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

Abstract

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), surveying mildews and rots on both foliage and clusters. A first set of analysis indicated that only downy mildew (DM) significantly affects yield (mass of grape clusters harvested) in our experimental conditions. We then leverage this situation to model the relationship between DM severity (measured by standardized AUDPC) and yield losses at vine stock scale. For AUDPC ranging from 0 to 10%, an increase of the severity of DM of 1% on the clusters decrease yield by 2% regardless of years and sites. These values ranges from 1.1% to 9% when considering the severity of DM on the foliage, depending on sites and years. This variability was correlated with a moisture risk index calculated between crop stages inflorescences visibles to pre-ripening of the berries. An increase of the severity of DM of 1% on the foliage decreases yield by 1.2% during dry year (quantile 10% of the index), by 3.3% during intermediate year (median the index) and by 7.3% during wet year (quantile 90% of the index). Regarding quality variables measuring grape ripeness on the must, we found that an increase of the severity of DM of 1% on the foliage decrease the potential alcohol content from 0.014 to 0.18 % vol depending on years while few effects on the pH were detected.