The American chestnut (Castanea dentata) is a large, monoecious deciduous tree of the beech family native to eastern North America. Before the species was devastated by the chestnut blight, a fungal disease, it was one of the most important forest trees throughout its range, and was considered the finest chestnut tree in the world. It is estimated that between 3 and 4 billion American chestnut trees were destroyed in the first half of the 20th century by blight after its initial discovery in 1904. Very few mature specimens of the tree exist within its historical range, although many small shoots of the former live trees remain. There are hundreds of large (2 to 5 ft diameter) American chestnuts outside its historical range, some in areas where less virulent strains of the pathogen are more common, such as the 600 to 800 large trees in northern Lower Michigan.
Castanea dentata is a rapidly growing deciduous hardwood tree, historically reaching up to 30 metres (98 ft) in height, and 3 metres (9.8 ft) in diameter. It ranged from Maine and southern Ontario to Mississippi, and from the Atlantic coast to the Appalachian Mountains and the Ohio Valley. It has several related chestnut species, such as the European sweet chestnut, Chinese chestnut, and Japanese chestnut, which are distinguishable from the American species by a few morphological traits, such as leaf shape, petiole length and nut size. C. dentata was once one of the most common trees in the Northeastern United States. In Pennsylvania alone, it is estimated to have comprised 25-30% of all hardwoods. The tree's huge population was due to a combination of rapid growth and a large annual seed crop in comparison to oaks which do not reliably produce sizable numbers of acorns every year. Nut production begins when C. dentata is 7-8 years old.
C. dentata can be best identified by the larger and more widely spaced saw-teeth on the edges of its leaves, as indicated by the scientific name dentata, Latin for "toothed". The leaves, which are 14-20 cm (5.5-8 in) long and 7-10 cm (3-4 in) broad, also tend to average slightly shorter and broader than those of the sweet chestnut. The blight-resistant Chinese chestnut is now the most commonly planted chestnut species in the US, while the European chestnut is the source of commercial nuts in recent decades. It can be distinguished from the American chestnut by its hairy twig tips which are in contrast to the hairless twigs of the American chestnut. The chestnuts are in the beech family along with beech and oak, but are not closely related to the horse-chestnut, which is in the family Sapindaceae.
The chestnut is monoecious, producing many small, pale green (nearly white) male flowers found tightly occurring along 6 to 8 inch long catkins. The female parts are found near base of the catkins (near twig) and appear in late spring to early summer. Like all members of the Fagaceae family, American chestnut is self-incompatible and requires two trees for pollination, which can be any member of the Castanea genus.
The American chestnut is a prolific bearer of nuts, usually with three nuts enclosed in each spiny, green burr, and lined in tan velvet. The nuts develop through late summer, with the burrs opening and falling to the ground near the first fall frost.
The American chestnut was a very important tree for wildlife, providing much of the fall mast for species such as white-tailed deer and wild turkey and, formerly, the passenger pigeon. Black bears were also known to eat the nuts to fatten up for the winter. The American chestnut also contains more nitrogen, phosphorus, potassium and magnesium in its leaves when compared to other trees that share its habitat. This means they return more nutrients to the soil which helps with the growth of other plants, animals, and microorganisms.
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. Merkel estimated that by 1906 blight had infected 98 percent of the chestnut trees in the Bronx alone. 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 high 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 trees over 60 cm (24 in) in diameter within its former range is probably fewer than 100. American chestnuts were also common part of the forest canopy in southeast Michigan.
Although large trees are currently rare east of the Mississippi River, it exists in pockets in the blight-free West, where the habitat was agreeable for planting: settlers brought seeds for American chestnut with them in the 19th century. Huge planted chestnut trees can be found in Sherwood, Oregon, as the Mediterranean climate of the West Coast discourages the fungus, which relies on hot, humid summer weather. American chestnut thrives as far north as Revelstoke, British Columbia.
At present, it is believed that survival of C. dentata for more than a decade in its native range is almost impossible. The fungus utilizes northern red oak as a host, and while the oak itself is unaffected, any American chestnuts nearby will quickly succumb. In addition, the hundreds of chestnut stumps and "living stools" dotting eastern woodlands may still contain active pathogens.
