURE 33.--Comparison of the Dutch scale for tie assessment of hlight on potato haulm with the B. M. S. scale for unsprayed Bintje ingen. variety on sand land, 1953.

78 TABLE 42.—Number of crops and percentage sprayed of Bintje, Eigenheimer, and Voran potatoes according to soil areas, Netherlands, 1953

Bintje Eigenheimer Voran Soil areas Crops Sprayed Crops Sprayed Crops Sprayed

Number Percent Number Percent Number Percent i^pfl clav- ••...••.•••••••• 76 94 44 93 0 0 River clay 13 46 9 7Ö 0 0 Sand lands••••••••••••••• 85 50 47 23 79 30

but throughout western Europe. The data for ESTIMATION OF LOSSES FROM PREMATURE blight on the Bintje haulm in each of the DEFOLIATION 5 years are sunuiiarized graphically by the progress curves given in figure 31. The The method of estimating average losses curves for 1951 and 1952 are for the country from an average growth curve, as employed as a whole, but the *'unsprayed" curves refer In England and Wales, has not yet been In- chiefly to the southern sand areas, where vestigated In the Netherlands. From figures most of the unsprayed Bintje crops are obtained In periodic liftings on Bintje In the situated, while the *'sprayed" curves are the North West Polder In 1952 and In the North means for the sprayed crops in both the sand East Polder In 1955 on crops free from and the clay areas. For 1953 and subsequent blight or at most very little affected. It Is, years, when the survey results were analyzed however, possible to draw up an approximate according to region, '*sprayed'* and *'un- mean growth curve for Bintje In the Nether- sprayed" curves are given for the southern lands (fig. 34). Assuming, as In England and sand regions, with "sprayed" curves for sea Wales, that further Increase In the crop of clay regions. The surveys did not provide tubers Is arrested when blight reaches the data for the few unsprayed crops in the sea 7 5 percent (or "3" stage) on the haulm, the clay regions, but to complete the picture mean percentages of the full potential crop as far as possible, approximate "unsprayed" obtained, as shown by this approximate curve curves for these regions are added from a and In the absence of additional damage from study of the blight assessments on the con- the passage of sprayer wheels, will be as trols in a number of spraying trials. shown In table 43. The curves show that there was a good deal So far these figures are very tentative, of variation in the course of the blight attack but they may now be tested by reference to according to district in the 5 years (and the actual 75 percent dates and recorded yields especially in 1953), but the curves also give In spray trials. This Is done In table 44 for some indication (sufficient for this study) of trials carried out In each of the 3 years. the relative severity of blight on Bintje In the

several years for the Netherlands as a whole. PERCENTAGE In all 5 years widespread outbreaks (0.1 per- OF ^ cent stage) started on the unsprayed in the TOTAL / CROP second half of June or in early July. The 7 5 / / percent stage (*'3" on the Netherlands scale)

was reached by the third week of July over a ■ ^'/ wide area In 1953 (the worst blight year); < by about the end of the first week In August f / In 1951, 1954, and 1956; by mid-August In 1952; and In the second half of August In 1955. / There are not sufficient data to establish a frequency of "blight years" over a long-term period, but It would perhaps be reasonable to say that the curves for 1951 strike a fair 50 ,/ 1 Á ' 1 1 1 18 25 7 14 21 28 S 12 19 26 average for Bintje In the Netherlands over Sepfember the 5 years. That Is to say, the mean 75 per- July Auqust cent date for the unsprayed crops was about FIGURE 34.—Approximate mean growth curves for Bintje and Voran August 7 and for the sprayed crops about varieties in the Netherlands, showing percentage of the total crop August 24. of tubers produced by various dates.

79 1953, 1954 and 1955, at three centers, re- TABLE 43.—Es tima ted percentage of the spectively, at Emmeloord on sandy clay in the potential crop obtained when the 75 percent North East Polder, at Terneuzen on tl^e sea stage of blight on the haulm is reached by clay in Zeeland, and at Doetinchenn on the given dates, on Bint je, Netherlands sand in the east of Gelderland, The Plant Protection Service at Wageningen supplied the data on these trials. Where the original Date when blight on haulm Estimated percentages of potential (total) yields were given in kilograms per acre reaches 75 percent stage crop they have been converted to equivalent tons per acre; where they were given in kilograms per 100 plants they have been left in that form. July 18- 55 As the fiül potential yield (with complete con- 25, 64 trol of blight) was not known, it has been Aug. 1. 72 assumed that the percentage of the potential 7. 80 crop obtained in the sprayed plots was in U. 87 accordance with table 43, and the actual and 21. 93 estimated percentages in the unsprayed crops 28. 97 have then been compared. Sept. 5. 100 Throughout these 9 trials, imder widely differing conditions, there was a good practical

TABLE 44. —Checks of estimated against actual yields in 9 spraying trials on Bintje, Netherlands, 1953-55

1953 1954 1955 Place and data unsprayed Sprayed Unsprayed Sprayed Unsprayed Sprayed

Emmeloord

Date of 15 percent blight stage on haiilm July 15 Aug. 20 Aug. 3 Aug. 10 Aug. 16 Aug. 29 Estimated yield percent. • 50 92 75 83 87 97 Actual yield...tons per acre,. 69.2 ^ 1P1.8 67.2 ^ 79.6 17.8 20.5 Actual yield percent.. 53 92 74 83 ^5 97

Terneuzen

Date of 15 percent blight stage on haiilm Aug. 9 Aug. 28 Aug. 6 Aug. 24 Aug. 30 Sept. 9 Estimated yield percent.. Ö2 97 78 94 97 100 Actual yield...tons per acre.. 87.7 ^ 99.0 17.0 19.4 12.7 13.1 Actual yield percent.. 86 97 82 94 96 100

Doetinchem

Date of 15 percent blight stage on haulm July 26 Aug. 8 Aug. 8 Aug. 17 Aug. 4 Aug. 27 Estimated yield percent.. 65 81 81 89 74 96 Actual yield...tons per acre.. 45.0 ^ 62.3 96 ^ 101 10.5 14.7 Actual yield percent.. 59 80 Z5 89 69 96

Kilograms per 100 plants.

80 agreement between the percentage yield of the crop by about the end of September, the growth unsprayed crops as estimated from the 75 curve for the tubers is probably similar to percent dates and as determined by actual that for Majestic in England and Wales, and lifting. Further investigation of the mean rate indeed the results of one periodic lifting trial of bulking of Bintje in the Netherlands is available for Voran in North East Polder in obviously needed. It would appear that the 1955 agreed almost point by point with the tentative curve of table 43 and figure 34 English Majestic curve. fits the available evidence fairly well, and Figure 32 shows that blight on the un- loss estimates based upon it are probably sprayed Voran crops in the sand areas reached the best that can be given at the present the 75 percent stage about the end of August stage. in 1953 and 1954 and in the second or third The curves for 1951 (fig. 31), suggested week of September, according to district, in as representing a general average for Bintje the dry year 1955; blight reached only the 25 over the 6 years, indicate an average loss percent stage by the end of September in 1956. of 20 percent of the potential total crop in The defoliation losses as determined by sam- the unsprayed (75 percent date, Aug. 7) and ple lifting of sprayed and unsprayed portions 5 percent in the sprayed (7 5 percent date, of a number of crops in the surveys were in Aug. 24). In comnnercial crops, however, good accordance with those to be expected where the spraying is done by tractor-drawn from the growth curve for Voran (shown in machines (and not by hand as in the trials) figure 34). In 1954 with the 75 percent stage there is wheel damage also to be taken into on the unsprayed occurring at the end of account. The magnitude of this danmage has August and on the sprayed crop at mid- been investigated by trials in the Netherlands September, the gain from spraying was a (Plantenziektenkundige Dienst, Wageningen, little over 1 ton per acre; while in 1953, 1955, pp. 48-49) and estimated at 3 percent when the spraying appears to have been less of the total crop. This figure is in close agree- effective and the 75 percent date occurred ment with that obtained in England and Wales about September 7, the gain from spraying with similar machines. Accepting this figure, was only a little over 7 ton per acre. In 1956, the loss in the sprayed crops with 75 percent in five spraying trials, the mean 75 percent date of August 24 would be not 5 but 5 + 3, or date for the unsprayed was the end of August, 8 percent. and for the sprayed, mid-September. The Thus, in ware crops of Bintje, with a poten- mean total yields were 13.6 tons per acre tial yield (were there no blight) of 12^ tons for the unsprayed and 15.2 for the sprayed, per acre, the average yield of the unsprayed giving an increase of 1.6 tons for the sprayed would be 10 tons per acre and of the sprayed plots--in exact accordance with the curve. ^ ll| tons per acre--a difference of I7 tons per acre or 3.75 tqnnes per hectare. In 1954, BLIGHT IN THE TUBERS which according to the progress curves was about an average year and similar to 1951, The surveys provided good data on the mean the mean gain from spraying as determined percentages of blighted tubers at lifting in the on 31 crops of Bintje in various parts of the 3 years 1953-55, as shown in table 45. Each Netherlands in the blight survey was 3.9 figure in this table is a nciean for a good num- tonnes per hectare--again in very close agree- ber of crops, in all cases at least 10, and ment with the estimate figure. It must be sometimes 30 or more. stressed that all these estimates so far are The figures indicate a high level of tuber of loss of total yield due to defoliation; they infection in Bintje and Eigenheimer, par- do not include losses due to tuber infection, ticularly in the sea clay areas, in the wet which will be considered later. seasons of 1954 and 1956, and fairly high The survey data and trial figures available infection in the sand areas in 1953, while in do not permit a similar computation of aver- the dry season of 1955 the level of infection age losses in the variety Eigcnheimer, but was low. Tuber infection in Voran in all 3 it would appear that in general the 75 percent seasons was relatively low, though the infec- stage on both sprayed and imsprayed crops tion is still considerably higher than for of this variety can be expected to be reached Majestic in England and Wales. Protective about a week later than in Bintje. The date spraying gave some reduction of tuber infec- of maturity is also somewhat later. This may tion in all 3 varieties. counterbalance the slower progress of blight, so that in practice there may be no great CONTROL: MATERIALS AND METHODS difference in the defoliation losses with the The amount of protective spraying against two varieties. For Voran, the variety predominantly grown blight varies according to area and the potato variety grown. In general, the majority of In the sand areas, the surveys provided good growers in the sea clay regions spray, whereas data for both unsprayed and sprayed crops In each of the years 1953, 1954, 1955, and in the sand regions growers spray occasionally. 1956. For this late variety, producing a full This is in part a reflection of the differing 81 TABLE 45. —Blighted tubers at lifting, as percentage of total crop, for Bint je. Eigenheimer, and Voran in 2 soil areas, Netherlands, 1953-56

Blighted tubers

Year and soil area Bintje Eigenheimer Voran

Unsprayed Sprayed Unsprayed Sprayed Unsprayed Sprayed

1953; Percent Percent Percent Percent Percent Percent Sea clay. 4.8 2.0 Sand..... 8.1 7.4 6.2 5.0 2.5 1.4

1954:

Sea clay. 11.4 9.4 Saad 9.4 ^.5 4.3 3.4 2.7 1.0

1955:

Sea clay. .5 1.4 Sand 3.9 2.4 1.8 2.3 .8

1956:

Sea clay. 8.4 17.6 Sand 7.2 7.9 3.9 3.4 2.8 2.0

susceptibility to blight of the predominant steadily decreased in importance, although varieties in the two areas: Bintje on the sea it is still the connmonest fungicide. There clay and Voran on the sand. Figures fron:i has been a nmarked increase in the use of the the Netherlands surveys showing the propor- organic fungicide zineb, which is applied tion of potato crops sprayed illustrate this principally in the form of a split treatment variation and are given in table 46. followed by copper. The most recent trend is Over 95 percent of the Bintje crops on sea in the use of stickers to increase the effi- clay have been sprayed during 1953-56. There ciency of application. The results of numerous has been a considerable difference from year spraying trials in which the use of zineb and to year in the amount of spraying done on copper compounds are compared have been Voran in the sand regions, but so far as can reported in recent years (Ormel, 1954; Lint be seen this variation does not bear any clear and Meijers, 1955). In the earlier trials, relationship to the severity of the blight zineb was disappointing, but its performance attack in the particular years. It would appear was greatly improved by increasing the appli- that under one-third of the Voran crops on cation rate from 2.7 to 4.5 lb. per acre (3 to sand arc sprayed against blight. The number 5 kg. per ha.). The general experience has of spray applications given also varies. In been that with an equal number of spray the sea clay regions, Bintje for ware nor- treatments, zineb has given a greater increase mally receives 4 or 5 applications, although in total yield than copper, but that on soils seed crops of the same variety, which are where tuber blight infection is a problem (the killed prematurely in mid-July, are given only 2 or 3 sprayings. Voran on sand is TABLE 46. —Percentage of Bintje and Voran usually given 2 applications. crops sprayed, Netherlands, 1953-56 The surveys provide very full information concerning the materials used in the Nether- Variety and lands for protective spraying over the period soil area 1953 1954 1955 1956 1951-56. Table 47 sets out these materials and shows their relative importance in the several years. Per- Per- Per- Per- Table 47 brings out a number of important Bintje on sea cent cent cent cent points. The use of bordeaux and burgundy clay 94 98 mixture has been virtually abandoned in recent 95 98 Voran on sand.... 30 45 years. Copper oxychloride applied alone has 22 17

82 TABLE 47.—Spraying materials and percentage of total sprayed area on which they were used, Netherlands, 1951-56

Material used 1951 1952 1953 1954 1955 1956

Percent Percent Percent Percent Percent Percent Bordeaux or burgundy mixture., 8 1 1 Copper oxychloride 81 79 69 52 42 Cuprous oxide 11 A. 4 2 <1 Colloidal copper preparations 2 12 6 5 3 Zineb 2 2 7 9 2 Zineb followed by copper..r.. 2 14 26 31 Zineb followed by copper plus sticker 8 Copper plus sticker 12 Other materials 1 sea clay areas) a greater number of diseased by Maan (1954). He particularly mentions the potatoes has offset this effect. In other words» research work in spraying against potato zineb has proved considerably less effective blight of Fransen and Kerssen of the Instituut than copper In reducing tuber infection. This voor Plantenziektenkundlg Onderzoek, Wagen- can be illustrated by the results of 5 trials on Ingen. Ground spraying is carried out from sea clay and 5 trials on sand in 1954, a year the top only without the use of droplances. when tuber infection was very bad in the sea In recent years there has been a considerable clay regions. Average results for these trials development in spraying at low- and medium- are given in table 48. The variety under trial volume rates. Figures, based on a sample was Bint je. of 293 sprayed fields, show that in 1956, 22 Table 48 shows that on both soil types the percent of the growers were spraying at less total yield of potatoes was highest with the than 18 gallons per acre; 41 percent were zineb treatments. On the sand soils, net yield spraying at 18 to 53 gallons; and the remain- (total yield less diseased tubers) was 1.5 tons ing 37 percent at over 53 gallons. To some per acre greater with zineb than with copper, extent spraying against blight is combined but on the sea clay, due to the very high per- with DDT treatments for the control of the centage of blighted tubers in the zineb-treated Colorado beetle. plots, this result was almost exactly reversed. In the light of their experience the ECONOMICS OF SPRAYING Dutch reconunend that copper alone, or a split treatment (2 sprayings with zineb, fol- Leeuwenburgh (1955) has considered whether lowed by copper) should be used for protec- blight control is justified on economic grounds. tive spraying against blight in the sea clay He states that the cost of one spraying is areas. Zineb, alone, is considered satis- approximately 30 guilders per hectare (£1 4s. factory in the sand regions. per acre), of which, 20 to 22 guilders is the A high proportion of the spraying is carried cost of the material and 8 to 10 guilders the out by contractors, and most of it is done by cost of application. The question of justifica- ground machines. Fixed-wing aircraft were tion depends very much on the price obtained used to spray 13,000 ha. of potatoes against for the potatoes, and this varies considerably blight in 1955, but expansion of air spraying among the different sorts. For example, is- limited, owing to the small size of the Leeuwenburgh quotes prices of 0.15 guilder majority of the potato fields. Recent develop- per kilo for seed potatoes, 0.1 guilder per ments in aircraft spraying have been reviewed kilo for sea clay ware potatoes, and 0.06

TABLE 48. —Yield of tubers (over 1-3/8 in. diameter) from spraying trials with copper and zineb, Netherlands, 1954

Sound tubers Total -. Area Unsprayed Copper Zineb Unsprayed Copper Zineb

Tons/acre Tons/acre Tons/acre Tons/acre Tons/acre Tons/acre 11.6 14.7 15.1 16.2 10.4 12.9 Spn P1 nv ....•••.••••••••• 11.2 12.7 Sand 12.1 11.3 13.2 11.8

83 guilder per kilo for ware and manufacturing plasticity of the fungus. Since 1951, H. J. potatoes from the sand regions. (These prices Toxopeus at the Instituât voor Veredeling van are equivalent to £15, £10, and £6 per ton, Landbouwgewassen at Wageningen has been respectively.) Thus, the actual Increase In concerned with the breeding of blight-re si s tant potato yield required to meet the cost of one potatoes and the genetical analysis of im- spraying Is approximately 200 kg. per ha. for munity. In a recent paper (Toxopeus, 1956) seed potatoes; 300 kg.per ha. for ware potatoes he has discussed the present possibilities In from sea clay areas; and 500 kg. per ha. for breeding for blight resistance in potatoes. He potatoes from sand areas. In terms of tons notes the apparent breakdown of field re- per acre, these yield Increases become 0.08, sistance to blight of the Irish variety Champion 0.12, and 0.20 ton, respectively. and suggests that the same thing is happening These figures, taken In conjunction with the to Voran In the Netherlands. He associates. greater resistance to blight of the sand land this breakdown with the ability of the fungus potato varieties, and also the smaller impor- to adapt Itself even to field-re si s tant varie- tance of tuber Infection In the sand regions, ties, particularly when these predominate go a long way toward explaining the markedly over other varieties In the makeup of the snnaller amount of spraying that Is done In country's potato acreage. Toxopeus considers these areas as opposed to that done In the that the Incorporation of R-gene resistance sea clay areas. Into new varieties, where this is combined with a high degree of field resistance, may be very useful. This Is because the effect of FURTHER CONTROL MEASURES one group of genes Is likely to prevent the establishment of races of the fungus pathogenic A considerable amount of research work has to the other genes. Up till now, no R-gene been done in the Netherlands on the mechanical blight-resistant varieties of potato have been and chemical destruction of potato haulm. grown on a commercial scale in the Nether- Most of this has been In connection with haulm lands. destruction In seed crops (Lint and Crucq, Van der Zaag (1956) (in connection with his 1956), where roto-beatlng, followed by one or work in De Streck) has discussed the pos- two sprayings with a chemical such as sodium sibility of a number of phytosanitary measure s arsenlte, has been found necessary to effect for blight control. Including the destruction the complete killing of the vigorously growing of primary foci. The main difficulty here lies haulm. In ware crops attacked by blight, In the detection of the foci at a sufficiently destructive spraying of the potato tops Is early stage to make destruction worthwhile. also recommended under certain circum- Some consideration has also been given in the stances. One spraying is sufficient. Chemicals Netherlands to the effect of storage conditions used Include sodium arsenlte, the di-nitro on the development of blight in tubers. Work compounds, and sodium chlorate. Growers, of this kind Is the responsibility of the Stichting however, have been advised not to use sodium voor Aardappelbewarlng (Institute for Potato arsenlte on sand soils, owing to the risk to Storage) at Wageningen. succeeding crops of toxic residues remaining In the soil. Lint and Meljers (1955), reporting SUMMARY: NETHERLANDS on trials In 1954, concluded that on sea clay soils It was an econonnlc proposition to burn Countrywide blight surveys have been car- off the haulm of a blight-Infected susceptible ried out In the Netherlands each year since variety (Bintje) even when this was carried 1951, using a modlñcatlon of the B.M.S. key out at an early date, due to decreased tuber for the assessment of the disease on the haulm. blight. On sand soils, where tuber infection Severe epidemics occur on Bintje and other Is normally slight even in susceptible varie- mldseason varieties In most years, partic- ties. Lint and Meljers stated that haulm de- ularly In the sea clay areas, where, if the ware struction In ware crops was unwarranted. crops were left unsprayed, there would be an Further trials in 1955 confirmed these con- average defoliation loss of about 20 percent of clusions. the potential crop, with a further loss of 4 to Fundamental work on the genetics of blight 10 percent for Infection of the tubers. Losses resistance has been carried out by Mastenbroek in Voran, the principal variety grown on the (1953), and this worker has cooperated in the sand lands, are similar to those in Majestic setting up of an International nomenclature for In the southern zone of England and Wales. The blight races (Black, Mastenbroek, Mills, and valuable seed and ware crops of Bintje and Peterson, 1953). Mastenbroek and Bruin (1955) other susceptible varieties almost all receive have demonstrated that the present common Intensive protective spraying. This spraying field race in the Netherlands Is race 4. This usually reduces defoliation losses to about 8 research on blight races follows earlier work percent, and also gives some reduction of tuber by Bruyn (1951), which showed the alleged Infection. Much less spraying Is done on Voran.

84 A marked increase in the use of organic made many contributions of fundamental im- fungicides for blight control has occurred in the portance to breeding for blight resistance and Netherlands in recent years. The Dutch are to the understanding of the origins of blight pioneers in potato blight forecasting and have epidemics.

DENMARK Denmark grows annually about 250,000 acres Figure 35 shows the distribution of potato of potatoes, which produce an average yield of growing in Denmark. The main concentration about 8 tons per acre (Danish Agricultural is in Jutland, particularly the west, and there Organizations, 1954). According to Russell is a smaller concentration in north Zealand. (1954), human consumption of potatoes was 282 Ware potatoes are grown throughout the coun- lb. per head in 1950. In addition, potatoes are try, but the fodder, industrial, and seed potatoes grown for fodder and industrial use. There is are produced principally in Jutland. There is an important export market for Danish seed a center for early potatoes on the island of potatoes, and to a lesser extent, ware potatoes. Amager, immediately southeast of Copenhagen. Thus the structure of the industry bears a The most important variety is Bintje. This very close relationship to that in the Nether- is a midseason potato, grown for the home lands. ware market and also for export as ware DENMARK

Each dot= 680acres

FIGURE 35.—Distribution of potato growing in Denmark, 1951.

85 and seed. Other varieties grown on a fairly- Incidence of disease Description large acreage, all of which can be classified as late-maturing, are Tylstrup Odin (indus- 1 Negligible trial). Alpha (fodder and export), Dianella 2 - Slight (industrial), and Up-to-Date (ware). The crop 3 Moderate is grown principally on the lighter soils. The 4 Severe main planting takes place in early May and 5 Devastating lifting is carried out in September and October. Summers are cool, and rainfall is generally In table 49 blight attacks in Denmark are adequate, with a peak in August. This is assessed according to this scale for the period illustrated in figure 36. 1945-55. The average yield of potatoes in the country for each of the years 1945-54 (F.A.O. BLIGHT EPIDEMICS: DISTRIBUTION AND statistics) are Included in the table. FREQUENCY The average yield in the 6 •'blight years" Ernst Gram, who has contributed the infor- was 7.0 tons per acre, and in the 5 other years nnation on which this section is based, finds about 7.7 tons per acre. The slight evidence that blight occurs throughout Denmark, from this analysis tends to indicate that in wherever potatoes are grown, and there is no Denmark, unlike England and Wales, the in- evidence to show that the disease is worse in crease in potato yield resulting from increased one part of the country than in another. Attacks rainfall in the blight years is not balancing are most severe where susceptible varieties, or exceeding the losses caused by blight. in particular Bint je, are grown. The late varie- The number of **blight years" (scale 4-5) ties for feed and industry, with the exception of in the 11 years 1945-55 was 6, indicating a Dianella, show a high degree of blight re- frequency of about 1 blight year in 2. This sistance. agrees very well with an analysis made by Annual reports on plant diseases, including Gram (1935) of the years 1911-34. In 12 years potato blight*, based on the observations of out of this 24-year period, blight incidence plant pathologists and also of agricultural was described as negligible to moderate (scale and horticultural advisers, have been published 1-3), and in the remaining 12 years, the dis- since 1884. The present title of these reports, ease was described as severe or devastating which include a sunnmary in English, is (scale 4-5). Plante sygdomme I Danmark. An attempt has Blight is reported first on early crops, been made to establish a fixed terminology in usually in June. In 1949 (a *'severe ** year), describing the incidence of the various dis- the first observation of the disease in the eases, so that observations from all over the field was made on June 9, although this was country can be easily correlated and an over- said to be unusually early. In 1947 (a "slight" all picture obtained. The following simple year), first attacks were not reported until scale is used by all reporters: the middle of July. Little exact information on the progress of blight, after the initial ^West Jutland outbreaks, is given in the annual reports of

RAINFALL TABLE 49. —Yield of potatoes and blight inches incidence, Denmark, 1945-55

Incidence Average Year of blight yield

Blight years: J FMAMJ JASONO Rating Tons/acre 1945 4-4.5 BLICHT YEAR 1951 NO BLICHT YEAR 947 6.0 r 1949 4-4.5 6.6 1950 4-4.5 7.0 60V 5 60 F RAINFALL 1951.* 4-4.5 7.4 inches /^^"^^ ^ SO" F 1953 4 7.0 1954 3 4 8.0

2 Other years:

i 1946 ^// 3.5 7.0 M J J i 1 Ï 1947 2 6.9 1948 •. 2.5 8.5 FIGURE 36.—Mean monthly rainfall and temperature for West Jutland, 1952 3.5 8.5 Denmark: Above, loiig-term average; below, "blight*' and **no 1955 1 blight*' years.

86 Plantesygdomme I Danmark. It would appear, TABLE 50. --Increase in yield of potatoes by however, that in a severe blight year, the tops spraying as related to blight incidence, of midseason and early maincrop potato varie- Denmark, 1911-34 ties are completely killed between the middle to the end of August; and those of late maincrop varieties between early to mid-September. Blight incidence Increase in Years yield by In 1945 {*'severe"), it was stated that the tops (rating) of midseason and early maincrop varieties spraying were practically dead from blight as early as the end of July, but this seems to have been Number Percent an exceptional season. I..Negligible 2 1.5 2..Slight 6 10 BLIGHT LOSSES 3•.Moderate 4- 12 4••Severe••..•• 4 16 A very large number of spraying trials 5..Devastating 8 21.5 against blight have been carried out in all the potato-growing areas of Denmark during Total years 24- — the present century. Figure 37, which has been adapted from data supplied by Dr. Gram, shows the average increase in yield (cwt. per acre) Stapel (1953) has used the results of spray- in the sprayed plots as compared-with the ing trials from. 1910 to estimate the average untreated for each of the years 1934-53. These loss in Denmark caused by blight through results are based on figures from 287 separate premature defoliation of the potato plants. He spraying trials carried out with bordeaux concludes that the average loss, in the absence mixture. of spraying, is 15 percent of the crop (Ij tons If figure 37 is compared with table 49 it on a 10-ton crop). This is just about double will be seen that there is a very close cor- the average yield increase achieved in the respondence between the increased yield from spraying trials illustrated in figure 37. How- spraying in any particular year and the in- ever, it seems a fair assumption that only 50 tensity of the blight attacks in that year. Thus, percent of the total loss due to blight was re- in 1950 and 1953, attacks were severe and covered in these trials, particularly as in the yield increases high; in 1947 and 1948, attacks miajority of them only one spraying was given. were slight and yield increases from spraying There has been a marked tendency for yield small. Despite the factthatthis correspondence increase in the spraying trials to be less may not hold exactly for every year, e.g., during recent years, or to put it another way, yield increases were greater in 1946 than in for blight losses to be less severe. This point 1945, although 1945 was certainly the worse can be seen from figure 37, but it is further blight year. Gram (1935) has demonstrated illustrated by data supplied by the Plant that over a long-term period the spraying Production Office at Skanderborg in Jutland, trials give a very good measure of blight and quoted by Stapel. These data show the incidence. Dealing with the period 1911-34, average increase in yield for the four 10-year during which time 1,350 spraying trials were periods from 1910-50, and are given in table carried out, he expressed blight incidence 51. (as measured by the cooperative plant disease It is possible that the chemical situation in recording scheme) against yield increase re- the war and postwar years influenced the posi- sulting from the spraying trials. This analysis tion in 1940-50, but a change in the potato for the 24 years is shown in table 50. varieties grown is also, no doubt, having an effect. Acreages of Bint je have increased during recent years; and although this variety DENMARK TABLE 51.--Average yield increase for potatoes by spraying, in 10-year periods, CWT Denmark, 1910-50 PER ACRE

1910-20 1920-30

1934 5 6 7 B 9 I940 I 2 3 4 5 6 7 B 9 I950 I Z 3 1930-40 1940-50 FIGURE 37.—Average annual increase in yield from spraying with bordeaux mixture, Denmark, 1934-53. 87 Is very susceptible to blight, it is very early The relative frequency of severe blight maturing, and may often escape the full years in Denmark (1 year in 2) would appear severity of the attacks« to be connected with the generally high August Returning to Stapel's estimate that, on rainfall. In west Jutland, the long-term aver- average, 15 percent of the potato crop will age rainfall for August is 3.5 inches. Weather be lost in the absence of spraying, he develops conditions in a **no blight" year (1947) and a his argument further to estimate an actual **blighf year (1951) are compared in figure loss of 7 or 8 percent, basing this on the 36. Summer temperatures in 1947 were ex- assumption that half the potato acreage in ceptionally high, and this, with the very low Denmark is sprayed. Clearly, this assumes August rainfall, will have prevented the de- that the spraying that is carried out is 100- velopment of blight. In 1951, the very high percent efficient, which it cannot be, so that August rainfall, 5.8 inches, will have been this final figure is probably low. In addition, one of the main causes of the severe attacks. it appears that 35 percent and not 50 percent of the acreage is sprayed. Assuming, however, CONTROL MEASURES that spraying is concentrated on the potatoes that need it most, the probable annual average Spraying is much more commonly practiced loss caused through premature defoliation in than dusting. The amount of spraying varies Denmark is around 10 percent. in different parts of the country. Experts judge that about 35 percent of the total potato area is sprayed. Copper compoiinds are prin- BLIGHT IN TUBERS cipally used, of which copper oxychloride is the most popular. Experiments at Lyngby in In view of this high loss figure, it is in- 1941 and 1942, comparing the efficiency of a teresting to learn from Dr. Gram that ''our number of spray materials, including different main problem [in Denmark] is the tuber attack, formulations of bordeaux mixture, have been owing to its influence on quality, on keeping described by Stapel and Peter sen (1943). in pits or houses, atnd on export values." This Under commercial conditions, it is normal does not imply that the losses caused by tuber to apply one or, at the most, two protective blight are greater than the losses brought sprays, although Hammarlund( 1956)has shown about by premature defoliation. It would be that in certain circumstances up to four can be most surprising if tuber blight losses were applied economically. The need for correct anything like so high as an average of 10 tinning of the spray application to obtain percent, though no figures exist on which to maximum protection of the potato haulm has base an estimate. The statement does imply long been realized in Denmark. An experiment that tuber blight is a very real source of loss to show how important it is to apply a protec- in Denmark, and that it is all the more im- tive spray as soon as the disease appears, and portant because of the export market in not to delay this application, was made at potatoes. The blight losses through premature Lyngby in 1918, and is described by Gram defoliation are a more intangible quantity, (1924). This work appears to have been an and there is always a tendency for them to be anticipation of the re cent use of blight progress balanced by a higher potential yield in the curves. Because of its historic value. Gram's blight years. original diagram illustrating this experiment is reproduced in figure 38. BLIGHT AND WEATHER DATA The diagram shows the development of blight on the sprayed plots and on the unsprayed plots (Ikke spr^jtet) according to a The climate of Denmark is under both scale (0-10), where 10 equals complete haulm maritinne and continental influences. Through- destruction. Blight first appeared in the plots out the country, the main rainfall comes in on July 19, and the first set of plots was summer and autunui. Figure 36 shows the sprayed the next day, July 20. The heavy line long-term average rainfall and temperature shows the blight developrntent on the haulm in conditions for west Jutland, together with the plots sprayed on that date. Other sets of plots main planting and lifting times for potatoes. received a single spraying at intervals after In Jutland, the average annual rainfall is this. The dates of spraying are all shown by between 25 to 30 inches, and mean tempera- arrows, and a blight progress curve is given tures in July and August are around 61^ F. In for each set of plots. The columns at the top the Danish Islands, annual rainfall is somewhat of the diagram give the date of spraying, and lower, between 20 to 25 inches, and summer the yield per treatment in 100 kilograms per temperatures are a little higher than in hectare. The plots which were sprayed on Jutland. The whole country is low lying and July 20 (the optimum time) gave a yield of 20 exposed to strong winds blowing in from the tonnes per ha. (7.9 tons per acre). The un- west. sprayed plots gave 16.2 tonnes per ha. (6.4

88 tons per acre). Thus, with optimum timing, a single spraying gave 14 days' prolongation of growth in early August, and increased the yields from 6.4 to 7.9 tons per acre. (Compare with figure 8 for England and Wales.) Haulm destruction Is not much used In Den- mark. About 10 percent of the potato area Is treated, mostly with sodium chlorate, which Is not too satisfactory. Most of the killing Is done on the late varieties, which have a very luxuriant haulm, but this Is mainly to facilitate lifting rather than to protect the crop from blight. Roto-beaters are also In use.

SUMMARY: DENMARK

In Denmark, the results of numerous spraying trials carried out In all parts of the country have been used In assessing blight losses. These numerous spraying trials give a very good Idea of the technical Interest taken by Danish farmers In blight control over a long period of years. The average annual loss from blight through prennature defoliation has been assessed at 15 percent. In the absence of spraying. Even so, only about one- thlrd of the Danish potato growers take protective action against the disease. To a considerable extent, the growers appear to Ignore FIGURE 38.--Effect of date of spraying on yield, Lyngby, Denmark, this large Invisible loss. The visible losses 1918: Progress curves for blight on foliage are given on a scale caused by tuber blight seem of more Importance of 0 to 10; yields in 100 kg. per hectare for dates of spraying to thenn. Denmark has an Inmportant export (shown by arrows) are given in the columns above (Gram, 1924). trade In seed potatoes.

ICELAND ICELAND The occurrence of potato blight In Iceland Is of special Interest, as the temperature conditions are here at the lower limit for any ARCTIC_ÇIKÇLE^ epidemic development of the disease. The northern coast of Iceland touches the Arctic circle (fig. 39), and by far the greater part of the land area (some 40,500 sq. miles) Is an uninhabited high tableland with vast snow- flelds and glaciers. Potatoes are grown on the lowlands to the southwest of the Island about Reykjavik, and also on the coastal Snoefell lowlands and In the glens to the south and east. The potato has been one of the main vegetable crops since It was Introduced In 1759- About 2,000 acres are planted each year, and the yields, varying considerably with the severity of the season, " are In general fairly high, giving a total annual production of 12,000 tons for a population of 154,000. All the potatoes Myrdalssondur are used for human consumption, and are all the more Important In the food supplies because the climate I s unsuited for grain crops. In addition to the Iceland production some potatoes are Imported from Denmark, Hol- FIGURE 39.—Key map of Iceland. Shading indicates bwland agri- land, and Ireland. cultural areas.

89 An account of potato blight in Iceland has Icelandic varieties (the Red Potato, the Blue, been published by Davids son (1951), Sturla and the Akranes), the Gullauga (an old Swedish Fridriksson has carried out potato variety variety). Early Rose, Great Scot, Up-to-Date, trials at the University Research Institute Eigenheimer, Alpha, and Ackersegen. Ability at Reykjavik over a number of years (Frid- of the haulm to withstand some frost injury riksson, 1954) and has provided much supple- toward the end of the growing period is of mentary infomnation. great importance under Icelandic conditions. In trials of varieties from nnany parts of the CLIMATIC AND CULTURAL CONDITIONS world Fridriksson (1954) found only one indigenous variety, as well as Furore and the Blight is known in Iceland wherever potatoes American variety. Sequoia, to equal Kerr's are grown, but it causes considerable losses Pink in this respect. only in the southwestern district from the southern part of the Snaefellsnes Peninsula INCIDENCE OF BLIGHT south and east to Myrdalssandur. In figure 40 a climagram is therefore given for Reykjavik, as the center of this area. It will be seen that Blight appears so late in the season that on a long-term average the mean temperatures the foliage does not suffer until the beginning in the summer are above 50 F. only in the of September. There is therefore little loss months of July and August, and the growing of total yield due to premature defoliation, period is short--at most, 120 days. But this but ill some seasons there is considerable loss growing season is increased, in effect, by from infection of the tubers and consequent pre-sprouting of the seed tubers, which are wastage in store (Davidsson, 1951). '*Blight kept at temperatures of 50 to 57 , with some years'* must here be taken to mean, therefore, exposure to light, for 4 to 6 weeks before years in which there is heavy infection in the planting in mid-May. The lower temperatures tubers. Davids son reported that severe losses over the growing period may also be offset (amounting to 20 to 40 percent of the crop) to some extent by the long-day conditions, from this cause occurred in 11 of the 41 years with over 20 hours of daylight in June and from 1918 to 1950, and Fridriksson informs July. Sandy soils are preferred, and the plant- us that over the past 21 years the losses ing is done by hand, with a rather close spac- from tuber infection have been severe in 6 years ing (12 in. X 24 in.). The crop is lifted in (table 52). mid-September to escape the severe autumn frosts. Kerr's Pink is perhaps the most commonly grown variety, but there are many TABLE 52. --Blight infections of tubers in others in cultivation, including the indigenous Iceland, 1936-56

Severe Slight Negligible ^ Iceland REYKJAVIK 64°09'N - 1939 1948 1936 RAINFALL «in» F ^ so 1941 1949 1937 Inches ^^ N 1944 1950 1938 - ^ X 40 ^ 1945 1954 'y 1940 / 30 1947 1955 1942 / 1953 — - PLANT /. LIFT 20 1943 -INC y 7/, -INC — — 1946 — — 1951 — — 1952 """ — . BUCHT SEASON ^'NO BLICH r'SEASON I940 1956

60»F Climagrams for seasons in which blight

RAINFALL was severe (1939) and negligible (1940) are 50'F Inches given in figure 40. The severe blight season 40"F ^^o ^^^^cterized by mean temperatures of 55 , 54 , and 53 F. in the months of July, August, and September, respectively; while V/ in the **no blight" season the corresponding 1 r temperatures were 52°, 50°, and 45°. With an 1 average diurnal range from 5° above to 5° below the mean, it would appear that in such FIGURE 40."Monthly rainfall and mean temperatures for Reykjavik, years as 1939 in Iceland, numerous spells Iceland, in relation to potato cropping: Above, long-term average; may occur during these 3 months when the below, *'blight** and "no blight" seasons. Beaumont minimum temperature requirement

90 of 50 is satisfied. The rainfall is abundant, CONTROL and, occasionally, as in August 1939, very- heavy. The southwest of Iceland is warmed by it would appear that the infection of the the Gulf stream, and humid conditions fre- tubers occurs in part before lifting and in quently occur. The long-term average mid-day part at lifting. The rather close spacing (24 (1400 hr.) relative humidity at Reykjavik over inches between drills on maincrop varieties) the months of June to September is about may make it difficult to provide adequate 72 percent--a figure rather less than that for soil cover in earthing up; and the necessity Garvagh in Northern Ireland (78 percent) but to lift before the frosts must result in many much the same as that for Aberdeen. Low crops being harvested while the blighted haulm temperatures in July and August appear to is still green. This would afford great op- be the factor checking the development of portunity for infection at lifting and probably blight in the '*no blight" seasons. accounts for most of the loss from blight In trials at Reykjavik on the susceptible developing in store. With a growing season Gullauga variety, blight was first noted on already so short, haulm destruction at the the foliage during the third week of August, beginning of September to reduce blight in both in the '•blight year" 1939 and in 1940. tubers would involve too great a sacrifice of In 1939 it progressed to about 95 percent by total crop. September 20, just before lifting, while in Protective spraying of the haulm with 1940 the haulm at that time was almost free, burgundy mixture has been found to give hi both years the total yield was about 10 tons good control of blight in the tubers, as indi- per acre, but in 1939 4 tons of the potatoes cated by the results of trials at Reykjavik in were diseased. 1939, 1940, and 1941 (table 53).

TABLE 53. --Diseased tubers as a percentage of total yield, in sprayed and unsprayed plots, Iceland, 1939-41

Sprayed with burgimdy mixture Once Once Twice Year and variety Unsprayed July 18-25; August 1-7 July 18-27 iUigust 7-l¿

1939: Percent Percent Percent Percent Alpha.... 7 5 2 2 Gullajuga. 40 25 5 4

194Ö: Alpha.... 0 Gullaxiga. 0

1941: Alpha.. •. 3 2 1 0 -Gullauga. 20 11 3 3/2

It is evident that spraying between July 18 Among the varieties found to be most and July 27 was too early for outbreaks not susceptible to blight under Icelandic conditions occurring until late in August; but that spray- are Early Rose, Arran Pilot, Gullauga, and ing in early August must have been very- Red Icelandic. effective in keeping the disease off the haulm, so that opportunity for tuber infection both in SUMMARY: ICELAND the ground and at lifting was greatly reduced. The relatively low percentage of blighted tubers Despite the low summer temperatures and in the variety Alpha even without spraying short growing period, potatoes are an important points to the possibility of obtaining good crop in Iceland, and good yields are obtained. practical control by the choice of varieties Blight occurs in the lowland areas to the resistant to blight in the tubers. southwest of the island, where it causes

91 severe losses by infection of the tubers from these temperatures range about 50 to 52 , late attacks on the haulm in about 3 years out with a mean diurnal rangeof about 10 between of 10. The blight attacks, under favorable the maxima and minima. Protective spraying humidity conditions, occur when the mean of the haulm with burgundy mixture has been daily temperatures in July and August reach found to be effective in preventing blight in 54° to 55^ F. but appear to be inhibited when the tubers«

NORWAY

Norway forms the western part of the Region: Thousand acres (1955) Scandinavian Peninsula, stretching from latitude 58° to 71° North, so that a large part Central Norway of the country lies above the Arctic Circle (Hedland, Op- (fig. 41). It is-a mountainous country, and land) 32 only in the southeastern parts and in the Rogaland 12 region of Trondheim Fjord are relatively large Trpndelag 20 areas of level land to be found. The maritime Other counties - - 41 climate of the west coast is associated with a high rainfall, in many places up to 80 inches Total - - 140 per annum and more. In the southeast (Oslo) and in central Norway, the continental influence These figures are from Erling F^rsundof on the climate is felt, and this is associated the Norges Landbruksh^gskoles Akervekst- with a summer rainfall peak. Annual rainfall ioTSpk, at Vollebekk near As, who has also is lower than in the west, and in parts of the provided the information about blight epidemic s interior may drop to an average of 12 inches. in Norway on which this section is based. The modifying effect of the Gulf Stream on the temperature conditions of the country DISTRIBUTION AND FREQUENCY OF insures a reasonably long growing season in BLIGHT EPIDEMICS southern districts, but in the north the summer is necessarily much contracted, although the The importance of blight varies very much days are long. Climatic conditions at three from district to district» but all farms in centers in Norway are illustrated in figure 42. southern Norway, except those at the highest About 140,000 acres of potatoes are grown altitude, suffer from attacks at some time. annually and produce an average yield of 8 The disease has been found as far north as tons per acre (F.A.O., 1956). Human con- Lyngen in southern Troms at latitude 69 ° sumption of potatoes is about 224 lb. per 30' N.--probably the most northerly blight person each year, and there are large quanti- record in the world. It has also been recorded ties of potatoes used for stock feed, with 550 m. above sea level at Kvikne. small amounts for industrial purposes. The Ffirsund has gathered together the available most commonly grown maincrop potato variety evidence on the incidence of blight over the in Norway is As, which shows a fair degree period 1901 to 1951. The source of most of of resistance to blight, but it is of poor quality his information has been the annual reports for the ware market. Kerr's Pink, Prestkvern, of the Norwegian Ministry of Agriculture, Jj^ssing, and Up-to-Date (Grahm) are main- together with data from the reports of some crop varieties widely grown for ware, all of of the coiintry's experimental farms. In table which are susceptible or highly susceptible to 54 the percentage of years during the 51-year blight. The growing season of maincrop period when blight attacks have been severe varieties in southeast Norway is from May or medium, slight, and absent, are shown for to September, with lifting continuing into each of the four main potato-growing regions. October, Arran Pilot and Irish Cobbler are Not a single year has been quite free from two of the principal early varieties. Early blight in all parts of the country. The variation potatoes are grown mainly in southeastern between different years and different districts Norway and in Rogaland County. within a year has been considerable. The high Four principal potato-growing regions can percentage of years in which blight was be distinguished, which, together, account for absent in central Norway (1 in 4) and the low 70 percent of the total potato acreage. These percentage of such years in Rogaland (1 in 50) are: is noteworthy. These two regions represent differing climatic conditions; Central Norway Region: Thousand acres (1955) has a continental, and Rogaland a maritime (Atlantic) climate (see fig. 42 for Bergen, Southeast Norway which lies just north of Rogaland county). (Çistfold, Vest- Since 1952, F/4rsund has been in charge of fold, Akershus, an investigation into the connection between Buskerud 35 weather conditions and blight epidemics to

92 NORWAY

70'N

Potato Bliqht Sept. 15, 1956

None

Medium

mtttt Severe

250 miles

FIGURE 41.-rKey map of Norway, and incidence of blight on September 15, 1956. serve as a basis for a blight forecasting country from that year, and the B.M.S, scale service in Norway. Somewhat more detailed for the measurement of blight on haulm has records of blight have been kept over the been used since 1954. Maps to show the

93 NORWAY TABLE 55. —Incidence of blight in Norway, 1952-56 ,Oslo 6 LATITUDE 59°55'N Severe or 5 \ - Slight Absent y Region medium / 50 RAINFALL 4 50°F N Inches / //. \ \ 40 3 1956 1955 - / \ JO 2 \ ^ X" ^ Southeast Norway. V/ - 20 I - PLANT /LIFTING -INC / / YA V/1 A s O N D 1955 j F M A M j J i 1952 i Central Norway.. 1956 . Berqen LATITUDE 60''25*N

Rogaland. RAINFALL a Inches

1955 Tr)2$ndelag. 1956

No great degree of precision is claimed for the terms used to describe the epidemics. Although a severe attack can generally be JFMAMJJASOND taken to mean a severe attack on the haulm with a resulting yield reduction due to pre- . Trondheim mature defoliation, there has been no marked LATITUDE 63°26'N attempt to separate defoliation losses from tuber blight losses. The problem of tuber

RAINFALL 4 infection at harvest increases in the more Inches northerly potato-growing areas where the growing seasons and harvest periods are short. F^rsund gives as an example of a severe attack the data relating to a crop of Kerr*s Pink under^^observation at the College

M A M J J A S O N D of Agriculture at As in southeast Norway in 1953. Blight was first noted on July 21, and the crop was completely dead on September 10. FIGURE 42.--Long-term average monthly rainfall and mean tempera- Total yield was 7.4 tons per acre, of which tures in relation to potato cropping at three centers in Norway. 14 percent was blighted. In general, blight is important in all the main potato-growing regions of Norway, and TABLE 54.--Percentage of years when potato particularly severe in the southwestern part blight has been severe, slight, or absent in of the country. The biggest losses are caused 4 regions, Norway, 1901-1951 in the varieties grown for ware, as most of these are highly susceptible to the disease. Severe or Region Slight Absent medium CONTROL EXPERIMENTS

Percent Percent Percent Roer (1957) has described a series of Southeast Norway. 35 10 spraying trials made in southeastern Norway Central Norway,.. 43 31 26 during the period 1945-54« The variety used 51 47 2 during the first 9 years was Up-to-Date, Tr^ndelag 45 47 8 which is very susceptible to blight. In 1954, this variety was replaced by Kerr's Pink, which is much more commonly grown in the incidence of blight at different times through- area and is also less susceptible than Up-to- out the several years have been drawn, and Oate, particularly in the haulm. Fungicides one of these (for Sept. 15, 1956) is reproduced were applied twice with a 2-week interval in figure 41. Blight attacks in Norway over between applications. The percentage increases the period 1952-56 are shown in table 55. in total yield from this spraying were: 94 Increase in yield analysis of these results showed,that a routine from spraying control program against blight on Up-to-Date Year: Percent f was profitable. Similar results with Up-to-Date were 1945 23 obtained by Lunden (1947), who carried out 1946 17 trials in the same area during 1939-44, Again, 1947 1 two protective sprays were given. In these 1948 - 5 trials, however, the field-resistant variety 1949 - 8 As was also included. Average results for the 1950 19 8-year period are shown in table 56. Bordeaux 1951 -- 22 mixture was used throughout. 1952 7 ^ The difference between the field-resistant 1953 7 As and the blight-susceptible Up-to-Date is 1954 -- 8 most striking. Protective spraying of As on a routine basis was not profitable. The much Average 12 smaller percentage of blighted tubers in As should also be noted. The average yield of the unsprayed potatoes over the 10 years was 9.6 tons per acre, and SUMMARY: NORWAY of the sprayed, 10.8 tons per acre. This Blight is important in all the main Norwegian represents an average increase of 12 percent potato-growing areas. Losses are principally from 2 sprayings, or just over 1- ton per in varieties grown for the ware market. The acre. Protective spraying had little effect on relatively small effect of the disease on As tuber blight infection, which was high, averaging points the need for a field-re s is tant ware 22 percent on the unsprayed plots. An economic variety.

TABLE 56. --Results of spraying tests with bordeaux on yields of As and Up-to-Date potatoes, Norway, 1939-44

o As Up-to-Date Test Total yield Blighted tubers Total yield Blighted tubers

Tons per acre Percent Tons per acre Percent UnsBraved...•..•••...•••• 11.6 1.6 S.S 11.0 Süravpd...... ••.••••• 12.0 .7 10. Ö 13.0

SWEDEN

Sweden extends in a narrow strip about appear that the long-day conditions of the near- 1,000 miles long between latitudes 55^ and Arctic summer suit the potato crop, provided 69® N. Due mainly to the influence of the Gulf that the right varieties are grown. stream, climatic conditions in the country are Approximately 40 percent of the crop is not so severe as might be expected fronn its used for human consumption, giving an average northerly position. In the south, mean summer annual intake of somewhat over 200 lb. per head temperatures exceed 50® F. for about 4 1/2 of the population: Half the potatoes are used months, butinthenorthfor only about 2 months. in animal feeding and the rest for manufacture Potatoes are grown throughout the country (farina and alcohol). A very full account of (fig. 43), principally on sandy soils, with the the distribution of potato varieties in Sweden main concentration in the south: The three has been given by Hagberth (1951), from the most southerly counties--Blekinge, Kristian- results of a nationwide survey carried out stads, and Malmphus --have, between them, between 1937 and 1943. The leading variety a third of the total potato acreage. About was then Magnum Bonum, and other maincrop 300,000 acres of potatoes are grown each year varieties widely grown were Wohltmann, Up- and produce an average yield of 5 1/2 tons to-Date, President, Birgitta, and Gloria. The per acre (F.A.O., 1955). Curiously enough, principal early varieties were Early Puritan the potato yields in the most northerly pro- and Early Rose. In the south of Sweden the vinces are as high, if not higher, than in the potato-growing season is from the beginning south, despite the shorter growing season. of May to early October. Northward the season This must, in part, be a reflection of more is contracted; planting is later, and lifting intensive cultivation methods, but it would also earlier. (See climagrams, fig. 44.)

95 SWEDEN

Arctic Circle

MALMOHUS ,, . .VAkarpVKRiSTIANSTADS Molmo« "

250 miles Each dot = lOOO acres

FIGURE 4S.—Key map of Sweden, and distribution of potato growing, 1955. (After Jonasson. Höljer, and Björkman, 1938, and Sweden. Statistiska Centralbyran, 1956.)

96 BLIGHT EPIDEMICS: DISTRIBUTION from spraying (in tons per acre) are shown AND FREQUENCY in table 58. In each year a number of different spray materials were used, and the results Potato blight occurs throughout Sweden, for the sprayed plots in the table refer to but the disease is most important in the the most successful treatments. The variety southern half of the country, particularly in was Up-to-Date in most of the trials, although the western coastal area. It is there that the in the earlier years it was Magnum Bonum first outbreaks of the season are usually at both centers. recorded. In north Sweden, epidemics, when 0 In the 7 years that trials were held at they occur, are usually late. No special survey Akarp, blight incidence for Sweden has been work is in progress, but Dr. Lihnell of thç described by Lihnell (table 57) as severe in Statens Vaxtskyddsanstalt, Stockholm, has pro- 3 years, slight in 3 years, and negligible to vided his own appraisal of blight incidence in severe in 1 year. There does not therefore Sweden for the years 1946-55 (table 57), and appear to be any particular bias toward other information for this section. '•blight" or **no-blight'* years in the series The classification of the epidemics shown shown in table 58. Where no 75 percent blight in table 57 covers the losses caused both by date has been given for the sprayed plots in premature defoliation and by tuber infection. this table, it is because the 75 percent stage Dr. Lihnell points out the difficulty of con- was not reached before lifting. centrating in a few words the truth about The figures in table 58 can be used, with potato blight for a country the size of Sweden, some reservations, to estimate blight losses where conditions vary considerably between at the two centers. The losses can be estimated the north and south. In some years, as in as the difference between the yields of the 1947, practically no blight occurred in any sprayed (S) and unsprayed (U) plots (the gain part of the country. But in 1955, for example, from spraying) expressed as a percentage of the northern parts of Sweden were almost the "sprayed" yield. This is on the assumption entirely free from blight, while in the south that the ''sprayed" yield is approximately one of the worst attacks of the whole 10-year equivalent to the full potential yield of the period occurred, with heavy losses from tuber crop, and it is here that reservations need infection. to be made. In those years when the 75 percent stage on the sprayed plots was reached before BLIGHT LOSSES the „end of September--1949, 1950, and 1954 at Akarp--the "sprayed" yield will be lower In the absence of survey data, the results than the potential yield: In all years, the yield of spraying triéds give the best indication of on the sprayed plots will be somewhat lower blight losses. Figures have been published by than the potential yield due to damage caused Andren and Pettersson (1956) and Andren while spraying. With this in mind, the estimated (1946-55) from trials held at two centers in blight losses for the several years at the Sweden during the period 1946-55. These two centers, presented in table 59, can now centers were at Akarp, near Malmö, in the be considered. extreme south of Sweden, and at Nyckelby, Losses due to premature defoliation are near Stockholm, in central Sweden. In addition high and have occurred every year. This is to the yields of the un sprayed and sprayed partially explained by the evident late bulking plots, details of the progress of blight defolia- of the crop at these two centers. For e¡xainple, tion on the haulm have also been presented, at Akarp in 1955, the 75 percent blight stage measured by a 0 to 10 scale. Fronn this was reached on the unsprayed potatoes on information it has been possible for most years September 21, but there was still a gain of to estimate the time the 75 percent blight 2.5 tons per acre on the sprayed plots; at defoliation stage was reached on the un- Nyckelby in 1948, the unsprayed plots were sprayed and sprayed plots. These estimates 75 percent defoliated on September 20, but together with the plot yields and the gain the sprayed gained 2.8 tons per acre (table 58). Particularly in the area represented by this latter center, where the harvest period is TABLE 57. —Incidence of blight in Sweden, relatively late and short, it seems likely that 1946-55 many crops will be lifted before they are fully mature, so that the trial figures tend to Severe Slight Negligible give a somewhat exaggerated picture of the benefits to the commercial grower of spraying against blight. Nonetheless, the disease is 1950 1946 1947 obviously very serious in Sweden, and this 1951 1948 1955 (North) is borne out by farmers taking protective 1953 1949 spraying as a matter of course. 1954 1952 Tuber blight figures have also been given 1955 (South) for all the years of the trials at Nyckelby, and for some of the years at Akarp. The

97 TABLE 58. —Yield, date when 75 percent blight stage occurred, and gain from sprajring potatoes at 2 centers in Sweden, 1946-55

Date of 75 percent Total yield blight stage Gain from Location and year spraying Unsprayed Sprayed Unsprayed Sprayed

AKARP: Tons/acre Tons/acre Tons/acre 1946... Sept. 14 6.4 7.9 1.5 1948... Sept. 10 11 12. 1949... Aug. 21 Sept. 18 9 11. 1950... Aug. 10 Sept. 20 11 16. 1953... Sept. 10 Sept. 30 15 21. 1954... Aug. 22 Sept. 20 11 14, 1955... Sept. 21 14 16,

NYCKELBY:

1946... Sept. 15 6.2 8.1 1.9 1948... Sept. 20 8.2 11.0 2.S 1949... (^) 11.0 13.3 2.3 1950... Sept. 11 10.1 14.2 4.1 1953... S.2 12.3 4.1 1954... Aug. 30 S.S 12.8 4.0

No data for date of 75 percent blight stage.

percentage of infected tubers in the unsprayed curately these figures reflect the position plots at both centers is shown in table 60. In throughout Sweden, but it should be remembered all years the variety under trial was Up-to- that they are solely for tuber blight before Date, apart from 1946 at Nyckelby, when it lifting, and as such they are high. Tuber infec- was Magnum Bonum. tion at harvesttime is also a problem in The average loss through tuber blight infec- Sweden, which becomes more acute in the north, tion for the 6 years of trials at Nyckelby was owing to the shorter growing season and shorter just under 8 percent. At Akarp the chief point harvest period. The use of haulm killers should of interest is the very heavy loss in 1955 (nearly greatly reduce this source of loss. 60 percent). It is not possible to say how ac- BLIGHT AND WEATHER DATA TABLE 59. —Percentage loss of crop due to blight at 2 centers in Sweden, 1946-55^ The climagrams (fig. 44) for three centers in Sweden show the strong continental influence 0 affecting the country's climate. There is a Year Akarp Nyckelby TABLE 60.—Percentage of blighted tubers at Percent Percent 2 centers in Sweden, 1946-55 1946 19 23 1948 8 0 25 Year 1949 20 17 Akarp Nyckelby 1950 31 29 1953 28 33 Percent Percent 1954 25 31 1946 0.1 1955 15 1948 16.1 1949 Average 8.3 21 26 1950 6.8 1953 10.0 12.7 0_TT 1954 2.5 2.3 ■*■ Formula used to estimate loss: (-g- x 100)^ 1955 57.8 where S is yield of sprayed and U is yield of ' unsprayed plots. 2 No trial. No trial.

