my mare had something similar this summer when it was really hot. Is it accompanied by bumps?
I found that scrubbing it with a curry comb to get all the dead skin off while bathing her and then putting some betadine on it helped to keep it clean. There's an ointment you can get, similar to a neosporin for humans, that you can get for horses and apply once the area is dry after washing. My vet gave it to me, its really cheap.
Something you can also look into is a sweat spray that you can get at a local tack shop, that might help too.