Treating Beech Leaf Disease

A park employee drills a shallow hole into the side of a root flare on a BLD-infected beech tree at Cabin Camp 2 in Prince William Forest Park. A small orange injection plug was then used to administer a BLD treatment. NPS/Nortrup

Take Aways:

• American beech (Fagus grandifolia) is the most common forest tree in National Capital Region (NCR) parks
• Beech leaf disease is a serious, often fatal condition affecting American beech and European beech (Fagus sylvatica) trees
• A new experimental treatment can help slow disease progression, extending the life of priority trees
• Treating individual trees is time and resource intensive, so selecting which trees to treat should be a strategic decision based on ecological, cultural, and visitor value.

Buying Time for Beech

Beech leaf disease (BLD) is no longer hard to find. At Prince William Forest Park this year (2025), its quiet spread since 2021 has turned from scattered and subtle leaf striping to wide-spread leaf damage, branch dieback, and canopy thinning. This fatal tree disease is caused by the tiny nematode Litylenchus crenatae ssp. mccannii, which parasitizes the leaves. As of August 2025, it has also been confirmed at Catoctin Mountain Park, C&O Canal National Historical Park (near Fort Duncan), George Washington Memorial Parkway, Harpers Ferry National Historical Park, and Monocacy National Battlefield.

About the Treatment

An experimental injection treatment is now one of the few treatment options for managing BLD. Over the last 3 years, researchers at Bartlett Trees and Rainbow Ecoscience have been testing BLD treatments. They found the fungicide Arbotect 20-S, which has the active ingredient thiabendazole and is labeled for treating Dutch elm disease, to be effective at minimizing visible BLD symptoms in trees and preventing canopy dieback. The results of their study have been promising enough that 24 states, including Maryland and Virigina have issued Section 24(c) Special Local Need labels that allow for the use of Arbotect 20-S to treat BLD.

In July 2025, a team of plant pathologists and forest health experts from the US Forest Service, Virginia Department of Forestry, DC Department of Transportation, and Rainbow Ecoscience along with NCR park staff held a biological evaluation of BLD at Prince William Forest Park. They also did a demonstration treatment using Arbotect 20-S at Cabin Camp 2. They gave root flare injections to two large beech trees growing close to historic buildings that had moderate-severe BLD symptoms. The treatments took about an hour to administer for each tree and should protect each tree for approximately two years. After that, the trees will need to be re-assessed and likely re-treated to avoid reinfection and decline.

There’s Still More to Learn

While Arbotect injections for BLD are promising, research on the disease and its management is still ongoing. Many questions remain, including whether the treatment builds up within trees, if treated trees can provide protection to neighboring trees, or if Arbotect’s effectiveness diminishes over time. For now though, the treatment helps buy time in the hope that additional options become available, that trees develop a natural BLD resistance, or that a biological control agent is identified.

Arbotect injections are not a solution for treating entire forest stands. It is most appropriate for treating high priority trees like witness trees, high-value specimens in cultural or natural landscapes, or trees in visitation areas. Additionally, candidate trees should meet the following criteria for treatments:

• Diameter at breast height (DBH) greater than 10 cm.
• No more than 30% canopy die back (trees with greater dieback of canopy are unlikely to recover).

For more information on BLD and treatment options, please contact NCR Ecologist and Integrated Pest Management Coordinator Dorothy Borowy.

Losing These Trees is a Real Beech

American beech (Fagus grandifolia) are the most common species of tree in NCR park forests. Their numbers have continued to increase over the past decade (Table 1). The loss of beech would mean the loss of:

• a dominant canopy species that shapes light availability, understory composition, and overall forest dynamics
• nutrient dense beech nuts, a key food source for wildlife ranging from black bear and white-tailed deer to a number of small mammals and birds
• beech-adapted insects, that in turn support a variety of birds and maintain important food web links
• critical shelter and nesting spots provided by mature beech crowns and cavities
• high-lignan leaf litter that decomposes slowly, protects the soil from erosion, contributes to long-term carbon storage, and maintains forest floor microhabitats

For these reasons and others, efforts to save beech trees are definitely worthwhile. Their decline would alter canopy composition and cascade through wildlife populations, soil processes, and ecosystem resilience.

While we still don’t know how BLD will unfurl itself across our parks, other actions can help bolster forest resilience including continued deer management, and prioritizing invasive plant control to allow regeneration of a diversity of tree species that were previously suppressed.

Beech Abundance in National Capital Region Parks

Where this belongs

• Catoctin Mountain Park
• Chesapeake & Ohio Canal National Historical Park
• George Washington Memorial Parkway
• Harpers Ferry National Historical Park
• Monocacy National Battlefield
• Prince William Forest Park

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