ABOVE: Beech trees showing dark green bands between the veins, an early symptom of beech leaf disease

CARRIE EWING

A new disease, named for the tell-tale symptoms that appear on foliage, is killing American beech trees. Beech leaf disease was first spotted in northeast Ohio in 2012 and has since moved into 10 Ohio counties, eight Pennsylvania counties, one county in New York, and five counties in Ontario, Canada. Its rapid spread has led scientists to raise the alarm as they try to figure out the cause.

Beeches typically have paper-thin, bright green leaves. An afflicted, but otherwise healthy-looking tree first develops deep green patches between the veins of its leaves. In a later stage, the leaves become thick and leathery and eventually crinkle up. The buds on these branches die and stop producing leaves, says Pierluigi “Enrico” Bonello, a molecular and chemical ecologist of trees at the Ohio State University and one of the authors of a report on the emerging epidemic, as they refer to it, published December 25 in Forest Pathology.

The scientists don’t yet know exactly how the disease weakens the trees. “It seems to be cutting off some sort of photosynthetic pathway for the trees to maintain their leaf structure and produce new leaves over the years,” says coauthor Carrie Ewing, an environmental science graduate student. It’s still too early to tell how deadly beech leaf disease is, though younger trees seem particularly vulnerable. Within three years of showing symptoms, a young sapling can die.

Essentially, it’s impossible to eradicate and contain the agent. . . . We’re left with the host and the only long-term management option is to screen for, select, and breed resistant trees. —Pierluigi “Enrico” Bonello, the Ohio State University

According to Bonello, the disease was first spotted in 2012 by his coauthor John Pogacnik, a biologist at a park district in Ohio called Lake Metroparks. He recruited Bonello to help investigate what was happening to the trees. They kept an eye on the disease until 2016 when its spread convinced them that something had to be done. Bonello rounded up a group of plant scientists to think about how to tackle the mystery disease.

“In the meantime, the disease was spreading fairly fast. It was behaving in many ways like a typical invasive, alien, forest pest,” Bonello tells The Scientist. “It’s first noticed on a very local basis. And then it starts spreading in a rate that is sort of exponential. I think we’re still in the exponential phase right now,” he says.

Because beech leaf disease moves so quickly, Bonello and colleagues don’t think the cause is an environmental factor such as water availability, temperature, or soil conditions. And because the leaves don’t appear chewed up, it’s likely not an insect chomping away. It “looks very much like a typical communicable disease, an infectious disease,” Bonello says.

An assortment of possible pathogens is under consideration: fungi, bacteria, viruses, and plant parasites called phytoplasmas. To get at what is killing these trees, Ewing is doing microbial profiling, grabbing the DNA and RNA of microbes associated with the leaves to get a sense of who’s there. Comparing the communities of microorganisms that live on diseased trees and asymptomatic trees in the hot zone with those far away from the infestation will hopefully reveal a potential culprit. Then to actually confirm the identity of a causal agent, the researchers have to infect new plants with the microbial suspect and see if it causes the disease.

Ewing has already observed differences in the forest since the disease appeared. “It’s quite remarkable actually how the canopy changes . . . over time, going back to the same site and seeing how much more light is being let in because of the canopy cover becoming smaller,” she says.

“Beech trees dominate in lots of different forests and their loss would take out another tree that’s of enormous importance ecologically and for other environmental reasons,” says Steve Woodward, a plant pathologist at the University of Aberdeen in Scotland. It could be an environmental disaster, he says.

For one, the trees grow beechnuts, which are important sources of food for animals including squirrels and bears. In the northern hardwood forests of the northeastern US, beech is the only “mast-producing tree,” a tree that provides food, says Michael Preminger, a graduate student at the State University of New York who studies beech bark disease. “If there were no beech it would be harder for mammals in that ecosystem,” he says.

Humans also reap benefits from beeches. To put a number on what they do for us, the authors of the paper used an online tool to estimate the cost to people if half the beeches in Ohio vanished. The figure amounted to $225 million lost in ecosystem services such as filtering water and sequestering carbon.

The concern stretches overseas as well. Beeches are prevalent in the forests of Europe and Asia, says Woodward.

Preminger says he is hesitant to push the panic button because of all that is still unknown, including the infection rate and what fraction of infected beeches die. But he says that right now beech leaf disease looks pretty bad. Long term monitoring would help answer those questions, he says.

The researchers who have watched the leaves turn in Ohio are growing alarmed. “Now is the time that we really need to act,” says Ewing. With past threats, including the emerald ash borer, people waited years while important native species were essentially wiped out, she says.

The options to save the trees are limited. “Essentially, it’s impossible to eradicate and contain the agent,” says Bonello. “We’re left with the host and the only long-term management option is to screen for, select, and breed resistant trees.”

C.J. Ewing et al., “Beech leaf disease: An emerging forest epidemic,” Forest Pathology, doi:10.1111/efp.12488, 2018.