Sometimes photosensitivity issues can develop late fall in heavily clovered pastures as the rains return. It can look similar to a roaring case of rainrot....only the exposed skin generally has a purplish shade. Other feed ingredients higher in the chemicals can contribute to the issue.
It also may have started as rainrot and progressed to other. Sometimes we see rainrot and then skin fungal conditions develop during monsoon season. We spray the skin down or even bath with a Chlorohex solution and then later after the area is dry groom with lamisil spray powder.
For cases that have gone past being able to be treated topically in either size or intensity, I would get a shot of EqStim from your vet to boost the immune system - it has worked every time I have used it.
I agree too with what others have mentioned about minerals in the diet - I top dress our feed with Equimin, which is a vitamin/mineral supplement and we hardly have any rainrot issues in our barn.