Q What is common scab?

A This disease (Streptomyces scabies) is the most common tuber problem for gardeners. Yield is lowered slightly, but the real waste comes in the extra peeling needed to prepare the potatoes for eating. Common scab also affects other root vegetables such as beetroot, radish, turnips and swedes, but it is seldom serious.

Caption: Common scab may look ugly but the potatoes can be peeled and eaten

Q How do I recognise common scab?

A Look out for corky irregular wide and flat bumps, often in groups. These are frequently pitted and covered in scabs. Stems are also attacked, but not noticeably.

Q When should I expect it?

A Dry, alkaline, sandy or chalky soils and hot dry summers promote scab attacks. The disease is an actinomycete – like a bacterium, but with threads of mycelium-like fungi. It lives in the soil, infecting tubers through natural openings like lenticels. Scabs release more resting bodies into the soil, where they remain dormant, ready to infect the next crop.

Q What can I do about common scab?

A Once the crop is affected there is no cure for this disease. The tubers may look unappetising but they are still edible if peeled. Storage is not affected.

Q Can common scab be avoided?

A Yes: by avoiding using lime before planting potatoes. Use plenty of organic matter such as compost, leaf mould and manure, when preparing the soil. Water during dry spells and grow resistant varieties. 'Accent', 'Anya', 'Pentland Crown', 'Golden Wonder', 'Nadine' and 'Wilja' are relatively resistant, but 'Desirée' and 'Maris Piper' are very prone to attack.

Avoid growing infected seed potatoes, but as the disease is widespread in most soils it's hard to avoid. Also, don’t compost diseased peelings or old potatoes