Lilac Powdery Mildew

23 July 2007

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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Sometimes white stuff on green foliage is attractive, look at all the variegated plants available these days.  But sometimes the white stuff is a fungus called powdery mildew that is not so good for plants.  Powdery mildew fungi attack a variety of ornamental plants grown in Virginia.  Photinia, crape myrtle, dogwood, lilac, Japanese euonymus, wintercreeper euonymus, and many crabapple cultivars are highly susceptible and can be seriously damaged by powdery mildew infection. Many herbaceous ornamental plants are also susceptible to powdery mildew.

The fungi that cause powdery mildew all belong to the family, Erysiphaceae. Some powdery mildew fungi attack several different host plants, but most attack only a single host or, at most, only a few species. Most powdery mildew fungi produce a conspicuous white to grayish growth of fungal mycelium on the surface of the diseased plant part. Conidia, or spores of the fungus, are produced on the mycelium. In late summer most powdery mildew fungi also produce fruiting bodies called cleistothecia, which are dark-colored at maturity and about the size of coarse grains of pepper. These appear as dark specks on the white mycelium.

The prolific growth and sporulation of the fungus on the surface of leaves and other plant parts give the affected tissue a talcum powder-like appearance, hence the name "powdery mildew". The new growth of infected plants may be completely covered with powdery mildew. As a result, new growth may be dwarfed, and the leaves, stems and young shoots may be curled. In addition, shoot tips may be killed and buds may fail to open. Powdery mildew can also be severe on older leaves or plant parts. Leaves that are heavily infected with powdery mildew may become chlorotic and senesce early. The unattractive appearance of nursery plants infected with powdery mildew may make them unsaleable.

As a general rule, powdery mildew diseases are most severe under dry climatic conditions; however, fog or high relative humidity is necessary for spore germination. Free water on the leaf surface actually inhibits spore germination of most powdery mildew fungi; however, splashing water can spread spores. Warm, dry days and cool nights are most conducive to infection. On dry days conidia can be blown to healthy tissue. As air cools at night and humidity rises, the spores absorb moisture, germinate, and infect. Under favorable climatic conditions, a single spore can cause a new visible, sporulating lesion in 3-5 days. Thus, the disease can appear to develop very rapidly.

In geographical areas with mild winters, the mildew fungus overwinters as conidia or mycelium in infected buds, or on leaves, stems, and other plant parts. The fungus resumes its growth the following spring and is first observed on the new growth. In areas where winters are severe, the fungus overwinters as cleistothecia on plant debris. In spring the cleistothecia produce ascospores, which, like conidia, are blown to healthy tissue and cause new infections.

Cultural practices that reduce humidity, such as pruning for improved air circulation and avoiding shady locations, help prevent powdery mildew.

Frequent application of fungicides throughout the period of the year when the plant is making rapid growth minimizes serious damage. A number of fungicides have proven effective for control of powdery mildew diseases. Call your Extension office for details. Fungicide labels should be consulted for instructions for use on specific ornamental species.

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Before using fungicides you should attempt to limit powdery mildews by other means. The following cultural practices should be beneficial for controlling powdery mildews.

• Purchase only top-quality, disease-free plants of resistant cultivars and species
• Prune out diseased terminals of woody plants, such as rose and crabapple, during the normal pruning period. All dead wood should be removed and destroyed (preferably by burning). Rake up and destroy all dead leaves that might harbor the fungus.
• Maintain plants in a high vigor.
• Plant properly in well-prepared and well-drained soil where the plants will obtain all-day sun (or a minimum of 6 hours of sunlight daily).
• Space plants for good air circulation. DO NOT plant highly susceptible plants--such as phlox, rose, and zinnia--in damp, shady locations.
• Do not handle or work among the plants when the foliage is wet.
• Water thoroughly at weekly intervals during periods of drought.
• Avoid overhead watering and sprinkling the foliage, especially in late afternoon or evening.

Cultivars with resistance to powdery mildew are available for some ornamental species. Because chemical control for powdery mildew often involves season-long applications, use of resistant cultivars is highly desirable. On cultivars with high levels of resistance, sprays for powdery mildew may be reduced or even eliminated, but fungicides may still be necessary to control other diseases.

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For more information about this and other landscape topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.