First Report of Stem Canker Caused by Diaporthe eres on Quercus glauca Thunb. in Korea

Quercus glauca Thunb., a warm-temperate tree species, is increasingly recognized as a promising candidate for climate change adaptation with an expanding distribution range. In April 2024, stem canker symptoms were observed in naturally infected Q. glauca seedlings, grown from seeds collected in Gotjawal, a natural Q. glauca habitat, and cultivated in a greenhouse at in Jeju, Korea. Diseased stems were collected and surface sterilized with 1% NaOCl (1 min) and 70% ethanol (30 s), rinsed three times with sterile distilled water, and cultured on potato dextrose agar (PDA) medium with 0.05% streptomycin sulfate. Emerging mycelia were hyphal-tipped and five morphologically identical isolates were obtained. All isolates were identified as Diaporthe eres based on ITS sequencing (ITS5/ITS4) (White et al., 1990). A representative isolate JSB24 produced white mycelia with wavy margins, formed black conidiomata in a circular pattern, and developed viscous conidia after 1 month of incubation. Alpha- and beta-conidia were harvested from the conidiomata. The alpha conidia were ellipsoidal, aseptate, hyaline, and contained two guttules, measuring 5.10 ± 0.70 × 2.20 ± 0.25 µm (n = 40). The beta conidia were relatively slender and elongated, aseptate, with both ends curved in a hook-like shape, and had an average size of 24.14 ± 2.17 × 1.40 ± 0.25 µm (n = 40). These morphological characteristics were consistent with those of the Diaporthe sp. (Yang et al., 2024). Translation elongation factor 1-alpha (TEF1) and beta-tubulin (TUB) genes were amplified using EF1-728F/EF1-986R (Carbone and Kohn, 1999) and Bt2a/Bt2b (Glass and Donaldson, 1995), respectively. Sequences of the representative isolate were deposited in GenBank (ITS: PV765850, TEF1: LC878875, TUB: LC878876.1). Pairwise alignment of each sequence from isolate JSB24 showed 94.98% (397/418), 98.82% (335/339), and 97.31% (217/223) identity with the ex-type D. eres strain CBS 138594. Multilocus phylogenetic analysis clustered JSB24 within D. eres, including the ex-type strain. For pathogenicity testing, healthy Q. glauca seedling stems were slightly wounded with a sterile blade, inoculated with mycelial plugs of JSB24 cultured on PDA for 7 d, and wrapped with Parafilm. Aseptic PDA plugs were used as the negative control. Six seedlings were used for each treatment group. Stem necrosis was observed in Q. glauca seedlings 3 weeks after inoculation with JSB24 including leaf abscission and brown necrosis of internal stem tissues, but no symptoms were observed in the control seedlings. The pathogen was re-isolated from symptomatic seedlings and identified as D. eres through sequence analysis. Based on USDA Fungal Database and Choi et al. (2023), D. eres has been reported to cause diseases on several host plants including Machilus thunbergii, Prunus mume, Ziziphus jujuba, and several Quercus species. However, this study is the first report of D. eres causing stem canker on Q. glauca in Korea. The findings are important in the sustainable production of Q. glauca, as a climate-resilient tree species, through early detection of threatening diseases at nursery stage, to facilitate their effective control and production of healthy seedlings, ensuring their successful establishment after out planting. Since the symptoms were only observed under greenhouse conditions, further investigations to determine the distribution and epidemiological significance of this disease under field conditions could important.