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

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... FSS 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.