98 SWEDEN Sweden are summer temperatures generally favorable for blight. For example, long-term -Malmö mean July and August temperatures at Malmb are 62° and 60° F., and at Stockholm 63° and 60°. The climagram for Harnbsand shows the RAINFALL 4 Inches shortness of the growing season for potatoes (approximately 110 days), but a fairly large acreage of the crop is grown even farther north than this. Under these conditions, tuber infection at harvesttime has proved to be the major source of loss caused by blight. JFMAMJJASOND CONTROL MEASURES

6 ^ Srockh olm LATITUDE 59°0'N " Dr. Lihnell has told the authors that prac-

5 tically all growers who produce potatoes on a . -X commercial scale spray their crop once or RAINFALL 4 50° N Inches twice, and growers that use better farming 3 \ methods spray 3 or 4 times in the season. / . - Haulm destruction has become popular in 2 -^ recent years. Investigations into the biology of 1 : PLANT y - Phytophthora infestans and on spraying tech- -INC /^ 1 ^ LIFTING niques are carried out at the Royal Agricul- V/ //x J FMAMJ 1 JASOND tural College, Uppsala. A recent paper from there by Bjorling and Sellgren (1955) has given r- Härnös and an account of field experimental work on blight LATITUDE M°3S'N infections through the upper and lower surfaces .-^ ^ - of potato leaves. Although these workers found that by far the greater proportion of the RAINFALL 50° F /' N, Inches \ infection took place through the upper sur- / \, faces, there was sufficient infection through the s lower surfaces for them to state that both / X surfaces need to be sprayed to give efficient :^~~ =^ PLANT / X protection against blight. Work on protective r^ 1JFTINC -INC / 1/ / j //Á spraying and potato haulm destruction is also JFMAMJJASOND1 i being carried out at the Institutet for Vaxtfor- skning och Kyllagring, at Nynasham, by B. FIGURE 44.--Long-term average monthly rainfall and mean tempera- Emilsson and his colleagues. tures in relation to potato cropping at three centers in Sweden, showing shortening of growing period at higher latitudes. SUMMARY: SWEDEN marked summer rainfall peak and a wide Although survey informiation on blight inci- difference between summer and winter tem- dence in Sweden is not available, the results peratures. The wettest month of the year is of spraying trials lead to the conclusion that August. Continental conditions are modified in losses from the disease both by premature the south by warm winds blowing across the defoliation and tuber infection are potentially land from the Atlantic, and in the north by the very heavy. Losses are, however, reduced by presence of the Baltic Sea and the Gulf of a universally applied annual spray program. Bothnia. These various sea influences, result- Tuber infection at lifting--a direct result of the ing in high summer humidity conditions, appear short growing and short harvest periods--is ta be one of the main factors leading to the the main problem in northern Sweden. In the relative frequency of severe blight years-- south, yield losses brought about by premature approximately 1 year in 2. Combined with the defoliation is the more important aspect of the high humidities in the southern parts of disease. WESTERN GERMANY

About 3 million acres of potatoes are Production for-- Million tonnes grown annually in Western Germany, Seed 2.9 producing an average yield of just over Ware 8.2 8 tons per acre (F.A.O., 1955). Total potato Fodder ■ 12.0 production is about 26 million tonnes Starch manufacture • .2 per annum, and Van Hamelen (1955) Alcohol manufacture .2 states that this is used as follows: Shrinkage loss • 2.5 Total 26.0

99 Human consumption accounts for less than south of Schleswig-Holstein, and the north of half of the crop. Even so, consumption is very North Rhine. This region contains a high pro- high, and in 1953 was 372 lb. per head per portion of medium-sized farms, ranging from annum. Enough certified seed is produced to 50 to 300 acres. A very large acreage of plant about one-third of the potato acreage, potatoes is in Bavaria. The acreage is chiefly and in addition some seed is exported, princi- in small or very small farms, and the potatoes pally to Switzerland, Italy, France, and Brazil. are for home consumption. Early potatoes are Fodder potatoes, grown mainly for pigs, ac- grown throughout the country, with the most count for about 50 percent of the total produc- important district centered on Dollbergen, tion. between Hanover and Brunswick (Braun- schweig). Seed potatoes, also, are grown in all POTATO DISTRIBUTION AND VARIETIES areas, with a concentration of crops in Lower Saxony, which produces about half of all the Potato acreages, by province (fig. 45) for certified potato seed grown in Western Ger- 1954, are shown in table 61. These figures were many. Much of this seed production goes on in supplied by the economist Intelligence Unit. the north of Lower Saxony in the area adjoining The principal areas producing potatoes for the North Sea. Stade is one of the main seed- the ware market are in Lower Saxony, the producing centers.

GERMANY

NORTH SEA S4°N

Sfade •^Hamburg \ MECKLENBURG '/ ••.•■../• •• I [•Ebersdorf A denburg • ^>* • ' ' ' • • / • Bremen ;. ^^ LOWER SAXONY • Uelzen '• BRANDENBURG ' POLAND »Berlin^ NETHER- ^ V• ■■■■••• ••••Honove?.°»'''.T"; Manöver« «Brunswick • . '■ :" <^ LANDS .^SanSruV" i.i \ _ W /^^ NORTH RHINE /SAXONY-ANHALT-

^^j'wESTPHALIA .-'HESSE / SAXON Y _ , ^\ •Bonn'--' / /THURINGIA BELGIUM- ,-,HiNE.:

f ^PALATINATE." ' ^ Darmstadt • • ^°^ Kissingen N ^ £ C -^ ^ ' -NSAAR- BAVARI A \ Ay O

•" BADEN • Nuremberg ^ >^ ¿ o FRANCE / / - ^ A K \ A I • Freudenstadt ^^ y ^f - iàlH \ WURTEMBURG • ^^ / \

^^y

200 miles

ncURE 45.--Key map of western and eastern Germany.

100 TABLE 61.--Potato acreage, by province, Western Germany, 1954

Variety

Province Mid-season Total Early and maincrop

1,000 acres 1,000 acres 1,000 acres Schleswig-Holstein 9.5 128.0 137 Hamburg >1 3.5 4.2 Lower Saxony U5 .9 665.7 711.7 Bremen .6 2.1 2.8 North Rhine Westphalia. 38.9 389.6 428.7 Hesse 11.8 255.2 267.0 Rhineland Palatinate... U.l 244.5 258.6 Baden-Wurttemherg 13.3 322.1 335.4 Bavaria 30.4 762.2 792.6

Total 165.2 2,773.0 2,938.2

Most of the varieties grown originated in SOIL AND CLIMATIC CONDITIONS: CULTI- Germany. In a recent review, Hix (1955) has VAT IONS compared the importance of the different midseason and maincrop potato varieties from the results of a nationwide survey. His figures, Most of the potatoes are grown on light, showing the makeup of the Western German sandy soils. The climate throughout the greater potato acreage in 1954, are shown in table part of Germany is continental in type, with relatively severe winters and the rainfall peak 62. coming in July and August. Summer tempera- Hix did not include first-early varieties in tures are not high; in most places the mean his survey. Erstling is the most important temperatures in July and August do not exceed early potato, and most of the seed of this 65^ F. In the north, true continental climatic variety has to be imported. German conditions are modified by the presence of the early varieties grown on a fair-sized scale North Sea and the Baltic; whereas in the south, are Oberambacher Frühe, Sieglinde, and the fact that most of the land is high leads to Vera. cool summer conditions. Table 62 shows the preponderant part played There is a difference of 1 to 2 weeks in the by maincrop (late) varieties as opposed to tinae of planting potatoes between the north and midseason potatoes. This is a somewhat dif- south of Germany. The earlier plantings take ferent state of affairs from that in France and place in the north of the country, from the Belgium, where a muchhigher.percentage of the beginning of April onward. Digging of early potato acreage is planted with midseason varie- ware varieties starts from the middle to the ties. The three most important varieties are end of June; and midseason and late varieties Ackersegen, Heida, and Bona. The part played are lifted from mid-August to the end of by these potatoes varies in different parts of September. Except in Schleswig-Holstein and the country. Ackersegen is the most widely parts of Lower Saxony, individual holdings are grown variety throughout almost the whole of mostly small and subdivided into numerous Germany and is particularly important in south strips, which may be widely scattered. This Germany. In Bavaria, it makes up half of the is a great hindrance to mechanization and, acreage; and in Hesse, about two - thir d s. Heida incidentally, to the blight control measures. is hardly grown at all in Bavaria and Hesse, Opitz (1949) describes potato cultivations in but in the other provinces it occupies a tenth Germiany in detail. to a half of the potato acreage. The midseason variety Bona is concentrated in north Germany, particularly in Westphalia, where it is even POTATO BLIGHT INVESTIGATIONS ahead of Ackersegen and makes up almost a third of the potato crop. Before the second World War, research and The two main blight-resistant varieties de- investigational work on the potato crop in rived from Solanum demis sum hybrids, now Germany was concentrated in what is now grown in Germany, are Aquila and Maritta. the Eastern Zone, where the larger part of the Aquila does not occupy a large acreage, but crop was grown. Workers on potato diseases Maritta is important, particularly in Bavaria. in Western Germany hada considerable amount Recent descriptions of Germian potato varieties of leeway to make up after the war, and are have been given by Snell (1952). only now getting fully into their stride.

101 out the majority of the trials on blight control TABLE 62.--Potato varieties, use, and in recent years. Individual Plant Protection percentage of acreage in Western Offices are also responsible for collecting in- Germany, 1954 formation on the incidence of plant diseases from local observers, and datgi on potato blight Varieties and use Acreage from this source, presented on a national basis, have been published. Midseason BLIGHT EPIDEMICS: DISTRIBUTION AND FREQUENCY For ware: Percent An attempt has been made by Voelkel (1954) Bona 9.4. to delimit those areas in Germany where Concordia 1.0 potato blight is most important. His method Flava .2 Olympia 2.5 embodies a conception of **pest or disease Others zones'' (Schadgebiete), which he defines as **areas in which a pest or disease in most of For ware, fodder, or industry: the years in a particular period, is always more or less severe." Figure 46 is from Augusta 1.2 Voelkel's map of the **Schadgebiete" for potato Mittelf rulie 1.8 blight. The darkly shaded areas, which are most concentrated in Lower Saxony, are the Total 16.9 real blight zones. Voelkel based his zoning upon a combination Mainerop of the total number and the frequency of **severe" blight reports received by the var- For ware: ious Plant Protection Offices from the different districts during the period. The bad i^gnes 3.6 blight areas in Western Germany are thus Heida 12.6 shown as (1) the south of Schleswig-Holstein, Virginia 1.2 (2) Lower Saxony (Oldenburg and Hanover), Others 1.9 (3) the western half of North Rhine-Westphalia, and (4) a small area in southeastern Hesse. For ware, fodder, or industry: Areas 1 to 3 form a continuous zone, comprising practically the whole part of Western Ackersegen 44.0 Germany on the great sandy plain that stretches Aquila , 1.1 across the north of Europe. In this northern Cape 11a , 1.0 blight zone, the disease is most serious in the Magna , 1.0 districts near the coast. Maritta 6.4 Annual accounts of potato blight in Germany Voran 4.2 were published from 1920 to 1931 in the Others , 5.3 Mitteilungen aus der Biologischen Reichsan- stalt für Land und Forstwirtschaft. FromLl925 For industry: to 1931, these accounts included maps showing the distribution of the disease. Since 1948, Robusta general accounts of blight for most years have been compiled by Dr. Harle from the Total 83.1 records of the Plant Protection Offices, and published in Nachrichtenblatt des Deutschen Pflanzenschutzdienstes, Braunschweig. (See In the late 1920's and 1930's K. O. Müller Härle, 1951.) It is possible from this material, (1928), with his W-races, and Schick, working together with information supplied by Dr. at the Kaiser-Wilhelm Institute, were among Johannes, to divide the years 1948-55 into two the pioneers in the search for blight-resistant categories--*'moderate" and '*severe"--ac- potato varieties. Since 1947, Rudorf and his cording to blight incidence. This has been done colleagues have continued this line of research in table 63, which also shows the average at the Max-Planck Institute, Voldagsen i^ber potato yields (F.A.O. statistics) in particular Elze, in Hanover. Also, since the war, funda- years. mental studies in connection with schemes for In no year was blight absent, and although forecasting blight have been carried out in the incidence of the disease varied consider- different parts of Western Germany, notably ably between different areas of Western Ger- by Johannes (1953a), and Uhlig (1955). many over the period 1948-55^ in each year The Plant Protection Service (Pflanzen- there was at least some part of the country schutzdienst) is organized on a provincial where blight was stated to be severe. The dif- basis, and has been responsible for carrying ference between a moderate and a severe blight

102 severe throughout the whole country in a particular year (as in 1950), the more general pattern is one of variation between different regions. The only satisfactory way of defining epidemics is on the basis of a nunnber of relatively homogeneous potato-growing areas, and, unfortunately, the data are not available to do this. The highest yield (9.8 tons per acre) during the period 1948-54 was in 1950--the worst blight year. Without a clearer method of defining the epidemic in that year, it is impossible to explore fully the reasons for this, but it indicates that a high loss from the

Bliqhf severe in more disease in a blight year may be amply offset than Syears in 20. by an actual increase over normal production.

Bliqht severe in mcxe fhan Syears in 20, BLIGHT ON HAULM AND LOSSES and over 25 'severe' reporfs per disfrict. Very few of the accounts of spraying trials in Western Germany include a description of FIGURE 46.--Blight zones inGermany, 1925-44. (After Voelkel. 1954.) the progress of the disease on the haulm. An year lies in the time of destruction of the exception is some recent work by Johannes potato haulm by the fungus. Generally, in a (1953b), who used a scale from 0 to 5 to esti- **severe" year, the haulm over a large part mate the amount of defoliation, but this scale of the acreage is killed before the end of is rather lacking in precision. It thus becomes August, with a considerable resulting loss in very difficult from published information to yield. In a ** mode rate" year, the blight attack make statements on the timing of blight comes late and defoliation los s es are generally epidemics in the different regions of the small* This can, perhaps, best be seen from country. However, Dr. Scheibe of the Pflan- Harle's account for 1950, which appears to have zenschutzamt at Hanover has given us a verbal been the worst year in the 8-year period; and account of blight progress on potato haulm in for 1952, when blight was least damaging: his area, which covers the eastern and larger 1950. Blight severe. ** Blight on haulm, half of Lower Saxony. From this account it is present in the Rhineland in May, was possible to get an idea of the effect of blight sporadic in June, and from July in one of the principal "Schadgebiete" of generally distributed and severe in the Western Germany. In this part of Lower Saxony whole of Western Germany. . • The blight occurs every year, most commonly disease was especially severe in appearing in July. In a bad blight year, it Schleswig-Holstein, Oldenburg and appears at the end of June and the tops of Westphalia. On some varieties, such unsprayed maincrop (or late) varieties are as Bona (midseason), the foliage was dead by the middle to the end of August. The completely killed within a few days. crop would normally ripen off by the middle It appeared that the spraying against to the end of September. The frequency of such blight was often done too late. In blight years is about one in three. Schleswig-Holstein and in part also Thus, in a bad blight year there may be a in Oldenburg, blight in tubers, (espec- loss of 6 or 7 weeks' growth on unsprayed ially on the lighter soils) was maincrops. By analogy with England and Wales, relatively slight despite the severe where the length of the growing season is very attacks on the haulm." similar, this will inyolve a loss of about 30 1952. Blight moderate. The summer in this percent of the potential yield. But this is putting year was dry and hot, and, up to the the picture at its blacicest. Even in the bad end of July, blight incidence was blight year, the yield loss is unlikely to average described as very slight. In August 30 percent on the unsprayed maincrops, and it *'although blight was long delayed with will be reduced in the area as a whole by outbreaks only here and there in this protective spraying. Moreover, the earlier month, the disease on haulm and maturing potato varieties areunlikely to siiffer tubers was in parts still very severe, so severely from the effects of premature de- especially in North Rhine Westphalia, foliation. Lower Saxony, Hamburg and Schles- The results of large numbers of spraying wig-Holstein. South Germany re- trials in many of the potato-growing regions of mained almost free of the disease." Western Germiany have been reported in the The attempt that has been made in table 63 literature since 1946. These trials have varied to define the incidence of blight in certain from properly replicated experiments to years for the whole of Germany, is, at best, simple demonstrations with one sprayed and unsatisfactory. Although the disease may be one unsprayed plot. It might be possible, as

103 has been done in Denmark, to use the results percent gave a yield increase as a result of of these trials, expressed as yield increases spraying, and this increase averaged over 13 from spraying, to estimate blight losses on a percent. To what extent the yield increases countrywide basis. To the outsider, however, quoted above for the three sets of trials reflect they present many difficulties of interpreta- the level of losses caused by blight during tion. Some of the most important of these dif- severe blight years in parts of Germany is a ficulties are: (1) Very often the results for a matter of conjecture. The efficiency of the number of years and/or a number of varieties spraying is a most important factor, and it is of differing maturity groups have been aver- most unlikely that full blight loss was pre- aged; (2) far too little precise information on vented. the progress of blight on the haulm is given, Malmus (1949) has made some overall blight and even general notes on the severity of blight loss estimates for the German potato crop.He in the year of the trial nnay be lacking; and (3) estimated that 33 percent of the crop was lost the results of the trials are often exprès sed as by blight in 1916, and 25 percent in 1926. In percentage increases in yield from spraying, years of less severe attack than these, Malmus without quoting the actual yields of the sprayed suggests that the loss is about 10 percent. It and control plots. seems most unlikely that the average annual An idea of the scale of the yield increases in loss by premature defoliation in Western Ger- some recent trials can be gained by quoting many will exceed 10 percent of the potato crop. representative figures. Johannes (1953b) has This is on the basis of the careful analysis by given the results of experiments carried out Stapel (1953) of blight losses in Denmark, in 1949 and 1950 at a number of centers in where losses were estimated to be of this order. Western Germany, where the miain object was The weather and other growing conditions in the to test the relative efficiency of different rates blight zones of Western Germany are very of copper application and amounts of water in similar to the general conditions in Denmark. low-volume spraying. In 1950, which was a Outside the blight zones, where half the total severe blight year, trial centers were at crop is grown, blight losses are likely to be Uelzen and Ebersdorf in Lower Saxony, at less than in Denmark. It is important to see this Sandrup and Bonn in North Rhine Westphalia, loss of potential crop in its proper perspective. and at Darmstadt in Hesse. The average An analysis of West German potato yields in increase in yield(all varieties) over the 2 years moderate and severe blight years (table 63) for the optimum treatment was 26 percent. shows that in the years of moderate attack, the This figure corresponds very closely to an average potato yield was 8.2 tons per acre; in average increase of 23 percent of the unsprayed the years of severe attack it was 8.8 tons per yield, which was the result of trials by Ext acre. (1950) during 1950 in Schleswig-Holstein. Two Little data exist on the extent of tuber infec- sprayings with a copper compound were given tion, either in the soil before lifting or at to a number of different varieties. The varia- harvesttime. Dr. Johannes has said that in tion in yield increase between some of the Western Germany, as a whole, tuber blight is varieties in Ext*s trials are shown in table not a very important problem, and this he 64, and the figures illustrate some of the associates with the generally sandy soils on difficulties of interpreting results of this kind. which potatoes are grown and the relatively In southern Germany, Malmus (1954) has long growing season. Speadcing of Lower Saxony, reported on 412 spraying trials in different Dr. Scheibe stated that although trouble may be parts of Bavaria during 1949, when blight was caused in a wet harvest year, tuber blight is severe in the province. Of these trials 72 not usually a problem.

TABLE 63.—Incidence of potato blight and average yield of potatoes in Western Germany, 1948-55

"Moderate" blight years Average yield "Severe" "blight years Average yield

Tons/a Tons/acre 1949 , 7.4 1948 §^2 1952 Ö.3 1950 9.8 1954 9.0 1951 8.6 1955 1953 8.4

Average. 8.2 Average B.S

104 TABLE 64. —Increases in yield of certain potato varieties from copper sprasring, Schleswig-Holstein, Western Germany, 1950

Average increase Variety Mattirity in yield from Range of spraying increase

Percent Percent Flava Midseason 17 5 to 29 Bona - .fin 36 -2 to 66 Heida Maincrop 27 S to 43

BLIGHT AND WEATHER DATA great difference in the average monthly rainfall and temperature figures between the Uhlig (1955) has pointed out that the main centers occurs. A greater annual and summer blight zones in Western Germany (Voelkel's rainfall is found in the higher parts of Ger- •'Schadgebiete") lie predominantly in those many; at Freudenstadt (2,395 ft.), for example, parts of the country where the mean relative the average yearly rainfall is 58 inches, but humidities in July and August are over 75 these wetter districts lie outside the main percent. But so far no detailed studies of blight potato-growing regions. The higher summer conditions in relation to macro-climate have humidities due to sea influences, which occur been made. The long-term rainfall and mean in the north, are not correlated with a higher temperature data shown in figure 47 for three rainfall in these regions; and there is no centers in different potato growing areas simiple relationship between blight and rainfall. reveal a very similar pattern, and no very Summer temperatures throughout the country WESTERN GERMANY are generally favorable to the disease. A study of blight incidence in relation to . Hamburq weather at Braunschweig (Brunswick) over a 82 ft. period of 3 years (1952-54) has been made and is given in table 65. Climate at this center is RAINFALL inches very similar to that at Hanover, and may be considered as reasonably typical of weather conditions in the great sandy plain of northern Germany. In this table the descriptions of the blight attack in the several years have been given by Dr. Johannes. During both the *'slight" and the '*severe" attacks in 1952 and 1953, respectively, rainfall was below average, although low rainfall was , ^ Hanover -, *• 187 ft. most marked in the "slight" year. The most 5 - y""^ ^\^^ unusual year of the 3 from the point of view of •F relationships between blight and weather was RAINFALL Inches 1954. Despite extremely wet conditions in the / \ J - ^ V summer (July rainfall: 7-8 inches), the very low temperatures in July (5® F. below average) 2 ^"*~" -^ delayed the onset of blight, amd the attack was 1 - - PLANT / 1 ;; LIFTING late. -INC / / 1/ / / V/ //A 1 1 CONTROL. MEASURES

.Nuremberq Dr. Johannes has given a general account l,OI4ft. of chemical control measures against blight in Western Germany. In the north of the

RAINFALL country, protective spraying is commonly inches practiced. Seed potato growers (export and home consumption) are obliged by their con- tracts to use protective sprays on their crops; also, there are a number of organizations of large growers that insist on protective spraying as a necessary operation if the organization J F M A M J is to buy the member's potatoes. Inmost cases FIGURE 47.—Long-term average monthly rainfall and mean temperatures in relation to potato cropping for three centers in Western the work is done by the farmer himself; for Germany. the small farmers, communal machines are

105 available. Spraying against blight is much less original blight resistance. Recent West Ger- common in the south of Germany. man work on the resistance of numerous wild An estimate of the amount of spraying done Solanum species and on the incorporation of in the area covered by the Pflanzenschutzamt this resistance into commercially acceptable at Hanover has been supplied by Dr. Scheibe, potatoes, has been reviewed by Rudorf, and his figures illustrate some of the factors Schaper, Ross, Baerecke, and Torka (1950) and at work. The area falls into three main di- Rudorf (1950). visions. In the seed and ware-growing district Much attention has been paid in recent years in the northwest, about 50 percent of all to the development of weather rules by which to growers spray each year; the growers of early forecast the appearance of blight epidemics.' varieties for seed usually spray three times, The operation of a blight forecasting service to and the growers of maincrop varieties for growers is the responsibility of the individual seed usually only once. In the central ware- provincial plant protection offices, and it is growing district, 20 to 30 percent of the unlikely that a centralized system of issuing growers spray once or twice. In the early warnings will be developed. At the present ware-growing district centered onDollbergen, time, the plant protection office at Oldenburg no spraying is done. (W. Holz) is the only center in Western Ger- The most commonly used spray materials many issuing regular blight forecasts based in Western Germany are, in order of impor- on meteorological data, although other offices tance: Copper oxychloride (45 percent Cu); send out advice on spraying against blight when copper oxides (35 or 70 percent Cu); and organic first reports of the disease in their areas comte fungicides, in particular, zineb. Dr. Scheibe in. The main centers for research into fore- has said that in Lower Saxony about 90 per- casting methods are at Braunschweig and Bad cent of the spraying is with copper materials, Kissingen. Johannes (1953a) and Uhlig (1955) and about 10 percent with organic fungicides. have both tested the Beaumont criteria and also Most of the spraying is done at high volume, the vapor pressure rule of Thran (1952), which but the low-volume rate is increasing in was developed in Schleswig-Holstein. They popularity. From the results of trials in 1949 considered that both these methods give in- and 1950 at centers in different parts of sufficiently accurate forecasts under West northern and central Germany, Johannes German conditions, and have formulated more (1953b) concluded that the best low-volunne complicated rules based on crop humidity results were obtained by an application of 4 to 6 lb. of 50 percent Cu in 40 gallons of 5 At an international cxjnference on forecasting (Ullrich, 1957) held at Bninswick in December 1956, Uhlig explained that according water per acre. Sommer (1951) has given to the "field-humidity rules" employed in a trial warning service in details of the spraying techniques employed 1954-56, a critical period was one in which the relative humidity, in the important seed-growing region around measured in screens 2 m. (6| ft.) above ground level, was over 88 Stade in Lower Saxony. Throughout Germany, percent for 15 hours in the absence of rain, and for 12 hours following spraying against blight is to some extent rain, with minimum night temperatures falling during the period to 12°-16° C. (53°-61° F.) but not below, and the maximum day tempera- combined with spraying against the Colorado mre rising to 20° C.(68° F.)orover, either during the period or with- beetle, but in most years the first two appli- in the following 12 hours. The rule was used in conjunction with 3 cations against the beetle are given too early hourly weather telegrams. The critical periods from a network of for it to be worthwhile to combine them with stations were plotted on an operations chart to reveal the areas an antiblight treatment. affected; and epidemic spread of blight was not anticipated until three Haulm destruction is not practiced on any critical periods had occurred. At the same conference Thran explained that according to the scale, even in seed crops. Where it is used, •'air-mass and accumulated temperature rule" employed in Schleswig- the chemicals most, commonly employed are Holstein, a critical period is one of 2 consecutive days with vapor D. N. O. C. and D. N. B. P. pressure 12 mm. or more, minimum night temperature 10° -17° C. Over 80 percent of the West German potato (50°-6aP p.), cloud five-tenths or more, and maximum temperature acreage is planted with maincrop or late on at least one of the days reaching 23° C. (73° F.) or more, the varieties (table 61), most of which, and notably readings being taken in screens at 2 m. above ground level. It was found that potatoes were attacked by blight following such a critical Ackersegen, show a high degree of field resis- period only when certain accumulated temperatures had been reached. tance to blight. Schaper (1950) has described Figures for these (which ranged between 600 for chitted first-early the blight susceptibility of most of the com- varieties and 1,650 for very late varieties)could be obtained approxi- monly grown German varieties. An interesting mately by multiplying the number of days from April 1 by the mean conclusion that he came to was that there is day temperatures in °C. (With long-term mean temperatures for no correlation between the foliage and the tuber Hamburg, this would indicate early June as a zero date for first earlies and early August for the very late varieties.) resistance of potato varieties. The same In his very detailed microclimatic investigations, Johannes (1953a) worker at the Max-Planck Institute is engaged found that a critical period for the spread of the fungus occurred when on research into the biotypes of the fungus. the relative humidity, measured in the upper third of the crop, ex- Western Germany is one of the few countries ceeded 90 percent for a period o fat least 33 hours, with temperatures in the World where named ^. demi s sum x S. between 10° and 20° C. (50° -68° F). A fall in hunüdity to not less than tuberosum hybrids, in this case Maritta and 82 percent for 4^ hours did not break the period; but 2\ hours at a minimum of 72 percent did. Two critical periods were found necessary Aquila, have become accepted into commer- for epidemic spread. Johannes' critical period refers to conditions in cial practice on an appreciable scale, although the crop; its relation to conditions in screens has not yet been deter- these two varieties have not retained their mined.

106 TABLE 65.--Blight incidence in relation to weather at Braunschweig, Western Germany, 1952-54

Rainfall Temperature Average and Blight year attack June July hig. Sept. June July Aug. Sept.

In. In. In. In. ix- :2- Long-term average... 2.4 3.3 2.9 2.2 61 64 63 57 Difference from average for— 1952 , -.2 -2.3 -1.4 + .2 -2 +1 +2 -4 Slight. 1953 , +.2 -1.1 -1.0 -.7 +2 +1 0 0 Severe. 1954 , -1.0 +4.5 +1.7 +1.1 0 -5 -1 +1 Late.

periods. One of the main troubles with these principally in the north of the country, where new rules, however, is that they are very air humidities in July and August are highest. difficult to apply at the practical level of a But the present system of blight surveying is large-scale warning service to farmers. Dr. not sufficiently precise to define epidemics in Ullrich is continuing with the work" on this particular years satisfactorily. It is also very problem that was started by Johannes at difficult on the basis of present data to esti- Braunschweig. The need for blight forecasts mate blight losses, but there is st:»n)ngevidence varies in the different parts of the country. that in recent years these losses have been out- In the north of Germany (Schleswig-Holstein balanced by increased potato yields due to the and the north of Lower Saxony) a routine annual additional rainfall in the blight years. Chemical spray program is the general rule, so that there methods of control are generally employed in is little need for a blight forecasting service. the north, but to a muchless extent in southern In mid-Germany, it is usually a problem to Germany. Apart from the smaller need to spray know whether to spray or not, and blight fore- in the south, the mechanization of spraying is casts would be most valuable. Blight occurs greatly hindered by the small size and awkward infrequently in southern Gernaany, and a serv- layout of the holdings. Potato varieties showing ice is less necessary. a high degree of field resistance to blight occupy a large part of the West German potato SUMMARY: WESTERN GERMANY acreage. The acceptance of Solanum demis sum hybrids into commercial practice reflects It is possible to define fairly well the bad pioneering German work in the search for blight areas in Western Germany. These lie blight-resistant potato varieties.

AUSTRIA The greater part of Austria is mountainous Average yield is about 6 tons per acre (fig. 48) and includes the eastern extension of (F.A.O., 1955). Hunrian consumption at some- the Alps. Arable farming is carried on mainly what over 200 lb. per head each year accounts in the lower land of the east of the country, for less than half the total potato production, which is also the driest area (fig. 49). Climatic and there is a large usage for fodder and the conditions vary considerably over short dis- manufacturing industry. tances, according to altitude, but in the east the Short notes on the occurrence of blight are climate is typically continental. About 400,000 included in an annual account of plant diseases acres of potatoes are grown annually. Acreage and pests given in Pflanzenschutz Berichte, figures by province in 1952 (Austria, Statisches For the year 1951 and those following, a short- Zentralamt, 1953) were: hand notation of describing blight epidemics according to their severity and distribution Province: 1,000 acres ^^^ ^^^^ adopted (see Schreier, 1957). There Vienna 12.4 is a 1 to 4 scale for severity (1, negligible; Lower Austria 198.3 2, medium; 3, severe; 4, very severe) and a Upper Austria 82.3 1 to 3 scale for distribution (1, local; 2, Burgenland 29.9 fairly extensive; 3, general). Table 66 shows Styria 48.4 the description by this method of blight epi- Carinthia 28.4 demies in Austria during the period 1951-56. Salzburg 7.2 Thus, the epidemic in 1956 was severe and Tirol 9.6 fairly extensive. No measure of the epidemics Vorarlberg 3.5 in 1952 and 1954 was given. Dr. Beran, to whom we are indebted for the following notes, Xotal 420.0 has told us that no special blight investigations 107 AUSTRIA

í , / /\. r / \' MU' LOWER •Ünz J^Vicnna ^ UPPER I AUSTRIA ...<) f \ .'AUSTRIA

/ \ I f I- / S T Y R I A TIROL • Innsbruck / -BERG/ SALZBURG •'

.--^N.^ .V ^i.^- v^. ( TIROL •. C A R I N T H I A

10 o miles ^ ^ f 4yN

FIGURE 48,--Key map of Austria and altitudes. are in progress at the present time. The serious in years of above-average rainfall. special problems of the disease under alpine The attack varies greatly, according to variety. conditions have, however, been extensively in- Early and midseason varieties are most se- vestigated in the neighboring country of verely attacked, those commonly grown being Switzerland, notably by Hänni (1949) and by Erstling, Sieglinde, Saskia, Oberambacher Aebi (1956), who has employed progress curves Frühe, Bintje, and Bona. American varieties and used Beaumont criteria in fore casting. introduced after 1946 proved to be very sus- Blight is of economic importance in the whole ceptible, and this was one of the main reasons of Austria, but in the eastern parts, which are why they quickly went out of cultivation. Under the principal potato-growing areas, it is only normal weather conditions, loss in yield in the

108 . Vienno 656 ft TABLE 66. --Blight incidence in Austria, 1951-56 RAINFALL Inches Year Blight epidemic^

1951 3/3 1952 1953 3/3 1954 1955 3/3 6,- Innsbruck 1956 1,968 fr. 3/2

RAINFALL Inches First figure indicates severity and second figure distribution of disease. (See p. 107.)

In the west, it has been necessary for some years to spray against the Colorado beetle, and between 10 to 20 percent of the potato area is also sprayed against blight. Spraying against the beetle-has only recently become necessary FIGURE 49.—Long-term average monthly rainfall and mean tempera- in the east; there, not more than 5 percent tures in relation to potato cropping for two centers in Austria. of the growers take action against blight. It blight areas ranges from 5 to 20 percent, is principally the earlier varieties and valuable according to variety. seed crops that are sprayed. Haulm killing is The proportion of growers spraying against not practiced as a control measure against the disease is greater in the west than the east. tuber infection. POLAND Although relatively small in total area, is 22.3 inches, but there is a marked summer Poland comes third among the countries of the rainfall peak, and the wettest months of the world in order of potato production. (The year are July and August. The mean temper- U.S.S.R. comes first, and Germany second.) ature at Warsaw in July, the hottest month, Nearly 7-1/2 million acres of potatoes were is 65 OF, (fig. 50). According to an analysis of grown each year over the period 1948-52, and survey data on blight incidence dating back to these produced a yield of about 5 tons per 1926, sent to us by H. Dubniak of the Polish acre (F.A.O., 1955). Potatoes are grown Ministry of Agriculture, the areas of Poland throughout the country; they form a major where blight attacks are most frequent and item of human diet and are also grown ex- severe are those where the average August tensively for industrial purposes and animal rainfall is 80 mm. (3.2 inches) or over. fodder. These **blight" areas can be divided into three About 70 percent of the potato area is main regions: (1) A mountainous region in the planted with varieties showing a high degree south of the country; (2) the center of Poland, of field resistance to blight. Examples of these where blight occurs along the river valleys; are Ackersegen, which occupies about half of and (3) the northern region, which is sub- the total potato acreage, Wohltmann, Switez divided into an eastern and western zone. The Dolkowski, Silesia, and Lenino. About 10 map (fig. 51) shows the approximate boundaries percent of the area is made up of varieties of these different regions. The losses caused that are highly susceptible to blight, such as by blight vary in the three regions. The great- Early Rose, Alma, and Flava, but these are est losses occur in the north, where blight principally early or midseason varieties and attacks are earliest and most frequent; in the so normally escape the worst effects of blight southern mountainous region, the attacks are epidemics. The remaining 20 percent of the generally late. potato acreage is occupied by varieties showing During recent years, the worst attacks have an intermediate degree of resistance, e.g.. been in 1930, 1931, 1934, 1942, 1949, 1952, Mittelfrühe. The potato-growing season is and 1954. This gives a frequency of *'severe" generally from early May to the end of Septem- blight years of about 1 year in 4. In such ber. years, the disease is present throughout the The climate of Poland is predominantly whole country. From the point of view of continental in type, though the maritime in- blight epidemiics, 1955 was an exceptional year, fluence of the Baltic Sea is felt in the north owing to an abnormal distribution of rainfall. In of the country. Average annual rainfall through- that year, the attack was relatively slight in out Poland is relatively low; at Warsaw, it the north, but severe damage was caused

109 POLAND

RAINFALL Inches ,1 """ i"'n''•1'' ' '.i\ "' ",.'.I..!."„: ,,',ir. iin;!""',::^, ■""":-'li".!'^

'••'.'"iMl

: II i'",' 'i' FIGURE 50.--Long-term average monthly rainfall and mean tempera- DefolioMon per cent tures in relation to potato cropping for Warsaw, Poland. II 0-20 POLAND 20-50 ".I BALTIC SEA I"'! ',:'^-'"i''""i I - ^//A^V^ •I ..'.'-. I" ' ''-I, , 'if '-!]•••' •" '•■111.'":

j^-r--, 7'%\ FIGURE 52.--Extent of defoliation by blight at end of growing period on Ackersegen, Poland, 1955. most o\fectcd yy-^'.^y ç-^'' j by bliqht ,r TABLE 67. --Defoliation of Ackersegen at end of , \ »Warsaw August, 1954 and 1956, Poland }/ Ackersegen crop >^-i Percentage of defoliation by blight iA- at end of August

None., 0-10.. 10-40, 50..., FIGURE 51.--aight areas of Poland, according to survey data for 1926-55. variety is around the end of September, so that approximately 3 to 4 weeks growth will have through the center of the country, in areas been lost. On the other hand, in 1956, over where it normally has little effect. Figure three-quarters of all Acker se gen potatoes were 52 shows the zoning for blight damage in 1955. still unaffected by blight at the end of August. The damage was measured by the amount of The relationship between premature defoliation defoliation caused by blight on the variety and loss of crop yield has not yet been worked Ackersegen toward the end of the growing out, but it has been estimated that in "blight" season (the exact time is not specified). It is years, crop loss from this cause falls between of particular interest that the survey informa- the limits of 10 to 30 percent. Dubniak points tion is available to enable the Polish Ministry out the difficulty of using average potato growth of Agriculture to produce a map of the type figures to estimate blight losses in particular shown for 1955, which can form the basic years. Thus, in 1955, when planting was late material for the assessment of countrywide because of a cold wet spring, losses due to blight losses. blight would tend to be overestimated. Blight The normal time for the development of seldom causes widespread or heavy losses in epidemics is during the second half of August. Poland due to tuber infection. Widespread attacks of the disease earlier or Research on Phy t ophtho r a inf e s t an s is later than this are out of the ordinary. A very carried out at the Laboratory of the Institute of rough comparison of the amount of defoliation Cultivation and Acclimatisation of Plants at caused by the disease on Ackersegen in a Gdansk Wrzeszcz. This includes work on the '*blight" year (1954) and in a year when the biology of infection, the aggressiveness of par- disease was slight (1956) can be made from ticular biotypes, the length of the incubation figures provided by H. Dubniak. These are period, and the relation of time of infection by shown in table 67. the fungus to the chemical composition of In 1954, the 50 percent blight defoliation potato leaves. Breeding of new potato varieties stage was reached in general at the end of resistant to blight is also in progress at this August. The normal time of maturity for this Institute.

110 UNION OF SOVIET SOCIALIST REPUBUCS With an area of over 8-1/2 million square miles, the Soviet Union is the largest connected land mass under one central government in the world. The estimated population in 1954 was about 215 million (F.A.O., 1956). Potatoes are one of the country's principal crops and over the period 1948-50, 31 percent of the total world production of potatoes was in the U.S.S.R. This compares to 5 percent of the total world production in the U.S.A. Apart from ware and animal feed, potatoes are grown extensively as an industrial crop, particularly for the manufacture of alcohol, much of which is used in the synthetic rubber POLAND : WHITE industry. Figures relating to human consump- ■.■' RUSS I A tion of potatoes are not published, but the crop forms a large part of the diet. From 1934 to 1938, the average area under potatoes was about 7 million hectares (17-1/2 million acres) with an annual average production of about 74 million tons. This means that the average yield of potatoes was around 4 tons per acre and it has continued to remain at about this figure. The potato-growing area has been considerably enlarged since the Second World War, owing to the incorporation of the Baltic Republics into the Soviet Union, and also the eastward and northward extension of potato growing. Under the fifth Five-Year Plan, the potato area was FIGURE 53.—Key map for European Russia, showing July isotherms. to reach 10 million hectares by 1955 (Oxford inches. The wettest parts of the country are in Economic Atlas, 1956). Transcaucasia, adjoining the eastern seaboard of the Black Sea (at Batum, average rainfall DISTRIBUTION OF POTATO-GROWING AND is 93 inches per annum), and parts of far CLIMATIC CONDITIONS eastern Russia, where areas are influenced by the summer monsoon winds. Over most of the The principal potato-growing areas are in country winters are severe and the length of European Russia, west of the Urals. Large the winter in the more northerly axeas sets a acreages are grown in the Baltic Republics, limit to successful potato cultivation. White Russia (Belorussia), and the Moscow In the main potato-growing areas of European Province. The northern Ukraine is an impor- Russia, annual rainfall is between 18 to 25 tant potato region. Roughly speaking, the 70°F. inches per annum. At the upper limits, rainfall isotherm for July (fig. 53), which divides the is generally adequate, as there is a marked Ukraine and continues in a northeasterly di- summer rainfall peak and the months of maxi- rection across European Russia, marks the mum rainfall are July and August. This is shown southern limit of intensive potato production. in the climagrams for Leningrad and Moscow In the north, latitude 60^ N., which passes (fig. 55). Long droughts however, do occur. At through Leningrad, fixes the approximate the lower rainfall limits, the crop may fre- northern limit of intensive potato culture, al- quently suffer from drought. The lower rainfall though small quantities of potatoes are grown areas are toward the south and have higher above this, even as far north as the Arctic summer temperatures, so that the rate of evap- Circle (fig. 54). oration of soil water is increased. Russell East of the Urals, the most important potato (1942) says **In the black soil region (of the areas are in western Siberia, and a scattered Ukraine) potatoes are grown on the flat, not on acreage of the crop is grown through the Si- ridges; this would cause too much los s of water berian Plain, and eastwards as far as the by evaporation, and also loss of soil by wind vicinity of Lake Baykal. Relatively small erosion.'* Despite the trend for rainfall to de- acreages of the crop are grown in the far east crease and temperature to increase in a of the Soviet Union, with some potatoes in the southerly direction, there is a remarkable area north of Vladivostock, and in the island general uniformity in the weather con- of Sakhalin. ditions of European Russia, due to the topog- Rainfall is generally low in Russia. In few raphy, which is that of a great undulating parts of the country does it exceed 25 inches plain with no mountains except at the per annum, and over vast areas it is below 10 edges.

Ill U.S.S.R.

y^Vladivostok ; 2000 miles

Each dot represents 100,000 tons. Heavy lines indicate potato qrowinq areas.

FIGURE 54.—Potato production in the U. S. S. R. Data are based on the Oxford Regional Economic Atlas: U.S.S.R.and Eastern Europe. 1956. (By special permission, Oxford University Press.) Lorkh (1945) has given data for the length of medium-late) varieties; Wohltmann (late) is the potato-growing season (May to September) grown for industrial use and animal fodder. in the Moscow Province, over the 8-year While the early varieties are normally highly period, 1934-41. This averaged 147 days from susceptible to blight, the late varieties gen- the time of planting till the first killing frost in erally show a fair degree of field resistance. A the autumn, with a range in time of 126 to 176 number of Solanumdemis sum hybrids have been days. Thus, there is normally ample time for put on the market. These include Kameraz, the maximum crop of potatoes to be formed. Hybrid 42, andPhytophthora-Resistant. Details In areas northof Moscow, the decreasing length of the principal potato varieties grown in the of the growing season begins to be a limiting U.S.S.R. have been given by Zaitsev (1950 and factor to potato yields. Outside European 1953). Most of the potatoes in the more northerly Russia, the generally low rainfall and the parts of European Russia are grown on sandy usually short growing season for potatoes are soils, while in the Ukraine, they are grown on unfavorable to the crop. In all regions, potatoes black organic soils. A feature of Rus s i an farm- are grown for early summer, autumn, and ing is the large amount of hand labor that is winter use. In the main regions of cultivation, still employed, and this applies to potato they are used for ware, animal feed, and in- growing, including planting and lifting. A recent dustrial purposes; in the remaining areas, development has been the planting of potato they are used mainly for ware. Seed potatoes crops on the square, with from 24 to 28 inches are grown in all potato-producing regions. between rows and plants. Before this, most of the crop was grown in 24 inch ridges, with POTATO VARIETIES,SOIL CONDITIONS, AND approximately 18 inches between plants. CULTIVATIONS DISTRIBUTION, FREQUENCY, AND TIMING Both Russian and foreign potato varieties OF BLIGHT EPIDEMICS are grown. Common early varieties are Epron, Epicure, and Early Rose. Berlichingen is a Potato blight is most damaging in those widely grown mid se as on potato, and Lorkh is parts of European Russia where conditions arc one of the most important maincrop (or most favorable to potato growing, i.e., in the

112 U.S.S.R. TABLE 68. --Incidence of blight in the . Leninqrod Leningrad Province, 1946-55

RAINFALL 5 Inches Severe Slight Negligible

1946^ 1950 1947 19482 1955 1951 1949^ 1952 1953^ 1954^

^ First appearance of disease: Early August. First appearajLce of disease: End of July.

In the severe blight years (1 year out of 2) in the Leningrad area, the disease normally appears at the end of July or early August, although it may sometimes occur earlier than this. Dr. Polykov mentions that in 1954 (a severe blight year), early varieties were completely defoliated by the disease at the end of August, and midseason varieties were _ Saratov dead by the middle of September. This is un-

RAINFALL likely to have resulted in a large loss of yield Inches through premature defoliation, as the times mentioned coincide fairly closely with the normal ripening times of the two classes of potato in the area. But there is evidence that destruction of the potato haulms by blight does occur earlier than this. Kameraz (1954) has stated that in 1953, on many farmis in the Leningrad Province the tops of maincrop as well as early varieties had been destroyed by mid-August. This will have resulted in about FIGURE 55.--Long-term average monthly rainfall and mean tempera- 4 to 6 weeks* loss of growth inmaincrops with tures in relation to pota to cropping for three centers in the U. S. S. R. a consequent heavy fall in crop yield. Data on Baltic Republics, White Russia, the Moscow the progress of blight, other than in 1953 and and Leningrad Provinces. In a southerly and 1954, are not available. easterly direction from this region, the importance of the disease diminishes, although BLIGHT LOSSES outbreaks continue to occur as far east as Rozhestvenski (1934) estimated the average western Siberia« In the southern parts of the annual loss of potatoes in the Soviet Union as U.S.S.R., attacks are infrequent, and in the 10 percent of the crop, due to attack by blight. Central Asian republics and southern This figure included losses from premature Kazéikhstan they do not occur at all. In some of defoliation, together with tuber blight losses. the potato-growing areas in the far east of the The same worker also gave estimates of the Soviet Union, as on the Island of Sakahlin, variation in losses from blight between dif- blight occurs regularly every year. ferent potato-growing regions, and his miap, Some idea of the frequency of severe blight setting out these variations, is reproduced here years in the Moscow Province can be obtained as figure 56. Although Rozhestvenski*s esti- from data given by the Research Institute of mates should be treated with reserve, as they Potato Growing (Kartofelnoe Khozyaistvo, 1946) were not based on systematic survey informa- for the 17-year period 1918-34, during which tion, the estimated varying losses between time there were 7 *'blight'* years. This is a regions appear to reflect, with reasonable frequency of 1 in 2-1/2 years. In this area, accuracy, the differing importance of blight in blight appears about the end of July to early the main potato-growing areas of Russia. Thus August in **severe" years, but information on the disease is most damaging in the regions the subsequent developmient of epidemics is immediately south and southeast of Leningrad. lacking. Table 68 sets out data on the inci- The Baltic Republics were not then part of the dence of blight in the Leningrad Province during Soviet Union, so are not zoned on the map. the 10 years 1946-55. This and much other It is not possible to state with any accuracy information for the U.S.S.R. has been provided the relative importance of premature defolia- by I. Polykov of the All-Union Research Insti- tion and tuber blight losses, although Dr. tute of Plant Protection at Leningrad. Polykov has told us that, for the Soviet Union 113 quently sxxffers from drought. Even in the higher rainfall potato regions of European Russia, drought is a problem in some years, and the average rainfall figures conceal a considerable variation in rainfall from year to year. It has been estimated that losses from blight are highest in the Leningrad region. This does SVERDLOVSK not have the highest summer rainfall, but summer humidity conditions are higher there than in the Moscow region, and summer temperatures are lower. Also, tuber blight infection at harvesttime is a greater potential threat in the Leningrad region than in areas farther south, owing to the shorter growing period for l2-l5«»/o 7-IO*/o potatoes. Although the long-term average tem- am 2-äX perature and rainfall figures for Moscow ODESSA might lead to the s uppo s it ion that blight attacks r J'^r^-^ are unlikely to be serious very often in this area, such an interpretation is too superficial. FIGURE 56.—Average annual losses from potato blight in U.S.S.R. Apart from other considerations, it ignores (After Rozhestvenski, 1934.) the variation in rainfall from year to year, which is illustrated by figures for the Moscow as a whole, the two problems are of about province during 1935 and 1938, shown in table equal importance. That heavy losses brought 69. These data have been given by Lorkh about by early defoliation of the potato plants (1945). do occur has been shown for the Leningrad The survey information is not available to Province in 1953, but how frequently this link the progress of blight epidemics in par- happens is not known. The available literature ticular years with weather conditions in the also gives little help in deciding to what extent Moscow Province. However, it seems likely tuber infection with blight prior to lifting is a from the rainfall figures alone, that in 1935 a problem; but the indications are that infec- early defoliation by blight in the area will have tion at lifting is a more serious source of loss. caused heavy yield losses; whereas in 1938, In most of the potato-growing regions of the the potatoes will have suffered severely from U.S.S.R the harvest period is short. Over drought. How frequently a year like 1935 occurs large areas, e.g., Leningrad region and western is not known. In the potato-growing regions to Siberia, the short harvest period will be com- the south of Moscow, represented by the clima- bined with a relatively short growing period, gram for Saratov, blight losses are estimated so that in many years there will be a con- as being relatively small. Apart from the low siderable likelihood of tuber blight infection rainfall of these regions, mean summer tem- occurring at harvesttime, unless steps are peratures are high, with a large diurnal range, taken to deal with blighted haulms. and the long growing season minimizes the risk of tuber infection at harvest. In general BLIGHT AND WEATHER DATA it would appear that, throughout European Russia, the problem of drought to the potato The climagrams for Leningrad, Moscow, and grower is very much greater than the problem Saratov (fig. 55) should be considered in rela- of blight. tion to figure 56. The general similarity in the weather conditions of the several areas of CONTROL MEASURES European Russia is brought out by the three climagrams. Summer temperatures increase It is characteristic of the Russian approach from north to south; in the south low summer to agricultural problems that in the control of rainfall and a high rate of evaporation often blight strong emphasis is placed on the sprout- limit potato yields, and the crop very fre- ing of seed tubers before planting. A main aim TABLE 69.--Average rainfall for Moscow Province, 1935 and 1938

Year and average July August September Total

Inches Inches Inches Inches 1935 5.6 3.9 4.8 14.3 1938 .6 .3 1.0 1.9 Lone"-term average • 2.8 2.9 2.2 7.9

114 behind this is to speed up the growth cycle of that the number, range, and variety of forms of the potato crop so as to avoid or minimize the a particular plant increase toward the locality effects of blight attacks. The process is in which it has originated. Between 1925 to 1932, usually referred to as **vernalization/* and Vavilov, Bukasov (1933), and Juzepczuk made normally requires the provision of heated and extensive collections of wild Solanum species lighted buildings. This is a major obstacle to and, as early as 1931, the variety, Phytoph- the widespread adoption of vernalization, par- thora-Resistant, a Solanum demis sum hybrid ticularly when regarded in relation to the that is still being grown, had been produced. In severe Russian winter. Arnautov and Novikov their breeding for blight resistance, the Russian (1946) have described a method of sprouting workers up to the present time have relied potatoes in open trenches, which has been suc- mainly on S. demis sumí x S. tuberosum hybrids. cessfully employed at the Research Institute To what extent they are still hopeful of tackling for Potato Growing at Korenevo in the Moscow blight control through breeding is not clear. Province. The potatoes were placed in the A recent account of potato breeding, including trenches during the second half of April. At blight resistance work in the U.S.S.R., has been night and during daytime frosts, the potatoes given by Hawkes (1957). were covered with straw or mats. Whether Little is known of methods used in the this method is at all widespread in commer- U.S.S.R. for the forecasting of blight and the cial practice is not known. It obviously re- issue of blight warnings to growers. Reference, quires a large amount of hand labor, but this however, should be made to a paper by Nau- is not likely to be a limiting factor to its mova (1935), in which the author states that adoption in the U.S.S.R. at present. The ad- field observations and controlled experiments vantages of drying tubers immediately after carried out during 1933 and 1934 in the Lenin- harvest as a prophylactic measure in the grad area confirmed the rules of Van Ever- control of blight has been discussed by Bor- dingen (1926) relating to the appearance of dukova (1947). She places considerable em- blight epidemics. Further experiments were phasis on the large quantities of potatoes that conducted by Naumova to ascertain the effect could be saved by adopting this measure, and of temperature on the incubation period of the this suggested practice indicates that a good blight fungus. These experiments took into deal of tuber infection must occur at lifting. account diurnal variation of temperature. The The development of chemical or mechanical results were used to construct a nomogram methods of haulm destruction might be a val- from which the incubation period could be ob- uable alternative approach to the problem. At tained for various minimum, mean, and maxi- the present time the chief method is hand mum temperatures during the days following cutting. infection. According to Dr. Polykov, spraying and dusting against blight as protective measures SUMMARY: U.S.S.R. are not widely practiced in the U.S.S.R. The potato crops most commonly sprayed are early Potato blight is most serious in those parts and midseason varieties grown for seed. of European Russia where summer rainfall is Bordeaux mixture is mainly used, although highest. Even in these areas, however, drought some copper oxychloride is used. Dusting is is a more serious actual and potential menace done with '*AB," a powder containing chalk, to the potato crop. There is an almost complete gypsum, and approximately 36 percent copper lack of evidence on which to base reliable loss carbonate. estimates, but the shortness of the growing The Russians were the pioneers in sending season for potatoes in some areas has made expeditions to Central America and South tuber infection a serious problem. In control, America to collect potato species possessing emphasis has been on an ** es cape*' method disease resistance and other valuable economic (vernalization) as opposed to chemical meth- properties for use in their breeding work. The ods. The use of chemicals is largely confined principle applied in this case was the to the valuable seed crop. The Russians were geographical method of N. I. Vavilov, then the first to realize the potentialities of wild director of the Institute of Plant Industry at Solanum species in breeding for blight resist- Leningrad. This method is based on the fact ance. CYPRUS

Potato growing in Cyprus exemplifies con- Morocco, parts of Spain, and southern Italy, is ditions under a Mediterranean climate. This is a very similar one. Two crops of potatoes are characterized by a summer drought period grown in the year, both under irrigation. The with high temperatures. The pattern of potato first crop is grown during the spring and early growing in the countries bordering the northern, summer months; the second during the late southern, and eastern shores of the Medi- summer, autumn, and winter. The period during terranean, which include Greece, Israel, Egypt, the naiddle of the summer is unsuitable for

115 potatoes owing to the high temperatures, and Famaqusfa in most of the countries, particularly in the north of the area, there is a period in the winter when conditions are too cold for potatoes. Cyprus is the third largest island in the RAINFALL Mediterranean Sea and has an area of 3,572 Inches square miles (fig. 57). About 13,000 acres of potatoes are grown each year, and the greater part of this crop (about 60 percent) is exported,

S O CYPRUS SPRING CROP WINTER CROP

FIGUEIE 58.-Long-term average nionthly rainfaU and mean temperatures in relation to potato cropping at Famagusta, Cyprus.

tures. Only two potato varieties are grown commercially: Up-to-Date and Ar ran Banner, of which Up-to-Date is the more important. Both these varieties are very susceptible to blight. Accounts of potato growing in the island are available (Cyprus, Department of Agri- culture, 1939; and Littlejohn, 1947). A feature of farming in Cyprus is the small size and fragmentation of the holdings. According to Allan (1956), the 1946 census of the island showed that the average farm consisted of 13 plots of an average size of 1.4 acres. These plots are often widely scattered and are commonly many miles apart. This is a great hindrance to improved farming methods, including blight control measures.

