Chestnut blight
Chestnut blight is a devastating disease caused by the fungus *Cryphonectria parasitica*, which primarily affects hardwood trees, notably the American chestnut (*Castanea dentata*). Introduced to North America in the late 1800s, chestnut blight rapidly decimated the population of American chestnut trees, which were once abundant in the eastern United States, with estimates suggesting that three to six billion trees were lost by the mid-20th century. The disease initially manifests as cankers on the bark, which girdle the branches and trunks, ultimately leading to the tree's death while the root system may remain alive, allowing for potential regrowth.
The significance of the American chestnut extends beyond its ecological role as a food source for various animals; it was also valued by Native Americans and European settlers for its sweet nuts and high-quality, durable wood used in a variety of products. In response to the near extinction of the American chestnut, scientists are actively researching methods to develop blight-resistant varieties through hybridization with Asian chestnuts, which are naturally resistant to the disease. Efforts are also underway to genetically modify trees to enhance their resilience, aiming to restore the American chestnut to its former prominence in North American forests. However, achieving this goal will be a long-term endeavor, requiring decades to cultivate mature trees.
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Chestnut blight
Chestnut blight is a disease that affects hardwood trees around the world but is particularly a problem in North America. The fungusCryphonectria parasitica causes the disease. The fungus kills branches and trunks of American chestnut trees and some other trees. Asia introduced the blight to the United States in the late 1800s. It spread quickly and killed nearly all of the American chestnut trees, which included between three billion and six billion trees, in the United States. Only a handful of American chestnut trees remain alive in the late 2010s, and these trees will most likely die in the future from the disease. Into the twenty-first century, scientists are working on a solution.
Background
The American chestnut tree (Castanea dentata) was at one time one of the most common tree species in the eastern United States. Before the 1900s, about one in four trees growing in the Appalachian Mountains was an American chestnut tree. The tree was a vital species that helped feed many animals and sustain its ecosystem. For thousands of years, these trees produced a large crop of nuts that many animals—such as turkeys, deer, bears, pigeons, squirrels, and hogs—relied on for food.
People also depend on the American chestnut trees. Native Americans valued them for their nuts and wood for thousands of years before settlers arrived and eventually introduced the blight. European settlers also relied on the trees. They ate the chestnuts, fed them to livestock, and sold them for an income. The trees grew quickly, and their wood had a straight grain. This made their wood cherished. Furthermore, the wood was highly rot resistant, so items made from it lasted for long periods. People living in North America used American chestnut wood to make an array of items such as furniture, musical instruments, log cabins, and fences.
Chestnut blight was first identified in New York City in 1904, but it most likely was introduced years earlier in the late 1800s. Asian chestnut trees, which were imported to the United States to grow in orchards, were determined to be the culprit. The blight spread quickly. People tried to stop the blight from spreading by using chemicals and by cutting and burning entire stands of American chestnut trees. Nothing successfully stopped the blight's spread. The blight spread at a rate of about 25 miles per year, although it was faster in some areas.
By 1950, about 99.9 percent of American chestnut trees were infected by the blight and had died. Before the 1900s, between four billion and six billion American chestnut trees grew throughout the forests of the eastern United States. Nearly all of these trees were dead by the mid-1950s. About fifty years after Cryphonectria parasitica was first detected, the American chestnut had mostly vanished from North America, and people and animals had to adjust to the loss of the source of food, wood, and income.
Chestnut blight not only nearly obliterated the American chestnut tree, but also it killed and infected other species of trees. The Allegheny chestnut (Castanea pumila) and the bush chinkapin chestnut (Castanea alnifolia) are two other chestnut varieties native to North America. They can be affected by blight, but some are able to survive the disease—unlike the American chestnut. The European chestnut (Castanea sativa) is also susceptible to blight, but it is not native to North America. It has been affected by blight in North America and Europe. Other trees such as white oak, scarlet oak, staghorn sumac, and red maple sometimes contract chestnut blight.
