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Effect of Plant Species and Environmental Conditions on Epiphytic Population Sizes of Pseudomonas syringae and other Bacteria. R. D. O’Brien, Department of Plant Pathology, University of California, Berkeley 94720, Present address: Department of Plant Pathology, University of California, Davis 95616; S. E. Lindow, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 79:619-627. Accepted for publication 21 November 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-619.

Selected biological and environmental effects influenced epiphytic colonization of plants by Pseudomonas syringae, Escherichia coli, Salmonella typhimurium, Aeromonas hydrophila, and Rhizobium meliloti when tested in a growth chamber at 24 C. Epiphytic population size varied with plant host, environmental conditions, and among strains of P. syringae tested. Strains of P. syringae achieved only slightly larger population sizes than strains from other genera when incubated on inoculated plants for 48 hr, and near 100% relative humidity (RH). However, the strains of P. syringae maintained populations at least 25 times higher after a subsequent 72 hr at 40% RH. Epiphytic population sizes of 15 different strains of P. syringae varied up to 10-fold on a given plant species, indicating epiphytic diversity within this bacterial species. Relative population sizes of three strains of P. syringae on plants under field conditions were predicted by growth chamber populations. Neither epiphytic strains, pathogenic strains, or toxin producing groups were associated with greater epiphytic population sizes. Different plant species varied up to 17-fold in the sizes of bacterial populations supported. Maceration of inoculated plant tissue increased bacterial population size estimates relative to cells removed by sonication, but only after low RH incubations.