One of America's favorite native ornamentals has been under siege. A virulent fungal disease has cause dieback and mortality in two of our indigenous dogwoods, flowering dogwood (Cornus florida) in the East and mountain dogwood (Cornus nuttallii) in the Pacific Northwest. Today, after almost two decades of research, scientists are making headway against this destructive new disease, called dogwood anthracnose. We now know what conditions or habitats seem to inhibit the spread of the disease, and scientists have developed a management plan to help gardeners protect their favorite trees. Although the origin of the pathogen has not yet been definitively established, much has been learned and today research programs are underway to develop disease-resistant selections of dogwood.
The geographical range of dogwood anthracnose is alarmingly extensive for both its North American host trees. The disease first appeared in 1976 on the mountain dogwood in the Vancouver area of Washington State. Until 1981, its spread had been limited to that state, but by 1983 the disease was reported in Oregon and British Columbia, as well as in one county in Idaho. By 1994, dogwood trees in northern California were affected.
Anthracnose was first reported on the flowering dogwood in 1983 in New York, Connecticut, Pennsylvania, and New Jersey. By 1987, the disease had spread north and south, to Massachusetts, Delaware, Maryland, Virginia, and West Virginia. By 1988, it was traced south, primarily along the Appalachian mountain range, and was discovered in high-elevation sites and cool, wet valleys in Georgia, North Carolina, and Tennessee. It was confirmed in Kentucky and Alabama in 1989, Ohio and the District of Columbia in 1990, New Hampshire in 1991, Rhode Island in 1992, Indiana, Vermont, and Michigan in 1993, and Missouri in 1994. Some confirmed cases in these states have been attributed to movement of dogwood nursery stock from areas with the disease. Even the non-native Korean dogwood (Cornus kousa), usually resistant to anthracnose, can suffer leaf infection when moisture conditions especially favor the disease.
Public concern and research has been greatest in the eastern United States, not surprisingly, considering flowering dogwood (C. florida) and its many cultivars are highly valued ornamental trees important to the nursery and landscape gardening industries.
Soon after the first reports of dogwood anthracnose, a fungal pathogen in the genus Discula was identified as the cause. In 1991, the pathogen was fully described and named Discula destructiva. Dogwood anthracnose had not been reported in pre-1970s surveys of dogwood diseases, and examination of herbarium specimens of Cornus species found no evidence of it prior to that time. Because of its sudden and coincidental appearance near ports of entry, specifically Seattle in the West and New York in the East, as well as its rapid spread and resulting widespread destruction, the disease is suspected of being an introduced pathogen. So far there is no evidence on Cornus hosts native to countries outside North America. For now, the origin of dogwood remains a mystery.
Some infected dogwoods, especially in shaded woodland conditions, are killed in one to three years. Others in sun-exposed sites often survive and, depending on moisture conditions, exhibit symptoms of varying severity year to year. Dogwood anthracnose affects all above-ground parts of the host tree: leaves, bracts, current-year shoots, trunk sprouts, bark and cambium, fruits and seeds.
Leaf symptoms are first noted on newly opened leaves in spring. Three kinds of leaf symptoms may develop: leafspot, blotch, and blight. Leafspots are localized dead or discolored areas scattered in the leaf blade. Blotch refers to irregular dead patches often observed on tips or along leaf margins. Blighting, the death of the entire leaf blade, commonly occurs throughout the canopy of infected dogwoods in shady areas, or within the lower, shaded part of the canopy of those in sun-exposed sites. Blighted leaves become withered and sometimes remain attached to branches throughout the winter. Even bracts can show telltale symptoms of the disease: reddish purple spots or brown blotches.
Twig dieback occurs when infection advances from blighted leaves directly into the stems. Twig death tends to be concentrated in the lower branches of dogwoods, hence the early name of the disease, "lower branch dieback." Epicormic branches (clusters of sprouts) appear on the trunk and main branches following dieback; these are readily infected, and infection spreads from them into the trunk and main branches, where it causes cankering (localized dead areas in the bark and cambium).
An understanding of how environmental conditions affect dogwood anthracnose has helped researchers develop control practices. Sunlight and moisture are the most important influences. Anthracnose is more severe, and tree mortality is greater, on dogwoods in the shade. In full sun, symptoms are more severe in the lower branches than in the upper, exterior canopy, and infected trees often recover with proper maintenance.
