Some dogs have food allergies, usually to grains (which can be the major part of Kibble) and even to some or most animal proteins - go figure on that one, dogs descend to us from wolfs that ear mostly animal protein.
There can be other environmental factors. Allergies can be hard to specifically identify. When feeding Kibble I always use grain free or at least elimination of corn and wheat. Look on "dogfoodadvisor" for a listing/rating of dog foods You'll find there the Tractor Supply house brand 4Health is highly rated and is medium to lower cost. You don't have to spend a fortune to get good dog food, the super market shelves are full of not-so-good dog food in my experience.
Yeast infection will result in lost of fur, itch, licking biting the infected areas, and a smell - ears are one area where yeast likes to form. There are prescription meds and over-the-counter shampoos for dogs containing keteconazole, the best way to treat yeast, and can be used as a maintenance deterrent, so you can use the shampoo even if not immediately needed. I have sound this type shampoo in Walmart available without a prescription.
Good luck and tell us more about the symptoms, breed and age.
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