Streptomyces acidiscabies

Taxonomic position and pathogen type: Streptomyces acidiscabies belongs to the kingdom Bacteria and the genus Streptomyces. It is a filamentous, Gram-positive soil bacterium known for its ability to produce secondary metabolites. As a plant pathogen, it acts as a facultative parasite that can alternate between a saprophytic lifestyle in the soil and a pathogenic stage when susceptible hosts are present.

Diseases and host crops: This organism is the primary cause of common and acid scab in potatoes. Beyond potatoes, it has been reported to affect other taproot vegetables including carrots, radishes, and parsnips. The infection leads to the development of necrotic lesions and scab-like crusts on the tubers or roots, which significantly impacts the marketability and quality of the harvested crops.

Biology and life cycle: The pathogenicity of S. acidiscabies is largely attributed to the production of a phytotoxin known as thaxtomin. This toxin interferes with plant cell wall synthesis, allowing the bacteria to penetrate the epidermis of developing tubers through lenticels. The pathogen persists in the soil for extended periods, utilizing organic matter and plant debris as a nutrient source until host tissues become available for colonization.

Conditions for development and damage: The bacterium thrives in warm soils (20–25°C) and can tolerate a wide pH range, which distinguishes it from other scab-causing species. Dry soil conditions during the early stages of tuber formation are particularly conducive to disease development. The economic impact is severe, often resulting in heavy yield losses and high percentages of cull tubers that cannot be sold in the fresh market.

Measures for protection and control: Managing this pathogen requires a long-term integrated strategy, as it is difficult to eradicate once established in the soil:

• Planting only certified scab-free seed potatoes.
• Implementing long-term crop rotation cycles.
• Irrigation management during the critical period of tuber initiation to inhibit bacterial entry.
• Maintaining soil health through the use of specific cover crops.
• Selecting and planting cultivars known for higher resistance to scab.