The effect of drought and ring nematode in predisposing Prunus domestica (plum) to infection by Pseudomonas syringae

Abstract

Bacterial canker is among the most common bacterial diseases affecting the production of stone fruit trees in South Africa. As reported in previous studies, the causal agents associated with bacterial canker of stone fruit trees include Pseudomonas syringae pv. syringae and P. syringae pv. morsprunorum. Since these studies date back to the1980s, a current study is required to investigate which causal agents are responsible for bacterial canker of stone fruit trees in South Africa. In addition, plum tree decline was reported in the Western Cape late 2015. The cause of the tree deaths was unknown as dying plum trees were of different ages and cultivars. The assumption was that several factors were responsible for the plum tree mortality including bacterial canker, drought and ring nematodes. Anecdotal evidence from a report in Franschhoek in the previous year suggested that ring nematodes were linked to a bacterial canker outbreak which resulted in plum tree mortality. Therefore, the overall objective in this study was to determine the effect of drought and ring nematode stresses in predisposing plum trees to bacterial canker infection. Firstly, the causal agents of bacterial canker of stone fruit trees in the Western Cape were identified using a multilocus sequence typing approach based on four housekeeping loci. The strains isolated from symptomatic plant material of stone fruit trees were delineated into two phylogenetic groups, namely, P. syringae sensu stricto and P. viridiflava. The delimitation into the two respective genetic clusters was further corroborated by whole genome-based analyses. Furthermore, the Pseudomonas strains in the present study were found to have a relatively increasing pangenome comprising a small set of core genes and a large accessory genome. These strains also consisted of a small T3SE repertoire complemented by detection of additional virulence factors, such as biosynthetic clusters of three phytotoxins, i.e. phaseolotoxin, syringomycin and syringopeptin. Most of the Pseudomonas strains comprised a locus encoding an insecticidal toxin indicating a potential entomopathogenic ability in these strains that is yet to be proven. A disease survey was also conducted in 12 major plum-producing farms in the Western Cape evaluating the link between high ring nematode densities and bacterial canker outbreaks. The findings from the survey revealed a weak to no association between high ring nematode densities and bacterial canker infection. On the other hand, drought was shown to significantly influence both the ring nematode densities and the occurrence of bacterial canker infection. In contrast, the greenhouse study undertaken in this study showed that drought did not influence the occurrence of bacterial canker infection as lesion development of inoculated plum seedlings of two cultivars did not significantly progress under soil water deficit. However, the physiology of these plum seedlings was altered under water stress as illustrated by reduced net rate of CO2 assimilation, which reduces the photosynthetic rate in these seedlings. This study has shown that drought and ring nematode had very little impact on bacterial canker infection. It is now apparent that additional factors also contributed to the plum tree decline.