886 YEARBOOK OF AGRICULTURE 1953 and with a nearly airtight pack or container. For prepackaged flowers or for flowers stored in water in a moder- ately tight cold room, the potential damage that can be done if ethylene- Apricot and producing diseased material is in- cluded is serious. Observations have indicated that once a blossom is in- Almond jured by ethylene it immediately becomes more liable to attack by Botrytis. Thus a chain-reaction type Brown Rot of response is initiated that will lead to more ethylene production and thus to more injury. E. E. Wilson The observed effects of ethylene produced by diseased plant tissues Probably nowhere else in the world emphasize the desirability of either are stone fruits grown in such variety near-perfect control or complete elim- and number as in the three States that ination of plant disease. If adequate border the Pacific Ocean. There are disease control is maintained in field grown the edible varieties of peaches, or greenhouse and in the storage nectarines, apricots, the three types of room, one of the important factors in cherries, plums, prunes, and almonds. successful long-term storage of cut There, too, are produced the numerous flowers is reduced to negligible pro- other species of stone fruits that are uti- portions. lized as rootstocks for the edible sorts. The orchards in the region are com- G. E. WILLIAMSON, assistant professor posed not of a few trees of miscellaneous of plant pathology in Cornell University^ is kinds grown in the back yards of scat- a native of Indiana. His work with dis- tered farm homes but many trees of the eases of ornamental plants began at Cornell same kind in contiguous blocks ex- in igsy. His studies on ethylene effects tending over hundreds of acres. One began with the demonstration of stimulated entire locality may be given over to ethylene production by rose leaves affected cherries, peaches, or prunes; another with black spot. He is a graduate of Wa- may be planted only to apricots and bash College and Cornell University. peaches; and another to almonds and A. W. fyiMOCK, prof essor of plant path- peaches. ology in Cornell University, has specialized The foundation stocks of all these in diseases affecting ornamental crops for stone fruits were introduced into the many years. His interest in ethylene the region. Some came from the Orient effects was a natural consequence of his and others from Europe; some were in- close association with Dr. Williamson troduced first into eastern United during his studies on ethylene production by States and later brought west; others diseased plant tissues and with Dr, C. W. came directly from their foreign home. Fischer, Jr., during his, studies on effects of With the establishment of the fruit ethylene on cut flowers in storage. industry in the Pacific coast region came disease problems: The frequent For further reference: shipments of nursery stock and other A. W. Dimock and Kenneth F, Baker: Ethylene propagative material provided ample Produced by Diseased Tissues Injures Cut opportunity for introduction of dis- Flowers, Florists Review, volume io6. No. 2'/^4, eases. Once introduced, the diseases pages 2y-2g, 1950. C. E. Williamson: Ethylene, a Metabolic found large numbers of hosts to afi^ect. Product of Diseased or Injured Plants, Phyto- Typical,of the maladies occurring on pathology, volume 40, pages 205-208, 1950. stone-fruit trees in this region are the APRICOT AND ALMOND BROWN ROT 887 two brown rot diseases. One of them, tively a fruit-rotting organism. Its ac- which I shall call apricot brown rot tivity in blossoms and twigs is much less for convenience, was introduced from pronounced than that of the apricot Europe. The other, which I shall call fungus. peach brown rot, apparently was native to the eastern part of the United States IN CERTAIN RESPECTS the two fungi and developed there on the wild spe- closely resemble each other. In the cies of stone fruits. Whether it occurred form and structure of their parasitic on the Pacific coast before the fruit stage on the hosts, they are indistin- industry was established or was intro- guishable. Taken from the host and duced later is uncertain. grown on certain artificial culture The two diseases, formerly thought media, however, they exhibit certain to be different manifestations of one differences. Nevertheless, for a long malady, are produced by two closely time most investigators considered related fungi. The one causing apricot such differences merely the normal brown rot is Monolinia {Sclerotinid) laxa. variations of a single fungus species. The cause of peach brown rot is Mono- Some still believe that they should not lima {Scier otinia) Jructicola. be called different species but should The latter fungus and the disease it be regarded as variant forms of the causes are discussed by John C. Dun- same species. Whether they be two egan in another article in this volume. species or two forms need not concern Consequently I shall confine my dis- us here; the important thing is that cussion largely to the apricot form. they behave differently with respect to This fungus, which is common in disease production and host prefer- England and on the European Conti- ences. Consequently when one is pres- nent, was found in Oregon in 1915 and ent there arises a particular type of dis- subsequently in Washington, British ease problem on a particular group of Columbia, and California. It is more hosts. When the other is present there common in California than the peach arises another type of disease problem fungus. Outside of the Pacific coast re- on another set of hosts. gion, however, it is