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Epiphytic Populations of Pseudomonas syringae on Susceptible and Resistant Bean Lines. Margaret E. Daub, Department of Plant Pathology, University of Wisconsin, Madison 53706, Present address: Department of Crop and Soil Sciences, Michigan State University, East Lansing 48824; D. J. Hagedorn, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 71:547-550. Accepted for publication 4 November 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-547.

Large differences were found in populations of virulent Pseudomonas syringae (cause of bacterial brown spot of bean) on resistant and susceptible bean lines in the field. About 106 cells per gram fresh weight were isolated from leaves of the susceptible cultivar Eagle compared to about 103 cells per gram fresh weight from leaves of the resistant plant introduction, WBR 133. Epiphytic populations on resistant breeding lines were intermediate to those on cultivars Eagle and WBR 133. These lines were also intermediate in reaction to the brown spot pathogen. There was, however, no direct correlation between populations of P. syringae and brown spot disease severity on the breeding lines. P. syringae was identified by morphology on Crosse’s medium and by production of typical brown spot symptoms on bean pods in two kinds of pod inoculation tests. Lower populations on the resistant lines did not appear to be due to the presence of antagonistic epiphytic microorganisms.