Bark on beech tree branches with numerous reddish-orange short tufts of fungal growth.

Signs of Nectria canker on a beech tree. This fungal growth, in conjunction with beech scale, is responsible for beech bark disease. Photo: Ansel Oommen, Bugwood.org

Updated: August 13, 2026

What causes beech bark disease?

Beech tree trunk with numerous areas of reddish fungal growth.
Neonectria coccinea fungal growth on American beech bark.
Photo: Richard Hamelin, University of British Columbia, Bugwood.org

Beech bark disease (BBD) is a condition that affects beech trees (genus Fagus) which is caused by two organisms: a scale insect and a fungal pathogen. The insect is a very small non-native armored scale species called beech scale (Cryptococcus fagisuga). The canker-causing fungus belongs to the genus Neonectria, including Neonectria faginata and Neonectria ditissima.

The insect's feeding creates tiny wounds (and after damage accumulates, cracks) in the bark, which in turn give the fungus the opportunity to start an infection. By themselves, the scale insects usually do not kill the tree, although the feeding of high scale populations may weaken them. The canker fungus can ultimately stunt and kill beech trees. In some forest areas, the mortality of beech has approached 50%, and the loss of timber value and food for wildlife is significant. The disease also poses a threat to ornamental beech trees (those planted in managed landscapes).

What are the signs and symptoms of beech bark disease?

The first detectable sign there is a risk of BBD developing is the presence of the scale insect's white body covering, which stands out against the gray bark. The scale suck sap from tissues under the bark, so will be found on the trunk and branches in scattered groups that become more extensive as the population builds.

Beech scale

Due to their miniscule size, wind is the main factor that transports the yellow-bodied scale crawlers (newly-hatched juveniles) from one beech tree to another. After landing on a suitable tree, the crawlers settle down and insert their stylets (elongated sucking mouthparts, like a flexible needle or straw) through the bark to feed. The juveniles will molt twice as they grow, becoming adult females in the spring. In midsummer, the all-female population reproduces asexually by laying eggs and then dying. The eggs hatch late summer to early winter to start the next generation.

Scattered clusters of white insects on the light gray bark of a beech tree trunk. Some areas of bark are slightly dark-stained.
High beech scale populations are noticeable by their white body coverings. A few tarry spots on the bark are associated with abundant scale. Photo: Joseph OBrien, USDA Forest Service, Bugwood.org
Groups of white scale insects on the gray bark of a beech tree. Near the scale clusters are several dark, wet-looking spots on the bark.
Tarry spots on the bark, indicating wounds or infection, become more prominent as the disease progresses. Photo: Joseph OBrien, USDA Forest Service, Bugwood.org

Canker fungus

After beech trees are infested by the scale and the insect population has had time to build, Neonectria fungal spores are deposited by the wind onto bark wounds created by the scale. Once the fungi invade the tree, they kill the inner bark and can rot the outer wood, girdling the tree and killing it.

An irregularly-shaped patch of bark removed from a beech tree showing blotches of dark brown-stained wood within lighter yellowish tissue.
Cankers on an American beech tree afflicted with beech bark disease. A thin layer of bark has been removed to reveal the dark-stained color of infected sapwood. Numerous white scale insects are visible on the bark. Photo: Joseph OBrien, USDA Forest Service, Bugwood.org

Where has beech bark disease spread?

The distribution of BBD is from Pennsylvania north to Nova Scotia, with isolated infestations in Michigan, Tennessee, North Carolina, Virginia, West Virginia, and Maryland. BBD can potentially affect beech trees wherever they grow, which is over a wide range for the native species (Fagus grandifolia).

BBD occurred in Europe before 1849. It was thought that the scale insect was the cause of tree death until 1914, when the fungus was found to be involved. Sometime in the late 1900s, the scale was introduced to Nova Scotia. By the early 1930s, mature Beech trees were dying from the combination of the scale and fungus in Canada and Maine.

Can beech bark disease be treated?

There are few control options for BBD. Ornamental beeches can be sprayed with insecticides (consult a certified arborist) to stop the scale, but there is no practical control method for a forest setting. Salvage cutting is the only way to reduce timber losses. Some individual beech trees appear to have resistance to the scale, which may help forests and beech plantings resist BBD in the future.

Additional resources

Beech Bark Disease | Ohio State University

Beech Bark Disease: The Oldest "New" Threat to American Beech in the United States | USDA Forest Service