What you see: Spots on your potato’s skin; they likely each have a larger outer circle and a dot at the center
What it is: An infection of the little structures potatoes use to breathe
Eat or toss: If you’re truly committed (#nofoodwaste!), give the potato a deep peel to remove the infection (which is usually only a couple of millimeters deep) and eat. Definitely boot it from your potato sack quickly; the infection will impact quality and can spread to other potatoes
During photosynthesis, plants take in carbon dioxide and release oxygen through their leaves. But, when they’re respiring, plants, just like us animals, take in oxygen and release carbon dioxide.
On some plant parts, special pores called “lenticels” conduct this gas exchange. On apples you may see them as pin-prick-sized spots. On potatoes too, you may see anywhere from dozens to more than 200 of the telltale little dots. You can definitely see them on the potato pictured above. And that’s because the lenticels are suffering from an infection.
Breathing underwater is hard!
Odds are good that the potato pictured spent time in a too-wet environment. It might have been waterlogged in the field, or it could have been washed, but not properly dried.
Can you imagine trying to breathe under water? Just as you might gasp to get enough air, the lenticels struggled. So they enlarged to increase their odds of taking in oxygen, explained Nora Olsen, potato specialist at the University of Idaho.
But, while that might have helped them suck in some more oxygen, it also enabled a plant-attacking pathogen to sneak in. The lenticels got infected.
The bacteria that hitchhiked in on a water wave is now hard at work rotting tissue. Just below the peel, you’ll likely see brown, degraded potato. And while that’s certainly a bit icky, according to a paper from the Washington State University Extension, infected tissue usually doesn’t go further than about 4 millimeters. Excavate another layer deeper and you may find some potato that’s still just fine.
Olsen said she’s never eaten a potato with a lenticel infection, though would consider it if desperate. She suspected that even after you peel away the infection, you may find a potato with a dehydrated, flabby texture.
A potato infection is not a people infection
In a more advanced lenticel infection, you might see the spots coalesce into a larger, sunken lesion, or, the lenticels might look “puffy,” the result of trapped gasses emitted by the busy bacteria. While that's pretty gross, if you see a lenticel-infected potato at any stage, keep in mind that this is a plant infection. The attacking organism goes after potatoes, not people. So, while the infected areas won't taste good and you'll want to quarantine this potato to prevent its disease from spreading, the threat to you lies primarily in the ultimate quality of your potato dish.
It’s also possible to encounter potatoes in which the lenticels enlarged, but didn’t get infected. In that case, in their effort to take in as much oxygen as possible, cells within the lenticels will expand and multiply, which make the lenticels look like thick, white dots. Some stick out so much that the condition is known as popcorn. Pass the butter!
...But aren't lenticels cool?
While it’s a bummer to find your potato so afflicted, perhaps one silver lining is spending a moment considering that those little dots on all potatoes, apples and other fruits and veggies aren’t just for decoration.
“Lenticels are pretty fun because they’re air exchange pathways,” Olsen said. “It just reminds us that the potatoes are alive and respiring and they need oxygen.”
Nora Olsen. Professor and Extension Potato Specialist. University of Idaho.
Managing Lenticel Spot on Potato Tubers. Andy Robinson, Extension Potato Agronomist at North Dakota State University and the University of Minnesota; and Gary Secor, Professor and Plant Pathologist at North Dakota State University. December 2016.
Bacterial Soft Rot and Lenticel Spot on Potato Tubers. Debra A. Inglis, Professor of Plant Pathology and Extension Specialist. Brenda K. Schroeder, Assistant Professor of Plant Pathology, Washington State University - Mount Vernon NWREC. Dennis A. Johnson, Professor of Plant Pathology and Extension Specialist, WSU Pullman. Published by Washington State University Extension.
Compendium of Potato Diseases. W. J. Hooker. International Potato Center. 1981
You Have Skin in This Game. Paul Bethke (USDA ARS Vegetable Crops Research Unit & University of Wisconsin Department of Agriculture). Badger Common'tater. September 2019. Vol. 71. No. 09. p. 49-51.
And the award for best-named publication consulted for this post goes to.... Badger Common'tater!