Hi Sandy-
Grain free and potatoe free have nothing to do with yeast in dogs unless your dog has an allergy to grain (fairly rare) or potatoe (also fairly rare). This is a popular internet myth. Dogs with yeast are not necessarily allergic to the food they are eating either. Yeast overgrowth on the skin is secondary to a larger problem like allergies, but environmental allergies can also cause yeast.
The best thing you can do if you want to rule in or rule out food allergy is a elimination trial. This is the only realiable why to diagnois a food allergy. Using a food from the vet like Royal Canin Ultamino or a homecooked diet of a novel protein and carb for 3 months are your best options. But the dog can not eat anything else, but that diet for the full length of the trial. Then the idea is to challenge the dog by putting him back on the old food and see if he has a reaction. If the symptoms went away during the food trial and came back with the old dry food, then you will know its a food allergy. If the symptoms show no improvement on the elimination diet, then food is not the issue. If food is not the issue the next step is a veterinary dermatologist for environmental allergy testing (if you can afford it).
I couldn’t afford the dermatologist, so I’ve been managing my dogs seasonal allergies with frequent bathing in Malaseb shampoo or Miconahex+Triz by Dechra. Both available on chewy.com.
