First Report of Alternaria alternata Causing Leaf Spot on Climbing Rose ( Rosa hybrida ) in Korea

Climbing roses (Rosa hybrida) are commonly planted in public gardens, arches, and landscape areas in Korea. In April 2025, leaf spot symptoms were observed on climbing rose plants in Suncheon Bay National Garden, Jeollanam-do, Korea (34°55′36.05″N, 127°30′44.35″E). About 5% of the plants in the surveyed area showed symptoms. Small dark-brown flecks enlarged into circular to irregular lesions with dark margins and pale necrotic centers, which coalesced and caused extensive leaf blight. Symptomatic leaf tissues were cut from lesion margins, surface-disinfested in 100% ethanol and 1% sodium hypochlorite for 1 min each, rinsed three times with sterile distilled water, and placed on 1.5% water agar supplemented with 100 ppm streptomycin. After incubation at 25°C in darkness for 7 days, emerging hyphae were transferred to potato dextrose agar (PDA). Three single-spore isolates were obtained and designated SYP-1773 to SYP-1775. Colonies on PDA were dark olive-brown near the center and grayish toward the margins after 14 days. Conidia were light brown, ellipsoid to obclavate, with roughened walls and 1–6 transverse and 0–3 longitudinal septa, measuring 24.4–56.6 × 5.3–14.7 μm (mean = 42.7 × 10.4 μm, n = 50). These morphological characteristics were consistent with previous descriptions of Alternaria alternata (van der Waals et al. 2011; Woudenberg et al. 2015). For molecular identification, ITS, gapdh, tef1, rpb2, Alt a1, endoPG, and OPA10-2 loci were amplified and sequenced using previously described primers (Woudenberg et al. 2013, 2015). The sequences were deposited in GenBank as PZ384195–PZ384197, PZ393708–PZ393710, PZ393711–PZ393713, PZ393714–PZ393716, PZ393717–PZ393719, PZ393720–PZ393722, and PZ393723–PZ393725, respectively. BLASTn analysis showed 100% identity with the ex-type strain CBS 102598 of A. alternata for ITS (562/562 bp), gapdh (588/588 bp), tef1 (240/240 bp), rpb2 (918/918 bp), and OPA10-2 (657/657 bp). Alt a1 and endoPG showed 96.45% (462/479 bp) and 99.34% (452/455 bp) identity, respectively. In a maximum likelihood tree based on the combined seven-locus dataset, the isolates clustered with reference isolates of A. alternata. Pathogenicity assays were conducted using isolate SYP-1775 on leaves of one-year-old climbing rose plants. A conidial suspension (1 × 10⁶ conidia/mL) prepared from 14-day-old PDA cultures was spray-inoculated onto wounded and non-wounded leaves. Three plants were used per treatment. After inoculation, plants were covered with transparent plastic bags for 48 h and maintained under natural field conditions (18–26°C, 75–90% RH). Symptoms similar to those observed in the field developed within 14 days after inoculation, whereas control plants treated with sterile distilled water remained symptomless. The same fungus was consistently re-isolated from symptomatic tissues, and its identity was confirmed by Alt a1 sequence analyses. The experiment was repeated three times with similar results. To our knowledge, this is the first report of A. alternata causing leaf spot on climbing rose (R. hybrida) in Korea. Similar diseases associated with this pathogen have previously been reported in Bangladesh (Hossain et al. 2021). Because climbing roses are frequently planted in densely managed ornamental landscapes, continuous monitoring of foliar diseases may be necessary to reduce aesthetic damage and maintain plant health in public garden settings.