known to occur To illustrate how the introduction of only in a few localities in Wisconsin the apricot fungus into California has and Michigan. affected the lot of the apricot grower, If one reads the early accounts of we might consider the situation in the brown rot in California, one is led to Santa Clara Valley, which lies just the conclusion that the apricot fun- south of San Francisco Bay. This valley gus occurred there long before the is extensively planted to apricot, the peach fungus. Most of those accounts only other stone fruit in comparable deal with brown rot in the apricot and amount being the prune. The peach mention certain characteristics, which fungus is occasionally found there on we recognize as those of the apricot the prunes and peaches. The apricot fungus. Both here and abroad, for ex- fungus, on the other hand, is wide- ample, the apricot fungus is noted for spread among apricot orchards and, if its preference for the apricot. The two not controlled, causes serious damage. German investigators who described Consequently, each year one or more and named it mentioned this feature. applications of a fungicide are required Another marked characteristic is its to hold it in check. Were it not for this propensity for blighting blossoms and fungus, therefore, the need for fungi- twigs. In California at least, the cides in the apricot orchards of this val- amount of fruit rotting it produces is ley would be comparatively slight, secondary in importance to the amount since coryneum blight, the disease on of blossoms and twigs destroyed. The which great amounts of fungicides are peach fungus, on the other hand, sel- expended in other parts of the State, is dom occurs on apricot and is dis tine- not prevalent there. 888 YEARBOOK OF AGRICULTURE 1953

HAVING COMPARED the two brown are resistant. In the past, most of the rot fungi, I shall now discuss in more brown rot of prune fruit in California detail the activity oí Monolinia laxa, tíie has been caused by Monolinia Jructicola, apricot species. First, however, a word Aside from the fact that the Early should be said about the other stone- Richmond and Montmorency varieties fruit hosts of this fungus. The almond of sour cherry are known to be affected, has already been mentioned as one, the there is little information on this host. others are the sour cherry, the plum, and the prune. Next, a word about sus- LET us NOW follow the life cycle of ceptibility among the different varie- the apricot fungus through one season. ties of these stone fruits. In spring the fungus enters the blos- Well over three-fourths of the apri- soms when they emerge from between cots grown in California are of the va- the scales of the winter buds. It grows rieties Blenheim and Royal, both of rapidly through the blossoms and down which are highly susceptible to blossom into the supporting twigs. Soon after infection by the apricot fungus. The the blossoms wither, the fungus pro- next most important variety, Tilton, is duces small ash-gray masses of conidia moderately susceptible but neverthe- on them. Usually the conidia perform less is abundantly infected at times. no further function, but occasionally Investigations at the California Agri- they may cause infection of some of the cultural Experiment Station revealed fruit as it ripens. Mr. Dunegan described some interesting effects of parentage on how the peach fungus produces spores the susceptibility of apricot hybrids. of a second type in the infected fruit Regardless of the susceptibility of the that falls to the ground. The apricot other member of the cross, the resistant fungus has been known to produce this St. Ambroise, Moorpark, and Tilton stage in Europe but has not been found varieties apparently transmit to their to do so in America. Therefore the rot- progeny a large degree of resistance. ted apricot fruit that falls to the ground The principal almond varieties in is not a source of infection the following California are Drake, IXL, Ne Plus spring. Fruits that hang in the tree, on Ultra, Nonpareil, Peerless, and Texas. the other hand, harbor the fungus over Of these, the Drake is by far the most the winter and in spring produce susceptible to brown rot blossom masses of conidia. Because of the blighting. The Ne Plus Ultra and IXL sporadic occurrence of fruit rot, such are moderately susceptible. The other fruit are relatively unimportant in three varieties are highly resistant. A California as a source of infection. relatively new variety, the Jordanolo, The most important sources of infec- which resulted from crossing Non- tion in California are the blighted blos- pareil with Harriett, is quite suscepti- soms and twigs, in which structures the ble. Apparently, therefore, this almond fungus mycelium remains alive but qui- hybrid, unlike the apricot hybrids escent during the hot, dry summers. I mentioned, did not inherit resistance After the winter rains begin, it starts a from its resistant parent, the Non- slow development. At numerous places pareil. Instead it inherited the suscep- beneath the corky outer bark the my- tible qualities of the Harriett. celium produces small compact masses Among the lo most important ship- by growth and segmentation. These ping plums, the Santa Rosa and Wick- small knots of fungus tissue, the sporo- son are the most susceptible to brown dochia, or conidium-bearing structures, rot blossom blight. Only occasionally gradually enlarge and push up through are plum fruits affected. Of the prune the overlying bark to the surface. Even varieties, the Burton is very susceptible before the structures emerge the co- to blossom infection; the