Beschreibung:
<jats:p>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.</jats:p>