Nematostoma Syd. & P. Syd., Annls mycol. 12(2): 161 (1914).
Index Fungorum number: IF 3443; Facesoffungi number: FoF 07906, 17 morphological species (Species Fungorum 2021), molecular data unavailable.
Parasitic on living leaves. Sexual morph: Ascomata superficial, globose to subglobose, dark brown, with apical ostiole, surrounded by long, septate, dark brown setae, tapering to acute at the apex. Peridium comprising 2–3 layers of dark brown cells of textura angularis. Hamathecium comprising numerous cylindrical, filiform, branched, septate, anastomosed, hyaline, pseudoparaphyses. Asci 8-spored, bitunicate, cylindric-clavate, sessile or with short pedicel, apically thickened and rounded. Ascospores 2-seriate, ellipsoid to fusiform, upper part wider, slightly curved, multi-septate, constricted at the septa, hyaline to light brown, smooth-walled. Asexual morph: Undetermined.
Type species: Nematostoma artemisiae Syd. & P. Syd.
Notes: Nematostoma was introduced by Sydow and Sydow (1914) with N. artemisiae as type species. Nematostoma resembles Acanthostigmella but differs in the ecology, as a plant parasite. von Arx and Müller (1975) included Nematostoma in Dimeriaceae based on its parasitic lifestyle. Barr (1997) revised and accepted Nematostoma in Pseudoperisporiaceae and this placement was followed by several authors (Kirk et al. 2008; Hyde et al. 2013) Boonmee et al. (2017) re-studied the type specimen N. artemisiae and retained the genus in Pseudoperisporiaceae based on its parasitic lifestyle, subiculum bearing ascomata, ostioles surrounded by long setae, bitunicate asci and 3-septate, light brown ascospores.