Phylogenetic Diversity of Fusarium Species Complexes Causing Wilt in Watermelon and Melon in Korea

Watermelon (Citrullus lanatus Thunb.) and melon (Cucumis melo L.) are globally important horticultural crops, cultivated widely for their economic and dietary value. However, their productivity is increasingly threatened by Fusarium wilt, which causes substantial yield losses. The genus Fusarium comprises over 400 phylogenetically distinct species, classified into multiple species complexes (SCs). Many of these species are soilborne and resilient in agroecosystems, complicating accurate identification and disease management. While Fusarium oxysporum formae speciales-f. sp. niveum (FON) and f. sp. melonis (FOM)-have long been considered the main causal agents of Fusarium wilt in watermelon and melon, an increasing number of regional studies suggest that diverse Fusarium spp. may also be involved. In this study, we examined the diversity and pathogenicity of Fusarium spp. causing wilt in watermelon and melon across key cultivation regions in Korea. Thirty-three Fusarium isolates were initially identified using internal transcribed spacer (ITS) sequencing and further characterized through multilocus phylogenetic analysis of the translation elongation factor 1-alpha (EF-1α) and the RNA polymerase second largest subunit (RPB2) genes. The isolates were classified into five distinct SCs: F. oxysporum (FOSC), F. nisikadoi (FNSC), F. fujikuroi (FFSC), F. incarnatum-equiseti (FIESC), and F. solani (FSSC). Morphological and microscopic features were examined to support SC-level classification. Pathogenicity assays confirmed that representative isolates from each SC induced wilt symptoms in both watermelon and melon. These results broaden the understanding of Fusarium diversity in cucurbits and underscore the needs for phylogenetically informed diagnostics and disease management strategies.