The Identification, Characterization, and Fungicide Sensitivity of Leptosphaerulina trifolii Causing Didymellaceae Leaf Spot of Elymus Plants in China

Leptosphaerulina trifolii (Didymellaceae) is a widespread phytopathogen commonly associated with leaf spot diseases on legumes. However, its occurrence on Poaceae hosts has rarely been documented. In this study, leaf spot symptoms on Elymus plants were observed in Gansu and Qinghai Provinces, China. Morphological characterization, combined with multi-locus phylogenetic analyses (ITS, LSU, and RPB2) and pathogenicity assays, confirmed L. trifolii as the causal agent. Phylogenetic reconstruction demonstrated that newly obtained isolates clustered with ex-type and reference strains of L. trifolii with high support, while inoculation trials reproduced typical field symptoms and fulfilled Koch’s postulates. Growth condition assays further revealed that the fungus exhibited optimal proliferation at 20 °C, with KNO3 and D-maltose as the most favorable nitrogen and carbon sources, respectively, and under either continuous darkness or a 12 h light/12 h dark regime. To our knowledge, this is the first report of L. trifolii causing leaf spot on Elymus spp. in China. This study provides the first evidence of L. trifolii on Elymus species, thereby expanding its known host range. Identification was confirmed through field surveys, morphological and molecular analyses, pathogenicity tests, and fungicide sensitivity, supporting the validity of this host record.