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 le af disk bait procedure, and 53 isolates were obtained from diseased so ybean plants. During 1990 and 1991, 282 isolates of P. sojae were obta ined from diseased soybean cv. Sloan or Amcor 89 seedlings planted in soil from 88 fields in northwest Ohio, and 27 isolates were obtained f rom field-grown plants with the Rps1-k resistance gene. All isolates w ere identified as to their race phenotypes by inoculation of different ial soybean cultivars having Rps1-a, Rps1-b, Rps1-c, Rps1-d, Rps1-k, R ps3-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 race s 9 and 3. In 1990, race 3 was most prevalent, followed by race 7, rac e 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, t he major gene used for control of Phytophthora rot of soybean in the n orthern midwest. Race numbers for five of these new races are proposed . It is concluded that race phenotypes that can defeat all existing Rp s genes or gene combinations now available for control of Phytophthora rot are already present in soil. Time course for increase of these ne w races and alternative sources of resistance are discussed.