Beech bark disease (BBD) is caused by the combined actions of an insect, the beech scale (Cryptococcus fagisuga), and a fungus (Neonectria faginata). The insect feeds on the tree creating holes in the tree bark which become an entry point for the fungus and, by stressing the tree, decreasing its resistance to the subsequent fungal infection. The beech scale, probably along with the fungus, arrived in Nova Scotia around 1890, where it arrived from Europe on infested beech seedlings. The disease was first noticed in Halifax in 1920 and by the early 1930's was found through the Maritime Provinces. It has since moved southwestward and was first detected in 1965 in Quebec and in 1999 in Ontario.

Biology
Beech scale is a tiny insect (up to 1 mm long), that feeds only on beech tree sap. There are only female scale insects, which reproduce parthenogenetically (i.e. the female reproduces without mating). The adult scale lays her eggs on the trunk of the tree in mid-summer, and nymphs (also called crawlers) hatch from the eggs later that summer or fall. Each nymph crawls to find a location to feed, and it inserts its stylet mouthparts into the bark to suck sap from the inner bark of the tree. Once the scale begins feeding, it becomes immobile, and eventually secrets a distinctive woolly white covering (see photo in sidebar). The nymph becomes an adult the following spring. Scale insects are spread by wind, by animals or with infested wood.

The scale itself does not cause BBD, but it weakens the tree and reduces its ability to resist the fungus. Within two or more years (up to 10) of infestation by the scale insect, fungal spores are moved to the tree bark by rain or wind, where they enter the tree through the feeding punctures made by the scale insect. The fungus grows in the tissue under the tree's bark (the phloem and cambium) killing this tissue. The disease stresses the tree, reducing its growth, making it more susceptible to other pests or pathogens, and can cause the tree to become deformed with multiple cankers. Over time the disease can destroy the inner bark around the circumference of the tree, girdling the tree and killing it; this may take many years or even decades. Beech trees killed by BBD are more susceptible to decay fungi and insects, become fragile, and can break in high wind ("beech snap").

Trees at risk
As the name suggests, BBD affects beech trees, both American beech (Fagus grandifolia) and European beech (Fagus sylvatica) are vulnerable here in Canada. The greatest impact of the disease is on larger trees - those 25 cm or more diameter at breast height (dbh) - but can kill trees as small as 10 cm dbh. Some beech trees (about 1% of them) are resistant to the scale and the fungus, and others are partly resistant.

Current range
Beech bark disease is now found through most of the natural range of beech in Canada.

Detection
When trees are infested with beech scale, the bark will look like it has woolly material on it. This is most often seen on rough areas of bark, near branch stubs, or under larger branches. As the infestation progresses, these white patches become more extensive and can cover most of the tree's trunk.

When infected with the fungus, cankers are usually seen on the lower part of the trunk. The fruiting bodies of the fungus can also be found on the cankers, though they may be difficult to see when infection is light. Initially small whitish patches are seen on the canker, this is followed by more conspicuous red fruiting bodies (side bar). The tree canopy may show signs of decline or dieback, especially in the branches immediately above patches of dead bark.

What you can do
Since BBD is widespread, the approach is to manage it. If you have beech trees in your woodlot, read about management recommendations for beech bark disease