Another doom-and-gloom story? There is an ever-growing lineup of new and invasive pests, weeds and diseases with which we, as tree care professionals, must keep up. For foresters and arborists in the mid-Atlantic, New England and parts of the upper Midwest, beech leaf disease (BLD) is one such problem. It has raced toward the front of the line due to its alarming severity, rapid spread and unique characteristics that require new management tactics. But there is hope.

Banding on American beech caused by beech leaf disease. All photos and graphics courtesy of the author/Bartlett Tree Experts.

How bad is the disease, and why is it unique?
BLD was first identified around 2012 in Ohio and has spread at an astounding pace, moving primarily east and north. Already it affects beech in at least 15 states, plus Ontario. Unfortunately, BLD has brought a second epidemic to much of the same region of American beech (Fagus grandifolia) forests already severely affected by the beech-bark-
disease complex. The beech-bark-
disease epidemic began around 1890 in Nova Scotia and has moved at a much slower pace south and west through much of the northeastern United States and adjacent regions in Canada.

BLD is caused by a nematode, Litylenchus crenatae mccannii, a type of microscopic roundworm. Unlike most nematode plant pests which infest roots, this species infests the developing buds on beech twigs and causes dramatic gall-like deformities in the leaves. As it is a relative newcomer to science, there is still much to learn about the nematode, particularly how it spreads and some aspects of the life cycle.

Evidence points to the species originating in Japan, where it causes a nonlethal, minor form of the disease on the native beech there. In North America, however, it causes a severe, progressive disease that deprives beech leaves of their ability to produce food and energy, turning from a net gain of resources to a net loss. Over the course of several seasons, and as the nematode populations build up, we see defoliation, bud abortion and eventual branch dieback. Other pests and secondary diseases may take hold once the tree is severely weakened, leading toward a decline spiral.

Bartlett Tree Experts and collaborators conducting research on beech leaf disease in the summer of 2021.

Why care about beech?
For the benefit of those not lucky enough to work with beech, these trees are the muses of art and lore. They are prized by landowners lucky enough to have a mixed beech forest or a majestic specimen tree accenting their property. They are iconic trees known for their smooth gray bark, muscular root flares, spreading canopy of vibrant leaves, shady understory and marcescent leaves that wither but persist over the winter.

In eastern North America, the American beech is sometimes called the “aristocrat of the forest.” Like oak, beech is considered a keystone species for its role in local ecosystems, providing vast crops of beechnuts in masting years once trees are mature, as well as nesting habitat and deep shade throughout the season. While American beech is grown as an ornamental shade tree in landscapes, the European beech (Fagus sylvatica) plays a prominent landscape design role. It is the European beech cultivars, such as weeping beech, copper beech, tricolor beech and fern leaf beech, that are prized for their architectural form, leaf color and stately nature.

So far, all beech species commonly grown in the United States are susceptible to BLD, although some natural variation in severity has been observed.

Litylenchus crenatae mccannii, a type of microscopic roundworm, causes dramatic gall-like deformities in the leaves.

Learning from the past
Many of us may recall sitting in a dimmed classroom, hearing case studies of the devastating tree killers of recent history. For me, the classroom was at Virginia Tech, where I listened to plant pathologist Anton Baudoin and “Tree Doc” Jay Stipes. Together with students from the horticulture and forestry programs – some of whom are now also arborists and landscape professionals – we heard about chestnut blight, Dutch elm disease, hemlock woolly adelgid and emerald ash borer. Undoubtedly, these are culture- and ecosystem-altering epidemics, leaving untold millions of native trees dead.

While fascinating, there was no denying that it was all rather bleak. But we endured the doom-and-gloom lectures to learn from the past in hopes of answering the question, “What can we do about it?” Until very recently, when asked of BLD, this question was met with a disappointing string of, “There are no known treatments.” Finally, with a series of discoveries and extensive research trials, this has changed! We are entering the next phase of BLD management.

