Powdery Scab

Powdery scab may sound like an adolescent skin problem, but it is a disease of potatoes that leaves them looking, well, scabby. You can cut away the affected bits and eat the rest of your potato, but the skin is where most of a spud’s nutrients are stored, so that’s a shame.


​Also known as corky scab, let’s see what we can learn about this disease and how to prevent it.

Potato infected with powdery scab (A.J. Gevens, UW Extension) CC BY-SA 3.0 US

The powdery scab disease

Powdery scab is caused by one-celled creatures known as Cercozoa. They are not bacteria, fungi, or viruses. Instead, they are parasitic amoebas. Specifically, it is Spongospora subterranea that causes powdery scab. These microscopic critters exist around the world. Tomatoes, nasturtiums, and other members of the nightshade family can also become infected with powdery scab. Once infected, plants are more susceptible to other diseases, such as potato mop-top and potato scab. Powdery scab spores can remain viable in the soil for up to 10 years.


Powdery scab symptoms

Small, purple lesions that look like pimples first form on a potato’s skin. Those lesions grow bigger and bigger and then they rupture, sending spores in all directions. Those spores are white. To the naked eye, these spore clusters look like scabs. You may also see galls on infected roots, leaf wilting, or discolored stems.

Red potato infected with powdery scab (Agronom) CC BY-SA 3.0

How to prevent powdery scab

The easiest way to prevent powdery scab in your potato patch is by planting only certified disease-free eye potatoes. I know it’s tempting to use spuds and other vegetable starts from the grocery store. They’re certainly convenient, and you already have them, but they can bring a world of hurt to your garden. Produce bought at grocery stores is certified safe to eat, but it can still carry soil-borne diseases that may linger in your soil for many years. If you are growing in containers, this is less of a problem. If a disease appears in that case, you can toss the soil in the trash and start over. Hopefully, you catch it before spores have been released. Other practices that will help prevent powdery scab include:

• Providing good drainage
• Letting the soil dry out between waterings
• Planting resistant cultivars
• Using the least amount of nitrogen needed

Infected potatoes aren’t the only way powdery scab can enter your garden. To prevent this disease, you need to monitor these other potential pathways:

• Bulbs, corms, rhizomes
• Firewood
• Planting containers
• Shoes and clothing
• Soil from introduced plants

I’m a big proponent of buying used garden gear from yard sales and thrift stores, but you have to wash everything thoroughly before using it. And always put new plants into quarantine.


Chemical controls are not effective against powdery scab.