Trouble for Maryland Beeches?

Dark bands between leaf veins are a telltale sign of beech leaf disease.

WAMU, A Mysterious New Pest Is Threatening One Of The Most Important Trees In D.C.-Area Forests

The deadly ailment is called beech leaf disease, and it was first detected in the D.C. area in 2021 in Prince William County, at Prince William Forest Park. Then in May, 2022, it was found in Fairfax County, at Burke Lake Park.

“I stepped into the forest from the road and I looked up and I saw it,” says Joan Allen, who is in charge of tracking forest pests in Fairfax County’s Urban Forest Management Division.

“It was a very sad day for me,” Allen says.

Since then, the disease has been found at two other parks in Fairfax: Fountainhead Regional Park and Hemlock Overlook Park. It has not yet been detected in Maryland, or D.C., though it will likely arrive in the near future, if it hasn’t already — lurking unseen in the tree canopy.

Beech trees are ubiquitous in this part of the country. In Fairfax, there are an estimated 4 million of the trees — outnumbering humans by more than 2 to 1. If those trees were all wiped out, it would be devastating to local forests. But beech leaf disease is still new, and there are a lot of unanswered questions.

Beech leaf disease was first discovered in Ohio in 2012. “The symptom expression was really unusual. We’d never seen anything like it,” says Constance Hausman, senior conservation science manager at Cleveland Metroparks.

If you know what to look for, the disease is easy to identify — it appears as dark bands between the veins of leaves, visible as soon as leaves emerge in the spring. Over the course of the summer, the leaves can grow brittle and brown and start to curl.

After more than a decade observing the disease in Ohio forests, Hausman says, “the decline has been rather dramatic.”

There’s a growing list of native trees that have been virtually eradicated from the landscape by invasive pests – the American elm, the American chestnut. Most recently, ash groves are collapsing, under attack by a beetle called the emerald ash borer. So, is the American beech next? 

“It’s not as clear-cut as emerald ash borer, where you experience over 99% mortality,” Hausman says.

For one thing, some beeches appear to be resistant — so it’s possible beeches could evade the pest. And, the pattern of decline is different, because so far the disease mostly kills saplings, not mature trees. This could mean a slower unraveling of beech ecosystems: if saplings can’t survive, mature trees won’t have a next generation to replace them.

Small trees often die within three or four years of being infected, Hausman says.

Hausman is part of a team of researchers studying the disease and its progress across North America. It has now been found in 11 U.S. states in the Midwest, Northeast and Mid-Atlantic. Hausman says the disease has been progressing at about 10 miles per year in the Midwest, but may be moving much more quickly on the East Coast.

So what causes it?

For several years, the cause of beech leaf disease was a total mystery. Now scientists believe the culprit is a parasitic nematode.
. . .
In the case of beech leaf disease, the nematode in question is called Litylenchus crenatae mccannii, and it was discovered in 2019.

“You have a large number of these nematodes in the leaves and in the buds of the trees, and that’s why they cause a lot of problems, because they use the bud and the leaf tissue as a source of nutrients,” says Vieira. In one individual bud, he says, there can be thousands of nematodes feasting away.

With such a newly discovered creature, there are still a lot of puzzles. How exactly does it travel, particularly across large distances? How did it get to Virginia, apparently skipping over neighboring states? Where did it originate, and how did it get to North America?

“So far, we don’t have a specific answer for those questions,” Vieira says. “Our best rational hypothesis is that this is potentially coming from Asia.”

Just looking at that map, I would guess the reason it hasn't been found in Maryland yet is that no one is looking hard enough. All the counties in Pennsylvania adjacent to Maryland have some level of infestation. 

Beeches are not a main contributor to the forest here, but they do occur, and they are quite striking. We have a couple in our back lots, and one coming up in our yard that we transplanted shortly after we had our house built. It would be a shame if something were to happen to them. I think we need to keep Virginians, and Pennsylvanians out of Maryland to prevent the transmission of these worms, which I assume happens with soil. Maybe we could just make them leave their shoes at the border.