Exosporium
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.
Associated with leaf spot. Sexual morph: Pseudostromata weakly developed, rudimentary. Ascomata depressed subglobose, immersed in the upper bark, somewhat elevating the bark, single, dark brown. Peridium pseudoparenchymatous, dark brown, black in KOH. Ostioles central, not protuberant above the bark surface. Hamathecium comprising densely packed filiform, septate, branched, anastomosing, pseudoparaphyses embedded in a gelatinous matrix. Asci clavate or fusoid, generally containing 8 irregularly biseriate, rarely 4 uniseriate ascospores. Ascospores at first light brown, turning dark brown at maturity, obovoid to fusoid, clearly asymmetric, initially 1-septate, becoming 2–3 transverse, sometimes oblique distosepta in the larger and 1(–2) in the smaller hemisphere, hardly with a longitudinal septum in the larger hemisphere, strongly constricted at the main septum, slightly or not constricted at the secondary distosepta, with subacute to rounded end cells, wall finely verrucose, brown, the contents granular, each hemisphere surrounded by a thick gelatinous sheath (adapted from Voglmayr and Jaklitsch 2017). 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 (adapted from Videira et al. 2017).
Type species: Exosporium tiliae Link
Notes: Exosporium is characterised by discrete and punctiform or effuse, hairy, brown to black colonies, 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. In the phylogenetic analysis of Crous et al. (2011), a strain of E. tiliae (CBS 484.77, CBS H-713, Québec, Canada) clustered in Pleosporales. Voglmayr and Jaklitsch (2017) emended the generic description of Helminthosporium and transferred some species previously classified in Corynespora and Exosporium to Helminthosporium. Voglmayr and Jaklitsch (2017) synonymized Exosporium tiliae as Helminthosporium tiliae. Exosporium differs from Corynespora in having short, 0–2-septate conidiophores with a single apical pore (Ellis 1960). In the phylogenetic analyses of Videira et al. (2017), the position of Exosporium fluctuates with species clustering between Schizothyriaceae and Dissononiaceae in different analyses and there is no strong statistical support. Videira et al. (2017) tentatively included Exosporium in Mycosphaerellaceae based on phylogenetic analyses but the placement might change when more taxa are added. The type species of Exosporium, Exosporium tiliae has not been sequenced yet. Exosporium is a tentative genus in Mycosphaerellaceae until its phylogeny based on the type species is resolved. In this study we illustrate a specimen of E. tiliae (CUP-020505) as we were unable to borrow the type specimen. Molecular markers available for Exosporium are ITS, LSU, BTUB, RPB2 and TEF-1.
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