PHYTOPHTHORA-SOJAE RACES IN OHIO OVER A 10-YEAR INTERVAL

During 1978 and 1980, 199 isolates of Phytophthora 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 Rps1-k resistance gene. All isolates were identified as to their race phenotypes by inoculation of differential soybean cultivars having Rps1-a, Rps1-b, Rps1-c, Rps1-d, Rps1-k, Rps3-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 Rps1-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.