One of the most significant forest and landscape trees, the American beech, is in trouble.
Dr. Andrew Loyd, plant pathologist in the research lab of Bartlett Tree Experts, is determined to keep the species healthy and thriving.
“Beech are an important species,” said Loyd. “They are found around the world.” The American beech tree range includes the Midwest, Southeast and Northeast.
Beech are long-lived, shade tolerant and produce nuts that feed wildlife. In addition to being an important forest species, the beech is a popular landscape cultivar, with attractive foliage and autumn color, often planted as shade trees and hedges.
Loyd provided background on beech leaf disease (BLD): In 2012, odd symptoms showed up on beech trees in northern Ohio. Most noticeable were dark bands between leaf veins. At the same time, distinctive banding on European beech became obvious.
Other symptoms included crinkled, leathery leaves, buds that failed to leaf out and subsequent canopy thinning. When beech trees die in forests, excess light comes through the forest canopy and changes the forest environment.
A 2018 Japanese publication described a novel leaf gall nematode on Japanese beech trees. “Around the same time, pathologists with Cleveland Metro Parks isolated nematodes from banded leaves,” said Loyd. “The nematode was similar to the one described in Japan.”
The nematode is Litylenchus crenatae and is listed as one of the top 10 most important plant-parasitic nematodes.
Since its original discovery in Ohio, BLD has been found in Michigan, New York, Pennsylvania, Maryland, Delaware, Virginia and throughout New England.
Healthy beech trees photosynthesize normally and produces one flush of leaves per season. All photosynthates are stored in the woody tissues, and stored tissue helps beneficial ectomycorrhizae fungi. Trees grow and store carbon, and carbon storage leads to growth and health.
Researchers working to solve BLD saw significantly higher photosynthesis occurring in healthy leaves. Stomatal conductance was higher in crinkled leaves, leaving them wide open to losing water and function. Water use efficiency became worse as banding and leaf crinkling occurred.
“In an infected tree, there’s inefficient photosynthesis, stomatal conductance and stomatal density,” said Loyd. “Trees will often send a second flush of growth and will produce more cells that are larger. They lose some aspects of ectomycorrhizae symbiosis, and this is a break in the positive feedback loop. Instead of producing and storing carbon, carbon is used and eventually depleted.”
Beech leaf disease. Photo courtesy of Matthew Borden, Bartlett Tree Experts, Bugwood.org
When the disease progresses and buds start to abort, auxiliary bud growth occurs, resulting in a new set of leaves. However, secondary leaves don’t function as normal leaves.
“Unlike emerald ash borer, Dutch elm disease or oak wilt that can kill trees relatively quickly, beech leaf disease is progressive,” said Loyd. “It’s a true foliar disease – infection occurs in the buds and it gets worse over time. It doesn’t necessarily affect the vascular tissue or root tissue, but it affects the photosynthetic machinery of the tree.”
Loyd said an anecdotal finding is that in diverse tree stands with a low density of beech among other trees, the disease progresses more slowly. In stands with high beech populations, the disease progresses much faster.
Environmental factors help spread BLD. Researchers found a positive relationship between wind speed, rainfall and humidity and infection rate. Nematode populations begin to rise in July, increase through August and peak in September.
“When beech trees produce nuts, they’re maturing around the peak of nematode migration from leaves – entering buds to overwinter and cause damage to the next season’s leaves,” said Loyd. “Around July and August, nematodes move from leaves to buds. It’s humid, with summer storms and wind, and there’s infection of new buds. Nematodes wiggle in between bud sheaths and start causing damage immediately. That continues throughout dispersal period into October, then nematodes overwinter in buds, mostly as females and eggs, until the following season when leaves emerge and are banded.”
In one experiment, birds were trapped within stands where BLD was present. The goal was to discover whether birds had a role in dispersing nematodes. Researchers found nematodes associated with several bird species including tufted titmouse, dark-eyed junco and woodpeckers. Although nematodes populations increased quickly, researchers didn’t find nematodes passing through the gut. The thought was that if birds are spreading nematodes, it’s happening outside their bodies on feathers, talons and beaks.
During the study, nematodes were also captured in spiderwebs and found on caterpillars. While wind and rain are significant factors in disease spread, Loyd said there are likely other organisms that move within forests to spread nematodes.
Managing BLD is a matter of targeting treatment before nematodes are active. “We know it’s a true foliar disease and doesn’t move into the vascular system,” said Loyd. “We also know the biology of when the nematode is moving from leaves to buds, and when it is most active. Target treatment when nematodes are at their lowest concentration and starting to become active. The goal is to manage before nematodes get into buds.”
He added that if the nematode lifecycle is disrupted, leaves function well the following season.
Loyd has conducted experiments with two different chemistries for BLD. One is fluopyram, fungicide FRAC group 7, labeled for landscape trees. Fluopyram disrupts the energy lifecycle in cell respiration to prohibit nematode growth and function.
“Make fluopyram applications in late summer or early spring,” said Loyd. “You won’t get good efficacy if you can’t get full coverage or apply the material after nematodes have started to migrate to buds. Once nematodes are in buds, damage is occurring.”
The other chemistry is thiabendazole (TBZ), an antiparasitic used in both veterinary and human medicine, and effective as a nematicide. The active ingredient disrupts mitosis (cell division) and affects nematodes’ energy production. TBZ, labeled as Arbotect®, is suitable for use as a root flare treatment of large trees where full coverage would be difficult. Loyd said the Arbotect label is narrowly written for elm and sycamore and requires label expansion for special use.
“Beech leaf disease is caused by a putatively introduced nematode, probably from Asia,” said Loyd, summarizing what is known about the problem. “There’s progressive decline due to reduced carbon storage within a tree. High-density stands have faster levels of disease progress; diverse plantings have less damage than beech monocultures. Wind and rain are factors for local pathogen spread, and birds spread it further. Fluopyram and thiabendazole are effective chemistries for treating trees.”
by Sally Colby
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