World RootstockSituation

w. P. Bitters

Introduction

A number of different citrus rootstocksare usedin the variouscitrus-producing areas of the world. The performanceof eachhas been selected as best adaptedto the areain which it is used. This variation in usageis understandablein view of the differencesin soil types, environment,water relationships,nutrition, the miscellaneousdisease complexes involved and other factors. Rootstocksonce established are not very susceptibleto change;however, occasionally something happens which re- quiresa change. Rootstocksdo change. Sometimesthe changeis slow and gradual,and at other times it is suddenand almost catastrophic. The slow, evolutionarychanges we are not worried about as the industry hastime to adjust to them with little consequence.The suddenor catastrophicchanges occur right now, and unlessthe industry is prepared,have dire consequences.

Many of the rootstock problemsfaced by citrus growersthe world over arethe sameregardless of the country or areain which they live. However, in a few cases,what has been a catastrophe in one area seemsto be of little economic importance in another. For example, tristeza almost overnight wiped out citrus planted on sour rootstock in Africa, South Americl and California; yet it seemsto be of little import.nce in some of the Mediterranean countries. However, it is probable thlt this disease,and many others, will become more widespread and disastrous in areaswhere they are now minor in nature.

The ravagesof tristeza in Spainat the presenttime are a good example. It hasalso beentrue in the past and will undoubted. Iy be true in the future that organismscausing disease can mutate to more virttlent forms,or new diseasesappear which necessi. tate finding or developingnew rootstockswhich will be better suited to meet new threats,whatever they may be.

A reviewof the variouscitrus areasand their current rootstock situation is thus very much in order.

Spain

Spainis the world's largestexporter of fresh citrus and is exceededonly by Japanand the United Statesin citrus pro- duction. Becauseof the prevalenceof , nearly all Spanishcitrus hasbeen planted on sour orangestock. However, tristezais rapidly spreadingin ,and presentestimates are that asmany asa million to a million and a half treesmay now be infected particularly in the areaaround Alcira and the Rio Jucar river. Much of the mandarinacreage in recentyears has beenplanted on 'Cleopatra',and experimentalplantings are beingmade on 'Troyer' and 'Carillo' ,sweet orange and trifoliate. However,most of the soilsalong the Mediterraneancoast of Spainare too calcareousfor the successfulgrowing of trifoliate. However,it may prove useful in the interior of the GuadalquivirValley betweenSevilla and Cordobawhere new plantingsare beingestablished.

Almost all scionwood in Spainis infected with exocortis,and a budwoodvirus indexingprogram is beingestablished. Success of this programwill be necessarybefore some of the newerrootstocks can be used. Psorosisand xyloporosisare also widespread. However,the most prevalentdisease isPhytophthora, and in someareas earth ridgesare thrown up in a circle around the trunk to keepirrigation water awayfrom the foot of the tree.

Recently,because of the threat of tristeza,the Ministry of Agriculture haslicensed nurseries and prohibited the sellingof treesbudded on sour orange. However,this doesnot preventthe private growerfrom raisinghis own sour rootstock. Many of the growersstill prefer sour and do not believethat tristezais a problem.

Italy

Italy is the world's fifth largest producerof citrus, the secondlargest grower of lemonsand the major supplier of citrus in the EuropeanCommon Market Citrus Irels Ire locatedin southernItaly and Sicily. -2-

All varietiesof citrus in Italy are grown on sour orange. Becauseof the fear of what tristezacould do to the orangeand mandarinindustry, researchworkers havebeen stimulated to searchfor and test new rootstocks.

Tristezahas not becomean economicthreat in Italy, but it hasbeen found in backyardplantings of 'Meyer' and Satsuma.Eradication of thesetrees has hopefully removedor delayedthe threat. An indexingprogram for exocortis end psorosisis now underway,and I virus-screeningprogrlm hasbeen established.

Researchis taking placeat Acirealeto developnucellar lines of local orangevarieties. Whensufficient virus-freebudwood becomes available, 'Troyer' citrange may become a recommended rootstock for oranges. Other trials are being carried on to develop lemon varieties and rootstocks more resistant to mal secco. 'Monachello' has been recommended becauseof its re- sistance to this disease,but bud union overgrowth limits tree life when used on sour rootstock.

Tests with Citrus volkameriana have shown advantagesin vigor and resistanceto mal seccoover sour, and it is being rec- ommended as a rootstock for .

The generalpractice in Italy is to bud at a height of about 1 meter, or 40 inches. This height is usedbecause of an extreme fear of gummosis,which practically wiped out beforethe turn of the century, when most treeswere prop. gatedon their own roots from cuttings.

Corsica

Corsica, with very limited citrus acreage,has a fine rootstock program underway, perhaps the best in the Mediterranean area. It has fairly extensive rootstock trials with most of the world's existing or potential rootstocks and has complimented it with the introduction of numerous nucellar lines. Their problem is to find a replacement stock for sour orange, one with both triste- za and Phytophthora tolerance. Cold resistance is a rootstock assetwhich cannot be overlooked for Corsica.

Israel

Israel, with slightly over 100,000acres, is the fourth largestcitrus-producing country in the Mediterraneanarea. 'Palestine' sweet and sour orange are the principal rootstocks. Soil types range from sandy loams along the coast and in the northern Negevto heavy clay in the Jezreel Valley. In some areas,light, alluvial soil overlays heavier soil.

It is commonpractice in light soilsto bud on sweetlime for early production and to inarchwith sour, usually beforethe treesare 10 yearsof age. This is done becausetrees left on sweetlime invariably declinefrom xyloporosis. In heavysoils, trees may be planted on sour from the beginning.

