Ganoderma Butt Rot of Hazelnut (Corylus avellana) Caused by Ganoderma adspersum in Türkiye

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Abstract In September 2022, hazelnut trees in six orchards in the Çarşamba district of Samsun province, a major hazelnut production area in the Black Sea region of Türkiye, showed symptoms of leaf yellowing, shoot dieback, decline, basal stem rot and the presence of brown bracket‐like sessile basidiocarps. Approximately 5%–10% of hazelnut trees were affected, highlighting the significant impact of the disease. A comprehensive morphological analysis was conducted to characterize fungal isolates obtained from symptomatic tissues as G. adspersum. This analysis incorporated features observed in basidiomata from declining trees and those reproduced after isolates were cultivated on wheat grain for spawn production and grown on an oak sawdust‐based substrate. Confirmation of identification was achieved through the utilization of the internal transcribed spacer (ITS), large subunit (LSU) and small subunit (SSU) loci, along with sequencing and subsequent phylogenetic analysis. The G. adspersum isolates generally exhibited no significant growth differences at similar temperatures but demonstrated enhanced growth at 30°C, while growth was completely inhibited at 40°C. Incompatible reactions between isolates confirmed that distinct genotypes were present, highlighting genetic diversity within the species. In the pathogenicity trials, employing a single infested wheat grain per inoculation at the wounded site, hazelnut suckers exhibited distinct brown discolouration surrounding the inoculation site following a 2.5‐month incubation period. Significantly, lesions exceeding 3 cm in length were observed, providing clear evidence of vigorous pathogenic activity by G. adspersum. This study is the first report of G. adspersum causing butt rot in hazelnuts, underscoring the significance of this finding for hazelnut cultivation and providing a foundation for future research and disease management strategies.