of Vermont Extension
Department of Plant and Soil Science
Powdery Mildew of Ornamentals
Ann Hazelrigg, Plant Diagnostic Clinic Coordinator
Most landscapes have plants that regularly become diseased with one of the
powdery mildew fungi. Although the fungi involved are usually different on
different plants, all of the powdery mildews are similar in appearance. In most
cases, prompt recognition and control actions can prevent severe damage to
plants from the powdery mildews.
Powdery mildews, as the name implies, often appear as a white or gray growth of
fungus over the leaves, stems, or flowers of affected plants. Young foliage and
shoots may be particularly susceptible. Leaf curling and twisting may be noted
before the fungus appears. Severe powdery mildew infestation will result in
yellowed leaves, dried and brown leaves, and disfigured shoots and flowers.
Although it usually is not a fatal disease, powdery mildew will hasten fall
dormancy, and the infected plant may become extremely unsightly. On roses,
uncontrolled powdery mildew will prevent normal flowering of highly susceptible
Powdery mildews are familiar to almost all ornamental plants. However, they are
commonly seen on only those plants more naturally susceptible to the disease.
Most mildews produce airborne spores and infect when temperatures are moderate
(60 to 80 degrees F); the disease is not favored during the hottest days of the
summer. Most mildews, especially those on rose, apple, and cherry, are also
favored by high humidity. Overcrowding and shading favor disease development
because the plants are cooler and the humidity is higher.
Before using chemicals you should attempt to control powdery mildew by other
means. The following are good ways to manage the disease culturally:
- Buy varieties of plants resistant to powdery mildew.
- Avoid shade and overcrowding of susceptible hosts. Pruning may help.
- Water in the morning instead of the evening.
- For mildews attacking young foliage, cut down on nitrogen applications
to decrease excessive growth in the late summer.
- On indoor plants try to reduce the relative humidity.
- In the fall, remove or destroy the infected plants.
- Chemical Controls
On some plants, powdery mildews will occur in spite of good cultural control
measures. Thus, fungicides must be used. For best results, spray programs
must begin as soon as mildew symptoms are seen. Spray on a regular schedule,
more often during cool, damp weather. Use a good spreader-sticker with the
fungicides. Be sure to cover both surfaces of all leaves with the spray.
Before using any pesticide, read the label and follow all precautions!
Based on material developed in 1992.
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