Tristezanow occursin Israelicitrus orchards. So far the plan hasbeen to eradicateaffected trees as they appearand some indexingis beingdone for symptomlesscarriers. This plan would delay the onslaughtof tristezabut is not prKtical for con- trolling it They will haveto switch to tolerant rootstocks. Somework hasbeen done on nucellarlines but they are not in generaluse as they havenot beenas fruitful and are more prone to alternatebearing.

An extensiveresearch program is beingcarried on in Israelin all phasesof citrus culture, rootstocksincluded. It is felt that more information is neededon the nutritional requirementsof different rootstock-scioncombinations, and researchon this is beingexpanded. Other researchis directedtoward predictingthe compatibility and performanceof rootstock-scioncombin- ations from juvenile charKteristics or chemicalanalysis of tissue. If this should provesuccessful, it would help eliminatethe long wait now necessaryto evaluatemature trees. A closewatch is beingkept on diseaseproblems which could becomea threat to the industry.

Greece

In 1963, Greece had approximately85,000 acresof citrus, of which about 48,000 were oranges,19,000 were lemonsand 18,000were mandarins. A greatdeal of Greece'scitrus production is used for homeconsumption, and that which is exported goes to eastern European markets and does not compete with the western European economic community. The major orange .3- variety planted in Greecesince 1955 hasbeen the 'Washington'navel. Many of the older, common orangeplantings have beentopworked to navelvarieties.

The predominant rootstock in Greece for all citrus is sour orange. Attempts have been made in the case of lemons to use an interstock of sweet orange in the hope that this will give resistance to mal secco. Such attempts have been unsuccessful. As in the rest of the Mediterranean area, psorosis and exocortis are also present. Since all plantings are on sour, tristeza could rapidly become a major problem. No major rootstock trials are underway. Frost can be a problem and water is somewhat limited and of poor quality.

Lebanon

Lebanon hasbetween 25 and 30 thousandacres of citrus. Areassuitable for citrus are limited, and little expansionis ex- pected. As in Israel, the principal rootstocks are sweet lime and sour orange. Important diseasesare Phytophthora, xylopor- osis and psorosis. No rootstock trials are underway.

Algeria

Citrus plantings in Algeria increased rapidly after World War II and reached 84,000 acres by 1960. However, during the civil strife of the 1960's, few plantings were made and some groves were abandoned so that citrus acreagedeclined. During the past few years, however, citrus acreagehas again increased. Algerian soils are mostly heavy loams, although there is a anall coastal lemon area north of Algiers which has light, sandy loam soil. In some areas, the water table is only a few feet from the surface, and drainage ditches and tiling are used. Many commercial varieties have been introduced into Algeria. These include the 'Hamlin', 'Washington' navel, 'Cadenera', 'Thompson' navel, 'Shamouti' and many Portuguese and Spanish varieties. 'Marsh' has been introduced as have the 'Eureka' and 'Berna' lemons. There are smaller plantings of 'Lisbon', 'Meyer' and Italian varieties of lemons.

Sour orangeis the predominantrootstock. However,'', which is the secondlargest citrus crop in Algeria,is often planted on trifoliate orangein heaviersoils where high water tablesexist. Phytophthoraand foot rot are presentin most Algeriangroves, and treesare buddedabout 2 feet aboveground level. No rootstock trials are evident.

Morocco

In ,citrus acreagehas grown at an increasingrate during the pastfew years,with production mainly concentrated on oranges.Varieties are similar to those in Algeria. The most important mandarinis the 'Clementine'. Grapefruit plantings areprimarily 'Marsh', and lemonsare the 'Eureka' type. Most Moroccancitrus soilsare light, well-drainedloams. Salinity is a problem in someareas. Diseasesare psorosis,Phytophthora, Rio Grandegummosis, stubborn disease,xyloporosis and exo- cortis. Tristezahas been identified but hasnot reachedeconomic significance. Present citrus acreageis estimatedto be 180, 000 acres.

In 1969, a MoroccanAgricultural ServiceSociety, which is an advisorybureau containing research and extensionpersonnel, wasestablished. Purpose of the society is to carry out researchwork which will benefit Moroccancitrus culture. The princi- pal rootstock is sour orange;however, new rootstocksare beingintroduced and tried.

Egypt

Egyptian citrus acreageis now estimated to be between 85 and 90 thousand acres. The principal orange varieties grown are local selections called 'Beladi'.

Egyptian citrus soils are fertile and consistof deep,alluvial silt and clay loams. Grovesare irrigated by permanentbasins or sprinkler systems.The principal rootstock usedis sour orange,although sweet lime is usedin lighter soils. Diseasesinclude Phytophthora,foot rot, psorosisand xyloporosis. It might be expectedthat tristezaalso exists, since it is presentin other areas 4 in the Mediterraneanbasin. However,tristeza has not becomean economicproblem. There are no major rootstock trials underway.

Cyprus

Major varietiesin Cyprusare the 'Shamouti' and 'Valencia' orange,'Marsh' grapefruit and the local sour lemon which re- ~mbles the Lisbon. The 'Clementine'mandarin, 'Temple' orangeand somevarieties of satsumahave also beenintroduced. Soilsare generallylight in texture in citrus areas,and the major rootstocksare the sour orangeand sweetlime. There is a major problemwith Phytophthora. Wateris scarceand of rather poor quality. A few new rootstock types haverecently beenintroduced.

Tunisia

While citrushas been grown in Tunisiafor a long time, it hasonly beenwithin the pastfew yearsthat a sizeablecommer- cial industry hasbecome established. Even this is small by presentday standards.

When budded, trees are usually on sour orange; however, most of the native groves are seedling trees which are densely planted,200 to 400trees per acre, small in sizeand heavily pruned.

