To address issues of labour shortage and pesticide reduction in vineyards, the combination of the fully mechanizable Semi-Minimal-Pruned Hedge (SMPH) training system with fungus-resistant grapevine cultivars (FRGCs) offers a promising solution. A three year field trial conducted in Germany investigated the impact of mechanical bunch thinning on polyphenolic composition in fruit and wine as well as yield of the two red FRGCs ‘Regent’ and ‘Rondo’ trained in SMPH. Two different bunch thinning intensities (i) SMPH light and (ii) SMPH severe were applied at pea size stage of fruit development using a harvester and compared to a non-thinned control (SMPH control) and the widely spread Vertical Shoot Positioning (VSP) training system. Across 2017–2019, yield of ‘Regent’ vines trained in SMPH was increased by + 43.8%, while displaying −6.7% less reducing sugar compared to VSP. Thinning regimes reduced yield by an average of −7.8% to −26.3%, accompanied by + 3.8% to + 9.6% increased sugar levels in SMPH light and severe respectively. Conversely, cultivation of ‘Rondo’ in SMPH resulted in lower yield for control vines (-16.4%), due to strong seasonal yield fluctuations, but higher yield in thinned treatments (light: +27.9%; +10.4%), compared to VSP. Berry morphology differed between training systems and was substantially altered by mechanical fruit thinning, characterized by a lower single berry weight in SMPH control (‘Regent’: −30.8%; ‘Rondo’: −36.7%), which further decreased with thinning regimes (‘Regent’ light: −16.9%, severe: −15.7%; ‘Rondo’ light: −8.6%, severe: −9.4%). While total anthocyanin concentration per unit of fresh berry skin did not differ statistically significantly between all treatments, higher anthocyanin concentrations per unit of fresh berry mass were obtained with SMPH control (‘Regent’: +4.1%; ‘Rondo’: +26.4%) and thinned vines of ‘Regent’ (light: +20.2%, severe: +26.6%) at delayed harvest dates. Anthocyanin profile in SMPH was composed of higher proportions of non-acylated total glucosides (‘Regent’: +2.0% points; ‘Rondo’: +1.5% points). Thinning regimes in ‘Regent’ increased proportions of monoglucosides (light: +2.9% points; severe: +4.6% points) and further decreased acylated diglucosides (light: −1.3% points; severe: −1.7% points) compared to SMPH control, contributing to enhanced colour intensity but might affect anthocyanin stability in wine. Finally, across 2017–2019, total phenol concentration in wine of SMPH was increased by light (+18.8%) and severe (+31.5%) thinning in ‘Regent’ and by severely thinned vines of ‘Rondo’ (+18.7%).
Schäfer et al. (Tue,) studied this question.