Potato Scab
Potato Scab is a disease caused by naturally occurring bacteria in the soil that affects the surface of potato tubers, causing corky or scabby lesions. Scab can also affect other root vegetables such as beets, turnips, carrots and parsnips.
The symptoms of scab depend on potato variety and environmental conditions, but they typically look like raised or slightly sunken dark areas that may feel rough or corky.
Potatoes with scab are safe to eat, but they can make crops less marketable, lower
yields, and reduce the ability to store them long-term.
Scab is caused by varieties of the bacteria Streptomyces, which occurs naturally in many soils but can also be introduced when infected tubers are used as seed stock. Alaska has laws that regulate the sale, use, and import of seed potatoes to prevent the spread of scab and other diseases. Seed potatoes sold and used in Alaska must be certified by the Alaska Seed Potato Program. Planting potatoes bought at the grocery store risks introducing scab to your garden. Never use infected tubers to produce seed pieces.
Scab bacteria can linger in the ground for years, but rotating crops can help mitigate the problem. Avoid planting potatoes and other root vegetables in the same spot more frequently than every three or four years. The longer the rotation, the better for controlling scab.
In 2021 and 2023, Cooperative Extension personnel identified six previously unknown varieties of scab after collecting scabby potatoes from Alaska growers around the state. They also identified instances of powdery scab, a fungus-like pathogen that is of concern due to a lack of treatment options. Powdery scab is also a known vector for other pathogens.
Identifying scab and their properties, such as favorable conditions for the disease, allows researchers and extension agents to pursue trials and experiments on the best practices for reducing the risk and prevalence of scab.
Preventative controls to help reduce the risk of scab include adjusting the pH of the soil the potatoes are grown in. Depending on the species of scab, raising or lowering the soil pH can help minimize the risk of scab affecting the potato crop.
Testing provided by the Alaska Plant Materials Center and surveys conducted by Extension Agents with the Integrated Pest Management program has helped map which scab species most commonly occur across different regions of the state. In general, scab found in the northern and interior regions of Alaska are more adapted to neutral and alkaline soils, while those found in the more southern and coastal areas occur in more acid soils.
Now that more information is available on the distribution and different types of scab in Alaska, the next step is to study the efficacy of different management practices, such as crop rotation, avoiding an overabundance of soil organic matter, adjusting soil pH and using cover crops, to determine their effectiveness in controlling and preventing both common scab and powdery scab.
While there aren’t any new studies yet underway, researchers are reviewing opportunities to conduct more research.
Contact Extension Agent Casey Matney with questions at 907-262-3443 or camatney@alaska.edu