The climate in Tunisia is arid with low humidity and greatextremes in temperature. Irrigation water is limited and salin- ity is a problem. Periodsof drought often occur. Tunisiansoils are light textured and covercrops are grown in someareas to preventwind erosion. It is reportedthat few virus diseasesoccur in seedlinggroves. However,both Phytophthora and Armillaria are presentand psorosisis presentin vegetativelypropagated orchards. No real rootstock trials exist.

Turkey

In , the citrus industry is being modernized, and today there are over 100,000 acres of citrus in this country. Orangesare the most important, followed by mandarins, lemons, and grapefruit. Nearly all budded citrus in Turkey is grown on sour orange rootstock, with the exception of satsuma mandarin which is grown on trifoliate. Major diseasesare Phytophthora on all citrus and mal secto on lemons. Psorosis,stubborn, xyloporosis, and exocortis are present. Tristeza- like symptoms have been observed, indicating the virus may be present and a threat to Turkish orange plantings. No root- stock plantings are in evidence.

Middle East

In Iran, orangesare grown on their own roots or are buddedon sour orange,, or sweetlime. Sour limesand lemonsare grown entirely from seedlings.A similar situation existsin Iraq, and nematodeshave been mentioned as a serious problem. The Ministry of Agriculture hasimported largequantities of 'Troyer' citrangeseed for trial. No nucellarbudlines are available.

India

India, which is secondonly to Chinain beingthe oldestcitrus-growing area in the world, hasprovided the industry with severalimportant commercialrootstocks. Among theseare rough lemon,sour orange,'' lime (which is really an Kid mandarin)and mitha the sweetlemon. Gajanimma,also called Citrus moi and Citruspennivesiculata, 'Kharna Khatta' (Citrus kharna) and nasnaran,Citrus amblycarpa are also rootstocksof lesserimportance.

There are about 250,000 acresof citrus under cultivation in India, most of which is in small plantingsand much of which is double-cropped or more.

Citrusdiseases affecting rootst~ks in India includePhytophthora, tristeza,psorosis, xyloporosis and . A seriousthre.t is citrus diebackwhich is a complex problemmaybe due to a mycoplasmsuch as greening but which is terribly complicatedby nematodes,fungi and nutritional problems. .5-

Roughlemon or Citrusjambhiri is the most widely usedrootstock for orangesand mandarinsin India. 'Rangpur' lime is alsoused as a rootstock, especiallyin BombayState where it is usedwith the 'Mosambi' orange. C. Kharnais alsoused ex- tensivelyexcept under blood orangeswith which it is incompatible. 'Malta red' is a1soincompatible on rough lemon stock. A vigorousrootstock of somelocal importanceis 'Gajanimma'or C.pennivesiculata. It hasalso attracted some attention in California,but is very susceptibleto tristeza,stem-pitting severely. It is alsoextremely susceptibleto cold. The 'Kharna Khatta' is very susceptibleto gummosis.The rough lemon is susceptibleto gummosisto about the samedegree or perhaps a little less than sweet orange or grapefruit. Rough lemon is also susceptible to Armillaria or oak root fungus. It is tolerant to tristeza,cachexia and exocortis. It is susceptibleto nematodes.Rough lemon will grow fairly well on all soils but is par- ticularly adaptedfor usein light sandyand sandyloam soils. It is averageas to salt tolerancebut will not do well in poorly drainedsoil asit will not stand "waterlogging" for any length of time. It is more drought resistantthan either sour or sweet orange. As a replant in former citrus soils its growth hasbeen average to good. Fruit quality is poor.

Many rootstock trials have been conductedin India. Their resultsare somewhatinconclusive and rough lemon remains the rootstock of note. There are many forms of the rough lemon and variable performance in part can be attributed to the genetic differences between these . Of all the varieties coming out of India the nasnaran or C. amblycarpa has shown the most interest and best performance in California. More will be said about the 'Rangpur' lime later.

Pakistan

About 92 per cent of the 40,000 acresof citrus grown in Pakistanis in west Pakistan. Rootstocks,varieties and disease problemsare similar to those in India. Fairly extensiverootstock trials havebeen conducted in Pakistanbut in generalthey were of the samenature asthose conducted in India and the conclusionsare similar.

China

Certainlymajor citrus diseases such as Phytophthora, tristeza, psorosis, exocortis and xyloporosisare presentin . In the past, Citrussunki has been the principal rootstock in China. C. sunkiis mentionedin the literature as being a dwarf- ing stock. However,in trials in Californiawhere C. sunkihas been grown on rich, loam soils, it hasnot provedto be dwarf- ing. It is, therefore,probable that the cultural conditionsunder which citrus is grown in Chinaare responsiblefor the dwarfing effectof sunki. In southeasternAsia, sunki is close-plantedon ridgesonly a few feet abovethe water table. Prin- cipal varieties grown on C. sunki are the 'Swatow' orange,also known asthe '', and anothermandarin orange, the 'Tankan'.

Citrussunki is a sour mandarinin Swingle'sclassification.

In China,sunki is reportedto be slow growingand similar to ponki, which is similar to the 'Cleopatra'mandarin.

Sunki is tolerant to tristezaand, if like 'Cleopatra',tolerant of exocortis and susceptibleto xyloporosisand Phytophthora. Sunki is short-lived, about 10-12 years, in the Orient, and fruit size and quality are averagefrom trees grown on it.

The Satsumamandarin is alsogrown, usually on trifoliate rootstock (Poncirustrifo/iata).

While a largepart of the southernplantings in mainlandChina are nearthe seain heavysoils, inland citrus-growingareas, which are usually in river valleys,have sandy, alluvial and loam soils.

In addition to nativecitrus, a numberof westernvarieties such as 'Washington'navel, 'Valencia' and 'Shamouti' orange havebeen introduced, as well as 'Marsh' grapefruit, 'Eureka' and 'lisbon' lemon.

Although buddingand arecommon, propagation by marcotting and cuttings is alsoused. No rootstock trials are known. .6.

