Races of Phytophthora sojae in Iowa soybean fields

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 = 1a,1c,1d,1k,7) was identified. Our results suggest that P. sojae has greater diversity in virulence than in 1966 when race 1 was first found in Iowa.