Answer: Even without seeing it, I think what you are describing is sooty mold. You can scrape it off with your fingernail or wash it off because it is a coating on the tops of the leaves. Sooty mold is a by-product of insect feeding. Aphids love crape myrtle trees so I think that's what is causing the problem. Aphids feed on the fluids within the tissues of the leaves and they excrete a substance called honeydew. This honeydew is high enough in sugar content to develop bacterial growth which turns it black. If you control the aphids, the sooty mold will disappear. Insecticidal soap will usually control aphids. Good luck with your crape myrtles.
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