Diaporthe spp. Is Confirmed as the Main Fungus Associated with Defective Turkish Hazelnuts

The aim of this study was to identify the fungi associated with Turkish hazelnuts and verify and confirm the role of Diaporthe in causing kernel defects. Hazelnuts were sampled from 10 hazelnut orchards in seven Turkish provinces (Samsun, Ordu, Girensun, Trabzon, Sakarya, Düzce, and Zonguldak) during the early and full ripening stages in 2018, 2019, and 2020. Fungal isolation and identification at the genus level were performed for healthy hazelnut kernels and those with visible or hidden defects. Several fungal genera were isolated; those with a mean incidence greater than 10% were Aspergillus, Botryosphaeria, Diaporthe, Fusarium, and Penicillium. The incidence of Diaporthe spp. was higher in the full ripening stage than in the early ripening stage; it was also higher in kernels with defects than in healthy kernels. A similar pattern was observed for Botryosphaeria, but the opposite pattern was observed for Aspergillus. Diaporthe positively correlated with both hidden (ρ = 0.80) and visible defects (ρ = 0.77), confirming its key role in causing hazelnut defects. The role of Botryosphaeria appears limited to hidden defects; however, additional work is needed to substantiate this last finding. There is also a need to clarify the eventual interaction of Diaporthe with Aspergillus. The low incidence of defective hazelnuts among the 3 years of sampling precluded the role of meteorological factors in the incidence of hazelnut defects from being elucidated.