Alaska researchers survey spread of potato scab

As Alaskans harvest their potato crops this fall, many will find that some of the tubers have what looks like a case of warts — raised, dark areas that may feel pithy. 

Those marks are called potato scab, which is caused by naturally occurring bacteria in the soil. They are not harmful, and the potatoes are safe to eat, but scab may make potato crops less marketable, University of Alaska Fairbanks Cooperative Extension Service agent Casey Matney said. 

“You can eat it, that’s not a problem,” said Matney, an associate professor based on the Kenai Peninsula. “Scab affects the marketability of the potato because it looks funky. In addition, if scab is really prevalent, it can reduce the ability to store them long-term.”

A 2009 survey across North America found 12 species of the Streptomyces bacteria responsible for the lesions. Five bacteria species were found to cause 98% of the scab.

Alaska’s contribution to the survey was made up of only five potatoes from the Matanuska-Susitna region. One species of scab was identified in that sample.  

In 2021, Matney worked with microbiologist Christine Macknicki and plant pathologist Todd Steinlage, both with the Alaska Division of Agriculture’s Plant Materials Center in Palmer, to conduct a widespread survey of scabby potatoes harvested across Alaska. 

They found five species of Streptomyces that cause scab in Alaska potatoes. Some potatoes had multiple species in the same lesions. Confirmation of species identifications is ongoing.

Scab treatments involve adjusting the pH of the soil the potatoes are grown in. 

“Now that we know the (species) that are here, we’re finding that some of the scab grow just fine at low pH and some of them do better at high pH,” Matney said. “That’s going to change how we do our recommendations on what to do about scab if you see it in different places.”

In the meantime, growers can reduce the prevalence of scab in their crops, Macknicki said. 

Some potato varieties naturally resist the bacteria, Macknicki said. The Cooperative Extension Service and the Plant Materials Center can recommend varieties. 

“Start with clean seed,” she said. “Don’t replant tubers that show signs of any disease, including scabby lesions.”

The bacteria can linger in the ground for years, but rotating crops can help mitigate the problem. 

“Don’t plant potatoes, beets, carrots, etc., in the same place a tuber crop was grown,” she said. “The longer the rotation, the better for controlling scab. We recommend four years, knowing that isn’t always feasible.”

Macknicki said national studies, which have not been duplicated in Alaska, suggest planting brassicas such as broccoli, mustard or cabbage in a location the year before planting a tuber crop. Mustards that are “hotter” or more bitter have better effects. After harvest, chop up the leftover stems and leaves and rototill under in the fall before planting potatoes the following spring. 

“Once we have identified the species of common scab and their properties — like what are the most favorable conditions for the common scab bacteria having the most deleterious effects on tuber crops — then we can pursue some experiments on the best practices for disrupting the pathogenic effects of the common scab bacteria,” Macknicki said. 

Matney said the study will be continued for another year. Alaskans can send scabby potatoes from this year's harvest to UAF CES, 43961 K-Beach Rd, Suite A, Soldotna, AK 99669-9728.

Wrap two or three potatoes in dry paper towels or newspaper and place them in a small sturdy box (a small flat-rate box is recommended) to avoid crushing. Include your name, email, phone number, location and variety of potato.

Kenai Peninsula residents can drop samples at the CES office on Kalifornsky Beach Road in Soldotna. Email questions to camatney@alaska.edu.

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