Answer: What you describe is called sooty mold. Sooty mold is a black fungus that grows from the excretion of scales, whiteflies and aphids. Sooty mold detracts from the beauty of crape myrtles and reduces their photosynthesis activity.
It can be washed off the leaves using soapy water. To completely control sooty mold, you'll need to control the whiteflies. Some recommended chemicals that will control aphids, whiteflies and scales are: Orthene or Merit. Please read and follow label instructions on use of insecticides.
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