Powdery scab

Pathogen and Disease Type. Powdery scab is a soil-borne disease caused by the protozoan-like organism Spongospora subterranea. It is a significant pathogen that affects the roots, stolons, and tubers of the potato plant, leading to the development of distinct powdery pustules on the skin of the tubers.

Host Range. The disease primarily affects potatoes (Solanum tuberosum). However, it is known to infect other solanaceous crops and weeds. These alternative hosts play a critical role in the pathogen's life cycle, allowing it to survive in the soil even when potatoes are not grown in the field.

Symptoms and Signs. Infection starts as small, raised spots on the surface of young tubers. As the disease progresses, these spots develop into pustules covered by the host epidermis. Eventually, the epidermis ruptures, releasing a mass of brown, powdery spore balls. On roots, the pathogen causes the formation of small galls which interfere with nutrient and water uptake.

Conditions for Development. Spongospora subterranea thrives in cool, wet environments. The zoospores require high soil moisture to swim through the soil pores and reach the potato roots. Optimal temperatures for disease development range between 12°C and 18°C, making poorly drained or waterlogged soils high-risk zones.

Harmfulness and Control. The economic impact of powdery scab is significant, as it severely reduces the marketability of tubers and can lead to secondary infections during storage. Control strategies include planting disease-free seed tubers, implementing long rotations of 4 to 6 years, ensuring good soil drainage to avoid water accumulation, and maintaining appropriate soil pH levels.