Date of Award


Degree Name

MS in Agriculture - Crop Science


Horticulture and Crop Science


Keith Patterson


A field experiment was conducted in the Rockpile appellation of Sonoma County during 2007 and 2008. The experimental design was a randomized complete block with five replicates. Zinfandel vines were subjected to three levels of cluster thinning during the lag phase of berry growth: vines were thinned to 2 clusters per shoot with shoulders retained if present (2CS), 1 cluster per shoot with shoulders retained if present (1CS), or 2 clusters per shoot with shoulders removed if present (2CNS).

Cluster thinning Zinfandel grapevines during lag phase led to reductions in yield and crop load and increased °Brix of the fruit. In both years, wines that were made from vines with the lowest crop load had consistently higher amounts of small and large polymeric pigments, tannins and total iron-reactive phenolics.

Cluster thinning vines to 2CNS at lag phase consistently decreased the number of berries per cluster and cluster weight. Thinning vines to 1CS resulted in fewer clusters per vine in 2008.

Botrytis bunch rot tends to develop between the shoulder or wing and the main cluster in certain varieties. Shoulder removal in areas with a high risk of Botrytis should conclusively determine if this thinning method helps to decrease disease incidence. Additional research is also recommended with other varieties to assess possible differences between shoulder thinning and removing entire clusters on berry composition, yield, subsequent wine composition and crop load.