Alternatives to the use of gibberellins for bunch thinning and bunch compactness reduction on grapevine

Bunches with a loose cluster structure show reduced susceptibility to Botrytis cinerea and especially to sour rot. For this reason the main purpose of all measures to reduce the compactness of bunch clusters is to increase their health and to improve wine quality. Over the last decade, wine growers in South Tyrol have been commonly applying gibberellins during full flowering in order to reduce bunch compactness on tightly clustered, thin-skinned cultivars. This application is combined with an early and intense defoliation of the bunch zone, the removal of the lower part of tightly clustered bunches, and pneumatic defoliation. At the moment, sustainable wine production is increasing in South Tyrol, and the use of gibberellins is strongly debated by the wine-processing industry. Moreover gibberellins have also disadvantages and their effects are difficult to control. At the research Centre of Agriculture and Forestry Laimburg several field trials have been conducted from 2008 to 2011 on the cultivars ‘Pinot Blanc’, ‘Pinot Gris’, ‘Chardonnay’ and ‘Pinot Noir’. In these trials, we investigated the efficacy of anti-transpirants (paraffin oil and Pinus oil) and different mechanical methods (tractor-mounted rotating grape brush, hand-held rotating grape brush, manual removal of bunch parts, manual berry thinning, intensive manual defoliation, pneumatic defoliation) in bunch compactness reduction. All methods evaluated in our trials showed a certain degree of efficacy in bunch compactness reduction, but partially a high variability in their effectiveness across study years emerged. The already established pneumatic defoliation reduced the percentage of tight bunch clusters. Yield losses due to the use of the tractor-mounted grape brush were excessive in some trials, and therefore further improvements of this device are necessary. The use of gibberellins led to a reduced percentage of tight clusters and to a slight reduction of Botrytis cinerea and sour rot incidence. On the other hand in some cases an increased berry weight was observed. Anti-transpirants showed not always consistent, but promising results. The compatibility of anti-transpirants with other plant protection products has not yet been evaluated. The relatively labor-intensive methods, that is intensive manual defoliation, manual bunch thinning and removal of bunch parts, can be considered interesting, but expensive options only for small farms.