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COSEWIC Assessment and Status Report on the Eastern Flowering Dogwood in Canada

Limiting Factors and Threats

The primary threat is dogwood anthracnose, a fungal disease that is causing a high level of die-back (typically 25-75% in infected populations; but 97% in one Illinois population (Schwegman et al., 1998)) and mortality throughout the range of Cornus florida in eastern North America. The symptoms were observed in the United States since the mid-1970s and the causal agent was identified and described as a new species in 1988 (Redlin, 1991). There is a suspicion that it originated in the orient since Chinese dogwood (Cornus kousa) demonstrates resistance to dogwood anthracnose (Ranney et al., 1994) similar to other geographic patterns of resistance, such as the various oriental chestnuts having resistance to chestnut blight. Symptoms were beginning to appear in Ontario populations in the mid-1990s and the disease was confirmed in 1998. The disease can kill flowering dogwoods of all sizes and ages but especially understory trees and seedlings. Infection appears to occur during cool, wet weather in spring and fall, as masses of white to salmon-coloured conidia ooze from infected twigs and leaves, but can happen any time during the growing season. Host susceptibility appears to increase with drought and winter injury (NRC, 2004). There have been reports of less severe incidence of dogwood anthracnose in areas of higher light or wind exposure (e.g., Chellemi & Britton, 1992); however, in Ontario there was not a noticeable difference: severely affected trees were observed at the edge of forest sites as well as within them.

The University of Guelph Arboretum (Henry Kock, pers. comm.) attempted to limit the movement of nursery stock from the U.S. when dogwood anthracnose was well recognized there but not yet in Canada. Currently there are still no special restrictions on the importation of nursery stock of this species from the U.S., other than routine certification from source and border inspections (Ken Marchant, Forestry Specialist CFIA, pers. com.).

Apparent dogwood anthracnose resistance, while rarely documented, was observed in a few individuals at the Catoctin Mountain Park, Maryland, after the disease swept through killing most of the trees (79% mortality in 1991). One of these trees was cloned and named ‘Appalachian Spring’ (Windham et al., 1998).

The steep decline in the two populations monitored over 11 years, as shown in Figures 3 and 4, suggests that we will likely begin to see extirpation of populations within a few years. From the experience of Dutch elm disease and chestnut blight, it is unlikely that extirpation will be 100%.

For additional consideration, Strobl & Bland (2000) list root knot nematode, defoliators and borers as other pests, but certainly of a minor level of significance compared to dogwood anthracnose. Habitat loss and fragmentation throughout the Carolinian Zone, but especially in the extreme southwest of Essex County and Chatham-Kent, reduce the amount of available habitat and hence its continuity, which is important for re-colonization. While it can not be concluded that habitat is limiting, habitat loss and fragmentation are additional stresses that may exacerbate the impact of the disease, mainly by not allowing as much opportunity for the species to adapt a natural resistance to the disease. This particular threat is reversible through region-wide restoration programs such as Essex County’s Natural Heritage Restoration Program.