IMPORTANCE OF POTATO BLIGHT

Nattrass (1937) records that blight was first found in Cyprus on a potato crop near Famagusta in April 1931. The disease was FIGURE 57.--Cyprus, in relation to eastern Mediterranean, believed to have been introduced in Irish seed. G. P. Georghiou has informed us that blight principally to the United Kingdom. The average appears more frequently and is usually more yield per acre is approximately 4 tons (F.A.O., severe in the main potato-growing areas in 1955). The most important potato-growing the southeast of the island. He says that it areas are in the plains (up to 750 ft.) in the is worse on the spring crop of potatoes. Over Famagusta and Nicosia districts, where there the 7-year period 1950-56, blight was re- is a light red soil (terra rossa). Figure 58 corded as **severe'* on the spring crops in shows the two cropping periods during the 1952, 1955, and 1956, and on the winter crop year, and these are of about equal importance. in 1951. Heavy yield losses sometimes occur The first crop is planted in February and lifted in the spring crop, which may be reduced by principally in May and June; this is the spring up to 25 percent. In blight years the disease or summer crop. The second--the winter usually appears when the plants are about 8 crop--is planted in August and matures about inches high, and there may be 75 to 90 per- the end of November. Larger yields are gen- cent defoliation about 1 month before the erally obtained from the spring crop, as more normal lifting time. Tuber infection is not irrigation water is available at this time. About much of a problem and is believed to occur 50 percent of the spring crop is planted with mainly at lifting. An analysis of recent severe certified Irish seed, whereas the winter blight years in relation to average annual crop is planted with once-grown seed potato yields in Cyprus appears to show up from the spring crop of the same year. a connection between low yields and blight Dormancy troubles do not appear to arise at years (table 70). Without further analysis, this time, no doubt due to the high tempera- however, any such relationship should be

116 TABLE 70. --Average yield and blight crop, temperatures are favorable to the dis- occurrence in Cyprus, 1950-56 ease until about the end of April; after this, conditions begin to get too hot. Thus pre- Year mature defoliation is the main problem, with Average yield Blight tuber infection of little importance. In the winter crop, temperatures are generally too Tons/acre high for blight until the last month of growth 1950. 4.1 (November). Chambers (1952) records that 1951. 3.6 Severe^ winter. in 1951 an attack of late blight occurred at 1952. 3.5 Severe, spring. the end of the autumn growing season and 1953. 5.4 caused tuber infections, which resulted in 195^. 5.4 severe losses. 1955. 4.3 Severe, spring. No work has been done on the humidity 1956. 3.7 Severe, spring. conditions associated with blight attacks on the spring crop. There is no evidence that the channel-type irrigation practiced has any viewed with extreme caution. Both potato effect on blight. Table 71 shows blight incidence crops in Cyprus are grown under irrigation, in relation to spring rainfall at Famagusta and the amount of irrigation water available during 1950-55. The figures have been obtained is likely to have a far greater effect on yield from the island's Annual Reports, which un- than the presence or absence of blight. In ad- fortunately do not include temperature data. dition, time of lifting has a pronounced effect In 1952 and 1955, the blight years, March on yield, and in Cyprus this is very much rainfall was about twice the average for that under the influence of a varying export de- month, but in 1950, when blight was of little mand for potatoes. or no importance, March rainfall was even A study of the long-term average weather higher. The high rainfall in April 1953 did data in relation to the growing periods of the not lead to severe blight. On the basis of two potato crops at Fama gusta (fig. 58) ex- table 71, there does not appear to be any plains the greater importance of blight in very close connection between total rainfall the spring crop. Temperature appears to be during the spring and blight attacks for this an important controlling factor. In the spring important potato growing center in Cyprus. TABLE 71.--Monthly spring rainfall and occurrence of blight at Famagusta, Cyprus, 1950-55

Rainfall Year Blight January February March April

In. In. In. In. 1950 2.4 0.3 3.0 0.5 1951 3.7 3.2 .4 1.9 1952 4.4 3.1 2.7 .2 Severe. 1953 1.8 1.6 1.8 3.3 1954 1.3 2.2 1.3 1.0 1955 .7 .2 2.7 .3 Severe.

Lonß^-term average 3.8 2.6 1.3 .8

G. P. Georghiou has told us that control harvest. The growing of varieties showing a measures against blight are rather limited. high degree of field resistance instead of Copper and dithane fungicides are used, but the blight-susceptible Up-to-Date and Ar ran only by about 5 percent of the commercial Banner might go a long way toward control. growers* Applications are made by knapsack However, finding the right variety is not easy. sprayers. In the spring potato crop, control Commenting on the performance of Majestic appears to resolve itself mainly into pro- (the principal maincrop variety in England tecting the crop over a relatively limited and Wales) in variety trials held in Cyprus, period, after which temperature conditions Littlejohn (1947) says, **Majestic, which gave are likely to become unfavorable to the fungus. consistently high yields, always developed a The problem in the winter crop seems to be serious fault in that the tubers cracked badly tuber infection at lifting, which could be pre- and tended to deteriorate very rapidly in vented by removal of the potato haulms before storage."

117 NORTH AMERICA

UNITED STATES OF AMERICA The potato acreage in the United States was siderable differences between the average just short of 1^ million acres in 1953. This is yields for the different States; the highest less than half the area that was grown in 1930, being around 360 bushels (10 tons) per acre Although there has been this marked decline in in Maine (United States Department of Agri- the acreage, total potato production has re- culture, 1954). mained at about the same level, owing to increased efficiency in cultural practices. POTATO DISTRIBUTION Production has not kept pace with increasing population, and a marked feature of the Amer- ican potato industry is the falling demand for Figure 59 shows the distribution of potato potatoes in competition with other vegetables. growing in the United States. A relatively few At the present time the total production of States produce the bulk of the crop, and of f»otatoes is between 350 to 400 million bushels these, the most important, in order, are Maine, approximately 10 million tons) per annum, Idaho, California, and New York. The greater and consumption is around 345 million bushels. part of the early crop comes from the Southern Demand for potatoes is relatively inelastic, States and California. Some certified seed and almost all the crop is sold for human con- potatoes are produced in over half the States, sumption. According to Smith (1956), the an- but the chief seed-growing States are Maine, nual consumption of potatoes per head in the North Dakota, and Minnesota. Maine is by far United States is about 104 lb. the most important of these; half the total The average yield for the whole country U. S. seed production comes from this State over the period 1942-51 is given as 190 bushels alone. The acreages of potatoes grown in some (5.1 tons) per acre. (1 U. S. bushel of potatoes of the leading States in 1953 are shown in weighs approximately 60 lb.). There are con- table 72.

Each dor = lOOO acres

HGURE 59.--Distribution of potato growing in the U. S. A,, 1953. (Based on Neg. U. S. D. A. 46360 and U. S. D.A. Agricultural Statistics, 1954.)

118 TABLE 72.—Acreages of potatoes in leading red-skinned varieties. Thus, in Maine, where States, U.S.A., 1953 white-skinned varieties are favored, the acreage is made up largely of Katahdin; but in State Potatoes North Dakota the predominant varieties are the red-skinned Red Pontiac and Triumph. A full description of American potato varie- 1,000 acres ties has been given by Clark and Lombard Lame 156 (1951). Idaho 153 California 124 SOIL AND CLIMATE New York 106 North Dakota... 94 Potatoes are grown principally on mineral 'vlinnesota 78 soils and to some extent on muck soils (re- Pennsylvania... 62 claimed peats) and other organic soils, such Wisconsin 61 as the prairie soils in the Red River Valley. ivächigan 53 Climatic conditions vary considerably in the Colorado 54 different growing areas. In the South the North Carolina. 46 winters are relatively mild. In Florida, for Florida 42 example, the main planting of potatoes takes place in December and January. These crops are lifted by the end of May; during the height Despite the very large area over which of the summer, temperatures are too high potatoes are grown, production within States for potato growing. Moving northward up the tends to be concentrated in relatively small eastern side of the United States, winters growing centers. Examples of these are the become progressively more severe and plant- eastern part of Aroostook County, Maine; Long ing of the potato crop correspondingly later. Island in New York State; the Red River Valley In the Northeastern States, average winter in parts of Minnesota and North Dakota; and temperatures are well below freezing, and Kern County, Calif. planting cannot take place before the middle of May. Summer tennpe rature s are relatively POTATO VARIETIES low and rainfall is high, providing ideal growing conditions for potatoes, but the growing American potato varieties are usually clas- season is short, as frosts start to occur from sified as early or late. The early varieties, about the middle of September onward. In the such as Irish Cobbler, correspond to the long- North Central States, winters are severe, and day types of temperate regions; the late va- the growing season is about the same length rieties are day-neutral or short-day types. as in the Northeastern States. Summer temi- Stevenson (1949) stated that 13 varieties (Told peratures are considerably higher, however, and 6 new) constitute about 96 percent of the and rainfall is generally adequate. In the total U. S. potato acreage. These varieties Western States, lack of rainfall is the prin- are shown in table 73. cipal factor limiting potato production, and Since 1949, there has been a decline in the most crops are grown under irrigation. In popularity of some of the older varieties, parts of California, potatoes can be grown notably in Green Mountain. The acreage of all the year round, but in the States farther Red Pontiac has increased, and Kennebec, a inland low winter temperatures occur, and late, blight-re sis tant variety, has become it is possible to produce a summer crop considerably more important. only. Some varieties are grown in relatively re- An account of potato growing conditions and stricted areas. An example of this is Red methods in the major growing areas of the McClure, which is grown principally in the United States has been given by Dykstra and San Luis Valley of Colorado. Again, there Reid (1956) for the South; by Lombard, Brown, may be a preference for white-skinned or and Dykstra (1948) for the Northeastern and

TABLE 73. --Most important potato varieties and percentage of total U. S. potato acreage, 1949

Early varieties Percent Late varieties Percent Late varieties Percent

Irish Cobbler 15 Katahdin 28 Red McClure 2 Triumph 13 Rasset Burbank... 7 Pontiac 2 White Rose 9 Chippevm 7 Red Pontiac 1 Bed Warba 1 Green Mountain... 7 Russet Rural 1 Sebago 3

119 North Central States; and by Edmundson, beetle, have to be dealt with by spraying or Schaal, and Landis (1951) for the Western dusting in most potato areas. Farmers are States. An important feature of potato growing much more aware of the need for spraying is the short growing season, which in most and spray very much more frequently than in districts does not exceed 120 days* Europe. Large storage houses, very often built par- POTATO CULTIVATIONS tially into the ground, are used over a great part of the United States to keep the potatoes Generally speaking, potato growers in the during the winter months. U. S. A. tend to be specialists, and as such are well equipped technically and mechani- POTATO BLIGHT: REGIONS CHOSEN cally to deal with the crop. Much greater FOR STUDY emphasis is placed on the mechanization of every job to do with potato growing than in A great deal of work on many aspects of European countries. For example, the table the epidemiology and control of potato blight stock (ware) crop is universally planted by has been carried out in the United State s under machine • a wide range of growing conditions for the The potato seed is cut prior to planting. potato crop. An intensive study has been made The rows are wide (36 inches), and the seed of the effects of the disease in four widely pieces are planted closely in them (10 to separated potato growing areas, among which 12 inches), although there is variation from there are marked variations in the incidence these figures, depending on variety, soil, or of blight and in the problems that the disease climatic conditions. Almost all crops are poses. The areas chosen were, Aroostook grown on the ridge. The wide rows enable County, Me; Hastings Section, Fla.; Red River interrow cultivations and spraying to be carried Valley, Minn, and N. Dak.; and South Fork, out with less damage to the plants. Particularly Long Island, N. Y. is this so in the case of spraying; also guards Following the discussion on these particular are fitted to the tractor and sprayer wheels to areas, an account is given for the United States lift the haulm. A wide range of potato pests, as a whole. The location of places named in including flea beetle, leaf hopper, and Colorado the text is shown on the key map (fig. 60).

U.S.A.

FIGURE 60.—Key map of United States of America.

120 Aroostook County, Maine pAroosfook County CARIBOU According to Simpson and Shands (1949), 12 to 15 percent of the total potato production of RAINFALL the United States is grown in Maine, and of Inches this 85 to 90 percent comes from Aroostook County. The farming land in Aroostook County lies in a narrow belt along the eastern and northeastern part of the State, adjoining the Canadian border. For the past 30 years, Aroostook has led the United States in potato production. About 130,000 acres of potatoes are grown annually, producing average yields of around 10 tons per acre. Aroostook is principally a late ware-producing area, and only a few early varieties (5 to 10 percent) are grown, some of these for RAINFALL seed. About 25 percent of the total acreage is inches planted for certified seed production. Schrumpf (1953) states that the chief variety grown is Katahdin, which over the last 15 years has largely replaced Green Mountain. Katahdin shows some resistance to blight, whereas Green Mountain is highly susceptible, both in the foliage and the tubers. The variety Kennebec, which has the Ri gene for blight resistance, is also widely grown in the area. FIGURE 61.--Monthly rainfall and temperatures for Aroostook County Maine, U. S. A,: Above, long-term averages; below, ''blight*' and The potato soils are loams. Of the culti- **no blight" years, vated land 60 percent is Caribou loam, which contains a high percentage of sand. Potatoes and from the second week of October, there are also grown on Washburn loam, which is is always the likelihood of severe damage a heavier soil and less well drained. from freezing to tubers left in the ground. Climatic data for Caribou in central Aroos- Thus there is an annual race against time took are shown in figure 61, which may be at harvest, and end-of-season growth of many taken as representative of the whole area. crops has to be sacrificed to insure success- Annual rainfall is between 30 to 35 inches. ful harvesting of the whole acreage. Summer temperatures are cool and rain is usually abundant and well distributed, naaking Frequency of Blight Epidemics for ideal potato-growing conditions. Winter temperatures are well below freezing. The Hyre and Bonde (1955) have described the growing season is about 120 days. Planting severity of blight epidemics in Aroostook takes place from mid-May to early June, and County over the period 1943-55. Their de- lifting from the second week in September to scriptions are set out in table 74, together the second week of October. After the middle with the average yields of potatoes in Maine of September, killing frosts are likely to occur. from 1943 to 1953. TABLE 74. --Severity of blight epidemics and average yields of potatoes in Aroostook County, Maine, 1943-55

Severe Moderate Slight

Year Yield Year Yield Year Yield

Bu./acre Bu./acre Bii./acre 1943 355 1948 3S5 1944 268 1945 261 1946 358 1947 350 1949 465 1950 4Ö0 1952 360 1951 445 1953 370 1954 1955

121 These blight assessments have been based yield of potatoes has been the same in the two on the results of specific surveys plus general sets of years. observation. They refer to the total effect of the disease in a particular season and not to Blight on Haulm defoliation alone. During the period 1943-53, potato acreages Blight seldom occurs to any extent in Maine have fallen, and there has been a general rise before the end of July. The months in which in yields due to improved cultural practices. the first recorded field outbreaks have been The average yield over the years in this made in Aroostook County from 1943-55 are period, when blight has been described as shown in table 75. This has been compiled *'slight" has been 359 bushels per acre, and from material appearing in the Plant Disease in the 5 years when the disease has been Reporter and supplied to us by R. Bonde. The *'severe," the average yield has been 361 years in which the disease has been assessed bushels. Thus, for all practical purposes, the as *'severe" are indicated.

TABLE 75. --Time when blight was first recorded and years when severe, Aroostook County, Maine, 1943-55

July August September Blight outbreaks outbreaks outbreaks generally absent Year Severe Year Severe Year Severe

1944 1943 X 1947 X 1948 1945 X 1946 1949 1952 1954 X 1950 X 1953 1951 X 1955

In 7 years out of 13 the disease was either TABLE 76.--Amount of defoliation and per- generally absent or its effect was slight. In 2 centage of total fields (2,160) affected, years (1943, 1947), when the disease was de- Aroostook County, Maine 1954 scribed as ''severe," Cassell (1944a) and Bonde and Schultz (1949) have said that the Percentage of main damage was caused by tuber infection. Amount of defoliation In 4 years out of 13 (1945, 1950, 1951, 1954) (percent) total fields there were fairly heavy losses due to premature defoliation. In the same years tuber Percent blight infection was also heavy. The 2 years 0 39 9 (1945, 1954) that damage to potato tops by < 1-5 17 0 blight was the worst were both years of July 5-25 outbreaks. An early outbreak date, however, 18 8 25-75 10.5 is not necessarily followed by severe de- 75-100 foliation losses, and this is shown in 1944. 13.8 In that year, a field outbreak was recorded on July 8, but according to Cassell (1944b) it This is intimately bound up with the very did not develop, owing to the onset of hot dry intensive spray program against the disease, weather in early August. which is followed by practically all growers Very full information on the amount of de- in the area. foliation during the 1954 season is available from a survey carried out in late August and early September 1954 and reported by Bonde Blight Losses Due to Premature Defoliation (1955b). Defoliation in this year was probably the heaviest of any year in the 1943-55 period. The growing season is short (120 days), The results of the survey of the whole potato and the growth curve (fig. 62), adapted from area are summarized in table 76. Akeley, Stevenson, and Cunningham (1955), Generally, the fungus appears relatively of potatoes in Aroostook County is correspond- late in Aroostook, and rarely causes complete ingly steeper than in areas where potatoes defoliation of potato plants on a large scale. have longer to grow. These workers have

122 summarized the results of 3 years of vine- When these figures are related to the growth killing trials at Aroostook Farm, Presque curve of potatoes in the area, they indicate no Isle, held between 1950 and 1952, in which yield losses due to premature defoliation on they have given details of potato yield in- the sprayed plots. In 3 years out of 6 (1952, creases during the later stages of growth. 1953, 1955), losses on the unsprayed plots An extract from these results shows that in due to premature defoliation will have been the 4 late varieties (Sebaso, Katahdin, Kenne- negligible. In 1950 and 1951, there will have bec, and Green Mountain), approximately 45 been about a 15 percent yield loss, and in 1954, percent of the final yield was formed after almost a 40 percent loss on the unsprayed mid-August, and approximately 15 percent plots. It may be concluded that premature after early September. defoliation caused by blight at Aroostook Farm, U.S.A. which may be taken as representative of the whole area, is not a serious problem on the sprayed crop, but blight will cause heavy losses in unsprayed crops very frequently. ÜO In the area as a whole, the only data on which an approximate yield loss due to prennature defoliation can be based are the survey figures for the bad blight year of 1954, already 80 referred to in table 76. This survey took place over a period of 2 weeks, August 26 to Sep- tennber 10, and during this time the blight 60 / position in individual fields will have changed / very radically. If it is assumed, however, that very much the sanne state of affairs would have been revealed if the whole survey had 40 been carried out on one day, Septennber 2, which is the mean of the two end-dates, the following loss from blight would occur:

20 1 „ .. lO 20 30 9 19 29 Defoliation of AUGUST I SEPTEMBER Percentage of haulm by blight FIGURE 62.—Mean rate of bulking curve for late varieties at Aroos- total number on Sept. 2 Yield loss took Farm, Presque Isle, Maine, 1950-52. of crops (percent) (percent) Figures have been supplied by Dr. Bonde that give the dates at which the 75-90 percent 9 90 - 100 30 blight defoliation stage was reached on the 9 50 - 90 15 unsprayed potato plots at Aroostook Farm 25 5 - 50 10 over the period 1950-55 (table 77). 57 0 - 5 0 TABLE 77. —Percentage defoliation due to blight on unsprayed and sprayed potato plots, Aroostook Farm, Maine, 1950-55

Date of 75-90 percent defoliation Defoliation on sprayed plots Year on unsprayed plots at lifting

Percent 1950 Aug. 31-Sept. 2 (^) 1951 Sept. 3 1 1/2 1952 Sept. 8^ 1953 Sept. 10^ 195^ Aug. 20 5-10 1955 Sept. 14

^ Trace. ^ Only 20 percent defoliation at lifting. Over the whole acreage, this works out as centage of salable tubers in the sample. This an average loss of yield, due to premature is particularly so, if defoliation occurs very defoliation, of 6.5 percent. Since this is an early. Stevenson, Akeley, and Webb (1955) approximation, it would be better to state it give figures for a trial with artificial inocula- as a loss of 5 to 10 percent, with a reasonable tion held in 1954 at Aroostook Farm. An ex- certainty that average losses did not exceed tract from their results is shown in table 78. 10 percent. A crop, maturing normally, produces about The effect of premature defoliation, in ad- 95 percent United States No. 1 tubers. In a dition to lowering yield, is to reduce the per- highly competitive market, such as that in 123 TABLE 78. --Date when 90 to 95 percent defoliation from blight was reached in a potato trial and yield of U. S. No. 1 tubers, Aroostook Farm, Maine, 1954

Date when 90-95 percent U.S. No. 1 grade tubers Variety defoliation reached in sample

Percent / Aug. 13 35 Green Mountain \ Aug. 20 69

/ Aug. 13 51 Katahdin , \ Aug. 20 Ö0

the United States, it is usually only the No. 1 Blight and Weather Data tubers that the farmer is able to sell. Early defoliation also reduces the percentage of The mean air tennperature and rainfall solids in the tubers. This, however, is not figures for Caribou in Central Aroostook, a simple cause-and-effect relationship, as together with the approximate planting and the percentage of solids is also intimately lifting periods of potatoes in Aroostook County bound up with seasonal weather effects. are shown in figure 61. The low winter tem- I>eratures insure that in most years '*ground- Blight in Tubers keepers" are killed, and therefore cannot act as a possible source of blight inoculum. Occasionally, however, heavy falls of snow The largest losses in Aroostook County before severe weather conditions set in enable from blight appear to be the result of rot some **self-sets" or **groundkeepers" to sur- following tuber infection with the fungus. vive. Sumnner temperatures are cool. Neither Cassel (1944a) estimated an overall loss of low nor high temperatures during the growing 10 percent of the 1943 crop in storage due season are factors likely to hinder the de- to blight. Large losses from the same cause velopment of blight. Hyre and Bonde (1955), were observed in the 1944 crop by Cassel in a survey of weather data for a series of (1944c) and in the 1945 and 1947 crops by non blight and blight years over the period Bonde and Schultz (1949). Bonde and Robinson 1902-53, concluded that at no time did high (1950) reported that considerable loss from temperatures limit the effect of blight. Seven- blight tuber rot occurred in some fields in day mean temperatures were below 75 ^F. Aroostook in 1949. throughout the years. Tuber infection may take place in the soil Although there is a broad correlation be- before lifting, and this occurs to a greater tween rainfall and blight epidemics in the extent in the Washburn loam soils, which are area (e.g., 1954, a wet year, was a bad blight usually less well drained than in the Caribou year; and 1953, a dry year, was generally loam. Bonde (1955a) reported that blight in- free from blight), many other factors, infection in Washburn loam soils may be ac- cluding source of inoculum, need to be con- centuated by primary infection of the tubers sidered. In 1956, rainfall in June, July, and with powdery scab, Spongospora subterránea. early August was high, but blight appeared According to Bonde, there is generally less late in the area, and this was ascribed to than 1 percent tuber blight at lifting time in lack of inoculum following a year (1955) the unsprayed plots at Aroostook Farm, which largely free from blight. Hyre and Bonde is on Caribou loam. Cassel (1944c) attributed in drawing a **critical rainfall line" took a some of the loss in the 1944 crop to the heavy cumulated rainfall of 11.25 inches over the rain preceding and during a hurricane of Sep- period June 2 to September 15 as the base tember 14, which washed fungus spores into above which blight was likely to be severe. the soil and into contact with the tubers. The rainfall and temperature figures for 1954 However, Bonde and Schultz believe that (a blight year) and for 1953 (a no-blight year) most of the rot found in Aroostook is from are compared in figure 61. infection that occurs while the crop is being It is impossible to study true weather-blight harvested. The combination of a short grow- relationships in this area, because, apart from ing season and a generally late attack of unsprayed control plots at Aroostook Farm, blight makes the Aroostook farmer partic- all potatoes are sprayed. Only in years when ularly vulnerable to this. The advent of haulm rainfall is so heavy that growers cannot get killers during recent years has not entirely through their crops with sprayers is a severe removed the danger, as heavy rains over the blight epidemic likely to develop. Such a state short harvest period may wash off the mate- of affairs occurred to some extent in 1954, rials and thus to some extent nullify their when there were 15 rainy days in June, 22 in effect. July, and 19 in August. 124 Control Measures A spray warning service for Aroostook County was started in 1931, and reviews of It has been considered for some time that possible methods on which to base a fore- potato cull (or dump) piles are a very im- casting service for potato blight in the area portant source of primary blight infection in have been made by Hyre and Bonde (1955) and Aroostook County. Bonde and Schultz (1943), Hyre (1955). It is now proposed to introduce a as the result of extensive work carried out forecasting service to growers based on mov- between 1935 and 1942, concluded that "the ing graphs derived from rainfall and tempera- cull pile is the chief source of ... infection.'* ture data. A certain amount of circumspection These cull piles occur beside potato storage has to be followed in issuing spray warnings houses and merchants' warehouses. Over the and blight forecasts, as there is always the last 10 to 15 years an intensive program has danger of frightening off the buyers of the been conducted to get farmers to clear them seed crop, which forms a very important up, but a survey carried out by Webb and part of the total potato acreage. Bonde (1956) shows that this has not been It was at first thought that the growing of entirely successful. One of the chief methods blight-resistant varieties would provide an of dealing with the cull pile is to spray it with answer to the problem of the disease in this a haulm killer early in the growing season. area. The results of promising trials with the Some form of protective spraying against variety Kennebec in 1949 and 1950 have been blight is followed by all growers in the area published by Akeley, Lombard, and Stevenson every year. The first sprays are put on in (1952). It was soon found, however, that spe- early- to mid-July (before the crops start to cialized races of the blight fungus were pres- meet in the drill), and spraying is continued ent and that these were capable of attacking at weekly to fortnightly Intervals, depending Kennebec and other blight-resistant varieties. on weather, up to lifting tinne. On ave rage, 6 to The presence of 11 races of blight in Aroos- 7 protective sprays are put on, but under pro- took County has recently been shown by Webb longed conditions of blight weather, up to 12 and Bonde (1956). Kennebec, which is widely sprays are given. grown now receives a series of protective The use of organic fungicides (nabam plus sprays, similar to that given to other varieties. zinc sulfate, for instance) is universal. There Potato haulm killing, to protect the tubers are two nnain reasons for this: (1) They are at lifting, is now general. Farmers start cheaper than the copper spray materials, and killing the haulm, beginning with the seed as practically all growers do their own spray- crops, in the first week of September. Some ing, the cost of materials is one of the prin- growers still rely on frosts to kill the haulm, cipal items in the total cost; and (2) there are while others use roto-beaters. But the tops no phytotoxic effects from the use of organic are chiefly killed chemically with sodium fungicides. The effect of copper in a **no arsenite, and little trouble from vascular blight" year can depress yield as a result of discoloration of the tubers follows the use scorch injury. In a spraying trial reported of this nnaterial. Some crops are killed me>- by Stevenson, Akeley, and Webb ( 1955), carried chanically by roto-beating. out at Aroostook Farm in 1953, the yield of U.S. No. 1 tubers for 6 varieties was depressed Summary: Aroostook County an average of 8 percent by spraying with basic copper. Blight does not cause heavy yield losses The main disadvantage of using organic due to premature defoliation of the potato fungicides lies in their poorer adherence to plants, except occasionally, as in 1954. Even the potato foliage; as a consequence they in 1954, little or no blight occurred in the need to be applied more frequently than the majority of fields toward the end of the grow- copper sprays. ing season; where large losses occurred, In general, potato crops are sprayed from they fell on the individual grower rather than the top only; droplances are not used to any on the area as a whole. That this is the general extent. Since 1950 the use of low-volume state of affairs in severe blight years is spraying has very much increased. Peikert illustrated by a comparison of average yields in and Bonde (1954) have reviewed trials with severe blight years (361 bushels per acre) low-volume machines carried out during the with yields in slight years (359 bushels). seasons, 1950-53. The normal rate of ap- Tuber infection in the soil is not likely to be plication is about 40 U. S. gallons per acre. heavy, except on some of the poorly drained At the present time approximately 7 0 percent fields. Tuber infection at lifting time, how- of the farmers in Aroostook County are using ever, has in the past been a serious problem, low-volume spraying. About half the antiblight due to the short growing season and short sprays are combined with treatments, prin- harvest period. The use of vine killers should cipally DDT, against pests. The effect of these now greatly reduce this source of loss. antipest sprays is to prolong the growth of the The universal spray program with organic fungicides has an undoubted effect in reducing potato tops.

125 yield losses as a result of early defoliation. U.S.A. But the fact that severe blight years only .Hostings Secfion occur once in 2 years and the results from JACKSONVILLE the unsprayed plots at Aroostook Farm must lead one to question the need for such an in- RAINFALL f, tensive program. One of the aims behind Inches spraying has been to keep the foliage as completely as possible free from blight, to prevent tuber infection at lifting. As a result, heavy tuber losses would not occur when potatoes were lifted. Protective spraying continues up to lifting, but the need for the last one or two sprays has now gone. At the present time, the spray program does not appear to have FMAMJ J ASOND adjusted itself to the changed conditions brought about by the use of baulnn-killing methods. . BLICHT YEAR 1944 NO BLIGHT YEAR 1951

Hastings Section, Fla. ■ 80 RAINFALL ^ - 70° Inches The Hastings section, situated in north- 60 eastern Florida, is the oldest and largest potato-growing area in the State. In 1952, 50 potato plantings totaled about 16,000 acres. Yields vary considerably from year to year, but on average, are about 190 bushels (5 tons) per acre. *'New crop" (or early) potatoes are produced, chiefly for sale outside Florida, from the end of March to early June. During Jan F M A M J F M A M this tinne, the crop has to meet connpetition from the stored potatoes of the Northeastern and Midwestern States and the early varieties .Red River Valley coming from other Southern States. The bulk CRAND FORKS of the crop is planted with seed from the

Northern and Eastern States. The variety RAINFALL 4 Sebago accounts for 95 percent of the acreage. Inches Details of potato growing in the area have been given by Brooke and Spurlock (1950) and Eddins, McCulbin, Ruprecht, and Steven- son (1950). The soil is principally Bayboro-Bladen fine sand and fine sandy loam, and many of the fields FMAMJ JASON D^ are rather poorly drained, s o that waterlogging of the soil is not uncommon. Drought conditions BLICHT YEAR 1948 NO BLIGHT YEAR 1955 are also likely to occur during the growing season; most farms have artesian wells, which 70V y^^ - 70 F RAINFALL 60 60 can be used for irrigation. The climate is sub- Inches \- tropical, and potatoes are grown under short- Á-nrYZ~/ I//J 50 day conditions. There are considerable dif- ^ ferences in rainfall from year to year. Thus, V/ in March 1945 the rainfall was 0.23 inches, -'//, V/, and in March 1948, 9.47 inches; the April -//. AV. '//. V/ rainfall in 1942 was 0.28 inches, and in April 1947, 5,97 inches. Very heavy rainstorms VÄ'A/MA^ ^ % ^ ^ May J J A s iM J J A S may occur: In 1944, 3 inches of rain fell within 2 hours on April 3. The crop is also liable to frost damage, but very high temperatures do not usually occur, except occasionally at lift- FIGURE 63.--Monthly rainfall and temperatures for Hastings section, ing time when there may be sunscald of the Fla., and Red River Valley, Minn, and N. Dak.: Above, long-term tubers. Average rainfall and temperature averages; below. **blight" and "no blight** years. figures for the area are set out in figure 63. The growing season is 90 to 100 days, and Blight Epidemics: Frequency and Losses crops are dug before they are mature. Plant- ing takes place from December 20 to February A.. H. Eddins has summarized the damage 1, and lifting from April 1 to May 15. caused by blight, and the dates of the first

126 field outbreaks of the disease, over the period defoliation plus loss from tuber infection. 1931-55. These data are set out in table 79. The various categories of damage, together **Damage*' in this table refers to the esti- with their equivalent in percentage loss of mated reduction in yield due to premature crop and the number of years out of 25 (1931- TABLE 79. --First field outbreaks of blight and amount of damage to potatoes, Hastines section, Fla., 1931-55

Year First field report Damage Year First field report Damage

1931. Apr...., Moderate. 1944. Mar. 1. Severe. 1932. Jan. 28, Do. 1945. None..., None. 1933. Feb. 5, Severe. 1946, Mar. 13. Moderate. 1934. Do. 1947. Jan. 25. Do. 1935. Apr. 6, None. 1948, Feb. 25. Do. 1936. Mar. 15. Moderate. 1949, Feb. 15. Do. 1937. Jan. 2, Severe. 1950, Apr. 20, Little. 1938. Mar. 1, Moderate. 1951. None.... None. 1939. Mar. 8, Do. 1952. Apr. 16. Moderate. 1940, Apr. 15, Little. 1953. Mar. 20, Do. 194-1. Mar. 15, Moderate. 1954, Apr. 9. None. 1942, Apr. 1, Do. 1955, May 1. Do. 1943, May...., Little.

55 period) that each has been recorded are as in part due to a high percentage of blighted given in table 80. tubers in the seed, which had come from the Estinnated losses caused by blight are seen Eastern States. to be small. In summarizing losses in potato Very early field outbreaks are the excep- crops in Hastings from 1932 to 1943, Eddins tion, however, and the disease is usually first (1943) estimated an average annual loss of 4.4 noted in March or April. This will be seen percent, and since that time losses have from the summary set out in table 81, where tended to decrease. outbreak dates have been arranged, by nnonth, There is no point in comparing potato yields for the period 1931-55. in this district for *'blight" and **no blight'* years. Large fluctuations in yield occur from TABLE 81. --Months in which outbreak year to year, due to drought, flooding, and occurred and frequency during the period frost damage. Economic considerations play 1931-55, Hastings section, Fla. a big part in deciding when potato crops are lifted in an early district, and the time of Outbreak occurring in— Years' outbresLk: harvesting will in turn have a marked effect reported on yield.

Blight on Haulm Number January 3 Blight may attack the crop throughout the Febioiary 3 growing period. A severe attack, which started March 7 soon after planting in 1937, has been described April 7 by Eddins and Clayton (1943). The attack was May 2 No blight , 2 TABLE 80. --Rating for '^damage*' as related No data (1934-), 1 to loss of crop and number of years each type of damage occurred, Hastings section, There is no general association between Fla., 1931-55 the earliness of the first outbreak and the amount of damage done to the potato haulm. In 1947, for example, sharp frosts followed the initial outbreak on January 25; potato tops were killed, and the disease did not reappear until mid-March. The course of the attack in None.... this year has been described by Eddins (1947). Little.. Dr. Eddins has also supplied figures show- Moderate ing the amount of defoliation caused by blight Severe.• in his spray plots at Hastings over the period 1949-54 (table 82).

127 TABLE 82. --Amount of blight on haulm at lifting time, Hastings section, Fla., 1949-54

Blight on unsprayed Blight on nabam sprayed Year plots at lifting plots at lifting

Percent Percent 1949 100 7.5 1950 13.8 0 1951 0 0 1952 15.0 (M 1953 57.5 (^) 1954 0

^ Trace. In only 1 year (1949) out of the 6 did the 63. An account of an unusual and very early sprayed plots show a significant increase in attack by blight in 1937, given by Eddins and yield over the unsprayed, and serious defolia- Clayton (1943), has already been mentioned. tion is the exception. Sometimes, à prolonged A week after planting, the disease was found period of rain after the disease has become in seed pieces, and a high proportion of these established prevents farmers from taking rotted in the ground. Many of the sprouts that protective action, and then widespread severe were produced were killed by blight before defoliation may occur. This happened in 1944. or shortly after emerging. Stands were reduced 10 to 20 percent in most of the fields Blight in Tubers in the Hastings area, and in individual farms up to 90 percent of the plants were lost. The Overall losses from blight infection of disease continued to attack the plants after tubers are not serious. According to Dr. emergence and became very destructive in Eddins, *'Tuber infection during recent years some fields. Rainfall, although not particularly has caused little loss, as the foliage has heavy in January 1937, was well spread out, been kept almost free of late blight. If rains and air temperatures were abnormally high occur during epidemics, and the foliage is (13*^ F. above average). In table 83 a com- infected, a trace to 10 percent of the tubers parison of the January figures for 1937 and may be infected." Only occasionally, as in the succeeding years up to 1945 brings out 1944 when heavy rains exposed the tubers, these points. does a serious amount of infection take place Rainfall and temperature data for 1944 (a while the potatoes are still in the ground. blight year) and 1951 (a no-blight year) are Infection at lifting is not usually severe. If compared in figure 63. The course taken by the potato tops are blighted at this time, the blight in 1944 has been described by Eddins conditions of lifting play an important part. (1944) and by Rhoads (1944). The season was When the land and potato tops are wet, in- favorable to growth until March 21, at which fection is likely to be heavy, but if the fields date a little blight was general through the are dug under dry conditions little loss should occur from tuber blight. TABLE 83.--Number of rainy days, total The crop is washed and dried before ship- rainfall, and mean temperatures in January ment for sale. However, the effect of this on at Jacksonville, Fla. , 1937-45 the development of tuber blight has not been determined. No figures have been published Total Ufe an Year Rainy days on the amount of infection developing during rainfall Temperature transit of potatotes from the Hastings section. That such development in transit can be a serious problem in early potatoes has been Number Inches Op. shown by Ramsey (1942), who found that con- 1937 12 2.60 68.1 signments of Florida potatoes inspected on the 1938 9 1.17 56.5 Chicago market in January 1940 frequently 1939 5 1.59 58.2 showed 5 to 10 percent tuber blight. 1940 7 1.61 47.4 194-1 11 3.18 56.2 Blight and Weather Data 1942 6 1.30 55.1 1943 4 1.62 59.4 Average temperature and rainfall figures 1944 7 3.16 56.7 for Jacksonville together with planting and 1945 7 3.93 57.6 lifting dates for potatoes, are set out in figure

128 fields. During the last 10 days of March, Summary: Hastings 5.43 inches of rain fell, followed by 3.16 inches in a 2-hour period on April 3. Blight Blight does not cause a heavy average loss. spread rapidly during this time, and by April Even in the absence of control measures (as 20 the plants in nearly all the potato fields shown by spraying trials at Hastings), there had been killed by the disease and by water was a serious loss of yield due to the disease damage. During and after the rain, it was in only 1 year in 6. It appears that farnners impossible to get into the crops to protect in the area consider the expense of a regular them against blight and the growing season in annual control program necessary to insure most fields was reduced by 2 to 3 weeks. In themselves against occasional serious loss. 1951, the no-blight year, rainfall was below Even so, the control program broke down in average for each of the 5 months, January to 1944 under conditions of prolonged rain. May. The weather conditions associated with Red River Valley, Minn, and N. Dak. severe blight losses appear to be a fairly prolonged period of heavy rains, but this The Red River, which is formed by the happens only exceptionally. Eddins, Ruehle, juncture of the De Sioux and the Otter Tail and Townsend (1946) have stated that blight Rivers just North of Wahpeton, N. Dak., is at Hastings **has caused maximum losses the boundary between Minnesota and North when the rainfall has ranged between 5.56 and Dakota. It flows north for 533 miles and 8.31 inches during the last 30 to 40 days of eventually discharges into Lake Winnipeg in the growing period." Temperature conditions Manitoba, Canada. The width of the valley are generally favorable to blight development varies considerably along this length and in during the growing season, though frosts, places it is 200 miles wide. which may occur as late as March, have been There were 78,000 acres of potatoes in known to kill off the haulm and completely Minnesota in 1953, and 94,000 acres in North stop an epidemic (as in 1947). It is possible Dakota (U.S.D.A., 1954). Separate acreage that the short-day conditions under which the figures are not published for the Red River crop is grown, leading to heavy dew fornna- Valley, but in both States the valley is the tion, play a part in blight development in most important potato area. Average potato this area. yields are below 190 bushels (5 tons)per acre. About 40 percent of the potatotes are grown for seed. Red-skinned varieties are pre^ferred, Control Measures and the chief variety is Red Pontiac. The soil is a black, organic sandy loam; the better Protective measures are taken every year soils are on the land nearest the river. The by the majority of growers. According to whole area is very flat and drainage is a major Brooke and Spurlock (1950), most growers problem, particularly as short, very heavy in 1949 were not equipped to spray against falls of rain may occur during the growing blight, and were using dusts as a control. season. The mean annual temperature and Since then, there has been an increase in rainfall figures for Grand Forks, N. Dak., in spraying as opposed to dusting, and a change the center of the valley, are shown in figure from copper dusts to organic fungicides. In 63. Planting takes place from the end of 1956, the use of nabam sprays with zinc sulfate April to the end of May, and lifting is in was general. The danger of copper dusts September and early October. The full grow- causing scorch injury to potato foliage in dry ing season is about 120 days. seasons with a consequent reduction in crop yields has been pointed out by Eddins and Frequency of Blight Epidemics; Extent of Clayton (1943). Spraying starts when the Losses plants are 6 to 10 inches high, and 3 to 6 sprays are given during the growing season. Over the 10-year period 1946-55, blightwas This regular annual spray program is also a serious in only 1 year. This was in 1948, but feature of the **new crop" potato growing even then, damage due to the disease was areas in Florida and Alabama. According to confined to the northern part of the valley Dykstra and Reid (1956), control measures and did not occur to any extent south of Grand are more haphazard in other Southern Forks. In a series of fungicide trials carried out during the 6-year period, 1945-50, at 3 State s. Haulm killing before lifting is not carried centers in the North Dakota half of the valley, out. It is difficult to see how this could become blight was present only in 1948, and then in a regular practice in an early area, as the no more than trace amounts. The results of early grower cannot afford to wait the 2 weeks these trials have been summarized by Wallin between top killing and lifting, which is neces- and Hoyman (1954). In the Minnesota side of sary to prevent tuber infection. The market the valley, Buchholtz (1948 and 1949) has for his crop may quite easily disappear while given reports of spraying trials on Bliss Triumph in 1947 and 1948. In neither year he is waiting. 129 was there any significant increase in yield are shown in figure 63. The progress of the as a result of spraying. disease in 1948 has been described by Eide A feature of the disease in this area is and Brentzel, quoted by Miller and O'Brien its usually late first appearance. Wallin and (1948). Eide (Minnesota) stated that **during Hoyman (1954) have given outbreak data for July 30-August 3 blight was found scattered the North Dakota part of the valley. The in an area about 7 5 miles long and 25 or less earliest occurrence on record was in the wide, extending from a short distance south latter part of July in 1952. In 1944 and of East Grand Forks to north of Kennedy. By 1948, blight was reported on August 3 and 2, August 11-17, it was present practically over respectively. During the period 1949-53, the the entire area. By September 1, it had killed disease was first observed in the last 2 weeks the vines in many fields, being especially of August or the first weekin September. First severe around East Grand Forks." Brentzel outbreaks usually occur some little time (N. Dak.) noted that the weather was favorable earlier in the Minnesota part of the valley for the spread of blight during the end of July than in the North Dakota part. and early part of August, but after that it was Loss of yield through premature defoliation unfavorable for the spread of the disease. In is not a serious problem in the Red River 1955, blight caused little or no damage. The Valley. There is, however, a danger that main difference between the 2 years was the severe infection of the tops in individual fields considerably higher rainfall in August 1948. may occur under certain weather conditions. The somewhat higher temperatures in 1955 The generally high day and the low night may have retarded the development of the temperatures during the latter part of the disease in this year. growing season lead to heavy dew formation, The factors operating against blight in this and if blight inoculum is present in a field area are the high temperatures, low rainfall, it will usually be able to survive. Hoyman low humidities, and rather short growing and Wallin (1957) have been working with season. These factors also operate against blight isolates from this area, which have large potato yields. been able to survive in leaves at considerably higher temperatures (over 100° F.) than those Control Measures at one time considered possible. In a field where blight **smolders'* along in this way, Dusting rather than spraying is carried out 3 or 4 days of cool, cloudy weather will give in the Red River Valley, owing to a general the disease the opportunity to develop rapidly. shortage of water. In the Minnesota part of the valley, dusting appears tobe more common Blight in Tubers than in North Dakota, where at the present time little is done. There was a marked reduc- Tuber infection is the main problem in the tion in the quantities of fungicides used in the Red River Valley. Infection in the soil is not area following the withdrawal of price supports usually important, though individual growers for potatoes by the Federal Government after may have heavy losses. Heavy losses are asso- 1950. ciated with the generally poor drainage condi- Dusting with DDT and Toxaphene is a tions in the valley. Reporting the result of a routine measure against insect pests, partic- survey carried out at harvesttime in 1948 (the ularly leaf hopper, in the valley. Early blight bad blight year), Cook and Lutz ( 1950) recorded (Alternarla solani). although present in most that **tuber rot was worse in the low wet areas years, is not considered a serious problem. than in the higher better drained parts of the Organic dusts are most commonly used against fields. In the low areas up to 100 percent of late blight and a high proportion of the appli- the tubers were affected with blight, and had cation is done by airplane. rotted. Only 1 percent or less of the potatoes Wallin and Hoyman (1954) consider that a were affected in the better drained areas." lengthy control progrann against blight is un- These low areas, however, form only a small warranted in the North Dakota part of the proportion of the total acreage planted to valley, and the results of spraying trials both potatoes. in North Dakota and Minnesota confirm this. Infection of tubers from blighted tops at The same writers, and also Wallin, Eide, lifting is the main source of loss. Even this is and Thurston (1955), have discussed the value not very important, owing to the general of a blight warning service and a method of absence, or negligible amount, of blight at basing this on an analysis of temperature and harvest and the generally dry conditions at humidity data. An effective warning service lifting. for blight would be of great value in an area like this, where the disease occurs only Blight and Weather Data occasionally. However, there are a number of factors operating against a service: (1) The Temperature and rainfall figures for the large size of the valley and fairly considerable growing season in a blight year (1948) and a variations in weather conditions within the no-blight year (1955) at Grand Forks, N.Dak., valley; (2) the difficulty in obtaining weather

130 data; (3) the difficulty of getting the forecasts coast. Seed comes from Maine and Prince out quickly enough to the individual growers. Edward Island. Soil type is a sandy loam, At the present time the forecasting service which tends to be low in organic matter. obtains its weather data from instruments op- Figure 64 sets out the mean annual tempera- erated by interested individuals (plant pathol- ture and rainfall figures for the area. Plant- ogists, growers, etc.) and not from the main ing extends from the end of March up to mid- Weather Bureau stations. These stations do April, and lifting, which starts in the middle not give sufficiently good coverage of the area of August, goes on through September into and many do not supply humidity figures. The October. problem of notifying growers is one of a small trained personnel that has to deal with a . Lonq Island potato crop thinly scattered over a wide area. SOUTHAMPTON One factor that operates in favor of spraying

according to a warning service as opposed to RAINFALL 5 routine spraying is that growers are already Inches carrying out an annual control program against insect pests. Thus, the equipment is available to control blight in the odd year that conditions naake it necessary. Hoynman (1947) has discussed the value of different haulm killing methods under Red River Valley conditions. Most haulm destruction is now carried out by roto-beating, and it has been found that this does not cause vascular discoloration of the FIGURE 64.--Long-term average monthly rainfall and mean tempera- potato tubers, unless there have been drought ture in relation to the potato growing season for South Fork, Long conditions preceding killing. Island, N. Y.

Sumnnary: Red River Valley Frequency of Blight Epidemics Blight is not a serious problem. A routine The severity of blight attacks over the 10- control program against blight does not appear year period 1946-55 in the South Fork area of justified in the area, and many growers take Long Island has been summarized in table 84, no action against the disease. A blight warning The first field outbreaks of blight are not service would be of great value in the Red usually recorded before mid-July. Table 84 River Valley, but there are many difficulties shows that there was severe blight dannage in the way of this. In its absence, it naight pay in 1 year out of 10, and that in the remaining growers to put on 2 to 3 applications of organic 9 years the disease was either absent or fungicides in August as an insurance against caused very little damage. the odd blight year. Applications in July appear Overwintering of the fungus in this area unnecessary, in view of the generally late has been investigated by Peterson (1947). He appearance of blight, and September applica- concluded that infected seed pieces and cull tions would almost certainly be too late to piles were the chief sources of inoculum in have any beneficial effect on yield. the spring. Although diseased groundkeepers were readily found in grainfields that had produced a potato crop the year before, none South Fork, Long Island, N. Y. of them appeared to have been produced from In 1953, 55,000 acres of potatoes were grown diseased tubers. It was thus not possible on Long Island, a slightly higher acreage than to incriminate groundkeepers as inoculum that grown in up-State New York. The average sources. yield was 320 bushels (8-1/2 tons) per acre. Potato growing on Long Island can be divided Blight in Tubers; Blight Los se f roughly into 3 areas: (1) The western area, The amount of infection with blight taking which is rapidly being built up and has never place while the tubers are still in the ground been a bad area for blight; (2) the North Fork is usually from none to a trace. Skaptason, where almost all the potatoes are grown under Peterson, and Blodgett (1940) stated that the irrigation; and (3) the South Fork, where the high copper content of the soils in the district greater part of the acreage is grown without was probably afactor in inhibiting the germina- irrigation. Most of the work on blight under tion of the blight sporangia. The copper has Long Island conditions has been carried out in accumulated under conditions of continuous the South Fork area, which has, therefore, spraying with bordeaux mixture over a period been selected for more Intensive study. of years, as potatoes were very often grown In the South Fork, potatoes are produced on the same fields year after year. Tuber in- principally for table stock purposes. The chief fection at lifting is not a serious problem, varieties grown are K^.tahdin and Green owing to the longer growing season here than Mountain, with some Irish Cobbler along the in many other parts of the U.S.A. The potato

131 TABLE 84. --Blight incidence and source of data for South Fork, Long Island, N. Y. , 1946-55

Year Blight incidence Source of data

1946 Blight present but little or no United States Plant Disease damage. Survey (1946).

1947 Probably worst year for blight Miller and Wood (1947). ever recorded in the State.

1948 Blight present but loss practi- Miller and O'Brien (1948). cally negligible.

1949-55 Blight damage slight or nil. Information supplied by L. C. Peterson and confirmed for 1951-54 by Hyve (1955).

haulm has usually been dead a week or more, plots in this year was 27 percent below that before lifting. of the plots sprayed with bordeaux, and this The only recently published estimate of figure represents approximately the loss losses was given for the bad blight year of that blight caused in the trial. There was 1947. In this year Miller and Wood (1947) no significant increase in yield from spraying quote an estimate that there was an overall in 1948. loss in New York State of 20 percent of the potato crop. Fifteen percent of the loss was attributed to reduced yield as a result of Blight and Weather Data premature defoliation, and 5 percent to tuber Long-term average temperature and rainfall infection. data for Southampton, situated in the South Results have been given by Buchholtz (1948, Fork of Long Island, are shown in figure 64. 1949) of spraying trials against blight, carried Despite the high summer rainfall, blight is out at Sagaponack, South Fork, Long Island, severe only occasionally, and one of the main in 1947 and 1948. These have been set out reasons for this appears tobe the high summer graphically for 1947 in figure 65. This figure temperatures. Mean monthly temperatures at incorporates some additional material sup- Southampton in July and August are 70^ and plied by Lr. C. Peterson, showing the progress 69 F., respectively. However, this is not the of the blight on the tops of the treated and complete story; in 1947, the only bad blight the control plots. The yield of the unsprayed year during the 10-year period 1946-55, mean

U.S.A.

lOO M M i M I I i i Tons/a BLIGHT sprayings Unsprayed ON Unsprayed 7-7 HAULM Nabam+Zn 9'I per 50 Bordeaux 10-5 cent Bordeaux J o PLANTED i FTED MARCH May June July Auqust ^\ SEPT 23 3

FIGURE 6.5.-Progress of blight on haulni and corresponding yields of sprayed and unsprayed Green Mountain variety at Sagaponack, Long Island, N, Y.. 1947.

