There's not nearly enough information to diagnose the problem. A picture might help. Many types of crape myrtles are susceptible to powdery mildew, so, if I had to give a wild guess based on just what you wrote, that would be it.

brandon is right, we need a picture to better identify the problem. Lots of CREPE myrtles are susceptible to powdery mildew.

My sister live miles from here, and is unlikely to get a photo to me, but excessive rain must be bringing on powdery mildew. Seems exactly the problem. Thanks.