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Dogwood Tree

Anthracnose disease

Dogwood refers to certain species of trees and shrubs in the dogwood family (Cornaceae). The dogwoods are in the genus Cornus, which mostly occur in temperate and boreal forests of the Northern Hemisphere.

Species in the dogwood family have seasonally deciduous foliage. The leaves are simple, usually untoothed, and generally have an opposite arrangement on the twig. The flowers of dogwoods develop in the early springtime, often before the leaves. The flowers are small and greenish, and are arranged in clusters at the terminus of twigs. The flowers are sometimes surrounded by whitish leaves that are modified as petal-like, showy bracts, giving the overall impression of a single, large flower. The fruit is a drupe, that is, a hard-seeded structure surrounded by an edible pulp.

Several North American species of dogwood achieve the size of small trees. The flowering dogwood (Cornus florida) of the eastern United States can grow as tall as 43 ft (13 m), and is a species of rich hardwood forests. The flowering dogwood is an attractive species often cultivated for its large and showy, white-bracted inflorescences, its clusters of scarlet fruits, and the purplish coloration of its autumn foliage. The Pacific dogwood (C. nuttallii) is a component of conifer-dominated rainforests of the Pacific coast, and is also a popular ornamental species, for reasons similar to the flowering dogwood. Other tree-sized dogwoods of the western United States include western dogwood (C. occidentalis) and black-fruited dogwood (C. sessilis), while stiff cornel dogwood (C. stricta) occurs in the east.

Many other species of dogwoods are shrub sized, including alternate-leaved dogwood (C. alternifolia) and roughleaf dogwood (C. drummondii) of eastern North America. The widespread red-osier dogwood (C. stolonifera) is sometimes cultivated for its attractive, red twigs, which contrast well with the snows of winter.

The bunchberry or dwarf cornel (C. canadensis) is a diminutive species of dogwood that grows in the ground vegetation of northern forests.

The wood of tree-sized dogwoods is very hard, and has a few specialized uses, for example, in the manufacturing of shuttles for fabric mills, golf-club heads, and other uses where a very durable material is required. However, the major economic benefit of dogwoods is through their attractive appearance, which is often exploited in horticulture.

Dogwood stems are an important food for wild animals such as rabbits, hares, and deer that browse on woody plants during the winter. In addition, many species of birds and mammals feed on the fruits of various species of dogwoods.

Dogwood anthracnose is a disease that affects the flowering and Pacific dogwood (Cornus florida and C. nuttallii). Pacific dogwood infections have been reported in Washington, Oregon, Idaho, and British Columbia. In the eastern states, flowering dogwood infections have been reported in Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, and Delaware. More recently, anthracnose has been detected in Maryland, Virginia, West Virginia, North and South Carolina, Tennessee, and Georgia.

A dogwood in bloom, Georgia. JLM Visuals. Reproduced with permission.

The disease is caused by the anthracnose fungus, Discula sp. Infection most often occurs during cool, wet spring and fall weather, but may occur throughout the growing season. Trees weakened by drought or cold are most likely to suffer severely from the disease. When heavy infection occurs for several years, woodland and ornamental dogwoods frequently die.

The origin of anthracnose disease has not been determined. It may have been introduced, or it may have resulted from an altered host/parasite relationship that transformed an innocuous fungus into a significant pathogen.

Cultivated dogwoods that are well cared for are better able to withstand anthracnose during years of widespread infestation. To build up their resistance, dogwoods should be watered during periods of drought. Mulching may help conserve water, as well as protect trees from physical injury. Overhead watering may contribute to leaf infections.

The only way the disease can be effectively controlled is if the disease is detected before extensive dieback has occurred. The removal of diseased twigs and branches helps to reduce potential sources of infection. It may also help to remove any fallen leaves. Succulent growth, which is encouraged by high nitrogen fertilization, can lead to trunk canker formation. To encourage trees to grow, a balanced fertilizer may be applied in early spring.

Applications of the fungicides chlorothalonil and mancozeb in the spring protect against leaf infections. When conditions favor development of the disease later in the growing season, additional applications of fungicides may be beneficial.

Randall Frost

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