Assessing Susceptibility of Grapevine Cultivars to Latent Plasmopara viticola Infections Using Molecular Disease Index

Traditional assessments of grapevine susceptibility to grapevine downy mildew (GDM) caused by Plasmopara viticola rely on the visual evaluation of leaf symptoms. In this study, we used a well-established quantitative real-time PCR TaqMan assay (real-time PCR) to quantify the number of P. viticola infecting 12 grapevine cultivars under controlled conditions. The molecular disease index (MDI), derived from molecular detection methods, reflects the relative abundance of pathogens in plant tissues during the latent infection phase. Our application of MDI revealed a progressive increase in latent P. viticola levels over time, indicating dynamic levels of latent P. viticola infection across the inoculation processes. We calculated the area under the disease progression curve in terms of MDI (AUDPCMDI) to evaluate the susceptibility of each cultivar to GDM. Cultivars with lower AUDPCMDI values consistently exhibited reduced pathogen establishment, suggesting higher levels of innate resistance. Correlation analysis revealed a significant correlation between the visual disease index (DI) and the AUDPCMDI values (r = 0.790, P = 0.002), indicating that higher levels of latent P. viticola infections were associated with higher disease severity. Grapevine cultivars were clustered into distinct groupings, indicating variability in their susceptibility to the pathogen. Cultivars with similar levels of susceptibility were grouped, highlighting that the real-time PCR assay used in this study represents a robust, rapid, and standardized method for quantifying pathogens, which significantly improves the efficiency of evaluating the susceptibility of grapevine cultivars to GDM This quantitative protocol provides practical guidelines for selecting resistant cultivars and implementing effective disease management strategies.