Zucchini (Cucurbita pepo) is worldwide affected by Pseudomonas syringae, inducing vein clearing, stunting, and necroses during plantlet development. A collection of 58 P. syringae strains isolated from diseased zucchini plantlets was characterized by multilocus sequence analysis (MLSA). A subset of 23 strains responsible for vein clearing of zucchini (VCZ) was evaluated for pathogenicity on zucchini, and their genomes were sequenced. The host range of six VCZ strains was evaluated on 11 cucurbit species. Most VCZ strains belong to clades 2a and 2b-a within phylogroup 2 of P. syringae species complex and are closely related to other strains previously isolated from cucurbits. Genome analyses revealed diversity among VCZ strains within each clade. One main cluster, once referred to by the invalid pathovar name (peponis), gathers VCZ strains presenting a narrow host range including zucchini and squashes. Other VCZ strains present a large host range including zucchini, squashes, cucumber, melons, and in some cases watermelon. The VCZ strain pathogenic features are strongly associated with type III effector repertoires. The presence of avrRpt2 and absence of hopZ5 are associated with a narrow host range, whereas the presence of hopZ5 and absence of avrRpt2 are most generally associated with a large host range. To better detect the different clusters identified with whole genome sequence and pathogenicity analyses, we used a specific-k-mers approach to refine the MLSA scheme. Using this novel MLSA scheme to type P. syringae isolates from diseased cucurbits would give insight into distribution of worldwide strains and origin of epidemics.
Keywords: Cucurbita pepo; Pseudomonas syringae; bacterial seedborne disease; cucurbits; ecology and epidemiology; etiology.