132 temperatures in both July and August were areas of the United States. The most favorable 2 to 3 above normal. The attack in 1947 areas for the development of the disease are seems to have been associated with very high also those which provide the best conditions humidity conditions. for potato growing, namely a relatively cool summer with abundant rainfall. These areas Control Measures are the New England States, we stern New York All commercial growers follow an intensive State, and Pennsylvania--in short, the north- spray program at South Fork. First applica- eastern region. tions are put on when the potato plants are In the remaining Eastern States, as far south 4 to 6 inches high. The initial sprays are as North Carolina, potato blight is not so combined with an antipest treatment, usually important. However, a distinction should be DDT. The most important insect pests are drawn here between the relatively low-lying the Colorado beetle, flea beetle, and potato land stretching in from the coast and the aphid. mountainous areas of some of these States. Most of the spraying is now done with In the mountainous regions of North Carolina organic fungicides. The organic fungicides are and West Virginia, for example, blight occurs not so efficient as copper when applied at the to some extent every year. The Southern same intervals, but there has been no serious States, which concentrate on the production of blight year since 1947. There may be danger a spring and early sumnner crop of potatoes of overcompacting the sandy soil in this area are only occasionally affected to a serious by the large number of spray treatments that extent. The two States from which the largest are commonly given. Some growers follow a losses have been reported are Florida and split spraying schedule, starting off with an Alabama. The importance of blight also falls organic fungicide and finishing with copper off in a westward direction across the country. over the latter part of the season, when the The disease can be severe in parts of some danger of blight is greater. Practically all the of the North Central States; e.g., Wisconsin spraying is at high-volume rates, and the and Minnesota, but high summer temperatures fungicides are applied from the top. If it is and relatively low rainfall linnit its develop- necessary to kill the haulm before lifting, the ment. In the arid and semiarid conditions of most popular method is with mechanical the Western States, where potatoes are grown beaters. Sodium arsenite and the di-nitro under irrigation, blight is not a serious compounds are used for chemical destruction. problenn, although it is sometimes reported. An account of control methods against blight The first report of Phytophthora infestan s has been given in a publication of the New York in the United States dates back to 1843, when State College of Agriculture (1955). Stevens (1933) states that it was found near A blight warning service is operated in the Philadelphia, but it is of comparatively recent State by R. S. Dickey, Plant Pathologist of occurrence in some States. Walker (1950) the New York State College of Agriculture writes that it was reported on potato for the first time in Florida in 1903; in California at Cornell University, Ithaca. This service works on the basis of first and later confirmed in 1904; in Texas in 1931; in Kansas in 1936; and in Colorado in 1941. The first year that reports of the disease, which are then sent blight developed so extensively as to become to county agents, who in turn inform the an econonnic problem in the North Dakota part growers. Approximately 95 percent of the of the Red River Valley was 1942, according potato growers are informed of the presence to Brentzel (1943). and progress of blight in this way. SEVERITY OF BLIGHT EPIDEMICS Summary: South Fork, Long Island In a country so vast as the United States, Only in 1 year out of 10 (1946-55), has with potatoes being grown under all kinds of blight caused serious damage in the area. It varying climatic conditions, generalized state- is unlikely that this has been mainly due to ments about the severity of blight on a year-by- the spraying program followed, as experience year basis mean very little. Within nnost States, in this and other parts of the United States too, conditions differ widely, and this has a indicate that a spray program may breakdown marked effect on the development of blight. under severe blight conditions, particularly In Minnesota, for example, which is roughly when organic fungicides are the chief materials the same size as Great Britain, blight was used. In this area, it appears that the amount severe in the Red River Valley in 1948, but of protective spraying could be reduced. was hardly found anywhere else. In 1952 in the same State, there was widespread infection BLIGHT EPIDEMICS IN THE U.S.A.: GENERAL with blight in south Minnesota, but only traces of the disease could be found in the Red River The preceding sections have shown that there Valley. are considerable variations in the importance Short accounts of blight epidemics from the of late blight in the different potato-growing different States were included with data on other

133 plant diseases over the period 1917-39 in METHODS OF BLIGHT ASSESSMENT Supplements to the [U.S.] Plant Disease Re- porter (Plant Disease Bulletin until 1923). In assessing the percentage of defoliation Supplements giving accounts of the severity caused by blight in fungicide trials, most of blight on potatoes and tomatoes, again on a U. S. workers now use the method described State-by-State basis, were published for each by Horsfall and Barratt (1945) and Horsfall of the years from 1946 to 1952. These reports (1945), which is based on a probit scale. In show the differing importance of blight for field survey work, however, nnany plant path- particular areas, which can be expressed as a ologists still prefer to describe the degree of frequency of severe blight years. Thus the blight infection in words, i.e., as *'trace," frequency of severe blight years in Aroostook "slight," ''moderate," or **severe." In the County, Maine, is about 1 in 2. Cook (1949) summer of 1956, an opportunity was taken of has summarized data for a number of States. comparing this latter method of assessment He gives the frequency of years in which the with the British method, using the B.M.S. disease has been important as 7 out of 20 in scale. This comparison was naade on numerous Vermont; 13 out of 34 in New Jersey; 5 out blighted crops in eastern and central United of 20 in South Carolina; and 2 out of 31 in States. Table 85 shows the approximate equiv- eastern Virginia. alents. SOURCES OF BLIGHT INOCULUM TABLE 85. --Descriptive designation of blight It is generally agreed that the most important attacks compared to B. M. S. scale for blight sources of blight inoculum are infected seed pieces and cull piles. In the Southern States, Designation of Equivalent percentage practically all the potato acreage is planted blight attack defoliation with northern-grown seed, and the degree to which this is infected with blight will play an important part in the development of epidemics Trace Up to 0.1 in the South. Peterson (1947) has investigated Slight. 0.1 - 1.0 the possibility of a number of weed and culti- Moderate 1-50 vated solanaceous plants acting as inoculum Severe Over 30 sources of the fungus under Long Island conditions. He concluded that **none of the perennial LOSSES FROM BLIGHT members of the Solanaceae found growing wild on Long Island function as overwintering Over the period 1917-39, estimated losses to suscepts for P« infestans.** There is no evidence that solanaceous weeds are important the potato crop caused by blight for each State in other parts of the United States. Similarly were published in Supplements to the [U.S.] groundkeepers have not been incriminated as Plant Disease Bulletin and the [ U.S.I Plant inoculumi sources even where they occur, Disease Reporter. These losses were ex- although over large parts of the United States pressed as the percentage of total crop lost the severe winters make the survival of per State, which was converted into a loss groundkeepers exceptional. of so many thousands of bushels. No attempt was made to differentiate between the losses Outdoor tomatoes may serve as a source of due to premature defoliation and the losses infection to potato crops; but, in general, the caused by tuber infection. Chester (1950), transference is the other way round. About writing about these loss estimates, which were 50 percent of the tomato transplants in the also given for numerous other plant diseases, United States are grown near Tifton in southern states that **the annual disease loss reports Georgia and from here they are shipped to all were based on estimates of loss in the various parts of the north. In 1946, these plants were States made by the principal Survey Collabora- infected with blight and proved a potent source tors, usually with the assistance of other State of disease inoculum to tomato and potato crops, specialists. These loss estimates were some- in what turned out to be a disastrous blight times little better than guesses . . . Yet they year. Tomatoes may serve as an oversum- mering host for the disease in the Southern represented the only available comprehensive States. In wet summers, a number of outdoor body of data on losses from plant diseases in the United States, and they are undoubtedly tomato fields are abandoned by the growers more reliable than some other published and blight surviving on these can be carried over on to the winter potato crop. An account disease loss estimates." The map (fig. 66) of the importance of tomato blight in the shows the sort of use made of these estimates. United States is included in a report on the Since 1939, no systematic countrywide at- world distribution and present status of the tempt has been made to estimate losses due to blight. The survey carried out by Bonde disease compiled by Miller and O'Brien ( 1955). (1955b) on the amount of defoliation caused by

134 ñ I to 2-4% [7\| 0.5 to 0.9%

FIGURE 66.—Percentage losses from potato blight for the period, 1920-29. U. S. A. (After Bar rus, Boyd, and Wood, 1931.)

blight in Aroostook potato crops reported season, coupled with wet lifting conditions. It above provides the type of information on which has, for example, been an important problenn yield loss estimates can be based. The Aroos- in Aroostook County (growing season 120 took potato-growing area is relatively compact, days), but much less important on Long Island however, and it would be very much more (growing season 150 days). The use ofinsecti- difficult to carry out a survey of this nature in cides, principally DDT, tends to intensify this the Red River Valley. problem, as potato tops remain green longer The intensive study made of four potato- where these are used. Efficient haulm killing growing areas in the United States leads methods should largely prevent this type of to the conclusion that heavy losses due to blight loss. premature defoliation by blight are the exception. Even when losses occur, the overall BLIGHT AND WEATHER DATA picture may be a bigger potato crop in the **blight" than in the **no-blight'* years. This Among the most innportant factors that fact, however, will be of little consolation to influence the development of epidemics in the individual grower who does suffer a heavy particular districts are the weather condi- loss in the bad blight year. The short growing tions in the Z months preceding the month of season in many of the potato areas means that lifting the crop. Thus, in Aroostook County, a loss of a few weeks growth has a big effect where the main harvest month is September, on total yield, and also, most important in a the weather in July and August largely decides country where the supply of potatoes almost the severity of the blight attack. Table 86 sets always exceeds demand, that it causes a out the average temperature and rainfall fall-off in the percentage of salable-sized figures for the 2 months before the lifting month tubers. as against the frequency of severe blight year s Tuber infection in the soil does not appear in 5 potato-growing areas in the United States. to be a serious cause of loss, except under These are the 4 areas that have already been badly drained soil conditions. Infection at considered in detail, together with the North lifting time has been responsible for heavy Platte Valley in Nebraska, an irrigated district losses in the past, and this has been largely in one of the Western States in which severe a consequence of a short growing and lifting blight epidemics do not occur.

135 TABLE 86. --Average monthly temperatures and rainfall before lifting and frequency of blight years in 5 U. S. areas

Average monthly Average monthly temperature-- rainfall— Frequency of severe Area 2 months 1 month 2 months 1 month blight before before before before years lifting lifting lifting lifting

°F. Inches Inches Percent Aroostook County, Kfeine 64 62 4.0 3.3 50 Hastings, Fla 63 69 2.9 2.4 16 South Fork, Long Island, N. Y 70 69 3.0 3.8 10 Red River Valley, Minn, and N. Dak.. 68 66 3.4 2.9 10 North Platte Valley, Nebr 71 62 2.4 1.3

The frequency of severe blight years has than in Aroostook County. In the Red River been expressed as a percentage. Thus, a Valley, the diurnal range is from 25 to 30^ F., percentage of 50 means that 1 year out of 2 and very hot days are quite frequent, with the is a severe blight year. A "severe blight temperature rising above 100 on occasional year" is defined as a year in which there is days. Rainfall is well distributed, but relative a loss of more than 10 percent in yield on humidities are low. The area is also subjected unsprayed potato crops as a result of pre- to strong drying winds during the potato- mature defoliation. growing season. AROOSTOOK COUNTY: The temperature NORTH PLATTE: Temperature conditions and rainfall conditions are very favorable to are similar to those in the Red River Valley. blight. The diurnal range of temperature This and the low rainfall account for the absence during the potato-growing season is about of blight. 20 F. Very hot days are uncommon. Rainfall The general picture for the U.S.A. is that is well distributed and relative humidities are high temperatures are one of the chief factors average. limiting blight development. In the Western HASTINGS: Temperatures are higher than States, high temperatures are normally com- in Aroostook, but are not so high as to limit bined with low rainfall to make conditions even seriously the developnnent of blight. However, more unfavorable to the fungus. The coolest it should be noted that temperatures are con- growing areas, which are also wet ones., are tinuing to rise here at the end of the growing in the New England States, and these are also season, whereas in Aroostook they are falling. the areas in which potato blight is most severe. Diurnal range of temperature is approximately 20 F. Rainfall is low and badly distributed, CONTROL MEASURES and this together with the fact that it is an early crop being grown in the area are The two most marked features of the control probably the chief factors limiting blight of potato blight in the United States are (1) the losses. intensive control program followed by almost SOUTH FORK, LONG ISLAND: High tem- all growers in those areas where blight occurs peratures limit blight development. Rainfall to any extent, and (2) the preponderant use of is higher than in Aroostook, and relative organic fungicides (Cox, 1957). humidities are also high. Conditions here are The justification for an intensive control similar to those in the potato-growing region program must be considered on economic of eastern Virginia, where Cook (1949) has grounds, but there are a number of basic reported that only 2 years out of 31 were reasons why the United States growers are severe blight years. Average total rainfall for spray conscious. Many of them, particularly the 2 months before lifting at Norfolk, in eastern in the important potato areas, such as Virginia, is 8.0 inches, but average monthly Aroostook County, are potato specialists. They temperatures at 66° and 74° F. limit the keep no livestock, and the other crops they the development of the disease. grow are there primarily to give the land RED RIVER VALLEY: The average a rest from potatoes or to build up soil temperature and rainfall figures sug- fertility. Another type of specialist is the gest a greater frequency of blight years man who produces two crops a year. He grows than, in fact, occurs. The average tem- a summer crop of potatoes in the north and perature figures are obscuring a much then moves south with his equipment to grow greater diurnal range of temperature a winter crop. The result of this specialization

136 is that most growers have their own spraying an annual average loss of 12 percent represents equipment and, concentrating most of their a loss of 43 bushels per acre. In monetary efforts on one crop, wish to make the best terms, putting the value of 1 bushel of potatoes possible job of it. In addition, there is the need at $1 (a low figure), this equals an annual loss to deal with a wide range of insect pests, so of $43 per acre. This loss has been prevented that in the potato-growing areas of the United by spraying. Assuming an annual average States, spraying would go on even in the application of 6 sprays at $5 per spray ($2 for absence of blight. It thus becomes very diffi- materials), the annual cost of spraying amounts cult to decide what is the true cost to the to $30 per acre. This cost is seen to be well- grower of spraying against blight, since he justified, particularly as one benefit not taken will be using the same equipment as he uses into account in these calculations is that there against insect pests, and in some treatments is a higher proportion of Grade 1 tubers from antipest and antiblight sprays will be combined. the sprayed potato crop in blight years. In addition, if he is a specialist with specialized At the other extreme, the value of a routine equipment and stops spraying against blight, control program in the South Fork of Long about the only thing he saves is the cost of the Island or the Red River Valley in Minnesota fungicide, as the productive alternatives for and North Dakota appears very questionable his men and equipment are probably nil. when viewed against the fact that the frequency Whether a routine program for blight con- of recent blight years has only been 1 year trol is needed in any particular area in the in 10. The application of 4 antiblight treat- United States can best be decided by consider- ments at $5 each for a period of 10 years costs ing the effect of the disease on unsprayed $200 per acre, which exceeds the average crops. No large-scale surveys of the amount value of an acre of Minnesota potatoes. Wallin of defoliation caused by blight on unsprayed and Hoyman (1945) have stated that a lengthy crops have been carried out. Indeed, in areas control progranni is not warranted in the North like Aroostook County, where all the growers Dakota part of the Red River Valley. spray, they cannot be carried out. In the Cook (1949) wrote that '*the routine applica- absence of survey material, the results of tion of fungicides that is so often reconamended spraying trials must be used, but it is per- is economically sound only for those areas tinent to ask how true a picture of the un- where disease losses occur with sufficient controlled disease the unsprayed plots in these regularity so that the benefits over a period trials give, surrounded as they are by sprayed of years will profitably counterbalance the potatoes. cost of spraying or dusting every year.*' He With these points in nnind, the yield losses stressed the need for an effective blight caused by blight on the unsprayed plots at warning service that would tell the growers Aroostook Farm, Maine, over the period the years in which spraying is unnecessary, 1950-55 (table 77) can be considered in rela- and thus save them considerable sums of tion to the cost of a routine control program. money. By advising potato and tomato growers Table 87 sets out these losses, which have in eastern Virginia not to spray during 1947, been estimated from the 75 percent blight Cook considered that he saved them up to defoliation dates in relation to the potato $2,000,000. growth curve (fig. 62). No account has been One criticism of the U. S. control program that has been developed in Aroostook County TABLE 87. —Percentage loss of yield due to (see p. 126) is that spraying goes on too long blight on unsprayed potatoes Aroostook Farm, in an endeavor to keep the haulm completely Maine, 1950-55 free from blight up to lifting. With the present use of haulnn killers, the last 1 or 2 sprays Loss should be unnecessary. Year The advantages or disadvantages of using organic fungicides have been discussed under Percent Aroostook County. One of the most important 1950 15 advantages of these materials is that in hot 1951 15 dry summier s when blight is absent, they do 1952 not have a phytotoxic effect with a consequent 1953 lowering of the yield of the sprayed crops. 1954 40 Stevenson, Akeley, and Webb (1955) and Eddins 1955 and Clayton ( 1943), among others, have referred to this phytotoxic effect where copper ma- Average. 12 terials are used. However, copper is still generally recognized as being a more effective control of blight in bad blight years, and when taken of tuber blight losses, which in any case these years occur there is a marked tendency were very small. for growers to switch back to copper sprays The average yield of potatoes in Maine is during the bad season. approximately 360 bushels per acre, so that 137 METHODS OF SPRAYING (1949) have reviewed these and other methods used in the United States. Tops are also killed The U, S. grower appears to have very with chemical dusts (calcium cyanamide), by effectively cut down the losses normally as- rotobeaters and flame burners. sociated with spraying. In addition to relying A number of writers have reported a chiefly upon organic fungicides, he grows his browning of the stem end of tubers in crops potatoes in wide rows (36 inches) and fits treated in this way, and also a vascular wheel guards to his tractor and sprayer discoloration of the tubers. Peterson and wheels. If there is a tendency for yield per Gwinn (1952) have summarized the literature acre to fall to some extent as a result of these on this subject, which contains a large amount wide rows, it should be remembered that of conflicting evidence. In general, however, with its vast land resources, output per man workers agree that one of the principal factors and not output per acre is the primary con- involved in the use of chemical sprays is the sideration in North America. Another factor speed with which the haulnn is killed; and the minimizing wheel damage losses is the grow- faster the rate of killing, the more likely is ing of upstanding potato varieties,like Katahdin vascular discoloration to occur. The fact that and Red Pontiac, which have replaced sonme of this trouble has been widely reported in North the older more sprawling varieties, such as America and not from the British Isles would Green Mountain. Against this, however, it indicate that it is associated with the relative should be remembered that the very large immaturity of the American crops at killing. number of spray treatments so often given will At the present time (1957), vascular discolora- have a tendency to overcompact the soil tion brought about by vine killing appears to between the potato drills with a consequent be of little innportance in the United States. adverse effect on the growth of the crop. This may be associated with a greater use of In general, high-volume spraying (100 toi25 sodiunn arsenite, which kills the tops slowly. U.S. gallons per acre) is used, although in Aroostook County low-volume spraying has POTATO BLIGHT FORECASTINGS gained very rapidly in popularity during the last few years, and now 70 percent of the growers The first attempts in the U.S.A. to fore- are spraying at low volume. Dusting is carried cast blight outbreaks by an analysis of weather out in a few areas where water is in short data were made by Moore (1937) for the supply, but spraying is generally recognized Charleston area of South Carolina, and Melhus as being the more efficient technique. (1945), who has reported on a blight forecast- Top spraying without the use of drop lances ing service that issued warnings during 1943 is universal. The grower aims to protect the and 1944 to potato growers in the Upper lower foliage by starting his spray program Mississippi Valley States. early and keeping pace with the growth of the Cook (1949) developed a blight forecasting crop. But in view of the known poor adherence method for eastern Virginia based on weekly of the organic fungicides, his outlook on this nnean temperatures and cumulative rainfall. point appears sanguine. There seems to be a The critical temperature above which blight tendency for the growers to make rather a would not be important was considered to be fetish of spraying, without considering too 7 5 F. Cumulative rainfall data from past closely the efficiency of the job they are making. blight and nonblight years was used to fix a *'critical rainfall line," which by definition HAULM, OR VINE, KILLING was **a straight median line ... constructed The interest in **vine" killing is of com- between the lines for rainfall for the blight and paratively recent origin and nnost of the work nonblight years." on the subject has been reported since 1945. Blight forecasts were based on a 2-week Apart from preventing blight infection of the period during which the mean weekly tempera- tubers at lifting time, the practice has a number tures remained below 75° F., and the actual of other advantages, including (1) the control of cumulative rainfall line remained above the tuber size in seed crops; (2) prevention of "critical rainfall line." Growers weretoldnot late-season spread of virus diseases in seed to spray when weekly mean temperatures were crops; and (3) easier harvest by destroying above 75 F. and/or the actual cumulative top growth and weeds. rainfall line fell below the "critical rainfall Haulm killing is now widely practiced line" for a period of 2 weeks. The method throughout the potato-growing regions of the was said to be 84 percent accurate for eastern United States. In an area like Aroostook County, Virginia. where it has obvious advantages on a number Cook (1947) considered that ''the purpose of of counts, it is more or less universally the forecasts is to eliminate needless spraying carried out every year. Various methods are or dusting in years whenblight is not imiportant used; of these, spraying with sodium arsenite and to insure prompt and adequate protective or with the dinitro compounds, in particular measures in years when it threatens to cause DNBP, is the most common. Rose and Cook serious damage." The emphasis here, on

138 preventing needless spraying is interesting, but is readily explained by the fact that only U.S.Â. 2 out of 31 years in eastern Virginia had been bad blight years. Cook used his method successfully in 1947, when he issued four warnings to potato and tomato growers not to spray. His forecasting charts for 1946 1946 (a **blight" year), and 1947 (a "nonblight" year), are reproduced as figure 67. More recently, studies have been made in the North Central States by Wallin and Wag- goner (1949); Wallin and Samson (1953); and Wallin, Wade, and Darling (1953); and in the Northeastern States by Hyre and Horsfall ' 7-OAY MOVING AVERAGE TEMPERATURE /' (1951); Hyre and Bonde (1955); and Hyre DAILY MEAN TEMPERATURE AND «' (1954 and 1955), to test the efficiency of CUMULATIVE RAINFALL Cook's method under differing climatic conditions. Wallin found that in Iowa, Indiana, and Wisconsin, cumulative rainfall and temperature data alone did not give a sufficiently reliable basis on which to issue blight forecasts, and this he attributed to the lack of correlation between periods of high humidity favorable to the fungus and the amount of rainfall. In further studies in North Dakota and Minnesota, Wallin and Hoyman (1954) and Wallin, Eide, and Thurston (1955) have used the frequency of 10-hour periods during which the temperature does not exceed 75 F. and the relative humidity does not fall below 90 percent with subsequent daily temperature maxima ren:iaining below 95 to predict the development of blight. Hyre (1955) has reported that a modification of Cook*s method has been tested out successfully in a number of the Northeastern States. The chief difference between this and Cook's original method is the replacement of a fixed '^critical rainfall line" by moving graphs based on the total precipitation over a 10-day period. FIGURE 67.--Forecasting charts for a blight year (1946) and a non- The fact that cumulative rainfall forms a blight year (1947), at Norfolk. Va. (From Cook, 1949.) reasonable basis for making forecasts in eastern United States, whereas it is unreliable the full picture back from Beltsville in the in the North Central States, may be associated form of twice-weekly newsletters. with the greater frequency and larger amounts of rain that occur in the east. There is one BREEDING FOR BLIGHT RESISTANCE great advantage in the use of simple rainfall and temperature data for making forecasts in The United States Department of Agriculture that this information is readily available and the State experiment stations cooperating through the United States Weather Bureau, in the National Potato Breeding Program are while humidity data is not generally available. now responsible for the very large majority At the present time (1957), potato blight of new varieties released to the farming com- forecasting in the United States is still in a munity. The ainm is not to produce a number of very experimental stage. Some of the diffi- varieties that can be grown throughout the culties with whichithas to contend have already country but to breed varieties suited to the been discussed in the section on the Red River particular needs of the various States, which Valley. The collection and the dissemination of differ widely. However, the cooperative nature information on blight occurrence, spread, and of this work makes it possible to find varieties severity are accomplished through the U.S.D. A. that have a wide adaptability, e.g., Kennebec. Plant Disease Survey, of which Paul R. Miller Stevenson (1956), who was in charge of this is the head. This is centered at Beltsville, work until recently, has described the prog- Md. Cooperating State pathologists provide the ress made in producing potato varieties resis- information on which the blight situation for the tant to various diseases, including late blight, country as a whole is built up, and they receive and resistance to insect pests.

139 The story of the search for American in recent years. The principal workers in the blight-resistant potato varieties has been given United States now dealing with these aspects in detail by Peterson and Mills (1953). Pioneer of the disease are W. R. Mills of the Pennsyl- work in the use of Solanum demis sum as a vania State College, L. C. Peterson of Cornell source of blight resistance was carried out University, and M. E. Gallegly of the Univer- by Reddick at Cornell University; and Reddick sity of West Virginia. Mills and Peterson have and Peterson (1945) have described the charac- cooperated with Black of Scotland andMasten- teristics of Empire, the first **blight resistant" broek of Holland to produce a universally variety to be introduced in the U.S.A. Ennpire accepted, international nomenclature for blight was first grown in 1940 at the Cornell Uni- races (Black, Mastenbroek, Mills, and versity Agricultural Experiment Station, Peterson, 1953). Ithaca, N. Y., and placed in the hands of SUMMARY: UNITED STATES OF AMERICA commercial seed-potato growers in 1945. At the time of its introduction, Reddick and Potatoes are grown under a great range of Peterson wrote of its blight resistant quali- conditions, and the incidence of blight varies ties, *'• . • it must be assumed for the present considerably between different regions. An that Empire ultimately may break down.*' intensive study has been made of blight epi- This, in fact, proved to be the case; and this demics in four widely separated areas and breakdown has been repeated with more recent this has illustrated the variation in blight introductions, where resistance has been based problenns throughout the country. The disease on major genes, owing to specialization of the is most serious in northeastern United States, blight fungus. The most successful of the where summers are cool with abundant rain- "blight-resistant" varieties to come onto the fall, i.e., the best conditions for maximum market has been Kennebec, which contains the potato crops. The overall yield losses asso- R^ gene for blight resistance. This variety has ciated with severe blight years, however, may been described in detail by Akeley, Stevenson, not be great, as has been shown by a com- and Schultz (1948). Although Kennebec was parison of yields in a number of recent slight quickly attacked by blight, both the haulm and and severe blight years in Aroostook County. the tubers, the variety possessed other de- Average yields for the two sets of years were sirable commercial properties, and it is now very similar. widely grown, particularly in Maine. The Possible loss of crop yield through pre- position is, however, that where Kennebec is mature defoliation does not appear to have grown, it now almost always receives a full played such an important part in shaping the antiblight spray schedule. farmer's control program against blight as Breeding for field or partial resistance to the fear of tuber blight infection. Until re- blight is now considered to be the most hopeful cently, heavy tuber losses were common, approach to the problem. Some of the newer and these followed the lifting of crops while American potato varieties, in particular, some green, blighted haulm remained alive. Sebago, show a higher degree of field resis- That the haulm was still green reflects the tance than the old varieties like Green Moun- short growing and harvest periods for the tain, which they are replacing. Up to the crop throughout large parts of the United present, this trend cannot be said to have had States. Thus, the farmers' intensive antiblight any effect on the intensity of the farmters* schedule aimed and still aims at keeping the blight-spraying programs. potato tops free from blight to prevent in- Much research into the genetics of blight fection at harvest. The present use of haulm resistance and the various physiologic races killers should enable farmers to cut down on of Phytophthora infestans, including the * "plas- the number and frequency of their blight- ticity" of the fungus, has been carried out preventive spray treatments.

CANADA

The area planted with potatoes in Canada is stock (ware) potatoes, and the remaining 15 approximately 300,000 acres per annum. In percent is devoted to seed-potato production. 1940, just over half a million acres were grown, There is an important market for Canadiçin but plantings have fallen steadily since then. seed potatoes, principally in the United State s, On the other hand, production methods have Argentina, Uruguay, and Cuba. shown a consistent improvement, so that total The average annual consumption of potatoes production of potatoes, which is normally be- is somewhat higher in Canada than in the United tween 60 to 70 million bushels per annum States. It is now about 150 lb. per head per (1 bushel weighs approximately 60 lb.), has year, but there is a marked downward trend remained at about the same level over recent in the face of competition from other vege- years (Parks, 1955a). Approximately 85 per- tables. The annual yield is just under 6 tons per cent of the acreage planted produces table acre.

140 CANADA

FIGURE 68.--Distribution of potato growing in Canada. 1953. Each cJot equals 1.000 acres.

POTATO DISTRIBUTION AND VARIETIES important being Katahdin, Sebago, Irish Cob- bler (early), and Green Mountain. The only Potatoes are grown to some extent in all the varieties of importance that do not appear Canadian provinces, but conditions are not in the United States list are Canso and Keswick, very favorable to the crop in central Canada, which were bred specifically for blight re- and the largest concentrations are in the east sistance and have been derived from initial (see fig. 68, after Hudson, Stutt, Van Vleit, crosses using Solanum demi'ssum as one of and Forsyth, 1949). Important growing areas the parents. are the Maritime Provinces, notably Prince Edward Island and the St. John Valley in New SOIL, CLIMATE, AND CULTIVATION Brunswick; the Gaspe"" Peninsula and an area round Montreal in Quebec Province; and the Potatoes are grown over a wide range of counties north and west of Toronto in Ontario. soil types fronn the red sandy loams on Prince About 85 percent of the total Canadian certified seed crop is grown in Prince Edward Island TABLE 88. --Acreage and yield of potatoes and New Brunswick; each province grows in Canada, 1953 about 25,000 acres of seed potatoes per annum. In western Canada there is an important Province Acreage Yield potato district in the Lower Fraser Valley of British Columbia. Table 88, taken from figures published by the Dominion Bureau of 1,000 acres Bu./acre Statistics, shows the breakup of the Canadian Prince Edward Island 39 273 potato acreage by province in 1953, which was Nova Scotia 12 231 a year of generally high yields. These illus- New B27unswick 49 289 trate the better growing conditions for potatoes Quebec 100 166 that are found in the areas nearest the oceans. Ontario 63 189 Potato statistics are not published for New- Manitoba 19 170 foundland. Wart disease (Synchytrium endobi- Saskatchewan. 13 131 oticum) exists there and potatoes cannot be Alberta 15 179 exported from Newfoundland to any other of British Columbia.... 11 300 the Canadian provinces. The varieties grown in Canada are almost Total or average.. 321 209 all the same as those in the U.S.A., the most

141 Edward Island to the dark organic soils of Kennebec, Keswick, and Canso. There is a very characteristic red sandstone soil on the the Red River Valley in Manitoba. The length Island. The most intensive farming is carried of the growing season is almost everywhere on in Prince County in the northwest and short (not more than 120 days). This is mainly most of the potato crops are grown there. due to a late spring, and it affects not only The potato-growing season extends from central Canada, but also the Maritime Prov- inces, which are subjected to the cold Labrador the end of May to the end of September (about current of the North Atlantic and the cold 110 to 120 days). The Island is subjected to winds that sweep down from the interior of cold air currents from the north untü late northern Quebec. The longest and most equable into the year, which accounts for the late growing season occurs in the limited area of planting time. Average temperature and rain- the Lower Fraser Valley and on Vancouver fall data for Charlottetown are set out m Island in British Columbia, and this is reflected figure 69. Summer temperatures are not high, in the high yields of potatoes grown there. and it is unusual for the thermometer to go The same degree of specialization does not above 85° F. Annual rainfall is about 42 exist among potato growers in Canada as in inches with a slight peak in the autumn and the U.S.A. Nonetheless, even on so-called winter. In general, apart from the shortness mixed farms where potatoes are grown, the of the season, growing conditions are very range of crops is probably smaller than on favorable for potatoes. European farms producing potatoes. Growers are usually well equipped mechanically, and Frequency of Blight Epidemics nearly all of them have their own spraying machines. Potato cultivations are very similar Descriptions of blight epidemics on Prince to those carried out in the U.S.A., and have Edward Island are included in the Annual been described in detail by Parks (1955b). Reports of the Canadian Plant Disease Survey. In a number of years estimates of yieldlosses POTATO BLIGHT INVESTIGATIONS caused by premature defoliation due to blight Research on the disease in Canada has tended and estimates of tuber blight losses have been to concentrate on two main problems. First, made. These, together with the field outbreak the chemical control of blight: This includes dates and an appraisal of the severity of the the testing of different fungicides and haulm disease supplied by L. C. Callbeck of the (vine) killers. Most of this work is now done Science Service Laboratory, Charlottetown, at the Science Service Laboratory, Charlotte- are set out in table 89 for the period 1945-54. town, P.E.I. Second, the identification of blight (In those years where no estimates have been races and the breeding of blight-resistant given, yield losses were described as low.) varieties: This work has been carried out at A clear picture of epidemics on Prince the Science Service Laboratories at Frederic- Edward Island is shown up by this table. In 4 ton, New Brunswick, and Ottawa. years out of 10 (the **severe" years), blight The Dominion Laboratory at Charlottetown occurred early, continued to spread, and is famous as a center of research into potato caused heavy o ver all los se s through premature diseases. It was founded in 1915, and the first defoliation of the potatoes; in 1 of these 4 years, Plant Pathologist-in-Charge was Paul A. tuber blight losses were also heavy. In4 years Murphy, who, both on Prince Edward Island, out of 10 (the ''severe (late)*' years), the and later in Eire, did much fundamental work disease was widespread at the end of the sea- on the biology and control of Phytophthora son and caused little loss through defoliation, infestans. A case study treatmient of Prince but in the 2 out of the 4 years it brought Edward Island is now given, followed by a about large tuber blight losses. In 2 years generalized account of blight, covering the out of 10, the disease was present in only whole country. trace amounts and losses were negligible. There is no doubt that blight is a most serious Prince Edward Island

.Charlorretown -1 70 RE.I. ^^.^^^^ Prince Edward Island has a total area of RAINFALL 60 about 1,000,000 acres. Between 35,000 and Inches X X / N. . °F 40,000 acres of potatoes are grown annually, 50 producing an average yield of approximately / r// \ - / - 40 6| tons per acre. About 70 percent of the / ^ Y/, ^ y/. \ / ^ 30 total potato acreage is used for the production / of certified seed. / / ^ % iè v/ \ 20 PLANT / /LIFT The most important variety grown is Sebago, ^ -ING / /-INC most of which is exported as seed. Other VA ^ varieties grown are Green Mountain (prin- FIGURE 69.--Long-term average monthly rainfall and mean tempera- cipally for home consumption), Irish Cobbler, ture for Charlottetown. Prince Edward Island, Canada. (For Katahdin, and the blight-resistant varieties climagrams for Fredericton. Lethbridge, and Agassiz, see fig. 71.)

142 TABLE 89. --Outbreaks and losses from blight on Prince Edward Island, Canada, 1945-54

Defoliation Year Outbreak date Severity Tuber blight losses

Percent Percent 1945 July 14 Severe (late) 0 2-5 1946 End Aug. Trace (late) 1947 July 22 do (") 1948 July 26 Severe 5-10 8 1949 End Aug. Severe (late) 10-12 1950 Fid-Aug. do 10 1951 July 20 Severe 20 1952 "" July 21 Severe (late) "" 1953 July 10 Severe 20 "" 1954 July 19 do 15

Trace.

problem on the Island, particularly when it is anomalous. Nevertheless, it will be seen that remembered that the losses described above blight losses are very heavy, and heavy occurred despite an intensive control program. nearly every year.

Blight Losses Due to Premature Defoliation Blight in Tubers Estimates have been quoted above of yield losses from premature defoliation. These The amount of tuber blight in the sprayed estimates referred to the Island as a whole, (bordeaux) and unsprayed potato plots atChar- and losses on unsprayed crops would have lottetown are set out in table 90. been considerably more serious. This can be TABLE 90 --Tuber blight in sprayed and seen from figure 70, which shows the progress unsprayed plots, Charlottetown, Prince of blight on potato haulm, and the yield of Edward Island, 1948-55 sprayed and unsprayed potatoes (over the period 1948-55) from trials held at Charlottetown. Amount of tuber The results of these trials have been published blight in— in the Annual Reports on Potato Fungicides by Year and L. C. Callbeck for 1949-55, and for 1948 by average Sprayed Unsprayed Callbeck (1949a). If the difference in yield plots plots between the sprayed and unsprayed plots is taken as being approximately equal to the yield Percent Percent loss due to blight (a fair assumption in these 1940 0.4 6.6 trials), then the average loss from blight over 194.9 .3 20.4 the 8-year period (1948-55) at this center 1950 1.3 18.1 equaled 26 percent of the potential crop, and 1951 1.1 9.3 in no year did it fall below a 10-percent loss 1952 .3 S,S of crop. 1953 .6 4.1 The variety each year for these trials was 1954. .9 1.6 the susceptible Green Mountain, whereas the 105^- 1.0 7.2 variety most connmonly grown on the Island is Sebago, which shows a high degree of field Ave rage. .7 9.5 resistance to blight. The results on the sprayed plots shown in figure 70 refer to the plots sprayed with bordeaux mixture. In 3 out of the 8 years (1948, 1953, and 1955), the sprayed Here again, the variety in the trials was plots were artificially inoculated with Green Mountain, which is highly supceptible Phytophthora infestans, but these 3 years were in the tubers; Sebago is resistant in the tubers. all years of severe blight epidemics on Prince Thus, the high degree of infection while the Edward Island, and the inoculation is unlikely potatoes were still in the ground can to a large to have had much effect in increasing blight extent be explained by the susceptibility of the losses. The yield loss in 1952 seems very variety under test. Even so, it is remarkable high in relation to the progress of blight on that tuber rot losses in the unsprayed plots the haulm of the unsprayed plots, even though should be so high almost every year, and this planting was very late that year. The results would appear- to be connected with the generally in 1952 must therefore be considered high moisture content of the Island soils.

143 CANADA

May June July August September

loo BLIGHT ON HAULM 50 - per cent

i i i 1 1 \ \ loo 7sprayinqs '^ A 1949 /Burnt 50 y off Planted y Sprayed II-8 ^^ ♦ Unsprayed 10-5 O 1 1 1 1 i i \ \ lüü 8 sprayings 1950 ^ Burnt 50 y off Sprayed ll'O Pía nted Unsprayed 9*4 O 1 \\ i 1 i i üü 6s jrayinqs 1/^ Burnt 1951 / off 50 • Sproyed 9-0 Plant ed • Unsprayed 7-O O \ \ ), \ \ \ too 6 sprayings -^^"^ / 1952 /Burnt 50 y off Planted 1 Sprayed 12-1 i ^. Unsprayed 8-3 \ { \ \ \ \ i loo 7 sprayings 1953 1 r 50 - / Burnt 1 otf Planted Sprayed 8*5 Unsprayed 7*1 O II i 4 i i i J i )0 7 sprayings 1954 X Burnt O / off Plañí -ed I sprayed 9-8 Unsprayed 4-7 O i| — 1 1 i i loo 1 i i i 7s prayings Burnt 1955 so - / off / Planted Sprayed 8'l o 1 Unsprayed 5'5 May June July August September

FIGURE 70.--Progress of blight onliauîm oí sprayed and unsprayed Green Mountain potatoes, with corresponding total yields in tons per acre. Charlottetown. Prince Edward Island, 1948-55.

144 The estimates of tuber blight losses in the of above average might be made up largely commercial crop quoted in table 89 show that of infrequent heavy rains, each of which was in some years (as in 1948, 1949, 1950) losses quickly followed by conditions favoring rapid are very heavy. In the past, a high proportion drying, and thus making it impossible for the of this loss will have followed the iniection fungus to become established. On the other of the tubers by blighted haulm at lifting time, hand, the summer might be characterized by and this is understandable in an area with a numerous light rains and drizzles whose total short growing season like Prince Edward Is- fall would be considerably below average, but land. Haulm killing has now greatly reduced which coupled with dews and periods of high tuber blight losses in the commercial crop, relative humidity, would keep the foliage damp but heavy tuber infection via the soil can still for many hours at a time." occur on the Island, if the combination of an Since 1951, in an attempt to find a closer unsprayed crop and a blight-susceptible potato relationship between weather conditions and variety exists. the severity of blight epidemics, studies of weekly weather records, including humidity Blight and Weather Data data, have been made in relation to the occurrence of the disease. Table 92 shows the Mean air temperature and rainfall figures weather summary for 1954. for the experimental station at Charlottetown The first field outbreak of blight in 1954 are shown in figure 69, together with the plant- occurred on July 19, and in unsprayed fields ing and lifting times of potatoes. Temperatures potato tops were dead by early August. Those are usually favorable to the development of farmers who sprayed got through this period, blight throughout the growing season. and conditions became less favorable to blight The average total rainfall for the July-to- after the middle of August. September period at Charlottetown is 10.53 From the records kept since 1951, it seems Inches. L. C. Callbeck has set out the July-to- that blight makes its appearance about a fort- September rainfall against the severity of blight night after the first week following July 1 in for each year over the 34 years, 1922-55. which the average relative humidity remains The results are shown In table 91- at 80 percent or above. A week at this humidity The precipitation for the period ranged from level once blight has started increases the 6.38 inches in 1945, when blight was severe disease. This fits in well with what happened at the end of the season, to 20.1 inches in 1942, in 1954. Weekly humidity figures are now a year of an early and severe attack. In 1935 being used on Prince Edward Island as one there was no blight with 15.7 inches of rain, of the main criteria on which potato blight but it was severe in 1951 when the precipitation forecasts are based. was almost identical, 15.6 inches. Both 1954 and 1955 were very bad blight years, although Control Measures rain was below normal. In 1954 there were 8.85 inches during July to September, and in The importance of cull piles as a source of blight inoculum has been recognized for a 1955, 8.78 inches. Although there is a broad correlation be- number of years, and a successful campaign tween the amount of rain and the severity has been waged against them. In the summer of blight, the number of years that are of 1956, so far as was known, there was only exceptions to a general rule indicate that total one potato cull pile in existence on the Island. rainfall figures alone are insufficient to divide During recent years. Green Mountain has become considerably less important on Prince the '•nonblight'* from the •'blight'* years. Edward Island, and Sebago increasingly im- Callbeck has given the main reasons for this: ''A total July to September precipitation portant so that it now dominates all other varieties. The use of Sebago may be looked upon as a partial control measure against TABLE 91. --Severity of blight in relation to blight, as this variety exhibits a high degree of rainfall, Charlottetown, Prince Edward field resistance to the disease. However, the Island, 1922-55 differing susceptibility of Green Mountain and Sebago must not be looked upon as the main Years with-- reason for the substitution of the one variety Total rainfall. by the other. Green Mountain fell into disrepute July to September Severe with table stock growers, owing to net nee rosis, No Trace of which develops in the tubers following primary (inches ) blight blight blight leaf roll infection. The chief advantages from the point of view of blight control in growing Niimber Number ^fumber Sebago are that the progress of the disease can be kept in check on this variety with fewer Below 9 5 2 6 5 1 2 7 spray applications than are required on Green 9.5-11 5 Mountain, and that tuber blight infection is Ovp-r 11 5 1 1 9 considerably less.

145 TABLE 92. —Weather summary at Charlottetown, Prince Edward Island, 1954

Mean Average Rain Days with rain Week temperature humidity

°F. Percent Inches Number July 1-7 64.9 80.0 0.35 July 8-14 66.6 70.6 .17 July 15-21 65.8 86.0 2.25 5 July 22-28 66.3 88.4 1.21 3 July 29-Aug. 4.. 66.7 84.1 1.18 3 Aug. 5-11 62.6 88.6 .93 5 Aug. 12-18 62.6 80.1 .47 3 Aug. 19-25 63.0 78.2 .04 2 Aug. 26-Sept. 1. 62.1 71.2 .5S 3 Sept. 2-8 62.4 75.3 .44 1 Sept. 9-15 55.¿. 76.5 .60 2 Sept. 16-22 56.8 81.8 .18 1 Sept. 23-29 54.2 82.9 .45 4

Total B.S5 37

■'■ In addition^ there were 11 days over the period on which rain fell, though the amount was too small to be recorded.

The majority of potato growers on the Island 1953 Report on Potato Fungicides. A brief follow j an annual protective spray program summary of the findings follows. against blight. Most of this spraying is carried 1. 77 percent of growers used a wet spray; out by the farmer, and there is very little con- 8 percent used a dust; 15 percent ap- tract (custom) spraying. Where dusts are used, plied no fungicide. however, they are usually applied by contrac- 2. Of those growers who used a spray, 48 tors. Practically all spraying is carried out at percent used carbamates; 31 percent used high volume (80 to 125 gal. per acre), and bordeaux; 9 percent used fixed copper; sprays are applied from above the crop only. 12 percent used a split schedule (carba- The chief materials used are the carbamates mates followed by copper), (dithiocarbamates), but a fair proportion of 3. From 1 to 9 spray treatments were given; the growers are still using bordeaux mixture, average 3.4 treatments* From 1 to 4 dust owing to its better adherence to the potato treatments were given, average, 2.8 plants. The phytotoxic effect that occurs with treatments. the use of bordeaux in hot dry summers in 4. The average number of treatments given other areas in North America has not been according to the type of material used noted on Prince Edward Island. This, together were: Carbamates, 3.9; bordeaux, 3.2; with the fact that bordeaux is still being used split schediole, 3.0; fixed copper 2.1. extensively, despite the obvious disadvantages 5. The number of sprays applied varied with in preparing the material, emphasizes the potato variety: Average figures for usually wet and humiid conditions under which Green Mountain were 4.8; for Canso, 4.3; potatoes are grown. for Katahdin, 3.5; for Sebago, 3.4; and on About half the antiblight sprays are com- Irish Cobbler, 3.2. bined with treatment against insect pests. 6. 24 percent of the growers used a haulm These are the Colorado beetle, potato flea killer. bettle, and aphids. Early blight (Alternaria L. C. Callbeck, commenting on this survey, solani) is not a problem. Haulm killing is now stated that the average number of spray a common practice, and the principal chemical treatments given (3.4) was about half the opti- used is sodium arsenite. A few growers use mum required to control blight in this year. calcium cyanamide in dust form. A considerable amount of experimental work A survey of the blight control practices of with potato fungicides and on haulm killing Prince Edward Island potato seed growers was methods has been carried out for some years carried out in 1953 (a severe blight year). now at the Science Service Laboratory at Questionnaires were sent out by post to 300 Charlottetown. This includes annual screening fanners and 107 replied, which represents 2.4 tests of new materais. Callbeck (1950a) has percent of all seed growers on the Island. The reported on investigations into the copper/lime results of this survey were presented in the ratios in bordeaux mixture, and the same

146 worker (Callbeck, 1954) has given details of western Quebec and eastern Ontario; and in the tne etfect of incorporating zinc salts in potato Lower Fraser Valley of British Columbia. fungicides. Haulm killing trials have been These are also the principal potato-growing described by Callbeck (1949b). It has been areas in the Dominion and the areas where found that less tuber blight has occurred potato-growing conditions are most favorable. m plots sprayed with bordeaux mixture than There is an obvious link between the blight in those sprayed with organic fungicides in areas in eastern Canada, and those in the each year since 1945, when the organic s were Northeastern States of the U.S.A. first tested at Charlottetown. Mean tuber Blight has not been recorded to any extent blight percentages for the period 1945-51 north of latitude 52°, but only a few potatoes were bordeaux, 0.7; nabam, 5.1; zineb, 6.9Í are grown in the north of the country, owing unsprayed, 13.1 A possible explanation is the to the shortness of the growing season. H. N. lethal effect on blight spores of copper residues Racicot has estimated that 30 to 40 percent of accumulated in the surface soil, whereas the losses due to blight in Canada are caused dithiocarbamate residues remain toxic for a by premature defoliation of the potatoes, and much shorter time. 60 to 70 percent of the loss is due to tuber A blight warning service for growers was blight infection. Most of the tuber blight is started in 1947. Weekly press and radio caused by contamination with infected haulm reports based on field observations of the at lifting time, and is therefore largely pre- disease and current weather conditions are ventable. Blight infection in the soil is not, issued. Average weekly relative humidities generally speaking, an important problem, exceeding 80 percent are considered to favor and this is possibly linked with the fact that the onset or increase of blight. The Island is most potatoes are grown on light soils. well placed to receive up-to-date reports on The Annual Reports on blight have been the progress of blight, since 70 percent of the examined in detail for the 10-year period potato crop is for seed, and 30 seed inspectors 1946-55, and much of the data for individual are out in the field during the summer months. areas given below, including quotations on the A good deal of circumspection has to be exer- course of the disease in blight years has been cised in the issue of blight warnings for the obtained fronn this source. very reason that seed production is so im- MARITIME PROVINCES. Prince Edward Is- portant, and it is essential not to frighten away land is the worst area for blight in Canada, and the buyer of the seed crop. L. C. Callbeck has described the frequency Summary: Prince Edward Island of severe blight years there as 3 years out of 4. The disease is not quite so serious in Nova Potato blight is a most serious disease on Scotia and New Brunswick, and the frequency the Island. Average annual yield losses due to in those provinces is about 1 year out of 2. premature defoliation following blight attack The principal potato-growing area in New on unsprayed plots at Charlottetown over the Brunswick lies in a narrow strip of about 60 period 1948-55 equaled 26 percent of the pos- miles in the St. John River Valley, along the sible crop. Premature defoliation also causes western boundary of the province. This area heavy losses in the commercial crop despite is opposite the potato-growing district in the control program. Tuber blight is a serious Aroostook County, Maine, which has been problem, particularly where a blight suscep- considered in detail in the section of this tible variety like Green Mountain is grown. study covering the U.S.A. The chief difference However, now that haulm killers are available between the two areas is the greater acreage there should be good control of infection at and concentration of potatoes on the U.S.A. lifting. This used to be a trouble due to the side of the border, but the importance of blight short growing season for potatoes. It is sur- and the problems that the disease presents prising that a survey carried out in 1953 are very much the same. In New Brunswick revealed that 15 percent of the seed growers 1954 was a bad blight year. The folloydng ex- had no control program for blight. tract shows how serious the disease was. **Late blight was first reported in N.B. on BLIGHT EPIDEMICS: FREQUENCY AND 16 July . . . The disease developed rapidly and by mid-August practically every field in LOSSES the potato growing areas showed sonne in- The first records of potato blight in Canada fection. Some fields, particularly Irish Cobbler go back to the early 1840's, when the disease were already completely killed . . .** On a was found in the Maritime Provinces. Since number of occasions, including 1956, the initial 1920, annual accounts of the severity of the blight outbreaks in New Brunswick took the disease in the different Canadian Provinces form of an attack concentrated in stennlesions have been published in the Reports of the and lesions on the growing points of the potato Canadian Plant Disease Survey. plants. No satisfactory explanation has been Blight is most serious in the Maritime put forward to account for this type of attack, Provinces (Prince Edward Island, New Bruns- and it does not seenntobe correlated with potato wick, Nova Scotia) and Newfoundland; in variety. However, once stem lesions have been

147 established within a crop and the weather be- stored the tubers in warm storage spaces. comes favorable for blight development, the Losses at digging time were as high as 10- potato field Is then destroyed very rapidly. 80 percent of the crop ... It Is conservatively The principal losses in New Brunswick estimated that 10 percent, or 1 1/2 million result from yield reduction caused by pre- bushels of the crop, was lost. Fields dug mature defoliation. Haulm killing has cut out a after the tops were killed with a herbicide large part of the tuber infection that used to showed little tuber rot." occur. Infection in the soil is not usually a ONTARIO. The disease is most important in serious problem due to the large **hills" eastern Ontario, but even there the frequency and light soils in which the potatoes are of severe blight years is only about 1 year in grown. J. L. Howatt has drawn a distinction 4 or 5. During 1946-55, the worst outbreak of between those years in New Brunswick when blight in eastern Ontario appears to have taken the disease has been severe and those years place in 1951, when it was described as fol- when it has caused severe losses. The disease lows: .**... late blight became epidemic by is said to be severe in 1 year out of Z, but in mid-August. It was estinnated that about 10 many of these severe years, farmers who percent of the acreage was slightly affected, follow a regular control progrann are able to 25 percent moderately affected, and 65 percent keep the disease under control, and overall severely affected. The reduction in yield, from potato yields for the province may even be the premature death of the vines, varied from enhanced by the wetter growing conditions. 0 to 25 percent, and averaged 5 percent. An Occasionally, the wet weather may be so additional 5 percent was lost from decay of the continuous that the spraying program breaks tubers," down because the farmer cannot get his machine MANITOBA, SASKATCHEWAN, ALBERTA. through the crop. This happened in 1954, with Blight is not important in these provinces. the result that most crops were prematurely The first record of blight in Saskatchewan was defoliated and there were heavy yield losses. made in 1946, and in southern Alberta in 1952. Years like this occur about 1 year in 8. They The main potato-growing area in Manitoba lies are reflected in high potato prices during the in the Red River Valley, which has been de- autumn and early winter. In fact, Howatt has scribed in the U.S.A. section of this study. gone so far as to define a bad blight year in BRITISH COLUMBIA. The most important New Brunswick as a year in which the price of potato-growing area is the Lower Fraser table stock potatoes exceeds $3 per bushel Valley, where the frequency of severe blight shortly after harvest! years is about 1 year in 3. Over the period QUEBEC. In this province, blight was severe 1946-55, serious losses in the potato crop in 5 out of the 10 years from 1946 to 1955. The have occurred in 1948, 1953, and 1954. The importance of the disease varies with district, greatest source of loss is due to blight infec- and it is worst in the area around Montreal, tion of the tubers, and much of this is caused where the frequency of severe blight years is by contamination with infected haulm at lifting. about 1 year in 2. Along the south bank of the St. Lawrence from St. Anne-de-la-Pocatiere BLIGHT AND WEATHER DATA to Mont Joli, the frequency is about 1 year in 3. The chief climatic differences between the Figure 71 shows the long-term average air Montreal and this area are the cooler growing temperature and rainfall conditions for three conditions along the St. Lawrence and the centers across Canada. These are Frederic- tendency for dew formation to be less there ton, New Brunswick; Lethbridge in southern than in western Quebec, owing to the more Alberta; and Agassiz at the eastern end of the northerly latitudes with shorter nights. Lower Fraser Valley in British Columbia. The Although the first field outbreaks of blight average temperature and rainfall at Charlotte- in the province may occasionally be reported to^Am, Prince Edward Island, has already been in early- to mid-July (as in 1954), the disease shown in figure 69. These climagrams il- in general appears later in Quebec than in the lustrate the very different conditions under Maritimes; tuber blight losses rather than which potatoes are grow^n in Canada. yield reduction following premature defolia- MARITIMES, QUEBEC, AND EAST ONTARIO tion constitute the main problem. The course are very similar as to temperature and rainfall of events described for 1952 seems to be regime in the potato-growing areas. Winters typical of a severe blight year in Quebec are severe, so that the survival of potato Province: **Late blight was first reported groundkeepers is unusual. During the summer in fields about August 6 . . ., and by August 18 months, rainfall is well distributed, and very it was known in many districts. Late blight high temperatures are unusual. The diurnal was slow in developing, but warm weather range of temperature is approximately 20® F. accompanied by heavy dews and rain favored Summer temperatures are higher in western its development in September, Many growers Quebec and eastern Ontario than in the Mari- dug their potatoes while the tops were still times, but very high temperatures are excep- green or partly destroyed by late blight and tional. The mean air temperatures for July,

148 CANADA Edward Island is most probably explained by -Fredericton NEW BRUNSWICK the higher humidity conditions on the Island. RAINFALL 5 This is not shown up by the table. Humidity Inches data for Prince Edward Island has been discussed on page 145 but, in general, Informa- tion on humidity Is scanty or lacking for the region. However, the folly of dogmatizing about humidity and blight are Illustrated by an In- teresting observation made In New Brunswick on the Interrelationship of temperature and humidity with blight attacks. The Science Service Laboratory at Fredericton maintains _ Lethbr dqe 70 a potato substation at Alma, on the coast about ALBERTA 60 miles west of St. John. Although humidity RAINFALL - 60 „ Inches / X conditions at Alma are high, due to frequent 50-F / \ 50 coastal fogs, blight Is almost always worse \ some distance Inland where hunaldlty Is lower. / 40 / \ This Is explained by the higher Inland tem- * k it - / /A \ - 30 peratures; the lower temperatures on the y^ V/ \ coast slow up the progress of the disease. m\:„'/y1 s,- 20 LIFT The lower rainfall In eastern Ontario partly -INC -INC 1 y/, 1 i explains the lesser importance of blight there ♦ IrriqaHon practised as compared to the other centers (table 93). The lower summer temperatures at St. Anne, , Aqassiz BRITISH COLUMBIA compared to those at L*Assomption, play a part In the less serious nature of blight at St. Anne. Particularly Important In Quebec RAINFALL Inchcx are the September temperatures, as the blight attacks tend to be late In the province and the main source of loss arises from tuber blight Infection. The higher September temperatures In western Quebec, combined with greater dew formation, would seem to be an Important factor In making blight losses greater than In eastern Quebec. MANITOBA, SASKATCHEWAN, ALBERTA. Weather conditions In central and western Canada, apart from the coastal areas, are FIGURE 71.—Long-term average monthly rainfall and mean tem- unfavorable to blight. Generally speaking, peranires in relation to growing periods for three centers in precipitation during the growing season Is low Canada. and evaporation high. Dally temperature maxima are high and the diurnal range of August, and September, together with the temperature Is around 25°to 30°F., as com- average total rainfall for the July-September pared to nearer 20°ln eastern Canada. Rela- period are compared in table 93 for five tive humidities are low during the summer, centers in the region. The frequency of severe and this Is partly the outcome of the dry blight years at each center is also given. warming winds that blow across the region The greater severity of blight on Prince and the abundant sunshine.

