Soybean Phytophthora root rot is a significant disease in Iowa and its management relies on the use of resistant varieties. However, the Phytophthora fungi has developed many races that can defeat different resistance genes.
Phytophthora races in Iowa are identified by a disease-monitoring program at Iowa State University supported by check-off dollars. In late 1995 and early 1996, we tested isolates collected during the 1995 season. Our major findings are: 1) Races 1, 3, and 4, are still dominant in Iowa; 2) Race 25 was detected at a higher level than in 1994; 3) A race new to Iowa was found in a sample from Humboldt County.
Plants with Phytophthora root rot.
Results of our testing showed a slowly changing picture of Phytophthora races in Iowa. Compared to two years ago, we found more samples were either new races or race 25. This is because wide use of Phytophthora-resistant varieties produces pressure on the fungi, thus resulting in a change in races. Currently, more than 43 percent of the cultivars or brands used in Iowa have genes that give resistance to Phytophthora root rot. Of the 168 Phytophthora-resistant varieties used in Iowa in 1995, 82 had the Rps1-k gene.
Currently, most soybean varieties in Iowa use resistance genes Rps1, Rps1-c, and/or Rps1-k. Race 25 can defeat all of these genes. The table gives the responses of resistance genes to the four races in Iowa. If your resistant soybean varieties are infected by Phytophthora, you may want to send the plants to us, especially those with the Rps1-k gene. Your samples help our disease-monitoring program.
This article originally appeared on page 113 of the IC-476(16) -- July 1, 1996 issue.