Hot spots can also be an allergic reaction to something in the dog's food, bedding or environment (pollen, fleas etc.). My old rottie use to get them badly. Vet recommended Benedryl which eased the symptoms but did nothing for the cause and I couldn't see keeping him on it long-term. A friend suggested that I switch to a dogfood without soy, because that had caused problems for her son's dog. On a what the hell, that's a cheap fix basis I gave it a try. Problem went away in about 2 days, and never came back. Soy may not be your specific problem, there are other ingredients and contaminants that may be the culprit. But I would recommend feeding a low allergen food, preferably one made with human-grade ingredients. A small bag won't break the bank, and can give you a handle on one possible source of irritation.

Note: the more you learn about supermarket dogfood the more revolting it gets: feathers, grain, contaminated meat...nothing that should be fed to anybody or anything. The premium human-grade food may be more expensive per bag, but it costs far less per serving (you only need about 1/3rd as much) and in pays in terms of lower vet bills too.