Identification of <i>Epicoccum nigrum</i> causing soybean leaf blight in Northeast China and its sensitivity to selected fungicides

Soybean [Glycine max (L.) Merr.] holds significant global economic importance due to its considerable nutritional and commercial value. Since 2022, a leaf blight of unknown etiology has been observed in soybean-growing areas of Nehe City and Hongxinglong Farm in Heilongjiang Province, China. Although the disease has not yet reached epidemic levels, it warrants attention due to its potential to spread under favorable environmental conditions. Initial symptoms on infected leaves appear as small brown spots that gradually expand to form streaks of necrotic tissue delimited by the veins, with a brown necrotic center surrounded by a yellow margin. Through systematic sampling and isolation, twenty-six fungal isolates with similar morphologies were obtained. Morphological characterization, molecular identification, and phylogenetic analyses were used to identify the pathogen of this new soybean leaf blight as Epicoccum nigrum. This study represents the first report of E. nigrum as a pathogen of soybean in China. Pathogenicity tests showed that most isolates displayed moderate to high virulence on soybeans. In greenhouse spray inoculation assays, E. nigrum isolates infected several legume species but did not cause disease on maize or hyacinth bean under the conditions tested. In vitro sensitivity assays showed that E. nigrum isolates exhibited high sensitivity to pyraclostrobin and azole-tebuconazole, with an EC₅₀ value ≤ 0.50 μg mL-1. In pot experiments, both fungicides provided significant disease control, achieving 90.2% control efficacy at 450 μg mL⁻¹ (pyraclostrobin) and 91.7% at 600 μg mL⁻¹ (azole-tebuconazole). These results support the use of chemical fungicides as a key strategy for managing soybean leaf blight caused by E. nigrum. Overall, this study clarifies the causal agent of soybean leaf blight, thereby providing a scientific basis for its management in soybean production. Additionally, the identification of effective fungicides provides practical guidance for targeted disease management, which is essential for safeguarding soybean yields and mitigating economic losses associated with this emerging disease. Furthermore, this study also establishes a foundation for future investigations into the pathogenic mechanisms and epidemiological dynamics of E. nigrum.