Dogwood anthracnose thrives in moist conditions. For dogwoods in the Northeast, leaf symptoms begin to appear following leaf expansion, provided there is enough rainfall to keep the leaves wet for several hours at a time. Foliar infection can recur throughout the summer except during very dry periods. Moisture is also key to the production of fungus spores, which emerge from pinhead-sized fruiting bodies on dead leaf and twig tissues of infected dogwood. Under moist conditions, blighted dogwood leaves that often remain hanging over winter are a source of spores for new infections in the spring.
Most short-distance dispersal of the fungus within and among trees is thought to occur via wind driven and splashing rain. Seed-carrying birds are also suspected of being carrier of dogwood anthracnose since the fungus has been isolated from fruit and seed of infected dogwoods.
During the onset of severe dogwood anthracnose in the late 1970s and early 80s in the Northeast, almost all dogwoods became infected regardless of tree health prior to infection. For infected dogwoods, however, any additional stresses like droughts or low fertility reduce tree vigor and hasten tree decline.
The greatest hope for long-term management of dogwood anthracnose is the discovery of genetic resistance in this host. Surviving seedlings of C. florida in woodland sites have been identified as candidates for resistant stock; eight were discovered in Catoctin Mountain Park, MD, in 1991 - 1992. Clones and seed progeny of these selections are being subjected to high disease pressure by University of Tennessee researchers Dr. Effin Graham and Dr. Mark Windham to confirm their resistance. I have also identified potentially resistant survivors from a population of C. florida devastated by anthracnose in the late 1970s in the Mohonk Preserve, New Platz, NY; I am currently testing them for susceptibility and resistance to anthracnose. Other species of Cornus that have shown resistance to anthracnose are the North American native C. racemosa and C. canadensis, as well as non-native C. amomum, C. alternifolia, and C. mas.
The non-native C. kousa, which has large floral bracts similar to C. florida, can show leaf symptoms under moisture conditions especially favorable for the fungus, but this dogwood is considered relatively resistant to anthracnose. The cultivar C. kousa var. chinensis, selected for its larger bracts, carries similar resistance. Hybrids of C. florida x C. kousa, released as the Stellar series by Rutgers University, also produce showy bracts and have exhibited resistance to anthracnose in ongoing field tests. Their trademarked names are Ruth Ellen, Constellation, Aurora, Galaxy, Stardust, and Stellar Pink. Except for Stellar Pink, all bloom with white floral bracts about the size of those of flowering dogwoods, blossoming on the heels of this native species.
In the Future
In southwestern New York, the damage to flowering dogwoods from anthracnose is far less severe today than during the 1980s. Many landscape and roadside dogwoods have recovered from earlier infection, and leaf symptoms often are limited to scattered spots and blotches. Epicormic branches and new annual cankers appear less often. This apparent trend of decreasing disease severity in one region is puzzling and inconsistent with other introduced diseases in North American forest trees, most notably chestnut blight and Dutch elm disease. There are plausible explanations for this apparent decrease of anthracnose: environmental conditions in the Northwest may have become unfavorable for the disease or some as yet unidentified change may have occurred in the pathogen itself. Anthracnose is still having an impact on C. florida in some areas at the southern edge of its range, and on C. nuttallii in the West.
The long-term effects of anthracnose on native dogwood populations, and on the wildlife they support, are not known. In woodland sites in Maryland and New York, where mortality of dogwoods was high over a decade ago, dogwoods are surviving, some bearing viable seed. Researchers are monitoring the sites for any natural regeneration of dogwood by seedlings
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Dr. Craig R. Hibben was a research plant pathologist and chairman of BBG Research Center, Ossining, NY, conducting much of the original research on dogwood anthracnose in the Northeast. After retiring he became a research associate at Mononk Preserve, New Paltz, NY, and at Lasdon Park and Arboretum, Somers, NY, and a research consultant at NY Botanical Garden. This article is based on: M. L. Daughtrey, C. R. Hibben, K. O. Britten, M. T. Windham and S. C. Redlin, 1996, "Dogwood Anthracnose: Understanding a Disease New to North America," Plant Disease 80:349-358.