Comparison of Irrigation Management Strategies to Optimize Wine Grape Productivity and Fruit Composition
A study was conducted in a Cabernet Sauvignon vineyard at J. Lohr Winery, in the Paso Robles area. Treatments included four irrigation strategies: sustained deficit irrigation (SuDI – where vines are irrigated at some fraction of vineyard water use throughout the season), partial rootzone drying (PRD – where vines are deficit irrigated on one side of the vine for two weeks and then switched to the other side for two weeks), regulated deficit irrigation (RDI – where vines are deficit irrigated as some time during the growing season) and depletion of soil moisture (water is depleted in the soil profile until a critical value of vine water status is reached and an irrigation event then takes place). Applied water amounts at various fractions (0.375, 0.56, 0.75 and 1.12) of estimated Etc were included in each of the irrigation strategies, with the exception of the soil water depletion treatment. Vine water status was monitored throughout the growing season. The results indicated that the leaf water potential of vines irrigated a specific fraction of estimated Etc were similar regardless of irrigation management technique. For example, if the vines were irrigated at 0.375 times Etc, midday leaf water potential was similar regardless if sustained deficit irrigation (SuDI) or partial rootzone drying (PRD) was being used. Stomatal conductance also was similar at a specific irrigation amount between the two irrigation techniques. Based upon the original PRD work conducted in Australia this should not have occurred. The lack of significant differences in berry size, soluble solids and yield between vines irrigated with PRD and SuDI at the three irrigation amounts would also indicate that PRD had no distinct advantage over deficit irrigating vines at some fraction of estimated Etc seasonally or at a specific phenological stage.This study also included RDI as an irrigation management technique. The results indicate that deficit irrigation between berry set and veraison and then irrigating at greater applied water amounts thereafter, is a good as deficit irrigating throughout the growing season with regard to berry size. Deficit irrigation from veraison to harvest was only minimally useful in reducing berry size under the conditions of this study. Berry size of vines irrigated only once every two weeks was similar to that of the 0.357 Etc irrigation amount using SuDI, PRD and RDI from set to veraison. This may indicate that this less precise method would be useful in reducing berry size. Lastly, there were few effects of the treatments on yield the first year of the study. Berry anthocyanin and phenol compounds have not been measured as of the date this report was written. It is unknown how the above-mentioned irrigation amounts and management techniques will influence those two important characteristics of the fruit. Wine was not made from fruit of any of the treatments in 2002. The cooperator has indicated that small wine lots will be made from the fruit in 2003.