Karstenula Speg., Decades Mycologicae Italicae 7–12: no. 94 (in sched.) (1879).
Index Fungorum number: IF 2549; MycoBank number: MB 2549; Facesoffungi number: FoF 00042; 22 morphological species (Species Fungorum 2020), 1 species with molecular data.
Saprobic on dead wood and stems in terrestrial habitats. Sexual state: Ascomata rarely small, usually medium-sized, immersed usually under thin clypeus, scattered to gregarious, with flattened top and rounded pore-like ostiole, coriaceous. Peridium 2-layered, outer layer composed of reddish-brown to dark brown small cells, with an inner layer of pale compressed cells. Hamathecium of dense, cellular pseudoparaphyses. Asci bitunicate, fissitunicate, cylindrical to cylindro-clavate with short furcate pedicel. Ascospores ellipsoid to fusoid, reddish-brown to dark brown muriform. Asexual state: Microdiplodia (Constantinescu 1993).
Type species – Karstenula rhodostoma (Alb. & Schwein.) Speg., Decades Mycologicae Italicae 7–12: no. 94 (1879).
≡ Sphaeria rhodostoma Alb. & Schwein., Consp. fung. (Leipzig): 43 (1805).
Notes – Karstenula is an ambiguous genus due to its morphological similarities with different families (Zhang et al. 2012b, Ariyawansa et al. 2014d). For instance, Karstenula species share some similarities with Didymosphaeria in having ascomata seated in a subiculum or beneath a clypeal thickening and sometimes apical cells become reddish or orange-brown (Barr 1990a). The ascomata of Karstenula rhodostoma are similar to those found in Byssosphaeria and Herpotrichia, such as the paler area around the ostiole and even in peridium and development under a subiculum. However, based on multi-gene phylogenies Ariyawansa et al. (2014d) revealed that Karstenula is in Didymosphaeriaceae, but further collections are needed to resolve the ambiguity of this genus.