First Report of Fusarium fujikuroi Causing Wilt of Norway spruce Seedlings in Korea

Norway spruce (Picea abies (L.) H. Karst.) is a coniferous species introduced to Korea for its ornamental and economic value. Seedlings of P. abies were grown from seeds obtained from the Korea National Forest Seed and Variety Center and cultivated in commercial potting mix for research purposes in a greenhouse at Chonnam National University, Gwangju, Korea (35°17′27″ N, 126°53′31″ E). In May 2024, the two-year-old seedlings exhibited browning symptoms in the inner foliage, which progressed to the outer needles within two weeks and subsequently wilted within a month, with a disease incidence of 33% (30/90 seedlings). For pathogen isolation, 1-cm segments were excised from the transition zone between healthy and diseased tissues of leaves, stems, and roots of three symptomatic seedlings (total nine samples). Samples were surface-disinfected in 1% sodium hypochlorite (1 min) and 70% ethanol (30 s), rinsed thrice with sterile distilled water, and air-dried. For each tissue type from each seedling, 3–5 segments were placed per potato dextrose agar (PDA) plate with 0.05% streptomycin and incubated at 25 °C for 5 days (Leslie and Summerell, 2006). Emerging fungal colonies were purified through repeated hyphal-tip transfers onto fresh PDA plates. Six isolates with identical morphologies were obtained from stem, root, and leaf tissues of all three symptomatic seedlings, exhibiting aerial cottony mycelia, whitish surfaces, and pinkish reverse sides. Microconidia and macroconidia were examined on carnation leaf agar after 16 days of incubation (25 °C and 12 h photoperiod). Microconidia were abundant, elongate-oval to cylindrical, predominantly 0–1 septate, and measured 4.6–11.5 × 2.2–3.4 μm (n = 50). Macroconidia were slender, slightly falcate, 3–5 septate, and measured 24.3–46.3 × 2.6–3.6 μm, with curved apical cell and foot-shaped basal cell, consistent with descriptions of Fusarium fujikuroi (Leslie & Summerell, 2006). The internal transcribed spacer (ITS; ITS5/ITS4), translation elongation factor 1 alpha (tef1; EF1/EF2), RNA polymerase II second-largest subunit (rpb2; 7cF/11aR), and β-tubulin (tub2; Bt-2a/Bt2-2b) regions of the six isolates were sequenced, and the resulting sequences were deposited in GenBank (ITS: PZ039069–PZ039073 and PZ230286, tef1: LC939472–LC939476 and LC922555, rpb2: LC939477–LC939481 and LC922556, and tub2: LC939482–LC939486 and LC922557). Multilocus phylogenetic analysis (tef1, rpb2, and tub2) showed that all six isolates clustered within the F. fujikuroi clade together with reference strains, including the ex-type strain (CBS 221.76). Pathogenicity assays were performed on healthy one-year-old P. abies seedlings following Koch’s postulates. Five seedlings were inoculated with F. fujikuroi (1-mm mycelial plugs), inserted in the stems with a sterile scalpel and sealed with parafilm, while five control seedlings received sterile PDA only (Akgül et al., 2024). All inoculated seedlings developed leaf browning and wilting within 7 days after inoculation and died within 20 days. F. fujikuroi was re-isolated from all five symptomatic seedlings, whereas control seedlings remained asymptomatic. Although F. fujikuroi is a well-known pathogen causing bakanae disease of rice (An et al., 2023; Shin et al., 2023), this is the first report of its pathogenicity on a woody host, P. abies in Korea. This finding highlights its potential threat to P. abies seedling production and provides useful information for its management.