Common scab
A soil bacterium that causes rough, corky, scabby patches on potatoes and other root crops. It spoils the look but not the eating quality.
Also known as: Potato scab, Scab
Symptoms and identification
Raised, rough or sunken corky patches appear on the skin of tubers and roots, brown to dark grey. The flesh underneath is usually unaffected, and the damage can be peeled away. Leaves and yield are normally not affected.
Causes
Streptomyces bacteria live in soil and are most active in dry, warm, alkaline soil with a pH above about 5.5, particularly when tubers form during dry spells.
Organic treatment
There is no treatment once scab has formed. Peel affected roots and eat them as normal, and improve soil moisture next season. Green manures such as mustard may help reduce scab over time.
Prevention
Keep soil moist while tubers form, especially in the first 4 to 6 weeks after flowering. Avoid adding lime or fresh manure, rotate crops and choose scab-resistant varieties.
Chemical treatment (use with care)
No chemical control is generally recommended in home gardens. Lowering soil pH slightly with sulphur is an option on alkaline soil, guided by a soil test.