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Issue No. 17, Article 7/July 21, 2006

Odd Wilt Symptoms in Soybeans

Timely rain has been welcome in many parts of the state. Although the crops benefit from the rain, fungal pathogens do, too. Especially across the central part of the state, Phytophthora has reared its head. Nancy Pataky, director of the University of Illinois Plant Clinic, reports samples testing positive for Phytophthora sojae.

Phytophthora disease is a tricky one. It doesn't have a limited season of infection like many of our key pathogens. Phytophthora sojae can attack and kill soybeans throughout the season. What we are seeing now is the phase of the disease that attacks mature plants. Typically, an infection of a mature or nearly mature plant is expressed as wilting and death of the top of the plant, significantly rotted roots, and a diagnostic dark brown-black canker extending from the soil line up the stem.

As was true three years ago and is again this season, the symptoms we are seeing do not fit the textbook description of Phytophthora infection. The typical stem canker seems to be missing, and the roots of affected plants aren't quite as badly rotted as one would expect. Also, foliar symptoms on the lower leaves of these affected plants look typical of very early SDS symptom expression--odd. Now it could be that the plants were caught early in Phytophthora symptom development, and the unfortunate plants could potentially also have early SDS, but the symptoms are again remarkable, and we need to keep an eye open for this somewhat nontypical symptom development when scouting.

Wilting of Phytophthora sojae-infected soybean and unusual foliar symptomatology (photo courtesy of Stephanie Satterlee).

Management of Phytophthora relies on several factors. Seed treatment is beneficial, and for future seasons drainage needs to be addressed in problem fields, but the primary method of managing the disease is with the use of resistant varieties. Soybean varieties with specific resistance to Phytophthora should be selected to grow. The major resistance genes, Rps1c or Rps1k, should be effective in most fields. However, research by Dean Malvick while he was at the University of Illinois (he is now extension plant pathologist at the University of Minnesota) found that there are resistance genes no longer effective in some parts of Illinois. Races (pathotypes) of Phytophthora occur in some parts of Illinois that kill soybeans with Rps1a, Rps1c, and Rps1k. Many of the Phytophthora isolates from Illinois soybean fields can defeat Rps1a, and a smaller number can defeat 1c. A smaller number can defeat all of the common resistance genes (Rps 1a, 1c, and 1k) available in commercial varieties for Illinois. The research showed that the aggressive isolates are causing damage but do not seem to be widespread in Illinois.

The complete story of the odd symptoms, however, is not yet complete. Stay alert for symptom expression in the next week.--Suzanne Bissonnette

Suzanne Bissonnette

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