Mechanization of pre-flowering leaf removal under the temperate-climate conditions of Switzerland

Introduction

Grapevine leaf removal (LR) in the cluster area is typically done between fruit set and cluster closure to create an unfavorable microclimate for fungal diseases, such as Botrytis cinerea and powdery mildew. Grape growers are now turning their attention to pre-flowering LR, which has additional benefits under certain conditions. When applied before flowering, LR strongly affects fruit set and thus the number of berries per cluster. It is therefore a good yield control tool, replacing time-consuming manual cluster thinning (Poni et al. 2006). It also improves berry structure, that is, skin thickness, skin-to-pulp ratio, and berry composition (total soluble solids, titratable acidity, and polyphenols) (Palliotti et al. 2012; Komm and Moyer 2015). By exacerbating competition for assimilates between reproductive and vegetative organs, pre-flowering LR also poses some risks. Excessive yield loss at the same year’s harvest due to a too low fruit set rate is the main concern: intensive pre-flowering LR (100% of the cluster area) can induce up to 50% yield loss in potted vines (Poni et al. 2005). Other parameters, such as cool climatic conditions during flowering, also affect fruit set rate and make it difficult to predict potential yield at harvest. Repeated and overly intensive preflowering LR can have repercussions over time and induce a decline in bud fruiting and plant vigor (Risco et al. 2014).

The effects of timing and intensity of LR were experimented on five cultivars (pinot noir, merlot, gamay, chasselas, and doral) over six years under temperate Swiss climatic conditions and yielded interesting results (Verdenal et al. 2018). An intensive pre-flowering LR (removal of six basal + lateral leaves) confirmed its huge impact on the agronomic performance of the vine, mainly at the expense of fruit set. Yield was therefore strongly affected (about -35% of that of the non-defoliated control treatments). This yield loss was proportional to the initial yield potential, which depends on genetics. The intensity of LR modulated its impact on yield. Preflowering LR also had a positive impact against millerandage, sunburn symptoms and Botrytis cinerea development. In terms of berry structure and composition, skin thickness doubled and polyphenol concentration increased significantly. Due to pre-flowering LR, red wines were often preferred for their color and mouthfeel. However, this practice had a negligible impact on the composition of white wines. Pre-flowering LR had no negative impact on wine parameters.

Pre-flowering LR represents a prophylactic solution that reduces both chemical applications and cluster thinning costs. However, the considerable time required for its manual implementation limits its popularity among wine growers. Knowing that mechanical LR is delicate before flowering, as shoots are fragile, the choice of method is essential for optimal results. Mechanical LR by rotary suction was tested at flowering and resulted in the loss of shoots and inflorescences (Intrieri et al. 2016). In comparison, LR by low-pressure dual airflow (Collard, Bouzy, France) seems more suitable for pre-flowering LR. Two trials were conducted in Switzerland for five years on two cultivars to test mechanical pre-flowering LR.