Phylogenetic And Taxonomic Studies On Didymellaceae

The genus Phoma is ubiquitous and species-rich, and includes many important plant pathogens, which often cause heavy loss of the plant production. Some of the Phoma species are of quarantine concern. Ascochyta and Phoma have long been considered closely related since members from both genera are often highly similar in morphology, physiology, pathogenicity and nucleotide sequences. Research efforts attempting to distinguish these genera have been carried out since Saccardoan times, but the taxonomy of these two genera remains confused. The genus Didymella is the only genus that is correctly linked to Phoma s. str. and Ascochyta, which is also poorly understood now and numerous species remained phylogenetically unresolved. The Didymellaceae was established in 2009 to accommodate Ascochyta, Didymella and Phoma, as well as several related phoma-like genera. The family contains numerous plant pathogenic, saprobic and endophytic species associated with a wide range of hosts. Ascochyta and Phoma are morphologically difficult to distinguish, and species from both genera have in the past been linked to Didymella sexual morphs.The aim of the present study was to clarify the generic delimitation of Didymellaceae by combing multi-locus phylogenetic analyses based on ITS, LSU, rpb2 and tub2, and morphological observations. The resulting phylogenetic tree revealed 17 well-supported monophyletic clades in Didymellaceae, leading to the introduction of nine genera (viz. Allophoma Q. Chen & L. Cai, Calophoma Q. Chen & L. Cai, Heterophoma Q. Chen & L. Cai, Neoascochyta Q. Chen & L. Cai, Neodidymelliopsis Q. Chen & L. Cai, Nothophoma Q. Chen & L. Cai, Paraboeremia Q. Chen & L. Cai, Phomatodes Q. Chen & L. Cai and Xenodidymella Q. Chen & L. Cai), five species (viz. Allophoma nicaraguensis Q. Chen & L. Cai, Phoma odoratissimi Q. Chen, Phoma neerlandica Q. Chen & L. Cai. Phoma segeticola Q. Chen and Stagonosporopsis helianthi Q. Chen & L. Cai), two nomina nova(Ascochyta medicaginicola var. medicaginicola Q. Chen & L. Cai and Didymella senecionicola Q. Chen & L. Cai) and 84 combinations [viz. Allophoma labilis (Sacc.) Q. Chen & L. Cai, All. minor (Aveskamp et al.) Q. Chen & L. Cai, etc.]. Furthermore,11 epitypes (viz. Ascochyta hordei var. europaea Punith., Ascochyta pisi Lib., etc.) and seven neotypes (viz. Ascochyta desmazieri Cav., Phoma heteromorpha Schulzer & Sacc., etc.) were designated to help stabilise the taxonomy and use of names.As a result of these data, Ascochyta, Didymella and Phoma were delineated as three distinct genera, and the generic circumscriptions of Ascochyta, Didymella, Epicoccum and Phoma emended. Furthermore, the genus Microsphaeropsis, which is morphologically distinct from the members of Didymellaceae, grouped basal to the Didymellaceae, for which a new family Microsphaeropsidaceae was introduced.