Potato scabs

What are potato scabs?

Common scab is caused by Streptomyces scabies and powdery scab by Spongospora subterranea f. sp. subterranea. Both are pathogenic micro-organisms and cause rough, scabby patches. Scabs appear during summer and persist on harvested tubers throughout storage.

Common scab is most serious on potatoes, but also affects beetroot, radishes, swedes and turnips. Common scab is worse when soil conditions are dry when tubers form.

Powdery scab is worse under wet conditions and also infects tomato roots.

Symptoms

You may see the following symptoms:

• Common scab: Raised, rough patches of skin on the tuber surface
• Powdery scab: Irregular brown depressions containing masses of dusty brown spores on the surface of tubers

Severe attacks of scab can lead to massive distortion of tubers, reminiscent of wart disease caused by Synchytrium endobioticum, a serious but now very rare potato disease.

Control

Non-chemical control

• Select seed carefully and avoid planting seed potatoes that have visible signs of scab. Legislation controls the amount of infection allowed in seed, but low levels are permitted
• To control common scab, do not allow the soil to become dry during tuber development. Raise organic matter levels to improve water retention. Water the developing crop if necessary, starting two to three weeks after plants emerge and continuing for about four weeks and applying 20 litres per sq m (4 gallons per sq yd)
• Unfortunately, common scab is worse on alkaline soil, so liming the soil to prevent club root of brassicas will predispose to common scab in potatoes. Apply lime after the potato course of the rotation
• Choose resistant cultivars. ‘Accent’, ‘Arran Pilot’, ‘Juliette’, ‘Golden Wonder’, ‘King Edward’, ‘Pentland Crown’ and ‘Pentland Javelin’ show some resistance to common scab. ‘Desiree’, ‘Hermes’, ‘King Edward’, ‘Pixie’ and ‘Sante’ show some resistance to powdery scab
• The British Potato Council has produced the British Potato Variety Database which lists pest and disease resistance on a one to nine scale for many varieties and is a valuable guide to variety selection if any particular pest or disease is known to be a likely problem. For more information visit The British Potato Variety Database

Chemical control

There are no chemical controls available to gardeners for either disease.

Biology

Both the common and powdery scab pathogens are sometimes described as fungi, but in fact Streptomyces scabies is more closely related to bacteria and Spongospora subterranea f. sp. subterranea is related to the slime moulds.

Both organisms exist in the soil, either free-living (S. scabies) or as spores (S. subterranea f. sp. subterranea). They invade the surfaces of potato tubers and the plant responds by growing corky scabs, which actually limit the spread.

Powdery scab is worse under wet conditions because its spores germinate to produce infective swimming spores which need water for infection. By contrast, severe attacks of common scab can occur if the soil is dry during early tuber development.

Light attacks of both diseases are only superficial and do little to affect eating quality, but they are commercially important because cosmetic damage lowers the value of the crop. Severe attacks can lead to cracking of the skin and rotting of the tubers.

Very severe attacks, especially of the cankerous form of powdery scab, may be mistaken for infection by the serious disease known as wart. This is now very rare in the UK, but is notifiable and if it is suspected, the local Plant Health and Seed Inspectorate (a part of Fera) should be notified.