Interactive Effects of Mechanical Canopy Management and Regulated Deficit Irrigation On Syrah Grapevines

Canopy microclimate of Syrah 06/1103P grapevines were altered and exposed to Regulated Deficit Irrigation (RDI) with varying severity and timing to mitigate the crop load.  Four crop load levels were imposed by dormant pruning to 22 spurs (control) with no further manipulation, and mechanically box-pruning others to 10 cm hedges and mechanically thinning the canopy to a density of 5 count shoots/30cm of row (CLL); 7 count shoots/30cm of row (CLM); or mechanically box-pruning to a 10cm hedge with no shoot thinning (CLH), respectively.  Control vines were irrigated to 70% of ET from fruit set until harvest (RDIC).  Other vines either received 70% of full vine ETo until to veraison, after which the rate was cut to 50% of ETo (RDIL) or had their irrigation cut to 50% of ETo before veraison (RDIE), but not thereafter.  Application of mechanical shoot thinning at Stage 17 of the E-L scale by removing 25% of the total shoots exposed with the CLM resulted in exposing about 70,600 shoots per hectare with count shoots spaced about 4.6cm along the cordon.  This exposure translated to about 4 leaf layers and about 12.6 m² of leaf area to ripen the clusters retained on the vines. The leaf area to fruit ratio achieved with the CLM treatment exposed just enough leaf area to ripen the crop level retained on the vines.  The combination of the CLM and RDIE treatments decreased the berry weight harvest by 12% without any decrease in harvest weight compared to hand-pruned control resulting 25 tons/ha yield.  To achieve vine balance with a crop load value of 9.9 kg/kg and a leaf area to fruit ratio of 0.75 m²/kg for Syrah in the San Joaquin Valley of California, a combination of CLM and RDIE is needed.  The combination of CLM with RDIE results in higher total iron-reactive phenolics, higher tannin concentration and similar anthocyanin concentration to hand-pruned control with RDIC at harvest.  This study provides important information for growers considering mechanizing canopy management operations while scheduling RDI where best results were achieved with the combination of CLM and RDIE treatments to achieve vine balance.