Effects of Citrus Tristeza Virus Isolates and a Citrus Viroid Isolate on Growth and Production of Delta Valencia on Yuma Citrange Rootstock

Delta Valencia trees on Yuma citrange rootstock were inoculated respectively with two mild (GFMS 12 and T55), an intermediate (GFMS 10) and a severe (GFSS 1) citrus tristeza virus (CTV) isolate prior to planting in the field. The same CTV isolates were also inoculated in combination with a citrus viroid isolate belonging to Group I11 of the citrus viroids (CVd). A virus-free control was included in the trial. All the CTV isolates were without the Seedling Yellows component of tristeza. Seven years after planting, canopy volumes of the trees with the two mild isolates and the control were smaller than those of trees with the intermediate and severe isolates. This was in contradiction to what was expected. Overall, the CVd isolate had an additional reducing effect on canopy size, but only those trees with mild isolate T55 and severe isolate GFSS 1 were significantly affected. Production on a per tree basis was according to the canopy sizes, thus, trees carrying the intermediate CTV isolate were the highest producers. The production efficiency (kgIm3 canopy), however, did not differ among trees with the CTV isolates and the control. The CVd isolate generally increased the production efficiency. The internal quality of the fruit was not affected by any treatment. Yuma citrange is not currently recommended as a commercial rootstock in South Africa, but, because of the dwarfing effect this rootstock has on scions, it has potential where high density plantings are being considered (7,14). Trees on this rootstock are not dwarfed in California and Florida and the dwarfing reported in South Africa (14) was ascribed to the rootstock's sensitivity to the endemic citrus tristeza virus (CTV) (10). No known viroids could be detected in these dwarfed trees using biological methods (9) or biochemically (4) by sequential polyacrylamite gel electrophoresis (sPAGE) of nucleic extracts (7, van Vuuren and da Graga unpublished data). The application of the biocharacterization index for CTV (5) revealed that the dwarfed trees were infected with an isolate more severe than the standard CTV isolate used for pre-immunization (van Vuuren and da Graga, unpublished data). This paper discusses the influence of mild and severe CTV isolates, and a viroid, on the growth and production of Valencia trees on Yuma citrange rootstock. MATERIALS AND METHODS Virus-free Delta Valencia trees on Yuma citrange rootstock were grown according to normal practices under insect-free conditions in a greenhouse. When the scions had developed to approximately 30 cm, they were inoculated with buds infected with CTV and a citrus viroid (CVd), singly or in combination. Three South African CTV isolates, GFMS 12, GFMS 10 and GFSS 1, and isolate T55 from Florida, USA, were used. The virulence of the isolates differed and their biocharacterization index values according to Garnsey et al. (5) were: T55 = 0.5 (ultra mild); GFMS 12 = 6.4 (mild); GFMS 10 = 13.7 (intermediate); GFSS 1 = 30.5 (severe). None of the isolates caused stem pitting in sweet orange and they were all free from the Seedling Yellows (SY) component of CTV. The CVd isolate (CD 12) was CTVfree, gave a mild reaction on Etrog citron and according to sPAGE results, belongs to Group I11 of citrus viroids. Three mo. were allowed for the CTV and CVd to distribute through the plants whereafter they Thirteenth IOCV Conference, 1996Citrus Tristeza Virus 159 were planted in an orchard and normal practices were applied. Trees planted virus-free and trees with the CVd only, served as controls. Each treatment was replicated five times in a randomized block design with single tree plots. Tree volume measurements were made annually (2). Each year, all fruits were harvested, sized and weighed. Samples of fruit were taken to determine the percentage juice, soluble solids and acid. External and internal fruit quality was assessed according to export standards (1). The trees were inspected for CTV and CVd disease symptoms. Data were analyzed as for a two-factor experiment according to Rayner