Early defoliation (hand vs mechanical) for improved crop control and grape composition in Sangiovese (Vitis vinifera L.)

It is well known that the number of pre-bloom source leaves is a primary determinant of subsequent fruit set. Accordingly, we tested whether pre- and post-bloom manual and mechanical defoliation is effective in limiting yield on a high cropping cultivar like Sangiovese in a three-year field study. The first six basal leaves and any laterals were removed by hand and the same area was subjected to mechanical defoliation by a suction unit, the latter removing just less than half the area removed manually. Both treatments significantly reduced fruit-set, yield per shoot, bunch weight, berries per bunch and bunch compactness. The effects of hand removal were more stable from year to year and had the biggest impact regardless of treatment date, respectively reducing fruit set and yield per shoot by 34% and 42%. Leaf-to-fruit ratios were unaffected by defoliation since source loss was fully offset by yield decline. Both treatments improved soluble solids and total anthocyanins on a fresh-weight basis as compared to non-defoliated control. Improved quality correlated to compensatory post-treatment photosynthesis and was likely linked to increased bunch sink strength and light exposure. While the results from the hand treatment reinforce the physiological basis of the technique’s effectiveness and its relative insensitivity to year-to-year variability, the positive outcome of the mechanical approach has the potential to regulate yield in a timely and cost-effective fashion while improving quality through enhanced berry composition and looser, less rot-susceptible bunches.