We rescued Roxy from some Craigslist nut jobs, and she had something that looked similar to that.

We had her allergy tested and a skin scraping done and the vet determined that part of the hair loss was a wheat allergy and the rest of it was a bacterial skin infection called pyoderma. She's on antibiotics now along with Benadryl and has to have baths in special medicated shampoo twice a week, and it seems to be really clearing up. It's now just a patch on the back of her neck that looks VERY similar to the picture you have up there, but it's steadily going away.

It might be something to check into!!!