ISSUE 4 June 5, 2008
POWDERY MILDEW DISEASES ON LAWNS AND ORNAMENTALS
Part of my lawn is almost entirely shaded. It lies under a huge silver maple (probably the biggest in Fargo!), and it is on the north side of my house, surrounded by a tall fence. This particular patch of lawn gets very little sun and even less air-flow. So, it should come as no surprise to me that my nice green lawn turns whitish-blue every year under cool, humid conditions. I noticed the white powdery growth coating my turf leaves, and it seemed to have appeared over night! This patch of lawn annually suffers from a disease known as powdery mildew. The close-up image shows infected blades of a lawn (from a newer lawn in West Fargo; photo by Sam Markell).
Powdery mildew diseases affect many different plants, including ornamental plants and field crops. The various related fungi that cause these types of diseases are fairly host specific, so, for example, a fungus that causes powdery mildew on lilac will not cause powdery mildew on turf. Below is a standardized report that you might receive in the mail if you submit an ornamental sample afflicted with powdery mildew:
"The sample is infected with a powdery mildew disease. The fungi that cause these types of diseases are fairly host specific, so it is unlikely that unrelated plants will be affected by the same genus and species of fungus causing powdery mildew on the sample. Powdery mildew diseases are common on many trees and ornamentals, often occurring in plantings that are concealed in dense shade or have poor air circulation due to many trees, shrubs, or fences. The disease is favored by cool, humid weather, and high levels of nitrogen. Free moisture (dew) is not necessary for powdery mildew fungi to infect the leaves, so infection can occur even during dry periods as long as humidity is high.
Powdery mildew diseases can be managed by improving aeration, by planting species in appropriate sites to reduce stress (for example, sun-loving plants should be planted where they get adequate sunlight), and by using resistant varieties when possible. Often, pruning and thinning surrounding trees and bushes can help minimize disease development.
Fungicides are not typically recommended for powdery mildew diseases in the
landscape. Some fungicides may help with the control of this disease, but most
fungicides available to the homeowner do not cure already infected plants; most
only protect leaves from new infections. Fungicides may be available at lawn and
garden centers, but they must be applied season long or as long as weather
conditions favor disease, and they will not cure powdery mildew diseases by
themselves. As a result, the use of fungicides is seldom warranted for managing
powdery mildew in a landscape. Neem oil or sulfur may also be somewhat
successful in controlling this disease by protecting new growth from infection.
If chemical control is used, be sure to read, understand, and follow the label
instructions carefully to avoid injury to yourself, the plant, or the
NDSU Plant Diagnostician