VARIATION IN VIRULENCE, PLASMID CONTENT, AND GENES FOR CORONATINE SYNTHESIS BETWEEN PSEUDOMONAS-SYRINGAE PV MORSPRUNORUM AND P-S-SYRINGAE FROM PRUNUS

Strains of Pseudomonas syringae pv. morsprunorum and P. s. syringae from Prunus were examined for virulence to cherry, plasmid DNA content, and coronatine synthesis genes. Michigan strains of P. s. syringae spread faster through cherry rootstock plantlets than did strains of P. s. morsprunorum. Twenty-six strains of P. s. morsprunorum, regardless of geographic origin, contained three to seven plasmids of variable size. Twenty-two strains of P. s. syringae contained zero to two plasmids. Among 319 field strains of P. s. morsprunorum from cherry (Prunus cerasus and Prunus avium) orchards in Michigan, 98% contained genomic DNA that hybridized with two DNA probes for coronatine synthesis genes cloned from P. s. tomato. Conversely, genomic DNA from strains of P. s. syringae from cherry, except one strain, did not hybridize with either probe for coronatine synthesis genes. Southern analysis of plasmid DNA indicated that the genes for coronatine synthesis were located on the same 105-kb plasmid in Michigan strains of P. s. morsprunorum as in English strain PM567, which was used as a standard. In a bioassay for coronatine, strains of P. s. morsprunorum with homologous DNA for coronatine synthesis, but not strains lacking homologous DNA, induced hypertrophy on potato tuber slices. These differences in virulence, plasmid profile, and coronatine production provide further evidence that P. s. morsprunorum is distinct from stone fruit strains of P. s. syringae.