The overall goal of this project is to develop a basic understanding of temporal and spatial patterns of root survivorship in the drip irrigation zone, and in dry soils outside the wet-up zone, and to assess how such heterogeneity in available water influences vine performance in a wine grape vineyard. The project is ongoing and is being carried out in an established 11-year-old Merlot vineyard at the UCDavis Research Station in the Oakville Region of Napa Valley and the vines have been subjected to deficit and dryland irrigation treatments for four years. The response is being assessed for two rootstocks (Vitis berlandieri x V. rupestris cv 1103P and V. riparia x V. rupestris cv 101-14 Mgt) that are popular in upland (1103P) and valley (101-14) sites in the North Coast and other regions. The project has been a collaborative effort between U.C. Davis (D.R. Smart) and the Pennsylvania State University (D.M. Eissenstat). During the first three years following the initiation of the study, we have achieved a better understanding of root growth and survival through pursuit of several objectives. These objectives and the insights they are providing are of great value to wine grape, table grape and raisin grape growers. In this investigation, for example, we have documented that water can be transported, through flow eversal, throughout woody tissues of the vine by moving it axially across the high resistance transport pathways of stems. These findings provide tremendous insight into water transport processes that may be of fundamental importance for other high priority research issues in California viticulture like berry shrivel, where one hypothesis is that reversal of water flow leads to loss of berry turgor. Our results concerning root survival are providing further insights into the role root system health, or sustainability, plays in yield decline under deficit irrigation. The two rootstocks have had diametrically opposite responses to irrigation deprivation. We have found that rootstock 101-14 has diminished capacity for roots to absorb nitrogen (N), and is not very responsive to N fertigation treatments (15 to 40 kg N ha-1). One major consequence of this difference results in lower yeast assimilable nitrogen contents in musts of 101-14, and this N deficiency has had a strong influence on fermentations and wine quality. We have documented that phylloxera plays a major role in the longevity of fine roots of the two rootstocks, even though they are known for good phylloxera resistance. The project is supporting efforts in other laboratories to understand how phylloxera feeding influences root health and the general longevity of these rootstocks. The project has produced no less than five peer-reviewed publications, each breaking new ground in viticulture and the information concerning rootstock behavior under the irrigation deprivation treatments is brought regularly to the organizations such as the Clear Lake Winegrowers Association, the North Coast Viticulture Technical Group, the Sonoma County Viticulture Tech group as well as the regular meetings of the American Society of Enology & Viticulture.

This research focuses on the influence deficit and dryland irrigation on root proliferation, temporal and spatial patterns of water and nutrient availability, fruit quality, and yield loss. The overall goal of this project is to develop a basic understanding of temporal and spatial patterns of root survivorship in the drip irrigation zone, and in dry soils outside the wet-up zone, and to assess how such heterogeneity in available water influences vine performance in a wine grape vineyard. The project is ongoing and is being carried out in an established 11-year-old Merlot vineyard at the UCDavis Research Station in the Oakville Region of Napa Valley and the vines have been subjected to deficit and dryland irrigation treatments for four years. The response is being assessed for two rootstocks (Vitis berlandieri x V. rupestris cv 1103P and V. riparia x V. rupestris cv 101-14 Mgt) that are popular in upland (1103P) and valley (101-14) sites in the North Coast and other regions. The project has been a collaborative effort between U.C. Davis (D.R. Smart) and the Pennsylvania State University (D.M. Eissenstat).

During the first three years following the initiation of the study, we have achieved a better understanding of root growth and survival through pursuit of several objectives. These objectives and the insights they are providing are of great value to wine grape, table grape and raisin grape growers. In this investigation, for example, we have documented that water can be transported, through flow reversal, throughout woody tissues of the vine by moving it axially across the high resistance transport pathways of stems. These findings provide tremendous insight into water transport processes that may be of fundamental importance for other high priority research issues in California viticulture like berry shrivel, where one hypothesis is that reversal of water flow leads to loss of berry turgor.

