Update of Phytophthora races present in Iowa

At this Monday's teleconference, Iowa State University field crop specialists continued to report damping-off of soybeans in their regions. Since the weather has warmed up, and therefore soil temperatures have increased, the most likely cause of such damping-off is Phytophthora. Phytophthora also can continue to develop after the damping-off phase, causing stem and root rot. Diseased plants have chocolate brown discoloration from the soil line up, a unique symptom of this disease (see photo). If we continue to get periodic rainfall patterns with one week of wet weather followed by dry weather, conditions will be ideal for the continued development of this disease.


Resistance is the most effective tool for disease control. Phytophthora sojae has different races that can be controlled by soybean with specific resistance genes. Studies have shown that races of P. sojae vary geographically. Therefore, it is very important to select cultivars with resistance genes that are effective for the races present in your fields. Also note that the predominant races in a field also can shift after a resistance gene is used for a number of years.

Phytophthora stem rot.

Researchers at Iowa State University surveyed Phytophthora root rot races in Iowa from 1991 to 1994, and 2001 to 2002 by isolating Phytophthora from either soil samples or infected soybean plants (Table 1). A population shift of Phytophthora sojae in Iowa was detected in the 2001 to 2002 survey. Three new races were identified, races 20, 28 and 35. In addition, the frequency with which race 25 was detected increased.

The races found in Iowa can attack several commonly used resistance genes. The gene Rps1k is very widely used because it is resistant to most races found in Iowa (Table 2). However, the 2001 to 2002 survey revealed that races that are virulent on Rps1k, namely races 20, 25, 28 and 35, are becoming increasingly important. This may be due to selection pressure by race-specific resistant varieties. Phytophthora races that are able to overcome Rps1k resistance were found in Buchanan, Dickinson, Fremont, Greene, Jasper, Johnson, Lee, Madison, Marion, Marshall, McNay, Sioux, Ringgold and Wayne counties

If Phytophthora occurs in your 1k-gene soybeans, take note. Check with your seed dealer and ask for varieties with the Rps-6 gene. This gene is a new Phytophthora resistance gene effective against races 1, 3, 4, 25, 28 and 35.

Table 1. Percentage of Phytophthora sojae races found in Iowa from 1991-1994 and 2001 to 2002

Race 1991-1994 2001-2002
1 26 15.4
2 <2 3.8s
3 39 11.5s
4 17 3.8s
8 <2 0
13 <2p 5.8s
15 <2p 0
20 - 3.8p
25 2 11.5
28 - 7.7
35 - 9.6
Unknown 14 17.3

p Only detected in infected soybean plants;

s only detected in soil samples

Table 2. Resistance gene reaction to races of Phytophthora sojae

Resistance gene
Race Rps1a Rps1b Rps1c Rps1d Rps1k Rps3a Rps6 Rps7
1 R R R R R R R s
2 R s R R R R R s
3 s R R R R R R s
4 s R s R R R R s
8 s R R s R R s s
13 R R R R R R s s
15 R R R R R s R s
20 s s s R s s R s
25 s s s R s R R s
28 s s R R s R R s
35 s s s s s R R s

This article originally appeared on page 74 of the IC-492(12) -- June 21, 2004 issue.

Updated 06/20/2004 - 1:00pm