Forest Health
Beech Bark Disease
Beech Bark Disease (BBD) is a serious threat to American beech trees (Fagus grandifolia) in Canada鈥檚 hardwood forests. It is caused by the non-native beech scale insect (Cryptococcus fagisuga) and a fungal pathogen (Neonectria species).
How It Spreads
The disease begins with healthy trees being attacked by the scale insect. Infestation is easily detected as the nymphs or crawlers secrete a white, waxy covering to protect their body, giving the tree a woolly or snowy appearance. They pierce the living bark, creating puncture wounds as they feed on the tree鈥檚 sap. The feeding holes kill the bark cells causing small cracks to form on its surface.听听
While the insect does not move far, some are transported by wind, mammals, birds, and the movement of firewood, causing long range dispersal. Usually, the larger trees in a stand are attacked first. Although the scales generally do not kill the tree, infestations reduce tree vigor and growth, and lower the tree鈥檚 resistance to fungal infections.听听
Three to six years after the introduction of beech scale, the causal fungus (Neonectria faginata) enters the tree through the wounds opened by the insects and begins to colonize and destroy the cambial layer. This stage of the disease produces circular or oval shaped cankers, deforming the bark surface.听听听
In fall, small orange-red fruiting bodies appear in and around the canker, releasing minute fungal spores that are dispersed to new sites by wind and splashing rain. Branches above large patches of dead bark often show signs of decline, producing yellowing leaves and reduced canopy. As the cankers become progressively worse, the tree is girdled, resulting in 90% of the trees succumbing to the disease within five to 10 years.听听
How Does the Disease Progress
The stages of progression of BBD have three distinct phases, representing different ecosystem states: the advancing front, the killing front, and the aftermath forest.听
The advancing front听begins when previously unaffected beech trees are colonized by the non-native beech scale and their populations start to increase. During this phase, ecosystem changes are minimal, but damage caused by the feeding scales make the stand susceptible to fungal infestation.听
A few years after the insects arrive, the disease progresses into听the killing front.听At this point,听stands of beech trees exhibit high populations of beech scale and the Neonectria fungus infects colonized trees, causing severe cankering. Within five to 10 years of infection, many of the larger beech trees die. 听 听听
After this first wave of beech mortality,听the aftermath forest听shows lower but consistent levels of beech scale and Neonectria fungus. In this phase, the remnant large trees continue to gradually decline, and stem breakage is common. Beech now makes up a lower percentage of the overstory and thickets of shade-tolerant saplings sucker from the roots around dead and dying trees. These younger beech trees subsequently become infected with BBD as they age and eventually die.听听
Ecological, Economic, and Social Impacts
American beech plays an important role in the forests of eastern Canada. BBD causes severe die-back in mature Beech trees, negatively impacting wildlife, biodiversity, and sustainable forests in Ontario. 听听
- The disease typically kills larger trees first, significantly reducing canopy cover and altering the stand structure and forest dynamics.听听
- Beech trees, like oaks, produce nuts essential for forest wildlife, including black bears, deer, squirrels, and birds. Their loss reduces food availability and habitat for cavity-nesting species.
- Beech wood, valued for its fine grain in flooring and furniture, becomes less marketable when infected by BBD, harming the forest industry.
- The loss of these beautiful trees decreases the aesthetic value of our hardwood forests.听
Trees That Are At Risk
Trees are more at risk of BBD when there is a nutrient imbalance in the forest soils. Drought and other pests can predispose the trees to the disease and weather conditions, such as winter temperatures that remain above -25 掳C and continual autumn rainfall can favour the build-up of beech scale populations and the Neonectria fungus. 听
On a positive note, heavy rains can wash crawlers off the trees and delay infestation, and some trees (about 1%) may exhibit resistance to the scale insect or tolerance to the fungus. 听Trees less than 30 years old also seem to have a natural immunity to attacks.
Contact
For more information, please reach out to Rob Davies, Forestry and Restoration Supervisor, 519-451-2800, extension 243.