Quantitative effect of leaf damage caused by downy mildew (<i>Plasmopara viticola</i>) on growth and yield quality of grapevine 'Merlot' (<i>Vitis vinifera</i>)

Abstract

The impact of downy mildew (Plasmopara viticola) epidemics on the plant growth and yield quality was analysed during three years under field conditions in order to show a relationship between disease severity on leaves and yield quality losses. Three different treatments were compared: 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 epidemic progress in the treatment A has shown the same tendency during the three years with an increase starting from the beginning of the ripening phase. The disease severity was more important on the lateral than on the main leaves. During the three years of the study, the disease did not influence the amount of total healthy leaf area per plant until veraison. From this phenological stage until harvest, the healthy leaf area per plant decreased rapidly at the same time as the epidemic increased. The yield quantity was not affected indicating that a single fungicide application at the finding of the first sporulation was enough to preserve the crop production. Among the yield quality parameters, the sugar content has been negatively influenced by the downy mildew leaf damage. The difference was particularly evident between the treatments A and C with differences from 1.4 to 2.04 °Brix. Generally, treatment B didn’t differ from C. Sugar uptake in the berries begun to show a different dynamic between 7 and 14 days after the onset of ripening. No correlation between disease severity progress on the canopy and sugar accumulation in the berries from veraison until harvest was found, indicating the capacity of the vine to compensate a stress situation induced by the downy mildew damage on leaf canopy.