Phytophthora root rot in soybean is an important disease in Iowa. The disease caused severe crop damage and lowered soybean production in Iowa during the 1980s. In recent years, it has not caused severe damage because of well-developed resistance breeding programs in Iowa and the rest of the North Central Region. Currently, 42 percent of varieties used in Iowa carry phytophthora resistance genes.

Phytophthora has different races that can defeat different resistance genes of soybean. The phytoph-thora races in Iowa are monitored through a disease monitoring program at ISU, which is supported by the Iowa Soybean Promotion Board. Last year, we identified 41 phytophthora isolates into races. It was found that 14 isolates were race 1, 12 were race 3, 13 were race 4, and 2 were race 25. If there is a history of phytophthora in a field and you want to use resistant varieties, choose varieties resistant to major races in Iowa (races 1, 3, and 4).

We should be alert for the occurrence of race 25, which was found for the first time in Iowa in Humboldt and Webster counties. Race 25 was first identified a few years ago in other states. Its population increased rapidly in other states according to the 1994 reports to the North Central Region-137 Soybean Disease Committee. Currently most soybean varieties in Iowa use resistance genes, Rps1, Rps1c, and Rps1k. Race 25 can defeat all these genes. Table 1 lists the responses of resistance genes to the four races in Iowa. Very few newly released varieties have the Rps6 gene, which is resistant to all current races.

Although the population of race 25 is still very low, we should keep watching for this new race. If you have soybean plants infected by phytophthora, you may send the plants to us to help our disease monitoring program. Send plants to the Plant Disease Clinic, Department of Plant Pathology, Iowa State University, Ames, Iowa 50011.