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Rapid Evaluation of Pathogenicity in Pseudomonas syringae pv. syringae with a Lilac Tissue Culture Bioassay and Syringomycin DNA Probes

Losses from diseases caused by Pseudomonas syringae pv. syringae occur on a large number of deciduous woody plants in commercial nurseries in the Pacific Northwest. Bioassays for pathogenicity are one step in the identification of P. syringae pv. syringae and are usually performed on the host of isolation; however, woody plants can take months to develop symptoms. A bioassay with highly susceptible lilac (Syringa vulgaris ‘Sensation’) tissue culture plantlets evaluated pathogenicity in strains of P. syringae pv. syringae isolated from 25 species of deciduous woody plants. DNA colony hybridization with the syrB probe for a syringomycin synthetase gene and the syrD probe for a syringomycin export gene was also evaluated as a method for identifying pathogens. Of 552 strains provisionally identified as P. syringae pv. syringae, 59% were pathogenic in the bioassay and hybridized with the syr probes, while 19% were non-pathogenic and did not hybridize with the syr probes, giving 78% agreement between the two methods. Nine percent of strains were pathogenic in the bioassay but did not hybridize with the syr probes, and 13% were not pathogenic in the bioassay but did hybridize with the syr probes. These methods detected pathogenic strains of P. syringae pv. syringae isolated from diverse woody plants in 5 to 16 days.