Sounds like your Crape Myrtle has Sooty Mold.

I had that on one of my young CMs one very wet year. I just sprayed it with fungicide and it was fine.

bspatial, is this black stuff also on the stems and branches? Can it be rubbed off with a wet finger? Or is it actually part of the leaf tissue?

You see, black sooty mold has absolutely nothing to do with a plant disease. As a matter of fact, it won't even be affected by a fungicide. It is a simple mold that feeds on the sugars found in what is called 'honeydew', the excrement of different sucking insects. In the case of Crape Myrtles, aphids are the likely culprit. The black sooty mold will go away ONLY if and when the insects are taken care of.

Crape myrtles are quite commonly plagued by aphids and black sooty mold is often a problem. I, too, would assume that you need to find those aphids and figure out how you are going to get them under control.

Are there hackberry trees near your Crepe Myrtles and do you have Wooly hackberry aphids?
Those aphids' poop can even turn stop signs black because the mold moves in and lives on the poop.

Thank you all for a great education. I will consider aphids the primary reason and spray accordingly. BTW, a few Hackberry trees are in the vicinity but not very close to the CM's.
Again you have all been most helpful.

Grateful

Just to help everyone understand this better:

Black sooty mold is a by-product of several different kinds of insects. Aphids of ALL kinds, some scale insects, whitefly, even some leafhoppers, planthoppers, and mealybugs. All of the these insects suck large quantities of plant juices and also excrete large amounts! This is what is called honey-dew; it's a clear, sticky, sweet liquid that literally rains on anything that may be sitting underneath the infested plant.

Outside, a simple mold feeds on this sweet stuff....this is the black sooty mold. BSM, thus, can grow on lawn furniture, car tops, and even on plants that are not infested with insects.

Proper identification of the insect pest is important in successful control of the infestation. The BSM will eventually go away once its food source (honey-dew) is eliminated.

By the way, aphids overwinter on Crapemyrtles as eggs. You can do a great deal to prevent another serious outbreak by applying dormant oil sprays two or three times over the winter and early spring.

I know this is an old thread, but someone may be interested in a link I found about black sooty mold on crepe myrtles.

http://www.theeagle.com/homegarden/060404horne.php

I hate to see an article that recommends the use of imidicloprid, a system pesticide that can create problems for insects and birds that collect pollen and nectar.