Thailand

In southeasternAsia marcottage(aerial layering) is the commonmethod of propagation. This is true in Thailandwhere 95 per cent of the citrus, whether pummelosor mandarins,is grown this way. One of the reasonsmarcotting is so popular is that it avoidsany unfavorablereaction often experiencedwhen the scionwood is virus-infectedand susceptiblerootstocks are used.

However,in recentyears Thailand, more than any other country in southeastAsia, hasbeen experimenting with modern me'dlodsof production which includesrootstock trials aswell asfertilization trials and the introduction of modern pesticides.

A considerablenumber of pummelosare grown in Thailand,principally on their own roots. Pummeloshave a high resistance to Phytophthora and are grown on soil bedsa few feet abovethe water table under conditionswhere brackishwater is a prob- lem. It is questionablewhether few other stockswould tolerate this type of environmentas long asthe pummelodoes.

In northern Thailandthere tendsto be drought at certain seasons.All of Thailand is characterizedby rather heavycover crops.

Pummelo hasnever been successfulas a rootstock in trials in Californiabecause of its susceptibilityto tristeza. Yields also tended to be light in relation to tree size.

Rootstocksin

Japan is the world's largestproducer of mandarin-typecitrus. Satsumais widely grown, and the 2 major varietiesare the 'Wase', an early type, and the 'Owari', a late type. A summer orange, called 'Natsudaidai', is also grown. Ninety per cent of the satsumasare on P. trifoliata. The remainder are on 'YUlU'. In many cases,'YUlU' is used as an inarching stock for satsuma on trifoliate after the trees are about 10 years of age and begin to decline.

While tristeza,psorosis, exocortis and other diseasesare recognized,they do not seemto be a seriousproblem. A virus- indexing program for scion budwood sources is administered on a national basis in Japan.

Trifoliate orange,the number 1 rootstock in Japan,is alsothe world's third most widely usedrootstock. Trifoliate is an excellentstock for mandarins,particularly on heaviersoils. It is resistantto Phytophthora andArmillaria and tolerant of tristezaand cachexia(xyloporosis). Trifoliate is, however,susceptible to exocortit. It is tolerant of nematodesand does well in heavy,poorly drainedsoils. It is very resistantto cold and makesgood growth asa replant in old citrus soils but has a low toleranceto salt and calcareoussoils. Fruit quality of orangesand mandarinsgrown on trifoliate is excellent. Trifoliate is not compatiblewith lemonsand is a poor stock for grapefruit.

'' is usedextensively in Japanas a rootstock for orangesand asa secondarystock for satsuma.'Yuzu' hasbeen used in Japanfor centuriesand wastheir principal stock prior to the acceptanceof trifoliate. Treeson trifoliate are more preco- ciousin bearingand initially grow more rapidly than treeson 'Yuzu'. However,in Japantrees on trifoliate stock sometimes beginto declineafter 10 to 20 years,and it is the practiceto inarch thesetrees with 'Yuzu' assoon asgrowth seemsto be retarded. The decliningtrees recover and becomelong-lived and productive.

'Yuzu' doesbest on deep,rich soils,since it producesan extensivetaproot. Gummosisresistance of 'Yuzu' is comparable to sour orangeor better. It is not affected by citrus cankerand showsconsiderable resistance to oak root fungus. It grows relativelywell asa replant. In California,its replant growth is comparableto sour, but it doesnot show resistanceto the citrus nematodeas does trifoliate. Treeson 'Yuzu' are nearly as resistantto frost astrees on . Since'Yuzu' doeswell in Japanwhere tristeza is widespread,one may assumethat it is tolerant of this disease.

Indicationsin California are that 'Yuzu' would probably be a good stock to try in calcareoussoils, since it picks up more iron and lesscalcium. On the other hand,it picked up more boron and sodiumthan other rootstocksand should not be used under salineconditions. 1.

In general,Japan seems to havevery little in the way of rootstock problems. They haveescaped the ravagesof tristeza, primarily becauseof their widespreaduse of tolerant rootstocks. Poor resultshave been attained with 'Vamamikan','Gir. imikan' and 'Tachibana'.

Taiwan

Taiwan is anotherimportant citrus-producingcountry in Asia. Sincecitrus is a valuableexport commodity, the industry has been expanding and will probably continue to do so since production efficiency is increasing.

'Ponkan' and 'Tankan' makeup 83 per cent of the commercialcitrus production. C. sunki, which we havealready dis- cussed,is the principal rootstock for thesevarieties. Satsumamandaril1s are grown,with trifoliate orangebeing the favored stock. Somepummelos are grown, and theseare propagatedby marcottingand are planted on their own roots.

Phytophthora is 1 of the principal diseases,although all of the common virus diseasesare present. Likubin, or yellow shoot is also prevalent. A nursery-tree certification program is operated by the Taiwan Department of Agriculture. Mother trees are selected for yield performance and are virus indexed as pedigreed sources of propagating material. Nurseries are registered and inspected. Little rootstock work is conducted.

Philippines

In general,citrus plantingsin the Philippinesare small, and many treesare grown on their own roots. However,there hasbeen some interest in trying new rootstocks,and theseare principally the rootstocksthat havedeveloped and tried elsewherein the world. At the presenttime, many of the treesthat are buddedin the Philippinesare on calamondin(Citrus mitis).

Major citrus diseasesare Phytophthora, foot rot, psorosis,xyloporosis (cachexia), tristeza, citrus cankerand mottle yellows. Phytophthora has caused the loss of many trees during recent years and has spurred the use of rootstocks. Because of Phytophthora, the height of budding has been increased to 12 to 18 inches above ground level. Cover crops also increase the incidenceof Phytophthora.

