Türkkan, M.Özer, G.Bozoglu, T.Erper, İ.Derviş, S.2026-02-152026-02-1520260931-17851439-0434https://doi.org/10.1111/jph.70243https://hdl.handle.net/20.500.12514/10319Hazelnut (Corylus avellana L.) is a cornerstone of Türkiye's agricultural economy, accounting for over 60% of global production. In 2022, a wood decay disease was observed in orchards of Samsun Province, affecting 3% (15 of 500) of surveyed trees. Symptoms included branch dieback and canopy chlorosis, while internal inspection revealed extensive brown rot of the xylem. Fungal isolates consistently produced white, fast-growing colonies with clamp connections. Multilocus sequencing of the internal transcribed spacer (ITS), large subunit (LSU) and small subunit (SSU) ribosomal DNA regions, and the partial translation elongation factor 1-alpha (tef1) gene identified the pathogen as Fomitopsis marianiae. Phylogenetic analysis of ITS + tef1 datasets placed the Turkish isolate within the F. marianiae clade. Pathogenicity was confirmed by inoculating 1-year-old hazelnut seedlings, which developed necrotic lesions averaging 3.88 ± 0.22 cm within 3.5 months; the fungus was successfully reisolated, fulfilling Koch's postulates. This study represents the first report of F. marianiae as a pathogen of hazelnut and, more broadly, its first documentation in any agricultural system. These findings identify F. marianiae as an emerging threat to a globally important nut crop and highlight the need for targeted disease surveillance and management strategies. © 2026 Wiley-VCH GmbH. Published by John Wiley & Sons Ltd.en10.1111/jph.70243info:eu-repo/semantics/closedAccessBasidiomycotaBrown RotCorylus AvellanaEmerging PathogenFomitopsis MarianiaeFungal DiagnosticsWood DecayArticle2-s2.0-105028724759