BLD management
BLD poses unique management challenges and is, in many ways, unlike any other pest or disease we currently deal with in forestry, agriculture or the turf and ornamental industry. Even strategies for managing pine wood nematode (Bursaphelenchus xylophilus) and other foliar nematode pests (Aphelenchoides spp.) common in herbaceous ornamentals were of little use for BLD. With no historical precedent to easily apply to the new problem, a team of scientists and technicians at Bartlett Tree Research Laboratories, led by Dr. Andrew Loyd, began the long process of lab work and field trials to investigate possible treatments.

The first exciting development was one standout treatment among many failures in our 2021-2022 season field trials. This was the first treatment to directly target the nematode with a foliar application program, preventing the nematodes from infesting beech buds, where the bulk of damage occurs. We published this in the first-ever collaborative BLD management paper earlier this year. Several subsequent field trials have since been completed to help fine-tune the foliar program, and a second publication is forthcoming. This program is intended for small- to medium-sized beech in landscapes and nurseries, and is not for forestry use.

Injection treatment
Our second breakthrough came during the 2022-2023 BLD season, where one of several root-flare injection-treatment options we have tested showed excellent results. After deploying two additional field trials, we introduced the program commercially this past summer. The injection treatment, using macro-injection systems, is capable of suppressing BLD in large, mature beech trees for up to several seasons. This research will be officially published this winter in ISA’s Arboriculture & Urban Forestry, but news has already been released. Rainbow Ecoscience, the distributor of Arbotect 20-S, has been particularly helpful in donating product, time and expertise to help us get the extensive field trials implemented and offer training for professional applicators.

While these discoveries are by no means a solution to BLD at the forest ecosystem level, it is immensely encouraging to know that some beech can be protected in places such as landscapes and arboreta, where they are valued as shade trees and historic specimens and represent genetically diverse collections. We hope these can be preserved for future work and future generations to enjoy. As with most invasive pests and diseases, the scene will change over time and new tools will become available, such as resistant or tolerant varieties and more treatment-rotation products. We also anticipate that, similar to management of hemlock woolly adelgid, these tools developed for individual beech will eventually find use in the preservation of select, mature beech in natural areas.

Bartlett-exclusive treatment?
We recognized BLD as a severe issue with immediate, dire need for management tools. Where possible and appropriate, we want beech to be saved and preserved, so it is important that treatment options use products that are widely available commercially. Therefore, we are developing treatment protocols and BMPs (best management practices) using products accessible to arborists and certified commercial pesticide applicators in our entire industry. We investigate many possible tools using field trials, and then adapt those tools to the new problem of BLD and new host beech trees.

Additionally, we strive to publish all research in open-access journals, so the tree care community can freely access the information. We also have a series of publicly available BLD research posters that provide a summary of the ongoing work online at Research Gate.

Injection best management practices.

Foliar control
For the management tools Bartlett has developed, what are the active ingredients and the modes of action used?

Our foliar program utilizes a next-generation nematicide known as fluopyram, one of several chemistries developed to help replace old, broad-spectrum nematicides that were highly toxic. Although various products containing this active ingredient are labeled for nematode management in turf and agriculture, the only product containing fluopyram that is available for use on ornamental plants in residential and commercial landscapes, as well as in nurseries, is Broadform.

In order to use Broadform (a formulation labeled as a fungicide) for managing the nematode causing BLD, we assembled our early field trial and bioassay data to request a new use against this invasive disease. The result was a 2(ee) recommendation (pesticide registration addendum) supported by the manufacturer, permitting Broadform “For Control of Beech Leaf Disease on Beech Trees.” Fluopyram works both as a fungicide and nematicide by a SDHI (succinate dehydrogenase inhibition) mode of action. In basic terms, it acts to rapidly deplete the nematode of energy, and tests have shown that it targets the BLD nematode exceptionally well.

Macro-injection tubing.

Injection control
Our root-flare injection program utilizes an older chemistry, but one with a fascinating history. Many arborists know the fungicide product Arbotect 20-S as a go-to for treatment of Dutch elm disease and sycamore anthracnose. Few, however, know that the active ingredient, thiabendazole (TBZ), was once widely used for its anthelmintic (anti-parasitic) properties in veterinary medicine, particularly sheep, horses and cattle. Thiabendazole is said to bind tightly with the target pests’ beta-tubulin, disrupting a key component of their cell-division process. However, we have performed a series of bioassays that strongly points to TBZ also having a rapid mode of action, likely by inhibiting a step in their metabolic processes. Either way, the nematodes are quickly incapacitated even at low doses.

