Common Scab

Scabby knees and elbows may be a normal part of childhood, but it’s not what you want to see on your potatoes or other root vegetables.

Common scab of potato (Clemson University, USDA Cooperative Extension, Bugwood.org) CC BY 3.0

Unlike apple scab, a fungal disease of fruit, common scab is a bacterial disease that prefers life underground. The actinobacteria responsible for common scab, Streptomyces, are found everywhere. Potatoes are the primary host, but beets, carrots, parsnips, radishes, rutabagas, and turnips may also become infected with common scab.
 
Common scab symptoms
 
Common scab looks exactly how it sounds. Surface russeting is the first thing you notice about plants with common scab. If you look deeper, you will also see deep pits in your tubers and root vegetables. Luckily, infected plants are safe to eat. Cut away the bad bits and toss them into the compost pile.
 
Common scab lifecycle
 
Common scab bacteria overwinter in infected roots and tubers left in the soil. Bacteria can also be spread via wind and rain splashing. They enter roots and tubers through natural openings, such as the stoma, and wounds created by insect feeding and garden tools. Infection begins on the surface and moves inward. These bacteria feed on the roots and tubers and will continue to reproduce as long as temperatures are warm enough.
 
Common scab management
 
Common scab is most common in dry soils. This means that regular irrigation goes a long way toward preventing common scab in root vegetables. Common scab is more likely in soils with a pH greater than 5.2. Acidifying alkaline soils may help prevent scabbing, but acidification can be a losing battle. Finally, rotate your potatoes and other root crops with non-tubers and watch for resistant cultivars when shopping for seeds and seedlings.