Emerging Verticillium Wilt of Napa Cabbage in Korea: Morphological, Molecular, and Pathogenic Characterization of <i>Verticillium dahliae</i> and <i>Verticillium longisporum</i> (A1/D1)

<i>Verticillium</i> wilt is an emerging threat to Brassicaceae crops worldwide, yet information on the species diversity and pathogenicity of <i>Verticillium</i> on Napa cabbage (<i>Brassica rapa</i> subsp. <i>pekinensis</i>) in South Korea remains limited. During surveys in 2020 and 2021, patches of wilted Napa cabbage plants were observed in highland cultivation regions, with disease incidence ranging from 1 to 10%. Twenty symptomatic plants were collected, and fifteen fungal isolates were obtained through tissue culture. Morphological observations and multilocus phylogenetic analyses based on four loci (<i>ACT</i>, <i>EF</i>, <i>GPD</i>, and <i>TS</i>) identified two species, <i>V. dahliae</i> and <i>Verticillium longisporum</i> (lineages A1/D1). Morphologically, <i>V. longisporum</i> produced longer conidia (>6.1 μm) and elongate microsclerotia, whereas <i>V. dahliae</i> produced shorter conidia (<6.0 μm) and spherical microsclerotia. Growth assays demonstrated that <i>V. longisporum</i> grew optimally at 22 °C and <i>V. dahliae</i> at 25 °C, with V8 agar supporting the best growth of both species. Pathogenicity tests on Napa cabbage seedlings confirmed that both species caused typical wilt symptoms, including leaf yellowing, senescence, and stunted growth. To our knowledge, this is the first report of <i>V. longisporum</i> associated with <i>Verticillium</i> wilt of Napa cabbage in South Korea. While <i>Verticillium</i> wilt has so far been confined to highland cultivation areas, considering the predominance of <i>V. longisporum</i> in autumn-winter cropping regions in Japan, continued monitoring in similar Korean production regions will be essential. These findings provide new insights into the distribution, morphology, and pathogenicity of <i>Verticillium</i> species, contributing to the development of effective management strategies for Napa cabbage production.