Characterisation of Pseudomonas syringae isolated from systemic infection of zucchini in Australia.

Zucchini plants, with symptoms including twisted petioles, necrotic leaves, crown-rot and internal fruit-rot, were found in Bundaberg, Australia at a commercial field for the first time during late autumn 2016, resulting in direct yield losses of 70 to 80%. Three Pseudomonas syringae strains that isolated from symptomatic leaf (KL004-k1), crown (77-4C) and fruit (KFR003-1) were characterised and their pathogenicity evaluated on pumpkin, rockmelon, squash and zucchini. Biochemical assays showed typical results for P. syringae. The three isolates differed, however, in that two produced fluorescent pigment (KFR003-1 and 77-4C) whilst the third, KL004-k1, was non-florescent. Multi-locus sequence analysis classified the isolates to phylogroup 2b. The SNP analysis of core genome from the Australian and closely related international isolates of P. syringae showed two separate clusters. The Australian isolates were clustered based on fluorescent phenotype. Pathogenicity tests demonstrated all three isolates moved systemically within the inoculated plants and induced necrotic leaf symptoms in zucchini plants. Their identities were confirmed with specific PCR assays for P. syringae and phylogroup 2. Pathogenicity experiments also showed the Eva variety of zucchini was more susceptible than Rosa for all three isolates. Isolate KL004-k1 was more virulent than 77-4C on pumpkin, rockmelon, squash and zucchini. This study expands the knowledge of P. syringae isolates that infect cucurbits and provides useful information for growers about the relative susceptibility of a range of cucurbit species.