I have three dwarf Crape Myrtles in a small landscape bed tucked away in a corner, which I don't really notice that much. Last week I checked them out and was dismayed at how awful they looked, whith black sooty mold on the leaves. Sooty mold is caused by a heavy infestation of aphids, which are tiny inects found on the underneath side of the leaves which suck plant juices from the leaves as they feed. The aphids then secret a sweet, sticky substance called honeydew, which then causes the black sooty mold to form on the leaves. So it's an ugly cycle, all caused by the aphids coming in and feeding on the Crape Myrtles.
If I was on our LawnAmerica Tree & Shrub Program, my Crape Myrtles would probably be looking just fine. Aphids can be controlled with an insecticide treatment applied, usually during the summer. Aphids typically come in and feed on certain plants, such as Crape Myrtles, vegetable plants, and certain trees such as Maples. If the poplulation is large, they can damage leaves with their feeding to the extint that plants are damaged, leaves turn yellow or drop, and even death. If inseciticide sprays are used, the plants much be drenched thoroughly, as the insects are found usually on the underneath side of leaves. Some insects such as Ladybugs are natural predators of aphids, so this will help if you have some of those good insects. However, there are usually not enough natural predators to keep the aphid population down in some cases, so spraying will be necessary. Several treatments may be needed also for best results.