Common scab of potato

The causative agent of common scab is the soil-borne actinomycete Streptomyces rimosus (and related species like Streptomyces scabies). These filamentous bacteria live naturally in the soil, utilizing organic matter for growth. They are highly persistent pathogens that can survive in the soil for many years, even in the absence of a host crop, making them difficult to eradicate completely.

While potatoes are the primary economic host, this pathogen also affects a wide range of root crops, including carrots, parsnips, and sugar beets. The economic impact is primarily related to marketability, as the unsightly lesions reduce the aesthetic value of the harvest, leading to lower prices and increased losses during storage due to secondary infections.

The symptoms are highly characteristic and easily identifiable on the surface of tubers. They appear as superficial russeting, raised wart-like scabs, or deep, corky pits. These lesions occur when the bacteria penetrate the lenticels of developing tubers, triggering a defense response where the plant produces suberized, corky tissue to contain the infection.

The development of the disease is heavily influenced by soil conditions. Streptomyces rimosus thrives in alkaline or neutral soils with a pH ranging from 6.0 to 7.5. Furthermore, the pathogen is most aggressive in dry, well-aerated soils. Moisture stress during the critical period of tuber initiation significantly increases the likelihood and severity of the disease, as the tuber skin is most vulnerable during this phase.

Effective management requires a multi-pronged strategy. Farmers should aim to lower the soil pH by using acidifying fertilizers, such as ammonium sulfate. Maintaining consistent soil moisture during tuber initiation is crucial to prevent the entry of the pathogen. Additionally, rotating crops with non-susceptible species and selecting resistant potato varieties are essential practices for integrated disease management.