University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

March 17, 2006

Figure 1. Powdery mildew on Millennium wheat plants collected on January 24, 2006 from a research plot at the South Central Agricultural Laboratory near Clay Center and grown in the greenhouse at the University of Nebraska, Lincoln. The picture was taken on February 10, 2006.

Powdery mildew of wheat: biology and management options

Powdery mildew is a common disease of wheat. It is caused by the fungus Blumeria graminis f.sp. tritici (synonym: Erysiphe graminis f.sp. tritici). The fungus is obligate, that is, it can grow only on living plant tissue. It forms white to gray powdery patches on the surface of the plant. These patches consist of mycelia and asexual spores and are most conspicuous on the upper surface of leaves (Figure 1), but can also occur on stems and heads. Infected leaves often turn yellow on the lower surface below the powdery growth and later turn tan or brown. With time, the powdery growth changes to a gray-brown color (Figure 2). As the plant matures, tiny round, black fruiting structures called cleistothecia form within older patches of powdery mildew. The cleistothecia contain sexual spores and can overwinter on crop debris. The fungus also can overwinter as mycelia on volunteer or fall-sown wheat in southern states where warm temperatures prevail in winter.

Favorable conditions
Powdery mildew is favored by high relative humidity (greater than 80%) and a temperature range of 59oF to 72oF. Because of the high relative humidity in the lower part of the plant canopy, the lower leaves are most severely infected by powdery mildew. High rates of nitrogen fertilization result in heavy lush growth which favors disease by promoting high humidity under the dense canopy. High nitrogen fertilization also increases susceptibility of wheat to powdery mildew. Spores of the fungus are spread by wind. Spore germination and infection are favored by near 100% relative humidity, although free water on the plant surface tends to inhibit spore germination. Under ideal conditions for disease development, a new crop of spores is formed every 7 to 10 days.

Timing of outbreaks
Figure 2. Powdery mildew on Millennium wheat in a research plot at the South Central Agricultural Laboratory near Clay Center on March 6, 2006.

In Nebraska, powdery mildew normally occurs from May to July. If overwintered cleistothecia are present, they produce sexual spores which cause initial infections in spring. Asexual spores blown in from southern states usually are the major cause of infection. This year we observed powdery mildew in wheat research plots at the South Central Agricultural Laboratory near Clay Center in January. The powdery mildew apparently overwintered due to the warm winter temperatures we have had. In early March plants in the research plots were still heavily infected by powdery mildew and we also observed the disease in a grower’s field near Clay Center. This implies that if we do not get prolonged subfreezing temperatures in March and April, powdery mildew pressure may be higher than normal during the growing season. Although in most years powdery mildew is not yield limiting in Nebraska, sporadic, moderate to severe outbreaks can occur, resulting in significant yield reduction.

Management options
Powdery mildew can be managed by planting resistant varieties and applying fungicide sprays when it’s detected. It is essential to scout fields for powdery mildew and other foliar diseases to determine whether the level of disease warrants spraying. The decision to spray should be based on the severity of disease and how widespread it is in the field. Also consider the stage of crop development (the flag leaf and the leaf below it should be protected from disease to prolong grain filling), the yield potential, and the selling price of grain. Choose a fungicide based on its relative efficacy and cost.

Stephen N. Wegulo
Extension Plant Pathologist, Lincoln

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Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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