VIRU E \ IT two or takes a time before the symptoms occurs andior a infection follows trees consist of a rootstock a scion of which can differ in their sensitivi the combin:jti1)!1 of varieties used. vims free materials are sorne many viruses can be transmitted viruses are kncj\vn t(f are trees. For these viruses the absence programs effective methods fur virus free varieties tor This virus was introduced into latent. In maruba kaido country as and necrosis of the new with CLSV onlo old cultivars on maruba-kaido rootstocks 1930's. After a few years virus is transmitted by grafting. mother trees is the best and the only many dwarfed rootstocks are infected careful not to introduce CLSV -infected materials from Japanese pears Some cultivars of pear react with infected by pear necrotic spot vi russ l appear from before. and then only on mature leaves. Leaves of affected during summer. After a few years trees seem to weaken. The most one of the best commercial cultivars in On the other hand, infection without symptoms. These trees seem to be tolerant to know whether :J. cultivar or a new clone is tolerant or sensi before distribution. As this virus is known to be transmitted * Plant Pathoiogist, Fruit Tree Research Station. Hiratsuka. Kanagawa
208 free scions for or nursery trees is the best rnethod to Stone fruits Prunus necrotic isolated from some appears to be restricted. These viruses seeds. Isolation or eradication of infected free trees must be used as donors for have Citruses Satsuma mandarin covers the greatest acreage among the various kinds of citrus grown in Japan. Satsuma dwarf virus can be transmitted infected soil as well as Elimination of the infected tree and fumigation of the soil are necessary for the of the disease. Replanting of virus free trees is recommended. Exocortis viroid has been found in a restricted area,?) but has increased in importance because of the use of exocortis-sensitive rootstock, trifoliate The viroid is very stable, resistant to heat, can persist a and spread knives. To control cxocortis. the use of virus free nursery and field are necessary. Grapevines Grapevine fanleaf virus is transmitted by grafting in the soil. To control the planting of virus free trees and soil nematodes present are necessary. Virus certification of fruit trees in Japan In J 961, the Japanese government settled on a mother orchard system to fulfill farmers' need for a continuous supply of healthy and true-to-type fruit trees. The legally controlled inspection of virus diseases of citrus and apple by government plant inspectors Since 1968, this measure has been extended to stone fruits and Mother trees are by prefectural government in orchards of farmers or horticultural station after inspection for known viruses by researcher. Government plant inspector Table 1. Indicator plants for indexing viruses of mother trees currently used by Plant Protection Station. Fruit tree Citrus Apple Prunus Sweet cherry Grapevine Diseases Satsuma dwarf Hassaku dwarf (CTV -stem pitting str.) Exocortis viroid Apple topworking (CLSV-type stl) (SPV) NRSV and PDV GRMV GFV Indicator plants Sesanum indicum (white sesame) Mexican lime (Citrus aurantifolia) Etrog citron (Citrus medica) Maruba kaido (Malus prumfolia var. ringo) Mitsuba kaido (Malus sieholdii) Shirofugen (Prunus serntlata) Cucumber ( Cucumis sativus) Squash ( Cucurbita maxima) Kwanzan (Pnmus serrulata) Chenopodium amaranticolor C quinoa Go mph rena globosa --------------- ~---~~------ CfV: Citrus tristeza virus: CLSV: Apple chlorotic: lcafspot virus: SPY: Stem pitting virus: NRSV: Prunus necrotic ringspot virus; PDV: Prune dwarf virus: GRMV: Green ring mottle virus; GFV: Grapevine fanleaf virus
209 observes ail mother trees or stemso After careful observation, some or indicator of shown in Table l 0 Horticultural associmions or some similar the presence of any virus symptoms on leaves trees are indexed for viruses using herbaceous used the Plant Protection Station is ask commercial nurseries to grow nursery trees scions from the inspected mother trees" Thus fanners can get certified scions for or nursery trees made with scions from the mother treeso Although it seems difficult to estimate with accuracy the exact ratio of the number of certified scions to all scions demanded in our country, designated mother trees of each cultivar of each fruit Table 2. Number of mother trees and the possible number of nursery trees made from them as compared with the number of nursery trees produced in 1975 in Japan. Fruit trees Number of mother trees Possible number of nursery trees made from scionwood of the mother trees Number of Nursery trees produced and soldin1975 Satsuma (Citrus unshiu) Natsudaidai (C natsudaidai) Navel orange (C sinensis) Hassaku (C. hassaku) 45,422 5,535!60952 1.674 3,529,500 375,000 251,100 317,500 3,130,600 1,039,600 406.200 877,600 lyokan iyo l 310 16,000 221,100 Apple (Malus pumila demestica) Peach (Prunus persica vu[f;aris) Sweet cherry (Prunus avium) Grapevine (Vi tis) Japanese pear (Pyrus serotina) 777 468 15 1,451 2,157 469.600 207,200 148,900 245,800 1,136,400 944,300 208,700 886,700 671,900 (Fruit and