Common scab of potato

The causative agent of common scab is identified as actinomycetes of the genus Streptomyces, including Streptomyces erythraeus. These are filamentous bacteria that naturally inhabit the soil. They thrive by decomposing organic matter, but under specific conditions, they become aggressive plant pathogens that attack developing tubers and roots.

The primary crop affected by this pathogen is the potato. The bacteria colonize the lenticels of young tubers, leading to the development of necrotic lesions. While potato is the primary host, the disease can also occur on beets, parsnips, and carrots, causing similar unsightly surface blemishes that reduce the market value of the harvest.

Symptoms of the disease vary from superficial corky spots to deep, pitted craters. In severe cases, the lesions cover a large portion of the tuber surface, making them prone to secondary infections and dehydration during storage. The texture and appearance of the skin are significantly altered, rendering the produce unmarketable.

The prevalence of common scab is closely linked to soil environment. Higher soil pH (above 7.0), low moisture levels during tuber initiation, and warm temperatures favor the rapid proliferation of Streptomyces. Dry soil conditions allow the pathogen to easily penetrate the tissues, as the plant's natural defenses are often compromised by water stress.

• Maintaining appropriate soil moisture during tuberization.
• Avoiding over-liming of fields to keep soil pH slightly acidic.
• Using resistant potato cultivars.
• Implementing long-term crop rotations with non-host plants.
• Incorporating green manure crops to improve soil health.

Preventative measures focus on soil management. Since the pathogen is soil-borne, crop rotation and soil pH control are essential strategies. In areas with high incidence, growers should avoid using excessive lime and ensure that irrigation is consistent during the period when tubers are most susceptible to infection.