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THERAPY and VARIETY IMPROVEMENT the Citrus Variety

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THERAPY and VARIETY IMPROVEMENT the Citrus Variety THERAPY AND VARIETY IMPROVEMENT The Citrus Variety Improvement Program in Spain after Eleven Years L. Navarro, J. Juarez, J. A. Pina, J. F. Ballester and J. M. Arregui ABSTRACT. The Citrus Variety Improvement Program in Spain (CVIPS) started in 1975 with the following objectives: a) to recover virus-free plants by shoot-tip grafting in vitro from cultivars selected in Spain; b) to release virus-free budwood to citrus nurseries and c) to establish a germplasm bank of healthy plants. In 1983 these objectives were expanded with the importation of citrus bud- wood from other areas through a quarantine station based on tissue culture in vitro techniques. Sixty- three species and varieties have been introduced through this quarantine station so far. Presently the CVIPS has 267 accessions and 112 have been freed of virus and virus-like diseases and are available to citrus nurseries. So far, over 12 million healthy trees from the CVIPS have been planted in commercial orchards. Index words. shoot-tip grafting in vitro, quarantine, viruses, indexing. Virus and virus-like diseases are 3) Release of healthy budwood to widespread in older Spanish citrus growers. plantings producing very important 4) Establishment of a citrus germ- economic losses to the citrus industry. plasm bank. Tristeza, exocortis, cachexia, concave gum, impietratura, ringspot, cris- RECOVERYOFHEALTHY tacortis, vein enation-woody gall, PLANTS SELECTED IN SPAIN crinkly leaf, and stubborn have been detected in commercial orchards (9), This part of the program is based where most trees are infected with on shoot-tip grafting in vitro (6, 12, more than one disease. 14), and it includes the following steps These diseases posed important (8, 9). limitations to new plantations, be- Selection of mother trees. Selec- cause virus-free budwood of Spanish tion of mother trees of each cultivar varieties was not available and virus is based on their horticultural perfor- and virus-like diseases restricted the mance regardless of virus content. use of many rootstocks (5). Commercial varieties were selected in To solve these problems the Cit- private orchards, whereas other vari- rus Variety Improvement Program in eties were selected in citrus variety Spain (CVIPS) was established in collections. They include varieties 1975 (4, 8, 9) with the general objec- presently grown in Spain, old vari- tive of producing virus-free, true-to- eties not currently grown commer- type plants of a wide range of species cially, new varieties obtained by and varieties for commercial propaga- spontaneous bud mutations in the tion and research purposes. field, and foreign varieties introduced Presently, the CVIPS has the fol- in to Spain before the establishment lowing activities: of the CVIPS (often without adequate sanitary control). Presently, 204 ac- 1) Recovery of bealthy plants cessions have been included in the selected in Spain. program (table 1). 2) Introduction of foreign species Indexing of mother trees. Index- and varieties through the citrus ing is performed by graft inoculation quarantine station. to the following indicator plants: Therapy and Variety Improvement TABLE 1 ACCESSIONS OF THE CITRUS GERMPLASM BANK OF INSTITUTO VALENCIANO DE INVESTIGACIONES AGRARIAS (IVIA) No. of accessions Selected Introduced Genus Species in Spain by quarantine Total Citrus amblycarpa aurantifolia aurantium bergamia clementina deliciosa depressa excelsa gmndis halimii hystrix ichangiensis karna latifolia limettioides limon macroptern. madurensis medica meyeri myrtifolia nobilis paradisi shunkokan sinensis sunki tachibana tangerina temple unshiu webberi hybrids Afraegle paniculata Atalantia bilocularis citroides ceylanica Eremocitrus glauca Fortunel la crassifolia hindsii margarita Glycosmis pentaphylla Microcitrus austmlasica Murraya paniculata Pamburus misionis Poncirus trifoliata Severinia buxifolia Triphasia trifolia TOTAL 'Introduced by seed YIntroduced by budwood Arizona 861-S-1 citron for detection of for citrus tristeza virus (CTV) and citrus exocortis viroid (CEV); Par- vein enation; Pineapple sweet Orange son's special mandarin for citrus and Dweet tangor for psorosis, con- cachexia viroid (CCaV); Mexican lime cave gum, impietratura and cristacor- 402 Tenth ZOCV Conference tis; Eureka lemon or Arizona 861-S-1 plants (psorosis, concave gum, im- citron for crinkly leaf, and Madam pietratura and cristacortis) are very Vinous sweet orange for stubborn. common on mandarins and sweet Inoculated plants of Pineapple sweet oranges, but are rare in lemons. Vein orange and Dweet tangor are chal- enation is widespread in sweet lenge-inoculated with psorosis lesion oranges, and also found in lemons and bark inoculum (13) to determine the mandarins. Citrus tristeza virus was presence of psorosis A. found only in mandarins; although Indicator plants for detection of this disease is widespread in Spain. CEV, CCaV and stubborn are incu- Declining trees were not selected for bated in a warm greenhouse at 27-32 the