Silver scurf – Helminthosporium solani, fungus
"Silver scurf is relatively common on tubers, including those grown for seed. However, the symptoms of this disease may go unnoticed unless tubers are very carefully examined. A fine coating of dark green to black spores, visible to the naked eye only in mass, can sometimes be seen on the surface of infected tubers that have been stored under conditions of high humidity. However, the best way to detect this disease is by washing the tubers and looking for a silvery sheen that occurs in patches on the tuber surface. These patches may cover a large portion of the tuber and are caused by air space, which results from growth of the fungus beneath the tuber periderm. Symptoms can be difficult to detect on some white–skinned cultivars, but are obvious on those with red skin. Heavily infected tubers may not sprout properly and are an important source of inoculum for subsequent infection of daughter tubers. Infection does spread in storage, but can be limited by maintaining temperatures of 40° F and providing forced–air ventilation."
Information and photo sources: Zitter, T.A., Loria, R. "Vegetable Crops", Cornell University Vegetable MD Online