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Published August 28, 2010, 12:00 AM

Watch for late-season soybean diseases

ST. PAUL — Although the weather this summer has been good for soybean growth in much of Minnesota, the same weather has favored development of soybean diseases.

By: Dean Malvick, University of Minnesota Extension, West Central Tribune

ST. PAUL — Although the weather this summer has been good for soybean growth in much of Minnesota, the same weather has favored development of soybean diseases.

Several root and stem diseases that have potential to significantly damage yields have been appearing. Sudden death syndrome and Phytophthora root and stem rot have been particularly common, but white mold is also a concern.

Sudden death syndrome has been rapidly appearing in many fields in southern Minnesota. It had been confirmed in 23 counties prior to this year, but is now appearing in counties and fields where it had not been seen before. Weather conditions have been very favorable for development of this destructive disease, and symptoms were seen earlier than normal.

Leaf symptoms appear as yellow, diffuse spots that expand between veins to become brown lesions surrounded by yellow areas. Tan discoloration also develops in just under the epidermis of the lower stem. The pith in the stem remains white, which distinguishes sudden death syndrome from brown stem rot. Brown stem rot occurs across Minnesota, and will soon start to appear and damage yields in many fields, including fields where sudden death syndrome is developing.

This year has also been favorable for Phytophthora root and stem rot, and this disease is more widespread than it has been for several years. It is favored by wet and warm soil conditions early in the growing season. Infected plants develop root rot and brown discoloration of the stem extending up from the soil line. Leaves turn yellow and brown and typically stay attached. Plants are often killed in patches or sections of rows. Some common resistance genes are not effective in many areas due to aggressive forms of the Phytophthora pathogen.

White mold can be a serious problem and has been developing in soybean fields throughout Minnesota. Wet and cool weather during flowering is required for this disease to develop. While most parts of Minnesota have had adequate rain for white mold, the warm temperatures of the past two weeks appear to have suppressed this disease. White mold is also favored by high plant populations, high fertility, narrow rows and fields where plants dry slowly.

Earlier this month, I requested samples of soybeans with symptoms of sudden death syndrome from Minnesota fields. With funding support from the Minnesota Soybean Research and Promotion Council, we were able to diagnose the samples free of charge. This opportunity to diagnose symptoms as they appeared has helped the University of Minnesota expand knowledge on diseases that pose a risk to soybean production in the state.

Information shared at a recent soybean diseases workshop that I held through University of Minnesota Extension, such as diagnostic photographs and disease management recommendations, can be found on Extension’s Minnesota Crop Diseases website at www.extension.umn.edu/go/1042.

Dean Malvick is a plant pathologist with University of Minnesota Extension.

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