Australia

In 1966 it was reportedthat there were 76,000 acresof citrus in ,and that acreagewas increasing at an annual rate of 4 per cent. Australiancitrus consistsof about 50 per cent 'Valencia' oranges,30 per cent navels,10 per cent lemons and limesand 10 per cent grapefruit, mandarinsand other minor citrus.

Theclimate in whichcitrus is grown in Australiaranges from humid tropical to arid subtropicalwith the greaterpart of the acreagebeing in the latter lone. In general,soils in the citrus areasare calcareous, alkaline, high in natural salinity and rangefrom sandto sandyloam in texture.

In the past, rough lemon hasbeen the main rootstock becauseof its vigor and drought resistance.However, other root- stocksare now under trial and are beingused in commercialpractice. Theseinclude sweetorange, 'Carillo' citrangeand trifoliate. Rootstocksunder trial in experimentalplantings include 'Cleopatra'and 'Emperor' mandarins,'Sampson' and 'Orlando' tangelosand 'Troyer' citrange.

Phytophthora root rot is present and its spread is mainly blamed on faulty irrigation practices. Virus diseasessuch as exocortis and psorosis occur but are avoided by a scion selection program. Tristeza virus is also present in Australia, but lack of susceptible rootstocks make~ it of little economic importance.

New Zealand

Total citrus acreagein New Zealand is relatively small, being about 2,000 acres. However, citrus plantings have increased by over 50 per cent during the past 5 years, primarily becauseof the introduction and use of trifoliate as a rootstock. -8-

Becauseof unfavorableclimatic conditions and the useof rough lemon as rootstocks,soluble solids in orangestended to be low and acid high, givinga poor quality fruit. However,the useof trifoliate asa rootstock causeda markedimprovement in the solublesolids content and improvedfruit quality. As a result, this fruit hasfound a market and there hasbeen an up- surgeof orangeplantings.

Southernand Central Africa

In the Republic of South Africa, almost all buddedcitrus treesare on rough lemon becauseof the devastatinginfluence of tristeza. Recentlya number of test plantingshave been made with new rootstockswhich include sweetorange, 'Empress' man- darin, 'Troyer' citrange,and trifoliate orange. There is recentinterest in a of 'Troyer' citrangeand 'Cleopatra'mandarin 4t639. In addition to tristeza,exocortis and xyloporosisare widely distributed in most old-line citrus varieties. So far, they have beenof minor economicimportance because of the toleranceof most scion-rootstockcombinations.

Psorosishas been almost completelyeliminated in South Africa becauseof an efficient eradicationprogram carried out by the South African Departmentof Agriculture. This programwas started in 1927,and at the presenttime hardly any psorosis- infectedcitrus treesare to be found.

In southernRhodesia, rough lemon is againthe primary rootstock, althoughexperimental plantings have been made with trifoliate orangeand 'Troyer' citrange. Trifoliate orangemay proveto be of value,particularly in view of the heaviersoils and extensiverainfall which occur in many citrus-growingareas.

Citrusis grown for local consumptionin many of the newly createdAfrican countries. In almostall cases,the principal rootstock is rough lemon. Ghanasupported a lime industry, the treesof which were almostentirely on their own roots. In 1941, tristezavirus decimatedthe industry, and thesetrees have gradually been replaced with healthy scionmaterial on rough lemon rootstock. This was alsotrue of the Coast(Ghana). Phytophthora and tristezahave been the 2 limiting diseases in most instancesin lime culture.

South America

The wide-scalecommercial development of citrus which beganat the turn of the century in South Americawas basedon citrus buddedon selectedrootstocks. Sincethen, the citrus industry of South Americahas experienced several setbacks, most of which havebeen related to rootstock problems.

In 1900, almostall plantingsmade in South Americawere on sweetorange rootstock. However,losses from Phytophthora were immense,and by 1915 sour orangewas recommended and beingplanted asthe best rootstock for citrus. For the next quarter of a century, sour was almostuniversally used. The onslaughtof tristeza in the mid-1930'scaused the loss of more than 20 million treesin Argentina,, Paraguay,and Uruguay. While badly hurt, the citrus industry was not wiped out New plantingswere establishedon rootstockstolerant to the tristezavirus. In Brazil, the 'Rangpur' lime hasbecome the ma- jor stock. Sweetlime and trifoliate orangeare alsoused, particularly in Argentinaand Uruguay,respectively.

In Peru, sour rough lemon and 'Cleopatra' mandarin are being used in the drier coastal areaswhere sandy soils prevail, and 'Rangpur' lime and 'Troyer' are used in the more humid tropic regions.

Major rootstock research studieswere startedin tf1elate 1920'swhen trials were installedsimultaneously in Brazil, Argen- tina, and Surinam. Sincethen over 100 rootstock trials havebeen established in theseand other South Americancountries, principally Brazil.

In usingtristeza-resistant rootstocks, it hasbecome very important that virus-freebudwood be used,and constant indexing programsare necessary.For example,tristeza-tolerant, 'Rangpur' and other mandarinlimes are intolerant of exocortis and xyloporosis(cachexia). Trifoliate and most of its hybrids are susceptibleto exocortis. Most tangerinesand tangelosare sus- ceptible to xyloporosis. Thus, there hasbeen an increasingtrend toward the useof nucellarlines of budwood. -9-

Original trials in the searchfor tristeza-resistantrootstocks were mainly interestedin diseaseresistance with little regard to fruit quality. Now that tolerant stocks have been found, attention has been turned to resistant stocks with improved fruit quality. In SouthAmerica, a greaterdiversity of tristeza-resistantrootstocks are in usethan in perhapsany other part of the world. Thisis primarilybecause such rootstocks are chosenfor 1fteirbest performance under a variety of climatic and ecologicalcon. ditions. 'Rangpur' lime, which we havesaid before is really an acid mandarin,is usedextensively in South Americaand very little elsewherein the world.

