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COSEWIC assessment and status report on the American Chestnut in Canada

Limiting Factors and Threats

The Chestnut Blight (Cryphonectria parasitica, formerly known as Endothia parasitica) was likely introduced on Asian nursery stock prior to its first observation in 1904 where it was observed killing American chestnut trees at the Bronx Zoo, New York City (Anagnostakis 1982). The blight spread rapidly throughout the range of American chestnut in the subsequent decades. It reached southern Ontario in the early 1920s and by the 1930s most American chestnut trees were infected and dying (Canadian Chestnut Council, website). The chestnut blight fungus has had a devastating effect on American chestnut in the last century, reducing the once locally dominant forest tree species to scattered remnant individuals or mere stump sprouts. Continuing loss of habitat in the Carolinian zone, including such activities as fenceline removal at sites 1 and 18 and the construction of a municipally approved housing development at site 36, are ongoing threats that result in the loss of trees. The lack of reproduction in many trees of potentially reproductive size is due to the considerable distance between such trees. Cross-pollination is thereby prevented and seeds are not produced.

Hybridization with exotic species may also be a significant factor, with preliminary evidence of hybrid origin of some individuals in some populations in Ontario. John Gerrath, a graduate student at the University of Guelph, is currently examining the collected specimens from the 2001-02 survey. However, it is uncertain whether this influence will be negative, by genetically swamping native stands and reducing their occurrence, or possibly positive, by giving some level of resistance to those surviving individuals. A comparison with the European chestnut is noteworthy here: it appears to have been strongly influenced by human activity, with movement beyond its original range as far back as Roman times, and hybridization with oriental species at an early time as well (Anagnostakis & Hillman, 1992). Selection of blight resistance in cultivated trees has been reviewed by Jaynes (1978). Hypovirulent strains of the blight fungus have moved through European wild chestnut populations and a natural recovery is occurring.