aphids do not make sooty mold... they produce sugars.. which ants like to 'milk' .. and the sugar usually turns black ...

so .. do you have aphids?? ... look for the ants... or do you have a mold ??? ...

proper ID of the actual problems goes a long way in correcting such ...

ken

Crape myrtles and aphids go hand in hand in the south. As a matter of fact, there is a crape myrtle aphid! Since you used the words 'older' to describe your crape myrtles, I wonder about their age. The older seedling varieties are very prone to aphid infestation (and black sooty mold) as well as powdery mildew.

You can certainly help prevent a major infestation for next year by using insecticidal soap applications now, and a hitting them a couple of times over the winter with horticultural oil. Be sure to soak all of the branches and stems with the soil spray. In my opinion, those are the two pest control products you should probably have, anyway.

Crape myrtles are awfully forgiving plants, so I'd suggest that something about your soil system might be challenging them. Does it drain well? Is it severely compacted? What's your pH? Were you able to water during the worst of the drought? (Healthy, well-established crapes can take a drought.)

The single most helpful thing you can do for your crapes is to establish a good sized mulch ring around them. Give them as much space as you can (imagine where the root system might be) and apply 2 to 3 inches of organic mulch (chips or pine straw). You might be amazed at the recovery of your trees, just by providing them with a good mulched area.