Xenostomella
Xenostomella Syd., Annls mycol. 28(1/2): 130 (1930).
= Plochmothea Syd., Annls mycol. 37(4/5): 395 (1939).
Index Fungorum number: IF 5826; Facesoffungi number: FoF 12535, 3 morphological species (Species Fungorum 2022), no molecular data available.
Biotrophic on upper surface of leaves. Mycelium superficial, light brown, anastomosing, and sparsely septate, with hyphopodia from which hyaline, multicellular haustoria penetrate the epidermis of the leaf. Sexual morph: Thyriothecia circular, gregarious, superficial, carbonaceous, brown to black, basal peridium poorly developed, with a central irregular dissolving ostiole; In section lenticular. Upper wall composed of irregularly interwoven cells. Peridium comprising a single stratum of 4–5 layers of brown cells of textura angularis. Hamathecium comprising asci embedded in mucilage inclined towards the center, pseudo paraphyses sparse and only apparent in section. Asci 8-spored, bitunicate, fissitunicate, obclavate or broadly fusiform, with short pedicel, ocular chamber not apparent, not bluing in IKI. Ascospores overlapping 3–4-seriate, hyaline, becoming brown at maturity, 2-celled, clavate to fusiform, upper cells wider than lower cell, lower cells tapering to rounded base, wall smooth. Asexual morph: Unknown (adapted from Wu et al. 2011).
Type species: Xenostomella tovarensis Syd.
Notes: Xenostomella is characterised by superficial, light brown mycelium, circular, gregarious, superficial, carbonaceous thyriothecia, obclavate or broadly fusiform asci and 3–4-seriate, hyaline ascospores. Chardón and Toro (1934) added X. meridensis and Müller and Arx (1962) added X. monninae. Müller and Arx (1962) placed Xenostomella in Microthyriaceae based on the presence of an ostiole in the thyriothecium, the cylindrical asci and the two-celled ascospores and this was followed by Kirk et al. (2008) and Lumbsch and Huhndorf (2010). Wu et al. (2011) examined the holotype of X. tovarensis (S 843) and reported that Xenostomella resembles Maublancia in having superficial hyphae and haustoria but whether these characters are taxonomically significant is not clear. Xenostomella differs from Maublancia in that the later has discrete ascomata and wider ascospores. Xenostomella differs from Microthyrium in having different arrangement of cells in the peridium. Wu et al. (2011) suggested the placement of Xenostomella in Asterinaceae. Lumbsch and Huhndorf (2010) and Kirk et al. (2013) referred Xenostomella to Microthyriaceae. Hyde et al. (2013) and Wijayawardene et al. (2017, 2020) referred Xenostomella to Dothideomycetes genera incertae sedis. Molecular data is lacking. Fresh collections with DNA sequence data are needed to confirm the systematic position of Xenostomella.
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