Leaf Trichome Density Rather than Phenolic Compounds Determines the Resistance of Hazelnut Cultivars to Erysiphe corylacearum

Erysiphe corylacearum, a newly emerging pathogen of hazelnut (Corylus avellana L.), has rapidly spread across Europe, causing severe outbreaks and threatening the sustainability of hazelnut production. This study investigated the biochemical and morphological traits associated with cultivar susceptibility to E. corylacearum. Eight cultivars representing a range of resistance levels were analyzed for leaf phenolic composition and trichome density. A total of 22 phenolic compounds were identified, with myricetin-3-O-rhamnoside, 1,7-bis-(4-hydroxyphenyl)-4,6-heptadien-3-one-hexoside, hirsutenone-hexoside, and quercetin-3-O-rhamnoside as dominant metabolites; however, no correlation was found between constitutive phenolic content/profile and disease resistance. In contrast, leaf trichome density showed a strong negative association with susceptibility, suggesting that denser trichomes act as a physical barrier to infection. Our results indicate that trichome density is a key morphological trait conferring resistance to E. corylacearum and could serve as a practical marker for breeding and selection of more resistant hazelnut cultivars.