Cabernet Sauvignon Responses to Prebloom and Post-Fruit Set Leaf Removal in Virginia

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Summary Goals: To evaluate how variations in the extent of fruit-zone leaf thinning at prebloom or post-fruit set would affect crop yield components, cluster compactness, primary juice composition, and Botrytis bunch rot of Cabernet Sauvignon grown under Mid-Atlantic conditions in the United States. We anticipated that our results would guide leaf removal practice to target crop yield, primary juice composition, and disease management goals. Key Findings: Prebloom leaf removal reduced crop yield compared with no leaf removal, and by a greater extent when eight (59%) rather than four (25%) basal leaves per shoot were removed. Prebloom leaf removal treatments reduced crop yield by 20 to 55% compared with no leaf removal in the inaugural season, and by 50 to 77% in the third consecutive season of treatment implementation. Post-fruit set leaf removal of six basal leaves per shoot did not affect crop yield when compared to no leaf removal. Prebloom leaf removal reduced cluster compactness compared with no leaf removal, and by a greater extent when eight (47% reduction) rather than four (24% reduction) basal leaves per shoot were removed. Regardless of timing or extent, leaf removal resulted in 19 to 22% lower Botrytis bunch rot incidence compared with no leaf removal. Treatment effects on juice composition were generally unremarkable, besides lower total titratable acidities observed in leaf removal relative to no leaf removal plots. Impact and Significance: Our results illustrate that exposed grape clusters have lower bunch rot incidence and produce musts with similar pH and with lower titratable acidity than musts produced from shaded grapes. Prebloom leaf removal should be judiciously used, however, as severe crop reduction may result, particularly if prebloom leaf removal is seasonally repeated. Our results complement previous findings by (1) reporting on the effects of multiple extents of prebloom leaf removal over several consecutive growing seasons in Cabernet Sauvignon, a cultivar that is relatively resistant to bunch rots, and (2) by illustrating generally positive primary (herein) and secondary metabolite (our unpublished data) responses in harvested grapes.