In 2012, Ohio authorities detected an apparently new disease attacking American beech (Fagus grandifolia) in the Cleveland area of northeastern. They continued to observe decline and mortality of beech saplings. In 2017 scientists at the USDA Agriculture Research Service identified the causal agent as the nematode Litylenchus crenatae, which had been described very recently in Japan. Due to morphological and host range differences between Japanese and North American populations, the North American population has been designated a separate, subspecies, L. crenatae mccannii (Carta et al. 2020). The common term for the disease became beech leaf disease (BLD).
The disease has spread more rapidly than any other nematode species reported to date, probably due to its dispersal by wind and rain (Kantor et al. 2025). The most rapid spread has been to the east and northeast. The disease was detected in Ontario in 2017, coastal Massachusetts in 2020, and Maine in 2021. Dispersal has been slower to the southeast; the first detection in Virginia was in a National Park unit in the northern part of the commonwealth in 2021. A single county in North Carolina was found to be infected in 2026. There has been little spread to the west; outbreaks in a few counties in southeastern Michigan were detected beginning in 2023. As of July 2025, BLD had been detected in 15 US states and one Canadian province (United Kingdom FERA). It is likely that the disease is more widespread than the confirmed locations because of the difficulty detecting it at the earliest stages (USDA FS Pest Alert 2026).
In Japan, the nematode appears to be widespread, although the extent of its range is unclear. The presumed native host, Fagus crenata, grows from Kyushu Island in the south to southern Hokkaido in the north. A second native, F. japonica, is limited to parts of Honshu Island (United Kingdom FERA).
Impact of the Disease
In the first years the disease’s impact was uncertain. Researchers at the apparent introduction location, Cleveland Metroparks, found high rates of infection, nearing 100% in saplings. The damage to larger trees varied; some had died and others bore only a few leaves. One study reported that sapling and pole-sized trees died within ~three 3 years after symptoms were observed (Pogachnik, 2016). McIntire and Vieira (2025) reported an even faster disease progression from leaf symptoms to severe decline and death of between 1 and 2 years after symptom expression.
More recent studies show some mortality of mature beech. Citing a study by McIntire and Vieira (2025), the British risk assessment reported that rates of mortality vary by the age and size of the host. Fearer et al. (2022) found that mortality of mature trees was relatively limited and occurred only after 6-8 years. Shepherd et al. (2025) analyzed data from long-term research plots established in 2009 just miles from the epicenter of the BLD outbreak. BLD was detected at the site 2014. By 2025, 29% of the 263 American beech trees tracked had died. Lead author Brianna Shepherd stated that researchers had seen “an exponential increase in mortality in just the last 3 years.” Saplings had mortality rates as high as 14% in 2022 alone.
This same study found that tree mortality was higher in plots with denser beech populations. Shepherd et al. (2025) suggested this finding might indicate that crowding may be compounding stress on the trees. Another explanation is that proximity of hosts facilitates disease transmission.
The disease slows tree growth even when death has not been the result. Shepherd et al. 2025 found that photosynthesis by nematode-damaged leaves was reduced up to 60%. Beech trees in the affected plots grew significantly more slowly after BLD arrived. The British analysis cited findings by McIntire and Vieira (2025) that trees infected for more than two years had 52.3% fewer buds per branch than control individuals in stand lacking BLD symptoms the previous year. Infected trees also had higher levels of second flush leaves—a stress response that drains energy from the plant.
Means of Spread
Several sources agree that spread within a tree and to nearby beeches is probably via water film across the surface of host tissues and wind-borne rain between trees, similar to other anguinid nematodes. The necessary moisture might come from dew, rainfall or overhead irrigation. In addition, high numbers of nematodes have been detected in detached leaves shed during the winter. These leaves might be dispersed by wind. This mechanism is probably responsible for some local spread of the nematode.
Wind is also the most likely form of spread over larger distances, carrying either the nematode itself or the leaves within which it is present. The easterly/north-easterly direction of more rapid spread fits prevailing wind conditions in North America. Goraya et al. (2024) found that higher relative humidity and wind speed both have positive effect on numbers of nematodes moved to traps at various distances. Precipitation has a negative effect on dispersal away from under the canopy. Under the canopy, high numbers of nematodes can fall and infest both leaves and smaller trees below. (See the British risk assessment for a summary of these findings). United Kingdom FERA (2026) suggested that the disease might also be transmitted through root grafts.
