Downy Mildew Caused by <i>Plasmopara wilsonii</i> on <i>Geranium carolinianum</i> in Korea

Geranium carolinianum (commonly known as Carolina crane's-bill, Carolina geranium, and wild Geranium) is an annual herbaceous plant native to North America and widely naturalized in temperate regions worldwide. In Korea, this plant is considered an invasive species due to its rapid spread and competitive displacement of native flora (Secretariat, 2025; Oh et al., 2021). In May 2025, symptoms of downy mildew were observed on G. carolinianum populations growing in Buan, Korea, with a high disease incidence of 50–80%. Infected leaves exhibited chlorotic lesions with angular margins and dense whitish sporulation on the abaxial surfaces, typical of downy mildew infections. As the disease progressed, necrosis and defoliation occurred on severely infected plants. Two representative symptomatic specimens were deposited in the Kunsan National University Herbarium (KSNUH 2478, 2486). Sporangiophores (n=50) were slender, straight or slightly curved, monopodially branched in 2-5 orders, and measured 163–378 (av. 214 ± 49) × 4–8 (av. 6 ± 1) μm. Ultimate branchlets were straight to substraight, 6.7–21.8 (av. 12.1 ± 3.4) μm long, with a cup-like or swollen tip. Sporangia were subglobose to broadly ellipsoidal, rarely ovoidal or almost oblong, and measured 18.6–28.6 (av. 25.0 ± 2.5) × 14.9–22.8 (av. 19.4 ± 1.8) μm, with a length/width ratio of 1.15–1.43 (av. 1.3 ± 0.1). The morphological characteristics match a previous description of Plasmopara wilsonii (Voglmayr et al. 2006). To confirm the pathogen identity, total DNA was extracted from infected tissues. The internal transcribed spacer (ITS) region and the large subunit (LSU) gene of rDNA were amplified using the primer sets ITS1-O/LR0 and LR0R/LR6-O, respectively (Bachofer 2004; Moncalvo et al. 1995; Riethmüller et al. 2002). The obtained sequences were deposited in GenBank (Accession No. PV988127–8 for ITS and PV972918–9 for LSU). A BLASTn search revealed that the ITS sequences shared 99.0% identity with Plasmopara wilsonii sequence from the United States (DQ131919.1), and the LSU sequences showed 100% identity with those from Germany (KR349458.1, KR349457.1). In phylogenetic analyses of ITS and LSU sequences, the Korean isolates formed a well-supported group with P. wilsonii sequences, confirming its identity. For a pathogenicity test, 10 mL of a sporangial suspension (4.0 × 105 conidia/ml) was sprayed on the adaxial surfaces of all leaves of five healthy G. carolinianum plants. Five control plants were treated with sterile water. All plants were kept in darkness for 24 h, followed by incubation in a growth chamber at 90% relative humidity and 22℃, with a 10 h light/14 h dark cycle. After 10–14 days, typical downy mildew symptoms developed on the inoculated leaves, from which the pathogen was confirmed as P. wilsonii via morphological and molecular analyses. Control plants remained symptomless, fulfilling Koch’s postulates. Plasmopara wilsonii has previously been reported on Geranium carolinianum in the United Kingdom and the United States (Farr & Rossman 2025), but to our knowledge, this is the first report of downy mildew disease caused by P. wilsonii on G. carolinianum in Korea. Given that this plant is considered invasive in Korea, the emergence of this downy mildew may have potential implications for both plant health and management.