Powdery Scab
Spongospora subterranea
Overview
A soil-borne disease of potatoes caused by a protist, producing raised, powdery pustules on tubers and root galls. Also serves as a vector for Potato mop-top virus. Powdery Scab is managed first with cultural controls suited to permaculture systems: sanitation, spacing, irrigation style, and resistant planting stock. Causal agent: Spongospora subterranea. In regenerative gardens, prioritize sanitation, airflow, and unbroken soil cover over calendar spraying, and intervene early when symptoms appear.
Symptoms
Raised, circular pustules on tuber surfaces that rupture to release powdery brown spore balls. Root galls. Cankers on stolons. Tubers may crack. Reduces tuber quality and marketability. Compare several plants and note whether symptoms spread after rain, insect feeding, or wet foliage. Photograph undersides of leaves and cut stems when wilt is present to refine the diagnosis. Rule out simple drought, fertilizer burn, and herbicide drift before assuming a pathogen - abiotic injury often mimics disease.
Environmental Conditions
Cool
Prevention
Use certified disease-free seed potatoes. Rotate potatoes on a 5+ year cycle. Improve drainage. Avoid overwatering. Plant resistant varieties. Maintain soil pH below 5.5. Mulch to reduce splash, clear season-end debris, and avoid working plants when wet. Design beds for airflow and diverse plantings that support beneficial insects without creating permanent humid pockets around susceptible crops.
Management
Long crop rotation. Improve drainage. Avoid overwatering. No effective chemical control. Remove infected tubers. Clean and disinfect equipment. Prefer drip irrigation, morning watering, and open spacing so leaves dry quickly. Disinfect pruners between cuts on woody plants. Use copper, sulfur, or biological products only as labeled protectants when pressure is high - they rarely reverse advanced infections alone. Destroy (do not casually compost) highly infectious debris.