Leaf Removal Effects on Cabernet franc and Petit Verdot: I. Crop Yield Components and Primary Fruit Composition

As part of a broader study of the effects of fruit exposure on fruit composition, this study aimed to evaluate how the timing and extent of leaf thinning impacted crop yield components and primary fruit composition of Cabernet franc and Petit Verdot. Treatments consisted of three post-fruit set leaf/lateral shoot removal treatments: (i) no leaf removal (NO); (ii) leaf removal from opposite the basal primary cluster and the node directly above (MED); and (iii) leaf removal from the node directly above the distal primary cluster down to the cordon (HIGH). An additional treatment applied immediately prebloom (P-B) involved removal of leaves and lateral shoots from the basal six nodes of primary shoots. Post-fruit set leaf removal had modest or no effects on crop yield components, whereas prebloom leaf removal reduced most crop yield components in both cultivars relative to the NO treatment. In both cultivars, P-B reduced crop yield by an average of 49 to 50% over two years when compared to NO, and that reduction was primarily driven by reduced cluster weight. Prebloom leaf removal implemented on the same vines in two consecutive seasons caused further reductions in crop yield in both cultivars. Both HIGH and P-B tended to depress juice Brix in Petit Verdot but not in Cabernet franc, and HIGH reduced titratable acidity more consistently than did P-B. Fruit zone leaf thinning after fruit set may balance the competing goals of improving fruit exposure, while avoiding precipitous reductions in crop yield.