Title:

Effect of Cluster thinning on Fruit yield and quality in ‘Canadice’ and ‘Mars’ seedless table grapes grown in two different training systems

The objective of the study was to evaluate the effect of cluster thinning on fruit quality and yield of seedless table grape cultivars grown in two training systems. We hypothesized that cluster thinning would reduce total yields while increasing fruit and cluster quality. Previous work done focusing on the effect of cluster thinning on V.vinifera table grapes has shown positive outcomes in terms of fruit quality and composition, but much less information is available on the effect of cluster thinning for interspecific (V. labrusca, V. vinifera, and others) seedless table grape cultivars. The experiment was conducted at Woodman Horticultural Research farm, Durham NH, in 2022 using a split-split plot design with four blocks, with training system as main plot, cultivar as the subplot, and thinning treatment as the sub-sub plot. Two cultivars, ‘Canadice’ and ‘Mars’, were grown using both Vertical Shoot Positioning (VSP) and Munson (M) training systems. Three treatments were randomly assigned to each of the three vines per cultivar in each plot. For C (Control), no clusters were removed from the vines. For treatment “two”, 2 clusters were left per shoot on the vine. For treatment “one”, one cluster was left per shoot. Cluster removal was performed on 18-19 July 2022, after berry set and before veraison. Parameters such as average cluster weight, berry weight and brix, and marketable and unmarketable yield were measured. There was no significant effect of treatments on total yield, cluster weight and berry weight, however cluster thinning had a significant effect on total soluble solids (TSS) measured at harvest. Fruit from treatment “one”, which had only one cluster left for each shoot, had higher total soluble solids than the other two treatments. This was consistent with our hypothesis that having a smaller number of clusters on the vine will improve fruit quality. We also found that ‘Canadice’ had higher brix than ‘Mars’. In conclusion, our results do show that cluster thinning can improve the berry composition in terms of brix without impacting the marketable and total yield. However, taking into consideration labor costs and resources required for cluster thinning, our preliminary results suggest that cluster thinning may not be worthwhile for ‘Canadice’ and ‘Mars’ table grape production.

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