Scabby potatoes? Yuck!
What causes this condition, and how can it be prevented?
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.
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:
You can still eat potatoes infected with potato scab, but you should probably cut out the lesions and toss them in the trash.
You can grow a surprising amount of food in your own yard. Ask me how!
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