Our results concerning root survival are providing further insights into the role root system health, or sustainability, plays in yield decline under deficit irrigation. The two rootstocks have had diametrically opposite responses to irrigation deprivation. We have found that rootstock 101-14 has diminished capacity for roots to absorb nitrogen (N), and is not very responsive to N fertigation treatments (15 to 40 kg N ha-1). One major consequence of this difference results in lower yeast assimilable nitrogen contents in musts of 101-14, and this N deficiency has had a strong influence on fermentations and wine quality. We have documented that phylloxera plays a major role in the longevity of fine roots of the two rootstocks, even though they are known for good phylloxera resistance. The project is supporting efforts in other laboratories to understand how phylloxera feeding influences root health and the general longevity of these rootstocks.

The project has produced no less than five peer-reviewed publications, each breaking new ground in viticulture and the information concerning rootstock behavior under the irrigation deprivation treatments is brought regularly to the organizations such as the Clear Lake Winegrowers Association, the North Coast Viticulture Technical Group, the Sonoma County Viticulture Tech group as well as the regular meetings of the American Society of Enology & Viticulture.

The 2005 research on Syrah Disorder on the Central Coast continued to look at the effects of environmental variables on symptom expression; in particular, water stress and salinity stress were analyzed in more detail. Work also continued to investigate a potential common virus in affected plantings.

The applied irrigation treatments and salinity treatments led to measurable differences in leaf water potential, stomatal conductance, and berry weight, but did not lead to notable differences in leaf symptoms at the two principal research sites. However, both of the experimental blocks as a whole displayed very different symptom expression in 2005 as compared to 2004; one block had very delayed and reduced symptoms as compared to 2004, while the other block had much more severe symptoms. This suggests that environmental influences on a larger scale (such as winter rainfall amounts, crop load, etc.) do play some role in symptom expression. Continued virus testing has not yet identified a common pathogen that could explain the symptoms, which in many instances resembles leafroll virus.

The last of a four-year study to investigate the effects of irrigation frequency on Thompson Seedless productivity was concluded in 2001. The irrigation frequency treatments included: a.) The application of water whenever the vines in a weighing lysimeter used 2mm (2.11 gallons) of water, b.) The application of water once a day (the amount being equivalent to the day’s use of water by the lysimeter), c.) The application of water every three days (the amount being equivalent to three days use of water by the lysimeter) and d.) The application of water once a week (the amount being equal to seven days use of water by the lysimeter). In addition to the frequency treatments, water was applied in three different amounts: 60, 80 and 100%of lysimeter vine water use. Lastly, three different trellis treatments were included. The trellis treatments consisted of a single wire, a 0.6 m (2 ft) crossarm and a 1.2 m (4 ft) crossarm. Vine density in the vineyard was 1318 vines per hectare (533 vines per acre).Irrigation of the vines in the lysimeter and the rest of the vineyard commenced on May 29th. Vine water use between budbreak and harvest was 4599 liters (1217 gallons). The water use between budbreak and harvest was equivalent to 609 mm (approximately 24 inches). Applied water between budbreak and harvest was 3915 liters (1035 gallons) or 518 mm (20.4 inches) of water. The amount of water applied to the 60, 80 and 100%of ET irrigation amounts was 58, 76 and 100%of actual vine water use (measured with the weighing lysimeter). There were significant effects of irrigation amounts, frequencies and trellis type on berry weight and soluble solids in 1999 and 2000, but no significant interactions. There were no significant effects of irrigation amounts or frequency on final yield in either 1999 or 2000. The results in those two years at the three irrigation amounts were similar to those obtained in previous studies in this vineyard when irrigations commenced prior to bloom, as done in 1999 and 2000. During the 2001-growing season, irrigations were delayed purposely to see how this could affect the results. There were significant irrigation amount by irrigation frequency effects on berry weight, soluble solids and yields in 2001. The delay in starting irrigations resulted in smaller berries for all treatments compared to data from 2000. In general, vines irrigated at the higher frequencies and/or irrigation amounts at 100%of ETc had the largest berries and highest yields in 2001. These results illustrate that deficit irrigation management practices for Thompson Seedless grapevines can maintain productivity similar to that of fully irrigated vines, if the irrigations are commenced prior to bloom under the conditions of this study.