yeast problems are often caused by food allergies. yeast cells are present even on healthy dogs, but the immune system is able to keep them from overgrowing.
when the immune system is assaulted by allergies, it doesn't have the resources to do both, fight allergies and keep the yeast from overgrowing, so you get problems.
i saw in another thread that you are feeding your dog nutro, and i'd recommend switching to a quality food with better quality ingredients, such as for example california natural lamb and rice.
nutro is popular, but overpriced and by far not as good quality as salespeople make it out to be. it used to be not great but still decent about four years ago, but since then it has gone downhill.
edited to add: nutro lamb & rice contains lamb, but it also contains "poultry fat", which can still contain chicken - you never know because "poultry" is a generic designation. if a dog has issues with chicken, you have to eliminate any chicken products in the food.