First Report of Fomitopsis marianiae as a Causal Agent of Brown Wood Rot in Hazelnut ( Corylus avellana ) Worldwide

ABSTRACT Hazelnut ( 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.