Long distance dispersal mechanisms are less certain. It is possible that various tree-associated organisms transport the nematodes; predatory mites, spiders, and birds have been hypothesized. Many bird species feed on leaf buds (the locus of infection) and on caterpillars that feed on trees. Nematode genetic material has been detected in bird feces and on their feathers. No direct evidence for this type of spread has yet been documented (Goraya et al. 2024). Experts think it is unlikely that nematodes could penetrate the hard husks of beech seeds (P. Vieira, pers. comm.), which are eaten by a wide variety of wildlife.
Long range spread is probably assisted by anthropogenic transport, especially of nursery stock. While American beech is not a major nursery crop, both European (F. sylvatica) and Asian (F. orientalis) are sold. Both have shown symptoms of BLD (Ewing et al. 2018). In the first years of the disease, an Ontario retailer received—and rejected—a shipment of diseased beech from an Ohio nursery (Reed, pers. comm.).
Symptoms
Early symptoms are dark striping on the leaves. The striping is visible on both very young “emerging” leaves and fully mature leaves. These are best seen by looking upward into the backlit canopy. The striping is formed by a darkening and thickening of leaf tissue between leaf veins. Later, lighter, chlorotic striping might also occur. Eventually the affected foliage withers, dries, and yellows. Bud and leaf production is also affected. Drastic leaf loss does occur for heavily symptomatic leaves during the growing season, as early as June, but asymptomatic and mildly symptomatic leaves show no or minimal leaf loss respectively.
Disease Progression
In Northeast Ohio, intensive monitoring of a subset of 13 plots in Cleveland Metroparks sites revealed a 7% mortality rate in 2018, primarily in saplings (Volk, pers comm). More than half of the plots now have dead trees that had previously been only symptomatic. While most of the dead trees are less than 4.9 cm dbh, some larger trees have died and others bear only a few leaves in summer. Sapling and pole-sized trees die within about three years after symptoms are observed. In areas where the disease is established, the proportion of American beech affected nears 100% (Pogachnik 2016).
Disease incidence does not appear to be influenced by slope, aspect, soil conditions, or weather. Also, while a wide variety of insects and pathogens is associated with symptomatic trees, these appear to be independent of beech leaf disease.
Leaves with light, medium, or heavy symptoms of infection – as well as asymptomatic leaves – can occur on the same branch of an individual tree.
The disease may spread through beech clone clusters along the interlocking roots, rain splash, and biotic vectors such as through contact with songbirds (external or via consumption of plant materials).
Long range spread of the disease is probably assisted by anthropogenic transport, especially of nursery stock. Both European (F. sylvatica) and Asian (F. orientalis) beech have shown symptoms (Ewing et al. 2018). In the past, an Ontario retailer received – and rejected – a shipment of diseased beech from an Ohio nursery.
Place of American Beech in the Forest
American beech, Fagus grandifolia, is the only native member of the genus in North America (Beckman et al. 2021), and it is one of the most widely distributed hardwoods on the continent. Its range reaches from Nova Scotia to Texas; and in the north, beech grow west to the Mississippi River (Houston in USFS GTR NE-331; Beckman et al. 2021).(A disjunct population in eastern Mexico is sometimes considered a separate subspecies, Fagus grandifolia var. mexicana (Beckman et al. 2021, Cogbill, 2004).
Beech averages 2.3% of total forest basal area in its range. Densities are highest in a broad band following the Appalachian Mountains from Kentucky to Maine. In this band, beech constitutes 11.1% or more of basal area, up to 51% in parts of the Adirondacks. At the beginning of the 21st Century, northern hardwood landscapes tended to have beech in 70 – 100% of stands; beech formed 5 – 25% of the basal area (Cogbill, 2004). Data from Canada are spottier. A survey in Quebec in 2004 found beech made up 17% of the forest (Cogbill, 2004). A later (2019) survey in southwest Ontario found American beech comprised more than 50% of basal area in 17 plots and 20-50% basal area in 16 other plots (Reed et al. 2022). In much of the southern and Midwestern portion of the range, beech constitute less than 1% of basal area (Cogbill, 2004). Still, about half of the 7.16 million acres of forest in Michigan had a beech basal area greater than 20% before arrival of beech bark disease (Heyd, 2004).
Throughout its range, American beech is found on most mesic sites which have escaped fire. Trees grow from sea level along the Atlantic and Gulf coasts up to 2000m in the southern Appalachians. At the northern edge of its range, beech occurs under locally moderated temperature and moisture conditions up to ~800 meters on mountain slopes. The species is especially abundant on well drained soils in valley sides or on hardwood ridges (Cogbill, 2004). Due to its extensive distribution, the species has not been considered at risk by the IUCN or The Nature Conservancy (Beckman et al. 2021).
