Low Incidence of Huanglongbing Fruit Symptoms in Valencia Sweet Orange Trees in the Presence of a Population of Citrus Tristeza Virus

Abstract

Huanglongbing (causal agent is "Candidatus Liberibacter africanus") (HLB) remains a limiting factor for citrus production in the cooler areas of Southern Africa notwithstanding present control measures. These include the establishment of new orchards with certified plant material, chemical control of the insect vector, Trioza erytreae, and the removal of infected plant material. Delta Valencia on Yuma citrange rootstock was planted in 1985 with GXI [greening cross-protecting isolate, formerly citrus dwarfing isolate 4 (CD 4)] as one of the treatments for dwarfing. HLB was first observed in 1988 and by 1990 fruit symptoms had increased to 100% in some trees which forced the termination of the dwarfing trial. Infected branches were removed by pruning and HLB continued to be monitored in 1991, 1993, 1996 and 1998 in the control trees and those with GXI. The percentage HLB fruit symptoms remained at a low level in the trees with GXI. Several attempts were made to identify the agent(s) that is present in GXI. Biological indexing was employed for citrus viroids, citrus tristeza, citrus psorosis, citrus impietratura and citrus tatter leaf. Only citrus tristeza virus (CTV) was found to be present in this isolate. Mass and single aphids were used to transfer the CTV component from GXI. The massaphid as well as the single-aphid sub-isolates reacted differently on Mexican lime, suggesting different strains within the GXI isolate. Differentiation between sub-isolates was also shown by amplifying the coat protein gene by the reverse transcriptase polymerase chain reaction (RT-PCR) followed by restriction fragment length polymorphism (RFLP) and single-strand conformational polymorphism (SSCP) techniques.