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Powdery Mildew

by Don Janssen, UNL Extension Educator

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The sight of a lilac bush loaded with flowers is great, and the fragrance of lilac in spring never lasts long enough. Even the foliage is a great asset when the plant is used as a screen. But when you first see a white powdery substance growing on the upper leaf surfaces of the lower leaves on the plant you know you have a problem. These leaves and then others, may become twisted, distorted, then wilt and die when they have been infected with powdery mildew.

Powdery mildew is favored by high relative humidity at night (which favors fungal spore formation), low relative humidity during the day (which favors spore dispersal), and temperatures of 70 to 80 degrees F. Powdery mildews are parasitic fungi that can only utilize the nutrients of a live host plant. Although the powdery mildews seldom kill their hosts, the fungi reduce the amount of photosynthesis taking place, increase respiration and transpiration, and cause slower growth.

It may be necessary to use cultural controls to reduce or prevent powdery mildews. Prune plants to improve air circulation. In extremely dense plantings it may also be necessary to remove plants. Proper fertilization will assure proper growth without producing excess production of lush growth.

If powdery mildew occurs in the late summer or fall, it is usually not necessary to apply a fungicide, since the plant will have stored sufficient energy to flower and put on foliage the following spring. But when powdery mildew attacks in the late spring or early summer it may be necessary to spray an appropriate fungicide in order to control the disease.

A fungicide can be applied as soon as the first symptoms appear, with follow-up sprays every 7 to 14 days while conditions are favorable for growth and spread of this disease. A number of compounds are labeled for powdery mildew control. Fungicides labeled on lilac include chlorothalonil, chlorothalonil + sulfur, copper, and sulfur. When applying fungicides, read the label carefully to make sure you are applying the right product at the right time and under the right environmental conditions.

This resource was updated June 2008 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement

PHOTO Credit: Soni Cochran, UNL Extension in Lancaster County

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office

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