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. Other strains of Streptomyces cause other potato diseases. S. scabies is found in the soil everywhere potatoes are grown. These bacteria can infect young seedlings of any plant but are most commonly associated with root and tuber crops, especially potatoes.

Potato skins

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

Symptoms of potato scab

After entering a potato through a lenticel or wound site, S. scabies gets to work. 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 lesions that can vary in size and color 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: 

• Pitted lesions look like moon craters.
• Erumpent lesions are raised corky areas.
• Russet lesions (not to be confused with russet potatoes) are just extra corky tissue. 

These lesions start circular but can spread into larger, irregularly shaped areas. Potato scab lesions look much like powdery scab. Powdery scab is a fungal disease caused by Spongospora subterranea.

How to prevent potato scab

Prevent potato scab by manipulating soil moisture, texture, and pH. And by planting healthy stock. You won’t get rid of the bacteria completely, but you can significantly reduce their numbers with these tips:

• Install certified pest- and disease-free seed potato varieties resistant to this disease.
• Maintain high soil moisture levels during early growth to stimulate populations of competing bacteria, slowing disease development. [Too much soil moisture can lead to other problems, such as fungal disease and rot, so it is a balancing act.] 
• Delay adding aged compost until after harvesting potatoes to make life more difficult for these bacteria.
• Aim for a soil texture that is loam rather than sand.
• Soil pH at or below 5.2 suppresses these bacteria.
• Crop rotation can break this disease cycle, but there are exceptions to that rule. Grains and other non-host species, such as alfalfa, can reduce S. scabies populations. Do not rotate with other root crops, such as beets, turnips, parsnips, carrots, radishes, or 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.