Overview
The fungus that causes chestnut blight is Cryphonectria parasitica, formerly known as Endothia parasitica. This fungus originally came from Asian chestnut trees that were shipped to the United States and sold. The fungus infects trees when it enters through wounds or cracks in the bark. The fungus begins growing under the bark and spreads. The fungus creates either a raised or a sunken canker on the bark. Yellow or orange spores grow from the canker. These colored cankers are the telltale indication of chestnut blight. After the canker forms, it girdles, or cuts through, the branch or the trunk where it formed. In many affected trees, the tree dies above the area where the canker formed. Eventually, the entire tree dies. Although the chestnut blight kills the trunk and branches of trees, it does not kill the root system. After a tree dies from chestnut blight, its root system often survives. Sometimes roots will continue to grow shoots. These shoots, however, will contract blight and die just as the original tree did. Since the root system continues, American chestnut trees are not officially considered extinct. Small American chestnut sprouts grow in many areas where the trees used to stand. Trees that are found to be infected with chestnut blight—even those that will not die from it—should be pruned or cut down to prevent the spread of the disease.
Since the roots remain intact, scientist can still grow American chestnut tree shoots. Scientists are using these original American chestnut plants to create new plants. Scientists hope to create plants that are very similar to the American chestnut tree but are resistant to chestnut blight. Scientists are using different methods to create blight-resistant chestnut trees. Some scientists are breeding American chestnut trees with other blight-resistant chestnut trees. Since Asian chestnuts are mostly impervious to the disease, scientists are breeding them with American chestnut trees. Scientists want the large size, strong wood, and sweet nuts from the American chestnut and the disease resistance from the other chestnuts. By breeding the trees, scientists have developed a chestnut tree that is 15/16 American chestnut and 1/16 Asian chestnut. They hope this tree will resist blight but maintain many positive characteristics of the American chestnut.
Scientists also are working to restore chestnuts to the American forest. These scientists are changing the trees' genes to make them more resistant to blight. Scientists hope to create a tree that is very similar to the American chestnut but is able to live when infected by blight. Scientists are very close to producing trees that could be planted to take the place of American chestnut trees. However, even if the new trees become replacements for the American chestnut, it will take hundreds of years and much work to restore mature chestnut trees throughout North America.
Bibliography
"The American Chestnut Research and Restoration Project." State University of New York College of Environmental Science and Forestry, www.esf.edu/chestnut. Accessed 5 June 2017.
"The American Chestnut Tree." American Chestnut Foundation, www.acf.org/the-american-chestnut. Accessed 5 June 2017.
Anagnostakis, Sandra L. "Chestnuts and the Introduction of Chestnut Blight." Connecticut Agricultural Experiment Station, www.ct.gov/caes/cwp/view.asp?a=2823&q=376754. Accessed 5 June 2017.
Anagnostakis, Sandra L. "Revitalization of the Majestic Chestnut: Chestnut Blight Disease." American Phytopathological Society, 2000, www.apsnet.org/publications/apsnetfeatures/Pages/ChestnutBlightDisease.aspx. Accessed 5 June 2017.
"Background on American Chestnut and Chestnut Blight." State University of New York College of Environmental Science and Forestry, www.esf.edu/chestnut/background.htm. Accessed 5 June 2017.
"Chestnut Blight." Missouri Botanical Garden, www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers/chestnut-blight.aspx. Accessed 5 June 2017.
"Chestnut Blight: Cryphonectria parasitica." Cornell University College of Agriculture and Life Sciences, plantclinic.cornell.edu/factsheets/chestnutblight.pdf. Accessed 5 June 2017.
"Chestnut Blight Fungus (Cryphonectria parasitica)." Columbia University, www.columbia.edu/itc/cerc/danoff-burg/invasion‗bio/inv‗spp‗summ/Cryphonectria‗parasitica.htm. Accessed 5 June 2017.