Neotyphodium coenophialum is a systemic and seed-transmissible symbiont (endophyte) of Lolium arundinaceum (=Festuca arundinacea; tall fescue), a grass endemic to Eurasia and North Africa, but widely naturalized in North America, Australia and New Zealand / Aotearoa. The endophyte has been identified as the cause of the "fescue toxicosis" syndrome sometimes suffered by livestock that graze the N. coenophialum-infected grass. Possible symptoms include poor weight gain, elevated body temperature, reduced conception rates, agalactia, rough hair coat, fat necrosis, loss of switch and ear tips, and lameness or dry gangrene of the feet. Because of the resemblance to symptoms of ergotism in humans, the most likely agents responsible for fescue toxicosis are thought to be the ergot alkaloids produced by N. coenophialum. 
Continued popularity of tall fescue with this endophyte, despite episodic livestock toxicosis, is attributable to the exceptional productivity and stress tolerance of the grass in pastures and hay fields. The endophyte enhances several of these characteristics, including drought tolerance, nitrogen utilization, phosphate acquisition, and resistance to nematodes. Recently, natural strains of N. coenophialum with little or no ergot alkaloid production have been introduced into tall fescue for new cultivar development. These strains are apparently not toxic to livestock, and also provide some, but not necessarily all, of the benefits attributable to the "common toxic" strains in the older tall fescue cultivars. 
Molecular phylogenetic analysis indicates that N. coenophialum is an interspecific hybrid with three ancestors: Epichloë festucae, Epichloë typhina and an undescribed or extinct Neotyphodium species that also contributed a genome to the hybrid endophyte Neotyphodium occultans, among others.
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