Silver scurf is caused by the fungus Helminthosporium solani. It is a very common potato disease which only develops on tubers. In the field early varieties usually become more severely affected than late maturing ones.
At harvest, symptoms of infection are usually barely visible. During storage however, the disease can spread rapidly. Silver scurf causes no symptoms in the foliage. This disease is becoming increasingly troublesome because in supermarkets potatoes are sold after having been washed and packed in plastic bags. Washing makes the silver scurf lesions clearly visible. Moreover, the moist microclimate in the plastic bag greatly boosts the development of silver scurf.
On the tuber, roughly circular shaped silver grey lesions occur (see photo at the top of this blogpost). At the end the entire tuber may become covered with silver scurf (see photo below). If stored under moist conditions black sporophores with spores are formed which are visible with the aid of a magnifying glass. During transport and in storage in closed containers at too high temperatures affected tubers turn sooty, due to an abundant production of the black spores. Due to excessive moisture loss severely affected tubers shrivel in the course of time and do not sprout well. Symptoms of silver scurf may be confused with those of black dot. However, silver scurf lesions are usually sharply defined, while the border of black dot lesions is normally defused.
Top photo: Silver grey patches on a tuber, due to silver scurf. (Plant Research International Wageningen, The Netherlands)
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