Common scab of potato

Streptomyces tumemacerans is a significant species of actinomycetes known as a causative agent of common scab, a widespread disease affecting potato tubers and other root crops. As a soil-borne bacterium, it is characterized by its ability to thrive as a saprophyte, utilizing organic matter and debris to persist in the field for extended periods.

The disease primarily targets the periderm of the tuber, leading to the formation of lesions that compromise the quality of the harvested crop. While it does not kill the plant, it significantly reduces the market value of potatoes and increases their susceptibility to shrinkage and secondary infections during storage, leading to substantial economic losses for growers.

The host range for this pathogen includes various root crops, with potatoes, beets, radishes, and carrots being the most commonly affected. The infection typically begins during the early stages of tuber formation when the bacteria invade the lenticels or small wounds on the skin, causing abnormal proliferation of cork cells.

Symptoms are easily identifiable on the surface of the tubers and can manifest in several forms:

• Surface scab: shallow, superficial russeting of the skin.
• Raised scab: wart-like pustules protruding from the skin.
• Deep scab: crater-like lesions extending deep into the tuber flesh.

Environmental conditions play a critical role in the severity of Streptomyces tumemacerans. High soil pH levels (above 6.0), warm temperatures, and dry soil conditions during the tuberization stage favor the rapid development of the pathogen. Effective management strategies involve maintaining proper soil moisture during the critical growth window, practicing crop rotation, and utilizing acid-forming fertilizers to lower the rhizosphere pH.