The protozoan called Spongospora subterranea.
Spongospora is a soil-inhabiting organism with a complex life cycle. It can survive in soil without hosts for several years; hosts include many solanaceous relatives of potato including nightshades.
Powdery scab occurs in most potato producing regions in the world, and seems to be getting more prevalent in the Northwest.
Spongospora is easily moved to new locations on seed tubers or in soil.