Inside the Longshot Experiment to Resurrect Elm Trees—With the Disease That Decimated Them
As many as 100 million elm trees died from Dutch elm disease in the span of 50 years. Now, decades of careful breeding may help them return.
Walking through a field in rural Vermont last July, Gus Goodwin passes row after row of young elm trees. He points out trees that appear healthy, with green leaves shooting from multiple branches. But nearby, another tree has drooping leaves and a sparse canopy, a clear sign that it has fallen victim to Dutch elm disease (DED). Looking around the field, he sees another diseased tree, and another.
For Goodwin, a naturalist and elm tree expert for The Nature Conservancy (TNC) in Montpelier, Vermont, these trees are the focus of his professional life. To see this disease spreading through so many is a heartbreak—but one that doesn’t surprise him. After all, he infected them.
Goodwin and his colleagues from TNC and the United States Forest Service (USFS) had injected every one of more than 5,000 trees with the disease six weeks earlier. It’s part of their plan to save the species by breeding it to fight back against a pathogen that, in just the past 50 years, has all but wiped out one of America’s greatest trees.
The American elm, a native of eastern North America, can live up to 300 years and reach as tall as 80 feet. Lining streets, their limbs once formed archways whose sprawling branches shaded sidewalks. By the early 20th century, it was widely planted throughout America’s rapidly growing towns and cities. And in its native northeastern environment, it tolerates both frigid weather and water-logged soil, making it uniquely well suited to anchoring floodplains.
But across most of the country, gone are the verdant city streets and forested river plains where elm trees once towered. The decline of the American elm has emptied floodplains and city streets alike. The National Park Service estimates that as many as 100 million elms have been killed since DED first arrived in the U.S.
For decades, scientists and others have been searching for a way to return the elm to something approaching its former glory. This quiet plot in rural Vermont may be their best hope.
How the Disease Took Hold
The disease is caused by a fungal pathogen transmitted by several species of bark beetles; it is believed to have originated in Asia but earned its name after it was first identified in the Netherlands in the 1920s.
As the fungus grows inside an elm tree, the tree attempts to prevent its spread by releasing cells and chemicals that block the channels trees use to move water and nutrients, essentially walling off affected tissue. As a result, however, that water and those nutrients can no longer effectively travel from the roots throughout the trunk, and the tree eventually starves. So effective is the disease at spreading that two waves of the disease—one in the 1920s and another in the 1970s—caused mature elms in North America to decline by 75 to 90 percent.
In a desperate attempt to stop the spread, city trees were cut down and their logs burned, transforming the North American urban landscape and denuding Elm Streets across the country of their namesake.
“There are still elms out there on the landscape, but they're very small,” explains Kathleen Knight, a research ecologist with the U.S. Forest Service. “A whole generation of people has grown up with going into the forest and seeing elms as a very small species, because once it gets to a certain size, it usually gets hit by Dutch elm disease and dies.”
But for reasons that remain mysterious, some large elms have survived unscathed. Goodwin and his peers are using them to help resurrect the species.
Saving the ‘Survivor’ Trees
In a conference room in Montpelier, Goodwin turns to a map of Vermont marked with little symbols showing elms that have made it through one or both DED outbreaks.
“When you drive around Vermont, you'll see some of those big farm fields, and out in the middle, you might see a huge elm all by itself,” explains Goodwin. “That tree is probably outside the 300-yard flight range of the beetles, so there's a decent chance it's just never been exposed to the disease.”
These trees are the “escapers."
Some trees, however, were exposed and survived anyway. They are the “survivors.” Goodwin points to an old photograph of a field that shows a large elm, still standing today, with infected and dying trees nearby. “There's reasonable certainty that the tree had been exposed to Dutch elm disease at some point in its life,” he explains.
Beginning in 2010, scientists from USFS worked with Goodwin’s predecessor at TNC, Christian Marks, to find 53 survivor trees growing in New England; Goodwin and colleagues have continued the search since then.
“We’re finding more trees with resistance than were found in those early days,” explains Cornelia Wilson, a Forest Service ecologist who, along with Knight, co-leads the elms project for USFS. “We don't know exactly why, but the assumption is that it’s simply from natural selection. There's been that disease pressure for 80 plus years now and many of the most highly susceptible trees have likely perished, leaving some trees with resistance.”
(Knight encourages anyone who wants to search for healthy, wild elms to download the TreeSnap app and help gather data.)
The genetic advantages or physical mechanisms that allow some elms to survive DED remain unclear. But to confirm their disease resilience, researchers from the Forest Service clipped branches from those initial 53 trees, extracted pollen from their flowers, and crossed them with elm trees growing in a plot in Ohio that had already been identified as having some disease resistance.
From those 53 trees, scientists produced more than 5,000 trees that were planted at a test plot in Vermont in 2018; because elms tend not to succumb to DED until they reach sexual maturity, the trees were then nurtured and watched over until they were mature enough to test. That time finally came this past May.
Carrying pipettes loaded with 10 microliters of a solution containing approximately 100,000 spores of the fungus, Goodwin, Wilson, and their colleagues drilled small holes into the trunks of adolescent elm trees and dripped the potentially lethal liquid into the tree’s wound.
It was a milestone moment but also a painful one.
“We spend years propagating these trees and nurturing them and caring for them and pruning them, and then we go and inject them with a huge dose of this deadly fungus that can kill them, so it definitely causes you to cringe a little bit as you're as you're doing that,” admits Knight. “It's necessary for the science, but I love these trees, and so it's tough to do that.”
Six weeks later, they returned to assess the trees’ condition, methodically logging each tree against a checklist of symptoms. The preliminary results were immensely promising.
Of the 5,300 injected trees, more than 800 showed little to no impact from the pathogen and an additional 300 or so showed only minimal decline.
Restoring Elm Trees for Future Generations
“What we often see, and what we hope to see over the next few years, is that many of those trees that have a small amount of decline recover and look completely healthy within a few years,” explains Knight.
Goodwin and colleagues will continue to monitor the trees’ progress during that time; after two years’ time, the trees that are clearly not resistant will be removed from the test plot, and grafts from the parents of those that are will be used to create an entirely new planting. After 10 to 15 years, the resistant trees will be old enough to pollinate each other—and, scientists hope, produce disease resistant seeds.
In addition to backyards and floodplains in New England, scientists hope to replant American elm trees throughout the Midwest and upper Mississippi River Basin where they can test the limits of how much elm trees can weather waterlogged soil and cold temperatures across a variety of climates.
In the meantime, the search continues for more survivor trees.
“I realized last year that I won't see any of this work finish,” says Leigh Greenwood, director of TNC's Forest Pests and Pathogens Program. “But I’m fine with that because I like to imagine that, in 50 years, when there's a reporter interviewing a scientist as they look out over a healthy restored floodplain full of mature elms, they are going to say, ‘Thank goodness they put in place the things that they did back in 2026, because that made it so we could be here today.’ That's our goal.”
