The Effect of Pruning, a Citrus tristeza virus Isolate and a Citrus Viroid Isolate on Huanglongbing Infection

Abstract

High density planting of citrus is a common practice in South Africa. To prevent overcrowding, the trees have to be shaped and pruned. The effect of this practice on Huanglong-bing (HLB) infection in an area where the disease is present was investigated. Virus-free Delta Valencia trees on Troyer citrange rootstock were grown under insect-free conditions and pre-inocu-lated with LMS 6 (the experimental control, a standard CTV cross protecting isolate for the industry), CD 6 (a citrus viroid dwarfing agent) and CD 4 (HLB cross-protecting CTV isolate) before planting in the field in 1996. The control treatment was planted at a 6 × 3 meter density while the other treatments were planted at 6 × 2 meters. Pruning or tree shaping treatments were applied to trees pre-inoculated with LMS 6 and CD 4 at high density. The standard vector control procedures were followed in the field. Over a 4-yr period, trees with CD 4 at high density without pruning achieved the highest production but this was not significantly better than the control trees. Pruning decreased production by 30% and 43% for trees with LMS 6 and CD 4 respectively. Production of the trees with CD 6 was 23% lower than that of the control trees but it had the lowest HLB infection (18%). HLB infection increased where pruning was applied in comparison with un-pruned trees with the same isolates (118% with LMS 6 and 46% with CD 4). CD 4 reduced HLB infection in pruned trees significantly in comparison with pruned trees with the control CTV isolate.