Once deposited, the fungus spore germinates and penetrates into the inner bark tissue, where the canker forms. It kills the cambium layer and causes a disfiguration in the tree as it tries to grow around.
Courtesy of Dale Bergdahl.
Anyone who has ever had their fingers glued to a sticky green butternut, or spent the time to crack one open to sample the savory meat inside, will never forget this unique species.
"The butternut is a tree that people seem to get very attached to," says Dale Bergdahl, a retired professor of forest pathology at UVM who has devoted much of his own time and attention to the butternut. "Butternuts are really a rare species, when you consider all the other species out there. They don't grow in large groups; they're very individual trees that grow in small clumps here and there."
Unfortunately, for the past five decades or so, the butternut has been devastated by the butternut canker disease.
The butternut is native to North America, and its range extends from the East Coast to the Mississippi and as far south as Georgia and as far north as southern Canada. "Within this range, there is no uniform distribution, but there used to be a lot more trees. The canker fungus has really been severe in eliminating those," Bergdahl explains.
Nobody knows for sure, the fungus is believed to be an exotic that was brought to North America. The first reports of the canker disease came from Wisconsin in 1967. "There wasn't a lot of interest in the disease at that time; there weren't a lot of people working on it, because the butternut is not a major tree species," he points out.
Bergdahl confirmed the butternut canker in Vermont in 1983. It wasn't until 1993 that the time and research funds were available to really conduct a survey of butternuts at different locations in the state. Some 1,300 trees were surveyed on 18 different plots, primarily in the Champlain Valley. "At that point, about 12 percent of the trees were dead. By 2002, we were at 41 percent mortality on those same trees," he recounts. "The last real solid numbers for those trees was from 2005-06, when we were at 55 percent mortality."
A subsequent project aims to take a "snapshot" look at butternuts in all counties of the state (as opposed to following them over time). That research revealed a 29 percent mortality rate, a number that Bergdahl says would probably be twice as high if all the butternuts in those areas had been tracked over time and dead trees that had already fallen/been removed were included.
The story has been the same in other Northeast states, says Bergdahl. Similar research conducted in 2009 found that butternuts in Maine were at 29 percent mortality; New Hampshire at 27 percent mortality and New York at 25 percent mortality. "Those numbers are outdated now; I'm sure we're at 35 to 40 percent mortality in those areas," he adds.
Today, Bergdahl runs Plant Technologies LLC, based in Charlotte, Vt., which consults with landowners on tree health problems. Part of his work is an ongoing research project with the U.S. Forest Service to identify trees that are good candidates for future genetic work, which can be used for grafting and the creation of a seed orchard of high quality butternuts.
"We're continuing to look for those good trees. I get inquires from people just about every week. They send me information about trees they think are healthy, and I keep track of those locations," says Bergdahl, who can then visit and check out promising leads. "Landowners have been great about getting the information to me. That information is critical, and the more we have, the better."
The belief is that some of these trees that remain generally healthy may have some natural resistance to the canker disease. "About 1 to 2 percent of the butternuts still out there seem to be relatively healthy. That doesn't mean that they are totally healthy, but it means that they may have some resistance." From these promising candidates, material is collected for DNA testing to first make sure they are true butternuts.
In some areas, particularly in coastal New England, butternuts have inter-bred with Japanese Heartnut, which was introduced in the late 1800s. These hybrids appear better able to survive the canker - with butternut mortality at just 15 percent in Massachusetts and 5 percent in Connecticut - but the genetic material of the true butternut has been polluted, says Bergdahl. "If we want to restore butternut to the system, we have to have true butternut and not some hybrid," he explains. "So, we make sure that the material we collect is pure."
From trees that are deemed healthy and which pass the DNA test, fine wood is collected from during the coldest part of winter for grafting. Already, some grafted trees have been created from healthy butternuts found in Iowa, Pennsylvania, Connecticut and Vermont.
After this phase of the project is complete, additional funds will be sought to continue looking for healthy butternuts, particularly across the northern parts of Maine, New Hampshire, Vermont and New York, where butternuts tend to be relatively pure without a lot of hybridization that's occurred. The next step would be to create a seed orchard "using the best of the best trees we could find," says Bergdahl. "Those will be allowed to grow and interbreed among themselves." The ultimate goal, he explains, is to start a restoration effort that would produce stock for planting. "It's quite a long-term proposition, but that's the hope," he states.
