Postponing First Shoot Topping Reduces Grape Cluster Compactness and Delays Bunch Rot Epidemic

Field trials investigating the impact of the timing of first shoot topping in a vertical shoot positioning (VSP) trellis system on (i) the cluster morphology, (ii) the canopy structure in the cluster-zone, and (iii) the progress of the bunch rot disease severity, were conducted in two white Vitis vinifera (Pinot gris, Riesling) cultivars in the years 2012 and 2013 in Luxembourg. The first shoot topping was carried out at seven different times from one week prior to beginning of flowering (BBCH 57) to four weeks after the end of flowering (BBCH 75-79) at approximately weekly intervals. Late first shoot topping reduced cluster compactness, delayed bunch rot epidemics, and thereby prolonged the potential ripening period compared to the standard treatment, in which the first shoot topping was performed one week postbloom. First shoot topping four weeks postbloom delayed the moment when 5% disease severity was reached up to eleven days and increased total soluble solids by 0.77 to 2.24 Brix compared to the standard treatment, with no significant impact on yield. Postponing the first shoot topping to the latest technically possible moment is recommended to improve grape architecture and health as well as to optimize grape maturity and potential wine quality under cool climate conditions. Late first shoot topping can be implemented in bunch rot control strategies without imposing additional costs and might support the reduction of pesticide use.