In trials in California,with 'Valencia' scion, it wasthe most prolific bearerof 25 rootstocksin the test. However,fruit quality was low, beingonly slightly better than fruit grown on rough lemon. Fruit sizewas good. 'Rangpur' is susceptible to exocortis and xyloporosis,a little lesssusceptible to Phytophthora than sweetorange, and resistantto tristeza. Its salt toleranceis moderate,and it is similar to sweetin cold hardiness.It hashad no observedcompatibility problemswith oranges and mandarinsbut should not be usedwith lemons. All in all, it hasbeen a good backupstock for usein South America where tristezahas become endemic. British Honduras- - Trinidad

In British Honduras, Jamaica, and Trinidad, as in many tropical areas of high rainfall, foot rot caused by one or more of Phytophthora is the major diseaseproblem connected with rootstocks. Sour orange is the principal rootstock, although some sweet orange is used. In some of the heavier Jamaican soils, a local shaddock known as "Hog Shaddock" servesas a rootstock.

Mexico

Sour orangeis the major stock usedin Mexico. However,sweet is alsoused and in someareas numerous seedling trees are grown on their own roots. Seedlingorchards are numerousin the state of Veracruzwhere citrus is interplantedwith coffee. Productionfrom such orchardsis low, rangingfrom 60 to 125 boxesper acre.

Some of the best cultural practices are said to occur in the state of Nuevo Leon. New plantings there are mainly 'Valencia' orange budded on sour. Again, foot rot and other diseasescaused by Phytophthora are serious, particularly in the areas of high rainfall and humidity. The 'Cleopatra' mandarin and the 'Troyer' citrange were recently introduced and as yet there is no report on their performance.

United States

In the ,the main citrus producingstates are , California,Arizona, and Texas.

Many of the early orangegroves in Californiawere seedlingtrees on their own roots, and a few of theseorehards remained until World War II. However, many of the early California orehards were planted on sweet orange. When Phytophthora be- came a problem, sour orange became a dominant stock and remained so until the advent of tristeza.

Most of the diseaseswhich attack rootstocksof citrus are presentin California. For example,during the wet years of 1938, 1941, and 1943, Klotz estimatedthat almost a million treesin Californiawere infectedwith brown rot gummosisor foot rot. Othertypes of gummosisare also present.

Psorosis and exocortis alsoinfect Californiaorchards, but xyloporosisis not a problem.

Sweetorange growswell on fairly heavy soils, but is best adapted for growth on rich, sandy loams. It does not do well on extremely sandy soils or extremely heavy or calcareous soils. On heavy, poorly drained soils it may show iron chlorosis symp. toms, as it also does on calcareous soils. It does not develop a well-differentiated taproot as does sour, and is moderately shallow-rooted. It does develop an abundant system of lateral roots which generally are deeper than those of rough lemon. .10-

Scionsbudded on sweetorange produce large, vigorous trees with all commercialvarieties. The treesare largerthan when similar scions are budded on sour orange. Yields on sweet orange are good --the highest with all scion varieties except when navel oranges are budded on sour or 'Morton' citrange which outyields sweet Trees budded on sweet orange are not as pre- cocious in bearing as trees on trifoliate orange or , but trees on sweet are long-lived and bear well into ad- vanced age of 50 or 60 years. from trees on sweet are thin-skinned, juicy, and of high quality, hotd up well on the tree, and also store well. Per cent , solub1esolids, and are essentially identical to those obtained on sour orange and are better than from fruits grown on rough lemon. However, fruit quality is not as good as when scions are grown on trifol- iate orange or 'Savage' citrange. Granulation of fruit from trees on sweet orange is not a serious problem. Fruit sizes from trees on sweet tend to run somewhat smaller than average,although they are larger than when 'Cleopatra' mandarin is a root- stock but smaller than when rough lemon and sour orange is used.

Trees on sweetorange are extremelysusceptible to gummosis.This is true evenwhen the treesare buddedhigh, planted high, and the best cultural practjces are used. This is the greatest disadvantagethat sweet orange has. It seemsremarkable, considering the susceptibility of sweet orange to Phytophthora, that so many seedling orchards survived in the early history of California and Florida. Sweet orange is the most susceptible commercial stock to this disease,being somewhat worse than rough lemon and not nearlyas tolerant as sour orange,C. macrophylla,or 'Troyer' citrange.

Sweetorange performs very poorly asa replant in old citrussoil, eventhough the soil hasbeen fumigated and good cultural practicesare used. Sweetorange stock is especiallyprone to collar rot and haspoor root regenerationcapacity once infected Nth Phytophthora. There is somevariation in susceptibilityto Phytophthorabetween various cultivars, and 'Indian River' Meet seemsmore resistantthan others. However,this may be of little practical importance. A sweetorange rootstock resis- 18nt to Phytophthorawould be a greatasset to the citrus industry.

Sweetorange rootstocksare resistantto sour orangescab and to mal seccowhich are not problemsin California. Psorosis or scaly bark is more seriouson sweetorange than it is on the resistant sour orangestock. lemon treeson sweetdo not ex- pressshell bark symptomswith the sameseverity that treesdo on sour orangeor rough lemon. Sweetorange is not affected by exocortis,cachexia (xyloporosis), or woody gall. Reactionof sweetorange to tristezainoculations have generally been negative,so that most cultivars of sweetare tolerant of tristeza. Sweetorange stock is very susceptible,however, to citrus nematode,although certain selectionslike the 'Pineapple'and 'SanguineGrosse Ronde' haveshown resistanceto the burrow- ing nematodewhich doesnot exist in Californiabut is presentin Florida.

Sweet orange is not an answerto the world's rootstock problems,but it-hasso many good qualitiesthat its germ plasm should be perpetuatedin other rootstockssuch as in hybridswith trifoliate orangewhich haveresulted in citrangessuch as 'Troyer' and 'Carillo'.

