A new disease of beech trees (Fagus spp.) called ‘Beech leaf disease’ (BLD) has increasingly been observed in landscaped and forested areas in the Northeastern USA and Canada. The disease was first reported on American beech, Fagus grandifolia, in Lake County Ohio in 2012, and has spread to Pennsylvania, New York, Ontario (Canada), Connecticut, Maine, Massachusetts, Rhode Island, New Jersey, West Virginia and Virginia. In New Jersey, the disease was first reported in 2020 and has been confirmed in 10 counties including: Bergen, Essex, Hunterdon, Mercer, Monmouth, Morris, Passaic, Somerset, Sussex, and Union.

New Jersey Beech Leaf Disease Detection Map

BLD primarily affects American beech,  F. grandifolia, however, it has also been observed on European beech, F. sylvatica; Oriental beech, F. orientalis; and Chinese beech, F. engleriana. All of the specimens seen in the Rutgers Plant Diagnostic Lab have been American beech.

A foliar nematode species has been isolated from beech leaves and buds. This nematode was initially identified as Litylenchus crenatae subspecies crenatae, a species described in Japan on Fagus crenata that does not cause disease. In 2020, the cause of BLD was proven to be a newly described foliar nematode, Litylenchus crenatae subsp. mccannii, which is closely related to L. crenatae subsp. crenatae. L. crenatae subsp. crenatae has not been found on ornamental plantings of American beech in Japan. Conversely, L. crenatae subsp. mccannii has not been found on ornamental plantings of Japanese beech in North America. While the origin of L. crenatae subsp. mccannii and its native distribution are not clear, all other known species of nematode in the genus Litylenchus are found in the Pacific Rim.

Litylenchus crenatae subsp. mccannii isolated from beech leaf tissues in the Rutgers Plant Diagnostic Laboratory. Photo: Sabrina Tirpak, Rutgers PDL

In 2019, research scientists at the USDA took nematodes from diseased trees and inoculated them onto the buds of young, healthy trees in a greenhouse, waited for symptoms to appear, then re-isolated the nematode from the affected leaves. This process is called Koch’s postulate’s, which is plant pathology’s gold standard for verifying the cause of a disease.

Despite these findings, some researchers question the role of the nematode as the only cause of BLD. Recent reports suggest that diseased beech leaves also contained a fungus and 4 species of bacteria that are also carried by the nematode. This research suggests that both the nematode and a pathogen that it carries may be contributing to the disease. The original research from the USDA, however, suggests that nematode feeding alone can cause the disease.

The BLD nematode predominantly overwinters in buds. The nematode migrates from leaves to the buds beginning in August. After bud-break in the spring, the nematodes cannot be found in symptomatic leaf tissue until late-June or early-July. DNA-based markers, however, can confirm the presence of the nematode, presumably due to eggs, in these leaves. By early summer, the nematodes appear in the leaves and are easily identified in a laboratory setting. The numbers of nematodes in the infected leaves increase through fall. It is assumed that some nematodes overwinter in leaves on the ground. The mechanisms of transmission and spread within trees, among trees, and from site to site, are unknown. Research is ongoing in the hopes of answering these questions.

The initial symptoms of BLD include dark-green striped bands between the veins of leaves. The dark-green bands are easily seen by holding leaves up to the light and/or looking up into the canopy. In many cases, the size of the affected leaves is reduced. The banded areas usually become leathery-like, and leaf distortions–curling and crinkling–are often observed. These symptoms are best seen by looking down on the top of the leaf.

As seen from below, dark-green striping is clearly evident between the leaf veins. Photo: Jerry Giordano, Cornell Cooperative Extension

As seen from above, affected leaves are smaller, leathery, and distorted. Photo: Jerry Giordano, Cornell Cooperative Extension

Affected leaves thicken and may yellow between the veins. Photo: Sabrina Tirpak, Rutgers PDL

As symptoms progress, aborted buds, reduced leaf production, and premature leaf drop lead to an overall reduction in canopy cover. This ultimately results in the death of saplings within 2-5 years of infection and of large trees within 6 years. In areas where the disease is established, the proportion of symptomatic trees can reach more than 90%. There is significant concern that this disease will dramatically reduce the numbers of beech and their ecological services in Northeastern forests.

Because the research on BLD etiology is ongoing, and questions regarding dissemination and spread remain unanswered, management and control options are limited. Several pesticide options are currently being investigated including: abamectin, emamectin benzoate, and potassium-phosphite as potential controls. The avermectin materials are known to be effective nematicides in other animal and plant systems and may eventually be determined to be efficacious for this disease. At this time, however, the efficacy of any of these materials is unknown and are not currently recommended as treatments. Be aware that much more work needs to be done to understand this new disease before effective treatment protocols can be discussed.

In general, the spread of invasive species can be prevented by restricting the movement of plant materials and monitoring trees closely for signs and symptoms. BLD has spread very quickly eastward in the United States and is concerning to all of us. The Rutgers Plant Diagnostic Laboratory and our partners at the New Jersey Division of Community Forestry are interested in tracking this disease within New Jersey. Please contact the lab if you suspect a problem. Samples can be evaluated free of charge for the 2022 growing season.