Ecotypes and Pathogenicity of Ice-Nucleation-ActivePseudomonas syringaeIsolated from Deciduous Fruit Tree Orchards

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Abstract
Ice-nucleation-active (INA) strains of P. syringae were isolated as epiphytes from pome and stone fruit orchards in the Pacific Northwest [USA] (PNW). Fifty percent of the 82 strains were pathogenic in immature pear and sweet cherry fruit. Pathogenic strains isolated from trees in either pome or stone fruit orchards had a corresponding degree of virulence in the 2 hosts. Nearly all INA strains, however, induced hypersensitivity in tobacco and produced syringomycin. An INA strain of P. syringae pv. syringae from pear colonized inoculated apricot trees, attaining 108 to 109 colony-forming units per gram (fresh weight) of flowers at full bloom, and expressed an in vivo frequency of ice nucleation at 6 .times. 103 cells per -5.degree. C ice nucleus. These high populations were detected after flower infection which was mediated by damage from several mild frosts (i.e., -1.3.degree. to -4.7.degree. C). Bacteriocin and phage typing demonstrated no appreciable differences between pome and stone fruit INA P. syringae. All INA strains produced at least 1 bacteriocin and were subdivided into 11 producer groups; groups 6C, 8B, 8F and 13 contained 88% of the INA strains. Nine phage sensitivity groups were identified, and 73% of the strains were classified in either phage groups 1 or 2. Phages (12B, S3 and 017), which had been reported to specifically lyse pear strains of P. syringae pv. syringae were either weakly virulent or avirulent on INA strains isolated from trees in either pome or stone fruit orchards in the PNW. Phage typing differentiated PNW INA strains from most strains from England whereas bacteriocin typing differentiated them from most California strains. Therefore, at least 3 major ecotypes of INA P. syringae were discerned.