Symptoms
Irregular brown depressions with raised papery margins. These scabs are filled with dusty brown spores, which are distinctive of this disease.
The disease can be confused with common scab (Streptomyces
scabies) if the spores are not clearly visible. In some cases the tubers need to be examined microscopically to confirm which disease is present.
When young tubers or eyes are infected, tuber distortion and swollen outgrowths may result. In these cases the symptoms resemble those of the disease wart, caused by the fungus Synchytrium endobioticum. Potato wart is very rare in gardens, but is a notifiable disease.
Roots of tomatoes and other solanaceous plants may also be attacked, but the effects are seldom noticed.
Cause
This disease is caused by the fungus Spongospora subterranea f. sp. subterranea. It is generally found on wet, badly drained soils. The spores remain in the soil for several years.
Control
Plant only healthy, powdery scab-free seed (note: plant health legislation allows a low level of infection on seed potatoes, but any sign of infection should be avoided if possible).
Dispose of infected tubers - do not compost them. Avoid replanting potatoes in the same position for three years. Improve soil aeration.
There are no fungicides that can be used by home gardeners.
Avoid planting the cultivar ‘Pentland Crown’ which is very susceptible on sites that are known to have a problem. ‘Home Guard’, ‘Desiree’, King Edward’ and ‘Pentland Dell’ show some resistance.