Yeast (candida) is a normal part of the digestive Flora and, in a healty dog or person, is kept 'under control' by a strong proliferation of the beneficial bacteria. Your dog will have a yeast problem when the necessary bacteria is destroyed by outside factors, most frequently vaccination reactions. That same bacteria, when present in healthy amounts, also accounts for as much as 70% of immune function; therefore, its destruction can desimate much of immune system capabilities.
There are multiple underlying causes for dog skin sores including parasites (fleas, mites), poor grooming, dog skin infection (bacterial pyoderma, fungal), endocrine diseases, skin or hair follicle disorders disorders such as seborrhea, medication side effects (from corticosteroids) and allergy.
Vitamin deficiencies, specifically vitamin B12, foliate (folic acid), and iron, may trigger canker sore development. Hormonal changes, notably during menstruation, pregnancy, and menopause, have been linked to canker sores. Canker sores often run in families. Genetics may be a factor for many canker sore sufferers. Allergic reactions and sensitivities to certain foods may cause a canker sore to develop.