Beech Leaf Disease Now Present in All 67 PA Counties

Adapted from an article by Calvin Norman, Assistant Teaching Professor of Forestry, Penn State Department of Ecosystem Science and Management

Calls from Pennsylvania forest landowners concerned about withering leaves and branches in their beech tree stands have ramped up this year. Four years ago, we ran an article asking our readers to report findings of beech leaf disease; in 2019, beech leaf disease was confirmed in 12 counties in northwestern PA. Today, it is found in all 67 counties of the commonwealth.

 Beech leaf disease (BLD) is a substantial threat to American beech (Fagus grandifolia) and all other species of beech (Fagus spp.) across the eastern United States. American beech plays an important role in forest ecosystems; as a late successional species it adds structural diversity to a variety of forest types, and its nut production makes it an excellent species for wildlife.

Beech leaf disease was first identified in Lake County, OH in 2012, and has since been found across the eastern United States. BLD is easily recognized by its distinctive dark green interveinal banding pattern on symptomatic leaves. It is caused by Litylenchus crenatae ssp. mccannii (LCM), a nematode that parasitizes tree leaves. Nematodes are tiny worms that feed on plant cells, bacteria, fungi, and other microscopic creatures. Their damage is seen in most species of beech. BLD can be more difficult to identify in its early stages in beech species with dark leaves, like "copper" beech (Fagus sylvatica ‘Purpurea’).

Symptoms

The dark bands in leaves become evident in early spring, as leaves emerge fully symptomatic. Under high levels of infection, leaf symptoms become more pronounced; leaves become leathery and crinkled. These bands are due to cellular damage caused by nematodes in the leaf buds. In forests heavily impacted by BLD, it may look like there is a drought that is impacting only beech.

Disease Progression

In spring, from May to June, there are few nematodes in diseased leaves. The number of nematodes increases over the summer (July-August) and reaches a peak in late summer through fall (September-November). As the numbers build up within the leaves, nematodes will migrate from the infected leaves into the newly forming buds. Once nematodes reach the new bud, they will feed on the bud tissues, which negatively affects the overall architecture of the developing young leaves by increasing their number of cell layers. This damages the next generation of leaves, and repeated damage will kill infected beech. BLD spreads quickly, causing leaves to wither and, often, the tree to eventually die. Currently, the method of transmission between infected trees is unknown.

Treatment Options for Yard/Street Trees

BLD can be treated in yard trees by injections of thiabendazole (like Arbotect 20-S). According to the label, inject 2 to 8 fl. oz. of thiabendazole for every 5 inches of trunk diameter and dilute with 16 to 32 parts water for one part chemical. For large trees (stems with a DBH of 8 inches or more), stems may be injected with up to 12 fl. oz. thiabendazole per 5 inches of trunk diameter. Always read and understand the chemical label prior to applying. Other products that show promising results against BLD are potassium phosphite, potassium polyphosphate, and other potassium fertilizers. The recommended application rate is 2 fl. oz. of phosphite products mixed with 14 oz. of water for every inch of diameter and applied at the base of the infected tree. Trees that have been damaged for multiple years might not be saved, as the damage may be too much for the trees to overcome. Long-term research is still underway.

Management in Forests

Currently there is no feasible large-scale treatment for BLD in forests. Individual trees can be treated using the treatments above, but this can be expensive.

The Future of Beech Trees in Pennsylvania

The USDA Forest Service estimates there are about 700 million beech trees in Pennsylvania. Beech trees make up about 9% of Pennsylvania's forests, according to the PA DCNR Bureau of Forestry. Before BLD, their biggest threat was beech bark disease, a disease caused when certain scale insects feed on American beech bark, leaving wounds that are then infected by a fungus. Beech bark disease has killed millions of American beech throughout the northeast and continues to impact beech today. In many areas of the state, these two diseases are co-occurring, with devastating consequences. With beech leaf disease and beech bark disease on the landscape, American beech is imperiled.