Septoria lamiicola Causing Leaf Spot on Lamium purpureum in Korea.

Lamium purpureum L. (purple dead-nettle), an annual herbaceous plant in the family Lamiaceae, is native to Europe and western Asia. It has become invasive in eastern Asia, including Korea and North America. Due to its rapid growth and tolerance of disturbed soils, it invades croplands and natural ecosystems, reducing crop yields (Mock et al. 2009). It also serves as an overwintering host for cucumber mosaic virus (Tomlinson et al. 1970). Leaf spots on purple dead-nettle were first observed in April 2022 in Gimje (35°43'10"N, 127°00'42"E) and in April 2023 in Jangsu (35°36'16"N, 127°22'43"E), Korea, with a disease incidence of 20-30% among surveyed plants. Symptoms were distinct, angular, pale brown to brown, and bordered by leaf veins. Numerous blackish conidiomata with cirriform conidial horns were visible on the spots. Conidiomata were numerous, scattered, pycnidial, amphigenous but abundantly epigenous, scattered, dark brown to rusty brown, globose, embedded in host tissue or partly erumpent, 60 to 110 µm in diameter, with ostioles measuring 28 to 56 µm in diameter. Conidiogenous cells lining the inner wall layer were hyaline and ampulliform. Conidia were filiform, straight or slightly curved, 32 to 48 × 2 to 3 µm, hyaline, 2 to 5-septate, but often 3-septate. Morphological descriptions were consistent with Septoria lamiicola Sacc. (Verkley et al. 2013). For monoconidial isolation, a conidial horn was collected, placed in a 1.5 ml tube containing sterile water, and streaked onto the surface of 2% water agar (WA). After four days, a hyphal tip was transferred to fresh potato dextrose agar (PDA). Colonies incubated for two weeks at 25°C on PDA were measured 10 to 15 mm in diameter, with a white to greyish at the center and a dark grey to black at the margin. Voucher specimens were deposited at the Korea University Herbarium (KUS-F32758 and F33612), and a representative culture was deposited at the Korea Agricultural Culture Collection (Acc. No. KACC 410468). Nucleotide sequences of 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) genes were determined (Verkley et al. 2013) and deposited in GenBank (Accession Nos: PQ061283, PQ106849, PQ120989, PQ120981, PQ120994, and PQ120985). BLASTn search of the sequences showed 100% identity with the reference sequences of Septoria lamiicola CBS 102328 for ITS (KF251441.1), LSU (KF251945.1), ACT (KF253745.1), and RPB2 (KF252438.1). The TUB2 sequences showed 99.7% similarity (1/311 nucleotides different) with KF252913.1, and the EF sequences showed 96.0% similarity (13/368 different) with KF253389.1. In a phylogenetic tree reconstructed using the multi-loci sequences, the Korean isolate formed a well-supported group with reference isolates of S. lamiicola (Verkley et al. 2013). Pathogenicity was tested twice by spraying a conidial suspension (1×104 conidia/mL) harvested from a four-week-old culture (KACC 410468) onto twenty leaves of three-potted plants (two months old). Three control plants were sprayed with sterile distilled water. Plants were placed in a dew chamber at 26°C for 24 h before being transferred to a greenhouse. After ten days, typical symptoms appeared on the inoculated leaves, while no symptoms were observed on the controls. S. lamiicola was re-isolated from the symptomatic lesion and confirmed by microscopy, fulfilling Koch's postulates. Although S. lamiicola has been recorded on L. purpureum in Bulgaria, Poland, and Portugal (Farr and Rossman 2024), there is no previous record of this fungus in Korea. To our knowledge, this is the first report of leaf spot caused by S. lamiicola on L. purpureum in Korea. Septoria lamiicola could be utilized as a biological control agent against this invasive weed, thereby supporting sustainable management strategies for croplands and natural ecosystems.