University of Vermont Extension
Department of Plant and Soil Science


Powdery Mildew of Ornamentals      GD 16

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.

Symptoms

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 cultivars.

Hosts

Powdery mildews are familiar to almost all ornamental plants. However, they are commonly seen on only those plants more naturally susceptible to the disease.

Environmental Conditions

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.

Control

Before using any pesticide, read the label and follow all precautions!

Based on material developed in 1992.



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