Potato Scab

Scabby potatoes? Yuck!


What causes this condition, and how can it be prevented?

Potato infected with scab (Clemson University - USDA Cooperative Extension Slide Series, bugwood.org) CC BY 3.0

First classified as a fungal disease, we now know that potato scab is a bacterial disease caused by Streptomyces scabies. There are other strains of Streptomyces that cause other potato diseases. S. scabies is found in the soil pretty much any place potatoes are grown. This bacterium can infect young seedlings of any plant, but it is most commonly associated with root and tuber crops, especially potatoes.


Potato skins

Delicious twice-baked and cut into wedges, served with sour cream and butter, potato skins are actually the cork, or periderm, layer normally found underneath bark. This layer normally provides protection from pests and disease. You may see tiny nicks of color in a potato’s skin. These are called lenticels and are used for respiration. This is also where the S. scabies bacterium gets in and starts infecting a potato.


Symptoms of potato scab

After entering a potato through a lenticel or wound site, S. scabies start setting up house. As they feed and reproduce, these bacteria release toxins into the surrounding plant tissue. The first sign of potato scab is nothing more than reddish-brown spots on the potato skin. These spots expand as the potato grows, becoming corky and necrotic. Then, the bacteria start reproducing (sporulating) in earnest, producing different types of lesions, depending on host resistance, time of infection, the aggressiveness of the bacterial strain, and other environmental conditions.


There are three basic types of lesions caused by potato scab: russet, erumpent, and pitted. Pitted lesions look like moon craters, while erumpent lesions are raised corky areas. Russet lesions (not to be confused with russet potatoes) are just extra corky tissue. In most cases, these lesions start out circular, but can spread out into larger, irregularly shaped areas. Potato scab lesions look a lot like another potato disease, called powdery scab. Powdery scab is a fungal disease caused by Spongospora subterranea.


​How to prevent potato scab

Being a seed and soil borne bacteria, potato scab is best prevented by manipulating soil moisture, soil texture, and soil pH, and planting healthy stock You won’t get rid of the bacteria completely, but you can significantly reduce their numbers with these tips:

• Resistant varieties  Plant resistant varieties to reduce the chance of initial infection.
• Seed potatoes Plant disease-free seed potatoes.
• Soil moisture High levels of soil moisture during early growth appear to stimulate populations of competing bacteria, slowing the development of potato scab. Of course, too much soil moisture can lead to other problems, such as fungal disease and rot. 
• Soil amendments Since the bacteria responsible for potato scab are involved with breaking down organic matter, adding aged compost while potatoes are developing is not a good idea. [Put that treatment off until after harvesting is completed.] 
• Soil texture The potato scab bacteria seems to prefer gritty, sandier soil, over loamy clay.
• Soil pH Potato scab bacteria are suppressed in acidic soil, with a soil pH at or below 5.2.
• Crop rotation Crop rotation can be used with grains or other non host species, such as corn or alfalfa, to reduce S. scabies populations. Do not rotate with other root crops, such as beets, turnips, parsnips, carrots, radishes, rutabagas, as these crops are also susceptible to scab. Rotating with spinach, cabbage, and red clover should also be avoided.

You can still eat potatoes infected with potato scab, but you should probably cut out the lesions and toss them in the trash.