Résumé
Grapevine downy mildew, caused by the oomycete Plasmopara viticola, is one of the most devastating diseases of grapevine worldwide. While primary inoculum plays a decisive role in disease epidemics, we still know very little about the abundance and the distribution of oospores, which are the overwintering forms of the pathogen resulting from sexual reproduction. In this study, we used ddPCR to describe the spatial distribution of P. viticola inoculum in a vineyard soil at the onset of the growing season. We found P. viticola oospores in all soil samples except one. The distribution of primary inoculum at the field scale was not random but characterized by 25 m-diameter patches of concentrically increasing oospore concentration. There was a positive coregionalization between soil inoculum and soil moisture spatial distributions, possibly mediated by disease incidence. The results indicated that oospores accumulated 5 times more in the ridge of soil below the vine stocks than in the inter-row. We conducted a leaf disc bioassay to assess soil infectious potential on a subset of samples collected in the field. Soil infectious potential estimated through infected leaf-disc area was positively correlated with our DNA-based quantification of oospores. Overall, the quantitative and spatially explicit survey of primary inoculum reservoir gained from these molecular and biological methods will contribute to the design of management strategies aimed at preventing primary inoculum accumulation in the vineyard from one season to the next.