I agree with Jaymi, I have a chestnut Arab and she used to get fungus PERMANENTLY regardless of the season. Eventually I started putting a fly sheet/rainsheet (weather dependant) and ever other day I would wash her down with medicated shampoo. ☺
Try Hey where's that blue stuff! You can find it in Facebook and oh my goodness I swear it will change your life! It works amazingly and leads the fungus within a day!!!!
Keep a rain sheet on him when ever he is outside. Wash in medicated shampoo often. We make a mix of white vinegar, methol and baby oil in a spray bottle to kill it as much as possible
Make sure he's not wet after you turn him out
BANIX! It was sent from the heavens. Just spray it whatever and it should go away. You can also try scrubbing parts or your horse that are kind of hidden away like the fetlocks and behind the ears.