First Report of Target Spot Caused by Corynespora cassiicola on Catharanthus roseus in Korea

Catharanthus roseus (L.) G. Don, commonly known as periwinkle, is a perennial herbaceous plant. The plant is drought-tolerant and can persist in harsh, well-drained habitats, such as sandy soils, dunes, and limestone rocks (Nejat et al. 2015). It is widely cultivated as an ornamental worldwide and is an important medicinal species due to its anti-cancer alkaloids (Kumar et al. 2022). In the fall of 2025, distinctive leaf spots were observed on periwinkle plants grown as bedding in public gardens in Seoul and Pocheon, Korea. Symptoms were first noticed as small necrotic spots occurring on leaves of all ages, usually along the margins. As the lesions enlarged, they turned brown and became circular to oblong with distinct concentric rings, resulting in a greyish brown, target-like appearance. In severely affected plants, a large number of leaves exhibited chlorosis and abscised prematurely. Voucher specimens were deposited in the mycological herbarium of Korea University (KUS-F34657 and F34790). Conidiophores on symptomatic leaves were erect, brown to dark brown, single or clustered, and mostly hypophyllous, 125 to 570 × 5 to 9 μm. Conidia were solitary or in short chains, obclavate to cylindrical, straight to curved, pale olivaceous brown, with 4 to 11 pseudosepta, and measured 59 to 219 × 6 to 16 μm, with a distinct thickened hilum. The morphological characteristics were consistent with those of Corynespora cassiicola (Berk. Vilgalys and Hester 1990), respectively. A BLASTn search of the resulting sequences against reference sequences revealed >99% identity to Co. cassiicola strains (MZ422956 for ITS; OP896225 for LSU). All sequences obtained in this study were deposited in GenBank under accession numbers PX855243 and PX855247 for ITS and PX855246 and PX855337 for LSU. Pathogenicity was assessed by placing mycelial agar plugs from the representative isolate (KACC 411262) cultures on PDA onto the upper leaf surfaces of two healthy Ca. roseus plants, while sterile PDA plugs were placed on the leaves of two additional plants serving as non-inoculated controls. All the tested plants were kept in a humid plastic greenhouse (>85% relative humidity) maintained at 24–30°C for 48 h. At the inoculation sites, target-like spots identical to those observed in the field developed after 4 days, whereas control plants remained symptomless. The pathogen re-isolated from the inoculated plants was identified as Co. cassiicola through morphological examination and the sequence analyses described above, thus fulfilling Koch’s postulates. This study provides the first report of Co. cassiicola causing target spot on Ca. roseus in Korea. Corynespora cassiicola is a fungal pathogen that infects a broad range of plant species, including cotton, soybean, tomato, and cucumber (Dixon et al. 2009). However, within the genus Catharanthus, Ca. roseus is the only species reported as a host to date, with reports from Palau and the United States (Dixon et al. 2009; McGovern 1994). Field observations further suggest that shade, excess moisture, and reduced plant vigor may predispose ornamental periwinkle to infection, highlighting the need for continued monitoring in landscape plantings.