First Report of Leaf Spot Caused by Colletotrichum camelliae in Mung Bean (Vigna radiata) in South Korea
2024-06-11

Mung bean (Vigna radiata (L.) R. Wilczek) is a legume with high nutritional and economic value that is cultivated widely across Asia (Kang et al. 2014). In March 2022, a leaf spot disease in mung bean was observed at the Gangneung-Wonju National University Experimental farm (Gangneung, South Korea, 37.77°N, 128.86°E). The affected plants had irregular brown-gray leaf spots, and the bottom of the leaves showed concentric brown-gray rings that eventually progressed to necrotic lesions. Regardless of the cultivar, approximately 30% of the plants in the field were infected. To isolate the pathogen, the affected leaves were surface-sterilized by washing with 70% ethanol for 1 min, followed by washing with 2% NaClO for 2 min, then rinsing with sterile distilled water. We placed 3-mm sized diseased lesions on potato-dextrose agar (PDA), then incubated them at 27 ± 1 °C in the dark for 7 days and we obtained 1 isolate (CC1). The fungus on PDA had white aerial mycelia that became gray toward the center. Single spore cultures were obtained from fungal isolate. Isolated conidia were single celled, hyaline, cylindrical, and measured between 20.75 to 22.07 μm × 5.85 to 6.92 μm (n = 20), similar to other reports of C. camelliae(Wang et al. 2016). Mycelium from the single spore isolate was used for DNA extraction using ExgeneTM Plant SV / (GeneAll®, Cat.No. 117-152), and we amplified the ITS region with primers ITS1 + ITS2 and ITS3 + ITS4, a portion of the actin gene with primers ACT-512F + 738R, and a portion of the beta-tubulin gene with primers BT2aF + BT2bR (Carbone et al. 1999; Glass et al. 1995; White et al. 1990). The amplified regions were sequenced by by Macrogen® (Seoul, South Korea). Sequences were deposited under GenBank accession numbers OR523262 (ITS), OR582483 (Actin), and OR566953 (beta-tubulin). MegaBLAST analysis of the ITS1, ITS2, ACT, and TUB regions showed 99% (216/217 bp) similarity with C. camelliae strain HNCS-26 (MK041440.1), 99% (303/305 bp) similarity with C. camelliae strain G3 (ON025203.1), 99% (242/244 bp) similarity with C. camelliae strain FWT41 (MN525820.1), and 99% (456/460 bp) with C. camelliae strain LF152 (KJ955239.1), respectively. To fulfill Koch's postulates, we conducted a pathogenicity teston the mung bean cultivar VC1973A (Seonhwanokdu) grown for four weeks at 25 °C with a 16-h day/8-h night cycle, simulating the field conditions when the symptoms were observed. We tested the pathogenicity on six plants , three control and three infected plants. Using three leaf replicates per plant resulting in total of nine leaves per group. Leaves were first injured using a sterile needle then either sterile 5 mm PDA plugs or plugs with C. camelliae were placed on the leaf for control and infected conditions, respectively. Irregular gray leaf spots were observed on the abaxial and adaxial of the infected leaf after 2 weeks, like the symptoms observed in the field. This was observed only on infected leaves and nowhere else on the plant and the control plants had no infection. We re-isolated the pathogen from diseased leaves and identified it as C. camelliae using the same molecular markers described previously, completing Koch's postulate. To the best of our knowledge, this is the first report of leaf spot caused by C. camelliae in mung bean plants in Korea, previously the fungi was reported to infect tea plants in Korea (Hassan et al. 2023). More studies are required to investigate potentially resistant mung bean varieties to minimize future damage caused by this fungus.