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chestnut blight
The pathogenic fungus ''Cryphonectria parasitica'' (formerly ''Endothia parasitica'') is a member of the Ascomycota (sac fungi) taxon, and is the main cause of chestnut blight—a devastating disease of the American chestnut tree that in the early 1900s caused a rapid, widespread die-off of this once plentiful tree from its historic range in the eastern United States.
== Overview ==
The chestnut blight is a fungal infection affecting the American Chestnut tree that had a devastating economic and social impact on communities in the eastern United States. It later spread to other parts of the world including Italy.〔http://www.fao.org/docrep/x5348e/x5348e03.htm〕 The fungus is spread by wind-borne ascospores and, over a shorter distance, conidia distributed by rain-splash action.〔http://www.forestpathology.org/dis_chestnut.html〕 In the first half of the 20th Century it killed an estimated 4 billion trees. Infection is local in range, so some isolated American chestnuts survive where there is no other tree within 10 km. There are at least two viral pathogens that weaken the fungus through a mechanism termed ''hypovirulence'' that helps trees survive.〔(Hypovirulence-Infecting the infection )〕
The root collar and root system of the chestnut tree have some resistance against the fungal infection; consequently, a large number of small American chestnut trees still exist as shoots growing from existing root bases. However, these regrown shoots seldom reach the sexually reproductive stage before being killed by the fungus.〔() History of the American Chestnut〕 They only survive as living stumps, or "stools," with only a few growing enough shoots to produce seeds. This is just enough to preserve the genetic material necessary to engineer an American chestnut tree using genes from any of the disease-immune Asiatic species to confer resistance to the disease.
American chinquapin is also highly susceptible to chestnut blight. The European chestnut and the West Asian species are also susceptible, but less so than the American species. Surviving chestnut trees are being bred for resistance to the blight, notably by the American Chestnut Foundation, which aims to reintroduce a blight-resistant American chestnut to its original forest range within the early decades of the 21st century.〔http://www.fs.fed.us/r8/chestnut/qa.php〕 The resistant species—particularly Japanese chestnut and Chinese chestnut, as well as Seguin's chestnut and Henry's chestnut—have been used in these breeding programs in the US to create disease-resistant hybrids with the American chestnut.〔''New RHS Dictionary of Gardening''. By A. Huxley ed. 1992. Macmillan ISBN 0-333-47494-5.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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