This PDF from the VegMD site provides photos of common potato diseases and descriptions of risk factors. Scroll down for a table of conventional fungicides and ratings of their effectiveness against common diseases. Scroll down further for a complete list of OMRI approved organic potato fungicides for soil application, seed treatment, at planting application and foliar application, rated for effectiveness against selected diseases. Note: in both tables "Tuberborne" refers to preventing infection of tubers. Always check with your organic certifier before using any material. The last table lists many potato varieties and their susceptibility against many diseases, as well as other attributes.