Reappraisal of <i>Didymella macrostoma</i> causing white tip disease of Canada thistle as a new species, <i>Didymella baileyae</i>, sp. nov., and bioactivity of its major metabolites

Bioherbicides are expected to be a supplement to integrated pest management, assisting in the control of problematic weed species. For instance, bioherbicides (Phoma and BioPhoma) were recently registered in Canada and the USA for the control of some perennial dicotyledonous weeds in lawns. These products are based on strains of the fungus <i>Didymella macrostoma</i> (syn. <i>Phoma macrostoma</i>) that causes white tip disease (WTD) in Canada thistle (<i>Cirsium arvense</i>). In this study, WTD was reported for the first time in the Russian Federation. Analysis of the internal transcribed spacer (ITS) region of nuc rDNA and secondary metabolite profiling confirmed the identity of Russian WTD isolates to Canadian biocontrol strains identified as <i>D. macrostoma</i>. Multilocus phylogenetic analysis based on sequencing of the ITS region, partial large subunit nuc rDNA region (28S), RNA polymerase II second largest subunit gene (<i>rpb2</i>), and partial β-tubulin gene (<i>tub2</i>) has differentiated the WTD isolates from <i>C. arvense</i> and <i>D. macrostoma</i> isolates from other plant hosts. Based on phylogenetic, morphological, and chemotaxonomic features, these WTD isolates were described as a new species named <i>Didymella baileyae</i>, sp. nov. This study also demonstrated the low pathogenicity of the ex-type <i>D. baileyae</i> isolate VIZR 1.53 to <i>C. arvense</i> seedlings and its asymptomatic development in the leaves of aboveground shoots. The organic extracts from mycelium and culture filtrate of <i>D. baileyae</i>, as well as macrocidin A and macrocidin Z, displayed phytotoxicity both to <i>C. arvense</i> leaves and seedlings. Macrocidin A was only detected in the naturally infected leaf tissues of <i>C. arvense</i> showing WTD symptoms. Macrocidins A and Z demonstrated low antimicrobial and cytotoxic activities, exhibiting no entomotoxic properties. The data obtained within this study on the pathogenicity and metabolites of <i>D. baileyae</i> may be important for the rational evaluation of its prospects as a biocontrol agent.