Tree Diseases: American Chestnut Blight
Chestnut blight, also called chestnut bark disease, is a fungal infection that has significantly reduced the chestnut population in North America. The fungus is spread by wind-borne ascospores and conidia disseminated by wind and rainstorms. In the first half of the twentieth century, American chestnut blight is estimated to have killed around four billion chestnut trees.
The root collar and root system of the chestnut tree exhibit some resistance to blight infection. The soil organisms react adversely to the fungus, effectively repelling it. Consequently, a large number of small American chestnut trees remain as shoots growing from existing root bases. These regrown shoots seldom reach maturity before being killed by the fungus. Instead, they survive as living stumps, with only a few developing enough shoots to produce seeds. Chestnut blight can occur on numerous other species of chestnut, albeit with less frequency. Some of the more common examples include American chinquapin, European chestnut, and West Asian chestnut.
History and Distribution
Chestnut blight was introduced to North America around 1904 when the pathogenic fungus Cryphonectria parasitica, then referred to as Endothia parasitica, was inadvertently transported into the United States via Japanese nursing stock. The disease was first reported on the grounds of the New York Zoological Garden. Herman W. Merkel, a forester at the Bronx Zoo, made the initial discovery. In 1905, an American mycologist named William Murrill was able to isolate the fungus. Through a series of experiments, Murrill ascertained that the fungus was the cause of the disease.
Meanwhile, infections of American chestnut trees became more widespread. The disease appeared simultaneously in numerous locations across the eastern United States. This was likely the result of Japanese chestnut becoming a popular import. By 1940, the American chestnut population had been nearly extinguished, with only a few clumps of trees remaining in California, Michigan, Wisconsin, and the Pacific Northwest. Prior to this devastating event, it is estimated that in some places, one in every four hardwoods was an American chestnut.
Approximately 2,500 chestnut trees have been discovered growing on 60 acres of land near West Salem, Wisconsin. This site is now recognized as the world’s largest remaining stand of American chestnut. The trees are descendants of chestnuts planted by an early settler in the area named Martin Hicks. Because the trees grew beyond the natural range of American chestnut, they were able to survive the initial outbreak. They remained free of the disease until 1987, when scientists observed blight in the stand.
On April 22nd, 2006, a small stand of surviving American chestnut was found in Franklin Delano Roosevelt State Park near Warm Springs, Georgia. The discovery was made by Nathan Klaus of the Georgia Department of Natural Resources.
Distribution in Europe
The first appearance of chestnut blight in Europe occurred in Italy in 1938. By 1939, the infection had spread from the province of Genova to the bordering province of Allesandria. Sanitary measures were adopted in an effort to halt the spread of the disease, but they were deemed ineffective. Chestnut blight can now be found along the Apennine Mountains, ranging from Liguria to Tuscany.
Disease Cycle and Symptoms
American chestnut blight is caused by the fungus Cryphonectria parasitica. The fungus enters through the wounds of susceptible trees, growing in and beneath the bark. Eventually, the fungus kills the cambium around the twig, branch, or trunk. The first symptom of chestnut blight is a discoloration of the tree’s bark. Infected areas turn orange and brown. As the mycelial fan spreads under the bark, it creates a sunken canker. As the hyphae expand, they produce several toxic compounds, the most notable of which is oxalic acid. The oxalic acid lowers the pH level of the infected tissue from around 5.5 to approximately 2.8. This increases the acidity of the tree, which is toxic to the plant’s cells. While this transpires, the canker enlarges, girdling the tree. This prevents water and nutrients from being properly distributed throughout the tree, effectively killing the foliage above it.
Distinctive yellow tendrils often referred to as cirrhi can be seen extruding from the bark during periods of heavy rainfall. Small fruiting bodies called perithecia also appear on the tree, expelling spores once they mature in fall. The spores are released once the perithecia become sufficiently moist. The fungus spreads to other nearby trees as the spores are disseminated by the wind. It may also be conveyed to other trees by animals and insects that have made contact with cankers.
The disappearance of American chestnut has had a devastating economic and social impact on the regions where it was once found. It eliminated chestnut wood from the market for decades. The reddish-brown wood had many desirable attributes. It was lightweight, soft, easy to split, and resistant to decay. This made it useful for various purposes. For three centuries, people living near the Appalachian Mountains used American chestnut wood to construct barns, cabins, furniture, and caskets. They also used it to manufacture poles, posts, piling, railroad ties, and split-rail fences. The fruit of American chestnut was an important cash crop; it was typically collected in bushels and sold to nearby towns.
Efforts to save the American chestnut population began in 1930. When the blight was first discovered, local and state governments commissioned the removal of infected trees to control the disease. This proved to be an ineffectual solution. Scientists then set out to introduce a hypovirus imported from Italy and France into the chestnut blight fungus. The trees infected with the hypovirus responded immediately, sealing over the cankered areas. But there was one major caveat: the hypovirus was so efficient, it was unable to spread from one tree to another. As such, only those chestnuts treated with the hypovirus recovered.
In the interim, a group comprised of the American Chestnut Foundation, the Wisconsin Department of Natural Resources, the USDA Forest Service, West Virginia University, Michigan State University, and Cornell University gathered to fund a program dedicated to restoring the American chestnut. The research they have conducted has helped scientists develop a more comprehensive understanding of the disease, and how it may be combated.
Hybrid Chestnut Trees
Surviving chestnut trees have been selected for breeding programs, notably by the American Chestnut Foundation. The intention of these breeding programs is to replenish American chestnut populations by introducing blight-resistant chestnuts to their original forest range. Resistant species, particularly Japanese chestnut, Chinese chestnut, Seguin’s chestnut, and Henry’s chestnut, are being hybridized with American chestnut to create new disease-resistant species. Chinese chestnuts vary considerably in their resistance to blight; some individuals are susceptible, while others are essentially immune to the disease. Environmental stressors may reduce a tree’s resistance to blight. At higher elevations in areas exposed to harsh weather, and extreme temperatures, normally resistant Oriental chestnuts can be weakened, or killed by blight.
The Forest Health Initiative, along with contributions from the New York College of Environmental Science and Forestry, Penn State, the University of George, and the US Forest Service, is currently attempting to use modern breeding techniques, and genetic engineering to create disease resistant tree strains. Their strategy is to select blight-resistant genes during back-crossing, while preserving the traits inherent to the American chestnut. The newly bred hybrid chestnuts are anticipated to reach the same heights as the American chestnut. Mature chestnuts often grew up to 100 feet tall, with a spread of fifty feet.
Transgenic Blight-Resistant Chestnut Trees
At the New York College of Environmental Science and Forestry, plant pathologists William Powell and Charles Maynard have developed blight resistant American chestnuts. Full resistance was attained by introducing a wheat gene coding for the enzyme oxalate oxidase into the American chestnut genome. The enzyme converts the oxalic acid secreted by the blight into carbon dioxide and hydrogen peroxide. Transgenic trees produced through this method have been shown to exhibit blight resistance equal to, or surpassing that of the Chinese chestnut. The New York Botanical Garden has planted several of the transgenic trees for public display.
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Photograph courtesy of Claudette Hoffman.