Several organizations are attempting to breed blight-resistant chestnut trees. The American Chestnut Cooperators Foundation breeds surviving American chestnuts, which have shown some native resistance to blight, and the Canadian Chestnut Council is an organization attempting to reintroduce the trees in Canada, primarily in Ontario. A technique called backcrossing is being used by The American Chestnut Foundation in an attempt to restore the American chestnut to its original habitat.
American Chestnut Cooperators Foundation (ACCF) is not using Oriental genes for blight resistance, but intercrossing among American chestnuts selected for native resistance to the blight, a breeding strategy described by the ACCF as "All-American intercrosses." John Rush Elkins, a research chemist and professor emeritus of chemistry at Concord University, and Gary Griffin, professor of plant pathology at Virginia Tech, think there may be several different characteristics which favor blight resistance. Both Elkins and Griffin have written extensively about the American chestnut. They believe that by making intercrosses among resistant American chestnuts from many locations, they will continue to improve upon the levels of blight resistance to make an American chestnut that can compete in the forest. Griffin, who has been involved with American chestnut restoration for many years, developed a scale for assessing levels of blight resistance, which made it possible to make selections scientifically. He inoculated five-year-old chestnuts with a standard lethal strain of the blight fungus and measured growth of the cankers. Chestnuts with no resistance to blight make rapid-growing, sunken cankers that are deep and kill tissue right to the wood. Resistant chestnuts make slow-growing, swollen cankers that are superficial: live tissue can be recovered under these cankers. The level of blight resistance is judged by periodic measurement of cankers. Grafts from large survivors of the blight epidemic were evaluated following inoculations, and controlled crosses among resistant American chestnut trees were made beginning in 1980. The first "All-American intercrosses" were planted in Virginia Tech's Martin American Chestnut Planting in Giles County, Virginia, and in Beckley, West Virginia. They were inoculated in 1990 and evaluated in 1991 and 1992. Nine of the trees showed resistance equal to their parents, and four of these had resistance comparable to hybrids in the same test. Many ACCF chestnuts have expressed blight resistance equal to or greater than an original blight survivor but so far, only a handful have demonstrated superior, durable blight control. Time will tell if the progeny of these best chestnuts exhibit durable blight resistance in different stress environments.
Backcrossing as a treatment for blight was first proposed by Dr. Charles R. Burnham of the University of Minnesota in 1970's. Dr. Burnham, a Professor Emeritus in agronomy and plant genetics who was considered one of the pioneers of maize genetics, realized that experiments conducted by the USDA to cross-breed American chestnuts with European and Asian chestnuts erroneously assumed that a large number of genes were responsible for blight resistance, while it is currently believed the number of responsible genes is low. The USDA abandoned their cross-breeding program and destroyed local plantings around 1960 after failing to produce a blight-resistant hybrid. Burnham's recognition of the USDA's error led to him joining with others to create The American Chestnut Foundation in 1983, with the sole purpose of breeding a blight-resistant American chestnut.The American Chestnut Foundation is backcrossing blight-resistant Chinese chestnut into American chestnut trees, 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 first backcrossed American chestnut tree, called "Clapper", survived blight for 25 years, and grafts of the tree have been used by The American Chestnut Foundation since 1983. The Pennsylvania chapter of The American Chestnut Foundation, which seeks to restore the American chestnut to the forests of the Mid-Atlantic states, has planted over 22,000 trees.
The Surface Mining Control and Reclamation Act of 1977 requires owners of abandoned coal mines to cover at least 80 percent of their land with vegetation. While many companies planted invasive grasses, others began funding research on planting trees, because they can be more cost-effective, and yield better results. Keith Gilland began planting American chestnut trees in old strip mines in 2008 as a student at Miami University, and to date has planted over 5,000 trees. 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 was still very healthy seven years later; it contains 98% American chestnut DNA and 2% Chinese chestnut DNA. This tree contains enough Chinese chestnut DNA that encodes for systemic resistance genes to resist the blight. This is essential for restoring the American chestnut trees into the Northeast The Northern Nut Growers Association (NNGA) has also been active in pursuing viable hybrids. From 1962 to 1990, Alfred Szego and other members of the NNGA developed hybrids with Chinese varieties which showed limited resistance.