French nidia are being produced on them. The (Agen) is moderately susceptible; and first sporodochia appear on the twigs in Imperial, Robe de Sergeant, and Sugar early spring and continue to appear for APRICOT AND ALMOND BROWN ROT 889 6 weeks or more, developing in greatest a disease of the blossoms, we are con- numbers just before the blossoming cerned with the conditions that favor period. Thus even before the buds open infection of those parts. If the conidia in the spring, many strategically locat- of the fungus are present, the disease ed sources are ready to supply conidia will develop whenever the blossoms for blossom infection. reach an infectible stage and the proper weather conditions prevail. As TEMPERATURE AND MOISTURE affect long as the blossom parts are protected both the earliness and the amount of by the scales of the winter buds, they sporodochial development. In seasons are not accessible to the conidia of the when rains begin early in the. fall and fungus. They become accessible and thereafter occur at frequent intervals, consequently infectible as soon as they sporodochia may appear first in late emerge from the bud. They reach a December and continue to appear for stage of greatest susceptibility when 6 or 8 weeks, so by the time the trees they are fully expanded and gradually blossom a very large number are pres- become less susceptible as the petals ent. In seasons when winter rainfall is fall and the young fruit start to grow. deficient, however, they may not begin Regarding the effect of weather con- to appear until late January and then ditions on the disease, it is known that in small numbers. Low temperature, the fungus grows most rapidly at a particularly during late winter, like- temperature of about 68° F., but will wise delays their appearance and re- cause blossom infection at a tempera- duces their numbers. ture as low as 41° or as high as 86°. The amount of conidia that results After it enters the blossom and is not so from the infection of one blossom is ex- directly exposed to outside conditions, traordinarily great, because the blos- it develops rapidly even though out- soms and the blighted twig as well side air temperatures may be below bear the sporodochia. Not uncom- the range at which it grows best. monly 30 or more of those structures Because the conidia will germinate develop on a single twig. Since each only in the presence of moisture, little sporodochium produces hundreds of if any infection occurs when the conidia, many thousands of the spores weather is dry. A rain lasting only a arise from a single infection. Multiply- few hours, however, will permit infec- ing this by the hundreds of infections in tion if the temperature is near the a tree, one has some conception of the optimum for growth of the fungus. If prodigious numbers available for blos- temperature is below the optimum, a som infection. longer moist period is required for in- Conidia are washed from the sporo- fection. The combined influence of dochia and to the flowers beneath by temperature and moisture, in large rain. They are also liberated from the measure, is responsible for the year-to- sporodochia by air currents; being year fluctuations in the severity of the very small, they are readily carried disease. long distances by the wind. As the air carries them through the tops of the WHAT MEANS can be employed to trees, they arc deposited on twigs, control the disease? Let us recall that branches, and blossoms. Under favor- the disease can develop only if the able conditions they germinate and fungus is present in the orchard or a produce a slender infection strand, nearby orchard, if the blossoms have which penetrates into the blossom tis- reached an infectible stage, and if sue, thereby completing the life cycle temperature and moisture are favor- of the fungus. able for infection. Since no way is known to modify the temperature and SINGE THE BROWN ROT disease of moisture conditions in such a way as to apricots and almonds is predominantly prevent infection or prevent the bios- 890 YEARBOOK OF AGRICULTURE 1953 soms from becoming susceptible to in- Monolinia laxa, it will be recalled, fection, the only approach by which produces only one type of reproductive we might control the disease is that of structure, the conidia. They develop in destroying the fungus or its conidia be- the tree on blighted twigs and blossoms fore they can initiate the disease. One and on the fruit, which the fungus way to do that is to cover the blossoms occasionally attacks and which re- with a chemical that will kill the mains on the tree until spring. Early conidia that fall upon them. investigators recognized the desirabil- We know that the blossoms become ity of eliminating the conidia and infectible as soon as they emerge from recommended that all blighted twigs between the scales of the winter buds and rotted fruit be removed when the and remain so until after the petals trees are pruned in winter. Removal of fall. We must therefore give them a the numerous blighted twigs proved protective covering of a fungicide as impractical, however. soon as they emerge. Many field trials Chemical sprays to eliminate the were necessary before it was learned conidia were developed. Monocalcium just when to apply the spray, because arsenite proves particularly efí'ective the emergence and opening of the for the purpose. When applied to dor- blossom is a progressive thing. The tip mant trees in mid-January, the com- of the unopened blossom first appears, pound destroys the conidium-bearing and as the stem elongates it is pushed mats (sporodochia) present when the out of the bud. Experiments