Response of Tannat (Vitis vinifera L.) to pre-flowering leaf removal in a humid climate

The aim of this study was to test the effect of pre-flowering leaf removal (PFD) on grape sanitary status, yield, source-sink balance and berry composition. A completely random block design experiment was conducted over four seasons in a commercial vineyard of the Tannat/SO4 combination located in the south of Uruguay (34 degrees 35'30 S, 56 degrees 15'23 O). The PFD treatment was compared with a commercial defoliation (CC) comprising partial leaf removal between the fruit set and cluster closure stages (four leaves from the first node), a traditional practice carried out by winegrowers. Both treatments received equal phytosanitary management during the study. The results were conditioned by seasonal meteorological conditions, particularly rainfall, temperature and evapotranspiration. The seasonal effect was significant for yield components, berry composition and source-sink balance. The PFD treatment resulted in the lowest yields in all four years and modified the primary and secondary composition of berries. It also showed a higher anthocyanin potential in 2016, 2017 and 2019, related to a higher percentage of exposed clusters, lower yield and/or higher leaf to fruit ratio. In 2017, environmental conditions were more favourable for the development of bunch rot, and lower values of incidence (3 %) were recorded for the PFD treatment compared to CC (22 %), due to greater exposure of bunches and less compact bunches. The PFD vines showed a steady reduction in dry matter production capacity, mainly explained by the steady reduction in yields. The Tannat variety showed a differential response to PFD depending on the weather during the growth cycle. Under humid climate conditions, pre-flowering defoliation proved to be effective for yield control, by reducing bunch rot incidence, and for improving some attributes in terms of grape composition. This work provides useful information for grape growers in humid regions on how to improve the sanitary status and quality of their production.