Definitions

chestnut-bark disease

Beech Bark Disease

Beech bark disease is a disease that causes mortality and defects in beech trees in the eastern United States and Europe. In North America, the disease results when the beech scale insect, Cryptococcus fagisuga, attacks the bark, creating a wound. Later, two different fungi (Nectria coccinea var. faginata and Nectria galligena) common to North America can invade the tree through the wound, causing a canker to form. In subsequent years, new cankers will continue to form, ultimately leading to the death of the tree. In Europe, N. coccinea is the primary fungus causing the infection. Infection in European trees occurs in the same manner as it does in North American trees. Though the disease still appears in Europe, it is less serious today than it once was.

History and Distribution

Beech bark disease was first documented in Europe in 1849. At first, it was believed that the insect was the main cause of the disease. It wasn’t until 1914 that the fungus was associated with the disease. The first outbreak of beech bark disease in North America appeared in American Beech (Fagus grandifolia) in Nova Scotia around 1920. The disease then started to spread to the south and west. In 1929, the first case of beech bark disease was found in the United States in Massachusetts. By 2004, the disease had spread as far west as Michigan and as far south as western North Carolina.

The Destructive Duo

In order for beech bark disease to occur, two components are required, an insect and a fungus.

The Insect

The beech scale insect can be observed on the trunk and limbs of the tree as woolly, white tufts that will later develop into broad strips. The woolly, white tufts and broad strips are the colonies of the beech scale insect that are formed in the tiny crevices along the bark. An adult beech scale insect has a soft body, is yellow in color, ranges from 0.5 to 1.0 millimeter long, and has an elliptical shape. The beech scale insect also has a very important stylet that it uses to penetrate the bark of the tree for feeding purposes. There are no male beech scale insects, which results in the insect reproducing parthenogenetically. In mid to late summer, the beech scale insect will lay its eggs and the adults will die. The pale yellow eggs are laid on the bark in strings of four to eight eggs. The first-stage nymphs (also known as crawlers) begin to hatch in late summer and continue until early winter. These crawlers hatch with developed antennae and legs. They will move or be moved around until they find a suitable and safe area to force their stylet into the bark and begin feeding. At this point, they begin to secrete the woolly wax cover that they use to survive the winter season. At this time, they are in the second stage of their metamorphosis. When the spring season comes around, they will molt and become adult females.

The Fungus

There are two fungi common to North America that are important to the beech bark disease process. They are Nectria coccinea var. faginata and Nectria galliengena. The primary fungus is N. coccinea var. faginata, though N. galliengena is very important in some areas. These fungi infect the tree through the wounds caused by the beech scale insect and then begin to produce spores. One type of spore that is produced is perithecia. Perithecia are red, lemon-shaped fruiting bodies that form in clusters on the bark. These perthecia mature in the fall, and once they have become sufficiently moist, they each release eight spores that are carried by the wind to other beech trees. Even though the perthecia occur on dead bark, they still have the ability to produce viable spores the next year.

Signs and Symptoms

The first sign that is visible is a woolly, white, waxy covering that the beech scale insect secretes. This sign can be observed covering small areas or most of the tree. The amount of waxy material observed depends on the population of the beech scale insect on that tree. The Nectria fungi also show signs of its presence. An early sign is what looks like a bleeding spot on the tree. A reddish-brown fluid will ooze from the wound site, giving it this appearance. Later, perithecia will form around the dead spot, which is another sign of the disease.

Symptoms of beech bark disease can be observed in the foliage and on the bole of the tree. Symptoms affecting the foliage are yellowing, becoming small and sparse, and remaining on the tree during the summer time. Trees that display a thin, weak crown may persist for several years but may also die without displaying any symptoms. Symptoms that are noticeable on the bole are the cracking of the bark, the formation of cankers, and beech snap. Beech snap is a result of the fungi and insect weakening the wood, which makes it susceptible to being blown over by wind.

Controls

There are a few controls for beech bark disease. One important control is the prohibition of movement of nursery stock or other materials that may harbor the beech scale insect.Insecticides, generally not applied under forest conditions, are primarily used on high-value ornamental trees. The use of other organisms as controls is also a possibility. The ladybird beetle is a beetle that preys on the beech scale insect. A fungus that parasitizes the Nectria fungus could also be employed. The problem with using these organisms to control beech bark disease is that their impact on the disease has not been evaluated extensively. In a forest setting, controlling the beech bark disease is too costly. Timely salvage cutting is the best way to reduce the losses of beech in a forest.

References

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