conducted spray is applied and prevents their by a number of investigators showed further development by killing the that spraying before the blossoms mycelium inside the twigs. One appli- emerged reduced infection to some cation of a preparation, containing 3 extent but not enough to be of prac- pounds of monocalcium arsenite to tical value. Spraying after the blos- 100 gallons of water, commonly re- soms had emerged completely and the duces sporodochial development 95 to petals were unfolding often proved too 98 percent and results in a decrease in late. Spraying just after the blossoms blossom infection. emerged gave the most satisfactory re- Since 1940 or so, growers in several sults. Even better control could be ex- localities have employed this treat- pected if the application was followed ment on apricots with comparatively by another after the petals began to little injury to the trees. unfold. On the whole, such a schedule Some safety measures must be ob- proves satisfactory once the grower served, however: Give the treatment becomes experienced in judging flow- only after the trees are completely er-bud development and in the me- dormant; in California the spray is chanics of applying the spray. Never- applied from mid-January up to the theless, even a two-application sched- time the flower buds begin to swell. ule proves inadequate in some years. Delay the treatment at least 2 weeks The most common cause of unsatis- after the trees are pruned, or, better factory results from spraying is a delay still, spray before pruning. Do not add in giving the first treatment. Often spray oils to the preparation and rainy or windy weather at the time the avoid their use after the monocalcium spray should be applied interferes with arsenite treatment is given. It is prob- spraying operations, and the fungus ably unwise to give the treatment to gains entrance to the blossoms. Once trees in a low state of vegetative vigor. that occurs, spraying will not prevent Yellowing of leaves accompanied by the disease from developing. Such defoliation followed the application of difficulties led to the development of monocalcium arsenite in the winter in another control method which does one orchard. not require such critical timing of the Monocalcium arsenite is extremely spray treatments. poisonous and great care must be SOME IMPORTANT DISEASES OF COFFEE 891 taken to avoid breathing or swallowing it.

ALTHOUGH monocalcium arsenite preparation does not materially injure the principal apricot varieties or the Some Important Santa Rosa and Wickson plums, it in- jures all almonds and some prunes. Consequently less injurious eradicative Diseases of fungicides have been sought. Of about 75 other compounds that were tested, the sodium salts of the chlorophenols Cofîee (particularly pentachlorophenol) elim- inated the conidia most effectively. Sodium pentachlorophenate is destruc- Frederick L, Wellman tive to the conidia present on the twigs when it is applied but is not highly It has been commonly said that effective in preventing their further de- coffee {Coffea arabica) is a tree practi- velopment. Being very soluble, more- cally free from disease. Actually, the over, it is sometimes washed from the coffee plant is subject to more than 40 twigs by rain before its maximum effect diseases—ailments due to lack of minor on the conidia has been exerted. Under elements, virus troubles, mild bacterial proper conditions, however, it destroys infections of roots and fruits, and at- much of the conidial inoculum. That tacks by fungi and parasitic flowering in turn results in a significant decrease plants. A century of effort has been in the amount of blossom infection. expended on agronomic and horticul- Neither the eradicative nor the protec- tural problems in coffee, but only in tive treatment alone satisfactorily con- the past 50 years has intensive work trols the disease under all conditions. A been done on its diseases. combined eradicative-protective pro- In 1952 the Office of Foreign Agri- gram is much more effective. cultural Relations (now the Foreign The protective fungicides most wide- Agricultural Service) of the Depart- ly used against the disease in California ment of Agriculture sent a mission to are the copper-containing materials, study coffee diseases in all parts of the bordeaux mixture and the fixed cop- world. The mission was sponsored by pers. Sulfur fungicides, although rela- the Point IV program and financed by tively effective under favorable condi- the Institute of Inter-American Affairs. tions, cannot be used on apricots be- Information gathered on the trip has cause of the "sulfur sickness" they pro- been incorporated into this chapter, duce in the tree. The newer types of which is based primarily on study and fungicides, many of which are complex experience in Latin America. organic compounds, are being tested. Some show promise, but further tests COFFEE RUST, also called the oriental are needed to evaluate their effective- leaf disease, is by all odds the most ness. serious disease of coffee. It does not occur in the Western Hemisphere, E. E. WILSON is professor of plant pa^ maybe just by pure luck. There are thology in the University of California at two species of rust: The classic Hemileia Davis, where he has been engaged in studies vastatrix, which is so destructive and is of fruit diseases since igsg. found in most of the coffee regions of Africa, the Near East, India, Asia, and the Pacific Islands, and H. coffeicola, Mr. Dunegan's discussion of peach brown an equally dangerous rust but still rot appears on page 684. confined to the Cameroons of West