Symptoms are normally present at harvest but the disease develops during storage. During storage, potatoes display silvery spots that enlarge into circles with defined brown margins. The lesions later coalesce and may turn brown, making them hardly visible on unwashed potatoes. The appearance of lesions also varies with the potato variety, mostly due to skin types. The outer skin layer of infected potatoes softens and wrinkles and eventually peel off. Secondary infections with other pathogens may occur.
Silver Scurf is caused by the seed-borne fungus Helminthosporium solani. It survives on tubers for long periods of time and infects the skin. The infection can arise from the soil, the use of infected seed tubers or remaining spores in storage rooms. Temperatures of 3°C and relative humidities lower than 90% during storage hinders the development of the disease. The formation of condensation on the tubers during storage (warm air meets cold tubers) exacerbates the problem. Even though the potatoes are still edible, their market value is significantly reduced.
Natural biocides (hydrogen peroxide) or biological products (Bacillus subtilis, clove oil) have shown limited or no efficacy in reducing the risk of infection with silver scurf.
Always consider an integrated approach with preventive measures together with biological treatments if available. The application of fungicides to seed potatoes prior to planting or at harvest may prevent infection. Thiabendazole, applied to tubers as dusts, can reduce the incidence of silver scurf in the next season or during storage.