In California,almost all plantingsof orangesbeing made today are on 'Troyer' or, in somecases, 'Carilla'. There is some interest in trifoliate orangein the heaviersoils of the SanJoaquin Valley. Almost all lemonsbeing planted are on C. macro- phylla. However,there is someuse of rough lemon for grapefruit and lemonsin the sandysoils in the desertareas.

The reasonfor the switch to 'Troyer' citrangeas a primary rootstock for orangesis its toleranceto tristeza,coupled with resistanceto Phytophthora. It is, however,susceptible to exocortis,and it must be usedwith budwoodfree from this virus.

'Troyer' is susceptibleto Armillaria and to nematodes.Fruit quality and production are good with oranges,mandarins, and grapefruit. Most lemons, with the exception of 'Lisbon', suffer from bud union difficulties when grown on 'Troyer'.

'Troyer'is aboveaverage as to cold hardiness,low in salt tolerance,and doesnot standup well under conditions of poor soil drainage. Its replant growth in old citrus soils is good,as is its longevity.

'Carillo' citrange,which camefrom the sameseries of crossesthat gaverise to 'Troyer', is very similar to 'Troyer', al. though in sometests it showsmore resistanceto citrus nematode. For this reason,it is now beingplanted in preferenceto 'Troyer' in someareas of California. .11

Researchtrials on rootstocksindicated that C. macrophy//amight be a good rootstock for lemonsunder Californiaconditions, and most of the lemons now being planted in California and Arizona are on this rootstock:

While macrophyllabuds well to all varietiesof citrus, its susceptibilityto tristezaand cachexiarule it out for usein most areas exceptfor lemonsand limes.

Macrophylla is highly resistant to Phytophthora and tolerant of exocortis. It does well on all soil types from sandy to heavy clay loams and has a high degree of resistance to calcareous and saline soils. It shows perhaps the highest boron tolerance of all citrus rootstocks, and it is also capable of absorbing certain other in slightly greater quantities than other citrus. Hence, trees on macrophylla are deep green in color and seldom show leaf patterns that denote deficiencies.

Young lemon treeson macrophyllaare vigorous and precociousin bearing. One problem is that fruiting is so heavythat se- vere limb breakage occurs. Bud unions have been good with all scion varieties tested, and yields have been heavy with navels, 'Valencias', 'Oancy' , grapefruit, 'Eureka' and 'lisbon' lemons, and lime. Fruit from trees grown on macrophylla are larger than average. However, fruit quality is below averageand with some varieties can even be classified as poor. '', for example, granulates badly when macrophylla is used as a rootstock. Solub1esolids and citric acid from fruit of trees grown on macrophylla are low, being comparable to fruit grown on rough lemon. However, as mentioned before, recommendations are for use only with lemons and limes.

Treevigor is goodwhile the treesare young, but older treesshow no more, and perhapseven less, vigor than treesbudded on rough lemon. lemon treeson macrophyllaare relativelyshort lived. 'Eureka' on this stock hasa productivelife of 20 to 25 years,and the life of 'lisbons' may be only slightly longer.

Macrophylla is very sensitive to cold, and trees on this rootstock do not fare as well as trees on rough lemon when subjected to freezing temperatures.

Macrophyllais an interestingrootstock, and hybrids with trifoliate orangemay combinethe good qualities of both. Testing of such hybrids is in progressin Californiarootstock trials.

Arizona

Citrusgrowing in Arizona is centeredin 3 districts. The lower ColoradoRiver Valley nearYuma, the Weliton-Mohawkarea eastof Yuma and the Salt River Valley areanear Phoenix.

Arizona now hasover 50,000acres of citrus and rough lemon is and hasbeen the primary rootstock usedfor oranges,lemons and grapefruit in the Yuma and Weliton-Mohawk areas. Otherwise sour orange was used. Recently 'Troyer' and mocrophylla have been tried for oranges and lemons respectively. Some 'Cleopatra' mandarin has also been used as a rootstock.

Diseaseswhich affect rootstocksinclude Phytophthora,psorosis, exocortis, tristeza and a severetype of gummosisknown as Rio Grande gummosis. So far this diseasehas primarily attacked grapefruit trees. Rio Grande gummosis also exists in the desert areas of California and has been a problem in recent years in some grapefruit orchards. Tristeza, while present, has caused no seriousproblems.

Soilsin citrus growingareas of Arizona rangefrom sandand silt alongt~e ColoradoRiver areasnear Yuma to sandy,clay loam soils in the Phoenixdistrict. Salt accumulationand iron chlorosisare problemsin someareas. Small rootstock trials exixt atYuma and Phoenix.

Texas

The citrus industry of Texashas been badly hurt severaltimes during the past century by severefreezes. Eachtime, however, the industry hasrebounded with increasedplantings of both orangesand grapefruit. The principal rootstock hasbeen sour or- angefor both grapefruit and oranges.Texas grapefruit is of high quality and the fruit hasbeen sold at a premium on the market. -12.

Plantingssince the 1962 freeze,which nearly destroyedthe industry, havebeen about two-thirds red ('Ruby') grapefruit and about one-third of other varietiesof citrus. The orangesgrown are primarily varietiesfrom Florida.

Salinityis frequently a probfemin Texasorchards and considerableresearch has been done toward finding more salt tol- erant rootstocks.

. Rio Grandegummosis and foot rot are seriousdisease problems in Texas. Psorosis,cachexia, and other virus diseasesare alsopresent

Florida

Florida is now the largestcitrus-growing area in the wortd. It produces almost 2/3 of the world's grapefruit and about 25 per cent of the world's oranges. Also, most limes, and which are produced in the United States are grown in Florida.

