First Report of Leaf Blight Caused by Didymella americana on Soybean ( Glycine max (L.) Merr.) in Korea

In September 2025, a total of 15 soybean fields were surveyed in Muan and Gongju, Korea. Irregular leaf blight symptoms were observed in five fields, with approximately 1–10% of soybean plants showing symptoms in each affected field. To isolate the causal pathogen, symptomatic leaf tissue was surface-disinfected with 1% NaOCl for 1 min, rinsed with sterile distilled water, and placed on potato dextrose agar (PDA). The plates were incubated at 20℃ for 7 days, and olive-brown hyphae growing from the tissues were subcultured twice on PDA. Five isolates with similar morphology were obtained from symptomatic leaves, and one representative isolate, DA17-37, was selected for further morphological, molecular, and pathogenicity analyses. The conidia were predominantly oval to ellipsoidal and aseptate, measuring 3.71-6.45 x 1.77-2.83 μm, with a mean size of 5.11 x 2.14 μm (n=30). The ascospores were mostly oval and 1-septate, measuring 12.49-18.25 x 4.13-5.41 μm with a mean size of 15.08 x 4.48 μm (n=30). Additionally, multicellular chlamydospores were observed. Overall, these morphological characteristics were consistent with those of the genus Didymella. To genetically characterize isolate DA17-37, genomic DNA was extracted, and the internal transcribed spacer (ITS), beta-tubulin (tub2), and RNA polymerase II second largest subunit (rpb2) genes were amplified by PCR using the respective primer sets described previously (Gorny et al. 2016; Liu et al. 1999; Woudenberg et al. 2009). The resulting sequences were deposited in GenBank under accession numbers PZ274701, PZ433240, and PZ439292. Maximum likelihood phylogenetic analysis of the concatenated dataset showed that isolate DA17-37 clustered with the reference strains of Didymella americana. To confirm the pathogenicity of isolate DA17-37, a conidial suspension (1 x 10 ⁶ conidia/mL) prepared from 30-day-old PDA cultures grown at 20℃ was sprayed onto 2-week-old potted soybean plants. Control plants were sprayed with sterile distilled water. The inoculated plants were incubated in a dew chamber at 26℃ for 3 days and then transferred to a growth chamber maintained at 26℃ and 70% relative humidity under a 16-h photoperiod. Symptoms first appeared at 7 days post-inoculation (dpi), and representative symptoms showing disease progression were photographed at 14 dpi, whereas the uninoculated control plants remained healthy. To satisfy Koch’s postulates, the same pathogen was re-isolated from symptomatic leaves and identified based on morphology and DNA sequence analysis. Didymella americana has been reported as a causal pathogen of leaf blight on lima bean in the United States. To the best of our knowledge, this is the first report of leaf blight caused by D. americana on soybean in Korea. Continued monitoring is needed to assess the potential spread of this disease in Korea and to support the development of appropriate disease management strategies.