The fungus, Oospora pustulans (syn. Polyscytalum pustulans), develops from infected seed tubers, although resting bodies of the fungus may remain alive in soil for at least eight years. Severe sprout infection may delay plant emergence.
The disease is more prevalent in cool, wet seasons and more severe in crops on heavy soil types than in those grown on sandy or organic soils. Skin spot is common in crops harvested under moist conditions and stored under cool conditions (4EC). Infection takes place in the soil before harvesting and may spread in storage. At harvest, infected tubers are symptomless; skin spots and dead spots in eyes develop about two months later in storage.
Roots, stolons and stems below ground develop light brown spots that enlarge, darken and crack. Sometimes it is difficult to differentiate from symptoms due to Rhizoctonia disease. On the stored tuber, crater-like depressions with raised centers form. These pimple-like areas are slightly darker in colour than the surrounding healthy skin, but when wet, they become purplish-black.