Key Features
- White, powdery spots on leaves
- Leaves turn yellow or brown
- Branches also affected
Symptoms
Powdery mildews are commonly encountered fungal diseases on ornamental plants. On most plants, powdery mildew is often 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. There are exceptions on some plants, like petunias, leaves drop with little development of "powder". On sedum, the disease may appear more like scabby lesions. And on some veronica varieties, leaves turn black, and drop from the base upwards. 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 asexual reproduction, with the fungus producing columns of conidia capable of causing secondary 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. Although powdery mildews are fairly host-specific when it comes to woody plants, many annuals and perennials are infected by closely related powdery mildews that have broad host ranges.
Management Recommendations
Increase air circulation and light penetration by pruning and thinning overcrowded landscapes. Select powdery mildew-resistant varieties for new plantings. Some plants, such as annual phlox rarely if ever get powdery mildew while it is a common disease of perennial species of phlox. Choose plants that will perform well given soil type, drainage, sunlight, etc. Plants in locations that do not meet that plant's growth requirements are predisposed to severe disease. Powdery mildew rarely requires chemical control, though many preventative options are available. Ensure plant species being sprayed are included on the fungicide label or the chemical may damage or kill the plant.
Effective Pesticides
Active Ingredients include: Chlorothalonil, Detergents, Horticultural oils, Myclobutanil, Propiconazole, Sulfur, Tebuconazole, Thiophanate-methyl, Triforine
