What causes the white powder on the leaves of a crape myrtle?
It is probably a fungus called powdery mildew, which is very temperature and humidity sensitive, making some years worse than others. You can choose to monitor the disease and clean up diseased leaves as they fall. Or you can spray weekly with a fungicide labeled for use on ornamental trees that says it controls powdery mildew. This disease generally disappears when the weather changes. Powdery mildew is unsightly, but it rarely damages an otherwise healthy tree. Also, consider pruning in mid March to allow better air movement. This can help control the disease.