Program. C, whereas indicator plants for detec- None of the Spanish varieties of tion of the other diseases are incu- the CVIPS were virus-free. Manda- bated in a cooler greenhouse at 18-25 rins had the highest rate of infection, C. followed by sweet oranges and le- Table 2 shows a summary of in- mons. All the mandarins of the dexing results for virus and virus-like CVIPS had at least two diseases, and diseases of mother trees of sweet 87% has three or more diseases. Sixty- oranges, mandarins and lemons of three per cent of the sweet oranges Spanish origin. The exocortis viroids were also infected by three or more (2) are the most common pathogens. diseases. Lemons are usually infected Only two sweet orange mother trees by one or two diseases. These high were found to be free of them. One rates of infection have been found in was a large and productive old Wash- selected trees that, in general, did not ington navel tree, that probably is a show severe disease symptoms in the very early introduction of the Wash- field. Most trees in commercial or- ington Parent navel from California. chards are in worse condition, par- The other one is of Foia Menuda, an ticularly due to tristeza and psorosis. old variety located on the island of These data clearly suggest the very Majorca. All plants found infected important economic losses produced with cachexia also were infected with by virus and virus-like diseases to the exocortis, except a Cajel sour orange Spanish citrus industry. which is the only plant found so far in Shoot-tip grafting (STG) "in Spain infected only with citrus vitro". The technique applied is the cachexia viroid. Cachexia is also quite standard procedure for STG (6, 12). widespread, particularly in manda- The infected varieties are propagated rins. The diseases producing psorosis- on vigorous rootstocks in the green- like young leaf symptoms on indicator house. When the resulting plants are TABLE 2 INCIDENCE OF VIRUS AND VIRUS-LIKE DISEASES IN THE VARIETIES OF SPANISH ORIGIN INCLUDED IN THE CITRUS VARIETY IMPROVEMENT PROGRAM IN SPAIN (CVIPS) Group of varieties" Disease Sweet orange Mandarins Lemons Exocortis 95 Cachexia 32 Psorosis 27 Concave gum Impietratura and/or CristacortisY 57 Vein Enation 75 Tristeza 0 "Data expressed as percentage of infected trees. These diseases are placed together because they produce similar symptoms on indicator plants. Therapy and Variety Improvement used for STG, they are completely de- characters between virus-free plants foliated by hand and placed in a and the original virus-infected plants growth chamber at a constant 32 C. of each cultivar. So far, the available Flushes produced after 10 to 15 days data indicate that the plants freed of are used as source of shoot tips. These disease-agents are true-to-type. are composed of the apical meristem and three leaf primordia, and meas- INTRODUCTION OF ure 0.1-0.2 mm. Troyer citrange is FOREIGN VARIETIES used as the rootstock for all species Although many citrus cultivars except lemons, which are grafted on are currently grown in Spain, intro- Etrog citron. Routinely, 60-70% duction of new species and cultivars grafting success is achieved and over from other areas is necessary to span 90% survival is usually obtained upon the production period. In addition, it transplanting. is also desirable to introduce many Indexing of micrografted plants. citrus species and varieties and citrus Plants obtained by STG are indexed relatives, to increase the germplasm by the same methods as described bank necessary for research work. above. Tests for pathogens producing However, the introduction of vegeta- psosoris-like young leaf symptoms are tive material may lead to the intro- repeated, because sometimes they duction of new pests and diseases. produce very mild symptoms which To solve this problem, a Citrus are difficult to detect. These tests are Quarantine Station was legally estab- usually conducted in spring, when lished in Spain in 1982 and a new tis- symptom expression is best. sue culture procedure was developed Following the pretreatment at 32 for citrus introduction (11). This pro- C previously indicated, more than cedure basically consists of culturing 90% of the plants recovered by STG the imported budwood in vitro at 32 were free of virus and virus-like C to induce formation of flushes, from pathogens, including psorosis, con- which shoot-tips are isolated and cave gum, cristacortis and impietrat- micrografted in vitro to obtain plants ura. These diseases were more dif- that are grown and indexed in a ficult to eliminate when flushes were quarantine greenhouse. produced in field trees or in plants This STG method has not been growing in greenhouses at 18-25 C or adapted yet for many citrus relatives, 27-32 C (10, 14). and these are introduced as seed. To Ninety accessions have been freed enhance seed germination, and to of virus and virus-like pathogens. exclude possible soilborne pathogens, They include all the cultivars pre- seed are germinated in vitro, accord- sently grown in Spain, and the most ing to the procedure used to grow promising new cultivars for commer- seedling rootstock for STG (6, 12).
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