Answer: The problem sounds like sooty mold, a problem caused indirectly by aphids and scale insects. These insect pests suck the plant juices and excrete a liquid called honeydew. The sugar content of this excrement is just high enough to encourage fungal growth. That's what you're seeing on the leaves of your crape myrtle. There are several control options: if the plant isn't too large, you can hose off the plant to control the pests. Or, you can spray with Insecticidal Soap. Repeat sprays are usually necessary to get good control of the insect pests. Once you've rid your plant of insects, the sooty mold will weather away with rainwater or water from the hose.
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