Thiabendazole (TBZ) treatment.

Like fluopyram, thiabendazole is a good example of a chemistry that has valuable uses against certain fungal diseases and also certain nematode parasites. Following our early field-trial results, we assembled the compelling data and requested a new use of the product for BLD and beech trees, again with support from the manufacturer and states. So far, we have obtained special-use labels in Ohio, Pennsylvania, New Jersey, New York, Connecticut, Massachusetts, Maine, Rhode Island, Vermont and Virginia.

BLD and climbing/rigging
How about untreated trees affected by BLD – do they immediately get crispy and fall apart? This is a valid concern stemming in part from conscientious climbers who are aware of the hazards of working in heavily infested ash trees, which quickly become dry and brittle. However, this is not a concern for beech trees affected by BLD. Remember, BLD is a progressive disease that reduces the tree’s ability to support itself through functional foliage and gradually depletes resources. Assess the tree as you would in the absence of BLD, paying attention to structural defects and secondary fungi that cause dieback and decay.

Other than defoliation, is there a degradation in the trunk tissues? The nematodes causing BLD have not been found within branches, trunks or roots. As such, they do not directly degrade woody tissues or cut off the vascular system, as do the pine wood nematodes that cause pine wilt disease, a rapid and lethal wilting of some non-native pines. As BLD worsens, there is bud abortion and twig dieback, which can lead to other secondary fungi and borers moving in, but this is not exclusive to BLD. One other concern we see on many mature beech in urban areas is the prevalence of brittle cinder fungus (Kretzschmaria deusta). Because of the difficulty in assessing the extent of basal decay for this organism, extra caution should be taken and tree removal is typically prescribed.

Prognosis
How much of an infestation is needed before the tree succumbs? With EAB, we have some thresholds to help make decisions. Do we know this for beech trees?

Over time, research and trial-and-error, thresholds become vital to help make pest-management decisions. For emerald ash borer, research has demonstrated that once roughly 50% of the canopy has died, it is too late to have reasonable treatment success and save the tree.

An equivalent threshold does not yet exist for BLD, but this is in part because BLD is a very different disease and severity varies by location and the individual tree. Indeed, we sometimes see some beech trees suffer near complete defoliation one summer due to BLD, but then put out a flush of leaves in relatively good condition the next spring. Presumably, this is due to the tree shedding the majority of its nematode leaf-dwelling parasites before they can move into the next season’s buds. Unfortunately, total defoliation is not a long-term, sustainable defense strategy that can be repeated without significant stress.

Instead, we recommend focusing on the health of the individual tree and paying close attention to the canopy. Both of our treatment programs require foliage to be present in order to perform the application. After all, you cannot perform a foliar application – nor expect good uptake from a root-flare injection – on a bare tree. Therefore, severe defoliation or the presence of other advanced health concerns should draw pause. Instead of rushing to treat or giving up hope entirely, wait until the next flush of leaves to reevaluate the tree’s condition and determine if treatment is possible and worthwhile.

Give yourself time to learn about BLD and observe how the disease will impact beech in your area.

Matthew Borden is a doctor of plant medicine (DPM) with The F.A. Bartlett Tree Expert Company, an accredited, 49-year TCIA member company headquartered in Stamford, Connecticut. Bartlett operates the largest shade-tree lab in the world, The Bartlett Tree Research Laboratories, in Charlotte, North Carolina. With continual research from its team of scientists and technicians in fields such as diagnostics, soil science, plant pathology, entomology and horticulture, the company provides species-specific tree and shrub-care recommendations for its customers to ensure optimum growing conditions and health for beautiful landscapes. Bartlett’s certified arborists provide more than 188 services to clients from 187 operations in 41 U.S. states, Canada, the United Kingdom and Ireland.