| Summary: | The beach vitex (<i>Vitex rotundifolia</i> L.), a member of the Lamiaceae family, is a salt-tolerant, woody perennial common in coastal areas worldwide. Plant–fungal association was monitored in the shoreline forest area of Wando Island in Korea in 2020, and leaf samples showing leaf spot disease were collected. Eight fungal isolates were recovered from the samples on PDA and identified based on the morphological characteristics and multilocus molecular phylogeny. Among the eight isolates, four were identified as <i>Alternaria alternata</i>, two as <i>Fusarium humuli</i>, one as <i>Colletotrichum aenigma</i>, and one as <i>Stagonosporopsis caricae</i>. Pathogenicity tests of the fungal isolates on the detached leaves of beach vitex revealed that <i>S. caricae</i> CMML20–2 and <i>A. alternata</i> (CMML20–7, CMML20–8, CMML20–9, and CMML20–10) caused disease lesions while the other species did not. The fungal species <i>S. caricae</i>, <i>C. aenigma</i>, and <i>F. humuli</i> are the first reported in the host worldwide, and <i>S. caricae</i> and <i>F. humuli</i> are first reported in Korea. In vitro fungicide sensitivity assays were carried out based on a measurement of diametrical mycelial growth on media amended with different doses of the fungicides fludioxonil, metconazole, and fluxapyroxad. Fungicide sensitivity varied significantly among the species, and the <i>A. alternata</i> and <i>S. caricae</i> isolates were more sensitive to fluxapyroxad than the other isolates. Our study contributes to the understanding of fungal diversity in forest mycology and demonstrates that pathogenic fungi including <i>A. alternata</i> and <i>S. caricae</i> might accelerate decline in leaf health.
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