hello and thank you for asking your question on just answer.
It sounds like she has "dermatophilus" or "rain rot."
It is a superficial bacterial skin infection caused by a strep bacteria (usually) and the skin being too wet.
It is contagious to other horses through brushes and tack. So you are going to have to soak all your brushes in betadine and water. Wash your saddle pads thoroughly in hot water and soap. Wash your halters and other tack too.
To treat your gelding, clip the areas that area affected. This is to allow air to get to the infected areas. This is a very important part of treatment.
Go to the tack store, drug store, or online and purchase "Betadine scrub" or "nolvasan scrub."
Apply enough water to get the skin wet. Apply the scrub to the lesions. Srub for about 5 minutes then let the product sit on the lesions for about 10 more minutes. Then scrub again and see if you can get anymore of the crusts off. I know you will be gentle of course.
Pat the area dry with a towel. Put the towel in the washing machine since it is infected with the bacteria.
Now make sure his back stays dry.
Do this treatment every day until the skin leasions have cleared up.
Severe cases can be treated with penicillin injections.
Do not share tack, blankets, or burshes with anyone.
Is he does not start to improve in a few days, contact your veterinarian. Stubborn cases can be treated with antibiotics.
I am going to change this over to another format. If you reply, I will see it. Thank you : )
If he has a short summer coat, you do not need to clip it.
If he gets this again, and he has a long winter coat, then I would clip the affected areas.
This is because the long hair holds the moisture which makes the infection worse.
I hope this helps : )