Potato scab is a common plant disease that affects potato tubers, causing unsightly blemishes on their surface. While it primarily impacts potatoes, certain types of scab can also affect other root vegetables, including radishes, beets, carrots, and parsnips. Understanding potato scab involves recognizing its appearance, identifying its causes, and implementing effective prevention and control measures.
Identifying Potato Scab
Potato scab manifests as various types of lesions on the tuber’s skin. Common scab often presents as corky, rough patches that can be superficial, raised, or pitted. These lesions may initially appear as small, brownish spots that enlarge and can sometimes merge to cover large areas of the potato.
Another type, acid scab, produces lesions similar to common scab, but it can occur in more acidic soil conditions. Powdery scab, caused by a different pathogen, results in small, purplish-brown, sunken lesions that develop into raised pustules. These pustules eventually rupture, releasing a powdery mass of spores. While these different types of scab can look similar, their specific visual characteristics and the conditions under which they thrive can vary.
Understanding the Causes
Potato scab is caused by soil-dwelling bacteria belonging to the genus Streptomyces, most commonly Streptomyces scabies. These bacteria are widespread in many soils globally. The disease cycle begins when these bacteria infect young, developing tubers, typically through natural openings called lenticels or through wounds on the tuber’s surface.
Environmental factors influence the development and severity of potato scab. Streptomyces scabies thrives in slightly alkaline soils, with a pH above 5.2, and particularly between 5.2 and 8.0. Dry soil conditions during the critical period of tuber formation, which occurs roughly 2 to 6 weeks after tuber initiation, also favor the infection. The bacteria produce a phytotoxin called thaxtomin, which breaks down plant cell walls and leads to the formation of corky lesions.
Strategies for Prevention and Control
Managing potato scab involves cultural practices to create an environment unfavorable to the disease. Maintaining optimal soil moisture levels during the initial 2 to 6 weeks after tuber initiation is important; consistent moisture near field capacity can inhibit infection by Streptomyces scabies. This is because high soil moisture promotes the growth of other bacteria that can outcompete the scab-causing organisms.
Adjusting soil pH is another important control method. Common scab is suppressed in soils with a pH of 5.2 or lower. Growers can aim to maintain soil pH between 5.0 and 5.2, possibly using acid-producing fertilizers like ammonium sulfate, while avoiding amendments such as lime or fresh manure that increase alkalinity. However, excessively low pH can impact nutrient availability for the potato crop.
Crop rotation helps reduce the population of scab-causing bacteria in the soil over time. Rotating potatoes with non-host crops such as small grains, corn, or alfalfa for at least three to four years is effective. Selecting potato varieties known to have resistance to scab, such as Russet Burbank or Superior, reduces disease incidence. Using certified, disease-free seed potatoes is a fundamental practice to prevent introducing the pathogen into clean fields.
Effect on Potato Quality
Potato scab affects the appearance of the tubers, making them less appealing for fresh market sales. While the blemishes can be unsightly, they do not impact the edibility or nutritional value of the potato once the affected skin is peeled away. Infected potatoes are still safe to consume.
However, the presence of scab can lead to economic losses for growers due to reduced marketability. Deep-pitted lesions can increase peeling losses during processing and detract from the quality of processed potato products like chips or fries. Although scab does not affect overall yield, severely scabbed tubers may experience some shrinkage in storage and can serve as entry points for secondary soft rot bacteria, which could lead to further decay.