Development of overwintering oospores of Plasmopara viticola and severity of primary foci in relation to climate *

The French Plant Protection Service uses different techniques to provide advice on treatments against grapevine downy mildew: observation of primary foci, development of overwintering oospores and climate‐based forecasting models. This system is generally satisfactory but is time‐consuming, and the models used sometimes give a false evaluation of the severity of primary foci. We have accumulated 30 years of observations on the latter, together with the maturation of overwintering oospores. The severity of primary foci is linked to spring and autumn rainfall. Oospore maturation is affected by low autumn and warm spring temperatures. Long dry periods can block maturation. While there is little prospect that oospore maturation can be modelled in the near future, success can be expected for primary foci.