Until the beginning of the twentieth century, the American chestnut (Castanea dentata [Marshall] Borkh) was one of the most prevalent and valuable trees in the eastern forests of the United States. The chestnut blight fungus (Cryphonectria parasitica), accidentally introduced around 1900, killed most of the mature trees in the natural range of the species, and today the species exists mainly as an understory shrub. Efforts currently underway in Georgia and other states, however, may eventually result in the restoration of American chestnut to eastern forests.
Before Piedmont. The trees were especially dominant on higher mountain ridges and flats. They grew very rapidly to heights of up to 100 feet, with trunks more than 6 feet in diameter. Chestnut was more useful than any hardwood in America, providing timber for houses, barns, and fences; tannin for the leather industry; and nuts for people and wildlife. The wood was straight-grained and easy to split and work with hand tools. The heartwood of mature trees was very high in tannin content, which made the wood so decay-resistant that chestnut timbers in structures built more than a hundred years ago are still as sound now as they were when the structures were erected. Abundant nut crops were produced almost every year and provided a major source of food for wild turkey, deer, bear, and smaller mammals. Those who have tasted American chestnuts say that they are sweeter than their Asian and European counterparts.
Thebirds. These spores spread the infection south from New York at the rate of about 200 miles every ten years, wiping out almost every mature chestnut tree in the natural range of the species.
The soil. However, the tree rarely reaches thirty feet or produces nuts before it is attacked and killed by the fungus, which also infects the Allegheny chinkapin (Castanea pumila) and some species of oaks.
The American Chestnut Foundation's backcross breeding program has produced trees carrying blight-resistance genes from Chinese chestnut but with a growth habit close to that of their American chestnut parents, which supply 15/16 of their genome. Scientists at the University of Maryland are testing an approach that may lead to more rapid spread of the virus responsible for hypovirulence in the fungus: they have inserted a DNA copy of the viral RNA into the fungus, allowing the virus to spread sexually.
Scientists at the University of Georgia and at the State University of New York have produced tissue cultures of American chestnut that are capable of producing thousands of structures called somatic embryos. These structures, which resemble seed embryos, can be germinated to produce seedlinglike plants. The embryogenic cultures are being tested as target material for inserting potential blight-resistance genes into American chestnut through genetic engineering. Marker genes have already been inserted into chestnut cells by microprojectile bombardment and by Agrobacterium-mediated transformation.
In 2006 a stand of American chestnut trees, estimated to be between twenty and thirty years old, was discovered in Pine Mountain near Warm Springs. Composed of six forty-foot-tall trees, the stand is the southernmost representative of the species able to produce flowers and nuts. Pollen from the trees is expected to help scientists produce a breed resistant to the chestnut blight fungus.
Media Gallery: American Chestnut