Identification and pathogenicity of species associated with leaf blotch disease and premature defoliation in French apple orchards.

Leaf blotch caused by spp. is a common disease in apple-producing regions. The disease is usually associated with one phylogenetic species and one species complex, and the species complex ( SC), respectively. Both taxa may include the apple pathotype, a quarantine or regulated pathogen in several countries. The apple pathotype is characterized by the production of a host-selective toxin (HST) which is involved in pathogenicity towards the apple. A cluster of genes located on conditionally dispensable chromosomes (CDCs) is involved in the production of this HST (namely in the case of the apple pathotype). Since 2016, leaf blotch and premature tree defoliation attributed to spp. have been observed in apple-producing regions of central and south-eastern France. Our study aimed to identify the species involved in apple tree defoliation and assess the presence of the apple pathotype in French orchards. From 2016 to 2018, 166 isolates were collected and identified by multi-locus sequence typing (MLST). This analysis revealed that all these French isolates belonged to either the SC or . Specific PCR detection targeting three genes located on the CDC did not indicate the presence of the apple pathotype in France. Pathogenicity was assessed under laboratory conditions on detached leaves of Golden Delicious and Gala apple cultivars for a representative subset of 28 isolates. All the tested isolates were pathogenic on detached leaves of cultivars Golden Delicious and Gala, but no differences were observed between the pathogenicity levels of SC and . However, the results of our pathogenicity test suggest that cultivar Golden Delicious is more susceptible than Gala to Alternaria leaf blotch. Implications in the detection of the apple pathotype and the taxonomic assignment of isolates involved in Alternaria leaf blotch are discussed.