Groups That may cause skin problems

Manges
Ear Mites
Ticks
Bacterial Diseases
The commonest skin pathogen is Staph aureus, but there are many others.
Wet Eczema
This tends to occur in warm weather, and it starts off with a minor scratching and in no time erupts into a wet, septic area of discharge.
Atopic dermatitis
Anal Furunculosis
This is a very bad disease , it involves the massive breakdown of tissue next to the anus, with secondary infection.
Fungal Diseases
Malasseza (see orange stripe)
Allergies
There are just so many allergies you dog can suffer from. Allergens (the things cause allergies) can be inhaled, ingested, or touched by the skin.
Contact Irritations
These problems are caused by the irritant which the dog has come in contact with.