Etiology and Epidemiology of Branch Dieback and Fruit Blight and Necrosis of English Walnut in France

English walnut (Juglans regia L.) is an economically important fruit crop worldwide and ranks second in France in terms of cultivated area, following apples. This study presents the first comprehensive investigation into the etiology and fungal community analysis associated with dieback symptoms in walnut trees in France, where this disease has been increasingly reported since 2015. Both symptomatic and asymptomatic twigs and husks were collected from 12 commercial walnut orchards located in the two primary production regions from 2020 to 2022. Several fungal species-among Botryosphaeriaceae, Colletotrichum, Diaporthe, and Fusarium taxa-were consistently isolated from symptomatic husks and twigs. Among these, Botryosphaeria dothidea and Colletotrichum godetiae were significantly associated with symptomatic husks, and Diaporthe eres and Fusarium juglandicola with symptomatic twigs, as confirmed by ITS2 metabarcoding sequencing. Additionally, Neofusicoccum and Diaporthe were significantly associated with warm and dry years, unlike Colletotrichum. Following pathogenicity assays on twigs and fruits, N. parvum emerged as the most aggressive species. These results suggest a complex interplay of factors influencing disease dynamics, further compounded by potential interactions among the main pathogenic species. Notably, in twigs, we found that the prevalence of D. eres was negatively correlated to that of N. parvum while the association between D. eres and F. juglandicola was the most frequent one, suggesting both competitive and facilitative interactions. This research underscores the importance of understanding the fungal pathobiome and its interactions in the development of walnut tree diseases.