At the end of last year, we reported the preliminary results of Phytophthora race tests in Iowa, covering 29 isolates. Complete results for 1994 follow.
In 1994, we collected isolates from soybean fields covering major soybean production areas in about 30 counties across Iowa. We tested a total of 137 isolates. Thirty-five of the isolates were from soil, and 102 isolates were from infected soybean plants. Among these isolates, 21 percent were race 1; 44 percent were race 3; and 12 percent were race 4. Races 8, 13, and 15 accounted for very small portions of the total population. About 25 isolates had intermediate reactions in the first test, and three isolates appeared to be new races. We will test these isolates further.
In 1975, Dr. Tachibana reported that the only race of P. sojae identified in Iowa from 1966-1973 was race 1. Phytophthora root rot (PRR) was not identified on any PRR-resistant soybean variety up to 1973. Evidence presented here suggests that P. sojae isolates found in Iowa soybeans and soybean fields are diverse and attack several commonly used Phytophthera-resistant alleles. The results also suggest that selection pressure from using race-specific resistant varieties has resulted in the increased virulence in Iowa. For instance, race 25, which can defeat most resistance genes used in Iowa, has been found in Humboldt, Webster, and Cass counties.
In recent years, the disease has not caused severe damage because of well-developed resistance breeding programs in Iowa. If you find Phytophthora in your soybeans, selection of resistant varieties is the most effective way to control Phytophthora root rot.
Currently, most soybean varieties in Iowa use resistance genes, Rps1a, Rps1b, Rps1c, and Rps1k. Table 2 gives the responses of resistance genes to races found in Iowa. Very few newly released varieties have the Rps6 gene, which is resistant to races 1, 3, 4, and 25. If you do not know what race is in your field, use varieties having Rps1k gene, because it is resistant to most of the races found in Iowa.