It works for my particular fungus. It may work on yours. Rainrot is a bacterial infection. Clean and DRY is very important to get rid of it. Use a blow drier if need be.
Severe cases benefit from a short course of antibiotics.
The crusts one peels off contain vast numbers of organisms, and are best disposed of in the trash rather than the horse's immediate environment. Then shampoo with a povidone-iodine and shampoo mix. Thoroughly dry the horse and apply Desitin. This works for scratches too. If you get a sticky substance like tree sap in your horse's coat, you can rub peanut butter in the hair.