American Chestnut - Chestnut Blight

Chestnut Blight

Once an important hardwood timber tree, the American chestnut is highly susceptible to chestnut blight, caused by an Asian bark fungus (Cryphonectria parasitica, formerly Endothia parasitica) accidentally introduced into North America on imported Asiatic chestnut trees. The disease was first noticed on American chestnut trees in what was then the New York Zoological Park, now known as the Bronx Zoo, in 1904, by chief forester Hermann Merkel. While Chinese chestnut evolved with the blight and developed a strong resistance, the American chestnut had little resistance. The airborne bark fungus spread 50 mi (80 km) a year and in a few decades girdled and killed up to three billion American chestnut trees. Salvage logging during the early years of the blight may have unwittingly destroyed trees which had low levels of resistance to this disease and thus aggravated the calamity.

New shoots often sprout from the roots when the main stem dies, so the species has not yet become extinct. However, the stump sprouts rarely reach more than 6 m (20 ft) in height before blight infection returns.

The total number of chestnut trees in eastern North America was estimated at over three billion, and 25% of the trees in the Appalachian Mountains were American chestnut. The number of large surviving over 60 cm (24 in) in diameter within the tree's former range is probably fewer than 100. Huge planted chestnut trees (featured in National Geographic) can be found in Sherwood, Oregon, since much of western North America is still free of blight. American chestnut thrives as far north as Revelstoke, British Columbia. Although large trees are rare east of the Mississippi River in modern times, it exists in pockets of the blight-free West, where the habitat was agreeable for planting: settlers brought seeds for American chestnut with them in the 19th century.

Several organizations are attempting to breed blight-resistant chestnut trees. One of these is the American Chestnut Cooperators Foundation, which breeds surviving all-American chestnuts, which have shown some native resistance to blight. The Canadian Chestnut Council is an organization attempting to reintroduce the trees in Canada, primarily in Ontario. Another is The American Chestnut Foundation, which is backcrossing blight-resistant Chinese chestnut into American chestnut, to recover the American growth characteristics and genetic makeup, and then finally intercrossing the advanced backcross generations to eliminate genes for susceptibility to blight. The goal is eventually to restore the species to the eastern forests of North America. Backcrossed trees were first planted back into the forest for testing in 2009. In 2005, a hybrid tree with mostly American genes was planted on the lawn of the White House. A tree planted in 2005 in the tree library outside the USDA building is still very healthy seven years later; it contains 98% American chestnut DNA and 2% Chinese chestnut DNA. This tree contains enough Chinese chestnut DNA to resist the blight which is essential for restoring the American chestnut trees into the Northeast.

Other researchers are trying to attack the blight with viruses or are creating trees that are genetically modified (GM) to resist the fungus, and could be the first GM forest trees released in the wild in the United States.