crape myrtles are sick
My crape myrtles have white spots on some of the leaves and others a milky looking substance on them causing new stems to wilt and fall off. What is going on? I haven't had this problem before. The trees are well established and have been pruned in past years with no problem.
It sounds like your Crepe Myrtle is infected with Powdery Mildew.
Here are some links with more information.
The milky substance, if it's sticky, could be honeydew from aphids and the white spots could be either powdery mildew (easily treated) or downy spot.
These articles may help you identify exactly what the issues are and treat them appropriately: