Biophysical and agronomical drivers of the distribution of Plasmopara viticola oospores in vineyard soils

Abstract

Grapevine downy mildew (GDM), caused by the obligate biotroph oomycete Plasmopara viticola, is one of the most destructive diseases in viticulture. P. viticola’s life cycle is comprised of two parts: the asexual phase, which occurs during the grapevine’s growing season and in which the zoospores propagate on green parts of the vine; and the sexual phase, in which the crossing of strains in the leaves leads to the formation of oospores that overwinter in grapevine leaf debris and the surrounding soil. Oospores constitute the primary inoculum for downy mildew epidemics during the following growing season. While conventional GDM management relies primarily on fungicides to prevent the asexual propagation of the pathogen, the primary inoculum is increasingly thought to play a major role in GDM epidemics. This suggests that measures based on reducing primary inoculum or interrupting the pathogen’s sexual cycle have the potential to significantly reduce epidemic pressure. Such a paradigm shift in management strategy will require a thorough understanding of the environmental drivers of GDM primary inoculum spatio-temporal distribution.

With this in mind, we carried out an extensive study of P. viticola oospore concentrations in vineyard soils across Saint-Emilion. In November 2023 and February 2024, soil samples were collected across 63 Bordeaux châteaux. The soils were analysed for GDM oospore concentrations using droplet digital Polymerase Chain Reaction (ddPCR) and soil physio-chemical properties. We also conducted surveys with the viticulturists to characterize vineyard management practices and the disease history of each parcel.

We analysed the effects of season, soil physio-chemical properties, cover cropping type and duration, vineyard treatments, soil management practices, and GDM sensitivity of the parcel on GDM oospore concentrations in the soil. Our results provide insight into the spatial distribution of oospores in vineyard soils and into which cover cropping and soil management modalities may influence their integration into the soil. For example, our data indicate that there is a reduction in oospores between November and February when a winter cover crop is implemented compared to other cover cropping modalities. This work identifies key biophysical and agronomical drivers of P. viticola spatio-temporal distribution, contributing to a broader project to make GDM management more sustainable by finding ways to interrupt the pathogen’s sexual cycle.