Powdery scab – Spongospora subterranea, fungus
"Immature lesions occur on the tuber surface and are white and wartlike, then gradually become darker. Mature lesions appear as open pustules, which contain olive brown to black powdery masses of spores, often surrounded by the remnants of the periderm. The presence of the granular–appearing spores in mature lesions is diagnostic, but they may require magnification for positive identification. Lesions are not corky in appearance as are those of common scab, but may be confused with this very different disease. Powdery scab is favored by heavy soils and high soil moisture. This disease can spread in storage or lead to dry rot. Inoculum from infected seed tubers can spread to daughter tubers and cause infection. Clean seed and long rotations are recommended for control of powdery scab."
Information provided by: Zitter, T.A., Loria, R. "Vegetable Crops", Cornell University Vegetable MD Online