Etiology and epidemiology of branch dieback and fruit blight and necrosis of English walnut in France

Abstract

Abstract (Level 1 heading) English walnut (Juglans regia L.) is an economically-important fruit crop worldwide, particularly in France, where it is the second most important fruit crop after apples in terms of cultivated area. Walnut orchards are targeted by numerous diseases, but new symptoms have been widely observed since 2015 in France, consisting in typical branch dieback and fruit blight and necrosis. Herein, both symptomatic and asymptomatic twigs and husks were collected from 12 commercial French walnut orchards from the two main production areas located in the South of France from 2020 to 2022. Overall, Botryosphaeriaceae, Colletotrichum, Diaporthe, and Fusarium species were consistently isolated from symptomatic husks and twigs. Among these taxa, B. dothidea and C. godetiae were significantly more associated with symptomatic husks, contrary to D. eres and F. juglandicola, more frequently isolated from symptomatic twigs, as also confirmed by ITS2 metabarcoding sequencing. Tissue type was the primary factor significantly impacting the composition of the fungal communities (R2 = 16.6%), followed by sampling campaign and production area (R2 = 11.6% and 4.6%, respectively). We also found that Neofusicoccum and Diaporthe were significantly more associated with warm and dry years, unlike Colletotrichum. Pathogenicity tests revealed that only Botryosphaeriaceae and Diaporthe species were pathogenic on walnut fruits and twigs, with B. dothidea and N. parvum being the most aggressive species. Several significant positive and negative correlations were identified based on SparCC co-occurrence networks and correlograms based on prevalence data. In particular, the positive correlation between Colletotrichum and Fusarium species on symptomatic twigs was highlighted by both methods. This work is the first comprehensive etiological and epidemiological study of branch dieback and fruit blight and necrosis of English walnut in France.

Competing Interest Statement

The authors have declared no competing interest.