The current distribution of American beech reflects interactions of three major factors: the species’ background regional abundance, land use changes (e.g., land clearance, regeneration on abandoned agricultural lands, forest management), and—since the mid-20th Century—spread of beech bark disease. Canopy beech promote soil and forest floor conditions that favor beech seedlings over competitors, e.g., sugar maple and hemlock (Beckman et al. 2021; Lovett et al. 2006). In some forests, e.g., in Michigan and Ontario, white ash also used to comprise significant proportions of the forest (Reed et al. 2023; Kearney et al. 2004) until arrival of the emerald ash borer (EAB). Still, beech seeds and sprouts are not dispersed far from the parent tree, which to some extent limits its ability to exclude other tree species. Soil moisture is important because beech is highly sensitive to drought stress. On moist, acidic soils, beech competes overwhelmingly in late-successional stands. Beech uninfected by BBD are highly resistant to wind damage. American beech is often said to be long-lived, but Cogbill (2004) found few older than 250 years. Beech leaf litter is high in lignin so it decomposes more slowly than that of its principal hardwood co-dominants, yellow birch and sugar maple. Litter quality influences the development of the forest floor and affects nutrient retention and cycling in the stands (Lovett et al. 2006).
Human activities had reduced beech populations significantly over the 19th and 20th centuries. (Population densities in the northeastern U.S. at the beginning of the 21st Century were 30% of that at the beginning of the 19th Century). Despite this historic decline and 100 years of BBD, the species is expanding in regenerating forests across many parts of the Eastern Deciduous Forest (Ducey et al. 2023; Miller et al. 2023; Payne and Peet, 2023). One factor might be deer avoidance. Deer play a significant, if not always dominant, role in regeneration of forest tree species (Miller et al. 2023; Blossey et al. 2024).
High proportions of beech are also found in parts of the Mid-Atlantic region. The National Park Service https://www.nps.gov/articles/000/treating-beech-leaf-disease.htm reports that mature beech constitute 14% of all trees in park units in the Service’s National Capital Region; up to 25% in Prince William Forest Park (site of the first BLD detection in Virginia). The proportion of saplings is much higher: 20% overall, 38% in PWFP. In both categories, Rock Creek Park in the District of Columbia ranks a close second: 21% of mature trees, 35% of saplings. Beech are more numerous as mature trees and saplings than any other species in both parks.
American beech has limited value as timber, but it is recognized as very important to wildlife. The tree’s large nuts are high in both protein (20% dry matter) and fat (50% dry matter) [see National Deer Association article here ], making them an important food source for at least 20 wildlife species, (perhaps as many as 40) including rodents, certain passerine birds, turkey, and bear among others (McNulty and Masters, 2004). The nuts’ importance is greatest in those parts of the northern hardwood forest where oaks are rare; there, beeches are the only species producing hard mast (Lovett et al. 2006). However, mast resources are apparently not as “reliable” as has often been said. In Maine and New York – after arrival of beech bark disease – large mast crops are produced on alternate years (Jakubas et al. 2004; McNulty and Masters, 2004). In Michigan, before arrival of beech bark disease, less than 10% of beech nuts examined were sound (Storer et al. 2004).
In addition to challenges from three non-native pests (beech bark disease, beech leaf disease, and European beech leaf weevil), beech might be struggling to adapt to elevated nitrogen deposited in eastern forests by air pollution (Latty, 2004).
Treatments
No operational, forest-scale treatment is currently available. Several chemical products, most notably those containing fluopyram or thiabendazole, have demonstrated efficacy in reducing symptoms in individual trees. Bartlett Tree Research Laboratories has led research on their efficacy and best application procedures under various scenarios. Check the website https://www.bartlett.com/blog/beech-leaf-disease-groundbreaking-treatment-strategies/ for most recent recommendations and information about regulatory restrictions in your state. These treatments are suitable for high-value landscape trees, but they are not practical for broad forest applications due to cost, labor, and need for repeated treatments.
The USFS is also evaluating phosphite products for potential use. Shepherd et al. (2025) note that tree mortality was somewhat lower in plots amended with limestone (to lower acidity) or phosphate (to increase nutrients).
For now, the Forest Service and others advise practicing common mitigation strategies important for reducing spread, including destroying infected plant material after removal and avoiding transport of soil or beech branches, twigs, leaves, or seedlings from affected areas (USDA FS Pest Alert).
Resistance Breeding
From the beginning of the outbreak scientists noticed that trees appeared to differ in susceptibility to BLD. They considered this fact promising for breeding resistance (Ewing et al. 2018).
