Powdery Scab

Powdery scab may sound like an adolescent skin problem, but it is not. It is a potato disease that leaves spuds 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 a disease 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 nightshade family members can also become infected with powdery scab. Once infected, plants are more susceptible to other diseases, such as potato mop-top and 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 the skin of an infected tuber. Those lesions grow bigger until 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

Infected potatoes are not the only way powdery scab can enter your garden. Other potential pathways include infected bulbs, corms, rhizomes, firewood, planting containers, shoes and clothing, and soil from introduced plants.

I am 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. But these practices can help prevent powdery scab:

• Install certified pest- and disease-free eye potatoes.
• Provide good drainage.
• Let the soil dry out between waterings.
• Plant resistant cultivars.
• Use the least amount of nitrogen needed.

Whatever you do, do not give in to the temptation to install spuds and other vegetables from the grocery store. These plant sources may be convenient and inexpensive, 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 decades. If you are growing in containers, this is less of a problem. If any disease appears in that case, toss it in the trash and start over. Hopefully, you catch it before any problems spread to the rest of your garden.