Helminthosporium Link, Mag. Gesell. naturf. Freunde, Berlin 3 (1-2): 10 (1809).
= Helminthosporiella Hern. -Restr., Sarria & Crous, in Crous et al., Persoonia 36: 437 (2016).
Saprobic or parasitic on host. Sexual morph: massarina- or splanchnonema-like. Pseudostromata formed in the upper bark, usually well-developed, dark (reddish) brown, pseudoparenchymatous, of thick-walled dark brown cells; margin composed of dark brown, verrucose hyphae; less commonly rudimentary and composed of thin-walled, smooth brown hyphae. Ascomata immersed in pseudostromata or upper bark, variably elevating the latter, singly or in small groups, large, globose to depressed globose, often strongly depressed, dark brown to black. Peridium pseudoparenchymatous. Ostioles central, inconspicuous, not protruding above the cortical surface. Hamathecium consisting of numerous filiform, septate, branched, anastomosing, narrow pseudoparaphyses usually embedded in a gel matrix. Asci clavate or fusoid, containing 8 ascospores in irregularly biseriate arrangement, rarely 4 in uniseriate arrangement. Ascospores mostly large, hyaline or first hyaline to pale brown and turning medium to dark brown at full maturity, fusoid, broadly fusoid, subellipsoid, obovoid, less commonly oblong, asymmetric, with 1 eccentric primary septum and often with transverse or oblique distosepta, less commonly ring-like thickenings, in one or both parts, rarely with a longitudinal distoseptum in the larger part, strongly constricted at the primary septum, slightly or not constricted at the secondary distosepta, with subacute to rounded end cells; wall hyaline or brown, smooth or verruculose, sometimes with longitudinal striae; with granular to guttulate contents; each part surrounded by a thick gelatinous sheath. Asexual morph: Colony on natural substrate conspicuous, effuse to punctiform and hairy, or pulvinate, brown to black. Mycelium immersed in the substrate. Stromata usually present. Conidiophores arising solitarily or in fascicles from substrate hyphae or stroma cells, erect, simple, straight or flexuous, brown, single-, few- to many celled, with a well-defined small pore at the apex, commonly also with lateral pores beneath the upper septa, ceasing growth with the formation of a terminal conidium, usually not proliferating. Conidia formed singly (rarely in short chains), sub hyaline to brown, obclavate, obpyriform to lageniform, commonly rostrate, distoseptate, usually with a distinct dark brown to black scar at the base. Cultures on MEA and CMD in most taxa slow growing (fast in H. quercinum and H. velutinum), white, shades of brown or grey, rarely orange on MEA (H. austriacum), occasionally (H. austriacum and H. tiliae) with pigment diffusing into agar, odour in most species unpleasant (Adapted from Voglmayr & Jaklitsch, 2017).
Type species: Helminthosporium velutinum Link
Notes: Helminthosporium was introduced by Link (1809) with H. velutinum as type species. Helminthosporium is characterized by immersed ascomata, inconspicuous ostiole not protruding above the cortical surface, clavate or fusoid asci and large, hyaline to dark brown fusoid, broadly fusoid, subellipsoid, obovoid ascospores surrounded by a thick gelatinous sheath. The asexual morph is characterized by effuse to punctiform colony, sub hyaline to brown, obclavate, obpyriform to lageniform conidia. Helminthosporium has a very complex taxonomic history. Several taxa in Helminthosporium are not congeneric with the generic type. Link (1809) described Helminthosporium and Exosporium in the same publication. Fries (1832) synonymized Exosporium with Helminthosporium and placed the generic type, Exosporium tiliae in Helminthosporium. Hughes (1958) mentioned that the difference between pleurogenous versus acrogenous conidia is inappropriate for generic delimitation and broadened the generic concept of Helminthosporium to include taxa with acrogenous conidia. Ellis (1961), Luttrell (1963, 1964) carried out detailed morphological analyses and restricted taxa characterized by porogenous, distoseptate conidia with conidial scars comprising simple, flat-ringed pores and acropleurogenous conidia produced on septate, erect conidiophores which stop growing after the formation of terminal conidia. Ellis (1961) synonymized several species with H. velutinum based on an extensive morphological description, and accepted ten species in the genus. Following this restricted circumscription, several pathogenic taxa on Poaceae were transferred from Helminthosporium to other genera such as Bipolaris, Curvularia, Exserohilum, Pyrenophora in Pleosporaceae (Sivanesan, 1987; Hyde et al., 2013; Tanaka et al., 2015). Siboe et al. (1999) provided a list of 27 accepted species for Helminthosporium, and summarized their main diagnostic morphological characters. There are only few records of sexual morphs of Helminthosporium, and most of them are doubtful as they have not been verified by sequence data. Hughes (1953) mentioned the asexual morph of Helminthosporium in a British Massaria species culture isolated from Quercus but he provided no morphological details of the observation. Barr (1993) mentioned Helminthosporium cf. velutinum as the asexual morph of Splanchnonema quercicola but without morphological details and this sexual-asexual link was not confirmed by culture or molecular studies. Shoemaker and LeClair (1975) reported the important morphological differences between Massaria and Helminthosporium and considered these two genera as distinct. Subramanian and Sekar (1987) described Splanchnonema kalakadense as the sexual morph of H. velutinum based on cultural studies. Tanaka et al. (2015) described a massarina-like sexual morph for H. massarinum based on cultural and molecular studies. In the phylogenetic analysis of Kodsueb et al. (2007), Hyde et al. (2013) and Tanaka et al. (2015), the generic type, H. velutinum formed a distinct lineage in Massarinaceae. While studying hyphomycetes, several authors reported that Helminthosporium comprises species complexes (Shenoy et al., 2006; Bärlocher, 2010; Baschien et al., 2013). Voglmayr and Jaklitsch (2017) collected several splanchnonema-like fungi which were closely linked with Helminthosporium, Corynespora and Exosporium-like asexual morphs and provided a taxonomic revision of Helminthosporium. Voglmayr and Jaklitsch (2017) emended the generic concept of Helminthosporium and included taxa classified in Corynespora and Exosporium. Taxa in the graminicolous “Helminthosporium” complex were separated in different genera namely Bipolaris, Curvularia, Drechslera and Exserohilum (Pleosporales) while others were transferred to families of Corynesporaceae, Massarinaceae, Mycosphaerellaceae (Dothideomycetes) and other distinct Ascomycetes (Voglmayr and Jaklitsch 2017). The sexual morph of H. massarinum is different from those of Massaria and Splanchnonema in its ellipsoidal hyaline ascospores, and resembles Massarina, even though, the type species of Massarina, M. eburnea does not have a hyphomycetous asexual morph like that of Helminthosporium. Hongsanan et al. (2020a) synonymized Helminthosporiella under Helminthosporium based on morphology namely terminal polytretic conidiogenous cells and distoseptate conidia and phylogenetic evidence of multigene analyses of ITS, LSU, RPB2, SSU and TEF1 sequence data. Helminthosporiella is similar to Helminthosporium in having terminal polytretic conidiogenous cells and distoseptate conidia but differs in that Helminthosporiella has catenate conidia while Helminthosporium has both terminal and intercalary conidiogenous cells and solitary conidia (Crous et al., 2016; Voglmayr & Jaklitsch, 2017). Helminthosporium is morphologically and phylogenetically a distinct genus in Massarinaceae but is currently heterogeneous. Further revision of the genus is needed to enable a natural classification as several species may belong to other genera. Molecular markers available for Helminthosporium include LSU, SSU, ITS, TEF1 and RPB2.