Actidium Fr., Observ. mycol. (Havniae) 1: 190 (1815).
MycoBank number: MB 58; Index Fungorum number: IF 58; Facesoffungi number: FoF 08102; 6 morphological species (Species Fungorum 2020), molecular data unavailable.
Type species – Actidium hysterioides Fr., Observ. mycol. (Havniae) 2: 353 (1818).
Notes – This genus was established by Fries (1823) to accommodate A. hysterioides, a stellate mytilinidioid fungus found on Pinus and Picea in Europe, with two-celled, symmetric ascospores, light olive to reddish brown, later noted to be faintly longitudinally striate (Barr 1990a). Fries (1823) noted its similarity with Glonium. Zogg (1962) and Barr (1990b) recognised four species, A. hysterioides, A. baccarinii, both from Europe, A. pulchra, from China, and A. nitidum. Due to similarities in ascospore morphology, Actidium may have affinities with other didymospored hysteriaceous genera (e.g. Actidiographium, Glonium and Psiloglonium), although molecular data are lacking.