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LINCOLN, Neb. - Hot, dry weather made the summer tough for ornamental landscape plants and lawns in many areas. Now Nebraska landscape contractors and lawn care operators can add powdery mildew to the list of problems they may face on properties, a University of Nebraska (NU) plant pathologist said.
Powdery mildew appears as a dust-like coating on the upper surface of a plant's leaves. Pumpkins, squash, lilacs and zinnias are particularly affected by the fungus, though the disease can occur on many types of plants.
"Often the plants look like they have been sprinkled with baby powder," Loren Giesler, NU plant pathologist, said. "On closer inspection, you can see the cottony threads of fungus, and the patches of fungus won't wash off or wipe away."
Powdery mildew rarely kills infected plants, but the disease does detract from the beauty of landscape ornamentals and can compromise the quality of fruits and vegetables, Giesler said.
"Powdery mildew can result in some thinning of plants if it is severe," the NU Institute of Agriculture and Natural Resources specialist said. "But, the fungus is a parasite, taking nutrients and energy from the plant, which can affect plant growth and development. The shading effect the fungus patches have on the leaves also can restrict photosynthesis."
In roses, powdery mildew appears most often on the flower buds, often resulting in bud abortion.
Humid weather is favorable for powdery mildew problems because humidity builds up under plant canopies and gives the fungus an ideal place to grow and spread, Giesler said. The fungus favors shady areas and is most severe in late spring and early fall but can occur anytime during the growing season.
"This year, the stress from heat and lack of rain, combined with high humidity, left many plants more susceptible than usual to diseases like powdery mildew," he said.
Unless affected plants are of high value aesthetically, treatment of severe powdery mildew is not economical, Giesler said. The best approach is to begin fungicide treatment early next spring, before the disease becomes a big problem.
"The exact treatment depends on the plant, but for most plants you can begin treating once the plant canopy is full," he said. "If you see a powdery mildew problem starting, treat it right then. It's not worth it to wait until the problem is severe and then try to treat it."
For high value plants, Giesler suggested finding the right fungicide to treat powdery mildew.
"Be sure to read the labels for application rates and to be sure the product will treat powdery mildew," he said. "Many people confuse powdery mildew with downy mildew. These are different diseases, and the treatments are not interchangeable."
Users also should read labels very carefully when treating plants with edible fruits, Giesler said. Some chemical treatments are not labeled for use on food products or may require a waiting period before the fruits can be eaten.