Quantification of grapevine yield losses as a function of downy mildew severity on foliage and cluster

To quantify the relationship between grapevine disease severity and crop losses at the plant scale, we conducted a three-year field experiment at two sites near Bordeaux (France), monitoring mildews and rots on both foliage and clusters. Preliminary analyses revealed that only downy mildew (DM) significantly impacted yield under our experimental conditions. We capitalized on this single-disease dominance to isolate the effect of DM severity (assessed via standardized AUDPC), on both foliage and clusters, on yield loss at the individual vine scale. At a cluster AUDPC of 5%, yield consistently declined by 24% across all years and sites. Conversely, yield loss at a 5% foliar AUDPC ranged from 9% to 60% depending on the site and year, reflecting a tenfold variation in the damage coefficient. This variability strongly correlated with a moisture risk index computed between the phenological stages of visible inflorescences and pre-ripening berries. Specifically, at 5% foliar AUDPC, the expected yield loss escalated from 9% in a dry season (10th percentile of the index) to 15% in an intermediate season (median), reaching 39% in a wet season (90th percentile). Future research should aim to validate these statistical models across broader climatic, geographic, and agronomic contexts, including the characterization of cultivar-specific responses.