Disease

Tue, 11/08/2011 - 09:23 -- cadoughe

Insects and pathogens are a natural part of ecosystems and are essential to ecological balance in natural forests. Population dynamics of insects and pathogens are influenced by climate, management activities, natural tree defenses, and natural enemies. Trees can be weakend by a disease or an insect, and that weakened tree is then more vulnerable to insects, disease, and changes in temperature and precipitation. The effect on forested landscapes can be tree mortality and/or reduced tree vigor. Examining trends of individual insect and pathogen populations is useful in understanding their dynamic nature.

Over the last century, several forest types in the region have experienced significant declines due to infestations of insects and diseases.

Beech bark Disease

Beech bark disease occurs when the bark of the beech tree is attacked and altered by an insect, the beech scale, and then invaded and killed by fungi. The first sign of the disease is isolated dots of white “wool” on the bole of the tree. As the insect population increases, the entire bole of the tree may be covered by the waxy secretion. The disease in forest stands cannot be controlled at a reasonable cost, and maintenance of apparently resistant trees is the best way to reduce disease losses. Virtually all high-elevation beech stands and beech gaps in Western North Carolina are already seriously impacted by beech bark disease.

Oak Decline

A high incidence of oak decline has been observed in Western North Carolina. When mature and over-mature oaks experience adverse climatic and site conditions, combined with stress from disease or insects, they are susceptible to decline and death. Losses are heaviest during extended periods of drought, on dry, south-facing slopes, and on shallow soils and rocky outcrops. Oak decline will probably increase as oak forests continue to age.

Dogwood Anthracnose

In the period 1984 to 2006, the volume of flowering dogwood declined by 48 percent. The increase in mortality is due, in large part, to anthracnose, a fungus introduced into Western North Carolina in the 1980’s. Dogwood anthracnose is expected to intensify in the future, and losses will be heaviest at higher elevations and on shaded, north-facing slopes. Eventually, trees in the forest setting will be largely eliminated above 3,000 feet. Trees in full sun exposure below 3,000 feet are expected to sustain little damage. Besides being admired for their beautiful spring flowers, dogwood trees are an important source of soft mast for many species of wildlife. Rabbits, deer, squirrels, and a remarkable variety of game birds and song birds rely on dogwood to build up energy reserves to survive the winter. There is not a ready substitute with fruit of equal value to make up this loss.

Chestnut Blight

A century ago, the dominant tree in the canopy of Western North Carolina forests was the American chestnut. In some areas, the species grew in almost pure stands. During the early 1900’s, the species was decimated by a fungal disease that destroys the bark tissue and kills the tree.

Early in the century and continuing to this day, groups of private and public cooperators have worked to find a way to save the chestnut. Following years of research, a disease-resistant American chestnut has now been reintroduced into eastern woodlands. New efforts to hybridize surviving American chestnuts with blight-resistant Chinese chestnuts resulted in a species that is about 94 percent American chestnut with the protection found in the Chinese species. In the next decade, the success or failure of the hybridized trees should be apparent.