The pathogenic fungus, Cryphonectria parasitica (formerly Endothia parasitica), is the causal agent of chestnut blight, a devastating disease of the American chestnut tree that caused a mass extinction in the early 1900s of this once plentiful tree from its historic range in the south east United States.
The chestnut blight was accidentally introduced to North America around 1900, either through imported chestnut lumber or through imported chestnut trees. In 1905, American mycologist William Murrill studied the disease, isolated and described the fungus responsible (which he named Diaporthe parasitica), and demonstrated by inoculation into healthy plants that the fungus caused the disease. By 1940, mature American chestnut trees were virtually wiped out by the disease.
Infection of Asian chestnut trees with the chestnut blight fungus was discovered on Long Island in 1904. The blight appears to have been introduced from either China or Japan. Japanese and some Chinese chestnut trees show some resistance to infection by C. parasitica: they may be infected, but the fungus does not usually kill them. Within 40 years the near-4 billion-strong American chestnut population in Northern America was devastated – only a few clumps of trees remained in California and the Pacific northwest. Because of the disease, American Chestnut wood almost disappeared from the market for decades, although American chestnut wood can still be obtained as reclaimed lumber. The root collar and root system of the chestnut tree are fairly resistant to the fungal infection, so a large number of small American chestnut trees still exist as shoots from existing root bases. However, the shoots are seldom able to grow enough to reproduce before the blight attacks them. So they only survive as living stumps, or "stools", with only a few growing enough shoots to produce seeds shortly before dying. This is just enough to preserve the genetic material used to engineer an American chestnut tree with the minimal necessary genetic input from any of the disease-immune Asiatic species. Efforts started in the 1930s and are still ongoing to repopulate the country with these trees, in Massachusetts and many other places in the United States.
In some places such as the Appalachian Mountains, it is estimated that one in every four hardwoods was an American chestnut. Mature trees often grew straight and branch-free for 50 feet (sometimes up to one hundred feet), and could grow up to 200 feet tall with a trunk diameter of 14 feet at a few feet above ground level. For three centuries most barns and homes east of the Mississippi were made from American chestnut. The chestnut blight caused by C. parasitica has destroyed about 4 billion American chestnut trees, and dramatically reduced the tree population throughout the East Coast. The American Chinquapin (Chinkapin) chestnut trees are also very susceptible to chestnut blight. The European chestnut and the West Asian species are susceptible but less so than the American species. The resistant species (particularly Japanese chestnut and Chinese chestnut but also Seguin's chestnut and Henry's chestnut) have been used in breeding programs in the US to create hybrids with the American chestnut that are disease-resistant.
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