Stressors of Hardwood Forests
Dogwood anthracnose is a serious disease of flowering dogwoods. Since it was first reported in 1978, dogwood anthracnose has spread rapidly west and south from its northeastern origins. Some of the worst losses have been reported in the South, where the disease continues to intensify.
Aside from the aesthetic impact to both forest and shade trees, dogwood anthracnose can contribute to significant ecological disturbance. Many wildlife species are dependent on dogwood for food and cover. Anthracnose infection begins in the leaves during the growing season (Figure 5) and spreads to twigs and branches.
Branches die back to the main stem, resulting in cankers that kill the tree. In the South, mortality is most likely to occur in elevations above 3000 feet on all sites. Below 3000 feet, the damage is most severe on moist, cool sites. Shade increases the risk of infection and mortality.
Since the disease was first reported in northern Georgia and western North Carolina in 1987 and 1988, respectively, it has spread very rapidly throughout the entire southern Appalachian range. Data from intensive evaluation plots in the South reflect the virulence of this pathogen. From 1988 to 1992, mortality rates jumped from 0 to 17 percent, and severe infection rates from 4 to 17percent.
From 1988 through 1994, surveys and impact assessments were funded in the South through a special technology development project. State cooperators throughout the Southern Region routinely reported confirmed dogwood anthracnose infections on a county-by-county basis. This information continues to be reported to the Forest Health field office of the USDA Forest Service in Asheville, North Carolina, which maintains the data and prepares an annual report.
As of the year 2002, dogwood anthracnose has essentially spread throughout the range of dogwood in the South, although losses continue to intensify within the infected range. Nevertheless, the four-year drought of 1999-2002 has inhibited proliferation of the disease that thrives best during extended, moist conditions.
Figure 6 shows the spread of the disease from 1988 to 2001 for counties in which suspected dogwood anthracnose infections were confirmed through laboratory cultivation of the fungus. Initial data were supplied by numerous State and Federal cooperators.
Dogwood anthracnose will intensify in the future. Losses will continue to be heaviest at higher elevations and on shaded, north-facing slopes. Eventually, trees in the forest setting will be largely eliminated above 3000 feet. Trees in full sun exposure below 3000 feet will sustain little damage. The disease is not expected to cause much damage in the coastal plain.
Shade and landscape trees will incur fewer losses because conditions are less favorable to the disease and because fungicidal and cultural control methods are highly effective in protecting these valuable trees.
No special technological advances that will facilitate dogwood anthracnose surveys and confirmation appear imminent. Nevertheless, as the public becomes increasingly aware of the disease, the frequency and accuracy of reports is likely to increase.