Fungus on my show horses tail???

I have a palomino gelding that i show regularly so yes i blue his tail and wash it all the time! I use a thickening shampoo plus MTG when needed. At the last show I noticed he was being really touchy to the tail and today when i gave him a bath i noticed large nasty scabs coming off and taking huge chunks of tail with it!!! I pulled all the scabs and unfortunately the hair with it and then i washed it with iodine shampoo bc the tail was inflamed and infected!!! after i washed it i rubbed it with MTG. It itches bc he was scratching when i was rubbing it!!! I have never seen this so could you help me? is it a sunburn, ringworm, alergy to the blueing? please help