by Carolyn Bernhardt
July 8, 2025
Photo courtesy of Domini Brown
Ben Held, PhD, still remembers the moment he noticed a mighty elm tree was suddenly absent on one of his usual walks around the St. Paul campus. Yet another of the towering giants that once shaded neighborhoods, lined city streets, and anchored floodplains across Minnesota had been removed after Dutch Elm Disease (DED) claimed it. “Without immediately noticing that the elm had been removed, you could just sense the difference without it; the shade, its presence,” he recalls. “They provide so much to the community.”
“There are so many stories about how important elm was to communities,” says Held, a plant pathology researcher at the University of Minnesota whose work focuses on fungal biology in the contexts of decomposition, forest disease, and environmental extremes. “People really care about those trees.” Their iconic cathedral-like canopies offered more than beauty.
According to Ryan Murphy, an urban and community forestry expert in the Department of Forest Resources, elms help manage stormwater, shelter wildlife, and even cut energy costs by cooling homes in the summer and buffering wind in the winter. But DED, a persistent fungal pathogen spread by bark beetles, has wiped out much of this once-ubiquitous species. And the disease is still on the move.
With funding from the Minnesota Invasive Terrestrial Pests and Plants Center, Held and Murphy are working to revive the American elm. The team is identifying elms that have survived DED outbreaks in the wild, and cloning and testing them to find those with natural resistance. Their goal is to reintroduce the more resilient clones across Minnesota’s urban and natural landscapes, restoring lost canopy and biodiversity while preparing forests for a future stressed by climate change.
Resistance and resilience
According to Murphy, rebuilding Minnesota’s elm population begins with four essential steps: identification, propagation, growth, and testing. The process begins in the field, where the researchers scout for “survivor” trees—American elms that appear to be withstanding Dutch elm disease even as others around them succumb.
Once the experts have pinpointed a promising tree, they collect cuttings and propagate a clone in a greenhouse. The scientists then nurture the young clones in their research nursery for years—often five or more—until the trees are large enough to challenge the pathogen in a controlled setting. According to the research team, using clones is essential. “Resistance is not reliably passed to the seed population,” Murphy explains. “So, using the seed would just muddy the waters.”
Held grows the fungus in the lab, and the research pair injects a spore sample into test trees to see how well resistance unfolds. To confirm their observations, the team typically tests each tree twice, since environmental factors like location and weather can influence outcomes. Trees that pass these tests move on to the next stage: reintroduction plantings in natural areas and parks. Sites like Big Woods State Park and Elm Creek Park Reserve are now home to some of these resilient elm clones, providing long-term testing grounds and offering a glimpse at what recovery could look like on a broader scale.
The project’s success, the researchers say, hinges on collaboration. State agencies, local governments, and community members all play critical roles. Organizations like the Minnesota Department of Natural Resources, the Izaak Walton League, and Three Rivers Park District have all been instrumental in expanding the project’s reach. Murphy and Held both say public interest remains high.
Test plots and teaching trees
The elm project goes beyond identifying and propagating resistant trees, however. Re-planting the clones in the field serves two important roles: active reforestation and long-term observation. These plantings help restore elm populations in natural and urban landscapes, while also creating living laboratories where researchers can monitor how trees perform over time.
Held explains that the approach also acts as a kind of a long-term, passive study “as Dutch elm disease might move through those areas in subsequent years.” Even if none of the trees go to the commercial market immediately, having a genetically diverse stock in these test plots means the team has a resource for future research and restoration.
While promising, the project faces hurdles. Firstly, early outcomes in Nerstrand Big Woods State Park and Elm Creek Park Reserve have shown the challenges of real-world conditions. For their most recent planting, Murphy says, “We planted [some clones], and right after that, there was a windstorm.” After the wind and some opportunistic deer damaged the newly planted elms, causing concern among local collaborators, the team installed larger protective tubes to better shield the young trees from wildlife. Other future threats, such as elm yellows disease, further complicate restoration efforts.
And, importantly, resistance does not necessarily mean immunity. “Resistant selections can still get the disease,” says Held. “They can still wilt, but they have the ability to compartmentalize the pathogen and grow through it.” He recalls one survivor elm that is currently withstanding infection two years after a DED diagnosis. Whether the public can trust the process also remains to be seen, though. According to Held, clear communication is crucial for setting realistic expectations. “Resistance is not immunity. Even our best selections can have some limited wilt. That’s important to know,” he says.
“Another challenge is that this is such a long-term research effort spanning multiple decades,” Murphy says, “and funding landscapes are always changing.”
Growing forward
While right now, resistance does not appear to pass down through a tree’s seeds, the team hopes to eventually develop seed orchards—diverse groves of disease-resistant elms that could produce enriched seed. Now, when the team first tests a tree for disease resistance, they use cloned cuttings to make sure they’re testing the exact same genetic makeup. While some of the tree’s seeds might also be resistant, it’s a very small number—so testing all the seedlings would be inefficient. Instead, if researchers can identify and grow trees that are confirmed to be resistant, they can plant them near each other. These trees can then cross-pollinate and produce seeds that are more likely to carry resistance, possibly even combining different resistance genes.
The orchards that could establish this way would capture a broad range of resistance traits, helping future generations of elms stand strong against Dutch elm disease. “[We would be] transferring in any number of resistance genes or different traits,” says Murphy. “That’s the dream.”
To make that dream more efficient, Held and Murphy are also hoping for breakthroughs in gene discovery. “There hasn’t been genetic basis for resistance identified yet,” Held explains. “Which would, hopefully in the future, make [producing resistant trees] a lot easier.” With known resistance genes or markers, breeders could create controlled crosses and build up genetic diversity more deliberately. Although complex, the researchers believe this genomics-driven approach could someday accelerate the breeding and selection process and expand the tools available for restoration.
As they build a robust database of field submissions and expand plantings across the state, they’re laying the groundwork not just for reforestation, but for broader ecological resilience because this work could offer insights for other plant–pathogen systems. “Understanding resistance mechanisms goes further than just elm,” says Murphy.
But the elm’s resilience matters, specifically, for various reasons. “When you lose that population,” Murphy says, “you’re going to change the hydrology of a natural system.” Elm can help restore balance in those floodplains and forests. And in urban spaces, Held adds, “It’s important we figure this out so we can reclaim those areas and start using elm more.”
Held and Murphy know the work won’t yield instant results, but they remain encouraged. “There is resistance out there—we’ve seen that,” Held says.
And, as Murphy reminds us, this is a research field that moves in tree time: “We live in a world with a lot of immediate gratification, but this work just takes a long time,” he says.