Common scab

Common scab is a plant disease that primarily affects potatoes, but can also impact other tuberous and root crops such as beets and carrots. The disease is caused by various species of the genus Streptomyces, with Streptomyces scabies being one of the most common pathogens associated with common scab. This bacterial infection is characterized by the appearance of scabby lesions on the skin of the tubers, which can affect the crop's marketability and, in severe cases, reduce yield.

Symptoms

The primary symptom of common scab is the appearance of corky, rough patches or lesions on the surface of the tuber. These lesions can vary in size and depth, ranging from superficial blemishes to deep pitted areas. The severity of the symptoms can be influenced by environmental conditions, particularly soil moisture and pH levels, with the disease thriving in alkaline, well-drained soils.

Causative Agent

Common scab is caused by several species of Streptomyces, a genus of Gram-positive bacteria. These bacteria are soil-dwelling organisms that infect tubers through wounds or natural openings. Once established, they produce toxins that lead to the formation of the characteristic scab lesions.

Management

Management of common scab involves an integrated approach combining cultural, biological, and chemical strategies:

• Cultural Practices: Maintaining optimal soil moisture during the tuber initiation and development stages can help reduce the incidence of common scab. Crop rotation with non-host crops and the use of resistant potato varieties are also effective cultural practices.
• Biological Control: Some beneficial soil organisms can suppress Streptomyces populations, thereby reducing the incidence of common scab.
• Chemical Control: The use of soil fumigants and other chemical treatments can be effective but is often considered a last resort due to environmental concerns and the potential for developing resistance.

Economic Impact

While common scab does not typically result in significant yield losses, it can substantially reduce the market value of affected crops due to the unsightly appearance of the lesions. This is particularly true for potatoes intended for the fresh market, where visual appearance is a key factor in consumer choice.

Prevention

Preventative measures include the selection of resistant varieties, proper irrigation management to avoid overly dry soil conditions, maintaining a neutral soil pH, and practicing good crop rotation to reduce the buildup of pathogenic Streptomyces in the soil.