Exosporium Link, Mag. Gesell. naturf. Freunde, Berlin 3(1–2): 9 (1809).
Index Fungorum number: IF 8240; Facesoffungi number: FoF 08499, 42 morphological species (Species Fungorum 2021), 3 species with molecular data.
Pathogenic on host. Asexual morph: Colonies discrete and punctiform or effuse, hairy, brown to black. Mycelium immersed. Stroma usually present, often very well-developed. Setae and hyphopodia absent. Conidiophores macronematous, mononematous, often caespitose, straight or flexuous, unbranched, or very rarely branched, mid to dark brown or olivaceous brown, smooth or verruculose. Conidiogenous cells polytretic, integrated, terminal, becoming intercalary, sympodial, cylindrical, or clavate, cicatrized, conidiogenous loci (scars) often dark and prominent. Conidia usually solitary, short catenate in one species, acropleurogenous, simple, mostly obclavate, pale to dark brown or olivaceous brown, smooth, verrucose or echinulate, distoseptate, generally with a thick, dark hilum at the base. Sexual morph: Unknown (Adapted from Videira et al. 2017).
Type species: Exosporium tiliae Link
Notes: Exosporium was introduced by Link (1809) with E. tiliae as type species. Exosporium is characterized by discrete and punctiform or effuse, hairy, brown to black colobies, macronematous, mononematous conidiophores, polytretic, integrated, terminal conidiogenous cells and solitary verrucose or echinulate conidia. Link (1809) described the genera Helminthosporium and Exosporium in the same publication and Fries (1832) synonymized Exosporium with Helminthosporium. Voglmayr and Jaklitsch (2017) emended the generic description of Helminthosporium and transferred some species previously classified in Corynespora and Exosporium to Helminthosporium. In the phylogenetic analysis of Crous et al. (2011), a strain of E. tiliae (CBS 484.77, CBS H-713, Québec, Canada) cluster in Pleosporales but is shown to be Corynespora taxa in the C. olivacea complex occurring on Tilia. Exosporium differs from Corynespora in having short, 0–2-septate conidiophores with a single apical pore (Ellis 1960). Voglmayr and Jaklitsch (2017) emended the generic description of Helminthosporium and transferred some species previously classified in Corynespora and Exosporium to Helminthosporium. Currently, Exosporium is a distinct genus in Mycosphaerellaceae but the type species E. tiliae has not been sequenced yet. In this study we illustrate a specimen of E. tiliae under the code CUP-020505 as we could not loan the type specimen.