Quantification and modeling of grapevine yield loss as a function of the severity of downy mildew on foliage and cluster

Grapevine is subjected to a range of pathogen attacks requiring a sustained phytosanitary protection during the vegetative phase to prevent yield losses and ensure harvest quality. However, the link between disease severities and crop losses has been little studied. We conducted, on two sites near Bordeaux (France) during three years, a field experiment to study the relationship between the severities of mildews and rots (on foliage and cluster) and crop losses (mass of grape clusters harvested and potential alcohol content). As expected under oceanic climate, downy mildew (DM) was the most prevalent disease in all sites and years. The three levels of grapevine phytosanitary protection used in the experiment generated contrasted epidemic situations. This dataset was used to fit phenomenological relationships describing how yield losses (modeled as the difference between the attainable and actual yields) increased with DM severity. We found that yield losses significantly increased with DM severity on clusters regardless of year or site. Yield losses also decreased with DM severity on foliage but to a variable extent depending on years and sites. This suggests that the transfer of DM epidemics from the foliage to the clusters is variable. We further show that such foliage-cluster transfer could relate to an humidity index during the period of major susceptibility of clusters to DM infections, between stages 53 to 79. Finally, we quantified how the severity of DM reduce the potential alcohol content while discussing the mechanisms underlying the annual variability of these relationships.