Flower Division, MAF, 1976) tree may not be sufficient to meet the demand of farmers. Table 2 shows the number of mother trees, the possible amount of nursery trees made from them and the number of maiden trees produced in 197 50 From the Table, it can be easily recognized that the farmer's demand is far beyond the supply of certitled trees except for satsuma mandarin. Therefore, it seems desirable that the government select and designate the virus free nuclear mother trees, maintaining and propagating them and distribute virus free materials to farmers as in the IR-2 project in America, 2 l the EMLA scheme in England 1 l on deciduous fruit trees and the CVI program in California 6 l on citrus fruit trees. How to obtain virus free materials? Some virus free cultivars can be found by indexing a number of trees for virus content. In the case of apples, it may take over 5 years to detect all known viruses. However, once some cultivars are certified to be free from all known viruses, those eultivars are very valuable and have to be maintained with care to prevent reinfection. Mother tree blocks of these cultivars will be propagated and scions for topworking supplied from them to nurseries. If all trees of some cultivars or clones of a variety are infected with viruses, it is necessary to produce virus free materials by heat treatment or meristem or tip cultureo Sometimes heat treatment is combined with tissue culture. Heat treatment Heat treatment of virus infected plants has been attempted by exposing whole plants or budded plants to high air temperature or by soaking plant parts in hot water.
210 Exposure Tsuchiya et a! strain -infected for 25 and 50 offers researchers Ohmori et a! virus (CTV)-stem plants in 50 C hot water for l-4 hours air temperature for 14 weeks. strain from citrus hybrid clone 6781 orange) which was clone by exposing the to aphid (Toxoptera citricidus ). almost all citrus trees me CTV-stem pitting strain greatly easily transmitted Therefore. the control of the disease. In some mother trees as early as 1960 and it was Later. it was discovered that these virus. Now it is to planting might interfere with a severe strain in the t1eld. are needed. Tissue culture The use of meristem or shoot culture has been tried successfully with many herbaceous and bulbous not yet applicable to fruit trees. this method ro fruit the Rutaceae, plants arising from of most viruses. Unfortunately, such associated undesirable qualities are manifested. Navarro of placing a 0.15 mm long shoot excized fr<)ll1 an infected tree onlu seedling under aseptic conditions, and obtained cultivars free from viroid. Kobayashi and Ikeda (personal Attempts to develop tissue culture of Afalus (Hirabayashi et a!, 1976 and Krul and Worley, of virus free materials. Conclusion Horticulturists have faced many troubles such as weakness of trees, incompatibility of grafting, early or late ripening of fruits and so on. some of these troubles have been shown to be due to virus infection. Breeding of fruit trees usually takes a time. Sometimes valuable hybrid clones suffer infection with viruses by pollination or of these troubles can be avoided with caution and some viruses are spread. Replacing all fruit trees with virus contribute' to increased production and good quality fruits. References 1. CUTTING, C. V. & MONTGOMERY, H. B. S. (Ed.) (1 More and better fruit with EMLA - A practical guide to virus free planting material Published East Malling Station and Long Ashton Res. Sta. pp. 29. ') FRIDLUND, P. R. (1971 ). IR-2. A project with a "blood bank" of virus disease free fruit trees. Washington Agric. hxpt. Sta. Circular 401, 14. 3. HOLLINGS, M. (1965). Disease control through virus.free stock. Ann. Rel'.
Discussion J. T. Rao, can take up cane seed material has disease Answer: am you were to get riel of virus diseases in cane. In conditions treatment for virus free on the combination of virus we face difficulties~ such as deleterious influence tissue each combination. repeat tests until we meet conditions in M. Kobayashi, Why have not you applied yet tissue culture techniques to get virus free fruit trees? Answer: Indexing a number of trees for virus content is rather economical and convenienl If all trees of some cultivars clones are infected with we will to tissue culture We are to apply tissue culture to Vitis. According to our can be useful in mass production of virus free fruit trees. Y. Ohta, : As you it. the of virus free plants is commendable. However there are drawbacks too. For instance, virus free plants have to face the risk of reinfection which may cause more damage than otherwise, as shown in the case of the other if one gives the crop some immunity against more virulent virus them to mild or less virulent strains or even by into them virus particles, one could better as indicated in case of tomato and citrus. Answer: I in the case of citrus virus, virus free for the control of confering immunity to the crop disease and others, in which transmission is known to take there is no reinfection and results can be obtained. The IR 2 Certification Program) conducted in the USA or the EMLA scheme in