World Citrus 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 orange 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 fruit and is exceededonly by Japanand the United Statesin citrus pro- duction. Becauseof the prevalenceof Phytophthora, nearly all Spanishcitrus hasbeen planted on sour orangestock. However, tristezais rapidly spreadingin Spain,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' citrange,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' lemon 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 lemons. 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 the citrus industry 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 lime 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' grapefruit has been introduced as have the 'Eureka' and 'Berna' lemons. There are smaller plantings