Prosthemium (teleomorph Pleomassaria ) and Asterosporium (teleomorph unknown) are coelomycetous genera with stellate conidia on Fagales trees. Their morphological resemblance suggests their close relationship, but phylogenetic relatedness remains unknown. They have been distinguished on the basis of either conidiomatal morphology (pycnidia in Prosthemium and acervuli in Asterosporium) or their differing conidial septation (euseptate in Prosthemium and distoseptate in Asterosporium). To reveal their phylogenetic affinities and clarify reliable distinguishing phenotypical characters, five species of Prosthemium and two species of Asterosporium were investigated using sequences of the small subunit, large subunit, and internal transcribed spacer region of nuclear ribosomal DNA and β-tubulin gene from 43 isolates of these species. The analyses revealed the following: (i) Asterosporium typified by Asterosporium asterospermum on Fagus is a member of the Sordariomycetes and is distinct from Prosthemium belonging to the Dothideomycetes; (ii) Asterosporium betulinum on Betula species should be excluded from Asterosporium and transferred to Prosthemium as Prosthemium neobetulinum nom. nov.; (iii) conidial septation does not seem to have a diagnostic value, whereas conidiomatal morphology is useful in distinguishing both genera; (iv) the number and length of conidial arms are useful criteria in distinguishing Prosthemium species. A new species, Prosthemium intermedium sp. nov., found on both Betula ermanii and Betula maximowicziana , is described and illustrated.

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Transfer of Asterosporium orientale to the genus Prosthemium (Pleosporales, Ascomycota): a common coelomycetous fungus with stellate conidia occurring on twigs of Betula spp.Asterosporium, typifi ed by A. asterospermum (Pers.) S. Hughes, is also a lignicolous genus occurring on Fagus and Betula as endophytes (Sutton 1980;Danti et al. 2002). No teleomorph was found for this genus. The important criterion to distinguish Asterosporium from Prosthemium is the acervular conidiomata rather than pycnidia (Morgan-Jones and Kendrick 1972). Sutton (1980), however, regarded the conidiomata of Prosthemium also as acervular to eustromatic and weighted to the difference of conidial septation, namely, distoseptate in Asterosporium instead of euseptate conidia.In this study, we compared the morphological features of two similar species in these two genera, namely, P. asterosporum T. Kowalski & Holdenr. and A. orientale Melnik. Asterosporium orientale was originally described from twigs of B. ermanii in Russia by Mel'nik (1988). However, except for the presence of stellate conidia, the general characteristics of A. orientale do not fi t within the generic concept of Asterosporium typifi ed by A. asterospermum (Hughes 1958;Morgan-Jones and Kendrick 1972). In terms of the presence of pycnidial conidiomata and euseptate conidia, A. orientale has close affi nities with P. betulinum, the type of Prosthemium. In Prosthemium, seven taxa have been previously recognized as species of the genus (