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Widely Virulent Isolates of Phytophthora megasperma f. sp. glycinea Causing Root and Stem Rot of Soybeans in New York. P. W. Tooley, Research Associate, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. G. C. Bergstrom, Assistant Professor, Department of Plant Pathology, and M. J. Wright, Professor, Department of Agronomy, Cornell University, Ithaca, NY 14853. Plant Dis. 68:726-727. Accepted for publication 10 April 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-726.

Phytophthora megasperma f. sp. glycinea was confirmed for the first time in New York State on the soybean cultivar Amsoy 71. Five isolates obtained from a single field showed wide virulence on single gene differential soybean cultivars; only one isolate (race 8) could be classified using the existing numerical race designation scheme. Virulence formulae (effective/ineffective host genes) were used to describe the isolates. A multirace-resistant cultivar containing gene Rps1c had been grown the year before these widely virulent isolates were detected, revealing that rapid selection for virulence may occur.