The Extension office has recently received numerous calls from homeowners concerned about a white powdery substance that has appeared on the leaves and flowers of some of their garden plants. What they are observing is a fungal disease called powdery mildew.
Several different types of fungi cause this disease. Some are host specific while others can infect a variety of plants. Some of the most susceptible plants are crape myrtles, dogwoods, roses, phlox and golden euonymus. The disease usually begins on the lower leaves and moves to the top causing them to curl upwards and eventually turn red or brown. It rarely causes plant death, but infected plants can be disfigured.
Powdery mildew occurs during dry weather unlike most other fungal diseases that prefer wet conditions. The fungus requires high humidity but does not need to have water on the plant surfaces for an infection to occur. The conditions that favor the development of the disease are dry, sunny days followed by cool, moist nights. The disease spreads by spores blown by the wind and by rain drops splashing them on to nearby plants.
Chemical fungicides are usually the last method of control since they only slow the development and spread of the disease. Cultural control methods should be used to reduce the incidence of the disease. Planting resistant plant varieties is one of the best ways to keep plants free of powdery mildew infections. Several crape myrtle cultivars have resistance, especially the ones with Indian names like Natchez, Sioux and Tuskegee. Increasing air circulation among plants such as pruning or removing ones planted too close together also helps reduce infections. Clean up and remove plant debris on the ground as it serves as a source for the fungal spores.
Make sure you purchase healthy plant material. Install plants properly and provide proper cultural conditions that favor healthy growth. Apply appropriate levels of fertilizer. However, excess amounts of fertilizer can cause lush new growth, which can promote powdery mildew infection.
One plant, golden euonymus, has severe problems with powdery mildew, as well as scale insects. Avoid planting it since it will need continual applications of chemical pesticides. Existing plants should be removed and replaced with ones that have fewer pest problems.
Several chemicals fungicides help in controlling the disease and, if used, should be applied when the symptoms are first observed. Products containing the chemicals myclobutanil and chlorotalonil are effective at reducing powdery mildew problems. Remember, read and follow all label directions and use safety precautions when using chemical pesticides.
In most cases, powdery mildew will run its course with minimal harm to the plants even without any chemical treatments. It is more of a “cosmetic disease” than one that threatens the health of the plant.