skin problems

I live in the tropics in Costa Rica. I have just recently put my horses to pasture on a farm. My 5 yr old Ibero always develops a skin rash on the face, neck and legs. It is apparantly due to a fungus or bug in the grass.  I have injected him with Dectomax, and am applying an ointment with several ingredients, sulfur, iodine, and an antifungal agent.  This condition consists of itchy small circles where the hair is lost.  We have no real large animal vets here and the vets we have just wing it.  Anyone have any suggestions for diagnosis and cures?

my horse gets something similar to that except it only happens in the summer time and hell ich like crazy until he bleeds i was constantly applying a salve until my farrier told me about super 14 it's a coat mineral that promotes shiny coats healthy skin and hair growth i swear just after a few weeks my horses coat looked 5 times healthy but i also use a fomula called flex which helps my horses immune system it prevents colicing and helps the joints in horses its an allaround vitamin it's some really good stuff