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Ecology and Epidemiology

Ecotypes and Pathogenicity of Ice-Nucleation-Active Pseudomonas syringae Isolated from Deciduous Fruit Tree Orchards. D. C. Gross, Department of Plant Pathology, Washington State University, Pullman 99164; Y. S. Cody(2), E. L. Proebsting, Jr.(3), G. K. Radamaker(4), and R. A. Spotts(5). (2)(4)Department of Plant Pathology, Washington State University, Pullman 99164; (3)Department of Horticulture, Irrigated Agricultural Research and Extension Center, Washington State University, Prosser 99350; (5)Department of Botany and Plant Pathology, Mid-Columbia Experimental Station, Oregon State University, Hood River 97031. Phytopathology 74:241-248. Accepted for publication 7 September 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-241.

Ice-nucleation-active (INA) strains of Pseudomonas syringae were isolated as epiphytes from pome and stone fruit orchards in the Pacific Northwest (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 two 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 × 10³ cells per - 5 C ice nucleus. These high populations were detected after flower infection which was mediated by damage from several mild frosts (ie, - 1.3 to - 4.7 C). Bacteriocin and phage typing demonstrated no appreciable differences between pome and stone fruit INA P. syringae. All INA strains produced at least one 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 ø17), 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 three major ecotypes of INA P. syringae were discerned.