Possibly my mare has ringworm? Any home made treatments? Or what to use?

My barn owner thinks it's ringworm, as do I. But I am not familiar with it, just with some research, some I am not too knowledgeable if it actually is or not. My mare has hair missing underneath her tail. All around under it. Her tail isn't rubbed though, just the skin underneath. I actually had to even lift the tail to even notice it. Another horse at the barn has it now too, but not as much as her. It's basically dry flaky skin. It doesn't look as bad as described. It also doesn't seem like it's bugging her. Anyhow, is it ringworm? Is there anything I can use for that? I heard that vinegar does... is that true? Or maybe buying some anti-fungal things? It's really cold here right now, as it's still winter, so I would really rather avoid bathing that area unless completely necessary. She is boarded outdoors 24/7. I will not be separating her into a stall if someone suggests it, as she has heaves.