
2 of my rotties get these and I clip/shave the area (to let the air get to it), give a good clean with hipiscrub, and then depending on where it is either use diluted tea tree oil (apparently neat could cause it's own problems, plus they don't like the smell), or daktarin atheletes foot powder.
My boy had one at the root of his tail a couple of months ago, did the above with the daktarin (sprayed every other day) and cleared up nicely within a few days.