Potato mop-top causes dark, internal blemishes in potatoes. Because it is an emerging disease in the United States and no control measures yet exist, growers need to know the basic facts to minimize its spread. This bulletin discusses the signs and symptoms of its viral origin, the virus’s disease cycle and the management strategies that might help your field avoid an outbreak. Full-color photography helps growers to more accurately identify potentially afflicted tubers; also, a table that compiles the results of studies of selected potato varieties’ sensitivities to the virus will help sharpen your crop plan each growing season.