First Report of Leaf Spot Caused by <i>Septoria cerastii</i> on Invasive Weed <i>Cerastium glomeratum</i> in Korea

Cerastium glomeratum Thuill., known as sticky mouse-ear chickweed, is native to Europe and has become naturalized in the wild on most continents. After its accidental introduction to Korea around the 1980s, it quickly became one of the dominant invasive weeds on the Korean peninsula and is now considered a significant threat to the Korean agroecosystem (Park et al., 2020). In June 2013, a leaf blight was first observed on C. glomeratum in Jeju island, Korea. Voucher specimens (n = 12) from several locations throughout Korea were deposited in the Korea University Herbarium (KUS-F). Symptoms appeared as pale-greenish discolorations in indistinct, poorly defined leaf lesions, which progressed into pallid to brownish lesions without definite margins. The leaf spots were rarely observed on stems and bracts. The black conidiomata became visible in the lesions. Conidiomata were pycnidial, epigenous, and rarely hypogenous, scattered, dark brown to rusty brown, globose, embedded in host tissue or partly erumpent, 60 to 130 µm in diameter, with ostioles measuring 20 to 40 µm in diameter, and released cirriform conidial masses. The conidiomatal wall was composed of textura angularis without distinctly differentiated layers, but the outer cells were darker and somewhat thicker walls than the inner cells. Conidiogenous cells were submerged in the conidiomata wall, ampulliform or elongated ampulliform with a distinct neck and hyaline. Conidia were straight to mildly curved, hyaline, 25 to 65 × 1.5 to 2.5 µm, and 2 to 4-septate. Based on the morphological characteristics, the fungus was consistent with Septoria cerastii Roberge ex Desm. (Priest 1997, Verkley et al. 2013). Pure cultures were obtained by collecting conidia from pycnidia on leaf lesions and streaking them onto water agar media. After incubating at 24°C for 48 hours, germinating conidia were transferred to potato dextrose agar (PDA) plates. An isolate was deposited in the Korean Agricultural Culture Collection under Accession No. KACC 410466. To confirm morphological identification, six phylogenetic loci, including the internal transcribed spacer (ITS), 28S rDNA (LSU), β-tubulin (TUB2), translation elongation factor 1-α (EF), actin (ACT), and RNA polymerase II second largest subunit (RPB2), were sequenced (Verkley et al. 2013) and deposited in GenBank (Accession Nos: PQ061282, PQ106848, PQ120988, PQ120980, PQ120993, and PQ120983). Results of the BLASTn search revealed a nucleotide difference with the ex-epitype culture (CBS 102323) of Septoria cerastii in ITS and LSU regions but a 100% match with reference sequences of S. cerastii (KF252834.1 for TUB2, KF253309.1 for EF, KF253666.1 for ACT, and KF252363.1 for RPB2). In a phylogenetic tree reconstructed using the multi-loci sequences, the Korean isolate formed a well-supported group with reference isolates of Septoria cerastii. Pathogenicity was tested by spraying a conidial suspension (2 × 105 conidia/ml) harvested from a four-week-old PDA culture (KACC 410466) onto the leaf surface of three healthy C. glomeratum plants. Three control plants were sprayed with sterile distilled water. All plants were kept in a dark dew chamber at 26 °C for 24 h and then moved to a greenhouse (22 to 24 °C, >70% relative humidity, and a 12-h photoperiod). After a week, symptoms identical to those observed in the field developed on the leaves inoculated with the fungus. No symptoms were observed on control plants. Septoria cerastii was re-isolated from symptomatic lesions of the inoculated plants, and its identity was confirmed by morphology, thus fulfilling Koch's postulates. This pathogen has been recorded on many species of Cerastium worldwide, including the occurrences on C. glomeratum reported in Australia, New Zealand, and Portugal (Priest 1997, Farr & Rossman 2024). To our knowledge, this is the first report of leaf blight on C. glomeratum caused by S. cerastii in Asia as well as in Korea.