Characterization and pathogenicity of fungal species associated with dieback of apple trees in Northern Italy

Severe dieback symptoms were recently observed on apple (Malus × domestica) trees in Northern Italy, representing a growing concern for producers. Surveys were conducted over a 3-year period (2019-2021) and five apple orchards, from 5- to 12-years-old, were monitored. A total of 33 fungal isolates isolated from symptomatic plant was selected for characterization. The species identification was achieved through multilocus phylogenetic analyses performed on sequences of three genomic loci (ITS, tub2 and tef1). Morphological features were assessed and the average growth rate at different temperatures was determined. Seven species were identified in association with dieback of apple trees: Botryosphaeria dothidea, Cadophora luteo-olivacea, Diaporthe rudis, Diplodia seriata, Eutypa lata, Kalmusia longispora and Paraconiothyrium brasiliense. All the species were pathogenic when inoculated on healthy apple plants. Botryosphaeria dothidea resulted the most aggressive. This study provides an insight into the fungal species diversity associated with apple dieback and provides basis for further investigations to assess the phytosanitary status of plant materials in order to recommend and implement effective management strategies.