First Report of <i>Rhizoctonia solani</i> Causing Leaf Rot Disease on Mung Bean (<i>Vigna radiata</i>) in China

Mung bean (Vigna radiata L.) is one of the most important legume crops worldwide (Dikr 2023). In August 2023, a severe foliar disease resembling leaf rot was observed on mung bean with ~30% incidence in the field located in Nanning (22°49'N, 108°22'E), Guangxi Province, China. Initially, the leaves develop irregular, water-soaked grayish lesions with a gray-brown center and dark brown edges. Then, the lesions expand and merge into cloud-like patches under high humidity. Lastly, the whole leaf decays. Five infected leaves were collected and cut into 2-3 mm pieces, then were sterilized and cultivated on potato dextrose agar (PDA) and incubated at 25°C under a 12 h photoperiod for 5 days. Three of 10 obtained isolates (LY1, LY2, LY3), were used for further studies. Morphological characteristics were consistent with descriptions of Rhizoctonia solani (Ogoshi 1987). The colony was white initially and then turned to brown with sclerotia, the hyphal were branched at right angles with a septum and a slight constriction at the branch base. DNA was extracted from the three isolates and the three nuclear ribosomal regions (ITS, TEF1, RPB2) were amplified (White et al. 1990; Matheny et al. 2007). The obtained sequences were compared by BLAST analysis and deposited in GenBank (Accession Nos. PV017458 to PV017460 and PV175332 to PV175337). ITS sequences showed 99-100% (688/688, 679/679, 659/661 bp) similarity with R. solani strains (MG387102.1, EF429216.1, OP497979.1). The TEF1 and RPB2 sequences exhibited showed 97 to 99% (1012/1042, 719/724 bp) with R. solani strains (ON042753.1; XM_043331355.1). The three isolates were grouped within the AG-1 IA clade by phylogenetic analyses of ITS sequences (Godoy-Lutz et al. 2008). The pathogenicity of the three isolates was tested on mung bean by placing 0.5 mm-diameter mycelium discs on the leaves of 2-week-old seedlings (n=5), and pure PDA discs were inoculated as controls. All plants were incubated in a moist chamber for 2 days and then transferred to a greenhouse at 25°C. The experiment was repeated twice with three replicates. All isolates were highly pathogenic to the original host, mung bean. Inoculated leaves presented typical leaf rot symptoms consistent with those observed in the field. Based on morphological characteristics and ITS sequences, the identity of the reisolated fungus was confirmed as R. solani, fulfilling Koch's postulates. No pathogens were isolated from controls. Furtherly, the R. solani isolate LY1 was confirmed the host range same as above pathoginicy test, on eight other legume crops (n=5), adzuki bean (Vigna angularis), cowpea (V. unguiculata), pea (Pisum sativum), soybean (Glycine max), lentil (Lens culinaris), hyacinth bean (Lablab Album Semen), common bean (Phaseolus vulgaris), and faba bean (Vicia faba). The experiment was repeated twice. The isolate LY1 exhibited strong virulence on all tested crops, causing symptoms similar to those observed on mung bean and finally leading to plant death. Previously, R. solani was often reported as soil-borne pathogen causing root related diseases (Sneh et al. 1996). To our knowledge, this is the first report of R. solani causing leaf rot on mung bean in China and worldwide. This finding indicates a potential threat to legume crops.