doggy rash

My dog has a rash that starts out red and pussy then turns to brown spots. Dosen't appear to be allergic. Any ideas what is causing it

There are so many things that could cause rashes, your vet is really the only one who can diagnose it.

Some dogs can have a strong reaction to a single flea bite...or it could be a staph infection, or parasites...why do you think it's not an allergy? You don't say where on his body he gets this - young dogs actually get a sort of acne sometimes, around the snout and chin.