Different susceptibility of local Apulian and Calabrian grapevines to downy mildew and characterization of Plasmopara viticola populations

Downy mildew, a severe grapevine disease worldwide, is caused by different cryptic species of Plasmopara viticola. Several approaches are under study to reduce the impact of chemicals in the disease management, including the selection of resistant genotypes for grapevine breeding. In this study, the susceptibility of six different local wine grape cultivars was assessed on leaves and bunches, in 2023, an year particularly favourable to the downy mildew epidemy. Based on McKinney's index (MKI: 37.3 to 52.6%), Marchione, Moscato reale, Maresco and Moscato di Terracina were more susceptible than Moscato di Trani, Trebbiano and Verdeca, classified as resistant ones (MKI: never exceeding 16.6%). Additionally, the genetic variability of P. viticola populations infecting grapevine in Apulia, Calabria and Lombardia was evaluated. Sporangia were collected from oil-spots in 26 vineyards (15 Apulia, 11 Calabria), during 2023 and 2024 years. The CAPS analysis was carried on restriction enzyme AseI profiles of the amplified DNA using the ITS1-O/ITS2, according to Rouxell et al. (2013). A total of 130 isolates and 20 DNA samples from oil spots sampled in Lombardia were analysed. All belong to the clade aestivalis, as also confirmed by BLASTn analysis of the ITS partial sequence of few representative isolate of Apulia and Calabria regions. These preliminary results suggest that the resistant behaviour of analysed grapevine genotypes is limited to P. viticola clade aestivalis and highlight the opportunity to preserve the local biodiversity for possible introduction of new P. viticola clades, currently absent.