I have never heard of it. Is it on the back of the pastern? If it is, it may just be mud fever that you are thinking of. It is scabby and hard to get rid of although it can occur in any horses, not just white legged ones.
My draft X used to get them. The vet gave me some cream for it-as I recall it was a silvadene creme (same stuff they use on burn victims). It worked. It also helped if I ket his feathers trimmed, so there wasn't so much moisture on his pasterns with dew, etc. You should be able to get the creme from the vet.
Another name for that is dew poisoning.... mix the desitin with neosporin and an antifungal creme after cleaning well. Make sure it's completely dry before applying that mix.
I use Neo-Predef but that can get expensive
Scratches can be a hard thing to get rid of. It takes daily cleaning and treatment. Some people may just use desitin morning and night. Keep it as dry as you can and clean as you can. Take off the loose scabs but be gentle. They are uncomfortable for your horse so be patient. :)
Not sure if it was called 'scratches' or not, but my guy had a yellowish soft-clumpy fungus last spring. I used did a daily cleaning and rinsing with Apple Cider Vinegar. It worked like a charm, dried it right up and came off bit by bit daily until it was gone. The area that was affected was about a couple of inches in height and half an inch in width, and it ended up taking just a couple weeks to disappear completely. He has not had a case of it since.
Yes just this year it has been accuring, now most horses get in in the summer in the fields but a neighbour of mine has been getting it in the winter. I personally havent tried anything on my filly yet until I hear of something that cures or helps it.