A tree disease first spotted 9 years ago in Ohio is now a leading threat to one of eastern North America’s most important trees. The poorly understood malady, called beech leaf disease, is spreading rapidly and killing both mature American beeches and saplings, new research shows.
“This study documents how rapidly [the disease] has spread since its first observation in 2012,” says Robert Marra, a forest pathologist at the Connecticut Agricultural Experiment Station who was not involved with the work.
American beeches (Fagus grandifolia) are found across the eastern United States and Canada. The trees, which can grow nearly 40 meters tall and live up to 400 years, are a major player in many forests. Beeches constitute more than 25% of forests in Vermont, for example.
Historically, a blight called beech bark disease has been the primary threat to the species. But now, beech leaf disease appears to pose a bigger danger. First spotted in northeastern Ohio, it causes parts of leaves to turn leathery and branches to wither. The blight can kill a mature tree within 6 to 10 years. It has now been documented in eight U.S. states and in Canada.
In Rhode Island, observers first spotted beech leaf disease in 2020, confined to a small area, says Heather Faubert of the University of Rhode Island’s Plant Protection Clinic who was not involved with the study. But, “This year, it’s everywhere.”
To track the disease, Constance Hausman, an ecologist for a network of parks called the Cleveland Metroparks, and colleagues surveyed 64 0.04-hectare forest plots within 224 kilometers of Lake Erie in Ohio, Pennsylvania, New York, and Canada’s Ontario province. An analysis of 894 beeches in the plots found nearly half had the leaf disease, whereas just 34 had bark disease. Earlier surveys elsewhere had found the disease mostly attacked saplings, but the new work finds it is attacking mature trees, too, the team reported last month in Forest Ecology and Management. In forests near Lake Erie, beech leaf disease has now “become pervasive,” the group says.
The disease is “attacking the life cycle of beech trees in both directions,” Hausman says. The number of trees could fall so much in some forests that the species no longer serves key ecological functions, she warns, such as providing food and shelter for birds and other animals.
Another recent study by a different team examines an ongoing mystery: What exactly causes the disease? Earlier work raised suspicions that a tiny, previously unknown nematode worm that feeds on beech buds and leaves, dubbed Litylenchus crenatae mccannii, plays a role in spreading the blight.
Now, researchers report in Phytobiomes that when they examined diseased beech leaves, the tissues contained a fungus and four bacterial species also carried by the nematode. That suggests both the nematode and a pathogen it carries are contributing to the disease, says study co-author Pierluigi “Enrico” Bonello, an ecologist at Ohio State University, Columbus.
Marra is skeptical, however. He says one of the study’s suspects, Wolbachia, is known only to help its hosts. So he thinks its role in beech leaf disease, if any, might just be to strengthen the nematode’s attack.
So far, researchers haven’t identified a practical, cost-effective treatment for the disease, although some beeches appear to be resistant. But using those trees to breed new resistant strains could take decades, researchers say.