Molecular Characterization and Pathogenicity of Diaporthe Species Causing Nut Rot of Hazelnut in Italy

Hazelnut (Corylus avellana), a nut crop that is rapidly expanding worldwide, is endangered by a rot. Nut rot results in hazelnut defects. A survey was conducted in northwestern Italy during 2020 and 2021 to identify the causal agents of hazelnut rots. Typical symptoms of black rot, mold, and necrotic spots were observed on hazelnuts. The prevalent fungi isolated from symptomatic hazelnut kernels were Diaporthe spp. (38%), Botryosphaeria dothidea (26%), Diplodia seriata (14%), and other fungal genera with less frequent occurrences. Among 161 isolated Diaporthe spp., 40 were selected for further analysis. Based on morphological characterization and multilocus phylogenetic analysis of the ITS, tef-1α, and tub2, seven Diaporthe species were identified as D. eres, D. foeniculina, D. novem, D. oncostoma, D. ravennica, D. rudis, and D. sojae. D. eres was the main species isolated from hazelnut rots, in particular from moldy nuts. The pathogenicity test performed on hazelnuts 'Tonda Gentile del Piemonte' using a mycelium plug showed that all the Diaporthe isolates were pathogenic on their original host. To our knowledge, this work is the first report of D. novem, D. oncostoma, and D. ravennica on hazelnuts worldwide. D. foeniculina, D. rudis, and D. sojae were reported for the first time as agents of hazelnut rot in Italy. Future studies should focus on the comprehension of epidemiology and climatic conditions favoring the development of Diaporthe spp. on hazelnut. Prevention and control measures should target D. eres, representing the main causal agents responsible for defects and nut rot of hazelnuts in Italy.