Understanding the Impact of Beech Leaf Disease

That is me at the Brooklyn Botanic Garden admiring a century old absolutely massive European beech tree.

The beech tree falls under the genus (Fagus) which is in the family (Fagaceae). The family is comprised of eight genera and around 1,000 or more species. Fagaceae is one of the most ecologically important woody plant families in the northern hemisphere. This is a family of flowering trees that includes chestnut, oak, and beech. All of which provide significant benefits to sustaining the native landscape.

Beech trees have been around for approximately 81-82 million years, with the oldest fossils dating back to the late Cretaceous period. The Cretaceous period, known for dinosaurs, was also known for the rise of flowering plants. Early settlers valued the American beech (F. grandifolia) because it was an indication of fertile land. Beech trees were used for wood, fuel, leaves for mattresses, and ashes to produce lye soap. American beech trees are sensitive to pH, salt pollution, urban pollution, and more importantly climate change. The largest beech tree found in the United States is in Virginia, and in 2019 was measured at a height of 90 feet, a circumference of 224 inches, and branching that spans 110 feet.  

In the late 1800s and early 1900s two ecosystem-shifting catastrophes were introduced to North America, chestnut blight and beech bark disease. By the mid-20th century the American chestnut was functionally extinct, with the blight having killed an estimated four billion trees by 1950. Beech bark disease has killed millions of beech trees, and was just the start of the beech nightmare. Fast forward 120 years and in 2012 a new deadly disease emerged named Beech leaf disease or BLD. This disease  primarily affects the American beech, European beech and several other beech species, and it is spreading at a shocking rate. So far, BLD has killed an estimated 30% of beech trees in some areas, particularly younger trees and saplings.  

Winter branch structure from a European beech - Fagus sylvatica 'pendula' at the Northeastern University Arboretum, Boston, MA.

Why are beech trees so important ? 

American beech (F. grandifolia) is one of the top three most dominant tree species occupying the northern hardwood forest. Massachusetts is covered by 60.6% or 3.1 million acres of forest. It is the 11^th most forested state in the country, with Maine being number one at 89.06% and New Hampshire number two at 83.04%. The American beech tree makes up approximately 10% of the Massachusetts forest, which is around 141 million American beech trees. The American beech is an important late-successional hardwood species. Late-

successional hardwood species are long lived, slow-growing trees that thrive in shaded environments. Other very important late-successional trees are sugar maple, yellow birch, hemlock, red oak, and white ash. These species are important because of their ability to regenerate under established forest canopies. Like oak trees, beeches are a masting tree, and once mature they produce a beech nut. Beech nuts are a vital food source for various wildlife, including turkey, black bear, foxes, deer, squirrels, and rodents. Beech trees also provide nesting, resting, and foraging sites for birds like woodpeckers, warblers, and chickadees. Beech trees have a smooth elephant-grey like bark, beautiful fall foliage, and massive canopies. The American beech tree is used mainly as a shade tree in the landscape, with gorgeous golden fall foliage. The European beech is mainly used as a specimen tree in North America due to beautiful cultivars. A few of my favorites are weeping, fern-leaved, copper, and of course, ‘Tri-color.’  

 Understanding Beech leaf disease, and where it came from ? 

BLD is caused by a foliar nematode known as (Litylenchus crenatae ssp. Mccannii) or LCM. This nematode is closely related to a species associated with beech trees found in Japan, leading to the suspicion that it originated there. The disease is thought to have been introduced through the movement of infected nursery stock or by other means such as birds, mites, or insects. BLD was first discovered in Lake County, Ohio, in 2012. Currently, BLD has been found in 15 states and is running rampant across Connecticut, Rhode Island, and Massachusetts. In addition, BLD continues to spread northward through New Hampshire, Vermont, Maine and Ontario.   

The banding is a result of nematode feeding (Bartlett Arboretum & Gardens, Stamford, CT)

What is a foliar nematode ? 

Foliar nematodes are microscopic, worm-like parasites that primarily infect the leaves of plants. They are colorless and typically measure around 0.8 mm long. To really understand the size of this nematode, if you were to punch a hole through an infected beech leaf with a hole puncher, that single piece would have thousands of nematodes in it. Nematodes require moisture to move around and they enter the leaves through the stomata. Stomata are very tiny pores found on the plant surface mostly on leaves that regulate gas exchange, allowing plants to take in carbon dioxide for photosynthesis and release oxygen. Stomata also regulates water vapor released from transpiration.  

How does BLD spread ? 

The nematodes enter beech buds during late summer into autumn, where they feed and overwinter. It is within winter that the most damage occurs within the bud. LCM feed on the developing leaf tissue within the bud, causing the banding and distortion seen on affected leaves. The banding is due to the nematodes feeding and the beech trees reacting to it. There are many vectors, but so far, birds and insects are providing the best means of dispersal. Also moving infected plant material such as firewood or nursery stock can accelerate the spread of the disease. Additional stresses such as drought, winter injury, site-related stresses, and other pests and pathogens help to accelerate the decline of infested trees.  

Is there treatment or a cure ? 

Currently there is no cure for BLD but there are treatments. Some treatment methods include systemic injections, foliar sprays, and applications of phosphites. For systemic nematicide injections, recent studies show one application from thiabendazole (like Arbotect 20-S) prior to the end of July or early August can protect entire canopies for two years. Beech bark is thin and susceptible to cracking, so the safest way to inject beech trees is from the side of the root flare. Beech trees can be injected every two years with this treatment. For foliar applications there have been trials and reports showing that the systemic nematicide fluopyram provides some control. Early season applications of fluopyram in June and July may be most effective once beech leaves are fully formed. But there are other reports indicating that later season applications around August and September have shown improvements. One important note is there is still testing being done to see if fluopyram will kill LCM (Litylenchus crenatae ssp. Mccannii) eggs. Soil or lower trunk applications of potassium phosphites may provide some level of BLD control, but multiple years of treatment is needed. Phosphites when applied can trigger defense mechanisms against the disease. There is new research coming out all the time so staying up to date is the best course of action.  

My conclusion

I think we can all come to the conclusion that BLD is a serious threat, and is here to stay indefinitely. Scientists are actively researching various aspects of BLD, such as the nematode's biology, spread, treatments, and possible resistance in some beech trees. While treating individual trees in a forest setting is impractical and costly, treating specific trees in the landscape is looking promising. Some things you can do to help mitigate the spread of BLD are staying vigilant in monitoring for infection and proactively supporting the health of your beech trees.