First Report of Leaf Spot on Rhododendron yedoense f. poukhanense Caused by Corynespora cassiicola in Korea

In September 2025, leaf spot symptoms were observed on Rhododendron yedoense f. poukhanense (Korean azalea) growing in landscape plantings in Jeonju-si, Jeonbuk State (35°50′52.944′′N, 127°07′31.188′′E), South Korea. Symptoms included withered leaves, premature defoliation, and small, dark brown spots with slight yellow halos that gradually enlarged into irregular necrotic lesions. To isolate the causal agent, five symptomatic leaves were collected from the field. Samples were surface-sterilized in 70% ethanol for 30 s, followed by 3% sodium hypochlorite for 2 min, rinsed three times with sterile distilled water, and then plated onto potato dextrose agar (PDA). Plates were incubated at 25 °C in the dark for 7 to 10 days. Two fungal isolates (Nifos-J25091 and Nifos-J25092) exhibiting similar morphological characteristics were obtained. The fungal mycelia were initially white to gray and turned dark gray as the colonies aged. Conidia (16.6–96.8 × 4.7–16.3 μm; n = 23)) were hyaline to light brown, cylindrical to obclavate, sometimes tapering towards the base, and multi-septate. Conidia were produced singly or in chains, consistent with the morphological characteristics of Corynespora cassiicola (De Silva et al. 2021). For molecular identification, genomic DNA was extracted from both representative isolates. The internal transcribed spacer (ITS) region was amplified using primers ITS4/ITS5 (White et al. 1990), the translation elongation factor 1-alpha (TEF1-α) region using EF1-983F/EF1-1567R (Rehner and Buckley 2005), and the RNA polymerase II second largest subunit (RPB2) region using 5F2/7cR (Reeb et al. 2004; Liu et al. 1999). The resulting sequences were deposited in GenBank under accession numbers for ITS (PZ017184, PZ017185), TEF1-α (PZ023635, PZ023636), and RPB2 (PZ023637, PZ023638). Based on BLAST searches in NCBI, the isolates showed high sequence similarity to Corynespora cassiicola, with 99–100% identity for ITS (MH857942), TEF1-α (OM892999), and RPB2 (OM937067). A maximum-likelihood phylogenetic tree was constructed using concatenated ITS, TEF1-α, and RPB2 sequences. The isolates clustered within the C. cassiicola clade, confirming their taxonomic identity. Pathogenicity tests were performed using both isolates on three potted Korean azalea plants. On each plant, two branches were selected as independent replicates, and three leaves per branch were wounded with sterile needles and inoculated with 10 µL of a conidial suspension (1 × 10⁵ conidia/mL). Control plants were treated with sterile distilled water. Inoculated plants were maintained at 25 °C under high humidity conditions. Typical leaf spot symptoms identical to those observed in the field developed within 10 days after inoculation, whereas no symptoms appeared on control plants. The fungus was successfully re-isolated from the inoculated lesions and confirmed as C. cassiicola based on morphological and molecular characteristics, fulfilling Koch’s postulates. Although C. cassiicola has been reported on several ornamental hosts in Korea, including Hibiscus syriacus and Kerria japonica (Seo et al. 2013; Lee et al. 2015), leaf spot caused by this pathogen on Rhododendron spp. has not previously been documented in Korea. While the disease on Rhododendron spp. caused by the same pathogen has been reported in other countries, including China (Gao et al. 2026), this is the first report of C. cassiicola causing leaf spot on Rhododendron yedoense f. poukhanense in Korea.