A study was continued in a Cabernet Sauvignon vineyard at the J. Lohr Winery near Paso Robles during 2005. Treatments included three irrigation strategies: sustained deficit irrigation (SuDI – where vines are irrigated at some fraction of vineyard water use throughout the season), regulated deficit irrigation (RDI – where vines are deficit irrigated as some time during the growing season [in this study between set and veraison or veraison and harvest]) and depletion of soil moisture (an irrigation event takes place once every two weeks). 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. The PRD irrigation technique was not used in 2005 in this portion of the trial as it was not shown to differ from the SuDI technique in 2002 and 2003. However, a separate study comparing the following three irrigation treatments 1.12, 0.56 SuDI and 0.56 PRD was continued in 2005.
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 regulated deficit irrigation (RDI) was being used at the time the measurements were being made.
Berry size of vines deficit irrigated (0.375 and 0.56 of ETc) between berry set and veraison and then irrigated at greater applied water amounts thereafter (1.12 ETc), was greater than those of vines deficit irrigated throughout the growing season and those deficit irrigated between veraison and harvest. However, berry size of vines that were deficit irrigated between veraison and harvest were larger that those of vines deficit irrigated season long. Berry size of vines irrigated only once every two weeks was not significantly different than those on vines irrigated at the 0.357 ETc amount using SuDI.
As applied water increased yield increased. Yields of vines irrigated at 0.375 and 0.56 of ETc using SuDI, were 70 and 83%, respectively, that of vines irrigated at 1.12 of estimated ETc. Deficit irrigating vines between veraison and harvest reduced yields approximately 23% when compared to the 1.12 treatment. Small lot wines were made at J. Lohr winery of all the irrigation treatments used in 2005. They have not been analyzed as of the date this report was written.