Researchers at the State University of New York College of Environmental Science and Forestry (SUNY ESF), have developed fully blight-resistant transgenic American chestnuts. This was done by inserting a specific gene from wheat, oxalate oxidase, into the American chestnut genome. The oxalate oxidase enzyme is an extremely common fungal defense in plants, and is found in strawberries, bananas, oats, barley, and other cereals. Oxalate oxidase breaks down the oxalic acid which the fungus secretes in the cambium to lower the pH and subsequently kill plant tissues. The chestnut trees which contain this resistance gene can be infected by the chestnut blight, but the tree is not girdled by the resulting canker and heals around the wound. This lets the fungus fulfill its normal lifecycle without the death of the tree. The blight resistance gene is passed down to the tree's offspring to provide subsequent generations with full blight resistance. In 2015, the researchers are working towards applying for government permission to make these trees available to the public in the next five years. These trees could be the first genetically modified forest trees released in the wild in the United States.
Hypovirus is the only genus in the family Hypoviridae. Members of this genus infect fungal pathogens and reduce their ability to cause disease (hypovirulence). In particular, the virus infects Cryphonectria parasitica, the fungus that causes Chestnut blight, which has enabled infected trees to recover from the blight. The use of hypovirulence to control blight originated in Europe where the fungal virus spread naturally through populations of European chestnuts. The reduced ability of the fungus to cause disease allowed the European chestnut to regenerate, creating large stands of trees. Hypovirulence has also been found in North America, but has not spread effectively. The "Arner Tree" of Southern Ontario, is one of the best examples of naturally-occurring hypovirulence. It is a mature American chestnut that has recovered from severe infections of chestnut blight. The cankers have healed over and the tree continues to grow vigorously. Scientists have discovered that the chestnut blight remaining on the tree is hypovirulent, although isolates taken from the tree do not have the fungal viruses found in other isolates. Trees inoculated with isolates taken from the Arner tree have shown moderate canker control.
The nuts were once an important economic resource in North America, being sold on the streets of towns and cities, as they sometimes still are during the Christmas season (usually said to be "roasting on an open fire" because their smell is readily identifiable many blocks away). Chestnuts are edible raw or roasted, though typically preferred roasted. Nuts of the European sweet chestnut are now sold instead in many stores. One must peel the brown skin to access the yellowish-white edible portion. The unrelated horse-chestnut's seeds are poisonous without extensive preparation. Native Americans used various parts of the American chestnut to treat ailments such as whooping cough, heart conditions and chafed skin.
The January 1888 issue of Orchard and Garden mentions the American chestnut as being "superior in quality to any found in Europe." The wood is straight-grained, strong, and easy to saw and split, and it lacks the radial end grain found on most other hardwoods. The tree was particularly valuable commercially since it grew at a faster rate than oaks. Being rich in tannins, the wood was highly resistant to decay and therefore used for a variety of purposes, including furniture, split-rail fences, shingles, home construction, flooring, piers, plywood, paper pulp, and telephone poles. Tannins were also extracted from the bark for tanning leather. Although larger trees are no longer available for milling, much chestnut wood has been reclaimed from historic barns to be refashioned into furniture and other items.
"Wormy" chestnut refers to a defective grade of wood that has insect damage, having been sawn from long-dead, blight-killed trees. This "wormy" wood has since become fashionable for its rustic character. The American chestnut is not considered a particularly good patio shade tree because its droppings are prolific and a considerable nuisance. Catkins in the spring, spiny nut pods in the fall, and leaves in the early winter can all be a problem. These characteristics are more or less common to all shade trees, but perhaps not to the same degree as with the chestnut. The spiny seed pods are a particular nuisance when scattered over an area frequented by people.