Question: It’s midsummer, and my crape myrtles look terrible. The trunk and branches are covered with a black sooty substance; the leaves started to grow quite late, and now the flowers are also more wimpy than usual. Is this some kind of a fungus, and how do I get rid of it?
Answer: Your trees are suffering from bark scale, a pest, not a fungus. If you look carefully at the blackened branches, you will see pinkish scale insects wherever you see the black stuff. These are Acanthococcus lagerstroemiae, more commonly, and simply, referred to as crape myrtle bark scale (CMBS). CMBS is emerging as an invasive pest in Virginia. It is the the CMBS that are responsible for exuding what you are seeing as black sooty mold. This substance is the abundantly-produced honeydew of these tiny insects. As the honeydew ages, it turns black and gives rise to this most noticeable symptom of infestation.
Ideally, CMBS should be treated with a soil drench of imidacloprid or dinotefuran in the early spring, in April, when the leaves begin to emerge. At that time, that insects are still in an early nymph stage. Earlier in the year, in the winter and very early spring, before there are any leaves, we think that CMBS might (this is not yet fully researched) be susceptible to insecticidal soap or dormant oil.
As the CMBS mature in the late spring and summer their bodies are become protected by a white waxy coating that is impervious to insecticides. It is tough to kill the scale at this point, not only because they are protected by their mature coating, but also because they may be lurking within crevices in the bark. At this later stage, we can only suggest that you lightly prune the flower clusters and the most affected branches. Then, using a stiff-bristled brush, you might then scrub the bark with soapy water made from of very diluted dish detergent, and finally that you then apply an insecticidal soap spray. Sprays should not be used in conjunction with the soil drench.
CMBS does not kill most trees, so you may have to wait 6 or 7 months before you can prevail against this scourge.
Photo Credit: Clemson University