Unique findings of <i>Phoma</i>-like fungi associated with soybean

Abstract

Ascochyta leaf blight of soybean is a widespread disease caused by several closely related Phoma-like species, this disease often leads to significant crop losses. Among Phoma-like species from Didymellaceae family, the most frequently associated with symptomatic soybean tissues are species of the genera Boeremia and Didymella. Currently reliable species identification in Didymellaceae relies on polyphasic approach based on consolidated species concept and combined molecular phylogenetic, micromorphological and cultural features. At least three loci are commonly used for reconstruction of the molecular phylogeny of Didymellaceae: internal transcribed spacer (ITS) of the ribosomal DNA, partial RNA-polymerase II gene (rpb2), and β-tubulin (tub2). As a result of long-term phytosanitary monitoring of soybean crops, soybean leaves with symptoms of Ascochyta blight were collected from major soybean producing areas of Russia. From surface sterilized plant tissues more than 100 isolates of Phoma-like fungi were obtained and stored in the collection of pure cultures of the Laboratory of Mycology and Phytopathology (MF, All-Russian Institute of Plant Protection). Most of them, as a result of multilocus phylogenetic analysis, were identified as Boeremia and Didymella species. Eight isolates were identified as species of other genera, suspected to be rare findings. The aim of this study was to identify these eight isolates based on multilocus phylogenetic analysis, as well micromorphological, cultural, and pathogenicity data. Multilocus phylogenetic analysis has resulted in identification of all eight isolates to species level. Single isolate from the Ryazan region was Neoascochyta graminicola. Three other from three different districts of the Amur region were Remotididymella capsici. Two isolates from the Primorskiy territory and Amur region were Stagonosporopsis heliopsidis. Another two from two districts of the Amur region were S. stuijvenbergii. Pathogenicity tests have resulted in conclusion, that all studied isolates were not pathogenic for soybean leaves. Probably, these Phoma-like species are associate with soybean as saprophytes or endophytes. For all these Phoma-like species Glycine max was detected as substrate for the first time. Neoascochyta graminicola is widespread in Europe in association with Poaceae plants. There are only two findings of Remotididymella capsici in the world, both from leaves of Capsicum annuum. First finding was made in the former USSR in 1977 and was identified based on only morphological features. Second findings was collected in the Fiji and verified with multilocus phylogenetic analysis. Stagonosporopsis heliopsidis isolates were revealed in the USA, Canada, Netherlands and Russia and this fungus was believed to be specific for Asteraceae plants. Isolates of Stagonosporopsis stuijvenbergii are known only from soil in the Netherlands. Thus, such species as Neoascochyta graminicola and Stagonosporopsis stuijvenbergii were revealed in the Russia for the first time. Studied Remotididymella capsici isolates were first confirmed findings of this fungus in Russia. Additionally to detailed phylogenetic data, the manuscript is supplement with a detailed description of the cultural and micromorphological features of all species.