You will seemuch of Florida citrus and its problemson the field tours so I will spendvery little time on the rootstock situ- ation here,since there are otherspresent who arecertainly more qualified to talk about local conditionsthan I.

The predominantrootstock in Florida is rough lemonwhich is believedto havereached here early in the 19th century. On deep,well-drained, sandy soils it produceslarger and more productivetrees in a shorter period of time than any other rootstock. Sour orangeis the secondmost important rootstock and is preferredfor low hammockand flatwood soils. Other rootstocks cominginto useinclude 'Cleopatra'mandarin, 'Rangpur' lime, trifoliate orange,and 'Troyer' and 'Carillo' citrange.

Major citrusdiseases include psorosis,exocortis, brown rot gummosis,tristeza, Armillaria, Clitocybe root rot aswell asothers. -13-

ROOTSTOCKSHORT COURSE

11:15 Discussion Monday,September 24

Questionsfor Dr. W.'. Bitters question: I would like to ask Dr. Bitters to giveus his experienceon lemonson roughlemon rootstock in California. How much doesscaly bark and psorosisreduce the overallperformance? answer: Depending upon the soil type and the areain which they are growing,such trees may beginto decline at 10 or 12 years of age and may be out of production by the time they are 20 years of age. They are generally very short-lived. Shell bark hasbeen a very seriousproblem on them. Shell bark of treeson rough lemon is perhaps severe as it is on any rootstock combination. question: The trees we have now are 6 years old. We are experiencing some severeshell bark and gummosis. Would you say that they will grow to be 20 years old? answer: They could survivethis long with the shellbark but I don't know with the gummosis. question: What is the bestway of keepingthe greening diseaseout of Florida and California? answer: Dr.E. C. Calavan answering: We will needcontinued quarantineand a very careful enforcementof regulations.

~e5tion: I hearthat lemon treeson sour orangeroot are not affected by tristeza. wonder if you had observedthat answer: I think this is correct. Lemontrees are not affected by tristeza on sour orangeroots. I think one of the prin- cipal reasonsis that the vector itself doesnot feed on the lemon foliage at leastunder California conditions and the secondthing is that the lemon is not a very favorablehost for the build-up of the virus. question: You mentionedthat in Israelthat they eradicatetrees to the point of eliminatingtristeza. thought that was a hopelesstask. answer: I think it may be a hopelesstask but I do think it is a delaying action until they can at least establish a new rootstock program. We have faced a similar situation in the central valley of California where we had an erad- ication program for removal of trees showing symptoms or adjacent trees which have been indexed and discov- ered to be symptomless carriers. So far the spread has been held down in the central valley. I would expect this program to continue. question: Vou mentionedthat lemon should not be usedwith Rangpurlime. Is that right? answer: We have had a little difficulty with Rangpurlime and I would like to seea little more experiencewith them. question: On trifoliate orange,how critical is the calcium level and what is the proper pH range? answer: I couldn't tell you what the proper pH rangeis. I think it is primarily a calciumproblem becausewe run illto problemswhere M havecalcareous soils where trifoliate orangejust becomesso hard that we cannot grow it. We may haveto treat with iron chelateat leasttwice a year to keepthe treesgrowing. At one of our field sta- tions wherethe pH is around 7.3 to 7.5 we havethis problem in the nurserybut oncewe get a scionvariety on it we can continue to grow the treeseven though we do havesome chlorosis showing on them. .14-

11:15 Discussion(cont) September24 question: In Spainand Portugaland in that areathe pH is very high. waswondering if that was one of the reasons why they put the bud union so high. anSMr: Not asfar as I know. que$tion: How will trifoliata work on tropical conditions? answer: To the bestof my knowledge,trifoliata hasnot beentried successfullyunder tropical conditions. I have somereselVations asto whetherit will do satisfactorilyunder tropical conditions. question: Would you expect trifoliata to be more cold sensitiveif it were grown under tropical conditions? answer: I think it would. I say this becauseeven in California where we have had warm falls and then experienced a sudden cold spell in the fall, I have actually seen worse frost damage on trifoliata orange then I have on rough lemon ill the same orchard at the same time. I suspect its hardin,ss is due to the fact that as the temperature i$ lower it gets a chance to harden off and become more resistant to cold. If it doesn't have this factor it doesn't have the resistance to cold. question: Is there any differencein lemon quality (I am referringto California lemon growers)with lemon on macro- phyllaas far asfresh quality is concernedas opposed to souror roughlemon? answer: Quality wouldn't be asgood but it is still acceptable. question: What is the relativetolerance or resistanceof grapefruitto tristeza on sour orangeas compared to sweetoranges on sour oranges1 answer: Grapefruit on sourorange in Californiais veryslow in showingsymptoms of tristeza.Under field conditions your treesof sweetorange and sour orangewould probably be showingsevere symptoms and many treeswould probably be deadbefore the grapefruit treeson sour beginto show symptoms. Whenwe inoculatedgrapefruit treeson sour orangein the field we got symptomsrather soon. But under field conditions it takesquite a while for symptoms to appearso they are more resistant. This is alsotrue for mandarins.Mandarins on sour orange aremore resistantto tristezathan sweetorange on sour orange. question: Is stock-scionincompatibility a problem of any significanceand if so hasvery much researchbeen done? answer: It is a matter of significancebecause while we generallyhaven't found it on the older rootstocks,by this I meanrootstocks like sweetand sour, but this is a problemwith the trifoliate orangegroup and in our recent rootstock trials wherewe haveabout 200 new rootstocksunder test we find that about 10%of thesewill show very severeincompatibilities when they havetrifoliate parentagein them. Evenwith trifoliate orange,we have found bud union creaseto be fairly common. I think it is somethingto considerparticularly in the trifoliate Drangeor trifoliate hybrid group.