Microthyrium Desm., Annls Sci. Nat., Bot., sér. 2 15: 137 (1841).
Saprobic mostly on the upper surface of leaves, appearing as very small black spots. Mycelium superficial, abundant, comprising almost colourless, branched, septate hyphae, with subcuticular haustoria. Sexual morph: Thyriothecia circular, solitary or gregarious, superficial, membranaceous or slightly carbonaceous, light brown to brown, with a poorly developed basal layer, easily separated from host surface, with a prominent darker central ostiole; in section lenticular. Upper wall comprising cells of textura epidermoidea, radiating outwardly in parallel rows from the darker central ostiole. Peridium comprising a single stratum of cells, outer cells of small heavily pigmented cells of textura epidermoidea, inner cells light brown textura angularis. Hamathecium comprising asci inclined from the base and rim towards the central ostiole, pseudoparaphyses not observe. Asci numerous, 8-spored, bitunicate, fissitunicate, narrowly obclavate to long fusiform, with small knob-like pedicel or pedicel lacking, ocular chamber not apparent, not staining blue in IKI. Ascospores overlapping 2-3–seriate, fusiform to ellipsoidal, hyaline, 1-septate, fine appendages sometimes present, smooth-walled. Asexual morph: Unknown (Adapted from Wu et al. 2011).
Type species: Microthyrium microscopicum Desm.
Notes: Microthyrium was introduced by Desmazières (1841) with M. microscopicum as type species. Microthyrium was placed in Microthyriaceae based on the ascomata with a radiating structure, called thyriothecia. Desmazières (1841) illustrated Microthyrium microscopicum from Castanea sativa and later broadened the host distribution by adding Buxus sempervirens and Quercus ilex (Ellis 1976; Wu et al. 2011). In a study of British Microthyrium species, Ellis (1976) re-described and illustrated the type species of Microthyrium microscopicum. Ellis (1976) studied the type material from a leaf Castanea vulgaris and reported mature ascospores with apical cilia pointing out that the cilia are at times noticeable depending on the material and slide preparation. Ramaley (1999) added three new Microthyrium species from dead leaves of Nolina and described the anamorph of Microthyrium guadalupensis as Zalerion guadalupensis A.W. Ramaley. Ramaley (1999) also highlighted the presence of the ascospore appendages in numerous species but he is not sure if this occurs in all species. Kirk et al. (2008) and Hyde et al. (2011) reported a Leptothyrium anamorph for Microthyrium. In the phylogenetic analyses of Schoch et al. (2009) based on 5 genes namely SSU, LSU, TEF1, RPB1 and RPB2, one strain of Microthyrium microscopicum formed a distinct lineage in Microthyriaceae. Wu et al. (2011) increase their taxon sampling in their analysis and reported that Microthyrium and Microthyriaceae are polyphyletic. Microthyrium is morphologically and phylogenetically a distinct and type genus of Microthyriaceae. Several species that lack sequence data might belong to other genera hence a monograph of Microthyrium is required in future studies. Molecular markers available for Microthyrium include LSU, SSU, ITS, TEF-1 and RPB2.