First Report of Verticillium Wilt caused by Verticillium dahliae on Chilli in South Korea.

Abstract

Chili pepper (Capsicum spp.) is an important crop in South Korea and is widely used in Korean cuisine, cultivated across a land area of roughly 29.8 thousand hectares, with a total production of 69 thousand tons (Lee et al., 2005; Statista, 2022). In September 2020, two farmer fields in Samcheok (37.444039°N, 129.135875°E; 37.633738°N, 128.911731°E), Gangwon province, South Korea, it is observed that chili pepper leaves showing yellowing and wilting symptoms, with an estimated disease incidence of approximately 10-15%. To identify the causal agents six affected plants were brought to lab. All the plants exhibited vascular discoloration in stem and root. After surface sterilizing small pieces of discolored tissue in 1% NaOCl for 30 s and rinsing twice in sterile distilled water, the tissue pieces were placed on water agar and incubated at 25°C for 10 days. Six pure isolates with consistent morphological characteristics were obtained by single spore isolation. Two representative isolates, NC17601 and NC20845 were selected for identification based morphological and molecular characters. Colonies on potato dextrose agar (PDA) during 10 days of incubation at 25°C in the dark were cottony white initially but progressively became dark as the formation of melanized microsclerotia. Conidiophores were hyaline, vertically branched or not, and had 2 - 4 phialides per node. Phialides were subulate and tapering from base to tip. The colonies produced abundant conidia, which were hyaline, single celled, smooth walled, cylindrical to oval, clustered on phialides, and 3.8 - 7.2 ×2.1 - 3.9 ㎛(mean ± SD: 5.2 ± 0.7 × 2.8 ± 0.5). Microsclerotia were aggregated form, various size and shape, and brown. These are the typical morphology of Verticillium dahliae (Inderbitiz et al. 2011; Yu et al. 2016). The molecular identification was later confirmed through PCR amplification, and sequencing targeting the translation elongation factor 1 alpha (TEF), actin (ACT), tryptophan synthase (TS), and glyceraldehyde-3-phosphate dehydrogenase (GPD) genes using the primer sets described by Inderbitiz et al. (2011). The resulting sequences were deposited in GenBank with accession numbers LC761935 to LC761942. The maximum likelihood tree based on combined sequences of ACT, GPD, TEF and TS was inferred using RAxML- HPC2 on XSEDE as implemented on the CIPRES web server. The phylogenetic tree showed that the isolates were sit together with V. dahliae isolates (Ex-type PD322, PD227 and PD502). Pathogenicity tests using two isolates (NC17601 and NC20845) were conducted in the greenhouse, where 10 two weeks old seedlings per isolates (cv. Bigstar) were root-tip cut and then soaked in a fungal spore suspension of 107 conidia ml-1 for 1 h, while 10 seedlings were treated with sterile distilled water as a control. All the treated plants were maintained at 25°C (night)/ 25°C (Day) under natural light. After three weeks, all inoculated plants exhibit growth stunting with vascular discoloration in the stem and roots as compared to asymptomatic control plants. The isolates of V. dahliae were consistently re-isolated from discolored root tissues and identified based on morphological characteristics, thus fulfilling Koch's postulates. In South Korea, V. dahliae has been reported to cause wilt disease in various crops, including Kimchi cabbage and radish (Dumin et al. 2020; Choi et al. 2023). To the best of our knowledge, this is the first report that V. dahliae causing Verticillium wilt of chili pepper in South Korea. Overall, Verticillium dahliae is considered to be a significant threat to agriculture in South Korea, and efforts are being made to develop effective control strategies to mitigate its impact on crops.