Removal of basal leaves early in the vegetative and reproductive development of grapevines is a tool used to decrease fruit set, lower cluster rot severity, and improve fruit quality. However, the considerable time required for implementation limits its use by grape growers. Efficient mechanization can potentially mitigate these issues, studies are lacking in a cool climate setting where short seasons and humid summers limit grape production. Therefore, the goal of these studies were to compare mechanical leaf removal with the manual removal of six leaves at the pre-bloom and after-bloom phenological stages over two seasons in Pinot Grigio, a tight-clustered cultivar susceptible to bunch rot, and Merlot, which reaches suboptimal fruit quality in some seasons. For Pinot Grigio (Chapter 3), the loss of fruit to gray mold was lowered by all leaf removal treatments in the drier 2017 season, but only manual treatments mitigated loss from sour rot in that year. This indicates that a clear fruit zone and reduced cluster compactness are both needed to lower the effect of cluster rot disease. Only pre-bloom treatments enhanced fruit quality, likely driven by a similar reduction in cluster compactness. The results suggest that mechanical leaf removal at pre-bloom may be used to enhance fruit total soluble solids, while pre-bloom manual removal can be an effective means to reduce fruit loss to sour rot severity.For Merlot (Chapter 4), berry total soluble solids were highest with pre-bloom mechanical treatment. Furthermore, metabolomics analysis revealed that this treatment favored the accumulation of significantly more disubstituted anthocyanins and flavonols and OH-substituted anthocyanins compared with manual application. Given that vine balance was similar between treatments, increased ripening with PB-ME is likely due to enhanced microclimate conditions and higher carbon partitioning through a younger canopy containing basal leaf fragments proximal to fruit. Despite these results, it was not clear which factor was controlling the increase in fruit quality in response to pre-bloom mechanical leaf removal. In Chapter 5, an experiment was established where 60% of leaf area was removed from shoots in three ways: 1) manual removal of 5 leaves (PB-MA), 2) mechanical removal (PB-ME), and 3) simulated mechanical removal (PB-SIM), which was implemented to understand whether PB-ME improves fruit quality via enhanced microclimate conditions, or stress. Major phenylpropanoid classes were enhanced by PB-ME, however neither ABA nor ethylene were similarly altered, suggesting their lack of involvement in promoting phenylpropanoid biosynthesis in response to ELR. Instead, the leaf area at nodes above the fruit-zone was lower in PB-ME compared to C, which increased post-veraison fruit temperature (+2.8°C). These parameters correlated with anthocyanins at harvest. In conclusion, skin phenylpropanoid concentrations are influenced by canopy density above the fruit-zone. Finally, in Chapter 6, the influence of vine balance and light exposure on fruit quality parameters were compared in two locations (Michigan, Italy). Primary metabolism was not significantly altered with the excepting of titratable acidity being decreased by LR8 having the greatest light exposure during ripening. Flavonol biosynthesis was significantly altered by light exposure in both locations, but not by vine balance. The results indicate that fruit exposure to light, rather than source-to-sink balance has a greater influence on flavonoid biosynthesis in grape berries.
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