Phylogenetic affinity of Mycochaetophora gentianae, the causal fungus of brown leaf spot on gentian (Gentiana triflora), to Pseudocercosporella-like hyphomycetes in Helotiales

Mycochaetophora gentianae, the causal agent of brown leaf spot on gentian (Gentiana scabra), is characterized by its hyaline besom-like sporophore, although its conidiogenesis and phylogenetic position have so far remained unknown. We isolated the causal fungus from a new host, G. triflora, in Iwate, Japan. Both the G. triflora isolate and the ex-type M. gentianae isolate produced symptoms on G. triflora but not on G. scabra. Microscopic observations of the diseased leaves indicated that conidiogenesis was blastic from short conidiophores, and schizolytic secession of conidia left unthickened and inconspicuous conidial scars on the conidiogenous cells. Conidia were catenate, in branched acropetalous chains; secondary conidia were blastically produced from the first or second cell at the base of primary conidium. The G. triflora isolate was identified as M. gentianae because of its identity to the ex-type in characteristics of culture, pathogenicity, and conidia. Phylogenetic analyses using three ribosomal DNA (rDNA) sequences combined [small subunit (SSU) + large subunit (LSU) + 5.8S rDNA] indicated that both isolates clustered with Rhexocercosporidium carotae, and the cluster was placed within Helotiales-Rhytismatales. Additional analyses using internal transcribed spacers including 5.8S rDNA sequences revealed that both isolates were monophyletic and that they were closely related to three helotialean Pseudocercosporella-like hyphomycetous genera: Helgardia, Rhexocercosporidium and Rhynchosporium.