First Report of Anthracnose Caused by Colletotrichum reniforme on Soybean ( Glycine max L.) in South Korea

Abstract

Soybean (Glycine max L.) is an economically important legume crop in South Korea (Lee and Ahn 2017; Chae et al. 2025; Cho and Yoon 2025). In October 2021, anthracnose symptoms were observed on soybean plants in two commercial (1 ha each) fields in Gimje, Jeollabuk-do, South Korea (35°80′N, 126°88′E), with disease incidence exceeding 30%. Symptoms consisted of irregular, dark brown necrotic lesions surrounded by yellow chlorotic halos on leaves. Six symptomatic leaves were collected from the two fields for pathogen isolation. Small pieces (approximately 5 × 5 mm), surface-disinfested tissues from lesion margins were plated on water agar amended with streptomycin. Five isolates (FD0006, FD0007, FD8, FD0033, and FD34) were obtained and purified on potato dextrose agar (PDA). Colonies were pale gray with entire margins after 7 days at 25°C. Conidia (n = 50) were hyaline, aseptate, cylindrical with rounded ends, measuring 14.4–21.0 × 4.1–6.2 μm (mean 16.4 × 5.3 μm). Appressoria were dark brown, irregular to strongly lobed, measuring 7.4–15.9 × 4.7–11.3 μm (mean 11.7 × 8.2 μm). Asci were cylindrical to clavate, 38.0–70.4 × 6.5–14.2 μm, and ascospores were hyaline, aseptate, and 13.5–22.1 × 4.6–6.8 μm. These characteristics were consistent with species of the Colletotrichum orchidearum species complex, including C. reniforme (Liu et al. 2022; Zhao et al. 2026). For molecular identification, partial sequences of gapdh, chs-1, act, tub2, and his3 were amplified and sequenced using primers described by Damm et al. (2019). Representative sequences were deposited in GenBank (gapdh: LC797748–LC797752; tub2: LC797788–LC797792; chs-1: LC797828–LC797832; act: LC797868–LC797872; his3: LC797906–LC797910). Multilocus phylogenetic analysis (Bayesian and Maximum Likelihood) of the combined dataset placed all five isolates within the C. reniforme clade with strong support. Pathogenicity was assessed using isolates FD0007 and FD00034 on four soybean plants (cv. Daewon and Cheongja 5) at the V2–V3 growth stage. Plants were inoculated by spraying approximately 10 mL of a conidial suspension (1 × 10 6 conidia/mL) containing 0.02% Tween 20 onto unwounded leaves until runoff. Control plants (four) were sprayed with sterile distilled water containing 0.02% Tween 20. After inoculation, plants were maintained in a dew chamber at 24°C in the dark for 24 h and then transferred to a greenhouse, where they were kept at approximately 95% relative humidity by enclosing the plants with plastic film. Necrotic lesions similar to those observed in the field developed within six days, with an 80% disease incidence. The fungus was reisolated from symptomatic tissues and identified as C. reniforme based on morphological and molecular characteristics described above, thereby fulfilling Koch’s postulates. The experiment was repeated twice with similar results. Colletotrichum reniforme was originally described from China as a member of the C. orchidearum species complex (Liu et al. 2022). To our knowledge, this is the first report of soybean anthracnose caused by C. reniforme and the first record of this species in South Korea. This finding expands the known geographic distribution and host range of C. reniforme and indicates that the pathogen may represent an emerging threat to soybean production in Korea. Accurate identification of Colletotrichum species associated with soybean anthracnose will be important for disease surveillance, epidemiological studies, and the development of effective management strategies.