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Powdery Mildew of Wheat

June 7, 2002
We have seen a number of foliar diseases on the wheat crop this season. Over the past 2 weeks powdery mildew has been particularly evident throughout the state. Ellen Phillips, Crop Systems Educator at the Rockford Extension Center, reports the disease in northern Illinois wheat fields. Matt Montgomery, always-vigilant Crop Systems Educator of the Sangamon/Menard Unit, reports it in the central part of the state as well.

The fungus Erysiphe graminis causes powdery mildew of wheat. High humidity and temperatures between 60 and 75 deg F favor the development of powdery mildew.

The disease starts as yellow chlorotic flecks on the leaves.

Chlorotic flecks of powdery mildew infection. (Photo courtesy of Ellen Phillips.)

The symptoms of powdery mildew appear first on the lower leaves. The first diagnostic symptoms of powdery mildew are patches of whitish gray cottony growth on the leaf surfaces.

Cottony growth of powdery mildew. (Photo courtesy of Matt Montgomery.)

If you turn over the leaf, you will see that tissue on the opposite side of the leaf becomes chlorotic. As the disease develops, small brown-to-black spherical fruiting structures (cleisto-thecia) develop in the cottony fungal growth. The fungus overwinters as cleistothecia on straw of infested wheat and grasses.

Dense growth and high nitrogen levels favor powdery mildew, so wider row spacing and a moderate balanced fertilizer program help to reduce disease severity. As with all fungal leaf blights, the most significant yield losses result from infections on the flag leaf and flag-minus-one leaves. Diseases may be present on the lower leaves without affecting yield. For future reference, remember that it is important to monitor weather conditions as the plants approach flag-leaf emergence (growth stage 8). If it is warm and wet (60 to 75 deg F) and diseases appear to be spreading onto the upper leaves, then a fungicide application may be warranted. Note that if the weather dries out, then a fungicide is not needed, even with diseases on the lower leaves. Since we can't control the weather, the best way to manage powdery mildew is to select and grow resistant varieties.--Suzanne Bissonnette

Author: Suzanne Bissonnette

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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