by Linda Geist – University of Missouri Extension
Hot days, cool nights and spotty showers create conditions for powdery mildew to thrive.
This fungal disease affects common vegetables and landscape plants, says Patti Hosack, extension associate and director of the Plant Diagnostic Clinic at the University of Missouri.
There are many types of powdery mildew fungi, and each species attacks specific plants.
It first appears as white, powdery spots on leaves, shoots, and sometimes flowers and fruit. Infected leaves may yellow and fall off, Hosack says. Leaves may twist, buckle or become distorted.
Spores spread by water and wind.
While mildew usually does not kill the plant, it reduces vigor, which makes it more susceptible to other diseases or pests, Hosack says. Yields also may be reduced.
Moderate temperatures and shady conditions create a favorable environment for the disease. It is sensitive to extreme heat (above 90 F) and direct sunlight.
Follow these steps to control powdery mildew:
-Plant resistant varieties when available.
-Space plants to increase air circulation.
-Plant in full sun.
-Avoid excessive fertilizer or use a slow-release fertilizer.
-Use drip irrigation to limit overhead water. Overhead irrigation can splash spores and create a humid environment.
-Water early in the day to allow leaves to dry before evening. Don’t let your plants go to bed wet.
Spray chemical fungicides as needed. See soilplantlab.missouri.edu/plant/diseases/mildewfungicide.aspx for a list of fungicides labeled for control of powdery mildew.