The greater American potato industry is closely watching Maine, as seed potato farmers prepare to control a new outbreak of an old disease.

The Maine Department of Agriculture, Conservation and Forestry is considering a proposal to set limits on the prevalence of the rotting disease blackleg in its certification of seed potatoes, which more than 100 Maine farmers sell to other farmers along the East Coast.

Over the last two years, the Maine Potato Board and some seed potato farms have been dealing with concerned customers — farmers growing certified Maine seed potatoes who suffered outbreaks of blackleg. Some farmers in states such as Maryland and Pennsylvania lost as much as half of their crop to the disease in 2015, said Tim Hobbs, the Maine Potato Board’s director of grower relations.

The outbreak led to the discovery of a new source of blackleg, a little-understood bacteria known as Dickeya, and although the problem isn’t just linked to Maine seed potato farms, the Maine Potato Board has pushed for a response to ensure the integrity of the brand — and keep customers.

“Last year, most of the other states kind of watched what we were doing,” Hobbs said. “Now, we’re leading the whole effort in reaction to, planning for and implementing controls for Dickeya.”

The new measure was hashed out over the winter by a council of seed potato farmers and officials with the Maine Potato Board, and it would add blackleg to a list of six other diseases with limited tolerances in certified seed potatoes. The blackleg tolerances, though, would be set by the seeds’ field years — the years the cloned tuber has been planted in native soil after greenhouse propagation.

Under the proposal, certified seed potato fields could have no more than 0.1 percent of plants with visible blackleg symptoms in their first field year and no more than 0.2 percent in their second. These early “generations” of the potatoes are considered to have higher overall vigor.

In the third field year, under the proposal, blackleg limits would rise to 1 percent of plants, then to 2 percent for field years four and five. The proposal also would cut off the sixth generation of seed potatoes. That late generation of seed has been allowed under Maine’s certification for some time, but isn’t a significant source of revenue for seed growers, according to Hobbs.

When the Maine Potato Board’s seed council was discussing what to do, some made the case for including tests for the Dickeya bacteria, not just blackleg. But Dickeya itself cannot be identified visually on a plant and blackleg can be caused by Dickeya and other bacteria, Hobbs said.

“By controlling the broader issue, you’re ultimately controlling the Dickeya. It’s a good start,” said Hobbs.

Until recently, it was nearly impossible for U.S. farmers to get accurate laboratory tests for Dickeya on potatoes, Hobb said. Now the Maine Potato Board is offering Dickeya lab tests for farmers, separate from the certification. Seed farmers can use those tests to address their infections, or if they’re found Dickeya-free to assure their customers.

When Dickeya first emerged, the potato board asked McCain Foods, the international potato processing giant, to put them in touch with potato scientists in Europe, where Dickeya-fueled blackleg has been a problem for more than a decade. The board ended up bringing in three potato researchers from Scotland and has since adopted their protocols in the lab the board operates in Presque Isle.

University of Maine at Presque Isle biologist Larry Feinstein has been hired under a part-time contract with the board to help oversee quality control as a portion of the lab is renovated to accommodate molecular-based tests for Dickeya. The potato board is seeking a federal permit to import live bacteria from Scotland — four strains of Dickeya and three of Pectobacterium, all of which can cause blackleg — that lab staff will use to produce deactivated samples for comparison in testing local tubers.

“We’re transitioning from a smaller, more casual type of lab to one that’s going to be pretty significant,” Hobbs said.

The potato board already has been performing “dormant tuber” tests for Dickeya since January. Part of the challenge with Dickeya is that it can be present on otherwise healthy-looking potatoes.

Of 251 samples from 26 different farmers, 48 samples from 16 farms had tested positive for Dickeya, said Hobbs at the potato board’s April 13 meeting. The board will be releasing more data on the results in the future, showing trends by variety, but the growers will not be identified.

Scientists studying Dickeya still don’t have any good ideas about how it spread to potatoes in the U.S. in the first place, only that the bacteria has been found on ornamental plants for several decades. A $3 million multistate research project is focusing on developing diagnostic tools for Dickeya as well as understanding its life cycle.

Hobbs said Dickeya has not been found at the Maine Potato Board’s Porter Seed Farm in Masardis, the source of seed for more than 80 percent of the potatoes grown in the state.

“We’ve tested everything and never have found Dickeya in the greenhouses, supply water or anywhere at the farm,” Hobbs said.

Among Aroostook County seed potato growers, there is a sense of urgency about controlling Dickeya, said Garrett Hemphill of Hemphill Farms, a family-owned farm in Presque Isle growing several varieties of seed potatoes, including the Atlantic chipping stock.

The tests they’ve had done at the potato board’s lab showed no evidence of Dickeya, nor have they seen any blackleg in the fields. But it’s still on their mind, reminding them of the importance of keeping their storage buildings and equipment clean.

“It’s something all of the seed growers have to work together to overcome,” Hemphill said. “The reputation of Maine is at stake.”