Raised black lumps or rough scaly patches are signs of common potato scab. It just looks unappetizing and the tuber is perfectly edible. Credit: Julia Bayly / BDN

A lot of potatoes are coming out of the ground this fall covered in warty-looking growths. But the good news is that, as unappetizing as it makes the tuber look, it’s perfectly safe to eat.

That’s because conditions were perfect earlier in the summer for the spread of common potato scab.

Potato scab is caused by a bacteria found in most soils in Maine. Other root crops including carrots, beets, radishes and parsnips are also susceptible to the pathogen.

“The bacteria lives in the soil quite happily doing its thing,” according to Steve Johnson, crops specialist with the University of Maine Cooperative Extension. “When it grows on the potatoes, it produces a toxin that wrinkles up and messes up the potato’s skin.”

The skin can appear rough, alligator-like, deeply pitted or scaly, depending on the subspecies of the bacteria, Johnson said.

Johnson said it’s been more of an issue this year with homesteaders and gardeners than in past years. A combination of July weather and soil chemistry are the major culprits, he said.

“It can be a pH thing,” Johnson said. “If a grower is putting the wood ash to the garden or raising the pH by adding lime it can increase the possibility of common scab.”

Conditions at the time the tubers’ skin forms and sets also play a part.

“It tends to be an issue if you have dry conditions when that skin sets,” Johnson said. “Generally the potatoes are going to enter tuber primordium 50 days after planting and in Maine that tends to be around the first week of July [and] if you remember, we were dry in July.”

Primordium is the earliest stage when an organism is recognizable.

“When it’s dry like that we expect [common scab] to happen,” Johnson said. “Yeah, I expected it, and yeah, we got it.”

Unfortunately, once scab has formed on your potato crop, there is no getting rid of it.

“The big processors don’t like it,” Johnson said. “It makes the potato not peel well and that makes a difference when you are peeling several tons per hour.”

It can also make a difference for small farmers or homesteaders when it comes to marketability.

“People buy potatoes based on appearance,” Johnson said. “But to be honest, scabby potatoes eat just fine.”

The best way to prevent common potato scab on next year’s crop is planting scab-resistant varieties, bringing down the pH in the soil and making sure the tubers are getting enough water during critical growth periods.

In the meantime, if you end up with scabby potatoes, be ready to spend a little extra time preparing them.

“You are just going to have to peel them a little harder,” Johnson said. “If you are going to eat them with the skin, scrub a bit harder to get off all the soil before you eat them because we do not have gizzards.”

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Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.