Q What is powdery scab on potatoes?
A This is a fungal disease (Spongospora subterranea), but not particularly common. It sometimes affects tomato and watercress roots. It has an free-living, mobile stage in the soil, during which time it can transmit mop-top virus.

Caption: Powdery scab usually occurs in wet seasons and heavy soils

Q How do I recognise powdery scab on potatoes?
A You'll notice raised irregular scabs which release a powder of spores. This powder is brown and the spores can survive for as long as 10 years in the soil. Before the scabs burst, there may be surrounding areas of discoloured skin. In extreme cases cankers form, disfiguring the tubers with large outgrowths. These aren't spreading and cauliflower-like, as in wart disease. Another feature of powdery scab is the tumour-like growths which form on the roots.

Q When is powdery scab likely to attack potatoes?
A It's associated with heavy soils and wet seasons, but if potatoes are overwatered, it can occur in any summer and on light soils. The spores germinate in the soil and release swimming spores that affect roots by travelling in the soil moisture. More spores are released from these roots; they enter tubers through natural openings such as lenticels and wounds. Here, new scabs are produced. If there's a dry spell followed by wet weather, cankers and tumours are produced. Spores from these cankers then attack newly formed tuber tissues, making more scabs.

Q What can I do about powdery scab on potatoes?
A There’s no cure. Spores are long-lived so don’t grow potatoes on infected sites for at least three years.

Q Can powdery scab on potatoes be avoided?
A Avoid manure from places where potatoes are fed to animals and reject affected or damaged seed.

Q Are there any varieties that are resistant to powdery scab on potatoes?
A 'Santé' is exceptionally resistant. Or try 'King Edward', 'Desirée', 'Maxine', 'Pentland Dell' and 'Record'.