Q: What ails my crepe myrtles? The leaves are already falling and those left on the trees are covered with a charcoal powder that stains everything underneath.
A: You have crapemyrtle aphids. These tiny insects slowly build their population during the summer. By fall, there are enough of them to coat nearby leaves and surfaces with honeydew, an undigested sugary substance that feeds the sooty mold you notice on the leaves. Don’t be surprised outdoors if you begin feeling the insects crawling on your neck and face. There are winged forms in fall that fall out of crapemyrtles onto passers by.
The best way to control them is with systemic insecticides containing imidacloprid (click for sources).
The black material on the leaves will eventually wash off as the aphids disappear.