Activation of systemic acquired resistance in citrus to control huanglongbing disease

Abstract

Huanglongbing (HLB) is considered the most destructive citrus disease and occurs in the major citrus producing areas around the world. In Brazil, HLB is associated with the bacteria ‘Candidatus’ Liberibacter asiaticus (CLas) and ‘Candidatus’ Liberibacter americanus (CLam). CLas is the most aggressive and widespread species of the HLB bacteria. Currently, there is no curative control measure for this disease, and the eradication of diseased trees and control of the insect vector are the basic management measures for HLB. The search for other measures with higher efficiency and lower economic and environmental impacts for control of HLB has become a research priority. The use of chemicals that activate plant resistance mechanisms is a novel method for disease control. Thus, the objective of this study was to investigate the use of systemic acquired resistance (SAR) inducers for control of HLB. Nursery trees of Valencia sweet orange (Citrus sinensis (L.) Osbeck) grafted on Rangpur lime (Citrus limonia Osbeck) were inoculated with CLas by bud grafting technique using diseased material from HLB citrus trees, seven days after the first treatment with the SAR inducers, under greenhouse conditions. The SAR inducers imidacloprid (IMI), thiamethoxan (TMX), or acibenzolar-S-methyl (ASM), as well as the combination of ASM with IMI or TMX, were applied as soil drench. The nursery trees were evaluated according to the presence of HLB symptoms and the bacterium in leaf tissue. The presence of CLas was determined by using the PCR technique, at 120, 180, 240, and 300 days after the HLB bacterium inoculation. The first symptoms of HLB were observed on mature leaves of the check citrus trees 60 days before these symptoms were observed on trees treated with the SAR inducers. Two years after inoculation, 67% fewer citrus trees with HLB symptoms were observed for the ASM + TMX treated trees as compared to the check ones. Citrus trees treated with IMI, TMX, or ASM also provided lower levels of PCR-detected HLB bacterium compared to the untreated trees.