Have you ever noticed a white, powdery substance on the leaves of plants such as squash, roses, dogwoods and crape myrtles?
The cause of the discoloration is powdery mildew, a group of closely related fungal diseases. The disease is characterized by a whitish fungal growth on the surfaces of leaves, stems and flowers. It sometimes causes leaves to curl up and turn red or brown, and occasionally the disease can kill herbaceous annuals, perennials, vegetables and plants. However, this rarely happens to trees and shrubs.
Powdery mildew occurs during dry weather. It does not need the plants to be wet to grow. Rather, it only requires high humidity - something we get a lot of here.
Dry, sunny days followed by cool, moist nights favor the development of the disease. The mildew spreads by spores blown by wind and splashing rain to other locations on the plant or nearby plants.
Several types of powdery mildew exist. The one that affects crape myrtles is specific to that plant and will not infect other plants or vice versa.
Chemical fungicides are usually the last method of control because they only slow, or prevent, the spread of the disease. Instead, it's best to try cultural methods to control powdery mildew.
One method is planting resistant plant varieties. Several crape myrtle cultivars, the ones with American Indian names like Natchez, Sioux and Tuskegee, are resistant to powdery mildew. Several cultivars of roses, dogwoods, phlox and other plants are also resistant. Increasing air circulation among plantings can reduce the incidences of disease.
Avoid planting the plants too close together or overplanting an area. Remove infected stems and leaves, and clean up debris because it can harbor disease organisms. Select healthy plants.
Install them properly and in the right location, and provide the necessary requirements for light, soil and moisture. Apply appropriate levels of fertilizer - excess fertilizer will lead to lush growth that is very attractive to powdery mildew infection.
One plant, golden euonymus, often has severe problems with powdery mildew, as well as scale insects. It's best not to plant it, because the mildew and insects will ultimately kill it unless you continually apply chemical fungicides.
Several chemicals can be used to control the fungus. The fungicides must be applied when the symptoms are first observed. The most effective ones include Daconil, Fung Away, Funginex and Immunox. Read and follow all label directions and use safety precautions when using chemical pesticides. Refer to the 2007 Georgia Pest Management Handbook at www.ent.uga.edu/pmh.
Remember, one infected plant will not threaten the whole landscape. In most cases, the disease will run its course with minimal harm to the plants.
Timothy Daly is an agricultural and natural resource agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or firstname.lastname@example.org.