Key Features
- White, powdery spots mostly on leaves
- Leaves that turn yellow or brown
- Occasional symptoms on branches
Symptoms
Powdery mildews are some of the most commonly encountered fungal diseases on ornamental plants. Powdery mildew is easily diagnosed by the characteristic powdery, white coating of fungi that covers infected leaves. Dark brown dead spots may form in the center of white covered tissue. Powdery mildew rarely causes serious damage to its host, but infection can produce discolored leaves that die or drop, disfigured shoots, and deformed flowers. Although ugly, powdery mildew is rarely fatal. The distinction between powdery and downy mildew is important as the fungicides effective against one are not usually effective against the other.
Biology
In the spring, spores are windblown to newly emerging, susceptible leaves. Successful infection results in the fungus producing thousands of spores per lesion, capable of causing repeated infections. The coating of spores that develops on plant surface gives the leaf a characteristic white to light gray dusting. Powdery mildew is most prevalent during cooler temperatures and high humidity or extreme dryness. Powdery mildew pathogens are one of the only foliar pathogens that can infect in the absence leaf wetness. We see increased levels of powdery mildew in the spring and fall of the year in the Midwest.
Management Recommendations
Select powdery mildew resistant varieties for new plantings. Powdery mildew resistant varieties are available for crabapples, lilac, dogwood, azalea, rose, and many herbaceous ornamentals. Choose plants that will perform well given soil type, drainage, sunlight, etc. Plants in locations that do not meet that plants growth requirements are predisposed to severe disease. Increase air circulation and light penetration by pruning and thinning overcrowded landscapes. Powdery mildew rarely requires chemical control, though many preventative options are available.
Effective Pesticides
Active Ingredients include: Chlorothalonil, Detergents, Horticultural oils, Myclobutanil, Propiconazole, Sulfur, Tebuconazole, Thiophanate-methyl, Triforine
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