The disease was first noticed on American Chestut trees in the Bronx Zoo in 1904. While Chinese Chestnuts evolved with the blight and are usually immune, the airborne bark fungus spread 50 miles a year and in a few decades girdled and killed billions of American Chestnuts. 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 meters (20 ft) in height before blight infection returns.
It is estimated that the total number of chestnut trees in eastern North America was over 3 billion, and that 25 percent of the trees in the Appalachian Mountains were American Chestnut. The number of large surviving American Chestnut trees over 60 cm (24 in) in diameter within the tree's former range is probably less 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.
American Chestnut field trial sapling from the American Chestnut Cooperators Foundation
Several organizations are attempting to breed blight-resistant chestnuts.
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 American Chestnut × Chinese Chestnut hybrids to American parents, to recover the American growth characteristics and genetic makeup, and then finally intercrossing the advanced generations in order to breed consistently for blight resistance. The goal is eventually to reintroduce the species to the eastern forests of North America. In 2005, a hybrid tree with mostly American genes was planted on the lawn of the White House, and to date is doing very well.
The United States National Arboretum also has taken an interest in the American Chestnut, using similar methods of backcrossing to create hybrids resistant to blight. Overall, it is anticipated that the species may be ready for trial plantings in forests by 2010.
Two of the largest surviving American Chestnut trees are in Jackson County, Tennessee. One is the state champion and has a diameter of 61 cm (24 in) and a height of 23 meters (75 ft) and the other tree is nearly as large. One of them has been pollinated with hybrid pollen by members of The American Chestnut Foundation; the progeny will have mostly American Chestnut genes and some will be blight resistant.
On 18 May 2006, a biologist with the Georgia Department of Natural Resources spotted a stand of several trees near Warm Springs, Georgia. One of the trees is approximately 20-30 years old and 13 meters (40 ft) tall and is the southernmost American Chestnut tree known to be flowering and producing nuts [1]. Another large tree was found in Talladega National Forest, Alabama on June 2006.[2]. It is 26 meters (85 ft) tall with a diameter of 35 centimeters (14 in). A combination of factors may account for the survival of these relatively large
American Chestnut trees including low levels of blight resistance, hypovirulence (the attacking blight fungus is weakened by a virus), and good site conditions; in some cases, large chestnut trees have been lucky, and have not yet been exposed to the blight-causing spores.