Powdery mildew attacks many different species of plants including crape myrtle, phlox, roses, rock rose, honeysuckle, snapdragons and zinnias to name a few. It causes a white powdery covering to form on leaves and stems and can cause severe distortion of new growth and stunting of plants when conditions are favorable for its development. Powdery mildew is most prevalent in mild weather conditions, and high humidity. Crowding of plants minimizes air movement and can increase problems with this disease.
The best management strategy is to avoid it with resistant plant species and varieties. There are many crape myrtles that are resistant to the mildew. Most have names of native American tribes such as Choctaw. If you are planting a crape myrtle it makes sense to choose the variety based on its mildew resistance and mature size, to minimize the need for pruning. For a list of crape myrtles with info on their bloom color, size and powdery mildew resistance see the following web site:
If you have a susceptible crape myrtle variety there are a number of spray remedies. Among the more effective organic spray options are horticultural oil and plant based oil such as Neem oil. Potassium bicarbonate (Greencure) is another organic spray remedy. These sprays must be applied the plant foliage periodically to prevent powdery mildew infection. Thorough coverage is important because they only protect the foliage covered with the spray. Once mildew has damaged foliage it is too late to control it with a spray, so spraying is just to stop the spread of the infection. Repeat applications according to the label instructions.
Synthetic options include products containing myclobutanil (Immunox), Triadimefon (Bayleton, Fung-Away), Triforine (Funginex), and Propiconazole (Banner). These sprays are very effective in preventing mildew infections but also have some locally systemic action so they can provide a longer period of control. They also may provide some curative action if applied early in the infection. During mild and humid conditions repeat spraying according to the label instructions to maintain control.
The following web site is an excellent resource for more information on managing powdery mildew on landscape plants: