Acrocalymma Alcorn & J.A.G. Irwin, Trans. Br. Mycol. Soc. 88(2): 163 (1987).

MycoBank number: MB 11008; Index Fungorum number: IF 11008; Facesoffungi number: FoF 07097; 7 morphological species (Species Fungorum 2020), 7 species with molecular data.

Saprobic on the terrestrial host plant and freshwater water habit. Sexual morph: Ascomata initially immersed, later becoming erumpent, globose, unilocular, covered with light pale grey hyphae, opening by a centrally located ostiole, with a long break. Ostiole cylindrical, filled with hyaline periphyses. Peridium composed of thick-walled, brown cells of textura angularis. Hymenium composed of numerous, hyaline, filliform, septate, anastomosed pseudoparaphyses and asci. Asci bitunicate, 8-spored, cylindrical, short pedicellate. Ascospores biseriate, initially hyaline, ultimately becoming brown or pale reddish brown, narrowly fusiform, straight or slightly curved, 1–3-septate, constricted at septa, thick-walled, guttulate, enclosed by 1–2 μm wide, hyaline sheath (adapted from Shoemaker et al. 1991). Asexual morph: Conidiomata dark brown to black, solitary to gregarious or confluent, immersed to semi-immersed, stromatic, pycnidial, globose to subglobose, pyriform, unilocular or multi-locular, ostiolate. Ostiole circular, cylindrical, straight or curved, centrally or laterally located. Conidiomatal wall composed of brown to paler, thick-walled cells of texture angularis to textura globosa or textura prismatica. Conidiophores reduced to conidiogenous cells. Conidiogenous cells hyaline, enteroblastic, phialidic, ampulliform to subcylindrical or lageniform, smooth-walled. Conidia hyaline, cylindrical to fusiform, aseptate or 1–3-septate, bearing helmet-shaped, mucoid appendage at each end.

Type speciesAcrocalymma medicaginis Alcorn & J.A.G. Irwin.

Notes – The genus Acrocalymma was introduced by Alcorn and Irwin (1987) for a single species, A. medicaginis, which was associated with a root and crown rot disease of Medicago sativa (Fabaceae). Acrocalymma is characterized by globose conidiomata with a long pycnidial beak, and hyaline, or becoming pale brown with age, cylindrical to fusiform, aseptate, or 1–3-septate conidia, bearing an appendage at one or both ends. Zhang et al. (2012a) described an additional species A. aquatica Huang Zhang & K.D. Hyde from Thailand on submerged wood in a freshwater stream. A third species, A. cycadis Crous & R.G. Shivas, was introduced by Crous et al. (2014b), and it is distinct from A. aquatica by its smaller conidia. Trakunyingcharoen et al. (2014) revised the genus Sphaerellopsis, and allied lichenicolous and other genera, and transferred an isolate previously incorrectly identified as S. filum (CBS 317.76) to Acrocalymma, as A. fici Crous & Trakun. They also synonymized the genus Rhizopycnis D.F. Farr under Acrocalymma based on morphology and DNA phylogeny, and proposed a new combination namely, A. vagum for Rhizopycnis vagum (type species of Rhizopycnis). The sexual morph of Acrocalymma medicaginis has been linked to Massarina walkeri by Shoemaker et al. (1991) in culture. However, Trakunyingcharoen et al. (2014) showed that the two isolates which produced sexual morphs in culture are not conspecific with Acrocalymma medicaginis, and made a new combination A. walkeri (Shoemaker, C.E. Babc. & J.A.G. Irwin) Crous & Trakun. (Shoemaker et al. 1991; Trakunyingcharoen et al. 2014). A new family Acrocalymmaceae was introduced in Dothideomycetes to accommodate Acrocalymma.

Distribution – Australia, India, Spain, Thailand, USA (Alcorn and Irwin 1987; Nag Raj 1993; Trakunyingcharoen et al. 2014, this study).