AbstractA new fungal species constantly associated with hazelnut (Corylus avellana) fructification starting from its primordia is described. The fungus is associated with hazelnut fruit during all their developmental stages, being consistently more present in spring (March–June). A 4-year survey has been conducted, from young fruit formation to full kernel maturity including also the post-harvest phase, to collect fungi associated with damaged/discoloured kernels. A collection of 60 isolates of a new species has been obtained in this study, which is here described as Didymella corylicola sp. nov. Multi-locus phylogenies based on four genomic loci (nuITS and LSU rDNA, RPB2 and TUB2) in combination with morphological data confirmed the fungus to represent a new species of Didymella (Didymellaceae). The occurrence of D. corylicola sp. nov. might have an impact on the quality of hazelnut production by contributing to kernel defects.