Many garden and landscape plants are susceptible to diseases caused by a group of closely related fungi—the powdery mildews. These diseases look very similar: white talcum powder-like fungal colonies on the surface of the plant. At this time of year we receive numerous reports and samples of powdery mildews. They are most severe in warm, dry climates and are frequently found in mid-summer through fall when warm days are followed by cool nights.
Powdery mildews are specific to their hosts. For example the fungus infecting a lilac does not spread to your rose and vice versa. You simply see the same symptom on different plants.
Although not usually fatal, powdery mildew can be ugly! It can turn a beautiful garden into a dusty white mess in a matter of days. It can also prevent flower buds from opening, and hasten leaf drop and fall dormancy.
Shaded sites with poor air circulation favor this disease. Prune or thin plantings to increase light and air circulation. Avoid overhead watering, especially in the late afternoon and evening, to help reduce relative humidity. Remove and destroy all infected plant parts. When selecting plants that are very susceptible to powdery mildew, such as phlox, zinnias and roses, look for varieties that are resistant to the disease.
Fungicides can help prevent the disease from spreading and are best applied before it appears or when it is in its early stages. On edible plants, neem oil, horticultural oil and sulfur can be effective. There are also several excellent fungicides labeled for use on ornamental plants.