Powdery mildew has been showing up on crape mrytles this year, even in resistant varieties. The following is more information.
Powdery mildew on crape myrtle has not been a problem for many years since the introduction of new cold tolerant varieties (devl. at the National Arboretum and other places) that also happen to have powdery mildew resistance. But, this season has been so favorable for powdery mildew (especially the last several weeks) that we have seen powdery mildew on flower trusses of some of these cultivars. Remember resistance is not immunity. While levels have been low, homeowners have probably never seen powdery mildew on their crape myrtles before. While infection doesn't affect overall plant health, it may be severe enough to prevent flower buds from opening.
Information from Bob Mulrooney, Extension Plant Pathologist, UD