The Great Lakes Basin Forest Health Collaborative https://holdenfg.org/great-lakes-basin-forest-health-collaborative/ includes American beech in its resistance breeding program. The goal is to produce planting stock that is resistant, able to survive despite both beech bark and beech leaf diseases. They are working with partners to collect seed from across the extensive range both to detect possible existing levels of resistance and to ensure that propagules will be able to thrive in the many different climates. Work to develop trees resistant to the older disease, beech bark disease, had progressed to the stage or producing clones when beech leaf disease entered the picture. They now are testing which BBD-resistant trees might also have some resistance to the new plague, beech leaf disease (J. Koch, USDA Forest Service. Presentation to the USDA Cooperative Forum on Invasive Species https://research.fs.usda.gov/nrs/news/events/usda-cooperative-forum-invasive-species February 24 – 26, 2026 Annapolis, MD).
European Concern
In 2018 media in the United Kingdom and Europe began reporting about beech leaf disease in North America. The British Plant Health Risk Register added Litylenchus crenatae to its risk list in March 2019. It also decided to respond with regulations if the nematode was intercepted. Finally it called for preparation of a risk assessment once more info was available (United Kingdom FERA).
The UK also recommended that the European and Mediterranean Plant Protection Organization (EPPO) also add the nematode Litylenchus crenatae to its Alert List. This request was supported by the EPPO Panel on Phytosanitary Measures. The species was so listed in April 2019. The Panel recommended amending the EPPO phytosanitary standard governing imports of trees in the genus Fagus and asked the 52 member countries to consider whether current phytosanitary requirements for imports of beech propagative material (“plants for planting”) were adequate EPPO, 2022). An EPPO working party in June 2025 agreed that the nematode is a priority for completing a risk assessment, working from the British national PRA (United Kingdom FERA).
The British risk assessors searched for Litylenchus crenatae at 561 sites in six European countries (Belgium, Republic of Ireland, Netherlands, Romania, Slovenia and United Kingdom) in July 2021 to November 2022. No sign of the nematode was detected. However, the sampling methodology might not have been optimum because the assessors did not then know that the nematode‘s presence in leaf buds could be detected before leaves become symptomatic (United Kingdom FERA).
Beech are a major component of forests across central and western Europe. The natural range of Fagus sylvatica reaches from southern Scandinavia to Sicily as well as from northern Spain as far east as northern Türkiye. Fagus orientalis grows predominantly in the Balkans, Anatolia, the Caucasus, and northern Iran. Distribution of F. orientalis appears to be increasing as the climate becomes warmer and drier. Hybridisation is common where the trees’ ranges overlap Hybridisation is common where the trees’ ranges overlap (United Kingdom FERA).
Fagus sylvatica is one of most important and widespread broad-leaved trees in the United Kingdom, especially in southern England and Wales. It is a major species in 18 Special Areas of Conservation across these regions. Beech are also planted in more northern regions and Northern Ireland. In 2008, Fagus sylvatica cultivars were the fourth most commonly planted large broad-leaved tree after sycamore (Acer pseudoplatanus), ash (Fraxinus excelsior), and oak (Quercus spp.) in parks and gardens (United Kingdom FERA). [European and British ash are undergoing severe depletion due to the spread of a non-native pathogen, ash decline, caused by Hymenoscyphus fraxineus.]
The UK has imposed a temporary prohibition on imports of beech plants for planting from Japan. However, the rule exempts artificially dwarfed (bonsai) trees. A review of British data showed imports of only six consignments since 2015, but they weighed a total of 12,300kg. Still, the risk assessors concluded that introduction of BLD via this route was Unlikely. This rating was Low confidence because of the difficulty detecting the nematode in planta, the unknown number of Fagus bonsai that owners might plant outside, and the possibility that the nematode might be more widely distributed than so far detected. The assessors concluded that if Litylenchus crenatae is found anywhere in Euro, the likelihood of entry in P4P would be rated as significantly higher (United Kingdom FERA).
In Europe, unlike in the United States, the wood is valued for furniture, flooring and interior joinery as well as a veneer (United Kingdom FERA).
Online references
Beech Leaf Disease Publications, Cleveland Metro Parks https://www.clevelandmetroparks.com/about/conservation/research-and-monitoring/forest-health-research-and-monitoring
US Forest Service https://www.fs.usda.gov/inside-fs/delivering-mission/sustain/beech-leaf-disease-emerging-forest-threat-eastern-us
Penn State Extension https://extension.psu.edu/guide-to-beech-leaf-disease-for-the-public
Invasive Species Centre (Sault Ste. Marie, ON) https://www.invasivespeciescentre.ca/invasive-species/meet-the-species/invasive-pathogens/beech-leaf-disease/
Beech Leaf Disease, New York Department of Environmental Conservation, https://www.dec.ny.gov/lands/120589.html
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Photo Credit: New York State Department of Environmental Conservation