Phytophthora sojae Races in Ohio Over a 10-Year Interval. A. F. SCHMITTHENNER, Professor, Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. M. HOBE, Former Graduate Research Associate, and R. G. BHAT, Research Associate, Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. Plant Dis. 78:269-276. Accepted for publication 19 November 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0269.

During 1978 and 1980, 199 isolates of Phylophthora sojae were obtained from soil samples from 16 fields in northwest Ohio using a soybean leaf disk bait procedure, and 53 isolates were obtained from diseased soybean plants. During 1990 and 1991, 282 isolates of P. sojae were obtained from diseased soybean cv. Sloan or Amcor 89 seedlings planted in soil from 88 fields in northwest Ohio, and 27 isolates were obtained from field-grown plants with the Rpsl-k resistance gene. All isolates were identified as to their race phenotypes by inoculation of differential soybean cultivars having Rpsl-a, Rps1-b, Rps1-c, Rps1-d, Rpsl-k, Rpsl-a, or Rps6 resistance genes. Rps7 also was used for the 1990 1991 isolates. Race 7 was the most prevalent in 1978 1980, followed by races 9 and 3. In 1990, race 3 was most prevalent, followed by race 7, race 4, and race 1. In 1991, race 3 was most prevalent, followed by race 7, race 9, and new races with varied phenotypes that could defeat Rps\-k. In 1978-1980, 1990, and 1991, 7.5, 11.7, and 18.2%, respectively, of the P. sojae isolated were new races capable of defeating Rps1-k, the major gene used for control of Phytophthora rot of soybean in the northern midwest. Race numbers for five of these new races are proposed. It is concluded that race phenotypes that can defeat all existing Rps genes or gene combinations now available for control of Phytophthora rot are already present in soil. Time course for increase of these new races and alternative sources of resistance are discussed.