Advice
RHS Help & Advice
Potato skin problems
Hosts
Common scab: potatoes, beets, radish and carrot.
Powdery scab: potatoes, tomatoes and other solanaceous plants.
Symptoms
Common scab produces unsightly scabby spots or patches on the skin of the tuber. These are often slightly raised, but the symptoms can be variable and in some cases common scab can cause quite deep pitting and cracking of the tuber surface.
Powdery scab is similar, but the scabby areas are usually slightly sunken with raised, papery margins and may contain dusty brown spores. In some cases attacks cause distortion of the tuber's shape.
Silver scurf and black scurf are less serious skin blemishes
caused by other fungi.
Biology
Common scab is caused by a soil-borne bacterium-like organism, Streptomyces scabies. The disease is more prevalent on alkaline soils which have a light sandy texture and which are deficient in organic matter. Infection is usually only superficial and the potatoes are fit to eat. Infection occurs when the tubers are just beginning to form, and if this stage of growth coincides with dry soil conditions then levels of the disease can be high.
Powdery scab is caused by a fungus (Spongospora subterranea) and is worse on wet soils and in wet weather. Light infections are not very damaging, but severe attack can render the tubers too distorted to eat.
Control
Planting clean seed without skin blemishes and practicing rotation will help prevent many seed- and soil-borne diseases, including powdery scab and black scurf. As the common scab organism is so common in soil, however, rotation will not help in this particular case.
Applying plenty of organic matter to light soils, to increase their moisture-holding capacity, will help to prevent common scab, as will ensuring that the soil is kept moist during the crucial stage of early tuber formation. Avoid growing potato crops after brassicas if the ground was limed to counteract club root on the brassicas.
Potato cultivars known to be particularly susceptible to common scab include ‘Desiree’, ‘Foremost’, ‘Majestic’, ‘Maris Piper’, and ‘Red Craigs Royal’. More resistant cultivars include ‘Accent’, ‘Juliette’, ‘Arran Comet’, ‘Arran Pilot’, ‘Golden Wonder’, ‘King Edward’, ‘Maris Peer’, ‘Pentland Crown’ and ‘Pentland Javelin’.
The cultivar ‘Pentland Crown’ is very susceptible to powdery scab. More resistant cultivars include ‘Home Guard’, ‘Desiree’, ‘King Edward’, ‘Pentland Dell’ and ‘Record’.
There are no fungicide treatments for any of these problems.
More information on powdery scab
More information on black scurf