First Report of Coniella quercicola Causing Leaf Spot on Palmate Maple ( Acer palmatum Thunb.) in Korea

Palmate Maple (Acer palmatum Thunb.) is a widely cultivated ornamental species valued for its high aesthetic appeal and distinctive autumn foliage coloration. In August 2025, leaf spots and blights were observed on several maple trees aged approximately 15–20 years, growing in Cheonan (36°47′34.548′′N, 127°05′59.424′′E) and Seoul (37°37′14.34′′N, 127°02′34.835′′E), Korea. Initial symptoms appeared as irregular brown spots that later expanded into grayish-brown blighted areas. Five symptomatic leaves were surface-sterilized and placed onto potato dextrose agar plates (PDA), and incubated at 28 °C for 5–7 days. Two fungal isolates showing similar morphological characteristics were obtained. Colonies on PDA were white, with sparse mycelia. Conidiomata produced numerous aseptate, hyaline, cylindrical to slightly curved conidia, measuring 9.4–18.2 × 1.2–4.1 μm (n = 38), consistent with those of Coniella quercicola (Alvarez et al. 2016). For identification of the isolates (S10 and HM3), the loci of the internal transcribed spacer (ITS) and the large subunit of nuclear ribosomal RNA (LSU), and the gene for translation elongation factor 1-alpha (TEF1-α) were amplified using primers ITS1F/ITS4 (White et al. 1990), LR0R/LR5 (Vilgalys and Hester 1990), and EF1-728F/EF1-986R (Carbone and Kohn 1999), respectively. Sequences were deposited in GenBank with the following accession numbers: ITS =PX642505, PX642550, LSU =PX642508, PX642681, and TEF1-α =PX736860, PX736859. Based on BLAST searches in NCBI, the isolates showed high sequence similarity to Coniella quercicola, with 100% identity for ITS (ON791176) and LSU (MH871258), and 98.33% (S10) and 98.42% (HM3) identity for TEF1-α (KX833698). In the maximum-likelihood phylogenetic tree based on concatenated ITS, LSU, and TEF1-α sequences, our isolates clustered with Coniella quercicola (CBS 904.69, neotype; MFLU 19-2800; CBS 283.76; and CC.CBG09), and this clade further clustered with other strains of C. quercicola with 100% ML bootstrap support. For the pathogenicity test, three branches (each bearing four leaves) were selected from a healthy A. palmatum tree, estimated to be about 30 years old. The leaves were artificially wounded with sterile needles and inoculated in situ with 10 µL of conidial suspension (1 × 10⁶ conidia/mL), with two replicates per isolate. Control leaves were inoculated with 10 µL of sterile distilled water. Inoculated branches were covered with plastic bags to maintain high humidity for 2 days and subsequently exposed to ambient environmental conditions. Typical lesions developed on leaves inoculated with the conidial suspension, whereas control leaves remained symptomless. The pathogen was re-isolated from symptomatic tissues and identified as C. quercicola based on morphological characteristics and molecular data, thereby fulfilling Koch’s postulates. Coniella quercicola has a worldwide distribution on various hosts. For example, in China it has been reported from A. palmatum, Elaeocarpus decipiens, and Eucalyptus cloeziana, and in Europe from Quercus robur (Alvarez et al. 2016; Wang et al. 2022; Zou et al. 2023; Tang et al. 2024). In this study, C. quercicola is isolated, and to our knowledge, this represents the first report of this species on A. palmatum in Korea. The occurrence of pathogen on ornamental Acer trees suggests a potential risk to landscape and forest ecosystems, highlighting the need for continued monitoring and management.