Scab (Potato)
Streptomyces scabies, S. acidiscabies, S. turgidiscabies
Overview
Potato scab is caused mainly by Streptomyces species, soil-dwelling filamentous bacteria that produce corky lesions on potato tubers and sometimes on other root crops such as beet and carrot. The organisms thrive in dry, alkaline soils rich in undecomposed organic matter from previous root crops. Scab is primarily a quality disease: tubers remain edible after peeling, but fresh-market appearance suffers. Once Streptomyces populations are high in a bed, scab can return whenever susceptible potatoes are grown under favorable conditions.
Symptoms
Tubers develop shallow to deep corky, rough patches or pits on the skin. Lesions may be scattered or cover much of the surface. Russeting and raised corky scabs are common forms. Foliage usually looks healthy, so the problem appears only at harvest. Deep-pitted scab is harder to peel and more severe. Other root crops in the same soil may show similar corky spots. Wet soils during tuber initiation tend to reduce scab compared with dry soils at that stage.
Environmental Conditions
Dry
Prevention
Choose resistant varieties as the first line of defense. Use certified seed potatoes free of obvious scab. Irrigate consistently during early tuber set rather than letting beds dry hard. Build organic matter with well-finished compost, not raw manure, before potato seasons. Avoid overliming potato beds. Rotate at least two to three years away from potatoes and beets on problem soils.
Management
Maintain slightly acidic soil (often near pH 5.0 to 5.2 for scab-prone ground) when that fits your crop mix; lime carefully if you also grow acid-sensitive plants elsewhere. Keep soil evenly moist during tuber initiation and early bulking. Plant scab-resistant potato cultivars. Avoid fresh manure or undecomposed compost high in lime just before potatoes. Rotate potatoes with non-host crops and do not save seed tubers from scabby lots for replanting.