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Differential Reactions of Soybean Genotypes to Isolates of Phialophora gregata. G. R. Sills, Graduate Research Assistant, Department of Agronomy, University of Wisconsin-Madison, Madison 53706. E. T. Gritton, and C. R. Grau. Professor, Department of Agronomy; and Professor, Department of Plant Pathology, University of Wisconsin-Madison, Madison 53706. Plant Dis. 75:687-690. Accepted for publication 30 December 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0687.

Ten soybean (Glycine max) genotypes, three with the major genes for resistance (Rbs1, Rbs2, and Rbs3) and five that expressed resistant and two that expressed susceptible phenotypes in field trials, reacted differentially when inoculated by a root-dip technique with six single-spore isolates of Phialophora gregata obtained from a single field. Disease severity was determined by the percentage of internal stem discoloration (PISD) and percentage of necrotic leaves (PNL). Each isolate, obtained from symptomatic, field-grown plants, caused internal stem discoloration and leaf necrosis in susceptible cultivars. An important conclusion from this this study is that isolates of P. gregata from the same field differed for virulence against genotypes of soybean.