Answer: Sounds like your crape myrtle either has or had an aphid infestation. Crapemyrtle aphids are pale yellow in color with winged adults having black wings and black protuberances. They are primarily found on undersides of leaves and are particularly attracted to new growth. These insects damage crape myrtle by inserting mouthparts into soft tissue and extracting plant sap. Crapemyrtle aphids can reproduce and develop large numbers rapidly. Heavy infestations distort leaves and stunt new growth.
During feeding, aphids secrete droplets of a sugary substance called "honeydew". Drops of honeydew fall from the aphids onto leaves and stems below. This sugary substance promotes the growth of sooty mold fungi, Capnodium species. Sooty mold appears as a black staining or powdery coating on leaves and stems. The blackened leaves and stems are often the most obvious sign of aphid infestation.
Although unsightly, sooty mold itself does not directly harm crape myrtle. However, the black fungus shades the leaves and interferes with photosynthesis, potentially reducing the long-term vigor of the plant. Control of crapemyrtle aphid will halt further development of sooty mold. Existing sooty mold on leaves will wear off the leaves through the actions of sun, rain, and wind.
I would recommend spraying your crapemyrtle with insecticidal soap after the new leaves bud out in the spring. The aphids will reproduce quickly so I would spray a couple of times at 2 week intervals.
Once you control the aphids, you won't see any more sooty mold on the leaves of your crapemyrtle.
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