Combined Metabarcoding and Multi-locus approach for Genetic characterization of Colletotrichum species associated with common walnut (Juglans regia) anthracnose in France

Juglans regia (walnut) is a species belonging to the family Juglandaceae. Broadly spread in diverse temperate and subtropical regions, walnut is primarily cultivated for its nuts. In France, Colletotrichum sp. on walnut was detected for the first time in 2007; in 2011 the disease led to 50–70% losses in nut production. A combined approach of metabarcoding analysis and multi-locus genetic characterization of isolated strains has been used for taxonomic designation and to study the genetic variability of this pathogen in France. Evidence indicates that four Colletotrichum species are associated with walnut in France: 3 belong to the C. acutatum species complex and 1 to the C. gloeosporioides species complex. Results also show that C. godetiae is the most abundant species followed by C. fioriniae; while C. nymphaeae and another Colletotrichum sp. belonging to the C. gloeosporioides complex are found rarely. Representative isolates of detected species were also used to confirm pathogenicity on walnut fruits. The results show a high variability of lesion’s dimensions among isolates tested. This study highlights the genetic and pathogenic heterogeneity of Colletotrichum species associated with walnut anthracnose in France providing useful information for targeted treatments or selection of resistant cultivars, in order to better control the disease.