It's a project that's critical to the survival of the species. The disease represents a double-whammy for butternuts: Not only does it slowly kill the tree, it also prevents it from producing nuts (and, therefore, future trees) as the top branches fall off. "It can't regenerate itself," says Bergdahl.
The disease shows no signs of going away, either. "The fungus (Sirococcus clavigignenti-juglandacearum) is known to be rain splash-disseminated, and also insect-disseminated. We have found 17 different insects carrying these spores. They pick up the spores on their body and fly off to butternut trees," says Bergdahl. "The fungus gets into the trees through small wounds or some other natural opening on the small twigs of the tree." Something as small as claw marks from a squirrel running up and down the tree can create enough of a wound to let the fungus in, he adds.
Once deposited, the fungus spore germinates and penetrates into the inner bark tissue, where the canker forms. It kills the cambium layer and causes a disfiguration in the tree as it tries to grow around. "The earliest symptom of the canker is a slight crack in the bark and some 'bleeding' from it, or discoloration. That's very typical," Bergdahl explains.
Nearly 100 percent of butternuts have cankers, he says. It's just the number of cankers that makes a difference; once multiple cankers appear - along with other organisms that take advantage of stressed trees, such as root rots - "that does them in," Bergdahl explains. Cankers can occur high in the tree in smaller branches, as well as on major branches and the upper trunk and even on the root flare. "It's a downward spiral once the tree becomes infected with multiple cankers. It will usually be seven to 10 years before the tree will die, but that's what's going to happen," he says.
There is really nothing that can be done to slow or stop the disease. "By the time people see it, it's almost too late," says Bergdahl. The best thing that can be done to help butternuts is to keep them healthy and to keep the competition away. "The tree is really intolerant of shade," he emphasizes. If a butternut is being crowded by, say, maples or beech, these other species can be cleared back to give the butternut crown more room. "It can only help, it can't hurt," he concludes. "Trees growing in the open seem to have a better chance for longer term survival. I don't have any data on that, but just from visual observations, that's what I believe."
For woodlot owners who have butternuts, there are some additional considerations in the event a timber harvest is scheduled. Bergdahl says that the U.S. Forest Service several years ago produced some general suggestions saying that any butternut with over 50 percent of the crown still living should not be cut, while any tree with more than 50 percent mortality in the crown could be harvested. He says there theoretically may be some benefit to removing unhealthy trees because it could reduce the number of fungal spores in the forest, "but realistically, there are so many spores out there, it probably won't make a huge impact."
Another consideration is the value of butternut wood. With fewer butternut trees around, the wood has logically become more valuable. Ironically, though, wood from butternuts that have died is actually even more valuable than that from living trees, Bergdahl points out: "Once the tree has died, it is invaded by different insects that create what is known as 'character wood.' When the tree is harvested five years or more after it has died, that wood has greater value. So, why harvest a healthy tree when it will only be more valuable if it dies sometime down the road and is harvested then?"
Butternut trees have always been rare, but continue to be more so all the time. "Probably less than one-one thousandth of 1 percent of trees out there are butternut," says Bergdahl. It is listed as a "sensitive species" in the U.S., but it was recently re-categorized as an "endangered species" in Canada. The hope is that the work of Bergdahl and other researchers can someday turn the tide.
He says it remains important to locate and identify healthy butternuts that may have resistance to the canker disease. "We like to find trees that are in and amongst other butternuts that are dead and dying," Bergdahl explains. "I usually tell people that if they have an area with 12 to 15 butternut trees within a few acres, those are good candidate sites for us to visit, because within that small area, there may be a single tree that has had the opportunity to be infected, but is still healthy."
Woodlot owners in the Northeast who have butternuts they feel are generally healthy can contact Dale Bergdahl via e-mail at firstname.lastname@example.org.
Patrick White is a freelance writer based in Middlesex, Vt. Over the past 10 years, he has covered a wide range of agricultural operations around the Northeast. He is always on the lookout for unusual stories.