Reducing Grape Inflorescence Length to Decrease Cluster Compactness

Wine grape cluster compactness varies among varieties from loose to tight clusters, and can have implications in disease management.  Grape varieties with loose cluster architecture may allow for additional air circulation throughout the cluster, reducing disease incidence.  European studies have indicated that cutting the apical portion of inflorescences can reduce overall cluster compactness.  A two-year study was conducted in Prosser, WA to investigate the potential of inflorescence length reduction to decrease cluster compactness and improve fruit quality in Merlot grapes.  In 2008, treatments included:  1) control (unthinned), apical portion of the inflorescence removed at 2) pre-bloom, 3) bloom, or 4) post-bloom, and 5) half the clusters removed at bloom.  In 2009, the pre-bloom treatment was replaced with crop removal at shoot thinning (day of year 142 or pre-bloom).  In both years, yield was significantly reduced by crop removal, while inflorescence length reduction did not affect yield.   Reductions in inflorescence length during or after the bloom period reduced final rachis length when compared to the control, and this increased cluster compactness in both years.  Fruit composition (soluble solids, pH, and titratable acidity) was not affected in 2008, but soluble solids and pH increased for the crop removal treatment over the control in 2009.  Merlot grapevines appeared to compensate for the inflorescence length reduction by setting more fruit per unit cluster length, negating our hypothesized reductions in cluster compactness.  Crop reductions at shoot thinning or removal of half the crop at bloom were the most effective time period to decrease cluster compactness, a technique which growers can easily use to improve cluster architecture in compact varieties.