Vermont scientists spent the summer spreading Dutch elm disease to thousands of trees.
You read that correctly.
Researchers from the U.S. Forest Service and the Nature Conservancy want to find out whether a new generation of elm trees bred from disease survivors might have resistance to a fungus spread by bark beetles that has been decimating elm populations in the U.S. and Canada for decades.
The goal is to one day repopulate areas where tall elms were once everywhere, lining city streets and thriving along riverbanks.
“The loss of trees over the years has been devastating,” Gus Goodwin, senior conservation specialist at The Nature Conservancy, told The Associated Press.
Time will tell whether the Vermont experiment will result in a strain of trees resistant to Dutch elm disease (DED). Researchers working on similar efforts here in Canada hope science will provide solutions at a time when the disease is becoming a threat in new regions.
“We’re not losing the elm population,” said Alison Morrison, manager of the University of Guelph Arboretum, which is managing the elm restoration project. “What we have lost is the ability of our elms to age.”
In the shadow of Parliament Hill, stately elms fall victim to Dutch elm disease.
The Elm City That Lost Its Elms
Fredericton was once known as the “City of Stately Elms” until the disease spread there and other parts of eastern Canada in the 1960s and ’70s, causing the loss of an estimated 10,000 trees. Now only about 1,000 mature trees remain.
University of New Brunswick forestry professor Anthony Taylor has several elm trees on his property near Fredericton that survived the first wave of DED.
“But now I see that a new wave of disease is killing some of the trees on my property,” he said, “and it’s sad to see.”
When bark beetles carrying DED spores colonize a hospitable elm, the disease attacks its vascular system and blocks water-carrying tissue, starving the foliage and branches of water, preventing photosynthesis and ultimately killing the entire tree.
This can happen within one to four years after infection.
The beetles spread the fungus further by laying eggs on an infected tree, creating a new generation of vectors that can then fly to the next healthy tree. The fungus can also spread through roots.
To control DED, infected elms are immediately cut down and disposed of, Taylor said.
However, in past decades, other nearby elm trees were also removed, even if they were healthy.
“It’s called sanitation, where we try to kill the pest by killing the plant that carries it,” Morrison explained.
But this could “inadvertently” have had a detrimental effect when it came to DED tolerance, she says, by removing trees capable of resisting the disease from the population.
Edmonton, once free of Dutch elm disease, has seen an increase in cases since 2024. CBC’s Sam Brooks explains why some experts warn the city is taking the wrong approach to controlling the fungus.
A nightmare for streets overgrown with elms
While cities from Manitoba to the Maritimes saw elm trees decline in the 1960s and ’70s, the disease has become a threat in new places in recent years.
Edmonton has confirmed 17 infested trees since it was first detected in the city in 2024. Saskatoon has reported 23 cases in 2026, 11 years after DED was first discovered there.
“This is a serious threat to Saskatoon’s urban forests,” said Nolan Rumler, park ranger. There are about 26,000 elm trees in the city, he said.
City foresters act quickly to remove infested trees, but residents also have a responsibility to stop the spread, including enforcing an annual four-month ban on elm pruning and not storing or burning wood, he said.
“We have officially moved…to a more aggressive control strategy because the fungus is now considered imminent and it is widespread in the community,” he said.
Residents say more trees should be planted to replace those cut down due to Dutch elm disease. This year, almost two dozen cases of the disease have been confirmed in the city.
While the disease may persist, the problem is which trees it kills.
Losing mature trees means losing their shade and ability to absorb stormwater, Morrison said.
She says they are “another tool in our toolbox” when it comes to mitigating and adapting to human-caused climate change.
“If we can confidently bring elms back into our communities,” she said, “that will add another species … to create a complex ecosystem. And they are the most sustainable.”
Saskatoon Morning4:58A memorial tree affected by Dutch elm disease is being removed in the city of Saskatoon.
Test of time
Scientists and foresters across North America, including the Guelph Elm Restoration Project and the Fredericton City Project, are working to increase resistance to DED by crossing and cloning trees that have not succumbed to the disease.
In Guelph, Morrison says they have been working towards this goal for almost 30 years.
“We have 109 trees in the gene bank that we’ve essentially killed off from Dutch elm disease and several strains of it that persist here,” she said.
According to her, these trees have reached such an extent that They can now produce seeds, and the next step is to “see what these seedlings can do.”
That’s where the Vermont experiment is now—resilient tree seedlings—and on a much larger scale, involving about 6,000 trees.
Starting in the spring, U.S. Forest Service and Nature Conservancy researchers began injecting them with thousands of fungal spores to test which ones would survive and which would eventually die.
Because this breeding program involves generations of trees, it takes time.
“We … these trees will grow and love for about 15 years or so, after which they should produce seeds, and those seeds can be used for restoration,” Leila Wilson, a research ecologist with the U.S. Forest Service, told The Associated Press.
Taylor says if it works, this may mean that elms here can coexist with DED, as elms in Europe have done, with only minor negative effects from the disease.
Elm trees in Canada, he said, can also develop naturally, but that could take centuries, given that the disease only appeared in the county just under a century ago.
“We’re just kind of [expediting] this process of co-evolution,” he said.
A Toronto researcher who recently saw his first elm tree may find a cure for Dutch elm disease.