Effects of Citrus tristeza virus Isolates on Two Tolerant Commercial Scionson Different Rootstocks in South Africa

The effect of Citrus tristeza virus (CTV) isolates on Palmer navel and Delta seedless Valencia on five rootstocks, was evaluated for 8 yr (five were production years) regarding growth, production, fruit quality and health. The two scions were budded on rough lemon, Volkamer lemon, Troyer citrange, Yuma citrange and Gou Tou rootstocks. After the scions had grown, they were bud-inoculated with five mild isolates of CTV (GFMS 10, GFMS 12, T55, ST and Micveh T). Controls included a severe isolate (GFSS 1) and trees left uninoculated. ELISA was used to confirm infection whereafter the trees were planted at two climatically distinct sites (Malelane: hot, humid; Nelspruit: intermediate) in a split plot design with six replicates. Overall, the ST isolate had the greatest effect on both scion cultivars at both sites. Trees with this isolate were significantly smaller than trees infected with isolates GFMS 10 (both sites), GFSS 1 (both sites), T55 (Malelane) and Micveh T (Nelspruit), as well as trees planted virus-free (both sites). ST reduced the size of trees on all rootstocks except Volkamer lemon (Malelane) and Rough lemon (Nelspruit). The cumulative yield of the navel trees at Malelane was significantly reduced by the ST isolate and it was lower than that of trees with all the isolates except trees with T55. Significantly higher yields were obtained with the following rootstock/CTV combinations: Rough lemon/Micveh T, Volkamer lemon/T55, Troyer citrange/GFMS 12, Yuma citrange/GFMS 10 and GFSS 1. No significant effect on cumulative yield of the navel trees was apparent among the CTV isolates at the Nelspruit site. With the Valencia trees, no significant overall effect of the isolates was obtained at either site. Effects on the individual rootstocks were mainly present where the trifoliate orange rootstocks were used, but the effect was less at Nelspruit. The greatest effect of CTV on rootstocks was with Troyer citrange at Malelane where tree size was reduced by 62% (Micveh T), 57% (ST) and 44% (GFSS 1). Detrimental effects include tree size reduction, fruit size reduction, decline, bud union defects and bark cracking of the rootstock. Failure of sour orange as a rootstock for most citrus cultivars in South Africa in 1896, is probably the earliest recorded evidence for the presence of Citrus tristeza virus (CTV) in this country, although it does not necessarily mean that South Africa is the country of origin (13, 26). The sour orange rootstock was abandoned because of quick decline, and replaced by tolerant rootstocks such as rough lemon (9). This practice is no solution for sensitive scion cultivars such as grapefruit and cross protection with mild isolates has been the most successful approach to reduce the effect of the disease (10, 21, 22, 25). Since the establishment of the South African citrus industry on tolerant rootstocks, it was generally accepted that CTV has no effect on sweet oranges and mandarins. This situation can be partly ascribed to nurserymen who unwittingly applied cross protection by selecting parent trees showing the best health and production. These trees presumably carried strains of CTV that had the least effect on growth and production and protected against severe challenge. With the implementation of shoot-tip grafting, the virus-free material is vulnerable to infection by various strains, which are transmitted by aphids. Evidence of the presence of severe stem-pitting strains that can effect sweet orange exist in other countries (2, 15) as well as in South Africa (8). All citrus cultivars in the Southern African Citrus Improvement Program are freed from viruses and viroids by shoot-tip grafting (7). The abundance of the aphid vector, Toxoptera citricida (Kirk.), will result in virus-free trees becoming naturally infected with various strains (16) including virulent strains (2, 6, 12). 32 Fifteenth IOCV Conference, 2002—Citrus Tristeza Virus It is therefore necessary to protect the virus-freed plants from severe CTV strains by deliberately infecting them with mild strains (7, 24). The interaction of mild CTV isolates with regard to cross protection is specific with regard to biological activity (11, 22) and therefore, mild CTV isolates specifically for tolerant cultivars should be selected for cross-protection. This study was initiated to determine the effect of specific CTV isolates on commercial tolerant citrus cultivars regarding growth, production and fruit quality. MATERIALS AND METHODS Rootstocks. Rough lemon is the most commonly used rootstock in South Africa and the Wallace selection was used in this study. Volkamer lemon, Troyer citrange and Yuma citrange are equally used in the industry with the latter two gaining more interest. Yuma citrange is a semi-dwarf rootstock and it appears that transmissible pathogens (viruses and viroids) increase the dwarfing effect which makes it suitable for high density plantings (14). Gou Tou is a Chinese rootstock selection and it was included being a sour orange hybrid showing CTV tolerance and also tolerates higher levels of Phytophthora and citrus nematodes (20). Scions. Virus-free Palmer navel and Delta Valencia was used as scions for each rootstock. CTV isolates. The following CTV isolates which showed promise in previous trials were selected: GFMS 12 (the standard pre-immunizing isolate for sweet orange when the trial was initiated), GFMS 10, T55, ST, and Micveh T (17, 18, 19). Control treatments included a known severe isolate, GFSS 1, derived from grapefruit, and trees planted virus-free. Tree preparation. The rootstocks were grown under aphid-free conditions and budded according to normal nursery practices. When the scions were approximately 30 cm tall, they were bud-inoculated (two buds per tree) with the CTV isolates. The virus-free control trees were left uninoculated. Three months were allowed for systemic infection to develop, and this was confirmed by ELISA before the trees were planted in the field. Sites and lay-out. The trees were planted at Malelane (hot and humid) and Nelspruit (intermediate: between hot and cool) citrus production areas (3) according to a split-plot design with six replications and at a spacing of 7 × 3.5 m. Records. Growth of the trees was determined by measuring tree volumes according to Burger et al. (5). Fruit were sized according to export requirements and weighed (1).