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elm yellows
Elm yellows is a plant disease of elm trees that is spread by leafhoppers or by root grafts.〔"Elm Yellows." Elmcare.Com. 19 Mar. 2008 .
〕 Elm yellows, also known as elm phloem necrosis, is very aggressive, with no known cure. Elm yellows occurs in the eastern United States, and southern Ontario in Canada. It is caused by phytoplasmas which infect the phloem (inner bark) of the tree.〔Price, Terry. "Wilt Diseases." Forestpests.Org. 23 Mar. 2005. 19 Mar. 2008 . 〕 Similar phytoplasmas, also known confusingly as 'Elm yellows', also occur in Europe.〔Conti, M., D'Agostino, G., Mittembergher, L. (1987) A recent epiphytotic of elm yellows in Italy. ''Proceedings of the 7th Congress of the Mediterranean Phytopathological Union'', 208–209. Consejeria de Agricultura y Pesca de la Junta de Andalucia, Granada, Spain〕 Infection and death of the phloem effectively girdles the tree and stops the flow of water and nutrients. The disease affects both wild-growing and cultivated trees.
== Importance ==
Elms are very important to the American landscape, prized for their unique shade characteristics. Most native elms are susceptible to elm yellows and there are few resistant cultivars.〔 Large, healthy, landscaped elm trees can easily be worth thousands of dollars.
Penn State University is home to one of the oldest and largest elm stands in the country. Penn State has been battling Dutch elm disease for many decades, and the recent introduction of elm yellows into the PSU campus poses many threats.〔Ruskin, Paul. "Penn State Prepares for 'Elm Yellows' Disease." PSU Live 12 Nov.
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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