Without examined the spots it is impossible to tell what the problem is. Allergies (flea, environmental, and food) can cause skin lesions. Allergies decrease the skin's immunity and secondary infections start--both yeast and bacteria. Mites, insect bites, or ringworm are also possibilities. With mild allergies symptomatic treatment (fish oil, bathing, anti-histamines, and treating secondary infection) is started. With more severe allergies or if you just want the information allergy testing can be done to determine exactly what your dog is allergic to. Allergy testing can be done via a blood or skin testing. Have this dog examined by a veterinarian for appropriate diagnosis and treatment. Your veterinarian can give you more information about allergy testing.