Rootstocks and environment: influence on Liberibacter and Diaphorina citri multiplication, new flush dynamics, and on damages induced in citrus tree by Huanglongbing

Abstract

Huanglongbing (HLB) is the most destructive citrus disease. In Brasil it is associated with the bacteria Candidatus Liberibacter asiaticus and Ca. L. americanus, which colonize the phloem and are disseminated by the psylid Diaphorina citri, an insect highly prolific and of difficult control. All commercial citrus is susceptible to HLB and there is no cure for a diseased tree. Therefore, the only way to suppress disease spread is by reducing sources of inoculum and vector populations on a regional basis. This can be obtained through elimination of symptomatic trees and application of insecticides in large areas. Contrary to what has been observed with insecticide application, tree elimination, for obvious reasons, is not well accepted by the grower. As a result, infected trees remain in the field contributing to increase disease losses and pathogen dissemination. The development of genetically modified citrus (GMC) resistant to the pathogen is, therefore, anxiously expected as the most promising effective alternative to fight against the disease. But there is a consensus among specialists that even when resistant GMC is available the management of HLB will still continue relying on inoculum reduction and vector control. The reason is the high probability that the resistance eventually incorporated in citrus will not be complete. And even if it is complete, under high inoculum and vector pressures, its life span might be relatively short due to the selection of Liberibacter strains virulent to the new citrus material. Therefore, the current management practices need improvements and new strategies to fight against HLB must be pursued, which can only be obtained, as emphasized recently (Chiyaka et al 2012, PNAS 109:12213-12218), with a better understanding of all components of the HLB pathosystem - the host plant (a rootstock-scion combination), the pathogen, the vector, and the environment. Concerning the host plant, the investigation has so far been focused on the rootstock. Studies in Florida indicate the existence of rootstocks that promote considerable vigor to the scion, including those with HLB. Not clear is how this phenomenon works, if by increasing root development or reducing plant colonization by the pathogen. On an epidemiological perspective, we need to consider however a possible indirect influence of the rootstock also on the vector. By changing the dynamics of new flush growth on the scion (the preferable sites for D. citri feeding and laying eggs), the rootstock can make a tree more or less favorable to insect multiplication and, at the same time, more or less vulnerable to infection. Regarding the environment, studies have focused on the temperature, which may affect the host, the vector, and the pathogen. Milder temperatures favor symptom expression (more intense in fall and winter) while higher temperatures favor D. citri growth and, depending on the intensity and duration, can be detrimental to both Liberibacters. Not known is the influence of the temperature on pathogen multiplication inside the insect vector, as well as on Liberibacter establishment on a new flush. The detrimental effect of higher temperatures on Liberibacter helps to explain the irregular progress of HLB in São Paulo state. From the central region where the disease was first reported and when it reached the highest incidences, HLB spread more slowly to regions of hotter summers, despite the proximity of susceptible tissues (provided by the proximity of citrus farms) and the presence of the vector favoring uniform disease spread to all regions. This proposal was then planned to generate information for a better understanding of the influence of the rootstock and the environment on Liberibacter and D. citri multiplication, on the development of new flushes on the scion, and on damages induced in a citrus tree by HLB. (AU)