TABLE 93. --Temperature and rainfall during growing season and frequency of blight years, 5 locations, Canada

Mean tenrperature Frequency of Total rainfall, severe blight Centers July-Sept. July Aug. Sept. years

°F. ^F. Inches Percent Charlottetown. Prince Edward Island 66 66 58 10.5 75 Fredericton, New Brunswick 67 65 57 10.3 50 L'Assomption, W. Qiebec 69 67 58 10.5 50 St. Anne-de-la Pocatiere, W. Quebec 65 63 55 10.7 33 DP!hi F Ontario 70 69 61 S.5 20

149 The climagram for Lethbridge in figure 7 1 BLIGHT-RESIST ANT POTATO VARIETIES illustrates the type of rainfall regime typical AND BLIGHT RACES of thé prairie provinces, i.e., a marked summer rainfall peak. Even so, rainfall is deficient Two varieties, Keswick and Canso, contain- and potatoes are grown under irrigation in the ing the Rjgene for blight resistance and de- area. Summer temperatures are lower at rived from Solanum demis sum hybrids, have Lethbridge than in the potato-growing areas been grown fairly extensively in Canada. of Manitoba and Saskatchewan. However, blight Shortly after they had been introduced Into is not a problem. commercial practice they were found attacked LOWER FRASER VALLEY, BRITISH CO- by specialized races of the blight fungus and LUMBIA. The climagrann for Agassiz shows they are now sprayed against blight in the same way as other varieties. A comparison of the the longer growing season in this area than foliage and tuber resistance to blight of 64 in the rest of Canada. The mild winters allow varieties, including some British ones, was potato groundkeepers to overwinter and these made under severe blight conditions in 1954 may serve as a source of blight inoculum. at St. Anne-de-la Pocatiere by Campagna Blight is checked by the drier conditions of (1955). July and August, but high rainfall around lifting Much work on the identification of blight time has made tuber blight infection a serious races from the different Canadian Provinces problem. has been carried out at Fredericton by Howatt CONTROL MEASURES and his colleagues. The results of surveys carried out by them, showing the distribution The blight control program in Canada fol- of blight races throughout Canada, have been lows very much the same pattern asthatin the published in the 1954 and 1955 Reports of the U.S.A. Recent accounts of methods have been Canadian Plant Disease Survey. The 1955 re- given by Callbeck (1950b and 1953). As in the sults are shown in table 94. U.S.A., the spray program has to deal, in addi- The amount of material sent in by the dif- tion, with a number of insect pests. ferent provinces varied considerably, and the The main differences between the two coun- survey does not pretend to be a complete one. tries appear to be that (1) the control program Both surveys in 1954 and 1955 showed race 4 is generally less intensive in Canada (in com- to be predominant in Canada. Race 0, ori- paring similar areas in the two countries, such ginally considered to be the comnaon race, as Aroostook Co. with the Maritime provinces, cannot be determined with accuracy in the a greater number of sprays are applied in presence of other races, so that it is difficult Aroostook than in the Canadian region); (2) to show its importance. From the result of there is a relatively greater use of copper surveys carried out in 1952 and 1953 at fungicides in Canada than the U.S.A. (although Ottawa, Graham (1955) concluded that in organic fungicides are mainly used in Canada, Canada, **. . . the so-called common race of principally due to their lower cost and the ease P. infestans is actually composed of Races 0 with which they can be applied, copper ma- and 4.'' terials, in particular bordeaux (10:5:100), are Recent work at Fredericton by Howatt and still used by a large number of growers); and Hodgson (1954) and Howatt and Grainger (1955) (3) there is a greater relative use of dusts in has questioned the adequacy of the newly pro- Canada. Again, comparing the Maritimes with posed international nomenclature for blight Aroostook, dusting is carried out on a greater races (fig. 16) on the grounds of the erratic scale in the Maritimes. According to Howatt, behavior of certain blight isolates on the dif- up to 20 percent of the growers use dusts in ferential hosts. some years in New Brunswick, but in bad blight years, many of those who nor nnally dust, change SUMMARY: CANADA over to spraying. The Maritime Provinces are the worst area In general, blight control is good in the for blight in the Dominion. A close analysis of Maritime Provinces and the coastal areas of epidemics on Prince Edward Island has been British Columbia. In New Brunswick there is a possible. In other parts of Canada a frequency tendency to change over to low-volume spray- of **blight" years has been established from ing, though this does not seem to be happening the accounts given in the Reports of the much elsewhere in Canada. The blight control Canadian Plant Disease Survey. The major program is inadequate in many parts of Quebec source of loss from blight has been tuber Province; while in central Canada few infection, particularly infection at harvest- measures are taken to control blight, as the time. This is closely linked with the short disease is not usually a serious problem. growing and lifting periods for potatoes. The One feature that enmerges is that not enough control program is very similar to that in the growers are using haulm-killing methods. U.S.A.

150 TABLE 94. --Distribution of blight races in Canada, 1955

Prince Race of Phytophthora New Nova Mani- British infestans Edward Brunswick Quebec Saskatchewan Total Island Scotia toba Columbia

0.., 2 1.., 2 2.., 2 2 3.., 19 13 6 4 50 4.., 59 50 12 14 147 1,2.., 2 2 1,3.., 7 1 2 10 1,4.., 21 5 1 28 2,4.., 2 1 3 3,4.., 13 22 3 3 41 1,3,4.. 12 5 17

Total, 94 117 43 29 17 304

MEXICO Potatoes are not an important crop in tained. Growers found it impossible to produce Mexico. With a populationof nearly 30,000,000, potatoes at lower elevations in the wet season, the total annual production of potatoes is only owing to the ear lines s and rapidity with which about 150,000 tons (F.A.O., 1955). Atthenaost, they were destroyed by blight. this allows an average annual consumption of A program was established in Mexico in 5 lb. potatoes per head per annum, but this 1951 by the Rockefeller Foundation, with the figure takes no account of the portion of the object of improving potato cultivation methods total production which is used for seed. The and exploiting new areas of production. Among main reason for the low production of pota- the areas that show possibilities of becoming toes--about 80,000 acres are grown annually-- important centers of potato production are is the low yield per acre. In 1954, this was those known as the **high valleys*' of the Mesa 1.8 tons per acre. The result is that potatoes Central (Central Plateau)--principally the are very expensive and are considered a valleys of Mexico and Toluca. It has been luxury crop. demonstrated during the last few years that In Mexico (fig. 72), potato production can potatoes can be grown with much better re- be divided into two main types; (1) The culti- sults in these areas if use is made of the vation of indigenous varieties at high eleva- rains. To do this it is necessary to grow tions in the Sierra; and (2) the cultivation of potato varieties showing sonne resistance to imported varieties at lower altitudes, usually blight and to protect them with an intensive under Irrigation. spray schedule. The Sierra is the principal area of production. Two of the most important native varieties grown in this region are Leona and BLIGHT EPIDEMICS Amarilla de Pueblo. Inciported potato varieties come largely from Denmark and Holland and The greater part of the area in which pota- include Alpha, Furore, Gineke, Prunamel, toes are grown in the Sierra is situated above Bintje, and Up-to-Date. The imported varieties 3,000 meters, i.e., about 10,000 ft. Potato differ from the indigenous potatoes in that they blight Is not a serious problem at these have a shorter period of vegetative growth altitudes. The story is very different in the high valleys. At Toluca, where systematic and are potentially higher yielders. Potato blight is the chief factor limiting blight observations began in 1951, the disease has been devastatingly severe each year since potato production in Mexico. Potatoes are then, according to information from Dr. grown chiefly where and when blight is not Niederhauser. Its effect at this center can likely to be a problem, i. e., at high elevations be illustrated by what happened to an unsprayed in the Sierra and under irrigation during the crop of Alpha in 1956. The crop was planted on dry season, which occurs in the winter months May 15, and blight was first noted on the foliage (fig. 73). Although the se conditions are inimical on June 20. The tops were completely dead by to blight, they are also very unfavorable to good July 25, and only a few marble-sized tubers potato yields and are the main factors responsible for the present low yields that are ob- were produced.

151 MEXICO

FIGURE 72.--Mexico, showing potato growing areas referred to in text.

BLIGHT LOSSES cultural methods, this has meant the virtual elimination of potato growing from these It can be seen that the effect of blight in the valleys during the summer months. Thus, the high valleys Is to cause complete loss of the cultivation has had to be in areas where, unprotected potato crop. In the past, owing owing to unfavorable climatic conditions, both to the low level of technical development in potential and actual potato yields are very low. How serious this is can be seen from some ^ Puebla figures quoted by Niederhaus er and Cervantes 7000ft. (1956) in which they compare the national average yield of potatoes in Mexico with yield s of crops protected against blight and grown at RAINFALL Inches _ two centers in the high valleys. The original figures were given in terms of tons per hectare,

50 but have been converted in table 95 into tons per acre. 40 The figures in table 95 relating to Chapingo - 30 and Toluca are for small acreages of potatoes only. Even so, the differences between potato 20 yields there and the national average yields -HTTTV/VZ} are most striking. J F M A 1 M J *—CROPPING PERIOD-» According to Dr. Niederhauser blight infection of the tubers is very slight (usually hard FIGURE 73.—Long-term average naonihly rainfall and mean tempera- to find) in the Sierra, and it is only a little tures for Puebla. Mexico: The rainfall regime is typical for the more prevalent in the high valleys. There potato growing areas in Mexico, but temperatures vary greatly, appears to be no good explanation for this. according to altitude.

152 TABLE 95. --Comparison of national average yields of potatoes with those at 2 centers where potatoes were protected from blight, Mexico, 1952-55

Average yields of potatoes Year Mexico (national average) Chapingo Toluca

Tons/acre Tons/acre Tons/acre 1952 1.8 9 17 1953 1.9 7 13 195-4 1.8 11 15 1955 10 U

Thus, the main losses caused by blight in observed on these plantings, which are grown Mexico result from the premature defoliation under irrigation during the dry season. Mean of the potato plants by the disease. temperatures during the summer (which exceed 80°F.) are too high for potato growing. BLIGHT AND WEATHER DATA CONTROL MEASURES Figure 73 shows the average temperature It is apparent that the main way of con- and rainfall conditions for Puebla, a potato- trolling blight in Mexico up to the present growing district lying to the southeast of time has been to escape the disease. The Mexico City. The rainfall regime is typical of most successful of these escape methods has that in the potato-producing areas of Mexico, been to grow the potatoes in the dry season, i.e., a dry season during the winter months under irrigation, chiefly at heights above (November to April) and a wet season in the 5,000 ft. where growing conditions are cool. summer (May to October). Rainfall in the But irrigation is expensive and not generally summer is often very heavy and continuous. available. Mills and Niederhauser (1953) state that at The most hopeful means of obtaining eco- Toluca, in 195Z, rainfall was over 20 inches nomic yields of potatoes appears to be to make in both July and August. Temperature condi- use of the summer rains, and at the same time tions in the potato districts vary, mainly with to employ modern methods of blight control. differences in altitude. The workers of the Rockefeller Foundation at At Puebla (7,000 ft.) growers plant their Toluca and elsewhere have shown that this can potatoes in late January, during the dry season, be done. Niederhauser and Mills (1953) have hoping for just enough rain to grow a crop discussed the growing of potato varieties show- but not enough to favor blight development. At ing a degree of **field" or **partiar'resistance Leon (6,000 ft.) the farmer grows his crop in to blight (notably Alpha and Voran). It has been the dry season under irrigation. Work being shown that these varieties can be satisfac- carried on at Toluca (8,000 ft.) is designed to torily protected by ordinary spray schedules, show how potatoes can be grown during the wet when more susceptible varieties cannot be season (May to September or October), but this produced commercially even with regular can only be done if a rigorous blight control and thorough spraying. program is followed. Recommendations for the spraying program In the Sierra (above 10,000 ft.), potatoes have been made by Niederhauser and Cervantes are grown in the summer months. Niederhauser (1956). Generally speaking, the first spraying attributes the lack of blight under these con- should be at about the tinne the first rains are ditions to low temperatures. Unfortunately expected. If the plants emerge from the ground weather records are not available, but this some time before the rainy season, it should state of affairs accords well with observations be given while the plants are still small (4 to made in parts of the Sierra of Peru, as re- 5 inches high). Usually the spraying should be ported by Bazán de Segura. She tells us that repeated at intervals of 10 to 15 days. At blight has never been reported from the im- Toluca, 6 applications are normally required portant potato-growing areas of the Province over the growing season, and sometimes 10 of Puno, where temperatures average about are needed. Both bordeaux mixture and the 50° F. during the growing season. organic fungicides have given satisfactory In Mexico, potatoes have also been grown blight control in the high valleys. at sea level as a speculative crop in the POTATO BREEDING PROGRAM states of Veracruz and Tamaulipas, near the Gulf of Mexico, and in the state of Sonora, Mexico is the site of a most important near the Gulf of California. Blight has not been potato breeding program, sponsored by the

153 Rockefeller Research Foiindation. This pro- despite specialization In the blight fungus. gram alms at the production of new varieties The potato-breeding program In Mexico Is showing complete, or almost complete blight now working to Incorporate a high degree of resistance. A number of factors make the high such resistance In commercially acceptable valleys near Mexico City an excellent area for potato varieties. the direct field selection of blight-re sis tant A recent event In Mexico, of fundamental breeding nnaterial« Most, if not all, of the wild and historic Interest, has been the discovery species of Solanum that are highly resistant of circumstances In which Phytophthora In- to blight are found in central Mexico. These festans has aji active sexual phase (Nieder- species have been in close association with hauser, 1956; Gallegly and Gallndo, 1957. Not Phytophthora infestans over a long period of only have oospores been obtained readily In time. Mills and Niederhauser (1953) first culture, but at Toluca they have been foiind demonstrated that this association has given on potato foliage In the field. Oospores are rise to a number of naturally occurring bio- formed abundantly by the pairing of two types of the fungus in the field. Since then, compatibility groups of the fungus. In the Niederhauser, Cervantes, and Ser vin (1954a Initial experiments, these compatibility groups and 1954b) have reported the natural occur- were represented by sets of isolates from rence of 14 of the 16 races of blight named in Mexico, which were paired with isolates from the international nomenclature (fig. 16). In the United States and Canada. Both compati- addition, they have found an additional race, bility groups are present In the field In Mexico. provisionally termed ** 1,2,3,4,5,'* capable of Dr. Niederhauser writes: *'Having found attacking a clone of S» demis sum which ap- oospores in leaf lesions in the f?.eld, we believe parently possesses a fifth gene for resistance that the oospores probably do function in the to blight. Thus, in central Mexico, the highly field, and enable the fungus to survive In the specialized races of the blight fungus are field through the dry season."He continues: among the prevalent field races, and it is "The finding of oospores. . . suggests that any under these conditions that the search for partial resistance, we find may be effective blight-re sis tant potatoes Is being made. against any race [of blight], since the pathogen Field experience In Mexico has shown re- Is happily breeding as the season progresses peatedly that resistance to blight based only and sooner or later should come up with a race on major genes, either singly or In combina- that would take over. Furthermore, the partial tion, which have been derived fromS.demissum resistance that we have observed In^. or other wild Solanum species. Is of little or demis sum and other species has apparently no value. While this type of resistance confers been stable for many hundreds of years and immunity to specific races of the blight fungus, has enabled these species to survive." other races are either already present or quickly arise to attack the plants. Niederhauser High hopes, therefore, exist for the produc- says, **We have not yet found a tuber-bearing tion In Mexico of commercial potato varieties Solanum species that Is Immune to JP. In- showing a high degree of resistance to blight. festans at Toluca." Field or partial resistance To what extent such varieties would be adapt- shows much greater promise. Niederhauser, able to the main potato-growing regions of Cervantes, and Servln (1954a) have described the world, situated In more northerly latitudes, the Interaction between host and fungus, where remains to be seen. •'partial resistance" Is exhibited: *'Several species. Including some clones of S. demis sum, show a high resistance to P. Infestans even SUMMARY: MEXICO though foliage lesions are produced on them. These resistant-type lesions are slow spread- Mexico is an example of a country where ing, often turn ne erotic, and sporulation Is blight has largely been escaped. But the effect sparse. Petiole and stem lesions tend to be of this escape on potato yields has been little superficial and do not kill the distal portions short of disastrous. The use of modern protec- of the affected leaf or stem. Relatively few tive methods against the disease has been lesions develop on a plant, and these are more demonstrated at Toluca and elsewhere to be a often on the lower older leaves." The genetlcal more effective method of overcoming blight. basis of **fleld resistance" has not yet been The Imaginative action of the Rockefeller fully explored; major genes may or may not Foundation, In establishing a potato-breeding be Involved. The Importance of field resist- program In this country under conditions of ance lies In the apparent ability of this re- maximum severity of blight. Is paying hand- sistance of a variety to remain constant some dividends.

154 SOUTH AMERICA

PERU

Peru is of particular interest in any study all of these fall into one of two distinct zones. connected with potatoes. Lake Titicaca, which These are the '*Costa,'* or Pacific Coast zone, is partly in Peru and partly in Bolivia, is where potatoes are grown in the river valleys regarded as the World center of origin of the at heights up to 9,000 ft.; and the "Sierra," potato plant (fig. 74). In addition, potatoes or mountain zone, where they are grown from play a larger part in the diet of the population 9,000 to 13,500 ft. According to figures given of Peru than in any other country of South by Hawkes (1941), 30 percent of the total America. The combined weights of potatoes production comes from the Costa and 70 and other starchy roots consumed per head percent from the Sierra. There is no rea- in 1952 was 169 kg. (372 lb.), most of this son to believe that these proportions have being in the form of potatoes. This is a rate changed appreciably in recent years. The chief of human consumption comparable to that in characteristics of the two zones are as fol- some of the large European potato-producing lows: countries, such as Germany and France. The COSTA: The haciendas (plantations) are population of Peru is about 9 million persons, owned by Creoles or Europeans, and to produce enough potatoes for thena and worked on modern lines. Many requires a large acreage of the crop, as of the fields are large, and culti- the average yield per acre is very low. Over vations, including spraying, are the period 1948-52, 543,000 acres of potatoes mechanized. In general, potato were grown annually, and these produced an growing is possible all the year average yield of 2.3 tons per acre (F.A.O., round, owing to the absence of 1956). frosts; but, depending on altitude, Low yield per acre is in part a reflection the main growing season is either of inferior cultural methods, but a more in the winter or summer. At Lima important factor is the unfavorable climate (520 ft.) the growing season is in in which the greater part of the potato crop the winter (May to November); is grown. Production is carried out indifferent summer temperatures are too high areas throughout the length of the country, but for successful potato production. At Arequipa (7,600 ft.) the main PERU f[rowing season is in the summer January to June). Annual rainfall is very low throughout the Costa; at Lima, average annual rainfall is 1.8 inches, and at Arequipa, 4.2 inches, but sea fogs give rise to high humidity conditions. It is necessary to grow all potatoes with channel irrigation. The majority of the crops are planted with indigenous varieties, but some North American varieties are also gro^jvn. SIERRA: Most of the country's potato production is in these regions. Yields are lower than in the Costa. Cul- tivation is chiefly carried out by Indians, and methods are primitive. Part of the crop is still preserved as the traditional "papa seca*' or chuno. Most crops are grown during the sunnmer, without irrigation. Rainfall is generally adequate, but temperatures are low (mean monthly temperatures Puno*^VN['H"ca seldom exceed 55° F. during the •Arequipa growing season), and this is one of the main reasons for the low potato yields. Slow-maturing indigenous potato varieties are FIGURE 74.—Key map of Peru. grown.

155 Much of the information on potato blight in Under the Costa conditions, where seamists Peru has been provided by Consuelo Bazán de favor the disease, blight remains localized in Segura, of the Central Experiment Station of the aerial parts of the potato plants and no La Molina, at Lima. case has been observed of tuber infection. This is due to the small amount of rain during the growing season. On the other hand, in the BLIGHT EPIDEMICS: GENERAL Sierra, tuber infection is a most important problem. Msiny of the Indian growers in the Potato blight was considered of little or no Sierra think that the leaf and tuber attacks economic importance in the Costa until 1947, are caused by two completely independent and when it caused heavy losses. No satisfactory different diseases. Reasoning in this way they reason for the first appearance of the disease see nothing to be gained from saving the plants in epidemic proportions at this time has so by spraying or dusting if the tubers are to be far been put forward. Since 1947, there have attacked by the '*other" disease. Little pro- been a number of other years when severe tective action against blight is taken in the attacks of blight have developed, but the inci- Sierra, but since 1947, a high proportion of dence of the disease has shown a considerable the growers in the Costa have been spraying. variation from year to year. Bazán de Segura (1952a) states that potato BLIGHT ON THE HAULM blight has been troublesome in the Sierra since 1949- There are variations in the importance of the disease between the different COSTA: Details have been supplied by Bazan regions in the Sierra, and up to the end of de Segura of blight incidence during the period 1956 there had been no reports of blight 1951-56 in the Carabayllo Valley, a potato- occurring in the important potato-growing growing district, 16 kilometers to the north areas in the Department of Puno. of Lima. These data are shown in table 96. TABLE 96o--Blight incidence in the Carabayllo Valley, Peru, 1951-56

75 to 90 percent blight First appearance Year stage on unsprayed of blight potatoes by—

1951. July 15 1952. July 20 Sept. 8 1953. May 17 1954. (M 1955. (^) 1956. June 15 Sept. 2

■'■ Blight of little importance.

The disease was described as severe in 1952 The time at which the 75 percent blight and 1956. No record of the 75 to 90 percent stage occurs in relation to the stage of blight stage was made in 1951 and 1953, and growth of the potato plants decides the scale in the other 2 years, 1954 and 1955, blight was of the loss that blight produces through of little importance. premature defoliation. Bazán de Segura and The observations given in table 96 show Lamas Carrera (1953) have described the that even with this very limited number of results of a spraying trial carried out in the records there was considerable variation in Carabayllo Valley in 1952 (a severe blight the time of the first appearance of blight-- year), when the amount of defoliation caused between May 17 to July 20. The disease may by blight was measured at intervals, and the attack the potato plants from shortly after produce of the sprayed and the unsprayed emergence onward. Bazán de Segura (1950) plots was weighed. Blight assessments were states that during a number of bad blight made by Horsfall and Barratt's method. The years in the Lima area, it was very common effect of the disease on the control plots of to find blight attacking very small plants the two potato varieties in the trial is shown (5 cm. high). in table 97.

156 TABLE 97. --Blight record on untreated plots at Carabayllo Valley, Peru, 1952

Date when tops Time of Variety planting reached 75 percent Yield blight stage

Tons/acre Papa Blanca... June lÖ-July 2 Sept. 4 1.3 Papa Amarilla. June lÖ-July 2 Sept. 6 .3

From planting until 75 percent of the tops cool conditions and the slow-maturing indige- had been destroyed by blight was just over nous potato varieties that are grown. 2 months, and this accounts for the very low Bazan de Segura has told us that in a blight potato yields. Blight attacks of this kind, year in the Sierra, the disease appears some- which occurred in the Carabayllo Valley in time in January and the 75 to 90percent blight both 1952 and 1956, obviously have a com- stage occurs sometime in February. This is pletely devastating effect on some unprotected the most precise information that is available. potato crops. But the position is complicated A marked characteristic of blight in the by the wide range of tinne over which crops Sierra is the way that the foliage attack is are planted in the area. This stretches from localized almost entirely at the base of the May to July. Without detailed survey informa- leaf petioles and on the main stems of the tion on this and other points, it becomes im- plants. The leaf petioles become brittle and possible to assess with any accuracy the are easily broken by the wind. The name extent of blight losses in the Valley. **Palo Negro" ("The Black Stick") given There are wide variations in the incidence to the disease by the growers has arisen of blight among different growing areas in the from this form of attack. Costa within a particular year. This is so, even where the areas are in close proximity. BLIGHT IN TUBERS For example, 1956 was a severe blight year in the Carabayllo Valley, but the disease was only present in trace amounts in the outskirts Tuber infection before lifting the crop does of Lima. not occur in the Costa. But according to SIERRA: Blight attacks are not so severe Llaveria, Revilla, and Sanchez (1955), there on the potato tops in the Sierra as in the Costa. have been many cases in the Sierra where The disease does not usually appear until 3 or tuber blight, in conjunction with secondary 4 months after planting. There are still large rots, has caused up to 30 percent loss of areas from which it has not been reported. crop. It is not stated whether losses on this The course of a typical blight attack at the scale occurred before or after lifting, but Maco hacienda has been described by Llaveria, infection at lifting is a major problem. How- Revilla, and Sanchez (1955). Experimental work ever, infection in the soil also takes place; is carried out at this hacienda in conjunction in the trial at Maco carried out in 1953-54, with the Station of La Molina, and these workers the control plots showed 5 percent infection give the results of a fungicide trial against with blight at harvest. blight carried out during th.e 1953-54 growing Lack of knowledge about the disease is the season (table 98). Blight on haulm was meas- main reason for losses occurring after harvest ured by a simple scale numbering from Oto 4. in the Sierra. Very often the farmer's reaction The potato variety grown was Maco. to the appearance of blight on the potato tops Since the fungicide was applied by hand, has been to lift the crop, and this has resulted and the applications were repeated at frequent in severe tuber infection. intervals, it can be fairly assumed that the yield of the treated plots (11.5 tons per acre) BLIGHT AND WEATHER DATA shown in table 98 represents approximately the full potential yield of the potato crop. There was, therefore, a 36 percent loss in COSTA: Long-term average temperature yield on the control plots due to premature and rainfall figures for Lima are shown in defoliation by blight. This seems rather a figure 75, together with the main growing high figure, in view of the relatively long season for potatoes in the area. The crop is time (130-135 days) between the time of grown under irrigation during the winter planting the controls and the time the 75 to months. Total annual rainfall is extremely 90 percent blight stage was reached on them. low (average 1.8 inches), and the small amount However, it would appear that the normal of rain that does fall comes during the potato- potato growing season in the Sierra is a long growing season. At this time, however, Hum- one (around 170 days), and this is due to the boldt current effects give rise to more or less

157 TABLE 98. --Blight record of Maco potatoes in plots at Maco hacienda, Peru, 1953-54

Date when tops Time reached 75 to 90 Yield Plots of Blight outbreak percent blight planting stage

Tons/acre Treated^ Oct. 10 1st week of January After March 1 11.5 Control. Oct. 10 1st week of January- 3d week of February- 7.4

The figures refer to the treatment which gave the best results; in this case, a copper dust.

This rule is stated in very general terms, Costa Region 80 LIMA (520fr) and in particular, it lacks precision as to the 70 , number of nights of high humidity that are likely to give rise to severe blight conditions. 60 No mention is made in it of temperature con- RAINFALL - SO'F SO Inches ditions, and these are compared for 1945 and 40 1949 in table 99. Bazan de Segura considered that the large 30 diurnal range of tennperature in May and June 20 1945 probably had an inhibiting effect on the y^r¥77^77. blight fungus, which in part was responsible O N Dec I Jon F A M J for 1945 being a no-blight year, but her general LIFTING , , PLANTING H conclusion from the study of weather data in these and other seasons was that relative FIGURE 75.—Long-term average monthly rainfall and mean tem- humidity and not temperature is the principal peratures in relation to potato cropping at Lima, in the Costa factor in blight outbreaJcs. Temperature condi- region of Peru. tions, particularly duiing the main part of the growing season, are generally favorable to permanent sea fogs, and days with even a few blight. Table 99 illustrates the fact that the hours of sunshine are rare. diurnal temperature range is greatest at the Bazan de Segura (1950) has studied blight beginning and end of the growing period. incidence in the vicinity of Lima in relation The nocturnal humidity rule given above to weather conditions during the period 1945- was used with success to issue spray warnings 50. The main object of this work was to against blight to farmers near Lima in the formulate weather rules on which to base a winter of 1950. Unfortunately, however, the blight forecasting service for the area. Rela- personnel has not been available to continue tive humidity and temperature data were with this work since that year. recorded at the Central Experiment Station SIERRA: Precise weather data are not gen- of La Molina. The study included a detailed erally available for these regions. The winter comparison of weather conditions in 1945 (a (May to Septennber) is a dry, cold season. no-blight year) and 1949 (a blight year). Potatoes are grown in the wet season, during In both 1945 and 1949 average relative the summer months, from October to April, humidity figures remained above 75 percent but there are considerable variations in the practically every day (24 hours) during the amount of rainfall at this time in different growing season. The chief difference between parts of the Sierra, and temperatures also the two seasons was the higher relative humid- vary, mainly with altitude. ities at night in 1949. In 1945, there were very Temperature and rainfall have been recorded few occasions when a relative humidity of 96 at the Maco hacienda since 1953, and sonne of percent or over was reached during the night this information has been supplied by Bazan hours; whereas in 1949, relative humidity was de Segura. In figure 76 rainfall and tempe rature 100 percent for many hours on nnost nights. data for Maco are shown in a '*no-blight" From these observations, it was considered season (1955-56) and a *'blight'' season(l953- that a tentative rule based on nocturnal humid- 54). In 1953-54, blight first appeared at the ities could be put forward. This was as fol- beginning of January and reached the 75 to 90 lows: percent stage in the third week of February A nocturnal humidity of less than 95 on unsprayed potatoes. It seems likely that percent should not endanger the crop; but the low temperatures in 1955-56 played an night humidities greater than 95 percent important part in checking blight in this sea- will indicate the possibility of a severe son, as the monthly mean figures were below blight outbreak. 50° F. throughout the season.

158 TABLE 99. --Average monthly temperatures at Lima, Peru, in 1945 (no-blight year) and in 1949 (bligjit year)

Year and month Mean minimum Mean maYinrnm Diurnal range Mean

1945: Op- i^ril..... 55 §9 34 72 toy 45 84 39 65 June 46 80 34 63 July 50 74 24 62 August.... 52 74 22 63 September. 53 80 27 67 October... 47 83 36 65 1949:

J^ril 55 87 32 71 toy 53 82 29 68 June 51 77 26 64 July 4B 74 26 61 August.... 51 75 26 63 September. 4Ö 77 29 63 October... 50 77 27 64

Sierra Region infection in the soil does not occur. But it is MACO (lM60fr) BLIGHT SEASON 1953-54 not clear to what extent the disease is per- NO BUCHT'SEASON 1955-56 petuated by the succession of potato crops that 60'F 5 RAINFALL - 60 F are grown throughout the year in parts of the Inches % Costa. Also it is not known how important a 50 part infected seed potatoes brought in frona - 40 outside play in the development of epidemics. Control measures are not nearly so common - V\ 1 in the Sierra, where it has been estimated that -^ ^ i i not more than 5 percent of the growers take V, V/, ^1 Y/, ^ action against blight. Llaveria, Revilla, and N Dec Jon F M Sanchez (1955) have mentioned some of the factors unfavorable to blight control in these FIGURE 76.—Temperature and rainfall ponditions in a "blight** and regions. They say that the small peasant a **no-blight" year at Maco, in the Sierra region of Peru. The farmers are in the main against spending potato growing season is from October to April. money to combat blight, as they are not yet convinced that it is necessary. The topography Blight has not been recorded from the is also unfavorable for control measures. The potato-growing areas in the Department of application of sprays is very often impossible, Puno, which is one of the colder regions of owing to lack of water. The cultivated land is Peru, with an annual mean temperature of 42o situated on the slopes of the hills a consider- F. According to Bazan de Segura, it seems able distance from the streams, which flow reasonable to suppose that the disease does through the bottom of the valleys. The water not occur there, because of the low tempera- has to be carried in drums or cylinders, tures. either by mule or on trucks, and this is not always possible, owing to the poor condition CONTROL MEASURES of the hill roads. In the Costa, many of the growers use In these circumstances, the use of dusts fungicides (copper and dithiocarbamates) applied by hand machines has been advocated, against blight. Most of these fungicides are and successful control of blight with such applied as sprays with mechanical equipment. materials has been demonstrated in the trials Some growers start spraying when the potato at Maco in 1953-54. The prevention of tuber plants are only 2 to 4 inches high, and continue infection at lifting is one of the most urgent at 4- to 5-day intervals, even in the absence control measures required in the Sierra at of the disease. the present time. Recommendations to cut and It might be possible to prevent the carryover remove infected haulm 10 to 15 days before of the blight fungus from year to year under lifting have been made, but it is doubtful if this Costa conditions, in view of the fact that tuber advice is very often followed.

159 A suggestion for limiting blight losses in to blight has been reported by Bazan de the Sierra by "disease escape** hasbeenmade Segura (1952b). by Bazan de Segura (1952a). She has proposed SUMMARY: PERU that where it is possible to grow the potatoes under irrigation, planting should be advanced Severe losses from potato blight in Peru fronn the normal time (September-October) to were not reported before 1947. Precise data July-August. On the assumption that blight on the incidence of the disease from year to will continue to appear at about the same time year and between the different parts of the (January), the effect on yield through prema- coimtry are lacking; but available information ture defoliation may be much reduced. This shows that heavy yield losses through prema- method, however, would appear to have only ture defoliation occur at times both in the a very limited application, owing to the lack Costa and in the Sierra. The main potato- of irrigation facilities in the Sierra, and the growing districts are in the Sierra, where early-planted crops may suffer from frost it would appear that low temperatures result damage. Another approach to the problem on in a generally slow progress of blight on the sinnilar lines might be the growing of quicker haulm. These low temperatures are also un- maturing potato varieties to replace the pres- favorable to high potato yields. A major ent indigenous sorts. problem in the Sierra is the danger of tuber Work on the identification of a number infection at lifting time. This arises princi- of different blight races in Peru and the pally from the general ignorance of blight testing of potato varieties for resistance shown by the Indian growers.

CHILE

Potato blight was not found in Chile until Serena in the province of Coquimbo in the 1950, but in 1951 the Chilean crops suffered north, to the islands of Chiloe in the south a first and devastating epidemic, comparable (see maps, fig. 77). This region, which extends in some respects to that in Ireland in 1846. over a thousand miles, may be divided into The story of this latter-day catastrophe has three zones. In the northern dry zone, including been told by Müller (1952) who went out to the provinces of Coquimbo and Aconcagua, Chile as an F.A.O. expert in 1951 to review there is scarcely any rain in the summer the situation in the light of his European months, but there is potato cropping all the experience and to assist the Chilean Govern- year round with the help of surface irrigation. ment in the inauguration of control measures. In the southern zone, comprising the provinces The progress of the disease in relation to of Valdivia, Osorno, Llaiiquihue, and Chiloe, weather conditions, from the first outbreaks until 1956, has been the subject of a special CHILE and very detailed study by P. M. Austin Bourke, who, also under the United Nations Technical Assistance Program, but as a representative of the World Meteorological Organi- zation, went out from Ireland in 1955 to assist and advise on meteorological aspects of the problem. Before Mr. Bourke left on his mis- sion the authors discussed with him the particular data desired for Chile in the present study of blight epidemics. The information he has generously provided, in correspondence, and in his U.N. (W.M.O.) report (Bourke, 1956), has enabled us to compile a rather full section for Chile, tracing the course of establishment of the disease in a country where it was until recently unknown, and exemplifying the use of blight progress curves with the **Irish Rules" for the definition of blight weather, in world blight exploration.

POTATO DISTRIBUTION AND CULTIVATION

Each dot = lOOO ocrei In Chile, which has a population of about 6 million and a total land area of some 286,000 square miles, potato growing is almost entirely FIGURE 77.—Sketch maps of Chile, showing localities referred to in confined to a region extending from about La text and approxünate distribution of potato growing.

160 abundant sumjmer rainfall occurs, and with smaller islands of Chilene in 1949. It does much lower temperatures in the winter, the not appear to have been present in 1937, for principal cropping is in the summer, with a careful search was made for it in Chiloe early varieties in the late winter and spring, in that year by both Chilean and American much as in southern Ireland. In the central plant pathologist s in connection with a seed- zone, including all the provinces from Santiago potato certification scheme, and no trace of to Cautin, conditions are intermediate between it was then found. Bourke narrows down the the two extremes- date of introduction to some time between Of the total of 57,724 hectares (145,000 1937 and 1948, and considers that it probably acres) of potatoes grown in Chile in 1954-55, came from the Argentine; not, as has been as shown by the official statistics, 18,665 suggested, by way of migrant labor returning hectares were in the southern zone. This to Chilcie from the Argentine but by way of zone, in which the greatest losses from blight ships* stores. The most severe blight epidemic have occurred, is of special importance, not on record in- the Argentine occurred about only because of the high degree of local Buenos Aires in 1940-41, and it may have dependence upon the potato for staple food been that diseased tubers were first brought supply, but because, up to the present, it is to Chiloe at that time by ships plying between the only Chilean seed-growing area (Montaldo, Buenos Aires and Chile round the southern 1950). The acreage in the northern zone is tip of the continent (see map, fig. 77). relatively small, only 5,600 hectares. The Whatever the route of introduction, the annual production of potatoes for Chile as a presence of the disease in parts of Chilo'e whole, in recent years, has been about 6 by early 1949 has been fairly well estab- million quintals (59,000 tons), with a mean lished. yield of about 4.3 tons per acre. The mean From study of the available weather records annual consumption of potatoes per head is Bourke found that conditions were almost about 17 5 lb., but in the south, where the certainly unfavorable for the development of potato is the most important food crop, the the disease from then until early in February consumption is about 400 lb. 1950, after which the symptoms were probably Before the appearance of blight in Chile masked by the dying down of the winter crop. most of the potatoes grown were of the But conditions at that time, with many humid **Corahila" type--a varied mixture of indige- periods and much heavy rain, were ideal for nous varieties with red skin and firm yellow tuber infection both in the ground and at ñesh, all very susceptible to blight and rather lifting. Oehrens (1953), in his unpublished slow in bulking« Some European varieties, thesis, '*Tizón de la Papa,'* recorded that such as President and Industrie, were grown many serious cases of rotting of tubers in before 1951, and since that time there has storage occurred in the southern provinces been an extensive substitution of Ackersegen during the autumn months (March to May) in and Voran for the Corahila. Most of the potato 1950. Thus the disease, first recognized in plots or fields in Chile are small, and manual October 1950, was distributed throughout the cultivation is the rule. Planting distances are country in the seed potatoes from the 1949-50 variable, but generally they are about 30 inches crop in the south. It happened that the weather between the rows and 15 inches between sets. conditions were exceptionally favorable for Earthing up is not widely practiced. blight in the following season, and it was then The c limagram s in figure 78 show the that the first devastating epidemics occurred. long-term monthly average rainfall and mean The old story of alarm, scientific inquiry, temperatures in relation to the cropping and suffering of a peasant population was periods for La Serena in the north and Puerto enacted in miniature, but it was enacted Montt in the south. again. The parallel between the first epidemics in Chile in the years 1950 and 1951, and those THE BEGINNING OF BLIGHT IN CHILE in Ireland in 1845 and 1846, is quite instructive. In the first of the Irish Famine years the The first confirmed record of potato blight attack was also late--not until September in in Chile was on October 17, 1950, when the general; only a part (estimated at about a fungus was identified in the laboratories of the quarter) of the crop was lost, but the seed Ministry of Agriculture on the foliage from a was universally infected. It also happened crop growing in Mallarauco, a sheltered valley then that weather conditions in the following southwest of Santiago. In December of that year were exceptionally favorable for blight, year it was also found on tomato plants in the and so came the really devastating and tragic same district. It was then soon discovered epidemic, with the tops killed, not in September that the disease was present to a greater or but early in August, when the tubers were still less extent throughout all the potato-growing very small. It takes at least two blight years areas. Evidence later collected by Müller to build up a first epidemic, and in this sense, (1952) showed that this blight or "tizdn" had blight years are most serious when they come already been causing damage in some of the in pairs.

161 Northern Zone 70 CHILE LA SERENA RAINFALL H 60 Inches 50

3 - 40 IRRIGATION 2 h it X^*^^^^^^^^- 30

ÍZ1 Awv^-^ J I \t i i\ J A S O N Dec I Jan F M A M J WINTER CROP ^ iJWINTER CROP SPRING CROP SUMMER CROP

Southern Zone PUERTO MONTT

RAINFALL e Inches

FIGURE 78,—Long-term average montbly rainfall and mean temperatures in relation to cropping in the northern and southern potato growing zones of Chile. COURSE OF THE EPIDEMICS IN THE SOUTH- maincrops are planted about the end of October, ERN ZONE are aboveground by the end of November, beginning to form tubers in December, producing It is in the southern rainy zone of Chile that most of their crop in January and February, blight is of the greatest consequence. Bourke and dying down with little growth left in March. (1956) has collected and sifted all the available Thus the months of December, January, and information on the incidence of blight in this February in the southern zone of Chile cor- region for each year since 1949, and he has respond approximately with the months of June, presented this in the form of estimated blight July, and August in the south of the British progress curves. From these the probable Isles. (See tables 3 and 29 for England and for mean starting and 75 percent dates are taken Ireland.) and lined up in table 100, as against the mean The mean temperatures over the growing temperature and rainfall figure s for December, period in this part of Chile are very similar January, and February in each of the years. to those in southern Ireland, and the rainfall There is perhaps a month's range each way in figures, though perhaps varying more from planting and lifting dates, but typically the year to year, are also very similar.

162 TABLE 100. --Rainfall, mean temperatures, and deviations from mean temperatures in relation to blight at Puerto Montt, Chile, 1949-56

Rainfall Deviation from mean Blight in temperature southern zone Year Started Dec. Jan. Feb. Dec. Jan. Feb. 75 percent about-- date about--

In. In. In. OF. OF. OF. Average ■'■ 4.4 3.5 4.1 57 59 59 1949-50 -- 3.7 9.5 5.6 +1 +2 +3 Feb. 4 Mar. 31 1950-51 4.6 7.0 2.0 0 -1 -2 Dec. 8 Jan. 31 1951-52 3.7 1.1 1.7 +3 +2 +2 Nov. 25 Feb. 28 1952-53 .9 6.0 3.9 +3 +1 +3 Dec. 23 UhT, 26 1953-54 4.6 2.7 2.3 +3 0 +1 Dec. 27 VhT. 31 1954-55 7.4 5.4 5.1 +2 +3 +2 Dec. 3 J/feir. 10 1955-56 8.1 6.1 2.6 +1 -3 +1 Jan. 3 mr. 31

Long-term average ten^eratures are for 1911-40; rainfall averages are for 1911-48.

I950 1951 lO I950-5I BLIGHT ^ ^ ON y" HAULM o - ^ - per cent No records |<^ 23 33 X X X X X 1 o _■ m^ ^ ■ 1 ■ I ■ ■ ■ 1.

1951 1952 oo 1951-52 /^

^ 25 18 20 13 ^ O l..l^^ ^ ■ 1 ■ ■ ■ !■■■■ ■

1955 1956 lOO 1955-56

50 - ^ ^-

34 5 15 16 O ■ ■ ». 1 ■ ■ ■ 1 NOVEMBER DECEMBER JANUARY FEBRUARY MARCH

FIGURE 79.--Approximate progress of blight attacks in relation to blight weather spells for unsprayed maincrop potatoes in the southern zone of Chile. The effective duration in hours ofthe longer spells is given; the shorter spells ranged from 1 to 10 hours. (Data from Bourke, 1957.)

163 In each year since 1950 blight attacks in the ginning of January. It was stimulated by along southern zone of Chile have started at some- spell at that time, but thereafter there was time between the end of Novennber and the end only one short spell until the end of February, of December. In 1950-51 the disease made and the attack did not become serious until the rapid progress, and by causing 75 percent haulm was dying down in March. destruction of the haulm by the end of January, In the southern potato-growing zone of Chile it gave rise to a loss of about half the crop. severe blight weather conditions such as those In all the subsequent years the progress of the accompanying the epidemic of 1950-51 have disease has been retarded after the outbreak, not recurred in the subsequent 5 seasons, and so that it has not reached the 75 percent stage from a general analysis of past weather rec- until sometime in March, thereby causing little ords Bourke estimates that such exceptionally or no defoliation loss, but considerable infec- severe epidemics are unlikely to occur in tb^s tion in the tubers. region more often than 1 year in 8 or 10. To some extent the courses of the epidem- **Blight years," in which the attack is more ics in the 6 years were consonant with the moderate and much of the loss is by tuber simple mean temiperature and rainfall figures. infection, will occur more often, but there is as In the one severe blight year, 1950-51, the yet insufficient data to establish their fre- mean temperatures were below average. In quency. Although the mean temperature and four of the subsequent years the monthly rainfall figures during the growing period in mean temperatures were 2° to 3^ F. above southern Chile are very similar to those in average, and many of the rainfall fig- southern Ireland, and the mean diurnal range ures were low. In 1955-56, however, when the of temperature is also much the same (about least severe epidemic occurred, the January 15° F.), blight weather evidently occurs less mean temperature was well above average, and frequently than in Ireland, and the general the rainfall in December and January was high. incidence of blight over the years has so far Thus, although in southern Chile as in the been much lower. Bourke attributes this to the British Isles, high rainfall and nmean monthly lesser frequency in this part, of Chile of young temperatures only slightly below average tend atmospheric wave depressions with wide warm to favor the development of blight and temp- sectors such as are responsible for the longer eratures 2 to 3 F. above average with lower and more important periods of blight weather rainfall tend to hold it in check, rainfall and in Ireland. temperature figures alone are inadequate indi- cators of the occurrence of blight weather. EPIDEMICS IN THE NORTHERN ZONE By study of the available humidity records for southern Chile and the application of his The extreme north of Chile is one of the **Irish Rules" (see section on Republic of driest regions in the world; and the desert con- Ireland), Bourke was able to obtain a fair indi- ditions do not change until about latitude 27° cation of the **blight weather spells*' that S., where the mean annual rainfall is about probably occurred in each of the 6 years. In half an inch. From there southward, the rain- figure 79 these spells are here charted in re- fall progressively increases to about 5 inches lation to the estimated progress curves for at La Serena (30^ S.), 10 inches at Valparaiso blight in the years 1950-51, 1951-52, 1955-56. (33° S.), and 100 inches at Valdivia (40° S). Bourke stresses the importance of the **ef- The coastal potato cultivations in the northern fective duration" of the blight weather spells; potato zone about La Serena are dependent the longer spells are therefore distinguished upon irrigation, but even so, potato growing by taller blocks in the diagrams, and their would not be possible over much of the year effective durations in hours are also given in but for the influence of the cold Humboldt figures. Beaumont periods occurred in cor- current, which ^trikes the Chilean coast at respondence with most of these longer spells, about latitude 40 S., and sends a branch north- but were rarely associated with the shorter ward up to the coast of Peru. This ocean cur- spells of 1 to 10 hours. rent cools the air above it, giving rise to much Figure 79 shows clearly that in 1950-51 the sea fog and cloud and keeping the temperatures rapid progress of the disease was related to low and remarkably constant. Thus, although the numerous spells of blight weather, some of it i^ at about the same latitude as Brisbane long duration, which occurred at short inter- (30 S.), La^Serena has a mean temperature vals throughout December, January, and Feb- of^about 52 F. in midwinter, rising only to ruary. In 1951-52, with several spells of long 65 F. in midsummer. duration in November and the beginning of A mean temperature of 65° F., though rather December, the attack began earlier than in 1950. high, is by no means high enough to inhibit At first its progress was rapid, but thereafter, blight. Even so, the climagraun for La Serena until the beginning of February, there was only (fig. 78), with its striking dearth of rainfall, one short spell and the progress of the disease would not suggest much likelihood of blight on the maincrops was arrested. In 1956, with attacks. The low temperatures for the latitude only short spells in December despite the high and the proximity to the sea should give rise to rainfall, the attack did not start until the be- suspicion, but the diagram provides no direct

164 indication of the dense and wetting morning Voran, were brought in from Europe to replace fogs (camanchacas) which are common along the very susceptible Corahila mixture of varie- the coast and sometimes persist for much of the ties. At first these **white" potatoes were not day. popular, but the prejudice appears to have died In 1950 blight appeared in the winter crop down, and Bourke reports that the more re- about La Serena--the disease having been sistant varieties have now replaced the Cor- brought in with seed from the south--and ahila to the extent of 60 percent or more in although the attack on the winter crop was not the areas most subject to blight. Breeding work then serious as the winter was cold, the on other varieties is in progress. infection appears to have spread over, with Some protective spraying with bordeaux mix- favorable weather in October and November ture and other copper fungicides is done in the 1950, onto crops of the spring and summer La Serena and central areas, but the majority plantings. According to Müller the attack of ^the growers in the south, particularly in reached epidemic proportions from January Chiloe, where the yields are small, are not 1951 onward, dimiinishing in November 1951. convinced that it is an economic control The winter of 1951 was mild and the winter measure. Müller (1952) advocated all the usual crop in that year (sown about May) suffered cultural methods of control now practiced in severely. Montaldo (1953), cited by Bourke, re- Europe after a century's experience of the ported that a crop of the imported variety disease, notably earthing up of the drills to Sebago, near La Serena during this winter protect the tubers from infection, precautions season, was a complete failure because the at lifting, and measures to minimiize the in- newly emerged plants were heavily infected clusion of blighted tubers in seed stocks. He from neighboring crops of the Corahila po- also advocated the avoidance of winter cropping tatoes. Weather conditions were favorable for in the south, to break the continuity of infec- the infection of spring-planted crops in No- tion, and recommended the establishment of a vember and December 1951, but the disease **tizón look-out service." Bourke has carried disappeared around La Serena by January this latter suggestion much further and shown 1952. In subsequent years the disease has re- how a forecasting service can be operated. curred to some extent in the winter and spring He has also advocated quantitative blight sur- crops, but the attacks have been less and less vey work, on the lines of that carried out in severe and the summer crops have been vir- England and Wales in recent years, to es- tually free except in 1955, when Bourke found tablish the true significance of the disease in blight present on April 14, at the 10 percent the various regions and to guide control policy. stage by the B.M.S. key, on almost every crop he inspected in the La Serena region. This was SUMMARY: CHILE associated with quite abnormal rainfall in March. Potato blight was unknown in Chile until the No survey records or even the most tentative late 1940*s, when it was introduced probably of progress curves are available for the north- by infected ware tubers in ships' stores from ern zone, but it would appear that over the the Argentine. Outbreaks were identified in summer months, with fairly high temperatures many parts of the potato-growing regions late and virtually no rain, there is little blight, in 1950, and in 1951 there was a devastating despite the sea fogs and despite the surface epidemic that caused great distress and pri- irrigation (which Bourke considers to have vation, particularly in the Chiloe region, where no effect on the incidence of the disease). In the climatic conditions are rather similar to the La Serena district, however, blight weather those of southern Ireland. Fortunately, and can occur in the winter months with mean perhaps owing to less frequent persistence of temperatures about 55° F., and extremely little moist warm air masses over the area, severe rain. Whether they occur or not depends upon epidemiic conditions have not recurred since Humboldt current effects and not upon the 1951, although the disease has been present movements of atmospheric depressions, for every year and has caused some infection of this part of Chile is beyond the northern limit tubers. Other severe epidemics are to be of their track. expected in the future, but there is a hope that their impact may be lessened by the intro- CONTROL MEASURES duction that has taken place of more resistant varieties than the native Corahila. Severe Immediately following the first epidemic in epidemics can occur in northern coastal areas, 1950-51, varieties having some marked field despite very low rainfall, owing to the preva- resistance to blight, notably Ackersegen and lence of fogs.

165 AFRICA

UNION OF SOUTH AFRICA

In the Union of South Africa, with a total ground is from the end of September to mid- population in 1954 of 13-1/2 million, there is a April. Planting may be done in September, but considerable production of potatoes. The the first heavy rains are generally needed to F.A.O. (1956) figures for 1948-52 show some moisten the ground, and planting is not general- 140,000 acres each year, with a production of ly completed before mid-October. With nearly about 230,000 tons, indicating a mean yield of optimum temperatures and plenty of sunshine under 2 tons per acre. The TransvaalHighveld a 120-day growing period is adequate, and the to the east of Johannesburg is by far the most first crops will normally ripen in January. important center of production (see map, fig. When the crops are nearly mature they are 80), In this area, in which there are some earthed up well and normally kept in the ground 40,000 acres, the average yield is about 4 tons until they are to be marketed. This marketing (60 bags) per acre. There are lesser concen- period extends from January (for the very trations of production at similar cdtitudes earliest crops) through the rest of the summer, (5,000 to 6,000 ft.) to the east of the Orange autumn, and winter until August. There is little Free State, in Natal, and also on the lowland winter rainfall- The soil does not freeze to any along the extreme southern seaboard from great depth; there may be surface penetration Cape Town to Port Elizabeth. The Union is of of frost before the sun melts it during the day, particular interest in a world study of potato but there is no permanent freezing. Thus, there blight, (1) owing to the striking contrast be- is no clamping or other intermediate storage tween the clinnatic conditions at an altitude of of potatoes, as in Europe. They are left in the 5,600 feet in the Transvaal and those in the ground and lifted, sorted, and bagged as re- coastal areas of the south, and (2) because in quired for market. Planting is in rows 36 to the Transvaal the presence or absence of 45 inches apart, the widest rows (45 inches) primary infection appears to play a greater being those of farmers who like to earth up part in determining the occurrence of blight very high for the storage period. Spacing be- epidemics than is the case under European tween plants in the row is 16 to 22 inches. Many conditions. There is little in the literature growers start earthing up when the plants are concerning the incidence of blight in the Union, very small as a precaution against tuber moth. and for information the authors are indebted The climagram for Bethal suggests that with to J. E. van der Plank, who has carried out a 120-day growing period and with the abundant inquiries into recent epidemics, in part for the rain and favorable temperatures, late plantings special purposes of this study. would be made up to mid-January to obtain crops maturing in April before the frosts. CLIMATIC AND CULTURAL CONDITIONS But, in fact, the second half of the long season Figure 81 shows the long-term average favorable for the growth of potatoes is little temperatures and rainfall for Bethal, well used, owing to the incidence of blight. representative of the potato-growing areas Around George, 240 miles east of Cape Town, in the Transvaal Highveld; and also for George rain is well distributed throughout the year; the and Cape Town in the south. The climagrams climate is temperate, and near the coast the suggest that in the Transvaal, with abundant area is free from frosts. Potatoes are grown the summer and low winter rainfall and the mean year round, and blight is severe, especially in temperature line above 50° F. from August to the autumn and early winter. There isa constant May, there will be potato cropping only over carryover of inf e ction from one crop to another. this period; whereas at George in the south Around Cape Town the climate is notably there will be cropping all the year round, and different from that at George and also from around Cape Town winter cropping will be the that in the Transvaal; it closely resembles rule--the low rainfall and high temperatures that of Western Australia. The summers are in the summer months being unfavorable for dry, hot, and windy. The cropping period is the crop. fronn March to October or November, utilizing These indications are borne out in practice, the winter rains. Severe blight epidemics occur although at Bethal, with an upland continental over the winter and spring and are the limiting climate the diurnal range of temperature (from factors in potato production, which has now 54° to 80° F. in December and January) is fallen to a relatively small amount. greater than in the maritime regions. Fine warm days follow sharp cold nights, and the BLIGHT EPIDEMICS IN THE TRANSVAAL potato-growing period is limited by the oc- HIGHVELD currence of frosts. The average date for the In the 1955-56 season the potatoes, all Up- last severe frost is September 20 to 30, and to-Date except for a very few Kennebec, were for the first severe frost of the succeeding planted in September and October and were winter about April 20 to 30, so that the growing closing across the drills by about the end of period during which the plants may be above- November. On December 18-20, blight was 166 AFRICA : EAST 6 SOUTH

BE LG IAN CONGO

FIGURE 80.—Key map of East and South Africa: Shading indicates areas where potatoes are grown.

