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Nothing in this license impairs or restricts the author’s moral rights.available online at www.studiesinmycology.orgStudieSin Mycology 68: 211–235. 2011.doi:10.3114/sim.2011.68.10INTRODUCTIONThe genus Plagiostoma (Gnomoniaceae, Diaporthales) includes microscopic fungi that inhabit the leaves, stems, twigs, and branches of woody and herbaceous plants from a range of families including the Betulaceae, Euphorbiaceae, Geraniaceae, Hippocastanaceae, Oleaceae, Polygonaceae, Salicaceae, Sapindaceae, and Staphylaceae in temperate regions of the Northern Hemisphere (Sogonov et al. 2008). Although some species of Plagiostomacause diseases, most do not show symptoms prior to production of perithecia on dead tissues. Described by Fuckel (1870), the morphological concept of Plagiostoma remained relatively unchanged (Barr 1978, Monod 1983) until recently. Multigene phylogenetic studies suggest that the genus Plagiostoma forms a highly supported monophyletic clade that includes the type species of Plagiostoma, P. euphorbiae, and the type species of Cryptodiaporthe, C. aesculi, among others(Mejía et al. 2008, Sogonov et al. 2008). Sogonov et al. (2008) included 13 species in the genus Plagiostoma, several of which were previously placed in Cryptodiaporthe. A brief historical account of the major taxonomic treatments of Plagiostoma and Cryptodiaporthe illustrates the views of these genera through time. Fuckel (1870) proposed the genus Plagiostoma for sphaericeaous species characterised by flattened perithecia oriented horizontally having short, lateral, erumpent necks. Fuckel (1870) included the genera Ceratostoma, Gnomonia, Linospora, Melanospora, and Rhaphidospora together with Plagiostoma in the tribe Ceratostomeae of the Sphaeriacei. In his original description of Plagiostoma, Fuckel (1870) included four species, P. euphorbiae, P. petiolicola, P devexum, and P. suspecta. Fuckel’s concept of Plagiostoma was followed by Höhnel (1917) and von Arx (1951) who, like Fuckel, considered Plagiostoma to be relatively closely related to Gnomonia, the name on which the Gnomoniaceae is based. These authors differentiated Gnomoniafrom Plagiostoma mainly by orientation of the perithecial neck. Gnomonia was characterised by having central, upright, perithecial necks in contrast to species of Plagiostoma with eccentric, laterally oriented, perithecial necks. In her treatment of the order Diaporthales, Barr (1978) followed Fuckel’s concept of Plagiostoma and placed Gnomonia and Plagiostoma in the same suborder Gnomoniineae but in different families, i.e.Gnomonia in the Gnomoniaceae and Plagiostoma in the Valsaceae. The Valsaceaewas defined based on having “beaks oblique or lateral, erumpent separately or converging through stromatic disc” (Barr, 1978 p. 15). Barr (1978) made nine new combinations in Plagiostomaexpanding the number of species in the genus to 13. In his monograph of the Gnomoniaceae, Monod (1983) accepted most species treated by Barr (1978). However, Monod considered that the typification of Plagiostoma as P. euphorbiaeby Höhnel (1917) was not representative of Plagiostoma because the perithecial necks of this species are eccentric rather than lateral as stipulated by Fuckel (1870). Monod (1983) transferred P. euphorbiae to the genus Gnomonia and re-typified Plagiostomawith P. devexum. In agreement with Barr (1991) and Sogonov et al.(2008) the typification of the genus Plagiostoma with P. euphorbiaeby Höhnel (1917) is accepted here because this typification predates Monod (1983) and is in accordance with Article 10 of the International Code of Botanical Nomenclature (McNeill et al. 2006). AsystematicaccountofthegenusPlagiostoma(Gnomoniaceae,Diaporthales)basedonmorphology,host-associations,andafour-genephylogenyL.C. Mejía1, 2, 3*, L.A. Castlebury1, A.Y. Rossman1, M.V. Sogonov4 and J.F. White, Jr.21Systematic Mycology & Microbiology Laboratory, USDA-ARS, Rm. 304, B010A, 10300 Baltimore Ave., Beltsville, MD 20705, USA; 2Department of Plant Biology and Pathology, Rutgers University, New Brunswick, NJ 08901, USA; 3Smithsonian Tropical Research Institute Apartado 0843-03092 Balboa, Ancon, Republic of Panama; 4EMSL Analytical, Inc., 10768 Baltimore Ave., Beltsville, MD 20705, USA*Correspondence: Luis Mejía, MejiaLC@si.eduAbstract: Members of the genus Plagiostoma inhabit leaves, stems, twigs, and branches of woody and herbaceous plants predominantly in the temperate Northern Hemisphere. An account of all known species of Plagiostoma including Cryptodiaporthe is presented based on analyses of morphological, cultural, and DNA sequence data. Multigene phylogenetic analyses of DNA sequences from four genes (β-tubulin, ITS, rpb2, and tef1-α) revealed eight previously undescribed phylogenetic species and an association between a clade composed of 11 species of Plagiostoma and the host family Salicaceae.In this paper these eight new species of Plagiostoma are described, four species are redescribed, and four new combinations are proposed. A key to the 25 accepted species of Plagiostoma based on host, shape, and size of perithecia, perithecial arrangement in the host, and microscopic characteristics of the asci and ascospores is provided. Disposition of additional names in Cryptodiaporthe and Plagiostoma is also discussed.Keywords:Ascomycota, Betulaceae, epitypification, Fraxinus, new species, phylogeny, Salicaceae, Sordariomycetidae.Taxonomicnovelties:Plagiostomadilatatum L.C. Mejía, sp. nov., Plagiostoma exstocollum L.C. Mejía, sp. nov., Plagiostoma imperceptibile L.C. Mejía, sp. nov., Plagiostomaoregonense L.C. Mejía, sp. nov., Plagiostoma ovalisporum L.C. Mejía, sp. nov., Plagiostoma samuelsii L.C. Mejía, sp. nov., Plagiostomaversatile L.C. Mejía & Sogonov, sp. nov., Plagiostomayunnanense L.C.Mejía & Zhu L. Yang, sp. nov., Plagiostoma apiculatum (Wallr.) L.C. Mejía, comb. nov., Plagiostoma convexum (Preuss) L.C. Mejía, comb. nov., Plagiostoma populinum (Fuckel) L.C. Mejía, comb. nov., Plagiostoma pulchellum (Sacc. & Briard) L.C. Mejía, comb. nov.Open access under CC BY-NC-ND license.