Ascochyta
Ascochyta Lib., Pl. crypt. Arduenna, fasc. (Liège) 1(Praef.): 8 (1830).
Index Fungorum number: IF 7239; Facesoffungi number: FoF 07121, 723 morphological species (Species Fungorum 2022), 18 species with molecular data.
Pathogenic or saprobic on host. Sexual morph: Ascomata pseudothecial, immersed or erumpent, subglobose to flattened, or irregular, solitary or confluent, ostiolate, sometimes developing an elongated neck. Asci subcylindrical to subclavate, or saccate, sometimes slightly curved, 8-spored, bitunicate, sometimes short-stipitate. Pseudoparaphyses filamentous, hyaline, thin-walled, septate, conspicuous in immature fructifications, and disappear at maturity. Ascospores ovoid to ellipsoidal, slightly biconic, hyaline to yellowish into the ascus, may become brown when released, smooth, 1- septate, sometimes 3-septate, symmetrical or asymmetrical, constricted at the septum, uniseriate or 2-seriate. Asexual morph: Conidiomata pycnidial, subglobose or ampulliform to mammiform, sometimes irregularly shaped, superficial on or immersed into the agar, solitary or confluent, ostiolate or poroid opening formed at the end of the growing process. Pycnidial wall pseudoparenchymatous, 1–8-layered, outer wall pigmented. Conidiogenous cells annellidic or phialidic, hyaline, smooth, variable in shape, i.e., subglobose, cylindrical, flask-shaped, obpyriform, ampulliform to doliiform. Conidia variable in shape, i.e. ovoid, oblong, subcylindrical, ellipsoidal, cymbiform, allantoid, straight or slightly curved, hyaline or sometimes slightly coloured (yellow to pale brown), smooth- and thin-walled, aseptate or septate, mostly uniseptate, sometimes 2–3-septate, eguttulate or guttulate. Chlamydospores occasionally occur in old cultures (adapted from Chen et al. 2015).
Type species: Ascochyta pisi Lib.
Notes: Ascochyta is characterised by pycnidial, subglobose or ampulliform to mammiform conidiomata, annellidic or phialidic, hyaline, smooth conidiogenous cells, and ovoid, oblong, subcylindrical, ellipsoidal conidia with chlamydospores occasionally found in mature cultures. The sexual morph is characterised by pseudothecial, immersed or erumpent ascomata, subcylindrical to subclavate, or saccate asci and ovoid to ellipsoidal, slightly biconic ascospores. Ascochyta resembles Phoma as both genera are highly similar in morphology, physiology, pathogenicity and nucleotide sequences (Aveskamp et al. 2010). Ascochyta differs from Phoma in producing septate conidia both in vivo and in vitro while the latter rarely produces septate conidia in vitro but often in culture (Aveskamp et al. 2008; de Gruyter et al. 2009). Boerema and Bollen (1975) distinguished Phoma from Ascochyta based on differences in conidiogenesis and conidial septation. Ascochyta forms conidia that are produced from periclinal annellations and form holoblastic conidia while Phoma produce conidia from phialides with distinct collarettes and conidial euseptation forms independently from conidiogenesis, mainly after conidial secession (Boerema and Bollen 1975; Aveskamp et al. 2010). Punithalingam (1979) revised Ascochyta and stated that holoblastic conidiogenesis form for a short term while phialidic conidiogenesis occurs until the end of conidial development. Punithalingam (1979) mentioned that conidial development and septation should not be used as a morphological recognition criterion to delineate species in Ascochyta and Phoma. Kirk et al. (2001) and Hyde et al. (2013) accepted Ascochyta in Didymellaceae. Molecular markers available for Ascochyta are LSU, SSU, ITS, TEF-1, BTUB and RPB2. LSU sequences have less resolving power, followed by ITS (Chen et al. 2015). All Ascochyta taxa have nearly identical ITS sequences (Peever et al. 2007) while translation elongation factor 1-alpha (TEF-1) and β-tubulin (BTUB) genes seems to have greater resolving power in phylogenetic analyses (Aveskamp et al. 2010; Chen et al. 2015). Ascochyta is morphologically and phylogenetically a distinct genus in Didymellaceae but requires revision as several species currently lack sequence data and might belong to other genera.
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