If the entire plant is covered in it, sprays are suggested. Heavy infestations can cut down on photosynthesis, which in turn can hurt blooming. If there is only a light coverage, ignore it. Spray with Daconil, Funginex or similar general fungicide. I trim them every year, but I notice around town that some people have already trimmed them back for this year and I generally wait until close to springtime.
Many people begin pruning their crape myrtles as soon as they lose their leaves in the fall.
While this can be done, I too, prefer to wait until late February. I think they look more attractive, and secondly, if we should by some chance get any real winter weather, the plants have more protection, than if they have already been pruned back. Crape myrtles have been known to be winter killed, but not in the close past, due to our mild winters. Let them grow up and become mature, attractive trees. Do prune off suckers, small growths, and make some thinning cuts.