Résumé
IMPORTANCE Grapevine downy mildew caused by the oomycete Plasmopara viticola affects leaves and bunches and leads to important economic losses for viticulturists. Recently, evidence has accumulated that soilborne primary inoculum (i.e., oospores in the soil) importantly contributes to disease progress. The significance of our work is in presenting a direct and sensitive method for assessing soil oospore concentration, as well as quantitative and spatially explicit data on downy mildew primary inoculum. This opens the way to new research, the evaluation of new disease control strategies based on primary inoculum management and the improvement of epidemic risk models, which will potentially contribute to lower fungicide use in viticulture in fine.