167 SOUTH AFRICA The rainfall and mean temperature figures for .Transvaal Hiqhveld^ the season were: BETHAL , S,600f» , Rainfall Mean temperature Month: (inches) ^* RAIN FALL Inches September 0 57 October 5.5 60 Novemiber 5.3 63 December 8.4 64 January 1-3 65 The 1955-56 season was worse than average for blight, the attack being earlier than usual; this may have been associated with the high rainfall in December, but there were also dull I^CROPPINC PERIOD*|^-CROP IFFT IN CROUND- and overcast conditions in late December, continuing into January. The progress of the dis- lOO BLICHT ease was stopped by the dry weather later in ON 50 HAULM January, and the few leaves then remaining per cent O green were not further attacked. Precise information is not available for previous years. In general, blight starts toward .South Coastal the end of December and does not reach the 75 CEORCE X>ercent stage until the end of January, when growth is over in most of the crops. Indications RAINFALL of the probable mean blight progress curves Inches for most years, and for the bad blight year 1955-56, are given in the climagram (fig. 81). The estimated loss of 15 percent of the crop by defoliation in 1955-56 is fairly consistent, with some 2 to 3 weeks' loss of growth in this particular year. CROPPING ALL THE YEAR ROUND- In most years the loss from blight is slight in crops planted as early as possible (in September and October), as these are already Cape Town dying down (in January) before much defoliation by blight occurs. The major loss arises from the fact that the incidence of blight renders RAINFALL Inches it impossible to utilize the second half of an otherwise excellent season for potato growing. The market quality of potatoes left in the ground from January deteriorates by sprouting and other causes by July; and were it not for blight, second plantings would be made in December JASO or January to mature before the first frosts at SPRING CROPPING h«-WINTER CROPPING the end of April and to provide supplies of potatoes in good condition after relatively short FIGURE 81.—Long-term average monthly rainfall and mean temper- storage, to supply the winter market. The re- atures in relation to potato cropping for three centers in the Union of sult is a scarcity of good potatoes on the market Soutb Africa, with blight curves for the Transvaal. from July onward. Crops planted in December or January would be cut down by blight while the plants were still small, and thus there would found scattered over the whole area, and by be little or no yield. Experience noW being won January 12, 1956, only a few green leaves in the high valleys of Mexico (q.v.) seems to were left* The weather then cleared and further point a way toward overcoming this loss of spread of blight was stopped. On December 20, the second crop. the estimated crop for the whole area was 2,300,000 bags of 150 lb. each. After the epi- SOURCES OF INFECTION demic the actual yield was 1,850,000 bags. Of the drop, 350,000 bags, or 15 percent of the On the Transvaal Highveld it cannot be as- expected yield, was attributed to loss by pre- sumed that primary sources of blight infection mature defoliation; and 100,000 bags, or about are always present to start an epidemic when 4-1/2 percent,, to rotting of the tubers in the weather conditions become favorable for its ground from blight infection and other causes. development. As the climagram indicates, The loss in individual fields depended on the there is almost always abundant rain and favor- age of the crop at the time of the blight attack. able temperatures for blight from October

168 onward. Although many of the earliest planted on three sorts of resistance to late blight by crops are well forward and in flower by No- Van der Plank (1957) distinguishes between(1) vember, no blight has ever been seen in that R-gene resistance; (2) field resistance as- month. In the 1955-56 season, despite sociated with late maturity (as in Voran and in exceptionally heavy rain during October, No- the Dutch Robijn now grown in Kenya); and (3) vember, and December (49 inches in 11 weeks field resistance not associated with late ma- at Harrismith) and squelching fields, no spot turity (as in Se bago). of blight was found in the Highveld up to On the whole, the only important control December 31. measure in the Highveld has been to plant The seed from the Highveld crops saved for early for blight escape; but this is effective planting in the following year is riddled off only at the cost of a sacrifice of supplies for from the ware, or taken from crops specially the late market, which commands the best intended for seed. In either case it is lifted price. Attempts are being made to breed late, and it is often stored with a light straw varieties of about the same maturity as Up-to- covering under the dense canopy of evergreen Date but with tubers having a longer period of trees, which gives adequate protection from winter dormancy. One variety of this kind is the frosts. It may be that this combination of already under test and shows promise. It is circumstance s--the late harvesting of seed considered that breeding to this end may in after it has lain during the autumn in warm some ways be an easier task than breeding for soil and the temperate winter climate--is disease resistance, which is also under way. unfavorable to the survival of Phytophthora Dr. Van der Plank thinks the Highveld area infest an s in the local seed, so that in some may be so isolated from airborne infection years there is little primary infection from this in other areas and have storage conditions so source. unfavorable for the survival of the fungus in It is possible that the first infection may come the tubers, that epidemics might be suppressed in from other areas. It is known that the disease by growing only local seed or by heat treatment occurs in winter crops of potatoes and toma- on the Jensen principle of imported seed to in- toes in the frostfree. areas near the east coast. sure its complete freedom from blight infection. Some support for this idea comes from the Against this there is the fact that the humidity fact that blight epidemics normally occur in conditions, as apart from rainfall and temper- summer crops in the coastal province of Natal ature, of the Highveld have not yet been investi- before there are any outbreaks far inland, in gated. It is possible that records of blight- the main production area of the Highveld. weather spells by Bourke's Irish Rules (see Lastly, there is undoubtedly some blight section on Chile) or of Beaumont periods (see infection in imported seed. Much seed is still England and Wales) might reveal that the imported annually, mainly from Scotland. Be- relatively late outbreaks (in December) are due cause of the opposite seasons, the imported to a general absence of critical blight weather seed arrives late, and is mostly planted from before that time, rather than to any lack of the the second half of November onward, fully 2 very few and widely scattered primary infect- months after the planting of the local seed, ors that are necessary to start an attack over and this might explain the timing of the blight a wide area (see Van der Zaag, 1956). epidemics, which commonly begin, not in SUMMARY: UNION OF SOUTH AFRICA November as might be expected frominfection in the local seed, but in December. In the Transvaal Highveld, the principal potato-growing area of the Union, the major CONTROL MEASURES loss from blight is a loss of the whole of the Spraying plays practically no part in Highveld second half of an otherwise ideal potato-grow- farming. At least one farmer with a large ing season. Late-planted crops could be used to provide potatoes for the winter market if acreage and proper equipment sprays regularly the disease were controlled. Even with re stric- and says it pays him, but the spraying is tion to early planting for disease escape, losses basically with DDT to control tuber moth. Zineb is added for blight, but a part of the are still occasionally severe, as for example advantage may come from control of early in the 1955-56 season when the loss was about 20 percent of the expected crop. The frostfree blight. Alternarla solani, which is very com- areas of the Southern Cape, the only part of mon, and sometimes more injurious than the the Union.with adequate winter rainfall, should late blight caused by Phytophthora infestans. be the natural source of out-of-season crops; Some seed of Kennebec, an Ri resistant in the past it was usual for Cape potatoes to be variety, has been introduced, but in 1955- sent in quantity to northern markets in the 56 it suffered almost as severely from winter. For many years, however, blight has blight as the Up-to-Date variety. Two R1R2 limited potato production in this area, and the types (Onikwa 52 and Onikwa 53) have been Cape now imports potatoes from the north. In tried in the Cape Province, but are not Natal, crops of both the January and the Septem- promising. Blight races 1; 1.2; 3; and 4 are ber plantings are severely attacked in some certainly present in the Union, and skepticism years, and the damage is of particular conse- is felt about the value of varieties containing quence in seed crops for the Transvaal. the R-genes, singly or in combination. Anote 169 SOUTHERN RHODESIA

Conditions of potato growing in Southern Rhodesia until 1946, when it was found on Rhodesia (fig- 80), where according to F.A.O. Up-to-Date in the Enterprise district of Salis- (1955) there were some 5,000 acres in 1952, bury, on a first-irrigation crop about May 10. with a production of about 16,000 tons, are In the particular crop of 13 acres on which it in some respects similar to those in the was then found, there had been 13 irrigations Transvaal, but higher temperatures in the before flowering, and with 2 inches of rain in winter months permit cropping all the year. May, conditions were unusually wet. Although G. R. Bates reports that more than half of most of the foliage was killed, little if any the Southern Rhodesia crop is produced within loss of crop occurred, and all the tubers were a radius of about 35 miles of Salisbury. Since well developed. A little blight was seen on blight became established in that locality in neighboring farms that year. It is not known 1952-53, there has been an increasing tendency how the blight was introduced; the affected to explore new areas for potato growing, and crop was grown from local seed. Potato quite substantial amounts are now grown in supplies for troops during the war may have the Midlands, around Bulawayo, and on the brought in the disease. high ground between Salisbury and Umtali. Nothing further was seen of blight after Both seed and ware are also grown throughout 1946 until 1951-52. In 1952 there was asevere the mountainous eastern districts, where there attack in the eastern seed-growing area about are humus-rich friable soils. The tubers have Inyanga (fig. 80), and by 1953 the disease had an excellent keeping quality--a characteristic spread to the main Salisbury area. Dr. Hopkins rare in other areas of Southern Rhodesia. conducted a seed-potato growing scheme (summer crops) on the mountains about Inyanga, CLIMATE AND CULTIVATIONS and between 1940 and 1947 there was no blight in the area. In 1949-50 the Portuguese started The rainfall pattern in the Salisbury area potato growing with imported seed just over somewhat resembles that at Be thai (fig. 81). the Mozambique border, at over 7,000 ft. in There are 6 dry months and 6 rainy months. the mist belt near Manika. Blight attacks The rains start with thunderstorms in mid- becajne very severe, and it is possible that November, increase to January and February, southeasterly winds during the winter spread which is the wettest period, and end in March, the disease across the country. with a few thunderstorms in April. May is dry, and the weather then remains dry until mid- EPIDEMICS SINCE 1953 November. July is the coldest month, but here the mean July temperature (56 F.) is well G. R. Bates and W. E. Kerr have reported above the 50^ level, in contrast to that at that blight has been severe every year in Bethal (46° or 47°). Southern Rhodesia since the first general Three potato crops are grown during the epidemic in 1953. This has been due mainly year--the main, or summer crop, which is to a succession of very wet summers. In the usually planted in November with the onset of summer, or main, crop, overall blight losses the rains, and harvested in February; a first- have been estimated as never less than 25 irrigation crop, planted in March and harvested percent, and occasionally they have reached in June; and a second-irrigation crop planted 50 percent. Individual summer crops on some at the end of July and harvested in October. farms have been a complete failure. In each Frosts normally occur in June and July. of the 5 seasons blight was noted in the Salis- For irrigation crops a typical rotation is bury district between December 20 and Janu- potatoes, followed by maize--for two seasons ary 10, except in 1956-57, when, following an at least--during the summer, and wheat or unusually early and prolonged rainy spell in barley in the succeeding winter, followed by mid-November 1956, the disease appeared green cropping or summer fallow. Potatoes before the end of that month. are grown usually every third or fourth year The pattern of infection among the first- on the same land, but the rotation is now being irrigation crops has varied considerably. In widened. Until recently, the variety grown 1953 and 1954 it was sporadic and losses did has been almost invariably Up-to-Date, which not exceed 10 percent. Subsequently, losses is very susceptible to blight but has a strong of up to 30 percent have occurred. This has consumer preference, is well adapted to the been due to very wet seasons with extensions latitude, and has a fair tolerance to leaf roll. of the rains into May or even June, and also It is also drought resistant. to a tendency among the Salisbury growers to plant the first-irrigation crop earlier and BLIGHT INCIDENCE: 1946-53 earlier, with the result that the summer and first-irrigation crops have overlapped. Irriga- J. C. F. Hopkins, who was at Salisbury from tion is mainly by the furrow method, although 1926 to 1953, has told the authors that there overhead irrigation is becoming more common. were no records of potato blight in Southern With furrow irrigation, blight tends to attack

170 the stems and leaf stalks rather than the resistance has proved a failure for summer leaves; but this may perhaps be a characteris- cropping conditions in the varieties tested tic of the potato variety, for stem lesions are (Kennebec, Pentland Ace). Better results have very common on Up-to-Date, even when it is been obtained with fie Id-resistant varieties, grown under natural rainfall in the southwest of England. such as Furore, Voran, Robijn, and in particular. Pimpernel; but all these have their No blight has yet been observed on the disadvantages, and search is continuing for second-irrigation crop, but this matures during more acceptable field-resistant sorts. Recent the hottest period of the year and the keeping quality of the tubers is poor. records show that blight races 0; 1,3,4; 3,4; 4; and 1,4 are present in summer crops, and races 4 and 1,3,4 in irrigation crops. CONTROL Disease-escape methods are being used and explored. The seed crops along the eastern All attempts to control blight by spraying border, which once were threatened with have failed on the sunamer crops, chiefly extinction because of the devastating blight because of difficulty in obtaining adequate attacks, are now planted much earlier to protection under conditions of torrential rain. escape the most dangerous blight-period More promising results have been obtained (December-January). Elsewhere in the country on the first-irrigation crops, particularly the main (summer) ware crops can be planted with the dithiocarbamates. before about mid-October only where supple- A fair amount of work has been done on mentary irrigation is available. Field trials resistant varieties. Little prospect of success over the past 2 seasons have shown the value is expected until more attention is devoted, of planting in the third week of September as it has been during the past 2 years, to under such conditions, and heavy yields have cultural practices and the choice of varieties been obtained. Ulster Prince is being tried for the various croppings according to their for very early summer planting to escape several cropping characteristics. R-gene blight.

EAST AFRICA

The problems of potato growing in East able areas, and the cultivation of this crop Africa have been reviewed by Moreau (1944b), is becoming more important. Moreau (1944b) who has given an admirable account of the concluded that the short-day conditions were particular localities in the mountains of of no practical consequence, and that there Northern Rhodesia, Nyasaland, Tanganyika, was no reason to suppose that S. andigenum Kenya, and Uganda where the crop is grown, potatoes would do any better than suitable as well as a critical study of rainfall and varieties of S. tubero sum. According to F.A.O. temperature conditions. The production of (1955) the potato acreage in recent years has potatoes over the whole of this vast area is beenabout 12,500 in Kenya, 2,500 in Tanganyika, small and very localized, and with some 15,000 in Uganda, and 2,500 in Northern exceptions, the '^English*' potato is a minor Rhodesia. Mean yield figures are scanty and crop, providing but a small part of the native are not too reliable, but actual yields probably diet. In Uganda in 1955, for example, there range from 1 or 2 tons per acre in native were 500,000 acres of beans, 520,000 of cultivations interplanted with beans, to 4, or cassava, 1,585,000 of coffee, 425,000 of ground- very exceptionally, even 8 tons per acre with nuts, 380,000 of maize, 1,218,000 of millet, good European husbandry. Potato blight has 220,000 of peas, 1,414,000 of plantains, invaded most parts of East Africa only within 582,000 of yams and sweetpotatoes, and only the past 20 years. Watts Padwick (1956) has 15,000 acres of the ^'English" potato, or attempted to assess the approximate losses Solanum tubero sum. Most of the potato culti- now caused by the disease from data received vations are primitive, and often they are a in response to a questionnaire sent out as luxury vegetable that the natives can sell for part of a Colonial Plant Disease Survey. For a good price despite the small yield, which all parts of East Africa he has put the per- is often less than that of the earliest **new** centage loss at 10 percent or more. For the potatoes in Europe. Cans of such potatoes purposes of the present study, Kenya has are frequently sold by natives at the roadside been considered at some length, as typifying to passing long-distance lorry drivers. the equatorial mountain cultivations. The Nevertheless, with good husbandry excellent sections on Uganda, Tanganyika, and Northern crops of S. tubero sum can be raised in suit- Rhodesia are therefore shorter.

171 KENYA

KENYA

2°N

^^500ft. contour

Site of first outbreak,1941

^Meru KENYA EQUATOR 0° '{^'^-MT. KENYA

POTATO CROWING AREAS MI. KILIMANJARO

European

Africon

200n

FIGURE 82.—Key map of Kenya, showing principal potato growing areas and localities referred to in text.

Potatoes have probably been grown in Kenya in Kenya from its beginnings up to the present for as long as the Highlands have been settled time. by Europeans. The acreage, however, is relatively small and yields are low. According DISTRIBUTION OF POTATO GROWING to F.A.O. (1955) the mean area for the crop AND CLIMATIC CONDITIONS in the years 1948-52 was about 12,500 acres, with a mean production of about 21,000 tons, As shown by the key map (fig. 82), potatoes or 1 3/4 tons per acre. With a population of are now grown on both sides of the Rift Valley about 6 millions, the annual consumption at altitudes between 6,000 and 9,000 ft., of potatoes, together with other starchy roots, between latitudes 2° N. and 2° S. of the Equator. was estinnated at only about 20 lb. per person. The cultivation is in both native and ''settled" Blight was first observed in Kenya in 1941 areas within this altitude range. In all native by R. M. Nattrass, to whom the authors are areas the potatoes are produced as a food indebted for a first-hand account of the disease and as a cash crop. There is some specializa-

172 tion in potato growing here and there by the Rift the rains are heavier and more European farmers, but this is an indication evenly distributed over the year. There is of inclination rather than topography. Almost one planting in April or May, with lifting in all the area between 6,000 and 9,000 ft. is August or September. suitable, but the best parts lie within 7,000 During the rains at the higher altitudes, and 8,500 ft. especially east of the Rift, there are periods The climagrams (fig. 83) illustrate the of mist and much drifting light rain in addition mean rainfall and tennperature conditions at to the heavy falls. So far as temperature is two centers, respectively, west and east of the concerned, there cannot be said to be any Rift Valley. These figures show the general regular succession of spring, sununer, autumn, pattern of the rainfall, but the actual precipita- and winter. The seasons are dependent upon tion in individual months is exceedingly the advent of rains, and it is only lack of variable. East of the Rift there are two rain that stops potato cropping at any time plantings. One, the **long rains** crop, is of the year. The mean temperature lies planted in March and lifted about July, and between 60 and 65 F. the whole year ro\ind, the other, a *'short rains** crop, is planted and the controlling temperature variation in in October or November and lifted about the highlands is the diurnal range, which February. The **short rains** crop is often becomes excessive on all days when clouds light, as the rainfall is considerably less are absent. The mean diurnal range at Nairobi, than that in the **long rains** crop. West of however, is 18° (52° to 70°) in August. It rises to 23 or 24^ only in January and Feb- ruary. KENYA . Wesr of Rifr Valley THE BEGINNINGS OF BLIGHT IN KENYA SONCHOR Dr. Nattrass first found the disease in May 1941, about the middle of the east Rift on the **long rains'* crop (see climagram). The focal point appeared to be on high land

RAINFALL at about 8,000 ft. near the eastern escarpment Inches of the Rift Valley 60 miles north of Nairobi (see key map). The affected variety was Kerr*s Pink, in a crop for which imported seed had been planted. On his return to Nairobi, Dr. Nattrass found the disease in a garden at 7,000 ft., about 20 miles west of the city. Within the following few weeks reports of the **new disease** began to come in from practically all the potato-growing areas east of the Rift, particularly from the Aberdares Range and Mount Kenya--a large tract of country including extensive areas of Kikuyu reserve. By the middle of November, to quote one agricultural officer, **the disease had assumed epidemic proportions over the .Easrof Kift Valley whole of this area; squatters* shambas were NAIROBI heavily infected, and the crops in many parts, 5,700ft almost, if not wholly, a failure.'* The disease did not appear across the Rift Valley until a year after the original RAINFALL Inches outbreak; the first notification of the disease on the west escarpment was in June 1942. A month later it had reached the foothills of Mount Elgon on the Uganda borders. From this point it spread westward into Uganda and eventually into the Belgian Congo. South- ward, Tanganyika was reached in the same year (1942). One isolated area in the Usambaras in Tanganyika held out until 1943, although O N D heavy attacks occurred in the closely settled •^SHORT RAINS CROP — slopes of Mount Kilimanjaro. In 1944 the Tanganyika plant pathologist reported that the FIGURE 83.-Long-term average monthly rainfaUand mean tempera- disease was present in all the important areas tures in relation to potato cropping, west and east of the Rift VaUey of the Territory. in Kenya.

173 SOURCE OF THE FIRST INFECTIONS the raising of some fantastic hybrids with the ordinary domestic varieties this achieved Until 1939-45, potato growing was almost nothing. The main work has been with Black's entirely confined to African production, R-gene resistant hybrids from Scotland, of although one or two European farmers had which about 60 have been tried up to the imported seed and grown commercial crops present time. These R-resisters were at first from time to time. With the low prices then very successful, being immune from attack prevailing, however, commercial production by the blight races 0 and 2,4, then the only of potatoes by Europeans was much restricted races that appeared to be present. Some of or even nonexistent. Apart from squatters* these varieties were grown on a fairly large production on European farms, which supplied scale, and although they were eventually most of the European market demand, there attacked, they are still in cultivation. As new was (and still is) some growing of potatoes blight races appeared, seedlings withrnore at higher altitudes as a native food crop to genes for resistance came from Dr. Black, replace maize. The native crops were not and were tested against them. So far the blight pure varieties but mixtures of odd sorts long races identified from Kenya are 0; 2,4; 4; since gone out of cultivation by Europeans. 1,2; 2; and 1,3,4, (See fig. 16 for these race At the time of the blight outbreak a number of designations.) Race 3,4 has also been identi- Europeans were growing what had then be come fied from Kenya, but on tomato. Some of the a lucrative crop on account of Service demands. above races were found for the first tinne in Kerr*s Pink was the favorite variety, and a material from Kenya. large consignment of it was imported from In 1950 Phytophthora infestans was found the United Kingdom for the 1941 crop. This on the leaves of perennial woody Solanum s extension of potato growing coinciding with (S. indicum, S. panduraeforme, and S, incanumí an exceptionally wet season in 1941 may in widely separated localities of bush and possibly have given rise to circumstances forest (Nattrass and Ryan, 1951). The races more favorable for the introduction of blight of the fungus so far identified on these wild than there had ever been before. Dr. Nattrass shrubs are 2,4 and 4. S. incanum is suscep- is convinced that if blight had developed on tible to some races but resistant to others; any previous occasion in Kenya, it would almost another species, the treelike S, aculeastrum certainly have become endemic and could is susceptible to all races. The importance of scarcely have escaped observation. Except these wild hosts is that they flourish in the at high altitudes, there are no frosts to kill wetter forest regions where blight conditions the haulms; once potatoes have been grown on are very favorable, and they can act as a any piece of land it is practically innpossible perennial source of infection. (with ordinary cultivations) to prevent ground- A number of Black's resistant varieties keepers coming up. They grow like weeds and have been distributed over the country in both may persist for years. European and African areas. They have proved to be resistant to the dominant blight races CONTROL MEASURES and are now in considerable demand. Others are still being tested, and the two most The onset of the new disease in 1941 caught promising varieties to suit the local conditions the Europeans unprepared for any direct con- are a RjR^ and R3R4. trol measures, and native cultivators were in The most popular commercial variety in any case unable to do anything about it. The Kenya at the present time, however, is the effect was devastating on the European Kerr's Dutch Robijn, which was introduced from Pink, and some of the miscellaneous native John Clarke in 1945 and has no R-genes. It varieties were completely destroyed. Two of stands up to blight better than most varieties these native varieties, however, the **Kinongo in Kenya, other, than the R-resisters, and it Kerai*' (since identified as Northern Star) has largely replaced Kerr's Pink. Dr. Black and another, known locally as **Kigezi Blue'* has informed the authors that Robijn was (later identified as Skerry Blue), showed bred fronn Roode Star x Preferment, and considerable resistance to the disease. The that it is a variety with a fairly high field first step therefore was to advise that these resistance, which seems to suit the Tropics, two varieties should be collected and multiplied as it has also done well in trials in Southern with all possible speed. Rhodesia. On a scale from 1 to 5 for field The importation of blight-resistant hybrids resistance in which 1 denotes completely for test under the local conditions began in resistant and 5 completely susceptible. 1943, and this testing, centered at the Scott Dr. Black has placed Robijn, from the results Agricultural Laboratories at Nairobi, has of his trials, at 3.5, which compares with 3 continued since that time. In the early days for Voran and 4 for Kerr's Pink. It is thus a suggestion from Reddick and others that more resistant than Kerr's Pink, and, pre- Kenya should produce its own short-day potato sumably like Kerr's Pink, it is tolerant of was investigated, and some breeding work the short-day conditions. Kerr's Pink, so was done with Solanum andigenum. Beyond extremely popular in Ireland, appears suited

174 equally well to the extreme long-day con- greater resistance thaji other varieties to the ditions of Iceland (see Iceland section) and adverse conditions and to blight. to the extreme short-day conditions in Kenya. In 1952, owing to the failure of the short The second nnost widely grown variety in rains, plantings were small in the Nyanza Kenya is now the native Kinongo. A few Province. In the Central Province there was European growers plant a little Majestic and a shortage of planting material created by Up-to-Date. the heavy blight attacks of 1951, and through- Protective spraying as a means of control out Kikuyuland the short-rains crop suffered has been considered and full directions have a good deal from blight. In Meru good crops been issued (Nattrass, 1944) for the conduct were harvested. In 1953, yields were heavy in of the operation, both with freshly prepared the Meru district and moderate elsewhere. In bordeaux mixture and with cuprous oxide and the Nyanza Province a fair crop was harvested. copper oxychloride preparations. Only a very Owing to dry conditions, blight was less little spraying has been done, and the real severe than usual, and planting of Robijn was hope of control is through the introduction of increasing. In 1954, the crop did well in the resistant varieties. Blight has been a serious Central Province, with Kiambu coming into disease on tomatoes in Kenya ever since its production again. Production in Meru slumped introduction, and it is on tomatoes that control for security reasons. Blight and paucity of by spraying has real possibilities. A full resistant seed were reported as the limiting account of the disease on tomatoes is given factors to increased production. Yields in in Nattrass (1950). Kericho were above average. Stocks of Robijn were being built up gradually in all districts. In 1955, a steady increase of acreage was COURSE OF EPIDEMICS AND MAGNITUDE reported in the Central Province, except OF LOSSES Meru. Long-rains crops yielded well in all districts but short-rains crops wBre severely In Kenya the potato plants may be attacked hit by blight. Production was increasing in at almost any stage of growth, and conditions Nyanza, where the output at Kericho had are too variable over the range of potato- quadrupled since 1950. Sales from African growing areas for any estimate of loss of areas increased from 687 tons valued at crop from the date at which the haulm is £14,410 in 1954 to 800 tons valued at £17,300 destroyed. The worst blight years have been in 1955. 1941 (the first year of attack), 1946, 1951, and 1955. No surveys have been attempted to SUMMARY: KENYA establish the actual timing of the attacks or the extent of losses in various years; indeed, Blight was first found east of the Rift Valley such surveys would be extremely difficult to in 1941, and by 1942 it had appeared in both carry out, owing to the scatter of the cultiva- European and African cultivations in the tions and the many factors other than blight equatorial highlands on both sides of the Rift. contributing to low yields. In general, it can Mean temperatures between 60 and 65 F. all be said that slight attacks tend to coincide the year round, with heavy and variable rain- with partial failure of the crop from lack of fall, double cropping and perennial ground- rain; while with abundant rain and luxuriant keepers, all favor the incidence of the disease; crops the blight attack becomes severe. but attacks are very variable from district The introduction of potato blight has cer- to district and season to season. Much work tainly not prevented the continuation and ex- has been done on the determination of blight pansion of potato growing in Kenya; it has races, and Kenya has been found to be a country merely introduced yet another factor contrib- in which much specialization of the fungus uting to the variability of production in various can occur. Several races have been found on years and districts. The situation in 1951 to solanaceous forest trees and shrubs. R- resistant varieties have been extensively 1955, 10 years or more after the first epi- tested, many have gone down under local demics, may be seen from the notes on potato conditions, but some are still of value and production in the Annual Reports of the Depart- others of promise. Robijn, a variety with no ment of Agriculture. In 1951 excessive rains R-genes but with some useful measure of reduced yields in the Nyanza Province and field resistance has largely replaced Kerrys blight was prevalent, both there and in the Pink as the most popular commercial variety. Central Province. The Dutch Robijn showed

UGANDA

The authors are indebted to Margaret A. cultivators, and the acreage is very small Keay, of Makere College, Kampala, for a compared with that planted to other crops. At review of the available information for Uganda. one time there were some potatoes in Bug anda, Potatoes are grown entirely by African peasant but the climate in this central province is the

175 TABLE 101. —Potato acreage in Uganda, 1946-55

Eastern Western Bug anda Year Province Province

1,000 acres 1,000 acres 1,000 acres 1946 1.7 1.5 1947 2.1 1.1 1948 4.3 2.4 0.4 1949 6.1 .7 .7 1950 7.9 .7 4.0 1951 8.0 4.4 1952 8.9 .8 2.5 1953 9.0 2.6 1954 9.2 3.1 1955 11.0 4.1

least suited for them, and cotton and coffee provide all the cash crops required. At the UCANDA present time the largest acreage is in the ^S.W.Uqanda Kigezi district in the Western Province, at KABALE altitudes of 5,000 to 7,500 ft., but there is 6,138 ft. also a smaller acreage at the same altitude range in the Eastern Province near Mount Elgon. Table 101 shows the acreages in the RAINFALL three provinces over the 10 years 1946-55, Inches as given by the Annual Reports of the Depart- ment of Agriculture. According to the Annual Reports, blight spread rapidly into Uganda after it was first

found in Kenya in 1941, and in 1942 and A M J J succeeding years it destroyed much of the I CROPPING PERIOD - - CROPPING PERK»-» crop in the Kigezi district. In 1948, when the rainfall at Kabale in this area was 30.2 FIGUES 84.—Liong-term average monthly rainfalland mean tempera- inches, as against the long-term average of tures in relation to potato cropping at Kabale, in southwestern 39.2 inches, there was less damage. In 1945 Uganda. and 1946 the disease was recorded at Kawanda, near Kampala. Although blight appears to be a November. Mean temperatures are somewhat serious factor in reducing yields in Uganda, lower than at Nairobi, being between 61° and and particularly in Kigezi, it has not pre- 63 F. in every month of the year. The district vented a steady increase of potato growing iii is, moreover, notorious for its mists. It is that area, and this is clearly brought out by hilly, with very steep slopes to the valleys, the acreage figures. The increase is attrib- the bottoms of which are filled with swamps. utable partly to greater consumption of potatoes The valleys are narrow and there is con- by the African producers themselves, partly siderable shadow effect. A very high incidence to their need for a cash crop, and partly to of cloud occurs; the mean total amount of a greater demand for potatoes from an in- cloud, on a 0 to 10 scale, at 14.30 hr. of day- creasing urban population. Under good con- light being from 7.5 to 8.5 in every month of ditions yields are 2 to 3 tons per acre, but the year. Apart from the high diurnal' range most yields are probably lower, owing to of temperatures (from 49° to 74°, averaged the poor standard of peasant cultivation. over the year), the climate would appear to The long-term average rainfall and mean be almost ideally favorable for blight. temperature conditions for Kabale in the No information is available to indicate the Kigezi area are shown in figure 84. As at timing of the epidemics, or their varying Nairobi in Kenya, the rainfall distribution severity in differing years. Potatoes were is typically bimodal. There are two cropping introduced into Kigezi about 1913, and they are periods--planting in April for lifting in August considered to have done better before blight before the September rains; and planting in arrived. Sonne part of the increased acreage October for lifting in February before the may be due to the need to plant more acres March rains. In some places the main rains to compensate for blight losses, but the main occur about March-April and in other places reason for the increase is because more that are quite near they may be in October- potatoes can be sold.

176 Control by spraying is out of the question shown a useful degree of field resistance. for the very poor subsistence cultivators. The Blight on tomatoes has now been reported small plots are scattered over a wide area, and in Uganda. Baldwin ( 1956) records it at Kawanda it would be burdensome in the extreme to near Kampala, where he found it late in 1954, carry spraying or dusting equipment up the but not in 1953. Dr. Keay found it on tomatoes precipitious slopes. The variety Toro Blue on Maker ere Hill in the second half of 1954 (probably the old variety Skerry Blue) has and again in 1955.

TANGANYIKA According to F.A.O. (1955) the acreage of distribution elsewhere. Dr. Black has told potatoes in Tanganyika is very small indeed, the authors that this seedling when tested the figure given for 1948-50 being only about with race 1,2,3,4 was graded 1 for field 2,500 acres, with a production of about 2,000 resistance, and this bears out its performance tons. E. A. Riley, at Lyamunga, reports that in Tanganyika, where it has now been grown the real extent of potato cropping is probably for 7 years with 2 crops each year and no quite unknown. The cultivations are mainly by blight has appeared on it. Africans, and are located in areas about In the Annual Report of the Department of the Usambara and Ulugurus Mountains, Agriculture for 1955 it is stated that the main Kilimanjaro, Meru, the Porotos Mountains potato markets in Tanganyika are Dar es near M bey a, Bukoba, and North Mara by Lake Salaam and Tanga, and that these are sup- Victoria. (See key map, fig. 80.) plied from the Ulugurus and Usambara Blight first occurred in the territory in Mountains, respectively. Potatoes are a staple the Kilimanjaro area in July 1942, following food of Africans in the Porotos and Ukinga the outbreak in Kenya in 1941, and it spread Mountains of the southern Highlands, where rapidly to other areas in Tanganyika, some 7,000 tons were said to have been pro- practically wiping out potato growing in duced in 1955. There was no export market, the Usambara. but some were sold in tins to drivers of long- Control has been attempted by spraying, distance transport. The quality of the native- but this is almost impracticable under the grown potatoes has never been good, partly conditions of peasant cultivation, and nothing because of blight and partly because the crop is known of the circumstances that lead to is usually allowed to grow from tubers left outbreaks. A number of fairly resistant potato in the ground without any further cultivation. varieties were introduced into Tanganyika Nevertheless, the native growers expect a from Kenya about 1945, including one labeled very high price, and in some seasons they **Blight Résister No. 914.•' This later went lose their market, because it is cheaper to down with blight, and Dr. Black in Scotland import potatoes from South Africa. found blight race 1,3 on it. Before this only Moreau (1944b) records that in trials in the races 0, 1, and 2 had been recorded. After Kilimanjaro area potatoes of native type yielded 1945 eight of Dr. Black's R-resistant varieties 3 tons per acre, while in the Ulugurus up to 5 were introduced from Scotland. One of these, tons were obtained with Abundance, and in the 1521 (c), has withstood blight so far and has Usambara Mountains even 8 tons with Arran resulted in a revival of potato growing in the Chief. Usambara. Seed is being multiplied there for

NORTHERN RHODESIA Riley (1955) has reported that potato blight at Nkana on potatoes and tomatoes on April 1, was first found in Northern Rhodesia at Nkana, and at Luanshya on potatoes on March 3. It on the Copper belt, by Mr. Hadfield of the was also found on potatoes at 2 places and on Nkana Mine garden on April 19, 1955. On that tomatoes and potatoes at 1 place in the Broken date it was seen on the foliage of tomatoes but Hill area between April 4 and May 18, 1956; not on potato plants nearby, which were then and on tomatoes at 3 places and on tomatoes quite young (about 6 weeks). The disease and potatoes at 1 place in the Lusaka district, progressed rapidly on the tomatoes, and later between April 18 and September 26, 1956. the potatoes (Up-to-Date) were affected. Com- Moreau (1944b) notes that potatoes were plete destruction of the 4 acres of tomatoes grown along the railway line from Chôma to and 2 acres of potato haulm ensued. The Broken Hill (latitudes 17© to 14o S.)at altitudes temperatures during the period 12 to 19 days between 3,200 and 4,500 ft. The disease would before the outbreak were about 81° F. daily now appear to be established in this area, and maximum, and 55° daily minimum (mean 68°), also further along the railway to Nkana. In and there had been much cloudy weather. Northern Rhodesia there is also some potato A. Angus, at Mount Makulu Research Sta- growing at latitude 9° S. near Abercom. F.A.O, tion, Chilnga, has informed the authors that (1955) puts the total potato acreage of Northern the disease appeared again, on an epidemic Rhodesia at about 2,500 acres in 1953, with a scale in 1956. It was found on the Copperbelt production of about 2,000 tons. 177 WEST AFRICA

Few potatoes are grown in West Africa, but at the beginning of the dry season, about the the first occurrence of blight in native cultiva- end of November, and the surface soil is tions of the crop in the highlands of Baxnenda, scraped into raised beds, commonly about 6 in the Southern Cameroons, 6^ N«, so graph- feet by 4 feet. In this loose soil the potatoes ically described by T. A. Russell (1954), is which have been saved for seed are planted one of the incidents of special interest in the with the first rains in March, and seed of recent history of the disease about the world. other crops such as maize and kidney bean, J, M. Waterston at Ibadan and J. D. Tallantire is often inter-planted. The first crop is taken and his colleagues at Buea provided informa- in July, and the second crop, planted in late tion supplementing that given by Russell and August, is harvested in December. Yields of carrying the story up to 1956. 2 tons per acre are regarded as good. As recounted by Russell, potatoes are to Figure 85 shows the location of the area be found mostly above 6,000 ft. in small tilled and figure 86 the rainfall and temperature patches carved out of the prevailing grassland. conditions in relation to the croppings. The The cultivation is very primitive and is done climagram is for 1952, but the general pattern entirely by the women of the tribes with no of rainfall and temperature has been similar other tool than a hand hoe and a stout knife. in succeeding years, except that the months In the area to be planted the grass is burned with the heaviest rainfall (reaching 15 inches

CAMEROONS

8'N

// 3000ft contour NIGERIA

bOOOft. contour

FRENCH

C A MEROONS

GULF OF GUINEA 4**N

Fernando ISO miles

FIGURE 85.--Cameroons, West Africa, showing the Bamenda locality.

178 perature figures for May and June in each of the individual years 1952-56 are given in table 102. Potatoes were probably brought to Bamenda and the adjoining land about Dschang in the French Cameroons during the German occu- pancy of the territory before 1915. Since then, they have been adopted by the native tribes as an important part of their diet. Until 1952 the crops were free from the disease, but in May of that year there was a report of destruction of the first crop by a disease starting near the French boundary and sweeping over Bamenda Province. The symptoms suggested Phytophthora infestans, and during a visit in November 1952, the presence of the fungus was confirmed in every plot of potatoes throughout the area. Russell was able to ascertain that the disease appeared in the French part of the Cameroons in 1951, a year earlier than on the British side. Some ware potatoes had been imported from France, and occasionally some of these had been planted. There was therefore a possibility that the disease was brought in from Europe. Mr. Tallantire supplied the data in table 102, which show the incidence of. the disease on FIGURE 86.—Monthly rainfall and mean temperatures in relation to the British side since 1952. potato cropping in Bamenda Province, South Cameroons, 1952. In the early crops, lifted in July, weather conditions in May and June probably deter- or more) have been sometimes in July, some- mine the severity, or rapidity, of the blight times in June, or sometimes in September. attack. In 1956 the dry June may have been There is a dry period from December to partly responsible for the light attack, but in February. The niean temperature is near 70° the other years no obvious differences in F. for most months of the year, falling slightly rainfall or mean temperature can account for in May or June to September with a low of the differences, which were probably due to about 65° in August. The mean diurnal varia- variation in humidity conditions. With the tion in May and June is about 15° (from 60° to very high rainfall, and no doubt much mountain 75°). Comparisons with the climagrams for fog, mean temperatures up to 70° F. in May Kenya and Uganda (figs. 83 and 84) show that and June do not appear to have restricted the the conditions are similar to those in other development of the disease. equatorial mountain regions where blight now occurs, but mean temperatures in the POSSIBILITIES OF CONTROL Cameroons are distinctly higher, except in Russell records that spraying with 1 per- the June-September period. Rainfall and tem- cent bordeaux mixture at 10-day intervals TABLE 102. --Incidence of blight and climatic conditions in South Cameroons, West Africa, 1952-56

Mean Rainfall Year Blight incidence temperature May June May June

IÍ1. In. OF. OF. 1952 Disease aüDeared and croD wiDed out 4.0 7.6 69 1953 Widespread attack and heavy losses 5.9 70 67 1954 Widespread attacks destroying early crops^ September 10.3 9.2 es 67 plantings not so badly affected. 1955 Attack started in late May, 2 or 3 weeks later than 9.0 1Ö.6 67 65 usual. For the first time.(since 1951) fair-sized tubers were on sale in the markets. 1956 The lightest attack so far since the disease was 8.1 1.Ö 66 67 recorded.

179 over the growing period failed to prevent the Canada was then 0 percent, and in Cherokee destruction of the crop. With such heavy rain- 0.1 percent. Later news from Mr. Tallantire fall it may be impossible to protect the foliage in 1956 indicates that of the 18 varieties tried without an altogether uneconomic number of only 2 of Dr. Black's seedlings from Scot- applications. Control by spraying or dusting land, 1521(c) and 1521(d), have shown real is hardly a suitable measure for adoption by promise of standing up to the disease and impoverished and backward tribes. have given yields of up to 2 tons per acre. The introduction of resistant va^rieties SUMMARY: CAMERCX)NS appeared to offer more hope of success. In March and April 1953, a number of R-gene Blight, probably introduced from Europe, resistant seedlings or varieties from Scotland, attacked primitive potato cultivations in the Canada, and the U.S.A. were planted at the highlands of the Southern Cameroons for the Government Farm at Bambui within a field first timie in 1951-52, causing almost com- of the local susceptible variety. The B.M.S. plete loss of the crops. In subsequent years key was used for the assessment of blight on the severity of the attacks has decreased, the trial plots, and on June 16, 1953, where possibly owing in part to some natural selec- the local variety (planted March 26 to April 8) tion of the more field-resistant types in the showed 50 to 95 percent blight, most of the native varieties. Two R-gene resistant seed- resistant seedlings showed only 0.1 to 5 per- lings have shown promise in trials. The crop cent. The score in Kennebec, Pungo, Placid, is important, but even a good yield is only Canso, and Keswick from the U.S.A. and about 2 tons per acre.

180 ASIA

INDIA

Although the potato acreage in India (fig. 87) None of the other States, after Assam, has been increasing during recent years, po- grew more than 21,000 acres of potatoes. tatoes are still a relatively unimportant food Thus, production is seen to be concentrated crop. In 1950, for example, the total area of in northern and northeastern India. Of the cereals (rice, millets, wheat, etc.) in India potato crop 95 percent is grown on the plains was 193 million acres, as against just over of northern India. The remaining 5 percent half a million acres of potatoes (Russell, 1954). is grown in the hills, up to a height of 9,000 ft., partly to provide a source of seed for INDIA some of the plains' crop. Important hill- growing districts are around Simla and Dar- jeeling in the north, and the Nilgiri hills in the south. An account of potato-growing methods for the plains and hill crop in the Uttar Pradesh has been given by Mitra (1954). According to Pal and Pushkarnath (1951), 16

ASSAM local and 38 European potato varieties have been identified under cultivation in India. The main varieties, covering the greatest acreage, are Phulwa, Darjeeling Red Round, Up-to-Date, Magnum Bonum, Gola, Satha, Great Scot, and Italian White Round. The poor quality of much of the seed of these varieties is one of the reasons for the generally low level of potato yields.

CLIMATIC CONDITIONS AND CROPPING PERIODS

The most striking feature of the Indian climate is the rainy summer monsoon. The monsoon rains occur throughout the country, but with a wide range in duration between dif- Figure 87.--Key map of India. ferent localities. In general, winter is a dry Potatoes are grown for the ware market. They or very dry season. The potato crop on the remain something of a luxury crop, owing to plains is grown during the winter months, as their low yield, which averages approximately summer temperatures are too high for suc- 2-1/2 tons per acre. The average consumption cessful cultivation (see climagram for Patna, of starchy roots (principally potatoes) is 241b. fig. 88). Incidentally, even the winter temper head per year (F.A.O., 1955), but actual peratures are too high for potatoes on the consumption varies considerably over the low ground in most of peninsular India, and country, as the growing of potatoes is concen- this partly explains the concentration of potato trated in relatively few areas and transport growing in the north of the country. Two crops of potatoes are grown on the plains. The first, facilities from these areas are not good. planted shortly after the monsoons in Septem- DISTRIBUTION OF POTATO GROV/ING ber or October and lifted from December to AND VARIETIES March, is the more important. This is known R. S. Vasudeva, who has provided much of as the '*winter crop" and is planted with seed the information for this section, has included from a previous plains' crop, which has been detaüs of the acreages of potatoes grown in the stored during the summer. Formerly, storage was in sand in thatched mud houses, when different Indian states during the season 1954- losses through storage rots were very great, 55. These were as follows: but this method is now being rapidly replaced State: 1,000 acres by mechanically operated cold storages. The Uttar Pradesh ^"^^ second plains' crop--the "summer crop"-- Bihar - - - 10^ is grown from December to April, and the West Bengal 1^5 source of seed is usually the hills' crop. Both Assam 11 crops on the plains are grown under short-day Other States JLAl conditions and have to be irrigated. These factors tend to müitate against high yields. Total ^^5

181 INDIA takes place in late March or April and lifting is in August and September. This seed cannot .Plains normally be used for the ••winter crop*' on the plains, owing to dormancy difficulties.

DISTRIBUTION AND FREQUENCY OF BLIGHT EPIDEMICS Blight was introduced into India sometime between 1870 and 1880 in seed potatoes from Europe. In the north, it was first observed near Darjeeling in 1883, and the disease spread rapidly to other potato-growing areas in the hills and soon became prevalent in the Khasi, Sikkim, and Nepal hüls (Butler, 1918). The presence of the disease in the Simla hills was detected as early as 1902 (Butler, 1903). Since then blight has been widely spread throughout the entire sub-Himalayan region, wherever potatoes are grown, from Simla in the northwest to Darjeeling and Assam in the northeast. The disease causes heavy losses year after year, being particularly severe in the north-

J J A s o N D eastern hilly tracts, where the monsoons breaik early and are of longer duration. In the plains of northern India, the disease .Hills was first recorded in the Hooghly district of West Bengal in 1900. It was observed for the first time in Bihar at Bhagalpur in 1913; the first record of blight in the plains of Uttar Pradesh was not made until 1943, when it occurred at Meerut and Dehra Dun (Lai, 1948). Until recently, the only source of blight inoculum in the plains crop was infected seed from the hills; the high summer temperatures during the storage period of the plains seed crop did not allow the carryover of the fungus from one plains crop to the next(Dastur, 1948; Lai, 1948). Blight epidemics occurred only occasionally. The introduction of cold storage for the seed has enabled the disease to bridge the summer gap on the plains. In 1949, a severe attack of blight was recorded in a •'winter crop" of Phulwa at Hapur in Uttar Pradesh. This variety is confined to the plains, and the source of inoculum from which the crop became infected was undoubtedly carried over in the seed kept in cold storage during the summer. Since then, sporadic infection as well as severe epidemics of blight have been reported from many localities in the plains, including Meerut and Hapur, Allahabad, Benares, and Patna. In the Patna area, in particular, which is an important potato-grow- Figure 88,--Long-term average monthly rainfall and mean tempera- ing center, blight appears almost every year, tures in relation to potato cropping on the plains and in the hills of and sometimes causes severe damage. There northern India. would appear to be a strong connection between the more frequent outbreéiks during recent In the lower hills (3,000 to 4,000 ft.), where years and the introduction of cold storage for irrigation is possible and snowfall is either the seed crop (Vasudeva and Azad, 1952a). absent or over very early, potatoes are planted in February and lifted in June for the ware BLIGHT ON HAULM AND market. At higher elevations (5,000 to 9,000 ft.), BLIGHT LOSSES where the seed crop is produced, the growing season is during the summer monsoon period HILLS: There is no systematic survey data (see climagram for^ Simla, fig. 88). Planting on the timing of blight epidemics in the various

182 hül districts. Vasudeva and Azad (1952b), potential yields are very low (2-1/2 to 3 tons however, have given details for one center of per acre), bulking at the end of the growing blight progress on sprayed and unsprayed season is relatively very rapid. It would be potatoes during a severe blight year (1949) interesting to see the results of bulking rate and a year (1948) when the disease was slight. trials carried out under these conditions. Their trials in these 2 years were held at the PLAINS: Information is very scanty. Dr. Central Potato Research Institute Sub-Station Vasudeva has told the authors that at Patna, at Kufri near Simla, situated at a height of where the Central Potato Research Institute 8,500 ft. Figure 89 shows the blight progress is situated, blight generally makes its ap- curves on the control plots and on the plots pearance in the second half of December. sprayed with burgundy mixture--the most By February, when the daily temperatures successful of a number of different spray begin to rise, its further progress is stopped. The time that the disease is likely to cause damage is confined to about 1 month, from Planted 1948 mid-April mid-December to mid-January. Majid (1952) HAULM Tons¡Q per cent SO has stated that during the season 1950-51, blight caused losses in parts of the plains of Sprayed 2-5 Assam ranging from 10 to 65 percent of the Unsprayed 1-7 potato crop; protective spraying increased crop yields by 0.7 to 1.6 tons per acre. Planted 1 ^^^^ 1949 Little attention appears to have been paid in mid-April JT crop mature any part of India to the problem of losses due to blight in the tubers, and information on the ' / sprayed 3-0 extent of these losses is lacking. Unsprayed 0-7 Auqust Seprember Ocrober BLIGHT AND WEATHER DATA Figure 89.--Progress of blight on sprayed and unsprayed Darjeeling Red Round potato, with corresponding total yields in tons per acre, HILLS: The long-term average rainfall and Kufri near Simla, India, 1948-49. temperature figures for Simla are shown in figure 88. Total annual rainfall at this center treatments. In both years, the yields of the is 63 inches, of which over 80 percent falls sprayed potatoes given in figure 89 represent during the potato-growing season. This is approximately the full potential yields in the typical of the hill districts, where potatoes absence of blight. In neither year was the 75 are grown. Rainfall, particularly during July percent blight stage reached on the sprayed and August, is very high, although there are plots before the haulm died down naturally, considerable fluctuations in amounts of rain and spray treatments were applied by hand, from year to year. The very high rainfall at so causing the minimum of damage. The loss this time accounts for the almost annual oc- of growth due to blight (i.e., the period between currence of severe blight conditions in the the 75 percent blight stage and the time that hills. In table 103 humidity and temperature the tops matured) was about 2 weeks in 1948 data supplied by Dr. Vasudeva are given for on the control plots. In 1949 it was about 6 the 1948 and 1949 cropping seasons at Kufri. weeks. This premature defoliation led to crop The figures were obtained by averaging daily losses of 32 and 77 percent, respectively, readings made at 9-00 hours and 17.00 hours for the 2 years. The yield losses seem very (Indian summertime) of wet-bulb and dry-bulb high when considered in relation to the num- thermometers fixed in the potato-trial plots. ber of weeks' growth lost, but it may well Details of blight progress there in these years be that in crops such as these, where the have been shown in figure 89. TABLE 103.--Humidity and temperature data at Kufri, India, May-October, 1948-49

Average relative hiomldity Mean monthly temperature Month 194Ö 1949 1948 1949

Percent Percent OF. OF. 48 71 66 ^''^**0'i\JIav 25 June •• • ••••• • 42 59 69 67 julv 82 89 66 63 ÂiiPii5?'h ...... ■•.••••••••••••••••■•••• 96 86 62 62 55p"n+PTnl^py ...... •■••••••••••••••••••••• Ö8 87 61 60 Dn+oViPT* ...... ■■••••••••••••••••••••• 89 74 57 56

183 The bad blight year occurred in 1949. Higher jeeling Red Round, which Choudhuri describes relative humidities and lower temperatures as being very susceptible to blight. Satisfactory during the first half of the growing season in control of the disease was obtained in 1948 1949 as compared to those in 1948 no doubt and 1949 near Simla by 7 fungicidal applications partly explain the earlier appearance and given at 10-day intervals (see fig. 89). At greater effect of the disease in that year. But, Darjeeling a good control was obtained by a proper analysis of the 2 years on the basis applications at 6- to 8-day intervals, during of critical humidity periods, such as those the 3 seasons 1950-52. Even so, the final yields of Beaumont (1947) or Bourke (1955), is re- on the sprayed plots in these 2 sets of trials quired to make a worthwhile comparison were very low--in no case better than 3.1 tons of blight-weather relationships. Table 103 per acre--due to causes other than blight. It does, however, show the sort of variation in becomes obvious that the farmer can meet the humidity and temperature conditions that oc- cost of an intensive spray program, at this curs from month to month during the potato- low level of output, only if he receives a very growing season in the Indian hill districts. high price for his potatoes, which in effect PLAINS: Potatoes are grown under irriga- mcikes the potato a luxury crop. Potato-grow- tion during the dry winter months (climagram ing conditions in the hills of India are very for Patna, fig. 88). Apart from being very similar to those in the high valleys of Mexico, much less serious under these conditions than which have been described in this study. In it is in the hill crops, blight is more readily both areas control is very difficult, owing to controlled by chemical methods. heavy and frequent rains that wash the fungicides from the leaves. However, Dr. Nieder- CONTROL MEASURES hauser and his colleagues of the Rockefeller Foundation at Toluca, Mexico, have shown that No exact information exists on the extent of blight can be controlled by an intensive spray control measures practiced against blight in program. The sprays are applied to field-re- India, but it must be remembered that potatoes sistant potato varieties and yields of up to 17 are still largely grown under rather primitive tons of potatoes per acre have been obtained conditions, and it is unlikely that many of the there. The value of varieties ^that show a high growers are able to undertake a spray pro- degree of field resistance does not yet appear gram against the disease. The problem can to have been fully appreciated under the Indian only be seen in its proper perspective against conditions. the background of the present low yields of For the plains, information is required on potatoes. These are due to numerous causes, the need for spraying in the different areas, but among the most important of which are the the problem of control is nothing like so serious virus-infected seedstocks, poor cultivations, as in the hill districts. The use of cold storage and variation in water supply for irrigation. to keep the plains' seed during the summer Untu the general level of potato yields has months has greatly increased the amount of been raised appreciably, there can be little blight inoculum present in the plains' crop, and point in the widespread adoption of blight- thus the liability of the crop to infection. The control measures in^tke plains area. In the cold storages have so many advantages over hill districts, however, measures could be the older methods of storage, however, that adopted, but these districts account for only any return to them is out of the question. 5 percent of the total potato area. SUMMARY: INDIA Trials on the control of blight with spray and dust materials under hill conditions have Potatoe'ö are a relatively unimportant crop, been reported by Vasudeva and Azad (1952b) and yields^a^.re low. Blight is serious in the hill for the Simla Hills, and by Choudhuri (1954) districts, and although these districts provide for the Darjeeling district in West Bengal. only a smêill percentage of the total crop, they The variety chosen in all the trials was Dar- are an important source of seed potatoes.

184 AUSTRALASIA

NEW ZEALAND New Zealand is normally self-supporting in TABLE 104. --Potato acreage according to both ware and seed potatoes. The total acreage district, New Zealand of potatoes is smadl, about 20,000 acres per annum; but the population of the two islands is only 2 million, and with almost all the po- Land district Acreage-"- tatoes going to the ware market, this provides for an annual consumption of potatoes of about North Island: 100 lb. per head. This level of consumption is comparable to that in the U.S.A. The average Nnrth Anf^lcl and , , , 1,49/; potato yield for the country is 5.9 tons per South Auckland 731 acre (F.A.O. statistics). Gisbome 374. Hawkes Bay 1,075 POTATO DISTRIBUTION AND VARIETIES Tnranalrî **•••« 134 Wellington 3,064 The most important maincrop potato-producing areas in New Zealand are in Canter- Total 6,872 bury (chiefly around Christchurch), in the South Island and Wellington (Palmer s ton North South Island: area) in the North Island (fig. 90). Apart from Wellington, the North Island grows prin- Marlborough"\ cipally early and midseason potato varieties, Nelson ► 569 and there is an important early-growing dis- Westland J trict at Pukekohe, near Auckland. In the Canterbury 8,715 South Island, there is another early growing Otago 1,479 district near Nelson. A full account of potato Southland 646 growing in the various parts of New Zealand has been given by Claridge (1945). Total 11,409 Figures provided by P. W. SmcLLlfield of the New Zealand Department of Agriculture at G-rariri total ,,*.*...•«.. 18,281 Wellington are shown in table 104, which give s the average potato acreages for New Zealand on a land-district basis. ■^ 5-year average, up to and including 195¿i-55. The potatoes grown in New Zeédand are largely varieties that originated in Great conditions, and there is a considerable varia- Britain. More recently, a number of varieties tion in rainfall from district to district and in have been introduced from the U.S.A., and different years. The prevailing winds are some of these are now widely grown. Accord- from the west, and the wettest parts of the ing to Mr. Smallfield, 4 varieties have made country lie principally in the west, very no- up 71 percent of the total potato acreage during ticeably so in the South Island (see rainfall recent years. These have been: Aucklander map, fig. 90, based on Stamp, 1944). Generally Short Top (31 percent); Ar ran Chief (18 per- speaking, the potato crop is grown in the cent); Dakota (14 percent); and Ar ran Banner drier parts of the country, as in the Christ- (8 percent). church area of Canterbury, where the average Aucklander Short Top is a selection from annual rainfall is 26 inches. Climagrams for Sutton's Supreme. It is an early maincrop two potato-growing areas (Christchurch and variety that shows some resistance to blight. Pukekohe) are given in figure 91. Le itch (1947) has given a description of the Maincrop potatoes have a very long growing principal potato varieties grown in the country season in New Zealand (approximately and their behavior under New Zealand condi- 6 months). As the country is in the Southern tions. R-gene resistant varieties have not been Hemisphere, seasons are the reverse of those grown commercially anywhere in New Zealand. in the potato-growing regions of northern latitudes. Thus, in Canterbury, planting takes SOIL, CLIMATE, AND CULTIVATIONS place in October (late spring), and lifting is in May (late Autumn). A full description of The crop is grown over a wide range of soil cultivation methods has been given by Leitch types, with the largest yields on the heavy, (1947). Potatoes are grown in ridges, 28 to 31 well-drained silt loams of the Canterbury area. inches apart; in New Zealand, this has the ad- New Zealand has a mild equable climate of ditional advantage of protecting the tubers the temperate oceanic or northwest Europe from attacks by the potato moth. Up to the type. The sea has a marked effect on weather present time, control of blight by spraying is

185 NEW ZEALAND

borne

'30-50in

ANNUAL RAINFALL

Figure 90,--Key map of New Zlealand and annual rainfall.

