The dark black powdery stuff is sooty mold. It grows in the sticky substance given off by aphids, scale and whiteflies. I would swab the siding down with a product call Krud Kutter to help soften the stuff and then power wash it. If that does not work, swab it with mouth wash and scrub with one of those Mr. Clean eraser sponges. Best And Happy Yardening, Nancy.
Crepe Myrtles Have No Leaves And Dark Powdery Mold
Question From: N. Lee - TEXAS
Q: crepe myrtles on Galveston bay in Texas have no leaves, were not pruned, and now have a dark black looking powdery mold that is on the limbs and stems AND seems to be blowing off and getting all over white siding material. What is it and what do I do ! It is hard to wash off and is sticky. Thanks