First Report of <i>Colletotrichum sojae</i> Causing Anthracnose on Soybean in China

Soybean (Glycine max) is a crucial source of plant protein worldwide, fungal diseases are one of the significant factors affecting productivity during the soybean cultivation process (Boufleur et al. 2021). From July to August 2025, symptoms resembling anthracnose were observed in soybean fields in Changde City and Yiyang City, Hunan Province, China. Symptoms included brown lesions on infected stems and leaves, covered with irregularly arranged black specks. The pods exhibited black dots arranged in a rosette pattern. The disease incidence reached approximately 40% across nearly 30 hectares. Five diseased soybean plants were collected for pathogen isolation. Pod tissues were surface-sterilized (75% ethanol, 30 s; 1% NaClO, 1 min) and plated on potato dextrose agar (PDA) at 26℃ in darkness. The marginal mycelium of the colonies was picked and inoculated onto fresh PDA medium until a single colony isolate was obtained. The two obtained colonies (AX-1, NX-1) were circular with regular margins and showed minimal aerial mycelium. Colonies were circular, initially white, becoming light pink to pale orange with scant aerial mycelium, and produced numerous black masses. Acervuli were dark, clustered, and bore dark brown setae. Conidia were initially cylindrical, becoming falcate, measuring 11.2–39.4 × 3.8–7.7 μm (n=50). Appressoria were oval, 3.8–4.1 × 2.1–2.9 μm (n=50). Perithecia were dark brown, subglobose, 131.1–258.3 μm in diameter (n=50). Asci were clavate, containing eight ascospores. Ascospores were sickle-shaped, 28.2–35.4 × 2.5–3.1 μm (n=50), with two oil globules. These morphological traits matched Colletotrichum sojae (Hassan et al. 2022). The internal transcribed spacer (ITS) of rDNA, actin (ACT), chitin synthase (CHS-1), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes of two representative strains (AX-1 and NX-1) were amplified and sequenced using the PCR primers described by Weir et al. (2012). These sequences were deposited in GenBank (accession nos. AX-1: PX610531, PX666718, PX666722, PX666726; NX-1: PX610532, PX666721, PX666725, PX666729) and exhibited 99% to 100% identity with corresponding sequences from C. sojae strains. A polygenic tandem phylogenetic tree showed that both strains were clustered within the C. sojae clade. For pathogenicity assays, conidial suspensions (1×10⁵ conidia/mL) with 0.1% Tween 80 were uniformly applied to pods for inoculation until runoff, controls were inoculated with sterilized water using the same approach. Each treatment included three replicates, and all pods were cultured in incubator at 26°C under a 12-hour light/12-hour dark cycle. After 7 days, inoculated pods exhibited symptoms similar to those observed in the field, whereas controls remained symptomless. C. sojae was reisolated and identified from symptomatic pods, thus fulfilling Koch's postulates. C. sojae has been recorded as the anthracnose pathogen of multiple plants, including chili (Capsicum annuum) (Hassan et al. 2024), American ginseng (Panax quinquefolium) (Guan et al. 2021), and watermelon (Citrullus lanatus) (Guo et al. 2022). To our knowledge, this is the first report of C. sojae causing soybean anthracnose in China, posing a new challenge and potential threat to soybean production. Further studies on its epidemiology, occurrence, and integrated control are needed to improve disease management and reduce losses.