186 NEW ZEALAND usually a problem, and for Pukekohe, where f, p Sourh Island blight conditions are severe. CHRISTCMURCH AUCKLAND: Blight is normally present throughout the year. Crops are regularly RAINFALL sprayed from soon after emergence until ma- Inches turity. GISBORNE; Blight seldom occurs, because of the usually dry spring and summer. HAWKES BAY: The growing season is usually dry, but occasionally blight may cause some damage. Spraying is practically unknown. S O N Dec Jan F M A M J WELLINGTON: Blight occurs in some areas in most years, but rarely is damage severe • BUCHT SEASON ' |044-5 _' NO BLICHT SEASON * 1945-6 over a wide area. Sometimes it may occur in epidemic proportions during December, usually associated with very heavy rains and water- RAINFALL Inches logged soils, and thus considerable losses from causes other than blight. In 3 years out of 4, January and February are dry months with ample sunshine, so that blight does not develop. If blight does appear late, tuber infection may be high, as spores are washed

N Dec Jo" F M into soil cracked by earlier dry weather. Until a few years ago, no spraying was done in this area, but new contract growers, who lease large areas of land solely for potato growing, r North Island have invested in. spraying equipment. FUKíKOHt NELSON and MARLBOROUGH; BHght attacks are infrequent, mild, and late in the RAINFALL Inches season. WESTLAND: Rainfall and humidity are high. Blight is always severe. Potato requirements for the district are smaJ.1 and it is cheaper to ship them from Canterbury than to attempt to control blight. CANTERBURY: In general, summers are dry and blight is unimportant. Conditions are a little wetter in south Canterbury, so that blight occurs more frequently; even so, the main variety, Ar ran Chief, has sufficient re- Figure 91..—Monthly rainfall and mean temperatures in relation to sistance to carry it through the attack in most potato cropping for Christchurch and Pukekohe, New Zealand, and seasons. "blight** and "no-blight*• seasons at Christchurch. OTAGO and SOUTHLAND: Rainfall is higher in these districts than in Canterbury. There uncommon except in the Pukekohe district. are more days with rain and there is less The small amount of spraying is a reflection sunshine. Blight can be severe on King Edward of the generally sporadic attacks of blight and once in every Z years or more. However, the the scattered fields of the potato crop, which most common variety, Arran Chief, is seldom has usually made it uneconomical for the killed early enough to affect the yield no- farmers or contractors to invest in spraying ticeably. machinery. Christchurch District, Canterbury REGIONAL, INCIDENCE OF BLIGHT Ah account of potato growing in Canterbury C. M. Driver, of the Crop Research Division has been given by Dingwall (1953). He de- at Lincoln, has provided an account of the in- scribes the Christchurch district as the land cidence of blight in the various potato-growing area lying between the rivers Wainnakariri and areas. The disease occurs throughout New Rakaia. Over half the potatoes produced in Zealand, but except in the northwest of the Canterbury are grown in Christchurch. The North Island, the attacks are sporadic and district concentrates on growing maincrop usually only toward the end of the growing ware varieties, with some early varieties in season. Short notes on blight in the several sheltered situations near the coast. Farther districts are now given, and these are followed inland, seed crops are grown and these form by more detailed accounts for important the source of certified seed for the ware Christchurch district, where blight is not areas.

187 Potato crops are widely scattered through coln, in a article entitled, "Late Blight of the district. A high proportion are grown as Potatoes in North Canterbury, 1945." Table a sideline and occupy only a small part of the 105 presents some of the data from this farm acreage. The principal varieties are survey. Aucklander Short Top and Dakota, which fluc- It will be seen that heavy tuber blight tuate widely in yield between dry and wet losses occurred in 1944-45. Such years, how- seasons. Driver quotes 4 to 6 tons per acre as ever, have a frequency of only about 1 in 10 a typical yield for Aucklander Short Top in a in the district. Two of Blair's conclusions dry year. In a wet year, 14 tons per acre is from this admittedly small survey are worth typical for the same variety. quoting: (1) Dakota was less severely affected Potato soils are medium and medium heavy by tuber decay than the other varieties listed, silt and sandy loams, which are usually well whereas Arran Banner showed the heaviest drained. Planting extends from late September infection; and (2) the amount of tuber loss that to mid-November, with the majority of crops occurred was related to different soil types. planted in October. Varying with variety and The heavier and damper loams were associated season, most crops mature between mid- with more disease than the sandy loams. March and mid-Apr il. Harvesting starts in Infection of the tubers at lifting time by con- late April, and is carried out mainly in May. tact with blighted haulm is extremely rare. Most crops have matured some time before Blight Epidemics and Losses lifting, and the small importance attached to Blight is relatively unimportant in the this problem is a reflection of the long growing Christchurch district. During the last 20 season and also the long period over which years, the disease has been severe enough harvesting is possible. In fact, another problem to cause extensive damage in only 2 seasons. often arises. This is a strong weed growth, These have been 1935-36 and 1944-45. Every which makes lifting difficult. Dingwall (1953) third or fourth year, blight appears during says that some growers adopt the practice of February or March. Progress of the disease quickly grazing off this growth with sheep is usually slow (except in odd pockets), so before lifting. that a crop noticeably affected (10 to 25 percent leaf damage) may take a month to 6 Blight and Weather Data weeks or longer to reach 75 percent damage or more. The extra rainfall associated with The long-term average temperature and blight attacks results in greatly increased rainfall figures for Christchurch, together yields, so that, in general, yields are much with the planting and lifting times of potatoes higher in a blight year, and this, despite the are shown in figure 91. A marked feature of fact that very few growers make any attempt the disease in the district is its generally to control the disease. slow progress on the potato haulm after its Tuber blight is not a serious problem. In first appearance. Hot drying Fohn winds blow general, there is very little in the earlier across the district from the northwest during maturing varieties and usually not more than the summer, so that even after heavy rains, 10 percent on the late varieties (often nil). the haulms may be rapidly dried. In addition, There may be a few crops on heavy, poorly temperature conditions may not be too fa- drained soil where losses are high, but prob- vorable to blight. The diurnal range of tem- ably most of the loss is attributable to water- perature during the summer months is from logging rather than blight. 15° to 20° F., and cold nights with temipera- Results of a survey of tuber infection in the ture s dropping below 40° are common, with soil, made at the end of the bad blight season, occasional nights when the temperature goes 1944-45, have been reported by I. D. Blair below freezing. Conversely, high daytime tem- of the Canterbury Agricultural College, Lin- peratures, up to 90° are frequent; periods of

TABLE 105. --Distribution of tuber blight infection among certain potato varieties, North Canterbury, 1944-45

Range of infec- Average tuber Variety Crops examined tion between infection crops

Number Percent Percent Dakota 15 0-19 5 Aucklander Short Top. 16 1-42 15 King Edward 7-34 15 Arran Banner 13 11-77 34 Inverness Favourite., 5 5-20 11 Aucklander Tall Top.. 3 1-30 17

188 low humidity are also common. Hence, blight The soils are basalts and, although heavy, IS continually receiving severe checks. are generally well drained. Figure 91 shows In figure 91, temperature and rainfall con- the long-term average weather conditions at ditions from November to March in a blight Pukekohe, together with the planting and lift- season (1944-45) and a no-blight season (1945- ing times. Planting starts at the beginning of 46) are compared. In 1944-45, blight was first the winter (May), and continues into the spring; reported at the end of December, and most lifting takes place from August to January. potato crops were killed by the disease around The seed crop is usually planted in the early the end of February to early March. The very summer (November-December), following an heavy rainfall in December and February was early ware crop, and harvested the next April one of the main factors associated with the or May. epidemic. Blight Epidemics and Losses Control Measures Protective spraying is uncommon. Driver Blight is present in the district at all says, »'Spraying has not been done because it seasons. Severe epidémica of the disease has not paid. No damage is caused (by blight) would almost certainly develop every year three years out of four. In the fourth or fifth in the absence of protective spraying, which, year spraying may have the effect of increas- however, is carried out by all ware growers ing the surplus which cannot be sold.** Pota- from the time the potato plants emerge. Blight toes are being grown increasingly under spray is less severe on the seed crop, which is irrigation in the district. The change in the grown in the summer or drier part of the microclimate of the crop resulting from this year, and very often there is a time during is likely to give rise to conditions more fa- the growth oí this crop, when spraying can be vorable to blight, and protective spraying may curtailed. become common practice. In this case, far- There are two main reasons for the severity mers are already well equipped with low- of blight in this district. These are the volume spraying equipment, which they use weather conditions, and coupled with for weed control. Destruction of the potato them, the presence of ample sources of tops before lifting is generally unnecessary, blight inoculum throughout the year. The but occasionally a 10 to 12 percent solution sources of inoculum are (1) the potato crops, of sodium chlorate is used for this purpose. thennselves; (2) groundkeepers; (3) tomatoes; (4) honne gardens; and (5) weeds. Potatoes Pukekohe District, Auckland are being grown throughout the year, very often on the same field, so that there is a This district, centered on Pukekohe Hill, constant carryover of the disease. Unharvested concentrates on market garden crops. Out- potatoes sprout very rapidly and groundkeep- doors tomatoes are grown during the summer ers are extremely common. Driver writes months. The area is almost frostfree through- that **Large acreages of tomatoes are grown out the year, and potatoes are grown in the from November to May. Although in general winter to supply the early ware market. Dawe tomato blight races may be different from (1951) has described potato ' growing at potato blight races, yet the latter may carry Pukekohe. He says that during recent years, over quite well on the tomatoes, which often approximately 600 to 700 acres have been follow potatoes.'* Large numbers of home grown annually for the ware market. In addi- gardens throughout the district often grow tion, some 200 to 250 acres for seed are potatoes for the greater part of the year. In grown each year during the summer. The June 1955 Joan M. Dingley found blight in- main reason for this home seed production is fecting a common New Zealand shrubby weed, the difficulty in getting seed from elsewhere Solanum aviculare, in native bush near Auck- in New Zealand in time to plant the early land. Many other infected plants of this species winter crops. Some difficulty may be expe- have since been found and by May 1957 it had rienced in breaking the dormancy period of been shown to be susceptible to blight races 0; seed tubers and one method that has been 1; 4; 1,3; and 2,4; collected in New Zealand used successfully is dipping them in a carbide (Brienand Dingley, 1955; Driver, 1957). solution. There are no published records of blight Arran Chief is the most popular variety for progress on unsprayed potato haulm for the the earliest crops. The use of a matlncrop Pukekohe district. However, Taylor (1945) variety for this purpose is explained by has given some data for unsprayed plots of short-day conditions under which the crop King Edwards derived from a series of is grown. Northern Star (Gamekeeper) is the spraying trials carried out in the Auckland principal variety for the later crops. Yields district (site unspecified) between 1940 and vary according to the maturity of the crop 1945. Some of his results are set out in table and the month of lifting. Dawe gives 4 tons 106. per acre as an average crop lifted in Septem- In each season, blight appeared very early. ber. The speed with which the tops were killed,

189 TABLE 106. --Blight attack on unsprayed King Edward, Auckland district, 1940-43

Year Date of Date of blight Later observations planting appearance

1940-41 Oct. 15 Nov. 14 Dec. 27, plants severely infected. 1941-42 Sept. 15 After Nov. 14 Dec. Ö, plants severely infected. 1942-43 Sept. 16 End of Oct. Dec. 1, most plants killed.

however, was not particularly fast in view of early crop is being grown. For the 6 months. the obvious smallness of the plants, except May to October, total rainfall averages 28 perhaps in 1941-42. Conditions in these trials inches. Though rainfall is heavy, it tends to and those for the important early ware crop come in short, heavy showers, with dry, often at Pukekohe are not strictly comparable, sunny periods in between. This has an im- since the trials were planted in the spring, portant bearing on blight control, as it is whereas the early crop is planted in the unusual even in the wettest season to have autumn and winter. They do give a good idea periods longer than 3 days when spray equip- of the sort of blight conditions facing the ment cannot work. Mean temperatures during Pukekohe seed crop, which is usually less the growing season of the early crop are low. severely attacked by the disease than the On the other hand, humidities are high--in ware crop. part, a reflection of the long-night and short- Tuber infection before lifting is not a day growing conditions. problem in the district. Crops are usually dug quite clean. This is, no doubt, in part a Control Measures reflection of the good controlof blight achieved on the tops, but Driver has further suggested The early crop is sprayed from emergence that the high copper content of the soils in the onward at approximately 4- to 8-day intervals area, brought about by frequent copper spray until shortly before lifting. Up to 10 sprays applications, is protecting the potato tubers may be given over the growing season. The from blight infection. He says *'••• potatoes potatoes are lifted while they are still very may be grown 4 to 6 times in six years on one immature, at a time when the tops are meet- area. Tomatoes may have been grown for ing in the drill but have not yet made spraying several of those years, so the total number of difficult. Driver states, **U for any reason, sprays received over that period could quite harvesting is delayed longer than this, and the well be as many as 80 to 100, and is unlikely weather is wet, blight becomes very difficult, to be below 40." If this supposition is cor- if not impossible to control. However, this rect, it falls into line with work reported by seldom happens, growers tending to plant a Skaptason, Peterson, and Blodgett (1940), from succession of crops if they want to harvest Long Island, New York, and referred to in the over a period, rather than planting one area U.S.A. section of this study. and harvesting a portion at a time." Infection of tubers at lifting is also of little The most comnnonly used spray material innportance. Driver writes, **One would expect is copper oxychloride applied at the rate of some tubers to become inoculated from the 5 lb. in 100 gallons of water per acre. It is haulm at harvest but transport has been co- possible to produce satisfactory early crops ordinated so that potatoes harvested one nnorn- of potatoes by these control measures under ing can be in the shops in Wellington, 400 the very severe blight conditions at Pukekohe. miles away, by the next morning. Conse- But the cost of control is very high, and only quently a high proportion of the potatoes economic because of the correspondingly high would be eaten within 4 to 5 days of harvest, value of the early crop. It seems unlikely that not allowing tuber blight a chance to develop." maincrop ware potatoes could be produced The serious blight problem at Pukekohe is economically in such an area, particularly in therefore the foliage attack. This, however, view of the difficulty of controlling blight once would be so devastatingly severe each year in the potatoes have met in the drill, the absence of spraying as to make potato growing impossible. SUMMARY: NEW ZEALAND Blight is not generally a serious disease in Blight and Weather Data the country. The bulk of the maincrop potatoes are grown in the drier areas. The Pukekohe Mean temperature and rainfall figures for district in the northwest of the North Island Pukekohe are given in figure 91 in relation to is an example of an area where blight Is very the growing seasons of potatoes. There is a bad; but the production of a valuable out-of- consistently high rainfall each month during season potato crop justifies an intensive and the winter and spring, when the bulk of the expensive control program against the disease.

190 AUSTRALIA

Potatoes are grown in Australia at all POTATO VARIETIES, SOIL CONDITIONS, seasons of the year and under a wide range AND CULTIVATIONS of soil and climatic conditions. The acreage is relatively small. During 1939-45, the area According to White (1952), the most im- approached a quarter of a million acres, but portant variety now grown in Australia is in recent years it has been about 125,000 Sebago, which was imported from the U.S.A. acres. The only major outlet for the potato shortly before the Second World War. Sebago crop is the ware market; but annual consump- has displaced Carman (Green Mountain) and tion of potatoes is low, around 110 lb. per Snowflake, which used to be the leading person, which compares very closely with the varieties in Victoria and South Australia; in position in the U.S.A. The average yield is New South Wales and Queensland, Sebago has also low, no more than 3.5 to 4 tons per acre displaced Factor (Up-to-Date). Sebago shows a (F.A.O. statistics), and there is little sign high degree of field resistance to blight, and that present-day yields have improved appre- in this respect it is superior to the older ciably over those obtained during the late varieties it has replaced. Other important 1930's. Bald (1941) has stated that one of the varieties in Australia are Sequoia, Monak, major problems of potato growing in Australia and Adina. In Tasmania, Bismark (early) is lack of water supply. This is due to an and Brownell are still the chief varieties. unequal distribution of the rainfall, rather Most potatoes in Australia are grown on than insufficiency in the total amount, in the soils derived from basalts and adjacent transi- areas where potatoes are grown. In addition, tional soils. These are generally fertile, often there is a generally high rate of evaporation very deep, and permeable despite a high clay from the soil. content. Bald (1941) has described in some detail the relation of soil type to potato grow- DISTRIBUTION OF POTATO CROP; ing in the country. Methods of cultivation vary CLIMATIC CONDITIONS between districts; in general a surprisingly large amount of cultivation, in particular The most important potato-growing areas planting and lifting, is CJtrried out by hand are in the more temperate parts of the labor. Cutting of the seed is general, and the country (fig. 92). Victoria, with about 40 per- crop is grown in wide rows (30 to 36 inches). cent of the total Australian acreage, is the Potato growing under temperate conditions largest potato-producing State, followed by in Victoria has been described by Mattingley Tasmania with 25 percent. Figure 93 includes (1954); details of the cultivation of the crop climagrams for centers in these two States. under the subtropical conditions of Queens- The main potato-growing season is during land are given by CartmiU and Bechtel (1951). the summer months (October to April), and An important pest of potatoes in Australia is potatoes are grown both under coastal condi- the potato moth (Gno rimo s chema operculella). tions and at higher elevations; for example, Caterpillars of this moth feed both on potato in the Central Highlands of Victoria, around leaves and the tubers. The crop is sprayed or Ballarat. dusted with DDT to prevent caterpillar damage, New South Wales, with 15 percent of the and, in addition, good earthing up of the rows potato crop, and Queensland, with 9 percent, prevents the moths from laying their eggs on come next in order of importance. In northern the tubers. This latter preventive cultivation New South Wales and Queensland, climatic serves also to protect the tubers from blight conditions are subtropical, and the main infection. potato-growing season is in the winter or summer, depending on elevation. On the coast FREQUENCY OF BLIGHT EPIDEMICS near Graf ton. New South Wales (climagram, fig. 93), potatoes are grown during the winter. Not much attention has been paid to potato Summer temperatures, whichaverageoveT 75® blight, and the disease is usually considered F. for each of the 3 months, December to of little economic importance. According to February, are too high for potato growing. On Bald (1941), itis much les s serious in Australia the tableland near Glen Innes (climagram, than in most other countries where potatoes fig« 93), which is on the same latitude as are grown. G. C. Wade, Tasmania; S. Fish, Grafton, the main potato-growing season is Victoria; C. J. Magee, New South Wales; and during the summer. J. H. Simmonds, Queensland, provided the Potatoes are not an important crop in following notes on the incidence of the disease Western or South Australia. In Western in their respective States. Australia, early potatoes form a high pro- TASMANIA: Potato blight was first officially portion of the crop, and these are grown recorded in Tasmania in 1907, and a history during the winter and spring months. A summer of the disease in the State is given in an article crop is grown in South Australia,mainly under by Oldaker (1947). Since 1947, it has caused irrigation. fairly general losses in the 1947-48 and 1952- 191 AUSTRALIA

'^^ i ■•■,... ) : VICTOR lÁ

500 miles OtwoY Ranqes' Melbourne O Stanlcyi»^nYaj;C'

Midlands_^__N /J Area \~3w*"^°'*°'"* C-s/« TASMANIA

FIGURE 92.--Key map of Australia, showing distribution of potato growing and localities referred to in text.

53 seasons. In 1955-56 it was probably nnorc but the disease very seldom becomes of any generally serious than ever before, causing economic importance. It is reported most heavy losses in all districts, including many frequently from the Otway Ranges and South districts where it had previously never caused Gippsland. sxifficient damage to be noted. Except in a few NEW SOUTH WALES: Losses from potato localities, blight causes economic damage only blight in New South Wales are as variable in about 1 year in 5 in the important potato- as the season and the locality in which the growing districts of the northwest coast, while crop is grown. Whether loss will occur in any outbreaks rarely occur in the midlands and locality seems to depend on weather condi- south. Attacks are most frequent in the Table tions, the presence or absence of inoculum Cape district near Wynyard, and the Scottsdale not being an obvious conditioning factor. In district in the northeast. seasons favorable to blight it appears to be- VICTORIA: Isolated occurrences in individ- come well dispersed in the air spora. Annual ual crops are reported nearly every season. Disease Survey Records show that outbreaks

192 AUSTRALIA spring is normally dry. The appearance of blight is therefore sporadic and restricted 6 p Victoria to the somewhat rare occurrence of suitable BALLARAT weather conditions. OTHER STATES: Blight is of little or no importance in the greater part of the potato- growing areas in Western and South Australia. Cass Smith (1952), however, has reported that serious outbreaks of the disease have sometimes occurred in the winter-planted crop near Perth during the period August to mid-November.

LOSSES FROM BLIGHT

The small importance attached to blight by Australian growers and research workers is reflected in the paucity of precise data about the effects of the disease on the haulm and tubers. G. C. Wade and S. Fish have provided details of blight progress on unsprayed potato crops, grown in ''blight" years at particular centers in Tasmania and Victoria. Details of these two crops are shown in table 107. These records are, so far as is known,

.New South Wales (COAST) the only ones of their kind available for the CRAFTON whole of Australia. At Toolangi the period between planting and the 75 to 90 percent blight stage was 128 days. This is about the normal length of the RAINFALL 5 Inches growing period of Sebago, when it is grown in the U.S.A., although it may well have a somewhat longer growing season in Australia. The 13-ton yield of this crop, however, shows that the effect of blight on yield was not very- serious. The observations on the crop at Wynyard were made during a series of spraying trials, which have been reported by Williams, Wade, and Cartledge (1956). Bismark is an early 6 j. New South Wales (TABLELAND) variety, and the length of time between plant- GLEN INNES ing and 75 to 90 percent haulm destruction in the crop referred to in table 107 was 101 days. The unsprayed crop yield of 2.7 tons per acre compares with a mean yield of 6.2 tons per acre for the different spray treatments employed in this particular trial. The haulm on the sprayed plots was destroyed mechanically 112 days after planting, so that there was only approximately 11 days* difference in effective growth between the 2 sets of plots, but this FIGURE 93,--Long-term average monthly rainîall and mean tempera- resulted in a fall in crop yield of over 50 per- tures in relation to potato growing periods for four centers in cent on the unsprayed plots. To some extent Australia. this loss of yield was caused by tuber infection. Digging of the trial was delayed until all occurred in coastal districts of New South blighted tubers had rotted in the soil, so that Wales in 10 of the past 25 years (1930-31 to it is not possible to say how important this 1954-55). In the same period in upland or was. G. C. Wade has stated that blight was tableland crops, it was important in only 5 probably more serious in the 1955-56 season seasons, and in one of these it was recorded than in any season since its introduction into only on the northern tablelands. Tasmania. In similar trials carried out at QUEENSLAND: The main wet period occurs Wynyard in 1953-54 and 1954-55, blight caused during the summer when potatoes are not no losses. normally grown, or, if they are, tempera- It is impossible to make any accurate tures are too high for blight. The winter and estimates of the losses caused by blight in

193 TABLE 107. —Data on blight in Tasmania and Victoria, Australia, 1954-56

Date of 75 to First 90 percent Area and variety Date of appearance Yield- Year defoliation by planting of blight blight

Toolangi, VICTORIA: Tons/acre Sebago variety 1954-55 Dec. 20 i^ril 1 i^ril 27 13.0

Wynyard, TASMANIA:

Bismark variety 1955-56 Sept. 26-30 Oct. 30 Jan. 5 2.7

Australia, on the basis of the available evi- rainfall is poorly distributed. Heavy down- dence. All that can be isaid is that overall pours are followed by considerable runoff losses caused by the disease are very small« from the soil surface. In addition, there is a Nor is it possible to apportion the losses that high rate of water evaporation fronn the soil, do occur between yield losses resulting from associated with low humidity conditions. Low premature defoliation and tuber blight losses. humidities are chiefly responsible for the The good earthing up that the crop is given in relative unimportance of blight in Australia. Australia against potato tuber moth and the Mean monthly humidity figures during the generally long growing and long harvesting potato growing season for Melbourne and periods are factors operating against serious Ballarat in Victoria are shown in table 108. tuber blight infection. Bald (1941) has stated The dry conditions under which potatoes that, in aggregate, the losses caused in are grown suggest the possibilities of irriga- the Australian potato crop by early blight tion as a principal method to innprove crop (Alternaría solani) are probably greater than yields. Both overhead and channel irrigation those caused by late blight. The Australian of the potato crop are practiced in parts of potato growers show their lack of respect for Australia. It is possible that a marked in- both diseases by taking little or no protective crease in overhead irrigation would affect action against them. the incidence of blight attacks. BLIGHT AND WEATHER DATA TABLE 108. —Mean monthly relative humidity at Melbourne and Ballarat, Victoria The climagrams (fig. 93) for important potato-growing areas in Victoria and Tasmania Month Melbourne Ballarat indicate that low rainfall during the growing season is an important factor associated with the general absence of blight. Victoria and Percent Percent Tasmania together account for 65 percent of October.. 57 70 the total Australian potato acreage. At Ballarat, November. 56 66 the average monthly rainfall is at or below 2 December. 55 62 inches throughout practically the entire grow- January.. 53 60 ing season; at Stanley, the average rainfall is February. 56 59 below 2 inches for each of the 3 months, March.... 58 64 January to March, when blight is most likely i^rll 64 74 to develop. The clinmagrams for New South Wales show that the coastal crop at Grafton is also grown during a dry period; the table- CONTROL MEASURES land crop at Glen Inn es is grown during a relatively wet season. However, the frequency The replacement of the old blight-susceptible of blight years at Glen Inn es is only about 1 varieties. Factor and Carman, by the more year in 5, and it seems likely that high tempera- field-resistant Sebago has served as a partial tures are limiting the disease there, despite blight control measure. It may well be that in the wetter conditions. large parts of Australia where blight is The absolute amount of rainfall is only a normally either absent or slight in its effect, very rough guide to the likelihood of blight such action in itself is sufficient control, attacks, and in most of the potato-growing especially as Sebago is resistant in the tubers areas of Australia, totctl rainfall figures give as well as in the foliage. At any rate, very an exaggerated idea of the amount of water few Australian growers spray or dust their available to the potato crop. This is because potato crops against blight. In Victoria, New

194 South Wales, and Queensland, growers gen- reliable blight forecasting services for the erally consider spraying to be uneconomical. main potato-growing areas would help con- In Tasmania, matters are different. Blight siderably in preventing losses by the disease. caused fairly general losses in the 1952-53 Even in Tasmania, which is relatively com- season, and as a result, the Potato Marketing pact, there are considerable difficulties in Board asked the Department of Agriculture to the way of doing this, owing to variations in investigate the control of the disease. Spray- climatic conditions; and, it must be admitted ing trials were carried out in 1953-54, 1954- that in the larger States the present difficul- 55, and 1955-56, and the results of these trials ties facing a forecasting service are almost have been given by Williams, Wade, and insuperable. Until recently, a routine control Cartledge (1956). Some of the figures from program against blight, using copper sprays, these trials have already been referred to would almost certainly have led to a net loss above. Blight was not serious in 1953-54 and in potato yields, owing to the depression of 1954-55. Yield figures from the trial in this yields in **no-blight*'years. The position would latter season indicated that a considerable be different if organic fungicides were used reduction in yield can be caused by spraying every year, although provision would have to with certain materials in a **no-blight" year. be made to keep wheel-damage losses to a The yield on plots sprayed with bordeaux minimum. Potatoes are already grown in wide mixture was 30 percent lower than the yield rows. However, Australian growers at the on the unsprayed plots. In 1955-56 (a severe present time are making little use of any blight year), on the other hand, the unsprayed protective sprays. One reason for this is lack plots produced only 43 percent of the yield of suitable equipment; although low-volume on the sprayed plots. In connection with the sprayers, purchased prinaarily for weed con- spraying trials in two of the seasons, the trol, are on the increase. It is possible that temperature-humidity rules of Bourke (1955) in some areas of the country, such as parts and Beaumont (1947) were tested for blight of Tasmania, a routine spray program, based forecasting under Tasmanian conditions. The on the low-volume application of organic occurrence of blight-weather periods by these fungicides, may be found to be economical. rules in relation to blight developnnent at Little work has been done on haulm destruc- Wynyard is shown in figure 94. It was con- tion, as the need for this does not often arise. cluded that Bourke's Irish rules apply reason- A recent account by the Tasmania Agronomy ably well, but that a major difficulty in making Division (1956) has stated that pentachloro- general forecasts of blight in Tasmania lies phenol (P.C.P.) has proved an economical and in the considerable variations in weather that effective chemical for haulm killing, in experi- occur over short distances. Blight forecasts ments that they have conducted. Testing the were considered to be reliable only in the blight resistance of named and unnamed potato vicinity of the weather recording stations. varieties, including material from the U.S.A. and Scotland that could be incorporated in a lOO BLIGHT breeding program, has been carried out in ON Victoria. Phis work was reported by Chambers HAULM per cent SO (1954) and (1955). The emphasis has been placed on the incorporation of major gene resistance in commercial varieties, but up till the present time no Australian blight-

BOURKE PERIODS resistant varieties have come on to the BEAUMONT PERIODS market. 2 RAINFALL Inchei SUMMARY: AUSTRALIA

Potato blight is of small economic im- FIGURE 94.--Development oí blight on unsprayed Bismark haulm in portance in Australia. This reflects the gen- relation to Bourke periods (Irish rules), Beaumont periods, and erally low rainfall and humidity conditions rainfall at Wynyard. Tasmania, 1955-56. (After Williams, Wade, and Cartledge. 1956.) throughout most of the potato-growing areas. These conditions also lead to low yields. In Australia, where severe blight outbreaks Chemical methods of controlling blight are occur at infrequent intervals, the provision of rarely practiced.

195 CONCLUSIONS

The great interest of this study has been to given for each country, or to present any see blight epidemics, not in the abstract or resume of all the information that has been entirely from the viewpoint of plant pathology, gathered together. The general conclusions or but in their widest relations to the potato convictions of the authors that are reached husbandry of the world, the provision of food are stated broadly, leaving the reader to fill supplies, the distribution of the crops, and the in the innumerable necessary qualifications. diverse climatic conditions under which Names of countries given in brackets indicate potatoes are grown. It is not the intention to the preceding sections in which amplification recapitulate the sectional sumnnaries already of the various points, examples, may befo\ind.

THE DEFINITION OF CLIMATIC CONDITIONS

The kind of clinmagram evolved in the course year during which there can be potato crop- of this study may prove to be a useful device ping, and the favorability, or otherwise, of in other work on potato technology. Such the area for both potato growing and for blight, diagrams enable quick visual comparisons to at least as to mean temperature and rainfall. be nnade of some of the main conditions af- Diurnal range of temperature, distribution of fecting both potato growing and the incidence rainfall, and humidity conditions are broadly of blight in different regions. With the clima- dependent upon the type of climate, and day- gram for an area, it is possible to s3e with iength dz;pends on the latitude. fair certainty the period or periods of the

EFFECT OF MSAIT TEMPERATURE ON POTATO CROPPING

The entry of ascertained cropping periods v/here the highest potato yields are obtained. on the climagrams in this study confirms by The 2 most important months for the bulking world survey the finding of Bushnell (1925) of maincrop potatoes, during which the mean [Introduction] that maximum tuber production temperatures lie within this range, are, in occurs over the mean temperature range of general the Z months preceding the month of 59^ to 640 F. A slightly wider range (58^ to maturity. Those are also the 2 months during 65^) takes in most of the regions of the world which blight most seriously affects crop yield.

EFFECT OF RAINFALL ON POTA^^O CROPPING

Although lack of adequate rain limits potato show that regions with the higher potato yields cultivation in many parts of the world and the generally Imve, on a long-term average, 2 to4 rainfall during the growing period is the inches of rain during eachmonthof the growing principal single factor responsible for varia- period [Belgium, Netherlands, Ireland, Prince tion in yield from year to year in most of the Edward Island], but that potatoes are grown great potato-growing areas, there is no such where the rainfall in some months reaches 15 clear-cut relation between cropping and mean inches or more [ India, East and West Africa] rainfall as there is for cropping and mean and under irrigation in the virtual absence of temperature over the world as a whole. The rainfall, given suitable soil and temperature distribution of rainfall affects its utility, as conditions [Western U.S.A., Peru, northern also does the porosity of the soil and the rate Chile ] . of evaporation [Introduction] . The climagrams

DEFINITION OF BLIGHT EPIDEMICS

The losses caused by potato blight, both in effect of control measures can then be ascer- reduction of total yield and by infection of the tained by similar assessments on the treated tubers, are consequences of the attack on the crops. foliage. To define the incidence of blight in any The use of a standardkey, suchas the B.M.S. area, the necessity is first to assess the course key [England and Wales], which defines the of the unrestricted epidemic on unsprayed degree of attack by the approximate percentage foliage. If this is done over a sequence of years of the total foliage killed by the diseas'^e, it becomes possible to determine the frequency enables assessments of blight to be made with which epidemics of defined course and quickly and easily in potato fields. Such severity occur in the region. The modifying assessments can be made in any part of the

196 world, and workers in other countries can amount of blight that is left on the haulm at understand what is meant by them. If these lifting» assessments are nnade at intervals over the In the absence of such complete information, growing period, the whole course of the the eventual severity of the epidemic on the epidemic may be plotted on a time scale. This haulm may best be defined by the date at which progress curve will show the starting date of the disease h?5 alm.ost completely defoliated epidemic spread, the date at which further the plants and therefore stopped further growth growth of the tubers is arrested, the period of the tubers. This stage ic easily recognizable during which spores are washed down on to the in the field, even from the air, and the date soil and possibly will infect the tubers, and the introduces the num.erical element of precision. DETERMINATION OF THE EFFECTS OF BLIGHT EPIDEMICS ON YIELD Very frequently about the world, attempts out [England and Wales ] whereby the results have been made to base estimates of national of a few carefully conducted spraying trials losses from blight upon the results of sporadic with complete observations of the disease on spraying trials. Such trials, usually made for the foliage, coupled with periodic liftings to the purpose of comparing different spray treat- obtain mean bulking curves for the prinèipal ments, tend to be carried out in the worst varieties, can be used to interpret foliage districts for blight and on the most susceptible observations made on a large r imber of crops varieties. By the tacit and usually wholly over a very wide area. On test throughout incorrect assumption that similar results England and Wales over the past 10 years, would be obtained in all years and in all 1946-56, this method proved to give mean districts, grossly exaggerated and m.isleading estimates of defoliation losses at least as figures are obtained. accurate as those that can be obtained by In sonne countries [Northern Ireland, Den- lifting and weighing the produce from, sprayed mark] spraying trials to a uniform plan have and un sprayed plots. Application of the prin- been carried out at many centers year aiter ciple of the method for the interpretation of year over a long period, and the yields of available data from other countries [Nether- sound and blighted tubers have been recorded. lands, Republic of Ireland, Arcos took County, Such sequences of trials constitute excellent U.SeA] indicates its general applicability. surveys of the consequences of blight epi- Similar methods are alrepxly in use in Poland demics. If carried out at enough centers they and under trial in Norway « provide information on the average gain from The basic principle is to ascertain the time spraying that could scarcely be surpassed. at which blight on the haulm has progressed so They are very laborious, however; they do not far as to stop further growth of the tubers (75- reveal the defoliation losses unrecovered by 90 percent blight, according to the B.M.S. key) spraying, and unless they include systematic and then to read off the proportion of the poten- assessment of the disease on the foliage they tial yield nornaally present at this date from a provide no information on the courses of the mean bulking curve for the variety. epidenaics in the several years--except per- The mean bulking curve must be obtained haps by inference. for the variety in the region for whichloss as- Following the work of Murphy [ Ireland ] in sessments are to be made, and it must be used relating effects of spraying to the development in such a way that account istakienof the rela- of the potato plant, a method has been worked tive length or shortness of the growing season.

FREQUENCY OF ^'BLIGHT YEARS" IN DIFFERENT REGIONS The incidence of blight on the potato haulm but that the incidence of blight is also greatest in any region is a product of two factors: The within this range. There is little correlation severity of the epidemics and their frequency. betv/een the frequency of blight years and long- A "blight year" may be defined as one in which terra average rainfall, except at the extremes there is at least 3 to 4 weeks' loss of growth of the frequency scale. The centers fall, how- on maincrop varieties in along growing season ever, into quite distinct groups according to or 2 weeks in a short growing season, with a the type of climate. Blight years are most consequent loss in yield of 10 percenter more. frequent in maritime areas and at high altitudes The frequency with which such **blight years*' in tropical and subtropical countries where occur is ranked in table 109 for some of the there is much cloud and mountain fog. Where potato-growing centers of the world for which maritime influences modify continental condi- climagrams have been given. tions, as in Denmark and northern Germany, This ranking shows clearly that not only are blight years become less frequent; and where most of the world's potato crops grown where true continental climatic conditions prevail, the mean temperature range in the 2 months as in the Wa^rsaw and Moscow regions, the preceding maturity lies within 58*^ to 65° Fo, frequency is lowest«

197 TABLE 109. --Frequency of "blight years*' in some potato growing centers of world, ranked in relation to mean temperatures and rainfall^ in the 2 months preceding maturity of the maincrops

Frequency of Locations v/here mean temperatures in last 2 months before maturity were- "blight years" (years in 10) 5QP to 37^ F. 38^ to 63° F. 66° to 70° F.

10. Mexico City, Mexico (6-1/2)

Kabale, Uganda (3) Songer, Kenya (8) George, S. Africa (3-1/2) Birr, Ireland (3-l/2) 9 to 7. Penzance, England (3) Brest, France (2) Flushing, Holland (2-1/2) MaMo, Sweden (2-1/2) Chariottetown, P.E.I. (3-1/2)

Fens, England (2) 6 to 3. Jutland, Denmark (3) ( Aroostook County, U.S.A. (3-1/2) Cambridge, England (2) Berwick, Scotland (2-1/2) Ghent, Belgium (3 ) Gemert, Holland (2-1/2) 4 to 2. Hanover, Germany (3) Aberdeen, Scotland (3). Leningrad, U.S.S.R. (2-1/2) Moscow, U.S.S.R. (3) Warsav;, Poland (3) Reykjavik, Iceland Stanley, Tasmania (2) Glen Innes, (2-1/2). Australia (3)

Christchurch, New Zealand (2) Long Island, U.S.A. 'Puno, Peru (4-1/2) Ballarat, Australia (1-1/2) (3-1/2) 1 or less. Red River Valley, U.S.A. (2-1/2) Saratov, U.S.S.R. (1-1/2)

Numbers in parentheses after centers refer to inches of rainfall per month obtained from a long-term average for the 2 months.

INCIDENCE OF BLIGHT AND WEATHER CONDITIONS Although weather conditions in the early part the blight fungus can germinate, and it assists of the growing period affect the date of out- spread of the disease by washing the sporangia break of blight, it is, in general, the conditions about; but its greater effect, directly and during the 2 months preceding maturity of the indirectly, is to raise the relative humidity of crop that decide the progress of the attack on the atmosphere about the plants.Highhunnidity the haulm and the amount of defoliation loss. rather than rainfall is the main connponent of This is shown by the blight progress curves **blight weather,*' and the determination of available for different parts of the world, and the frequency and extent of occurrence of such it is substantiated under the previous head in weather demands detailed humidity observa- these conclusions. Long-term average rainfall tions [Netherlands, England and Wales, Ire- during these 2 months does not determine the land, Chile] . Average humidity records are of relative frequency of blight years in different little value for indicating the prevalence of areas, but in any given area annual variations **blight weather," and if broad indications are in rainfall profoundly affect both blight in- required they are best derived from considera- cidence and the level of crop yield. Rain pro- tion of the locality, the altitude, and the general vides water on the leaves in which sporangia of type of climate.

198 THE DUALITY OF THE DISEASE The duality of potato blight, as a disease of to overestimate the importance of losses the haulm and also of the tubers, complicates from blight in the tubers--which are im- the estimation of blight losses and makes it mediately obvious and sometimes spectacu- difficult for growers to grasp the rationale of lar--and to forget the more insidious control measures. Sonnetimes, as among the and unseen loss of yield due to premature Indians of Peru, the foliage and tuber infections defoliation. Very frequently, loss esti- are thought to be caused by two separate and mates for defoliation and tuber infection are distinct diseases. Often, and even among the lumped together and it becomes difficult most technically advanced of European and or impossible to disentangle them for sepa- North American growers, there is a tendency rate consideration.

THE DISEASE IN THE TUBERS

Estimates of tuber infection cannot be de- such as the sea clays of the Netherlands, than rived from climagrams or progress curves for in drier soils, such as those of the great sandy blight on the haulm. These often show condi- plain of northern Europe. The distribution tions under which tuber infection is likely to pattern and magnitude of losses from blight occur, but whether it occurs or not and the in tubers before lifting, with a long growing extent of its occurrence depend upon soil con- season, is exemplified by the results of de- ditions, the thoroughness of the earthing-up, tailed surveys in England and Wales. Here, the tuber suceptibility of the variety, the time even in a bad blight year, the mean loss, over of lifting in relation to the blight attack on the the whole country, is only 3 to 4 cwt. per acre haulm, and the weather conditions at the actual (less than 2 percent of the crop). This is made time of lifting. The incidence of tuber blight in up by a very few crops with high losses, relation to all these and other factor s has been sometimes of several tons per acre, numerous little explored. Two routes of tuber infection crops in which the loss is moderate or slight, must be distinguished: Infection in the soil and well over half the crops with no los s at all. before lifting and infection at lifting in the This pattern of distribution may be fairly presence of blighted haulm. Infection by the general. Certainly throughout the world, oc- first route mostly beconies apparent by lifting, casional heavy and spectacular losses from and as it is relatively easy to count or weigh blight in tubers tend to color the picture and the conspicuously blighted tubers at this stage, give rise to an unbalanced impression of the it is on this type of infection that most infor- relative importance of the losses caused by mation is available. Infection in the soil before tuber infection and those caused by premature lifting may occur where there is any blight on defoliation. Even in Ireland the mean loss from the haulm to provide inoculum. It is therefore blight in tubers (in unsprayed crops) is less a very general consequence of blight epidemics than a third of that due to premature defolia- on the haulm. Infection at lifting occurs only tion, and in many regions the proportion is where blighted haulm is still present or much less. In the Costa region of Peru, where sporangia are surviving in the surface layers coastal fogs favor infection of the haulm of the soil. Infection at lifting is therefore an and there is little or no rain to carry spores occasional consequence of blight epidemics on down to the tubers, tuber infection is un- the haulm. known. Where there is a long growing season, with Although the loss from blight in tubers may time for blighted haulm to die down completely often be much less than that from premature well before lifting, tuber infection before defoliation, it tends to occur more constantly lifting is the main cause of loss. Where the from year to year. Thus, quite a small general growing seasons and harvest periods are short reduction in the level of tuber infection, [ Iceland, northern Sweden, Prince Edward as by the adoption of more tuber-re- Island, Aroostook County, U.S.A.] infection at sistant varieties, may be of more value to lifting may predominate. It may also be a very the national economy than much larger serious source of los s in primitive cultivations. but sporadic reductions of defoliation Tuber infection before lifting tends to be losses in severe blight years [England and greater in soils of high water-holding capacity. Wales]. COMPENSATORY EFFECTS ON YIELD OF BLIGHT EPIDEMICS AND RAINFALL Throughout the great potato-growing regions ing from the study: The compensatory effect of the world, in middle to high northerly lati- of yield increase from the more abundant rain- tudes, potato yields in *'blight years" are not fall in the blight years. To some extent the markedly lower than in years when there is good yields obtained in blight year s are attribu- little blight, and they may indeed be higher. table to protective spraying. In some circum- This is perhaps the most striking single find- stances (see p. 204) protective spraying reduces

199 tuber infection, and this is part of the gain. frequency of blight years is higher and more But the overall effect of spraying may be much spraying is done. In Aroostook County, U.S.A., less than is generally supposed. In England and where all the crops are sprayed, it is likely Wales, where the situation has been very fully that yields in blight years might be lower than analyzed, it is found that only about a third of in no-blight years if no spraying were done. maincrops are sprayed, and that even if all But in Denmark and Germany, where the amount this spraying were fully efficient, it could not of spraying done is much the same as in Eng- be responsible for more than 0.35 ton per land and Wales, the contribution of spraying to acre increase of the mean national yield in a the production in blight years is probably about bad blight year; whereas the mean yield in bad the same. Certainly the general evidence shows blight years is about 8 tons per acre as against that throughout the main potato-producing 7 tons in the drier years when there is little countries of the world, blight years are years or no blight. On a long-term average over all of greater potential yield than the drier years types of seasons, protective spraying con- when there is less blight. In other words, tributes only about 1 percent of the total potato drought or shortage of rain is of far greater production. The part of spraying in maintain- consequence than blight in reducing potato ing high yields in blight years is greater in yields, and this is true not only in northern Ireland, in the sea clay regions of Holland, and Europe and North America, but throughout the some other maritime regions where the world as a whole. ECONOMIC EFFECTS OF BLIGHT LOSSES: TROPICAL AND SUBTROPICAL REGIONS Blight epidemics are occasionally cata- cessation of potato growing, and production strophic, and it has been common in the past to has increased in spite of it [India, Kenya, lay great stress on this fact. Even today, when Uganda]. The tropical and subtropical regions blight first invades an area where an im- in which blight now causes the most spectacu- poverished and backward peasantry has be- lar occasional destruction are in general ones come too largjely dependent on the potato for in which the potato is only a minor, and some- food supplies [Chile, Cameroons] , there may times only a luxury, crop and in which potato be acute temporary privation. In some regions yields are very low from causes other than [ Mexico, Africa] high incidence of blight may blight. Not infrequently, in such circumstances, prevent potato cultivation in a number of spectacular blight epidemics can do positive districts where it would otherwise be possible, good, for by focusing attention upon the potato and thus deprive the population of a valuable cultivations, they can lead to substantial im- supplement to other and more important food provements in the methods of husbandry, resources. Here and there the disease may manuring, choice of variety, and suppression cause the loss of part of an otherwise favorable of virus diseases, which are the real needs of growing season, with consequent loss of a the area, and can have much greater effect valuable market [Transvaal]. In most of the upon the level of yield than all the losses from recently invaded potato-growing areas, how- blight. ever, the disease has by no means led to any ECONOMIC EFFECTS OF BLIGHT: MAIN POTATO-GROWING COUNTRIES

The economic consequences of blight epi- industry, all reductions of yield by blight, demics in the great potato-growing countries even in the years when ware surpluses are are conditioned by the salient fact that yields greatest, may constitute real national loss. tend to be highest in the blight years and In the great potato-producing countries there lowest in the years with least blight. They also is very little likelihood of any recurrence of depend upon the national utilization of the famine due to potato blight, and shprtages of potato crop. In the majority of those countries potatoes, where they occur, are more likely to with a high human consumption of potatoes the follow the dry years with little blight, than the ware market is effectively buffered against wetter blight years. There nnay, however, be shortages and surpluses by a heavy usage of many blighted potatoes coming on the market potatoes for live stock feeding and manufacture. after intermediate types of season, where the Peru is an exception to this. Where outlets for total yield is maintained but the tuber infection the use of surpluses are limited [U.S.A., is heavy. Canada, England, and Wales] , reduction of In all years and in all countries there is a abnormally high yields by blight in some years national loss chargeable to blight equivalent may be positively advantageous to the national to the total cost in labor and materials of all economy, though not to individual growers. the spraying done; and in years with little or Early defoliation may, however, lead to a high no blight there may be quite a serious loss due percentage of small, unsalable tubers [Aroo- to reduction of an already light crop by stook County, U.S.A.]. Where there is a large mechanical and phytotoxic damage from un- and flexible usage of potatoes for fodder or necessary spraying. Where blight causes a

200 yield reduction regularly every year, or al- cent efficient. The economic consequences of most every year, there Is a national loss blight are thus highly complex. They benefit equivalent to the acreage that has to be planted some and cause hardship to others; Indeed, to compensate for the proportionof the regular they Involve the whole structure of society,' loss not recovered by spraying--and spraying, and they certainly cannot be expressed In where It Is done. Is rarely more than 50 per- ternns of tons per acre. COURSE OF ESTABLISHMENT OF BLIGHT IN NEW AREAS

Since 1935 potato blight has Invaded a nunn- fusely on the diseased haulm, with consequent ber of countries where It was previously un- further widespread Infection of the tubers. known [Chile, Peru, Kenya, Rhodesia, Came- In the following years the disease then acts roons]. The Introduction has almost certainly as an Instrument of natural selection among been brought about In each case through the the potato varieties that have become In- Importation of Infected tubers In seed or ware. digenous In the country. The more bllght- The courses of establishment of the disease In susceptlble sorts gradually pass out of culti- the new areas throw light on the first epi- vation, and this process Is aided by the demics In Ireland and other parts of Europe In Importation of more resistant sorts from other 1845-46, and they have followed a fairly con- parts of the world. In the meantime there may stant pattern. First, there Is aperlodofa year be quite a run of years during which weather or two when the disease Is present here and conditions are unfavorable for the recurrence there among the crops of the new.country, but of a severe epidemic [Chile]. Alarm then It passes unnoticed or unrecognized. Then subsides, and during these years the disease comes a year In which weather conditions are becomes endemic over the whole terrain; unusually favorable for the Infection of always with sufficient foci to start up the the tubers--It may be a late attack disease again on an epidemic scale when the causing no very great loss from premature weather Is favorable. Sooner or later, accord- defoliation. Then by 111-fortune the following ing to the climate, the next severe epidemic year Is also one In which the weather Is does occur, but with le s s susceptible varieties; favorable for blight, and favorable this time with some spread of education about the nature for an early attack. Many blighted tubers have of the disease, the losses are less than In the been planted In seed saved from the previous first year of epidemic attack. The cultivation season; there are many foci of Infection; and of potatoes then goes on, losses are con- the epidemic spreads over a wide area, siderable from time to time, but It Is usually causing spectacular destruction of the haulm a case of better blight than lack of rain, and and heavy loss of crop. At this stage there Is the extent of the cultivation comes to depend alarm, the Impoverished peasants who have rather upon nnarket demands and the general been subsisting In greater or less degree on suitability of the area for potato growing than the potato suffer acutely, and commissions of upon the Incidence of blight. Some old and Inquiry are set up--usually too late to prevent popular varieties continue to be grown no lifting of the crops while blight Is sporlng pro- matter what their blight susceptibility.

CARRYOVER OF BLIGHT FROM SEASON TO SEASON The cllmagrams for various parts of the possible. In other regions where potatoes are world show clearly the gaps between cropping grown in the winter [ Southern States, U.S.A.], periods In which the fungus naust survive. In tomatoes, for which the optimum temperature some areas [George, S. Africa; Pukekohe, range Is higher than for potatoes, may bridge New Zealand] potato growing goes on through- the summer gap. In still other areas [Trans- out the year, and new potato foliage Is always vaal; Australia] the winter gap Includes a subject to Infection by airborne sporangia from period, sometimes of several months, during older crops In the vicinity. In other areas which the tubers are left In the ground. In [Kenya, East and West Africa] where there are warm or hot soil, which causes more or less two cropping periods In the year, the short complete rotting of any with blight Infection, gaps between them may be bridged by potatoes thus effecting their eradication. Go spore s of left In the ground and growing on like weeds. the blight fungus have now been found In potato Blight on solanaceous trees and shrubs [Kenya; foliage In one country of the world [Mexico], New Zealand] may also have a bridging effect. and Is conceivable that these spores, If they In some hot regions [Plains, India] where pota- survive In leaf debris, may play some part In toes can be grown only In the winter, the fungus the carryover of the fungus. must survive a summer gap, with temperature s In the great northern potato-growing regions that may be lethal to the mycelium In the of the world (northern Europe, U.S.A., Canada) tubers, but the Introduction of cold stores for there Is a long winter gap during which there the seed In the summer has made survival Is no foliage on which the fungus can survive.

201 and so far as is known this gap Is bridged potatoes are grown, the infection appears first solely by survival of mycelium of the fungus on the foliage of the early potatoes and these in some of the infected tubers. The proportion contribute to the first infection of the main- of the infected tubers, in which the fungus does crops. Undoubtedly, the long period required survive is very small [Netherlands, England for the buildup of infection after the winter and Wales]; most, but never all, of the break retards the onset of epidemic spread infected tubers are riddled out of seed stocks; of the disease; there is some evidence that and the infected shoots arising from blighted when the level of blight in the tubers is low in tubers planted are generally few and far apart. a dry year, the outbreak in the following year Greater concentrations of blighted tubers occur will be later than usual. But in the main there in cull piles [ U.S.A.] or in waste heaps left by appears always to be sufficient inoculum to clamp sites [ England and Wales] , and thus the start a blight epidemic when weather conditions probability of primary foci occurring in grovsrth are favorable for its development. Although from such heaps is high. The proportion of important progress has recently been made in infected plants among groundkeepers also tends the study of the early stages of blighf epidemics to be high, as some of these arise from blighted [Netherlands, England and Wales], there is tubers rejected at lifting. In many districts much about this phase of the disease that is where both early varieties and maincrop still obscure.

EFFECTS OF VARIETAL RESISTANCE IN MODIFYING BLIGHT EPIDEMICS That sonne potato varieties are more resis- differing from each other in the potato varieties tant to blight than others was observed during that they can attack. New varieties, with ^. the first epidemics of blight in Europe in 1845- demi s sum in their parentage, have withstood 46. Ever since that time the choice, or the attack by all known races of the fungus for a breeding and introduction, of varieties having time, only to go down eventually with other some measure of blight resistance as one of races not previously distinguished or perhaps their desirable qualities has been the principal not previously existent. The situation has been factor in reducing the incidence of blight about analyzed in genetical terms (see fig. 16, the world. The introduction of Champion greatly Scotland). The ntiajority view of potato breeder s lessened the severity of the blight epidemics now appears to be that R-gene resistance is in Ireland during the nineteenth century, and not enough, and it may be of less value than the the same process is still going on. In recent long familiar kind of partial resistance pos- years the substitution of Sebagofor Up-to-Date sessed to a varying degree by such varieties in Australia, of Robijn for Kerr's Pink in as Champion, Voran, Kerr*s Pink, Alpha, and Kenya, of Sebago for Green Mountain in Prince Sebago. This kind of resistance is now dis- Edward Island, and of Alpha and Voran for tinguished by the name of '*field resistance." indigenous varieties in Mexico are all cases in Although only a partial resistance, this is point. For whatever reasons these substitutions effective against all blight race s. Its attractive - were made, and by no means all of them were ness to potato breeders has recently been deliberately made for the purpose of reducing enhanced by the discovery [Mexico] of an opera- blight losses, they have all had this effect. tive sexual phase in the blight fungus, which During the past quarter of the century the may be capable of producing an almost infinite long mooted idea of breeding potato varieties range of new blight races. totally immune from blight by crossing the A very important outconie of all the work domestic potato with Solanunn de mi s s um and with S. demis sum is the discovery that, apart other wild species has been very strenuously from R-gene resistance, this species also explored [u.S.S.R., Germany, Scotland, Nether- possesses field resistance in a very high de- lands, U.S.A., Mexico, England and Wales] . gree, and thatitmay beusedas a source of this The work has revealed that the blight fungus quality in potato breeding for field resistance. is not homogeneous, as was once supposed, but The nature of field resistance, now it has been that it has a number of races or strains. given this name, is under world investigation.

POTATO BLIGHT FORECASTING

A general relationship between the develop- ditions that would provide warnings of blight ment of potato blight epidemics and the oc- outbreaks for the purpose of spray timing. The currence of wet or humid weather has been work was advanced by the researches of recognized since the famine years of the last Crosier in the U.S.A. on the humidity and mois- century, but it was not until the formulation of ture requirements of the blight fungus, pub- the Van Everdingen rules [Netherlands] in the lished in 1934 [introduction], and by the der- mid-1920's that real progress was made in the ivation, over the period 1929-39 of the simple attempt to define specific meteorological con- Beaumont rule [England and Wales] .

202 In the past decade much practical success in form the basis of a valid forecasting system regional forecasting has been achieved by the can be worked out only by investigation and application of the Beaumont and other tempera- experience in the country concerned. Account ture-humidity rules--notably those of Post and must be taken of the number and distribution of Richel, Bourke, and Wallin--in ways already the weather stations available, the nature of summarized in this study under the countries the terrain, and the phenology of the crops. concerned [Netherlands, England and Wales, Effective and extensive collaboration be- U.S.A., Irish Republic, Germany]. The essen- tween meteorologists and plant pathologists is tially different cumulative-rainfall method of a prime requirement in the development of Cook has also been successfuly employed in a forecasting service, as the meteorological eastern Virginia, and with certain modifica- findings (whatever criteria of "blight weather** tions Hyre has found it successful under test are employed) must be checked and correlated in some Northeastern States [U.S.A.] . with the results of adequate field surveys of In general critical weather periods for the blight outbreaks in the region to be served. development of blight have now been em- The use of synoptic weather maps and air- pirically defined, and the recording of them mass analysis [irish Republic] may find an has been reduced to fairly simple and practical application in many parts of the world, where procedures. Critical periods, according to one forecasting has to be attempted for a wide system or another, can now be recorded in most area with no very close network of weather parts of the world, but the way in which the stations. observations are to be interpreted and used to

PROTECTIVE SPRAYING: MATERIALS The fungicides employed for potato spraying were not. By the 1950's practically all the have changed during the past quarter of a potato spraying in England and in Holland was century. Up till the early 1930's the use of being done with cuprous oxide or copper ox- bordeaux or burgundy mixtures was universal. ychloride. In the meantime, the use of organic Preparations of copper oxychloride and of sulfur fungicides, chiefly zinc or manganese cuprous oxide then began to come on the dithiocarbamates, had largely superseded cop- market, having the advantage that they greatly per oxychloride and cuprous oxide in the simplified the preparation of the spray fluid U.S.A. It was found that under the prevailing in the field, but the disadvantage that they did conditions in the U.S.A.--with frequent appli- not give such lasting protection of the foliage. cations, high temperatures, and much insect Bordeaux and burgundy mixtures, in point of damage conducive to copper injury--the dithio- retentivity under rain, are still unsurpassed. carbamates controlled the epidemics satis- The copper oxychloride and cuprous oxide factorily, and their use led to increased sprays made great headway, nonetheless. The yields. By 1957 the dithiocarbamates had taken introduction of low-volume potato spraying in a place among the available potato spraying the late 1940* s--following the development of materials of the world; they were replacing light low-volume machines for the application copper oxychloride to a considerable extent in of MCPA and 2,4-D weedkillers to cereals-- the Netherlands, at least for the first sprayings, was greatly to the advantage of the new sprays, and they were just being launched on the mar- as they were suitable for application at low ket for potato spraying in England and volunne while bordeaux and burgundy mixtures Wales.

