Silver Scurf

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Beginner

Scientific Name
Helminthosporium solani

Identification
This fungus only attacks tubers.

Symptoms usually appear first at the tuber stem end as small, circular or irregular spots of silvery appearance. Lesions may enlarge covering most of the tuber surface. The silvery appearance is more noticeable when the tubers are wet.
Tuber lesions that develop in storage are circular and dark brown to black. They increase in size and coalesce, rendering the tubers unmarketable
In storage, infected tubers lose water through the lesions, so some shriveling and wrinkling may occur.
Occasionally, silver scurf and black dot develop on the same tuber.

Often Confused With
Black dot

Period of Activity
Silver scurf develops late in the season. Delaying harvest increases disease incidence.

Scouting Notes
Look for silver scurf on tubers late in the season. Dig tubers at random after topkilling and wash them to assess silver scurf incidence.

Thresholds
No thresholds are established.

Advanced

Scientific Name
Helminthosporium solani

Identification
This fungus only attacks tubers.

Symptoms usually appear first at the tuber stem end as small, circular or irregular spots of silvery appearance. Lesions may enlarge covering most of the tuber surface. The silvery appearance is more noticeable when the tubers are wet.
Tuber lesions that develop in storage are circular and dark brown to black. They increase in size and coalesce, rendering the tubers unmarketable
In storage, infected tubers lose water through the lesions, so some shriveling and wrinkling may occur.
Occasionally, silver scurf and black dot develop on the same tuber.

Often Confused With
Black dot

Biology
The silver scurf fungus overwinters mainly on infected seed tubers. It can also survive for at least one year on decaying organic matter in the soil, indicating that soil may be a source of inoculum.

On infected tubers, the fungus produces spores only at the edges of the lesions. Thus, seed tubers with the surface completely covered by silver scurf do not spread the disease because the fungus cannot produce spores.

Spores produced on seed tubers wash onto new tubers. Infection takes place through lenticels and directly through the skin. The infection remains confined to the skin tissue, causing it to loosen. Air trapped under the loosened skin reflects light, resulting in the silvery sheen typical of infected potatoes.

Silver scurf lesions may be difficult to detect at harvest, particularly if the tubers are not washed. Disease incidence increases when potatoes are left in the ground after maturity and are harvested late. High humidity in the fall also increases silver scurf damage.