The best control comes from feeding animals a diet that does not contain synthetic toxic preservatives. The best commercial dog and cat food is Muenster Natural. The Missing Link supplement will also help. 
 
Comfrey tea can be use topically for dog skin allergies. Cook a mess of comfrey greens, dilute the cool pot liquor down to a light iced tea color and pour or dap onto the hot spots or into the into the dog’s fur. Comfrey is very powerful so don’t use more than once a month and only when needed. Cornmeal juice is also good and can be used more often. Put a cup of whole ground cornmeal in a 5 gallon bucket of water, strain out the solids and apply to the skin liberally.
 
Bathing the dogs in herbal shampoos that contain orange (d-limonene) and tea tree (melaleuca) will also help. Dry cornmeal, horticultural or whole ground cornmeal, dusted onto problem skin areas is also helpful, if the skin problems are fungal.