Powdery scab

Powdery scab is a soil-borne disease caused by the plasmodiophorid Spongospora subterranea. This pathogen is notorious for its ability to survive in the soil for many years in the form of spore balls called cystosori. Due to its persistence, once an area is infested, it remains a high risk for future potato crops.

The primary host for this pathogen is the potato plant. It attacks the roots, stolons, and tubers of the crop. The root infection is particularly detrimental as it creates galls, which interfere with the plant's ability to absorb water and essential nutrients, leading to reduced tuber quality and lower overall crop yield.

The symptoms on tubers are characterized by the development of raised pustules that eventually rupture to expose a powdery mass of brown spores. These lesions often become unsightly and can merge, creating larger, deeper open sores. Furthermore, the pathogen acts as a vector for the Potato Mop-Top Virus, causing further economic losses for farmers.

The development and spread of powdery scab are heavily influenced by environmental conditions, specifically high soil moisture and cool temperatures (between 12°C and 18°C). These conditions trigger the release of zoospores, which swim through the soil water to infect the young, developing potato tubers.

Management of powdery scab relies on an integrated approach since chemical control alone is rarely effective. Key strategies include:

• planting certified, disease-free seed tubers;
• ensuring well-drained soil to avoid waterlogging;
• implementing long-term crop rotations (at least 4-5 years without potatoes);
• avoiding the use of contaminated manure;
• selecting resistant or tolerant potato varieties.