First Report of Corynespora cassiicola Causing Stem Rot on Cowpea ( Vigna unguiculata (L.) Walp.) in China

Cowpea (Vigna unguiculata (L.) Walp.), a globally important legume and high-quality plant protein source (Jayathilake et al. 2018), is widely cultivated in arid and semi-arid tropical zones. In March 2025, a cowpea plantation in Chengmai County, Hainan Province, China (110.168808°E, 19.735978°N) showed unusual wilting, with over 30% disease incidence. Initial symptoms included irregular, dark brown dry necrotic lesions on stem bases and nodes, expanding bidirectionally, followed by leaf yellowing and plant death. Longitudinal sections revealed browned vascular bundles and partially occluded xylem vessels. Five typical diseased samples were collected. Stems were thoroughly rinsed with sterile water, and small stem fragments (4 × 4 mm) were excised from the lesion periphery. These fragments were surface-sterilized with 70% ethanol and 3% NaClO, rinsed three times with sterile water, and then cultured on potato dextrose agar (PDA) at 28 °C in the dark for 7 days. After purification via single-spore isolation and incubation under the same conditions (28 °C in the dark on PDA) for 7 days, the colonies displayed taupe-grayish centers grading to white margins; their reverse sides featured white margins, yellowish-brown inner rings, and dark brown centers. The mycelia were dense and cottony-flocculent. Conidia were solitary or catenate, straight to curved, obclavate to cylindrical, with 0-4 pseudosepta, measuring 15.4-114.4μm × 4.7-11.05μm. Their morphological characteristics aligned with those of Corynespora cassiicola (Pan et al. 2024). The internal transcribed spacer (ITS) region and β-tubulin gene of isolate CM01 were amplified using the primer pairs ITS1/ITS4 (White et al. 1990) and Bt2a/Bt2b (Glass et al. 1995), respectively. BLAST analysis showed that the ITS sequence (GenBank accession no. PX307421) and β-tubulin gene sequence (GenBank accession no. PX868915) of the isolate shared 100% identity with the corresponding sequences of C. cassiicola (KU204615, PP419028). Pathogenicity was confirmed using one-week-old cowpea seedlings. Stems were punctured and inoculated with 5-mm mycelial plugs; controls received sterile PDA plugs. Each treatment was set with 5 replicates. Seedlings were incubated in a growth chamber (12-hour photoperiod, 25 ± 2°C, 90% relative humidity). Inoculated plants developed dark brown lesions at 3 days post-inoculation, with lesions gradually expanding. After 14 days, vascular browning, leaf curling, yellowing, abscission, and stunting were observed; controls remained asymptomatic. Re-isolated fungi from lesions matched the original isolate (CM01) in morphology, fulfilling Koch’s postulates. It has been reported that C. cassiicola can cause leaf spot disease in many plant species, including cowpea (Li et al. 2014). However, to our knowledge, this is the first report of stem rot caused by C. cassiicola on cowpea in China. These findings not only expand our understanding of the pathogenicity of C. cassiicola on cowpea but also serve as a critical reference for cowpea disease monitoring and management strategies in China.