First report of Cercospora cf. sigesbeckiae causing Cercospora leaf spot of soybean in the Republic of Korea

During field trials conducted in 2022 and 2023, Cercospora leaf spot was observed on soybean ( Glycine max ) collected from Gimje-si, Jeollabuk-do; Yeoncheon-gun, Gyeonggi-do; and Muan-gun, Jeollanam-do, in the Republic of Korea. Lesions bearing black spore structures were observed on plants. To isolate the pathogen, infected tissues were surface-sterilized with 0.5% sodium hypochlorite for 1 min and cultured on potato dextrose agar (PDA). After seven days of incubation, white to gray fungal colonies with aerial mycelia were transferred to PDA and incubated at 25 °C under a 12-h photoperiod. Three isolates, 24YC2, MA35, and GJ3, were selected for morphological identification and pathogenicity testing. Conidiophores were brown to dark brown, straight to slightly curved, with multiple septa, measuring approximately 35–125 × 3.1–5.5 μm (n = 25). Conidiation was induced on soybean-based medium containing 16% (w/v) soybean tissue extract and 2% (w/v) agar under a 12-h photoperiod at 25 °C for 7 days. Conidia were hyaline to pale brown, cylindrical to filiform, and multicellular, measuring 45–120 × 3.5–5.7 μm (n = 25), consistent with previous descriptions of C. cf. sigesbeckiae . Genomic DNA was extracted from each isolate using the CTAB method, and sequences of the internal transcribed spacer region, actin (ACT), calmodulin (CAL), elongation factor 1-α ( tef1 α), and histone H3 (HIS3) genes were amplified (Bakhshi et al. 2018; Groenewald et al. 2013; Namiki et al. 2001). Sequences were deposited in GenBank (accession nos. PV915209–PV915211, PV921638–PV921640, PV921644–PV921646, PV921650–PV921652, and PV921647–PV921649); reference sequences used for phylogenetic analysis were obtained from GenBank. Maximum likelihood analyses using RAxML with the GTR + GAMMA + I model and 1,000 bootstrap replicates showed that the three isolates clustered with four reference strains of C. cf. sigesbeckiae with 94% bootstrap support (Edler et al. 2021; Stamatakis 2014). Cercospora cf. sigesbeckiae was phylogenetically distinct from the ex-type strains of the closely related C. cf. richardiicola and C. kikuchii . For pathogenicity tests, conidia from each isolate were suspended in sterile water with 0.01% Tween 20 and adjusted to 10⁴ conidia/mL. Suspensions were sprayed onto fully expanded leaves of 6-week-old soybean cultivars Jangol and Nuriol; mock plants received sterile water with 0.01% Tween 20. Five plants were used per treatment, and experiments were repeated three times. Plants were maintained at 25 °C and >80% relative humidity for 7 days and then transferred to a growth chamber at approximately 40% relative humidity for another 7 days. At 14 days post-inoculation, all inoculated plants exhibited Cercospora leaf spot symptoms, whereas control plants remained healthy. The pathogen was reisolated from symptomatic leaves and confirmed to match the original isolates by morphology and sequence analysis, thereby fulfilling Koch’s postulates. Among closely related taxa, only C. cf. flagellaris has recently been reported from soybean in Korea (Yang et al. 2025). To our knowledge, this is the first report of C. cf. sigesbeckiae causing Cercospora leaf spot on soybean in the Republic of Korea.