Other Exotic Forest Threats - Beech Bark Disease

Beech bark disease is a major threat to American beech (Fagus grandifolia) in eastern North America. The disease is the result of an interaction between an insect and a fungus, and does not occur if either is absent. The disease occurs when a tree is infested with a 'scale' insect (beech scale, Cryptococcus fagisuga), and then colonized by one of several species of fungi. One fungus, Nectria galligena, is a native North American fungus that causes a 'target canker' on weakened hardwoods (a canker is a localized region of dead tissue, often having a sunken appearance). The other common fungus is an introduced fungus, Nectria coccinea var. faginata. A third fungal species, Nectria ochroleuca, has been found in association with the scale in several states and Ontario.

Map of beech bark disease distribution
Disease distribution is in red, and blue counties have beech scale but not beech bark disease.
The native range of beech is in green
USDA Forest Service

Scales are tiny insects (0.5 to 1 mm) related to aphids. Like aphids, they have a long tube-like mouthpart called a 'stylet' that is inserted into the tree to extract nutrients from the sap. Reproduction is parthenogenic, meaning that females reproduce without mating. Adult beech scales lay eggs during the summer and the eggs hatch in the fall. Once the young insects find a suitable location on a beech tree, they insert their stylet into the tree and begin feeding. They lose their legs, become covered with a woolly wax, and will remain there for the rest of their lives. They overwinter in this stage, and become adults in the spring.

Beech scale was accidentally introduced from Europe into Nova Scotia, Canada around 1890. By the 1930s, the scale and an associated Nectria fungus were found to be killing trees in eastern Canada and Maine. The disease has continued to spread, and is now found as far south as Tennessee and North Carolina. It was discovered in Michigan in 2000. The European beech species do not get beech bark disease, having had thousands of years to develop resistance.

Disease Symptoms

Be on the lookout for beech trees covered in white wool. Report suspect trees or stands to the local DNR office or to DNR Forest Health staff.

Beech bark disease has three steps.

1. The first step in disease progression is the colonization of a beech tree by the scales. Initially, there are scattered woolly spots on the tree trunk, but as the scales reproduce, the entire trunk of the tree may become covered. Scale populations may rise to very high levels within as few as 2 or 3 years. In the northeastern states, natural movement of the scale has been estimated at about 6 miles per year. The immature scales are blown by the wind and are also moved by birds and people. Typically, the scale appears several years before disease symptoms.
2. After the scale is well established on a tree, the fungi become involved. The fungi use the insect's feeding wounds as an entry point into the tree, and as the fungi grow, several disease symptoms appear. The fungus kills the living tissue beneath the outer bark, and a distinct orange color can be seen at the leading edge of the fungal growth. On some trees a brown sap is produced at dead spots. A canker (region of dead tissue) develops since the surrounding living tissue continues to grow, and the result is a roughened appearance of the bark.
3. As the disease first invades an area, heavy mortality occurs as large beech trees usually die within a few years of scale buildup. This first wave of tree mortality is referred to as a 'killing front.' Cankers may girdle the tree and kill it, or the tree may remain alive but in a disfigured state. Other organisms such as decay fungi and wood-boring insects can also invade the diseased trees. These may kill the tree or physically weaken the wood so that the tree breaks during a strong wind. The loss of beech can dramatically alter forest structure, and can negatively impact many wildlife species because of the loss of the nut crop and loss of nesting habitat for birds. The loss of timber and ornamental trees is another consequence of this disease.
4. After the 'killing front' passes through, the remaining forest usually contains some surviving large beech and many small beech that have sprouted from the roots of the dead trees. Some of the large trees may be resistant to beech bark disease. The small trees are frequently disfigured due to the interactions of beech scale, the fungi, and another scale insect, Xylococculus betulae.

Management Options

Because the disease requires both the insect and fungus, killing the scales will prevent the disease from occurring. The scales can be controlled on ornamental beech trees through the use of insecticides, dormant oils, and insecticidal soaps. Alternatively, a soft brush or strong stream of water can be used to remove the scales on small trees. In forest stands there is no practical control option other than salvaging dead or diseased trees. In residential or recreational areas, the weakened trees are a safety hazard and should be removed since strong winds may cause them to break.

A small percentage of trees are resistant to the scale and do not develop disease symptoms even in heavily infected stands. Therefore, breeding resistant trees is a possible long-term management option.

Because beech is a very minor component of the overall forest in Wisconsin, there are no special recommendations for its management in preparation for the arrival of beech bark disease. However, when marking a stand for thinning during the next regularly-scheduled entry, consider beech vigor and bark texture in the order of removal. Typically, this would mean that beech with low vigor and/or rough bark would be harvested. Retain the vigorous trees with smooth bark, and keep the stand adequately stocked.

Don't remove all of the beech, because some trees are resistant to the disease and will continue to provide wildlife and timber benefits.

Be aware that emerald ash borer threatens ash that establish in openings. Thus, encourage the regeneration of other species such as oak, birch, maple, or conifers in stand openings through natural or artificial means. Oak and other mast-producing species may be particularly suitable as substitutes for the loss of beech nuts. Also consider the amount of deer browsing when selecting species to regenerate.

When harvesting, try to avoid damaging the root systems of the trees. Beech sprouts in response to root injury and a stand could become more susceptible to beech bark disease in the long term. Also consider the potential for sunscald or other injury to the residual beech in the stand.

Financial considerations and management objectives will affect the suitability of these recommendations. Management guidelines may change over time due to changing disease distribution and new research findings. The advice of a professional forester is recommended.

Beech Bark Disease in Wisconsin

Although we don't currently have beech bark disease, its presence in Michigan means that it could arrive in Wisconsin within the next few years. As in other states, the scale would be present several years prior to disease symptoms. Management options would depend on whether the infestation was a small, isolated infestation that could be eradicated, or a widespread infestation that could not be eradicated.

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