The itchy dog causes can be simplified using age of onset and symptoms.
Canine skin problems can start early in life. If your dog has ear infections and an itchy bottom and is scooting and is less than three months of age, you need to be thinking food allergy.
If your dog is scratching the lower back and base of the tail, it is probably a flea allergy.
If your dog is developing bald patches that are not itchy, you need to think about Demodex mange.
If your dog is going bald and is so itchy that it scratches night and day, then you need to consider scabies which is sarcoptic mange. It needs to be differentiated from the red mange which is Demodeciosis or Demodex mange.
If your dog did not have any skin problems until eighteen months or a few years of age, you need to consider atopy or canine atopic dermatitis.
This is a simplified account but if you want a more detailed account, you could consider our home diagnosis manual
For a thorough account of treating the itchy dog, you need to consider our itchy dog bible