Answer: The problem sounds like sooty mold and is caused indirectly by aphids or scale insects. These 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. Inspect the tree for brown lumps on the branches or trunks (scale insects), or watch the new growth for signs of aphids. Depending on what you find, there are several control options: if the tree isn't too large, you can use a gloved hand and squish the adult scales. Or you can scrub the scales off with a soft brush, then rinse with soapy water. Or, you can apply a dormant oil to effectively suffocate the scale insects. Apply dormant (horticultural) oil to all surfaces of the plant to get good control and don't spray if the air temperature is below 40F or above 65F. Be sure to read the label, and apply exactly according to directions to avoid damage to your tree. If you're dealing with aphids, hose them off the leaves with plain water, or use insecticidal soap.
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