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Issue No. 11, Article 4/June 17, 2010

Conditions Favorable for Phytophthora Root and Stem Rot

Phytophthora root and stem rot of soybean can occur when soil conditions are wet and the soil temperature is above 65°F. The disease is caused by a soilborne pathogen known as Phytophthora sojae. Symptoms can be observed as wilting of plants, root systems with dark lesions that may be rotted, dark stem lesions extending up from the soil line, and seedlings that have damped-off. Wet soil conditions are required for Phytophthora rot to be severe, because the pathogen produces spores (zoospores) that must "swim" through the soil to infect soybean plants. The most severely affected areas in a field generally are those that tend to hold moisture for extended periods (low areas, areas with higher clay content, etc.). Infection can take place throughout the season, and adult plants can be killed in severe conditions.

Soybean plants affected by Phytophthora root and stem rot.

With the high amounts of rainfall received throughout the state, it is likely that Phytophthora root and stem rot will be observed more frequently this year. No "in-season" management options are available for control. The best option is to plant a resistant variety. Two types of resistance are available: race-specific resistance and field tolerance. In addition, some fungicide seed treatments may provide some control.

Race-specific resistance. Soybean varieties with race-specific resistance use one or several Rps genes to provide control of specific races of the pathogen. This type of resistance provides complete control of the disease as long as the races of the pathogen present in the field are the same ones that can be controlled by the specific Rps gene(s) used in that variety. A race survey of Phytophthora sojae in Illinois was conducted in 2001 and 2002 by Dr. Dean Malvick (formerly with the University of Illinois). This survey indicated that many different races of the pathogen are present in Illinois, and some of these races can infect varieties that use some of the most common Rps genes for resistance (Table 2).

Field tolerance. Field tolerance is effective against all races of the pathogen, but this type of resistance does not provide complete control. Varieties with high levels of field tolerance can still be affected by Phytophthora, but they are not affected as severely as susceptible varieties. Different seed companies may use different terminology to describe "field tolerance."

Fungicide seed treatments. Seed treatments that contain either mefenoxam or metalaxyl can provide some protection against Phytophthora, but not season-long control. Higher rates of mefenoxam and metalaxyl will provide better control than standard rates that are typically used in Illinois. Results of research at Ohio State University indicate that the use of seed treatments on varieties with high levels of field tolerance may be an effective combination in helping manage this disease.--Carl A. Bradley

Carl A. Bradley

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