Hi Konrad; I have seen a similar disease in a seed potato right out of the bag. I didn't use it. The reason I discovered it was they were big seed potatoes, and I cut them up before planting, otherwise I would have never known and maybe started some problem like you have. Since the winters are milder we have potatoes volunteering every year now, they used to not make it through the winters before. Were the tops different ie wilted or yellowish? I would definatly give that part of your garden a rest from potatoes for a couple of years. Chic