Biological and molecular detection of toxic lipodepsipeptide-producing pseudomonas syringae strains and PCR identification in plants

Toxin-based identification procedures are useful for differentiating Pseudomonas syringae pathovars. A biological test on peptone-glucose-NaCl agar in which the yeast Rhodotorula pilimanae was used proved to be more reliable for detecting lipodepsipeptide-producing strains of P. syringae than the more usual test on potato dextrose agar in which Geotrichum candidum is used. A PCR test performed with primers designed to amplify a 1, 040-bp fragment in the coding sequence of the syrD gene, which was assumed to be involved in syringomycin and syringopeptin secretion, efficiently detected the gene in pathovars that produce the lipodepsipeptides. Comparable results were obtained in both tests performed with strains of the syringomycin-producing organisms P. syringae pv. syringae, P. syringae pv. atrofaciens, and P. syringae pv. aptata, but the PCR test failed with a syringotoxin-producing Pseudomonas fuscovaginae strain. The specificity of the test was verified by obtaining negative PCR test results for related pathovars or species that do not produce the toxic lipodepsipeptides. P. syringae pv. syringae was detected repeatedly in liquid medium inoculated with diseased vegetative tissue and assayed by the PCR test. Our procedure was also adapted to detect P. syringae pv. morsprunorum with a cfl gene-based PCR test.