Exploradio: On the Hunt For a Mysterious Tree Killer

Aug 12, 2019

A mysterious disease is killing one of the most majestic trees in American forests, the beech. 

Known for its smooth gray bark, the beech is an important anchor species. 

No one knows exactly what causes Beech Leaf Disease, but a team of tree scientists is narrowing down the list of culprits in this botanical whodunit.

It's a mile long hike through thick scrub to the spot where the first victims were discovered back in 2012 in Lake County.

We arrive at a bluff overlooking the Grand River where Lake Metroparks biologist John Pogacnik first noticed something was awry.

Lake Metroparks biologist John Pogacnik is bathed in sunlight, which is what first told him something was wrong with the shade producing beech trees.
Credit JEFF ST.CLAIR / WKSU

“It just looked different," he says, "you could tell right away something was up.”

What he saw was sunlight.

“Beech are usually a tree that create a lot of shade," says Pogacnik, "and these are no longer doing that.”

A slight breeze shakes the thinning canopy overhead as he points to a sad looking specimen. “This tree right here is a really good example," says Pogacnik. "You can see it’s probably 20 feet tall and there’s probably 50 leaves on it.”

The remaining leaves have mysterious dark bands between the veins.

Pogacnik was concerned, but alarm bells hadn’t gone off just yet. He thought they were just stressed by drought, but the following year the malady had spread to six other Lake Metroparks.

Now beech leaf disease is seen across Northeast Ohio, parts of Pennsylvania, Ontario, and as far east as Connecticut and Long Island.

“This had all the hallmarks of an invasion,” according to Enrico Bonello, a plant pathologist at Ohio State University.

He’s one of the first people Pogacnik called to figure out what was happening to the beeches.

Bonello contacted colleagues around the world, who were all baffled by the dark bands seen in the Ohio leaves.  He says, “nobody had ever seen anything like that.”

Bonello and his team are currently casting a wide net to identify a culprit, sequencing the DNA of everything living on the tree to see what stands out.

A new forest pest
Meanwhile other researchers think they have a prime suspect.

The Holden Arboretum's David Burke studies the damage from nematodes in the leaves of greenhouse beech trees.
Credit JEFF ST.CLAIR / WKSU

David Burke is head of research at the Holden Arboretum.

He’s focusing on a microscopic worm called a nematode.

Burke is testing whether nematodes found inside the leaves could cause the disease.

He gathered the tiny worms from infected leaves and put them on greenhouse saplings.

His test produced the tell-tale striping seen in diseased leaves.  "So you can see this dark green color that is between the veins," says Burke, "and it looks a little blistered too.”

It’s a perfect match for beech leaf disease.

Tracking down the rogue worm
Lynn Carta is a nematode expert with the U.S. Department of Agriculture who's been studying the samples from Ohio.

“This is basically unheard of," says Carta, ”to have thousands of nematodes in a tree that can actually be killing a tree."

An electron microscope captures a close-up of the nematode, Litylenchus crenatae, which plays a role in beech leaf disease. Exactly what that role is, and how it's spread remains a mystery.
Credit GARY BAUCHAN / USDA

Carta says the rogue worm is a subspecies of one recently discovered in Japan, but that may not be its country of origin.

“This thing might have come from China, or Korea, or anywhere over in the Pacific Rim," says Carta.

She estimates that it's spreading eastward at roughly 150 miles per year.

But how does a microscopic worm get around?

The Arboretum’s David Burke is trying to figure that out.

Accomplices to the crime 
“This is a nematode that has some assistance,” says Burke.

The nematode, he surmises, could be attached to a small mite that then hitchhikes on a bird or insect, “And I think the insects right now are the leading culprit in terms of a vector that are moving the nematode from one place to another.”

Heavily infected beech leaves at the Holden Arboretum carry tens of thousands of nematodes in each dark stripe that characterizes the disease.
Credit JEFF ST.CLAIR / WKSU

It also might carry a virus, bacteria or toxin that’s doing the dirty work.

All of it remains a mystery.

Along with how it got here.

One of the few instances of Asian beeches being imported is as bonsai trees. The USDA’s Lynn Carta says that could have been a route. 

Carta says nematodes are easily carried by wind-borne rain drops, and, “perhaps that could account for their transmission from a bonsai plant onto a beech that’s in the back yard.”

But she says we’ll likely never know exactly how they got here.

Federal regulators so far have not issued any restrictions on moving the wood from beech trees, and the US Forest Service is taking a wait and see approach, relying on state authorities to track the disease.

Meanwhile Canadian and European agencies are very worried.

They’re warning the public to keeping a close eye on a new disease that could devastate an already vulnerable species.