Races of Phytophthora sojae in Iowa Soybean Fields. X. B. Yang, Department of Plant Pathology, Iowa State University, Ames 50011. R. L. Ruff, X. Q. Meng, and F. Workneh, Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. 80:1418-1420. Accepted for publication 18 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1418.

During 1991 to 1994, Phytophthora sojae was isolated from plants or soils in 50 Iowa counties. Eighty-two isolates from soil and 124 isolates from diseased soybean plants were collected and tested. Of the 206 isolates tested, 178 were classified into races; the other 28 isolates could not be classified due to intermediate reactions. Isolates represented races 1, 2, 3, 4, 8, 13, 15, and 25. Race 3 was the dominant race from samples of diseased plants, consisting of 30 and 50% of isolates in 1991 and 1994, respectively. Race 1 was the dominant race from soil samples, ranging from 34 to 45% over the years. One new race (virulence formula = la,lc,ld,lk,7) was identified. Our results suggest that P. sojae has greater diversity in virulence than in 1966 when race I was first found in Iowa.