Spots tend to be bluish black and slightly raised. The mature fungal lesion is frequently sunken with a raised centre. Lesions may form either singly or in clumps and be distributed at random or aggregated around eyes, stolons and damaged skin. Infection can spread during storage. Early applications of CIPC sprout suppressant may exacerbate the problem. Skin spot can lead to necrotic buds in eyes and dense white mould on infected sprouts.
Cutting through a skin spot shows a dark thickening, which may extend up to 2mm into the tuber. Sealing tubers in a moist container for 3 days at room temperature should allow lesions to develop sporulating mycelia, visible as white fluff.