Om is right. We tried everything on our Bulldog. The only thing that worked was a diluted Hydrogen Peroxide and borax mixture that we spray on. After months of agony, this remedy gave improvement overnight and cleared up his skin completely in about a week. The recipe is:
1 part 3% Hydrogen Peroxide, 2 Parts water, add 50 mule Borax until solution is cloudy (i.e. no more granules will dissolve). Apply with a wet cloth to thoroughly soak affected area. Later you can use a spray a few times a day.