Common potato scab

Streptomyces griseus is a soil-dwelling bacterium belonging to the actinomycetes group, recognized as the primary causative agent of common potato scab. Unlike many other plant pathogens, this organism functions as a saprophyte in the soil, decomposing organic matter, but it can shift to a parasitic lifestyle when it comes into contact with developing tubers or root crops.

This disease primarily affects potato tubers, though it can also infect beets, carrots, and radishes. The bacteria infiltrate plant tissues through lenticels, stomata, or small physical abrasions on the skin of the tubers. Once inside, they release enzymes that degrade the tuber's outer layer, leading to the formation of necrotic lesions as the plant attempts to wall off the infection with corky tissue.

Symptoms are highly variable, ranging from shallow, superficial scabs to deep, pitted lesions or raised, corky mounds. These blemishes significantly impact the market value of the crop and can lead to increased water loss during storage. In severe cases, the entire surface of the tuber may become covered in scabby, rough tissue, making it unmarketable for retail distribution.

Environmental conditions play a critical role in the severity of the infection. Streptomyces griseus thrives in well-aerated, sandy soils with a neutral to alkaline pH (above 6.0) and prefers relatively dry conditions during the tuber initiation stage. Because the bacteria can survive indefinitely in the soil as saprophytes, they are extremely difficult to eradicate once a field is contaminated.

Effective management requires an integrated approach. Farmers should prioritize soil pH management, aiming for slightly acidic levels that inhibit bacterial growth. Crop rotation with green manure crops that naturally lower soil pH is recommended. Additionally, maintaining adequate soil moisture during the critical period of tuber formation is essential to suppress the colonization of young tubers by the pathogen.