Index Fungorum number: IF 8495
Synonym: Exosporium Link, Mag. Gesell. naturf. Fre- unde, Berlin 3(1–2): 9 (1809)
Saprobic on submerged wood, various dead plant material or rarely parasitic on plants or fungicolous. Sexual morph: Pseudostromata dark brown or reddish brown, pseudoparenchymatous, of thick-walled dark brown cells; margin mostly composed of dark brown, verrucose hyphae. Ascomata immersed in pseudostromata, solitary or in small groups, large, often strongly depressed, or globose, dark brown to black, with a central, inconspicuous ostiole. Peridium pseudoparenchymatous. Pseudoparaphyses numerous, cellular, septate, branched, anastomosing, narrow, usually embedded in a gel matrix. Asci 8-spored, bitunicate, fissitunicate, clavate or fusoid, short pedicellate. Ascospores irregularly biseriate, fusoid, ellipsoidal, obovoid, occasionally oblong, asymmetric, with one primary, eccentric, deeply constricted euseptum, upper cell longer and larger than lower cell, often with several transverse or oblique distosepta, rarely with a longitudinal distoseptum in the larger cell, slightly or not constricted at the secondary distosepta, hyaline when young, becoming medium to dark brown when mature, with subacute to rounded end cells, smooth or verruculose, sometimes with longitudinal striae, surrounded by a thick gelatinous sheath (Voglmayr and Jaklitsch 2017). Asexual morph: Colonies on natural substrate conspicuous, effuse to punctiform, hairy, brown to black. Mycelium mostly immersed, composed of branched, septate hyphae. Conidiophores macronematous, mononematous, arising solitarily or in fascicles, erect, mostly unbranched, straight or flexuous, cylindrical or subulate, with a well-defined small pore at the apex, and often with several lateral pores beneath the upper septa, ceasing growth with the formation of a terminal conidium, usually not proliferating, pale to dark brown, smooth or occasionally verruculose. Conidiogenous cells enteroblastic, polytretic, integrated, determinate, terminal and intercalary, subcylindrical. Conidia mostly solitary, acropleurogenous, obclavate, obpyriform to lageniform, mostly rostrate, straight or curved, pale brown to brown, distoseptate, smooth, with a paler apical cell and truncate base, often with a prominent, dark brown or black scar at the base (Ellis 1971; Zhu et al. 2016; Voglmayr and Jaklitsch 2017). (Descriptions from Dong et al. 2020).
Type species: Helminthosporium velutinum Link [as ‘Helmisporium’]
Notes: Helminthosporium is an old genus which was introduced in 1809 and currently comprises over 750 epithets in Index Fungorum (2020). Helminthosporium is morphologically similar to Corynespora Güssow and Exosporium Link as all of them have distoseptate conidia with tapering apex and truncate base, growing through a wide pore at the apex of the conidiophores (Ellis 1971; Voglmayr and Jaklitsch 2017). Exosporium has been synonymized with Helminthosporium based on their very similar morphological characters (Fries 1832) and this was accepted by Voglmayr and Jaklitsch (2017) with molecular data supported. Corynespora differs from Helminthosporium as conidiophores have successive proliferations (up to four), while they are not described in Helminthosporium (Voglmayr and Jaklitsch 2017). Phylogenetically, Corynespora has been placed in Corynesporascaceae, while Helminthosporium clustered in Massarinaceae (Voglmayr and Jaklitsch 2017).
The massaria- and splachnonema-like sexual morphs have been linked to Helminthosporium based on cultural studies and sequence data (Tanaka et al. 2015; Voglmayr and Jaklitsch 2017). Both sexual and asexual morphs can be observed in H. massarinum Kaz. Tanaka et al., H. microsorum D. Sacc., H. oligosporum (Corda) S. Hughes, H. quercicola (M.E. Barr) Voglmayr & Jaklitsch, H. quercinum Voglmayr & Jaklitsch. and H. tiliae (Link) Fr. (Tanaka et al. 2015; Voglmayr and Jaklitsch 2017). The sexual morph of the type species H. velutinum Link is still unknown. The sexual genus Byssothecium has a relationship with Helminthosporium in Massarinaceae, however, they can be distinguished by ascospore characters.
Helminthosporium species are commonly collected from terrestrial habitats worldwide as mentioned in Voglmayr and Jaklitsch (2017). Zhu et al. (2016) reported two species H. aquaticum H.Y. Su et al. and H. velutinum (type) from freshwater habitats in China (see list below) and no sexual morphs were observed. We do not make a key for these two species as they are morphologically very similar and they have overlapping size in all structures. Even though, multigene phylogenetic analyses supported them to be different species (Zhu et al. 2016; Voglmayr and Jaklitsch 2017). We believe that sexual morphs of H. aquaticum and H. velutinum will separate them clearly in future. (Notes from Dong et al. 2020).
List of freshwater Helminthosporium species (Dong et al. 2020)
Helminthosporium aquaticum H.Y. Su, Z.L. Luo & K.D. Hyde, Phytotaxa 253(3): 184 (2016)
Freshwater distribution: Yunnan, China (Zhu et al. 2016)
Helminthosporium velutinum Link [as ‘Helmisporium’], Mag. Gesell. naturf. Freunde, Berlin 3(1-2): 10 (1809)
Freshwater distribution: Yunnan, China (Zhu et al. 2016)
Dong W, Wang B, Hyde KD, McKenzie EHC, Raja HA, Tanaka K, Abdel-Wahab MA, Abdel-Aziz FA, Doilom M, Phookamsak R, Hongsanan S, Wanasinghe DN, Yu X-D, Wang G-N, Yang H, Yang J, Thambugala KM, Tian Q, Luo Z-L, Yang J-B, Miller AN, Fournier J, Boonmee S, Hu D-M, Nalumpang S, Zhang H (2020) Freshwater Dothideomycetes. Fungal Divers 105:319–575
Ellis MB (1971) Dematiaceous hyphomycetes. Commonwealth Mycological Institute, Surrey, England
Tanaka K, Hirayama K, Yonezawa H, Sato G, Toriyabe A, Kudo H, Hashimoto A, Matsumura M, Harada Y, Kurihara Y (2015) Revision of the Massarineae (Pleosporales, Dothideomycetes). Stud Mycol 82:75–136
Voglmayr H, Jaklitsch WM (2017) Corynespora, Exosporium and Helminthosporium revisited–new species and generic reclassification. Stud Mycol 87:43–76
Zhu D, Luo ZL, Baht DJ, McKenzie EHC, Bahkali AH, Zhou DQ, Su HY, Hyde KD (2016) Helminthosporium velutinum and H. aquaticum sp. nov. from aquatic habitats in Yunnan Province. China. Phytotaxa 253:179–190