Alternaria species associated with a new apple disease in North Italy

About 10 years ago, a new apple disease spread in North Italy, starting from Alto Adige, then in Trentino, Veneto, Piedmont and Emilia-Romagna. The disease affects leaves and fruits. Leaves show small circular brown spots, sometimes with concentric ring and purple-red halo, while fruits show rounded brown-black spots, often centered on lenticels, sometimes with a reddish halo. These symptoms loook like the "Alternaria blotch" (caused by Alternaria mali) but preliminary studies showed that the strains of Alternaria isolated in Italy don't belong to A. mali. Within the research project PRIN, 204 strains of Alternaria spp. were isolated from either leaves (40%) or fruits (60%) sampled in the most important apple-growing areas of northern Italy, and tested for pathogenicity. Different inoculation protocols were used, with either conidia or cultural filtrate (autoclaved or not), different inoculation methods on wounded or unwounded fruits or leaves, and incubation periods. Inoculation of drops of cultural filtrates on wounded leaves provided consistent results; 84 and 27% of the strains isolated from fruits and leaves were pathogenic, respectively. Highest frequency of pathogenic strains was observed in Piedmont (81% of strains) and the lowest in Emilia-Romagna (17%). Based on a morphological characterization, 17 out of 37 pathogenic stains belonged to the Tenuissima species-group, 12 to Gaisen, 4 to Alternata, 3 to Arborescens, and 1 to Armoraciae. This study made it possible to develop an efficient bioassay for pathogenicity of Alternaria strains and increased knowledge about etiology of this new apple disease which is spreading in North Italy.