Damage from Exocortis in Japan Results and Conclusions
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EXOCORTIS Damage from Exocortis in Japan S. YAMADA and H. TANAKA EXOCORTISWAS first recognized in Budwood of Etrog citron, us~cs60- Japan in 1963 by Dr. W. P. Bitters in 13, was imported from California some trees in the citrus variety collec- and propagated on trifoliate orange tion at the Okitsu Branch, Horticul- rootstock. Two methods of indexing tural Research Station (4). Since then were employed. In one, 2-year-old a survey for the presence of the dis- potted seedlings of rough lemon or ease and a study of its effects have Marumera were budded simulta- been made. The results are reported neously with Etrog citron and candi- in this paper. date tree buds according to the procedure of Calavan et al. (1). In Results and Conclusions the second, buds of Etrog citron EXOCORTISSURVEY.- From 1963 were topworked on field-grown sus- to 1969, citrus plantingsin the various pect trees at Okitsu and observed for prefectural experiment stations have about 1 year. been surveyed for exocortis by look- All 9 varieties of the affected field ing for stunting and bark shelling in trees mentioned above indexed posi- the trifoliate orange rootstock on tive for exocortis virus. They were: which the plants are grown. Eureka and Everblooming lemon, Symptoms of exocortis were; found common lumie, Marsh seedless and in 57 trees, representing 31 varieties Thompson grapefruit, Bergamot of 13 species of citrus (Table 1). The sweet orange, Temple orange, ta- trees were severely stunted and ex- kuma sudachi, and Cleopatra man- hibited characteristic bark shelling of darin. their trifoliate orange rootstocks. Al- The indexing revealed that trees most all these trees had been propa- having other virus diseases-sat- gated with scions imported from for- suma dwarf, citrus mosaic, Navel in- eign countries during the period fectious mottling, and natsudaidai 1920-40. dwarf -but free of exocortis bark Surveys have also been made in shelling, are free of exocortis virus. commercial plantings of satsuma Thus, the causal agents of these dis- mandarin and natsudaidai trees, but eases have no relation to exocortis no exocortis symptoms were seen in virus. Apparently healthy satsuma any of the trees. Orchards in Japan mandarin and hassaku trees are also have thus far escaped damage from free of the virus. the disease. In other experiments, Etrog citron INDEXING WITH ETROGCITRON.- buds were topworked on 3-year-old 100 PROCEEDINGS of the IOCV potted Temple orange trees not Volume of crown was calculated showing exocortis symptoms and on as height x maximum diameter several old trees of Eureka and Ever- X minimum diameter X 0.7. The blooming lemon, common lumie, results (Table 2) demonstrate re- Marsh grapefruit, and takuma sud- duced tree size and fruit yield of the achi. Two to 6 months later, new trees on trifoliate orange as com- shoots that grew from the citron buds pared with the trees on yuzu and developed severe epinasty. The re- yamamikan. By the citron test, these sults indicate that topworking with 2 healthy appearing trees also carry Etrog citron buds may be used as a exocortis virus, indicating that yuzu quick and convenient field test for and yamamikan as rootstocks are tol- exocortis virus just as topworking erant of exocortis virus. TABLE 1. CITRUSVARIETIES AFFECTED BY EXOCORTIS IN JAPAN Number of Common name of affected tree Group Spec~es Var~eties Trees Lemon 1 6 16 Eureka, Lisbon, Villafranca, Genoa, Everblooming, Orange fleshed Lumie 1 1 3 Common lumie Shaddock 2 5 6 Suzuka, Egami, Kao Panne, Bannokan, Kawachi bankan Grapefruit 1 3 7 Marsh seedless, Duncan, Thompson Sour orange 1 1 1 Kikudaidai Sweet orange 1 9 12 Bergamot, Fukuhara, Kanton, Malta, Mino, Navelencia, Pineapple, Valencia, Thompson navel Tangor 1 1 4 Temple Yuzu 1 1 4 Takuma sudachi Mandarin 4 4 4 Shikaikan, Yuhikitsu, Cleopatra, Sunki Total 13 31 57 with trifoliate orange and Rangpur EFFECTOF EXOCORTIS VIRUS ON lime can be. SATSUMA MANDARIN.-In 1963, six TREE SIZE AND FRUIT YIELD OF 2-year-old potted satsuma man- MARSH GRAPEFRUIT.-Three frees of darin trees on trifoliate orange were Marsh grapefruit were propagated inoculated with buds from trees on trifoliate orange, yuzu, and yama- showing severe bark shelling on mikan, respectively, in 1935 and their trifoliate orange rootstock. transplanted to the field at Okitsu Four noninoculated trees of the in 1937. Only the tree on trifoliate same kind served as controls. In orange shows exocortis symptoms May 1966, the trunk circumference on the rootstock; the other 2 trees 5 cm above the bud union and the are growing vigorously. Height of total length of new shoots produced tree, maximum and minimum diam- by these trees were measured. eter of crown, and trunk circum- Newly developed shoots were cut ference were measured in 1967. back to 20 cm to force new growth. EXOCORTIS TABLE 2. TREESIZE AND FRUIT YIELD OF EXOCORTIS-AFFECTED MARSH SEEDLESS GRAPEFRUIT ON VARIOUS ROOTSTOCKS Tree size Trunk Volume Degree Max. Mln. circum- of top Fruit yield of bark Height dia. dia. ference crown Weight Rootstock shelling (m) (m) (m) (cm) (cm) No. (kg) Trifoliate severe 2.2 3.4 2.8 48 13.8 936 127.7 orange Yuzu none 3.5 4.6 3.2 67 38.3 1993 360.4 Yamamikan none 4.3 5.4 4.6 84 74.8 2764 590.2 Each year the shoots were meas- TABLE 3. EFFECT OF EXOCORTIS VIRUS ON THE ured and cut back until final meas- GROWTH OF SATSUMA MANDARIN NURSERY TREES urements were made in 1968. The results (Table 3) demonstrate a reduction of from 10 to 40 per Months after Length of Trunk cent in length of new shoots and inoculat~on new shoot circumference trunk circumference in infected 37 0.65 0.88 trees as compared with the con- 45 0.76 0.90 trols. They support the reports by 56 0.82 0.89 68 0.62 0.91 Sinclair and Brown (2, 3) that Owari satsuma mandarin is susceptible to exocortis virus and that growth trifoliate orange in Japan is suscep- and yield can be reduced by the tible to exocortis virus-a very im- disease. They answer the question portant question for the citrus in- whether satsuma mandarin on dustry of Japan. Literature Cited 1. CALAVAN, E. C. et al. 1964. Rapid indexing Conf. Intern. Organization Citrus Virol. for exocortis of citrus. Phytopathology Univ. Florida Press, Gainesville. 54: 1359-62. 3. SINCLAIR.J. B., and BROWN.R. T. 1962. 2. SINCLAIR, J. B., and BROWN, R. T. 1961. Effect of exocortis on satsuma orange. Reaction of rootstocks to exocortis, p. Phytopathology 52: 581 -82. 50-55. In W. C. Price (ed.), Proc. 2nd ~.TANAKA,S. 1963. On theexocortisof citrus. Ann. Phytopathol. Soc. Japan 28: 88. .