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Martyn Davey, Head of Horticulture and Design, Easton College
Tuesday, February 21, 2012
My potatoes appear to be suffering from a scab disease, about half of the crop is affected. Are they edible? And how I prevent it next year? (Eric Maloney, East Bilney)
This is common scab, a bacterial disease of potato skins which causes rough, scabby patches, as is caused by Streptomyces scabies. Scabs appear during summer and persist on harvested tubers throughout storage.
Common scab is most serious on potatoes, but also affects some other root crops. It is worse when soil conditions are dry when tubers form. Common scab can occur if the soil is dry during early tuber development.
Light attacks are only superficial and do little to affect eating quality, but such cosmetic damage lowers the value of commercial crops. Severe attacks can lead to cracking of the skin and rotting of the tubers.
To help avoid future problems select seed carefully and avoid planting seed potatoes that have signs of scab.
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 (four gallons per sq yd).
Common scab is worse on alkaline soil, so liming the soil to prevent club root of brassicas will predispose to common scab in potatoes. So 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.
•This article was first published on September 3, 2011.