Hello, David. You are probably suffering from an aphid infestation. An example (photos) of such can be seen in the middle of this webpage:
http://insects.tamu.edu/extension/bulletins/b-6047.html Aphids feed by inserting their mouthparts into tender new leaves from which they suck sugary
plant sap. As they feed, the aphids excrete large amounts of a sugary liquid called honeydew. With a large aphid population, the honeydew can completely coat leaves, which serves as food for the sooty mold fungi, as well as various other insects like ants, wasps and flies, etc. Sooty mold is harmless so you should concentrate on controlling the aphid infestation instead.
In addition to switching to crape myrtles that have resistance to aphids, you can always wash them away with a strong jet of water or use natural predators like ladybugs, lacewings, hover fly maggots, parasitic wasps and entomophagous (insect feeding) fungi. Some of these are probably available now in your local organic minded nurseries.
If you want to use insecticides, be aware that these will also kill the natural predators listed above. You can try insecticidal soap (e.g., Safer Insecticidal Soap), horticultural oil (Sunspray Ultra-fine Spreay Oil), pyrethrins (Schultz
rose &
flower Insect Spray), neem oil (Bonide Bon-Neem, Green Light Neem Concentrate, etc) or products containing permethrin, cyfluthrin or malathion. Read the labels and take appropriate safety precautions. Also, consider rotating insecticides in order not to end with pests that resist any one insecticide. That way, if they do become resistant, when you rotate, the other insecticide will kill them.
I hope that helps. Good luck,
Luis
