Beech leaf disease poses a new and formidable challenge to an iconic American tree

Beautiful in landscapes and a vital component of New England forests and their ecology, the American beech is an iconic species long admired for its smooth gray bark, dense crown, and edible beechnuts. In recent years, American beeches and their counterpart species from Europe and Asia have been challenged at the Arboretum and across the continent by several damaging pests and diseases that target the genus. Now another aggressive and damaging malady, beech leaf disease, has arrived in our environment and places new pressure on beeches of every type and at every stage of their growth.

Much of the beech collection (Fagus) at the Arnold Arboretum grows on the south slope of Bussey Hill, across Valley Road from Rhododendron Dell and the north slope of Hemlock Hill. Visitors and researchers can explore the core of our nationally accredited collection of beech trees, nearly 140 accessioned plants representing one of the world’s most diverse groupings of beech species gathered from North America, Europe, and Asia. Frequent visitors to this part of the Arboretum have witnessed dramatic changes over the past decade, as disease- and drought-stressed trees in severe decline have been removed and mass plantings of native perennials added to the understory to enrich the environment for the beeches that remain. Radical changes to the collection and its landscape reflect the severity of the threats facing the beeches, and the commitment of the Arboretum to take all necessary steps to protect and preserve its plants against loss.

Herbaceous plantings in the beech collection at the Arnold Arboretum
Removal of dead and seriously compromised beeches in the collection at the Arnold Arboretum over recent years due to insect and disease issues left wide gaps in the landscape where the collection grows on Bussey Hill. Since a healthier environment boosts the outlook for infected trees, the landscape was enriched by the addition of native herbaceous perennial plants to the understory.

“Our beech collection has seen its share of hard times and robust interventions,” said the Arboretum’s Head Arborist, John DelRosso. “We’ve wrestled multiple threats, from ambrosia beetles to beech bark disease and beech canker. With beech leaf disease, these trees have certainly reached their tipping point.” Beginning in the early 2000s, a symbiotic combination of insect (beech scale, Cryptococcus fagisuga) and fungal (several species of Neonectria) pests began to send many Arboretum beeches into decline, a situation made worse by successive summer droughts. This season, the emergence of beech leaf disease—first reported in the United States in Ohio in 2012—prompts new concern for the beleaguered beech. Beech leaf disease is easy to recognize, evidenced by dark green, interveinal banding patterns on the lower canopy foliage, which eventually spreads throughout the tree. Symptoms progress through the buds and hinder leaf production, which over time can prove fatal to infected trees. Experts predict that the disease, left unchecked, can kill younger trees in two to four years, and take perhaps four to ten years for mature trees. “We first detected beech leaf disease at the Arboretum last year,” said Rachel Brinkman, Manager of Horticulture, “so this is our first season for planning strategies to confront it.”

Unfortunately, we don’t really know how beech leaf disease spreads, how new trees become infected, or even how long it takes before symptoms appear. Scientists suspect that a nematode worm, Litylenchus crenatae mccannii, plays a role in spreading the blight, as infected leaves have been found to contain a fungus and several bacterial species carried by the nematode.

Signs of beech leaf disease in the leaves include striping, curling, and/or a leathery texture. Symptoms may be visible from leaf out in spring until leaf drop in fall and may be detected by looking up and seeing gaps in the canopy.

American beech (F. grandifolia), European beech (F. sylvatica), and their counterpart from Eurasia (F. orientalis) all seem to be potential hosts for beech leaf disease, which has been observed in both urban and forested settings. To learn more and take action, the Arboretum connects and shares information with university-based cooperative extension services, organizations like International Plant Sentinel Network, and agencies like the US Forest Service to stay informed about potential and emerging plant pests and pathogens and follow best practices for treatment. Regular bulletins and pest alerts from these partners give our arborists and horticulturists a heads up to what they might find and possible steps to take.

“A key component of our integrated pest management program is just watching afflicted trees, monitoring their health, measuring the level of injury that’s occurring, and taking appropriate action,” Brinkman explains. A Plant Health Care Committee meets regularly to discuss some of the potential threats and to set thresholds to manage response. Together they assess what the institution’s tolerance should be regarding individual pests, and any action to take, from simply continuing to monitor their presence and any damage they incur to determining specific treatments or control measures. Trees deemed critical for their value to the collection or research become candidates for the most promising or proven treatments, or potential test subjects for trials recommended by our staff or their research partners. Clonal repropagation of these plants in our greenhouses provides an additional layer of for the future of infected plants in the collection.

Moth trap in the beech collection
The Arboretum partners with researchers at universities and state and federal agencies to track pests and diseases in the collections and to release natural controls. This moth trap in the beech collection is part of a collaboration with the USDA.

Massachusetts gardeners can report problems with their beech trees to the Department of Conservation and Recreation through an online form to contribute to our understanding of the scope and distribution of beech leaf disease in the Commonwealth. If leaves on your beech show evidence of interveinal banding, discoloration, or drying and crinkling, John advises reaching out to a reputable tree care company to investigate the problem and determine if the tree can be saved. The earlier the problem is addressed, the more successful any intervention is likely to be. However, John warns, saving an infected tree can be costly and may ultimately prove unsuccessful, as we still have much to learn about this disease and the most reliable treatments to combat it.

While the outlook remains uncertain for the beech collection, hope at the Arboretum is buoyed by successes in stabilizing the hemlock collection against hemlock woolly adelgid and recent neutralization of another serious threat—winter moth. Through constant monitoring, application of natural predators, and allowing time for other natural predators (particularly birds) to enter the mix, winter moth has been neutralized at the Arboretum and further intervention has been unnecessary. “We always try to stay on the forefront of what’s out there,” Brinkman offers. “It’s still early in the game with beech leaf disease, but we have a remarkable team and a long history of facing serious challenges to the health of our plants.”