Common potato scab

Common potato scab is a widespread soil-borne disease caused by actinomycetes of the species Streptomyces scabiei. These bacteria naturally inhabit the soil, feeding on organic matter, but when soil conditions are suitable, they infect young potato tubers, causing significant agricultural damage worldwide.

This pathogen has a wide host range, affecting not only potatoes but also beets, carrots, radishes, and parsnips. The primary infection occurs through the lenticels of developing tubers, as well as through wounds caused by insects or mechanical damage, where the bacteria colonize and disrupt the plant's skin cells.

Symptoms of the disease are characterized by surface blemishes on the tubers. The visual manifestations vary significantly:

• Superficial/Flat scab (shallow, corky lesions).
• Raised scab (pustule-like eruptions).
• Deep-pitted scab (deep craters penetrating the tuber flesh).
• Russet scab (network-like pattern covering the skin).

The development of Streptomyces scabiei is strongly favored by neutral to alkaline soil pH (above 5.5) and warm, dry conditions during the tuber initiation phase. Unlike many other plant pathogens, this organism thrives in soils with low moisture levels, making dry growing seasons particularly risky for scab development.

The economic impact of common potato scab is mainly due to the loss of market value, as affected potatoes fail to meet quality standards for fresh consumption. Furthermore, the lesions act as entry points for secondary infections, such as soft rot bacteria, which can lead to significant post-harvest storage losses if infected tubers are introduced into storage facilities.

Management strategies focus on soil modification and cultural practices. Maintaining a slightly acidic soil pH is one of the most effective ways to suppress the bacteria. Crop rotation with non-host crops, ensuring adequate moisture during the early tuber stages, and using certified disease-free seed potatoes are essential components of an integrated pest management program.