First report of Fusarium concentricum causing wilt disease on Balloon flower ( Platycodon grandiflorus ) in Korea

Balloon flower (Platycodon grandiflorus (Jacq.) A. DC.) is a perennial herbaceous plant belonging to Campanulaceae family, native to East Asia and rich in minerals with culinary benefits, and in medicinal compounds like saponins, which improve respiratory health like pulmonary inflammation and chronic bronchitis in animal models like rats and guinea pigs (Lee et al., 2020; Shin et al., 2002). Despite its tremendous value, P. grandiflorus is highly susceptible to fungal diseases such as Fusarium wilt, which causes wilting and lowers the root yield. In May 2025, wilt symptoms were observed in seedlings at Chonnam University arboretum (N 35°10’22” E 126°54’20”, Gwangju, Korea), with an incidence rate of 16.7 % (5 of 30) of seedlings exhibiting at least one of the symptoms, including vascular necrosis, followed by chlorosis and total seedling wilt. The associated pathogen was isolated from root tissues of symptomatic plants, which were surface sterilized in 70 % ethanol and rinsed three times in sterilized distilled water (DW). The air-dried tissues were placed on potato dextrose agar (PDA) medium containing 0.05% streptomycin sulfate and incubated at 25 ℃. The five isolated fungal strains were morphologically similar, and a representative isolate Y928 was selected for further characterization. The colony of Y928 on PDA was initially white but turned purple after 4 days. When inoculated on carnation leaf agar (CLA), Y928 produced monophialides and two types of conidia: macroconidia and microconidia. The macroconidia were straight and slightly curved with a 3 to 5 septa, 29.6 to 49.5 μm (average 38.9 μm) in length, and 2.5 to 4.5 μm (average 3.6 μm) in width (n = 30). The microconidia of Y928 were ovoid to elliptical in shape and aseptate, measuring 6.4 to 10.5 μm (average 8.1 μm) in length and 2.7 to 3.5 μm (average 3.0 μm) in width (n = 30), while the chlamydospores were absent. These morphological characteristics of isolate Y928 were consistent with Fusarium fujikuroi species complex (FFSC) (Choi et al., 2018). At molecular level, genomic DNA extraction, PCR amplification and sequencing of internal transcribed spacer (ITS; ITS5/ITS4 primer), translation elongation factor 1-alpha (TEF1; EF1/EF2 primer), RNA polymerase II largest subunit (RPB1; Fa/R8 primer) and RNA polymerase II second largest subunit (RPB2; 5f2/7cr primer) regions for species identification. Each of the sequences was deposited in NCBI GenBank under accession numbers PZ146033.1, LC922517.1, LC922518.1 and LC922519.1, respectively (O’Donnell et al., 2022). BLAST search of each sequence showed 100.0% homology for ITS, TEF1 and RPB1, and 99.79% homology for RPB2, with Fusarium concentricum CBS 450.97. The phylogenetic analysis of concatenated sequences of isolate Y928 and 23 type materials and 1 non-type material, using the maximum likelihood method confirmed Y928 as F. concentricum. To confirm the pathogenicity of representative isolate, a conidia suspension (1.6 × 106 conidia/mL) was poured onto the roots of P. grandiflorus seedlings, grown in a growth chamber set as 25 to 26 ℃ temperature, 85 % relative humidity and 12 h/12 h light cycle, while sterilized DW was used for the control group, with three replications in each group. After 14 days, vascular necrosis in the roots and seedling wilting were observed in the treatment group, while there were no symptoms in control group. The pathogen was re-isolated from symptomatic seedlings, and confirmed to be identical to the original Y928 through morphological and molecular analysis, thereby fulfilling Koch’s postulates. To the best of our knowledge, this is the first report of F. concentricum causing wilt diseases on P. grandiflorus in Korea. Our results provide valuable information for developing management strategies such as disease monitoring and customized control methods to improve P. grandiflorus cultivation.