Question
Hi Jim,
I recently purchased two Muskogee Crepe Myrtles and one of them started having yellow leaves with black spots on them.  The leaves are dropping like crazy and now the branches are turning black as well.  I have large black wasp type bugs flying around the trees.  It seems that the black mildew looking stuff is spreading to other plants in my landscaping.  Please help me figure out what this is and what I can do to get rid of it.

Thanks,

JoAnn

Answer
Sounds like sooty mold. Sooty mold grow on a secretion from aphids called honeydew. Honeydew is high in sugar content and sticky. The sooty mold will grow on the honeydew. This will attract ants and wasps. Aphids suck the plant juice from the plant and secretes the honeydew. Control the aphids and the sooty mold will go away.

There are several insecticides effective for aphid control when sprayed on plants. Perhaps most effective are those with systemic activity that allows them to move through the plant. Acephate (Isotox, Orthene) is the most widely available systemic insecticide.

Some insecticides can be applied to the soil and taken up by the roots of the plants. These are called systemic insecticides. The most recent, Imidacloprid, is sold under the trade name Bayer Advanced Garden Tree & Shrub Killer Concentrate. (Merit is the trade name of imidacloprid used by professional tree care companies.) It is applied as a drench over the root zone.