Quantification of the influence of the downy mildew (<i>Plasmopara viticola</i>) epidemics on the compensatory capacities of <i>Vitis vinifera</i> 'Merlot' to limit the qualitative yield damage

Abstract

A preview study on the analysis of the impact of downy mildew (Plasmopara viticola) epidemics on the plant growth and yield quality has shown no correlation between disease severity progress on the canopy and sugar accumulation in the berries from veraison until harvest, indicating the capacity of the vine to compensate a stress situation induced by the downy mildew damage on leaf canopy. In this study the compensation capacity of the plant was analysed during three years under field conditions comparing three different downy mildew control strategies: A = “Untreated canopy” (to prevent quantity losses, the clusters were treated once with a contact fungicide at the discovery of the first downy mildew sporulation); B = “Reduced fungicide schedule” (based on a first treatment at the appearance of the first symptoms, to avoid yield quantity losses followed by one or two additional fungicide applications during the early epidemic phase with the aim of delaying the epidemic). C = “Standard schedule” (schedule normally applied in the vineyard). The experimental plot was moved each year to avoid stress influence due to a repetition of the trials on the same place. The grapevine compensated for the carbohydrate requirements of the cluster by mobilizing the starch reserves stored in the woody parts. Roots were the most important site of carbohydrate accumulation used from the grapevine and the mobilisation was higher in treatment A followed by treatment B. This compensation did not completely exhaust the reserves of each woody part and lets suppose a hierarchical pattern of the mobilisation; first a mobilisation of the reserves stored in the roots, and secondly those from the trunk, cane and shoot. The downy mildew could be considered as a stress factor for the grapevine.