PROTECTIVE SPRAYING: EXTENT OF USE The amount of protective spraying done in mechanized, and numerous applications various parts of the world depends upon the already required for insect pest control make it nature of the terrain, the frequency and severity easy to incorporate fungicides [parts of U.S.A.]. of the losses caused by blight, the value of the The need for spraying against Colorado beetle crop, and the amount of technical development [France] may also promote its adoption. in the country concerned. The method is im- The value of spraying as a routine measure practicable in primitive cultivations, up steep is uncertain and its use is local and dis- mountains, and in areas where torrential rains cretionary in countries where there is an may wash off the spray deposits as soon as irregular alternation of blight years and years they are applied [East Africa] . It is employed with little or no blight [Germany, England and to best advantage in maritime areas where Wales]. It is in such countries that blight there is a high level of yield, and where with- survey and forecasting work is most needed, out it there would be heavy defoliation losses to distinguish the localities in which spraying almost every year [Netherlands, Ireland, is profitable and to save wasted expense and Prince Edward Island] . It is used, sometimes crop damage in years when it is not. In such to excess, where cultivations are very fully countries there is a strong tendency to spray

203 only the more blight susceptible and valuable objective and impartial blight surveys, there varieties. In general there is a tendency for is also a decided tendency for the amount of less and less spraying to be done as the spraying done to be influenced as much by the climate becomes more and more continental, commercial enterprise of spray manufacturers and the need decreases [U.S.A., northern and spraying contractors as by the incidence Europe], But, in the absence of guidance from of blight. PROTECTIVE SPRAYING: EFFECT ON CONTROL OF BLIGHT IN TUBERS Although it is common to regard protective tubers. The greater persistence of the copper spraying as a measure for the reduction of sprays, and particularly of bordeaux and defoliation losses rather than for the control burgundy mixtures, inpart explains the prefer- of blight in the tubers, one point that emerges ence for copper rather thanfor the dithiocarba- from this study is that protective spraying mates, as copper sprays provide the longest sometimes effects a very considerable reduc- possible protection of the haulm from the last tion in tuber infection. In some countries spraying and therefore the greatest possible [Ireland, sea clay regions of Holland] the avoidance of the chance of tuber infection. available figures show that protective spraying There is evidence [Prince Edward Island; Long generally about halves the mean percentage Island, U.S.A.; Pukekohe district. New Zea- of blighted tubers in the crops. Where spraying land] that the direct application of copper unduly prolongs the growth of partially blighted fungicides to the soil or accumulation of haulm at the end of the season, it often has the copper in the soil after years of protective opposite effect; but in some countries, spraying with copper may kill sporangia wash- especially those with a short growing season ing down to the soil, and therefore effect a [Iceland] and in some seed-growing areas direct protection of the tubers. If this is, in [Scotland, Brittany], protective spraying ap- fact, an important part of the action of late pears to be used for the main purpose of pre- copper applications in reducing tuber infection, venting tuber infection. There is a very notice- then the inferiority of the dithiocarbamates able preference for the use of copper rather for this purpose becomes understandable, for than organic sulfur fungicides for the later they may break down and lose their applications where the reduction of tuber potency as fungicides when in contact with blight is an objective; in Ireland strong appli- soi]. cations of burgundy mixture are recommended Where the growing period is short, and for this purpose. haulm destruction some 10 days or a fortnight It is understandable that if protective spray- before lifting would entail a considerable ing is continued late in the season and is so sacrifice of crop, reduction of tuber infection effective that it keeps blight off the haulm by late protective spraying with copper may be entirely, there will be no inoculum to wash a better technique. But it has the disadvantage down to the tubers and their infection will be that it nnay leave the haulm green and with prevented. This ideal state of affairs is rarely blight sporing on it at lifting, and thereby lead attainable in practice, and despite all prac- to infection of the tubers when they are ex- ticable spraying there will almost inevitably posed. Perhaps with copper in the soil, the be some blight on the haulm toward the end of usually stipulated 10 to 14 days' delay be- the season in a blight year. Then the effect of tween haulm destruction and lifting is not the spraying would appear to be one of degree, necessary, and haulm destruction much the longer blight is kept off the haulm by pro- nearer the date of lifting would suit tective spraying, the shorter the period during the case. This is a matter for investiga- which sporangia can wash down and infect the tion.

HAULM DESTRUCTION

Removal of blighted haulm before lifting to ical haulm-cutter s and pulverizers are als o in prevent infection of the exposed tubers is one limited use. of the oldest of blight control measures, but Haulm destruction is employed on seed destruction of the haulm by chemical or crops to prevent translocation of viruses fronn mechanical means is a comparatively new the haulm to the tubers and also for the technique. It is now employed extensively in limitation of seed size. It is often also useful North America, the Netherlands, Great Britian, on ware crops as a farm practice for clearing and to some extent in other European countries, the ground in preparation for lifting. and particularly on seed crops. The principal As a measure for the reduction of blight materials now employed are sodium and potas- Infection of the tubers it i s of most value where sium arsenites, sulfuric acid, s odium chlorate, partially blighted haulm would otherwise be DNC, and other di-nitro compounds. Mechan- surviving at lifting. This most commonly

204 happens where the growing season Is short infection in the ground before lifting--the chief (under continental conditions and at high lati- need where there is a long growing season and tudes) and the method is certainly used on a most crops die down naturally well before very large scale in North America at the harvest--haulm destruction is often disap- present time for the purpose of preventing pointing [England and Wales, Netherlands]. tuber infection at lifting. It has the disad- Unless the risk of tuber infection in the soil vantage that a period, generally taken as 2 is considerable, as when a variety very weeks, has to be allowed between haulm susceptible to blight in the tubers is being destruction and harvest. Thus, some sacrifice grown, it is generally uneconomic to employ of an already short growing season has to be haulm destruction for the purpose of pre- made if lifting is to be completed successfully venting tuber infection before lifting. Good within the limited harvesttime. earthing up and the avoidance of tuber-suscep- As a measure for the reduction of tuber tible varieties are better expedients.

CONTROL BY ^^DISEASE-ESCAPE" METHODS

One of the main findings of this study has Wales], or **vernalization'* [U.S.S.R.], may been that the optinrium conditions for potato also effect an important measure of disease- cropping are in general also the optimum escape, by obtaining as much of the crop as conditions for the incidence of blight. Oppor- possible before weather conditions become ' tunities for evading blight losses by moving to favorable for epidemic spread of blight. De- other production areas are therefore few. tailed study of the climagram for an area may Sometimes it may be preferable to grow help to reveal other possibilities. The choice potatoes in areas where yields are low, rather of early-maturing, or early-bulking, varieties than in those where yields would be nnuch [ Ar ran Banner, in Ireland; Bint je in France higher but for the heavy or catastrophic in- and the Netherlands] may contribute to the cidence of blight [Sierra versus the high same end. But early-maturing varieties usually valleys, Mexico]. Surface irrigation may pro- also sprout early and are not good keepers. vide the water required for good potato yields The combination of early bulking and late without giving rise to the humidity necessary sprouting, even if it is not attainable by for the development of blight on the foliage; breeding, could perhaps be secured by judicious thus, potato cultivation under surface irriga- use of chemical sprout-depressants. The op- tion in arid regions may be attended with posite method of disease-escape by choice of disease-escape [Western States, U.S.A.,]. variety is to grow certain late varieties, which Planting of maincrops as early as possible are not generally attacked by blight until late ^ and sprouting before planting [England and in the season [Ackersegen, in Germanvl.

INDICATIONS OF SUBJECTS FOR FURTHER INQUIRY

In the course of this study the authors have ing the full potentialities of disease- inevitably found many matters concerning the escape methods of control. epidemiology and control of potato blight on 4. In potato breeding there is scope for which there is a paucity of information in the •^improving" varieties popular for their world. Some of the directions in which further quality, such as King Edward in England investigation appears to be most needed are and Bint je in the Netherlands, by breed- as follows. ing in a greater measure of field resis- 1. There is a need in most countries for tance. systematically conducted field surveys 5. Breeding for tuber resistance deserves of the incidence of blight, over a number more attention. of years, to establish the true losses 6. The possibilities of breeding for disease- caused by the disease and to guide con- escape, e.g., for early bulking coupled trol policy. with good keeping quality (without early 2. Such surveys can be made most economi- sprouting), though perhaps intrinsically cally by organizing countrywide observa- difficult, should be further explored. tions of the disease on the haulm, and 7. Work in progress on the investigation interpreting the findings in the light of of the mechanism and true nature of field periodic lifting and spraying trials at key resistance is of fundamental importance centers. Aerial reconnais sanee - -with and should be extended. suitable checks from the ground--could 8. There is need for further investigation be employed in this work. of heat treatment of seed-potato tubers, 3. More joint agronomic and plant patho- to ascertain whether or not there is any logical inquiry could be devoted to realiz- practical possibility of eliminating in-

205 fection from seedstocks by this means of spread in the field have not been ade- and to reveal the effects of such treat- quately explored. nnent, at various times between lifting 14. The establishment of blight forecasting and planting, on dormancy breaking and services is particularly desirable in sprouting. Such inquiries would be rele- countries where there is great variation vant to observed effects of natural heat- in the incidence of blight from year to ing on seedstocks in several parts of the year. The purpose of such services world. should be as much to save unnecessary 9. In regions of the world where blight is spraying and sprayer damage in years still absent or confined to small areas, when the incidence is slight as to pro- a sharp watch should be kept for new mote correct spray timing in those when outbreaks to prevent the dispersal of it is likely to be severe. infected local seed, which generally pre- 15. The working out of a successful fore- cedes first and catastrophic epidemics. casting scheme in any country demands 10. Modern spray materials would be greatly investigation of the significance of criti- improved if their capacity to resist cal weather periods by direct blight washing off by rain could be brought up survey in the field and is best linked with to equality with that of bordeaux and surveys proposed in paragraph 1. burgundy mixtures. 16. The economics of chemical control need 11. The possible efficacy of copper applica- close examination in relation to the tion to the soil as a means of reducing frequency of blight years and to the loss blight infection of tubers deserves criti- from crop damage in protective spray- cal investigation. ing, and from crop sacrifice in haulm 12. The whole question of the need for 10 to destruction. 14 days' delay between haulm destruc- 17. There is need for much more spread of tion and lifting requires re examination. education about the nature of the disease There is also a dearth of real knowledge over the world as a whole. The need for concerning the relations between soil more enlightenment concerning the inter- conditions and tuber infection. actions between haulm and tuber infection is not confined to backward countries. 13. There is need for further study of the 18. In potato marketing and the organization early stages of blight epidemics. Con- of potato supplies, there is need for ditions governing spore dispersal, dis- economic study of the opposed effects tances traveled by sporangia without of rainfall and blight losses on potato losing their viability, and mechanisms yields.

206 LITERATURE CITED

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1944b. The climatic background to the problem of and Ryan, M. potato varieties for East Africa. East African 1951. New hosts of Phytophthora infestans in Kenya. Agr. Jour. 9: 127-135. Nature tLondon] 168: 85. Naumova, N, A, Müller, K. Q. 1935. [Qi forecasting the appea ranee of Phytophthora 1928. Über die Züchtung krautfauleresistenter infestans on the potato.] [Leningrad] Inst. Kartoffelsorten. Ztschr. f. Pflanzenzucht. 13: Zashch. Rast. Plant Protect. 3: 51-54. [In 143-156. Russian,]

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Ormel, H. A. Reddick, D., and Peterson, L. C. 1954. De bestrijding van de aardappelziekte, 1945. Empire--a blight resistant variety. Amer. Phytophthora infestans (Mont.) de Bary.Land- Potato Jour. 22: 357-362. bouwvoorlichting 11: 210-214. Rhoads, A. S. Oxford Economic Atlas. 1944. Late blight and other potato diseases in 1956. Oxford regional economic atlas. The U.S.S J^. northern Florida. [U. S.] Plant Dis, Rptr. 28: and Eastern Europe. 134 pp. Ocford University 438-440. Press, Oxford. Rigot, N. Pal, B. P., and Pushkarnath. 1953. Aspects de la recherche agronomique en 1952. 1951. Potato breeding investigations in India. Empire Station de recherches de l'état pour l'améliora- Jour. Expt. Agr. 19: 87-103. tion de la culture de la pomme de terre a Libramont. Rev. Agr. [Bruxelles] 6: 1739- Parks, N. M. 1749. 1955a, Canada's potato industry. Agr. Inst. Rev. [Canada] 1955. May-June. Riley, E. A. 1955. Late blight of potatoes and tomatoes in Northern 1955b. Potato growing in Canada. Canada Dept. Agr. Rhodesia. [U. S J Plant Dis. Rptr. 28: 438-440. Pub. 918. Roer, L, Peikert, F. W., and Bonde, R. 1957. Fors^k med spr^ytemidler mot ^Jrrlte (Phy- 1954. Potato disease and insect control with low- tophthora infestans (Mont.) de ^.) pa potet. gallonage sprayers. Maine Agr. Expt. Sta. [Norway] Forskn. Forsok i Landbr. 8: 125- Bui. 527, 16 pp. 138.

Peterson, C. E., and Gwinn, A. B. Roland, G. 1952. Influence of vine killing and 2,4-D on yield, 1946. La sensibilité de diverses varietés de pommes specific gravity, and vascular discoloration of de terre a l'attaque de Phytophthora infestans potatoes. Amer. Potato Jour. 29: 253-267. (Mont.) de Bary. Parasitica 2:121-124.

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219 INDEX

SUBJECT INDEX

Aerial reconnaissance, 205 Blight in tubers--Continued Africa, 5, 166-180, 196, 198, 200, 201, 203 Effect of storage conditions, 8, 84 Aircraft, use for spraying in General review, 199 England, 25 Incidence of. SEE under countries Netherlands, 83 Varietal resistance, 29, 35, 73, 106, 143, Red River Valley, 130 188, 194, 199, 205 Alabanna, 129, 133 Blight progress curves Alternarla solani> SEE Early blight Chile, 162, 164 Aphids, control of, 133, 146 Denmark, early use in, 88, fig. 38 Argentina, 5, 161 England and Wales, 11, 32, figs. 8,9, 12 Aroostook County, Maine, 121-126, 136, 197, France, 63, figs. 24, 25 198, 200 India, 182, fig. 89 Arsenites, use for haulm destruction in Long Island, 132, fig. 65 Canada, 146 Netherlands, 78, 79, figs. 31, 32 England, 32 Prince Edward Island, 143, fig. 70 Netherlands, 84 South Africa, 168, fig. 81 U. S. A., 133 Switzerland, use in, 108 World, general, 204 Tasmania, 195, fig. 94 Austria, 107-109, fig. 48 World, general, uses, 197, 198 Australia, 191-195, 201, 202, fig. 92 Blight races: biotypes of Phytophthora infestans Beaumont periods. SEE Forecasting Discovery of, 3, 34 Belgian Congo, 173 In Kenya, 174, 175 Belgium, 3, 67-71, 196, 198, fig. 27 In Mexico, 154 Blight assessment on haulm, methods In Netherlands, 84 B. M. S. key, England and Wales, 11 In Northern Ireland, 47 Dutch modification of, 78, fig. 33 In Peru, 160 Use in Norway, 93, fig. 41 In South Africa, 169 Use in other countries. SEE Blight In Tanganyika, 177 progress curves In U. S. A., 125, 140 Horsfall and Barratt probit scale In Western Germany, 106 U. S. A., 134 International nomenclature, 42, 140, 150, Use in Peru, 1 56 fig. 16 International needs, 196 Significance of oospores, Mexico, 154, 202 Scales for incidence and severity Survey of, Canada, 150 Austria, 107, 108 World, general review, 202 Denmark, 86 Blight-resistant potato varieties. SEE Breed- Western Germany, 104 ing, Potato varieties, and Field resist- Blight epidemics ance Definition of Blight weather By frequency and severity of blight years, Characteristics of, 9, 198 3, 134, 136, 197 Critical periods, 17, 56, 74, 138, 145, 158, By progress curves, 11, 196, 197 194 Determination of effects on yield, 19, 39, SEE Forecasting 54, 79-81, 87, 88, 97, 98, 103, 104, Blight zones 122-124, 197 England and Wales, 11, 12, figs. 4,5 Conapensatory effect of rainfall, 16, 86, France, 62-65 199, 200 Poland, 109, figs. 51, 52 Early stages of, 7, 17, 77, 147 Principles of zoning, 11 Economic consequences of, 200 Republic of Ireland, by rainfall, 49, fig. 18 In Ireland 1845 and Chile 1950 compared, '*Schadgebiete'' in Western Germany, 102, 161 105. fig. 46 In particular countries« SEE under country U.S.S.R., 113, fig. 56 In relation to weather. SEE blight weather B. M. S. key, for blight on haulm, 11 In world, general review, 197, 198 Bordeaux mixture Modification of, by resistant varieties, 202 Copper-lime ratios, 146 Blight in tubers Need for equal retentivity in other sprays, Conditions favoring. SEE Tuber infection 206 Control measures, 31, 204, 205. SEE ALSO Properties of, 203, 206 Haulm destruction; Heat treatment

220 Bordeaux mixture--Continued Use for potato spraying in Carryover of blight--Continued Belgium, 70 Primary foci from blighted tubers. 17 75 Canaeroons, 179 77, 128, 202 Canada, 143, 146, 147, 150 SEE ALSO Cull piles, Groundkeepers, Sola- Chile, 165 naceous wild plants Chile, 5, 57, 160-165, 196, 198, 200, 201, Cyprus, 117 fig. 77 ' England, 21-25 France, 66 China, 5 Kenya, 175 Christchurch district. New Zealand, 187-189 Long Island, 133 ** Chuño," 155 Mexico, 153 Climagrams Netherlands, 82, 83 Australia, 194, fig. 93 Norway, 95 Austria, 107, fig. 49 U.S.S.R., 115 Belgium, 68, 70, fig. 28 World, general review, 203 Cameroons, 178, 179, fig. 86 Bouisol, colloidal copper spray, 54 Canada, 142, 148, figs. 69, 71 Brazil, 5 Chile, 161, fig. 78 Cyprus, 117, fig. 58 Breeding potatoes for blight resistance Denmark, 88, fig. 36 Effect in modifying epidemics, 202 England, 15, fig. 6 History of, general, 3, 202 Explanatory notes, 3, 4, fig. 2 In Australia, 195 France, 65, fig. 26 In Belgium, 70 Iceland, 90, fig. 40 In Canada, 141, 150 In Chile, 165 India, 181, 182, fig. 88 Kenya, 173, fig. 83 In England and Wales, 34-36 Mexico, 151, 153, fig. 73 In France, 66 Netherlands, 7 3, fig. 30 In Kenya, 174 New Zealand, 185, fig. 91 In Mexico, 153, 154 Northern Ireland, 43, fig. 19 In Netherlands, 84 Norway, 92, fig. 42 In Northern Ireland, 48 Peru, 157, fig. 75 In Poland, 110 Poland, 109, fig. 50 In Scotland, 42 Republic of Ireland, 49, fig. 19 In South Africa, 169 Scotland, 40, fig. 15 In U. S. A., 125, 140 South Africa, 166, fig. 81 In U.S.S.R., 112, 115 Sweden, 95, 98, fig. 44 In Western Germany, 106 Uganda, 176, fig. 84 Need of breeding for tuber resistance, 205 U. S. A., 121, 128, 130, 132, figs. 61, 63, 64 Burgundy mixture, use for potato spraying in U.S.S.R., 111, 114, fig. 55 England and Wales, 25 Uses in potato technology, 3, 196 Iceland, 91 Western Germany, 105, fig. 47 India, 183 World, general review, 196, 197, 198 Irish Republic, 51, 52, 54 Climate, in relation to potato growing and blight Netherlands, 82, 83 General considerations, 1, 3, 9, 196-199 Northern Ireland, 45, 46 SEE ALSO under countries World, general review, 203 Colorado beetle and blight control Austria, 109 Calcium carbide, use for domancy breaking, 189 Belgium, 70 Calcium cyanamide, use for haulm destruction Canada, 146 Canada, 146 France, 66, 203 U. S. A., 138 Netherlands, 83 Cameroons, 178-180, 200, 201, fig. 85 U. S. A., 120, 133 Canada, 140-151, 200, 201, fig. 68 Western Germany, 106 Carryover of blight, from season to season World, general review, 203 By continuous cropping, 159, 166, 189, 202 Common scab of potatoes, surveys in By imported seed, 116, 127, 134, 169, 182 England and Wales, 19 By outdoor tomatoes, 134, 189, 201 Consumption of potatoes Early stages of epidemics, 17, 75, 206 In world, general, 3, 4, 200, 201 Effect of cold storage on. 182, 184, 201 SEE ALSO under countries Effect of soil temperature on, 169 Copper carbonate, use in potato spraying, In U. S. A., 131, 134 U.S.S.R., 115 In world, general review, 201, 202 Copper, in soil, effect on blight in tubers Indications from climagranns, 201 Long Island, 131 Investigations needed, 206 Prince Edward Island, 147 Life history of fungus, 7, 8 Pukekohe district. New Zealand, 190

221 Copper, in soil--Continued Dithiocarbamates - - Continued Need for further investigation, 206 Mexico, 153 Copper oxychloride sprays, use in Netherlands, 82, 83 Belgium, 70 Peru, 159 Cyprus, 117 South Africa, 169 Denmark, 88 Southern Rhodesia, 171 England and Wales, 22-25 U. S. A., 125, 129, 133, 136 France, 66 Western Germany, 106 Kenya, 175 World, general review, 203 Netherlands, 82, 83 DNC, DNOC, DNBP, di-nitro compounds, use New Zealand, 190 for haulm destruction in Northern Ireland, 46 Netherlands, 84 Scotland, 41 U. S. A., 133, 138 U.S.S.R., 115 Western Germany, 106 Western Germany, 106 World, general review, 204 World, general review, 203, 204 Dormancy breaking in seed tubers, 116, 189 Copper sprays, phytotoxic effect Dusting, for blight control in SEE Spray damage Canada, 150 Cost of potato spraying England and Wales, 25 SEE Economics of spraying France, 66 Cull piles, waste heaps, and clamp sites, as Northern Ireland, 45 sources of blight* infection, 7,75,125,134, Peru, 159 145, 202 Red River Valley, Minn, and N. Dak., 130 Cuprous oxide sprays, use in U.S.S.R., 115 Belgium, 70 England and Wales, 25 Early blight (Alternarla solani) in France, 66 Australia, 194 Kenya, 175 Canada, 146 Netherlands, 83 Prince Edward Island, 146 Northern Ireland, 46 Red River Valley, 130 Western Germany, 106 South Africa, 169 World, general review, 203 Economic effects of blight losses Cyprus, 115-117, figs. 57, 58 Compensatory effects of rainfall, 199, 200 In main potato-growing countries, 201 Daylength, effects of In tropical and subtropical regions, 200 Equatorial Africa, 171, 174, 175 Need for further investigation, 206 General considerations, 6 Economics of potato spraying Hastings section, U. S. A., 129 Belgium, 70 Iceland, 90, 175 England and Wales, 26-28, 33, 34, 36 Norway, 92, 94 Netherlands, 83 Sweden, 95, 97, 98, 99 U. S. A., 137 New Zealand, 190 Egypt, 115 DDT spraying, 66, 70, 83, 125, 130, 133, 135, England and Wales, 6, 10-36, 196, 197, 198, 169, 191 199, 200, 202, 203, 204, 205, figs. 3.4 Denmark, 85-89, 197, 200, fig. 35 Exploration, for wild Solanum species, 35, 115 Disease-escape nnethods of blight control England and Wales, 36 Field resistance of potato varieties to France, 66 Phytophthora infestans General considerations, 3, 205 As in Norway, 95 Mexico, 153, 205 Ackersegen and Voran in Chile, 165 Peru, 160 Alpha and Voran in Mexico, 153, 202 South Africa, 169 Black's Seedling 1521(c) in Tanganyika, 177 Southern Rhodesia, 171 Comparison with R-gene resistance, 154, U.S.S.R., 115, 205 169, 202 Distribution of potato growing, 1, 3, 196, 200, Definition of, 35, 154, 202 fig. 1 Further investigations needed, 205 SEE ALSO under countries Pimpernel and others in Southern Rhodesia, Dithane. SEE Dithiocarbamates 171 Dithiocarbamates, use for spraying in Robijn in Kenya, 174, 202 Australia, 195 Sebago in Australia, 191, 195 Belgium, 70 Sebago in Canada, 145, 2Q2 Canada, 146, 147 Skerry Blue (Toro Blue) in Uganda, 177 Cyprus, 117 Solanum demissum as source of, 34, 154, 202 England, 26 World, general review, 202 France, 66 Flame throwers, use in haulm destruction, 138

222 Flea beetle, 120, 133, 146 Haulm destruction--Continued Florida, Hastings section, 126-129, 136 Investigations needed, 206 Fohn winds, effect of. New Zealand, 188 Loss of yield caused by, 33, 34 Forecasting, potato blight Methods and objects of, 32, 204 Beaumont rule, 17, 202, 203 On cull piles, 125 Concept of zero date, 41 On seed crops, 39, 40, 41, 48, 66, 70 Cook's cumulative rainfall method, 138, 139, Practice in 203, fig. 67 Australia, 195 Cooperation between pathologists and mete- Canada, 150 orologists, 203 Europe, 66, 70, 84, 89, 106 In Aroostook County, Maine, 125 Peru, 159 In LK>ng Island, New York, 133 U. S. A., 125, 131, 135, 138 In Netherlands, 74, 75, 202, figs. 31, 32 U.S.S.R., 115 In Red River Valley, 130 SEE ALSO Sulfuric acid, Arsenites, Tar oil In Scotland, 41 fractions. Sodium chlorate. Calcium In Switzerland, 108 cyanamide, D.N.C, and Haulm cutters Investigations in Western Germany, 105, 106 and pulverizers Irish rules, 56, 203 Trials in England, 31, 32 Moving graph method, U.S.A., 139, 203 Vascular discoloration caused by, 125 Nocturnal humidity rule, Peru, 158 Heat treatment of seed tubers Nomogram for incubation period, 115 By high soil temperatures, 169, 201, 206 Operations charts, 17, 203, fig. 7 Lethal temperature for mycelium, 8 Requirements for success, 203 Need for reinvestigation, 205, 206 Thran accumulated temperature rule, 106 Proposed use in South Africa, 169 '•Tizon" (blight) warning service, Chile, 165 Holland, SEE Netherlands Trials of Beaumont and Irish rules Humboldt current effects In Chile. 164, fig. 79 Chile, 164 In Tasmania, 195, fig. 94 Peru, 157 Use of synoptic maps, 57, 203, fig. 21 Humidity conditions governing blight epidem- Van Everdingen rules, 74, 115, 202 ics. SEE Blight weather and Forecasting Warning services in France, 66, 67 Warning service in Prince Edward Island, Iceland, 89-92, 198, 199, 204, fig. 39 145, 147 India, 181-184, 196, 200, 201, fig. 87 World, general review, 202, 203 Introduction of blight into new areas France, 60-67, 198, 203, 204, 205, figs. 22, 23 Belgian Congo, 173 Frosts Cameroons, 178, 201 Effect on blight infection of tubers, 99, 121, Canada, 147, 148 150 Chile, 160, 161, 201 Effect on overwintering of blight, 124 Cyprus, 116 Frost-resistant varieties, 90 India, 182 Limiting potato-growing period, 6, 90, 99, Ireland, 160, l6l 121, 166, fig. 81 Kenya, 172, 173, 201 SEE ALSO Climagrams Need for lookout services, 206 Northern Rhodesia, 177, 201 Gains and losses from potato spraying. SEE Parallel between first Chilean and Irish epi- Spraying trials and Economics demics, 160, 161 Germany. SEE Western Germany Peru, 156, 201 Gnorimoschema operculella. SEE Potato moth Southern Rhodesia, 170, 201 Greece, 115 Tanganyika, 173 Groundkeepers as sources of blight infection, Tasmania, 191 131, 134, 174, 189, 201 Uganda, 173, 176 Growth curves for potato tubers U. S. A., 133 Bint je and Voran, Netherlands, 79, fig. 34 World, general review, 201 Late varieties in Maine, 122, 123, fig. 62 Ireland, 196, 197 Majestic and King Edward, England and Blight and weather in, compared with Chile, Wales, 19, fig. 8 160, 161, 164 Up-to-Date, Republic of Ireland, 54 Republic of, 48-59, 198, 199, 203, figs. 17,18 Use in loss estimations, 19, 20, 54, 196, Irrigation, practice and effect on blight 197, fig. 8 Australia, 194 Chile, 160, 164, 165 Haulm cutters and pulverizers, 33, 84, 89, Cyprus, 115, 117 125, 131, 133, 138, 204 General considerations, 6, 196 Haulm destruction India, 182, 184 For control of virus diseases, 48, 66, 71 Mexico, 153 In world, general review, 204, 205 New Zealand, 189

223 Irrigation- -Continued Phytophthora infestans - - Continued Overhead irrigation Survival Australia, 194 At higl temperatures, 8, 130 New Zealand, 189 In stems, 8, 147, 157 Peru, 155 In tubers. 8. 73. 77, 182 Southern Rhodesia, 170, 171 Tuber infection, routes of, 7 U. S. A.. 133, 135 Wild host plants. 134, 154, 174, 189 Israel, 115 World distribution, 1 Italy, 115 Poland, 109-110, 197, 198, figs. 50, 51 Population Japan, 5 In relation to potatoes, 5 SEE ALSO under countries Kenya. 172-175, 198, 201, 202, figs. 80, 82 Potatoes As world crop, 3, fig. 2 Leafhopper, 120, 130 Breeding. 3, 202 Leaf roll, 145, 170 SEE ALSO Breeding for blight resistance Long Island, 131-133, 136, 198, 204 Cultivations. SEE under countries Losses Factors affecting yields, 3, 5, 196. 197, 1?9 Due to premature defoliation by blight Human consumption Econonxic effects of, 200 Use for fodder and manufacture. 5. 200 In particular countries. SEE under country SEE ALSO under countries Methods of computation, 19-25, 38,39,55, Length of growing season, 3. 6 79, 86, 97, 98, 124, 142, 143, 156. Rate of bulking. SEE Growth curves 157, 183. fig. 8 Seed crops. SEE Seed-potato growing Due to tuber infection Potato moth, 191 Frequency diagram, England and Wales, Potato varieties fig. 11 As 92 95 General review, 199 Ackcrsegen, 60, 70, 90, 101, 102, 106, 109 In particular countries. SEE under country 110, 161, 165. 205, fig. 52 The duality of the disease, 199 Adina, 191 Due to spray damage. SEE Spray damage Agnes, 102 Low-volume spraying, technique, 22-25, 66, Akranes, 90 83, 104, 106, 125. 138, 189, 195, 203 Alma, 109 Alpha, 49. 68. 86. 90. 91. 151. 153. 202 MCPA and 2.4-D weedkillers, 203 Amarilla de Pueblo. 151 Mexico. 1, 151-154, 184, 198, 200, 201, 202. Aquila. 70, 101, 102, 106 205, fig. 72 Arran Banner, 10. 29, 35, 43, 48. 49. 52. 5 6 Morocco, 115 60. 116. 117, 185, 188, 205 Mozambique, 170 Arran Chief, 185, 187, 189 Arran Consul, 43 Netherlands, 3, 5, 71-85, 196, 197, 198, 199. Arran Peak, 10, 35, 43 202, 203, 204, 205. fig. 29 Arran Pilot, 10, 37. 91, 92 Net necrosis, 145 Arran Victory, 43, 45. 52 New Zealand. 5, 185-190. 198, 201. 204, fig. 90 Arran Viking, 35 Northern Ireland, 43-48, 197, figs. 17, 18 Aucklander Short Top, 185. 188 Northern Rhodesia. 177. fig. 80 Aucklander Tall Top. 188 Norway, 92-95, 197, fig. 41 Augusta. 102 Ballydoon, 43 Pentachlorophenol. use for haulm destruction. Berlichingen, 112 195 B.F. 15, 60 Peru. 5, 153, 155-160, 196. 198. 199. 200,201. Bintje, 60, 63-66, 67-70, 73. 76, 78-84, 85 fig. 74 87, 108, 151, 205 Phytophtho r a infestan s Birgitta, 95 Biotypes. SEE Blight races Bismark. 191 First records. SEE Introduction into new Black's Resistant Seedling 1521(c). 177. 180 areas Black's Resistant Seedling 1521(d). 180 Germination conditions, 7, 8 •'Blight Résister'' 914, 177 Incubation. 7, 8 Bliss Triumph, 119, 129 Infection through upper and lower leaf sur- Blue Icelandic. 90 face, 99 Bona, 101, 102, 105. 108 On tomatoes. SEE Tomatoes British Queen. 35. 43, 46, 47, 49, 52 Oospores. in Mexico, 1. 154, 201. 202 Brownell. 191 Primary foci, 7, 84, 212 Canso. 141, 142, 150, 180 Spore dispersal, 7, 8, 9, 77 Capella. 102 Sporulation. 7. 8 Carman. SEE Green Mountain

224 Potato varieties--Continued Potato varieties--Continued Champion, 49, 84, 202 Kinongo Kerai (Northern Star). 174. 175 Cherokee, 180 Krasava, 60 Chippewa, 119 Lenino, 109 Concordia, 102 Leona, 151 Corahila (nnixture), 161, 165 Lochar, 45 Craig's Alliance, 35 Lorkh, 112 Craig's Bounty, 35 Magna, 102 Craig's Royal, 35 Magnum Bonum, 95, 97, 181 Craig's Snow White, 35 Majestic, 10, 19-24, 29, 34, 35, 37, 39, 43, Dakota, 185, 188 46, 48, 49, 54, 81, 117, 175, fig. 12 Darjeeling Red Round, 181, 184 Maritta, 101, 102, 106 Dianella, 86 May Queen, 10 Dr. Mclntosh, 10, 35 Mittelfrühe, 102, 109 Duke of York, 10, 68, 73, 75, 77, 101, 108 Monak, 191 Dunbar Standard, 43 Ninetyfold, 10 Early Puritan, 95 Noordeling, 73 Early Rose, 90, 91, 95, 109, 112 Northern Star, 174, 175, 189 Eclipse, 10 Oberambacher Frühe, 101. 108 Eersteling. SEE Duke of York Olympia, 102 Eigenheimer, 68, 73, 77. 78, 81, 82, 90 Onikwa 52, 169 Empire, 140 Onikwa 53, 169 Epicure, 10, 37, 40, 49, 112 Orion, 35 Epron, 112 Papa Amarilla, 157 Erste von Nassengrunde, 66 Papa Blanca. 157 Erstling. SEE Duke of York Pentland Ace, 35, 42, 171 Etoile du Le^on, 60 Pentland Beauty, 35 Factor. SEE Up-to-Date Phulwa, 181, 182 Flava, 102, 105, 109 Phytophthora-Resistant, 112, 115 Furore, 90, 151, 171 Pimpernel, 171 Gamekeeper. SEE Northern Star Placid, 180 Gineke, 151 Pontiac, 119 Gladstone, 29, 35, 43 Preferment, 1 74 Gloria, 95 President, 95, 161 Gola, 181 Prestkvern, 92 Golden Wonder, 37, 49, 52 Prummel, 151 Grahm. SEE Up-to-Date Pungo, 180 Great Scot, 90, 181 Record, 68 Green Mountain, 119, 121, 123, 124, 131, Red Icelandic, 90, 91 138, 140, 141, 142. 143. 145. 147, 191, Red McClure, 119 194, 202, 212, fig. 71 Red Pontiac, 119, 129, 138 Gullauga, 90, 91 Redskin, 10, 37 Heida, 101, 102, 105 Red Warba, 119 Home Guard, 10, 35 Robijn^ 169. 171, 174, 175, 202 Hybrid 42 (U.S.S.R.), 112 Robusta, 102 Ijsselster. 68, 73 Roode Star, 174 Industrie, 70, 161 Russet Burbank, 119 Institut de Beau vais, 60 Russet Rural, 119 Inverness Favourite, 188 Saskia. 60, 108 Irish Cobbler, 92, 119, 131, 141, 142, 147, Satha, 181 157 Sebago, 119, 123, 126, 140, 141. 142, 143. Italian White Round, 181 145. 169, 191, 193, 194, 202 XeTssing, 92 Sequoia, 90, 191 Kameraz. 112 Sharpe*s Express, 10 Katahdin, 119, 121, 123, 124. 131. 138. 141, Sieglinde, 101, 108 142 Silesia, 109 Kennebec, 6, 119. 121, 125, 139. 140, 142, Sirtema, 66 166, 169, 171, 180 Skerry Blue, 174, 177 Keswick, 141, 142, 150, 180 Snowflake, 191 Ker Pondy, 60 Sutton*s Supreme, 185 Kerr's Pink, 37, 38, 43. 49, 52. 55. 90. 92. Switez Dolkowski, 109 94, 173, 174, 175, 202 Toro Blue (Skerry Blue), 177 Kigezi Blue (Skerry Blue), 174 Triumph. SEE Bliss Triumph King Edward, 10, 19, 22, 23, 24, 28, 29, 32, Tylstrup Odin, 86 34, 35,37,38,43,49,187,188,189, fig. 9 Ulster Beacon, 48

225 Potato varieties - -Continued Russia. SEE U.S.S.R. Ulster Chieftain, 10, 48 Ulster Dale, 35 Scotland. 5. 6. 36-43, 198, 202, 204, figs. 13, Ulster Emblem, 48 14 Ulster Ensign, 48 Seed potatoes Ulster Knight, 48 Chitting (green-sprouting), 10, 114, 115 Ulster Premier, 35, 48 Cutting of, 120, 191 Ulster Prince, 10, 48, 171 SEE ALSO Vernalization Ulster Supreme, 35, 48 Seed-potato growing in Ulster Tarn, 48 Austria, 109 Ulster Torch, 35, 48 Belgium» 67, 68 Up-to-Date, 35, 49, 52, 53,54,55,86,90. 92, Canada, 140, 141, 142, 146 94, 95, 97. 116, 117, 151, 166, 169, 170, Chile, 161 171, 175, 177, 181, 191, 194, 202 Denmark, 83, 88 Urgenta, 60 England and Wales, 1 0, 11 Varieties with field resistance to blight. France, 60 SEE Ackersegen, Arran Consul, Arran India, 181 Viking, Cherokee, Furore, Kigezi Blue, Netherlands. 71 Kinongo Kerai, Pimpernel, Placid, New Zealand. 185, 187, 189 Pungo, Robijn, Skerry Blue, Voran Northern Ireland, 43 Varieties with R-genes for blight resistance. Republic of Ireland, 49 SEE Aquila, Black's Resistant Seedling Scotland. 36 1521(c), Canso, Craig's Snow White, Ken- South Africa, 169 nebec, Keswick, Maritta, Onikiva 52, U. S. A.. 118, 121, 129 Onikiva 53, Orion, Pentland Ace, Pent- U.S.S.R., 112 land Beauty, Phytophthora-Resistant, Western Germany, 99, 100, 105 Ulster Torch Self-sets. SEE Groundkeepers Vera, lOl Sodium arsenite. SEE Arsenites Virginia, 102 Sodium chlorate, use for haulm destruction Voran, 49, 68,73, 77, 78,79,81,82, 84, 102, Denmark. 89 153. 161, 165, 169, 171, 174, 202 France, 66 White Rose, 119 Netherlands, 84 Wohltmann, 94, 109, 112 New Zealand, 189 Powdery scab, 124 Northern Ireland, 48 Primary foci of blight infection, 7, 8, 75, 77, Republic of Ireland, 52 84, 202 World, general review, 204 Prince Edward Island, 131, 142-147, 196, 198, Soil conditions affecting 199, 202, 203, 204 Potato yields, 6, 196 Pukekohe district, New Zealand, 189-190,201, Tuber infection, 7, 199, 204, 206 204 Solanaceous wild plants, as sources of blight, 134, 201 Rainfall Solanunn aculeastrunn, Kenya, 174 Compensatory effect on yield, 16, 86, 199, 200 Solanum aviculare. New Zealand, 189 Need for economic study, 206 Solanum incanum, Kenya, I 74 Data, in relation to potato yields and Solanum indicum, Kenya, 1 74 severity of blight Solanum panduraeforme, Kenya, 174 Cameroons, 179, 180 Solanum andigenum, use in potato breeding, Canada, 148 171, 174 Chile, 163 Solanum demis sum hybrids. Cyprus, 117 SEE Breeding for blight resistance England and Wales, 15 SEE Field resistance Netherlands, 74 South Africa, Union of, 36, 49, 166-169, 198, Northern Ireland, 43 200, 201, fig. 80 Republic of Ireland, 51, 53 Southern Rhodesia, 170-171, fig. 80 U. S. A., 128, 129, 135, 136 Spain, 36, 115 Western Germany, 105 Spongospora subterránea. SEE Powdery scab. Effect on potato cropping, 196 Spray damage In relation to frequency of blight years, 197 By copper injury, 26, 52, 55, 125, 129, 137, SEE ALSO Climagrams 146, 195, 203 Red River Valley, 129-131, 136 By sprayer wheels, 25, 26, 41, 81, 120, 133, R-gene resistance of potato varieties to 138, 195 Phytophthora infestan s Crop sacrifice in haulnn destruction, 33, 34, Comparison with field resistance, 154, 169, 91, 205 202 Spraying practice SEE Breeding potatoes for blight resistance SEE Low volume spraying

226 Spraying practice - - Continued Temperature - - Continued SEE under countries Requirements of Phytophthora infestons. 7, World, general review, 203, 204 o Spraying trials, results of Requirements of potato crops, 5 Australia, 193, 195, fig. 94 Significance of diurnal range, 196 Catneroons, 179, 180 SEE ALSO Clima g ram s Canada, 143, 144, fig. 70 Tomatoes, blight on Denmark, 87, 88, figs. 37, 38 Carryover to potatoes, 134, 189, 201 England and Wales, 20-25, 34, fig. 9 Chile, 161 France, 64, figs. 24, 25 Kenya, 174, 175 Iceland, 91 New Zealand, 189 India, 184, fig. 89 Northern Rhodesia, 177 Mexico, 153 South Africa, 169 Netherlands, 78-81, figs. 32, 33 Uganda, 177 New Zealand, 189, 190 U. S. A., 201 Northern Ireland, 45-47 World survey of, 1, 134 Norway, 94 Toxaphene, 130 Peru, 156, 157 Tuber infection, by blight Republic of Ireland, 52-54 Absence in lack of rain, 156, 157 Sweden, 97 At or before lifting, 28, 135 U. S. A., 123, 127, 128, 129, 131, 137, fig. 65 Conditions favoring, 30, 124, 125, 130, fig. Use in national surveys, 197 Western Germany, 103 Control measures, 31, 204, 205 Storage losses, caused by blight, 30, 31, 84, Duality of the disease, 156, 199 124 Effect of.copper in soil, 131, 147, 190, 204 Sulfuric acid, use for haulm destruction Effect of drying tubers after harvest, 115 History of, in England, 31, 32 Effect of protective spraying on, 52, 54, 91, In France, 66 204 In Scotland, 41 In relation to maturity, 28, 40 In world, general review, 204 In relation to soil conditions, 7, 29, 65, 69, Surveys for potato blight 81, 82, 83, 84, 104,188,199,204 Austria, 107 In seed crops, 39, 84, 134 Canada, 147 Incidence of, general review, 199 Denmark, 86 Routes of, 7 England and Wales, 12-25, fig. 5 SEE Haulm destruction Methods, 11, 74, 84, 107, 108, 109, 206, 216 Varietol differences, 29, 188 Netherlands, 78 Tuber moth, 169 New Zealand, 188 Northern Ireland 46 Uganda, 171, 173, 175-177, 198, 200, 201, Norway, 92, 93, 94 fig. 80 Poland, 109 U. S. A., 6, 118-140, 196, 197, 198, 200, 201, U. S. A., 121-124, 133-135 202, 203, 204, 205, figs. 59, 60 Western Germany, 102 U.S.S.R., 6, 111-115, 197, 198, 202, 205, fias. Sweden, 6, 95-99, 198, 199, fig. 43 53, 54 Switzerland, 107 Vernalization of seed pototoes, 114, 115 Synchytrium endobioticum. SEE Wart disease Vine killing. SEE Haulm destruction Tanganyika, 171, 177, fig. 80 Wart disease, in Newfoundland, 141 Tar oil fractions, use for haulm destruction, Weather conditions favoring blight. SEE Blight 33 weather Tasmania, 191, 195, 198 Western Germany, 99-107, 198, 200, 202, 203, Temperature 205, fig. 45 Range for severe blight incidence, 197, 198 Zineb. SEE Dithiocarbamates Ränge for tuber production, 5, 196, 197, 198

227 NAME INDEX Aebi, H., 108 Chester, K. S., 134 Akeley, R. V., 122. 123, 125, 137. 140 Chouard, P., 65 Allan, W., 116 Choudhuri, H. C, 184 Andren, F., 97 Claridge, J. H., 185 Andries, E., 68 Clark, C, F., 119 Angus, A., m, 177 Clarke, J., 48, 1 74 Amautov, V., 115 Clayson, A. M., 8 Azad, R. N., 182, 183, 184 Clayton, S. N., 127, 128, 129, 137 Clouston. D., 38, 41 Baerecke, M. L., 106 Cook, H. T., Ill, 130, 134, 136, 137, 138, 139 Bald, J. G., 5. 191. 194 Cox, A. E., 26, 136 Baldwin, B. J. T., 177 Crosier, W., 8, 56, 202 Bant. J. H., 21 Crucq, J., 84 Barbo tin. F., 63 CuUen, J. C, III, 35 Barratt, R. W., 134. 156 Cunningham, C. E., 122 Barrus, M. F., 135 Bary, A. de, 8 Darling, H. M., Ill, 139 Bates. G. R.. lU. 31. 170 Darpoux, H., Ill, 63, 65 Bazin de Segura. C. III. 153. 156. 158. 159, Dastur, J. F., 182 160 Davidson, W. D., 49 Beaumont. A., 17, 21, 74. 184. 195. 202. 203 Davids son, I., 90 Bechtel, W. H., 191 Dawe, T. C. R., 189 Beck, J., 70 Deasy, D., 49 Beer, W. J., 25, 26 Delaney, D., Ill, 49 Beran, F., in, 107 Demesmay, H., 60 Biggar, W. A., 38, 41 Dickey, R. S., Ill, 13 Bjórknnan, T.. 96 Dingley, J. M., 189 Björling, K., 99 Dingwall, A. R., 187 Black, W., III. 35. 42. 84, 140. 174, 177. 180 Doling, D. A., 48 Blair, I. D., 188 Doncaster, J. P., 19 Blodgett, F. M., 131, 190 Drew J, P. 49 Bonde, R., UI. 121, 122. 123. 124, 125, 134, Driver, C. M., m, 6, 187, 189, 190 139 Dubniak. H., Ill, 109, 110 Bordukova. M. V.. 115 Ducomet, V., 63 Bourke. P. M. Austin, UI. 56. 57. 58, 160, l6l, Ehimas, P., Ill, 63, 66 162, 163, 164, 165, 184, 195. 203 Dyke, G. v., 20 Boyd, A. E. W.. 28, 39, 40. 41 Dykstra, T. P., 119, 129 Boyd, D. A., 11 Boyd, O. C., 135 Eddins, A. H., Ill, 126, 127, 128, 129, 137 Brentzel, W. E., 130, 133 Edmundson, W. C., 120 Priejer, C. J., Ill Eide, C. J., Ill, 130, 139 Brien, R. M., 189 Emilsson, B., 99 Brooke, D. L., 126, 129 Ext, W., 104 Brown, B. E., 119 Bruin, T., 84 Fish, S., Ill, 191, 193 Bruyn, H. L. G. de, 84 Foéx, E., 63 Buchholtz, W. F., 129, 132 Foister, C. E., UI, 37, 38 Bukasov, S. M., 115 F^rsund, E., lU, 92, 94 Burton, W. G., 5 Forsyth, J. C, 141 Bushnell, J., 5, 196 Fridriksson, S., Ill, 90 Butler, E. J., 182 Galindo, J., 154 Gallegly, M. E., 140, 154 Cairns, H., 45 Georghiou, G. P., Ill, 116, 117 Callbeck, L. C., IH, 142, 143, 145, 146, 147, 150 Graham, K. M., 150 Campagna, E., 154 Grainger, J., Ill, 38, 41 Grainger, P. N., m, 150 Cartledge, E. W., 193, 195 Cartmill, W. J., 191 Gram, E., IH, 86, 87, 88, 89 Gray, E. G., HI, 38, 39, 41 Cassell, R. C, 122, 124 Cas s Smith, W. P., 193 Gregory, P. H., 19 Guntz, M., m, 63, 66 Caudri, L,., m Gwinn, A. B., 138 Cervantes, J., 152, 153, 154 Chambers, P. C, 117 Hânni, H., 9, 108 Chambers, S. C, 195 Hârle, A., 102, 103 228 Hagberth, N. O., 95 Martin, L. D., 31 Hammarlund, L., 88 Mastenbroek, C., 42, 84, 140 Hascoet, M., 66 Mathew, Father, 50 Hawkes, J. G., 6, 35, 115, 155 Mattingley, G. H.. 191 Henderson, J., 28, 40 Meijers, C. P., 78, 82, 84 Hirst, J. M., Ill, 8, 9, 17 Melhus, I. E., 138 Hix, H., 101 Miller, P. R., IH, 1, 130, 132. 134, 139 Hoctor, D., Ill Mills, W. R., 42, 84, 140. 153, 154 Hodgson, W. A., 150 Mitra, A. K., 181 Hôljer, E., 96 Monot, G., 66 Hogen Esch, J, A., 73 Montaldo, A., I6l, 165 Holz, W., 106 Moore, W. C., IH, 11, 29. 30 Honey, J. K., 19 Moore, W. D., 138 Hopkins, J. C. F., m, 170 Moreau, R. E., 6, 171, 177 Horsfall, J. G., 134, 139. 156 Müller, K. O., 35, 102, 160, 161, 165 Howard, H. W., 35 Murphy, P. A., 28, 49, 52, 54, 55, 142, 197 Howatt, J. L., Ill, 148, 150 Muskett, A. E., IH, 43, 45 Hoyman, W. G.. Ill, 129, 130, 131, 137, 139 Hudson, S. C, 35, 141 Nattrass, R. M., ni, 116, 172, 173, 174, 175 Hyre, R. A., 121, 124, 125, 132, 139 Naunnova, N, A., 115 Niederhauser, J. S., in, 151, 152. 153, 154, 184 Jaivenois, A., 70 Nijdam, F. E., 73 Jensen, J. L., 8, 169 Novikov, F., 115 Johannes, H., Ill, 9, 102, 103, 104, 105, 106 Jonas son, J., 96 O'Brien, M., 1, 130, 132, 134 Jouin, C, 66 Oehrens, E., 161 Journet, P., Ill, 66 Oldaker, C. E. W., 191 Juzepczuk, 115 Opitz, K., 101 Ormel, H. A., 82 Kameraz, A., 113 Keay, M. A., Ill, 175, 177 Pal, B. P., 181 Kendrew, W. G., 3 Parks, N. M., 140, 142 Kerr, W. E., Ill, 170 Patterson, J. B. E., 25 Klatzman, J., 61 Peikert, F. W., 125 Krostrowicka, M., 35 Petersen, H. I., 88 Peterson. C. E., 138 Peterson. L. C., HI, 42, 84, 131, 132, 134, 140, Lai, T. B., 182 190 Lamas Carrera, J. M., 156 Pethybridge, G. H., 35, 49. 52 Landis, B. J., 120 Pettersson. S., 97 Lansade, M,, 64, 66 Polykov, I., m, 113 Large, E. C., 11, 17, 19. 20, 22, ¿5, 26, 32, Post. J. J,, 74. 203 33, 35 Pouchet. J.. 66 Leeuwenburgh, J., 83 .Pushkarnath. 181 Leitch, C. G., 185 Lessens, W. J., 11 Racicot, H. N., IH. 147 Lihnell, D., Ill, 97, 99 Ramsey. G. B.. 128 Lint, M. M. de. III, 78, 82, 84 Raucourt. M.. 64. 66 Littlejohn, L. J. S., 116, 117 Reddick. D.. 140, 174 Llaveria, M., 157, 159 Reid, W. J.. 119, 129 Löhnis, M. P., 74 Revilla, v., 157, 159 Lombard, P. M., 119, 125 Rhoads, A. S.. 128 Lorkh, A. G., 112, 114 Richel. C.. 74. 203 Lunden, A. P., 95 Richter, H., HI Lutz, J. M., 130 Rigot, N., 69 Riker, A, J., IH Maan, W. J., 83 Riley, E. A., UI, 177 McCulbin, E. N., 126 Robertson, N. F., 8 Mclntosh, T. P., Ill Robinson, J. A., 124 McKay, R., Ill, 49, 50, 52, 54, 55, 56 Roer, L.. 94 Mackelvie, D., 48 Roland, G., 70 Magee, C. J., IH, 191 Rose, D. H., 138 Majid, S., 183 Ross, H., 106 Malmonté, M. J., 62 Rosser, W. R., 19. 22, 34 Malmus, N.. 104 Rozhestvenski, N.., 113, 114 229 Rudorf. W., 102, 106 Thran, P., 8, 106 Ruehle, G. D., 129 Thurston, H. D., 130, 139 Ruprecht. R. W., 126 Torka, M., 106 Russell, E. J., 60, 111, 181 Townsend, G. R., 129 Russell, T. A., 178. 179 Toxopeus, H. J., m, 84 Ryan, M., 174 Uhlig, S., 8. 9, 102, 105, 106 Ullrich. J., m, 106, 107 Salaman, R. N., 34, 35 Samson, R. W., 139 Sanchez, L,., 157, 159 Vandenplas, A., 70 Schaal, L.. A., 120 Van der Plank, J. E., III, 5, 166, 169 Schaper, R., 106 Vanderwalle, R., 69, 70 Scheibe, K., UI, 103, 104, 106 Van der Zaag, D. E., 8, 75, 84, 169 Schick, 102 Van Everdingen, E., 74, U5, 202 Schreier, O.. 107 Van Hamelen, M. G., 99 Schrumpf, W. E., 121 Van Vleit, W., 141 Schultz, E. S., 122, 124, 125, 140 Vasudeva, R. S.. lU, 181, 182, 183, 184 Sellgren, K. A., 99 Vavilov, N. I., 115 Servin, L., 154 Ventura, E., m, 64, 66 Shands, W. A., 121 Voelkel, H., 102, 103, 105 Siebeneick, H*. 73 Simmonds, J. H.. III. 191 Simpson. G. W., 121 Wade, G. C., m. 191, 193, 195 Skaptason, J. B., 131, 190 Wade, G. K., 139 Smallfield, P. W.. III. 185 Waggoner, P. E., 139 Smith, la. P., 17, 57 Walker, J. C., 133 Smith, O., 118 Wallace, R. W., III, 43 Snell, K., 101 Wallin, J. R.. m, 8, 129, 130, 137, 139,203 Sommer, K., 106 Waterston. J. M., III. 178 Spurlock, A. H., 126, 129 Watts Padwick, G., 171 Stamp, L. D., 185 Webb, R. E., 123, 125, 137 Stapel, G.. 87, 88. 104 Welvaert, W., m, 69, 70 Stevens. N. E., 133 White, L., 191 Stevenson, F. J., 122, 123, 125, 126, 137, 139 Wilkins, V. E., m 140 Williams, P. G., 193. 195 Storey, I. F., 21, 22 Wiltshire, S. P., 74 Stutt, R. A., 141 Wood, J. I., 132, 135

Tallantire, J. D., III, 178. 179, 180 Yule, A. H.. 22 Taylor. G. G., 25, 189 Taylor, R. E., 22 Ten Houten, J. G., III Zaitsev, N. D., 112

230 * U.S. GOVERNMENT PRINTING OFFICE : I9S9 O—S32688