Top up diet with Copper and Zinc to improve skin quality.
Treat immediately you see the first sign - hair standing on end, possibly clumpy - with Nizoral shampoo. You can buy it from the chemist, just don't tell them it's for your horse!
Dilute with hot water and lather up well, working well through hair to skin, wash well around the whole leg and well beyond affected region. Nizoral works really well especially if you get to the problem fast.