Phenotypic characterization and phylogenetic analysis of Pseudomonas syringae strains associated with canker disease on apricot in Iran within the context of the global genetic diversity of the P. syringae complex

Abstract

Bacterial canker of stone fruit is a destructive disease of Prunus species. Here we characterized five Pseudomonas syringae (Ps) strains isolated from bacterial cankers of stone fruits in East Azerbaijan province, Iran. Koch’s postulates were performed demonstrating pathogenicity on apricot cuttings. Ice-nucleation activity and syringomycin synthesis were determined phenotypically and confirmed molecularly in all five strains. Strains were variably resistant to copper. The strains were then compared to a sample of 227 strains of the Ps species complex using multilocus sequence analysis (MLSA) of the four housekeeping genes gap1, gltA, gyrB, and rpoD. The concatenated sequences of the four genes provided a deeply-resolved phylogeny. Iranian strains all belonged to Ps phylogroup 2 but clustered into three separate sub-groups. They had the greatest similarity to strains isolated from stone fruit trees in the USA, UK, and Lebanon but also to strains isolated from other plant species and the water cycle. We conclude that canker of stone fruit is caused by a diverse group of Ps phylogroup 2 strains with global distribution. Further sampling, host range testing, and genome sequencing will be needed to get further insight into the relative role of the water cycle and other modes of long distance movement in the dissemination of Ps strains that cause bacterial canker of stone fruit.