I've got white dogs and both did this the best course of action I took was to swap to a barf diet and feed lots of essential fatty acids.

Dr Billingstone's book claims that dry dog food is very low in vitamin a and EFAs and therefore its common for dogs to suffer with dry skin, so a supplement of some liver, oily fish and perhaps some corn oil would be very beneficial. Might take a week or so for you to see a difference.

My dogs don't itch as much now since I've been doing this feeding method.

hth