Diversity, Pathogenicity and Origin of Botryosphaeriaceae Species in French Walnut Orchards

Since 2015, there has been an increase in fruit damage, cankers, and tree mortality in French walnut orchards. Botryosphaeriaceae fungi are known, particularly in North America, for their role in causing these symptoms. In response to producer concerns, the project aims to characterize the fungal species associated and, given the difficulty of recognizing symptoms in the field, to develop specific and rapid detection tools. Samples of fruit and twigs from the two main production areas enabled the characterization of the pathobiome of symptomatic tissues and yielded 1,534 fungal isolates, 70 % of which belong to seven species, including Botryosphaeria dothidea and Neofusicoccum parvum. The origin of these ubiquitous species is probably local since these populations are genetically close to those isolated from French vineyards. After confirming their pathogenicity, a multiplex PCR-based tool was developed to detect them. This study suggests that a fungal complex is involved in the symptoms observed.