ARI SHAPIRO, HOST:
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 is causing beech leaf disease. A team of tree scientists is narrowing down the list of possible culprits. From member station WKSU, Jeff St. Clair reports on a botanical whodunit.
JEFF ST CLAIR, BYLINE: It's a long slog to a bluff overlooking the Grand River in Lake County, Ohio. It was here in 2012 that Lake Metroparks biologist John Pogacnik first noticed something was awry.
JOHN POGACNIK: It just looked different. You could tell right away something was up.
ST CLAIR: What he saw was sunlight.
POGACNIK: Beech are usually a tree that create a lot of shade, and these are no longer doing that.
(SOUNDBITE OF LEAVES RUSTLING)
ST CLAIR: A slight breeze shakes the thinning canopy overhead.
POGACNIK: This tree right here is a really good example. You could see it's probably 20 foot tall, and there's probably 50 leaves on it.
ST CLAIR: And most have mysterious dark bands between the veins. Pogacnik was concerned, but alarm bells hadn't gone off just yet.
POGACNIK: I thought maybe just drought was affecting them.
ST CLAIR: By the following year, he says, beech leaf disease had spread throughout the county. And now it's seen in parts of Ohio, Pennsylvania, Ontario and as far east as Connecticut and Long Island.
ENRICO BONELLO: This had all the hallmark of an invasion.
ST CLAIR: Enrico Bonello is 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.
BONELLO: Nobody had ever seen anything like that.
ST CLAIR: Bonello and his team are casting a wide net to identify a cause, looking at the genetic markers of everything living on the tree. Meanwhile, other researchers think they have a prime suspect. David Burke heads research at the Holden Arboretum. He's focusing on a microscopic worm called a nematode.
DAVID BURKE: So this is one of our greenhouses here at Holden Arboretum.
ST CLAIR: 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.
BURKE: So if you take a look right here, you can see that this is a leaf that the nematodes were added to. And what you can see is the disease right away. So you can see this sort of dark green color that is between the veins. And it looks a little blistered too, right?
ST CLAIR: It's a perfect match.
LYNN CARTA: This is basically unheard of.
ST CLAIR: Lynn Carta is a nematode expert with the U.S. Department of Agriculture. She says it's the first example she's seen of nematodes killing large trees. Carta says the rogue worm is a subspecies of one recently discovered in Japan. But that may not be its origin.
CARTA: This thing might have come from China or Korea or anywhere over in the Pacific Rim.
ST CLAIR: Carta says it's spreading eastward by about 150 miles each year. And to do that, it has to have an accomplice - an insect or a bird or both. And it's also not clear how the nematode actually kills the trees. It could be a toxin, virus or another pathogen it carries. So the mystery remains unsolved for now. As Canadian and European agencies issue warnings about beech leaf disease, American regulators are taking a wait-and-see approach as the investigation continues into this latest threat to our native forests.
For NPR News, I'm Jeff St. Clair.
(SOUNDBITE OF COURTNEY BARNETT SONG, "DEPRESTON") Transcript provided by NPR, Copyright NPR.