As planting season approaches, it's time to think about Phytophthora fungus on soybeans that causes both root rot and seedling disease. Phytophthora continues to be a problem in Iowa despite the widespread availability of resistant varieties. The disease was first found in Indiana in 1948 and since has spread throughout soybean-growing regions in the U.S. and Canada. Before 1996, at least 40 races of Phytophthora had been identified, although there are only a few races of concern for Iowa soybean growers.

Researchers first found Race 1 of the Phytophthora root rot in Iowa during a disease survey in 1966. A recent report, published in the journal, Plant Disease, showed that Iowa State University researchers found four major races and several miscellaneous races in a survey conducted between 1991 and 1994. They collected 206 samples of the fungus from locations throughout the state. The fungus had been isolated from either soil samples or infected soybean plants. According to the results (see Table 1), 26 percent of the samples was Race 1, 39 percent was Race 3, 17 percent was Race 4, and 2 per cent was Race 25. Races 2, 8, 13, and 15 comprised a very small portion of the total.

Table 1. Phytophthora races found in Iowa, 1991-1994.

The races of Phytophthora found in this study can attack several commonly used resistance genes (see Table 2). The gene Rps1k is very widely used because it is resistant to most races found in Iowa. Race 25, which can overcome Rps1k and many other resistance genes used in Iowa, may be increasing in importance. This may be due to selection pressure by race-specific resistant varieties. Race 25 has been isolated from Hum-boldt, Webster, and Cass counties.

Table 2. Resistance gene reaction to races of Phytophthora

S=susceptible, R=resistant

Many soybean varieties grown in Iowa use resistance genes Rps1a, Rps1b, Rps1c, or Rps1k. A few varieties include Rps6, which is resistant to races 1, 3, 4, and 25. Growers should use varieties that include Rps1k if the race in a particular field is unknown. If you find Phytophthora in varieties with the Rps1k gene, you may consider using varieties with Rps6 or with tolerance to Phytophthora.

The Phytophthora research program at Iowa State maintains isolates of most races found in Iowa. Isolates of specific races may be obtained by seed companies wishing to test lines for resistance to Phytophthora root rot either free of charge or for a nominal fee. We also collaborate with the ISU Department of Agronomy to check the tolerance of varieties entered in the ISU uniform yield testing with these isolates.