Assessment of the Variability in Soil Health Indicators and Incorporating Healthy Soil Management Practices into the Context of Napa Valley Terroirs

Adoption of sustainable soil management practices is becoming common in winegrape production in response to an increased awareness of the value of soil health to maintain environmental quality. Soil health is characterized by the functions it provides to the vineyard (i.e water retention, nutrient supply, carbon sequestration etc.) and is often impacted by vineyard floor management practices such as compost application, no till or the use of cover crops. However, the magnitude of the effect of healthy soil practices on soil organic matter and other soil health indicators is difficult to predict. A major impediment for the diagnosis and establishment of target values of soil health in vineyards is the large diversity of soils and physiographic conditions across and within growing regions. In this project, we carried out semi-structured interviews to growers paired with ongoing sampling efforts across 32 vineyards in Napa Valley to (i) establish a baseline of soil health indicators within the various Napa Valley soil types; and (ii) examine grower perception and comprehension of these indicators and the desired qualities of a healthy soil relative to production goals. Activities in this project were carried out though a collaboration between researchers, extension specialists and the Napa Valley Grapegrower Association. The data gathered so far shows that most grape growers consider soil health to be important for wine grape production due to perceived benefits to reduce soil erosion, improve vine health and grape quality. Through grower input, researchers were able to extract the soil properties that are the most important for these beneficial outcomes, namely a balanced soil structure and nutrient supply. Finally, data collected in the interviews revealed a strong grower knowledge of how the physical and chemical aspects of soil health are related to beneficial agronomic outcomes, as well as strong grower interest in understanding the role of soil organic matter and the soil biome on these outcomes. The research team is currently collecting soil samples to assess the variability and establish benchmarks for those soil health indicators that are desired for winegrape production. Furthermore, we will assess the role of soil organic matter and the soil microbiome with these indicators of soil health.

Physiological Responses of Grapevine to Salt Stress and Remediation by CaSO4 Amendments in Central Valley of California

Proper management of salt accumulation in soils is crucial to prevent long-term reductions in productivity, especially where irrigation water is saline or sodic. Reclamation of saline-sodic soils requires the removal of most of the exchangeable Na+ and the replacement by Ca2+ in the root zone. One of the most effective practices to achieve this result is the use of chemical amendments containing calcium, such as calcium sulfate (CaSO4).

This project started in 2019 intending to compare different forms and doses of calcium sulfate and their effect on the physiology of grapevine growing in a sodic soil located in the southern San Joaquin Valley. The first year was devoted to locate the experimental site, perform a baseline of soil and plant conditions, and applying the treatments over 17 acres in a randomized complete block design with 6 treatments and 4 replications. In the second year, we monitored the soil conditions, plant physiology, and soil composition both on the ground and with the aid of satellite imagery and started to observe the first effects of the treatments.

Rootstock Effects on Mature Pinot Noir Growth and Productivity under Cool Climate, Dry Farmed Conditions

(Year 1) Vine growth, yield, and fruit composition of Pinot noir grafted to 19 rootstocks and own-rooted vines were quantified during 2020. The vineyard was 23-years-old, and we hypothesized cumulative impacts of the rootstock on vine growth would be distinguished by rootstock. Specifically, we hypothesized that Riparia Gloire and other vigor-reducing rootstocks such as 101-14, 3309C, and 420A would have reduced canopy growth compared to other rootstocks not commonly planted in Oregon due to high vigor potential, such as 110R, 140R, 1103P, and 161- 49. Results show that the majority of rootstocks performed similarly for vine canopy growth and fruit production. However, there were some key differences noted in this first year of evaluation. Dormant pruning weights in early 2020 indicate that Riparia Gloire, 44-53, and 3309C had the lowest pruning weights, indicating low vigor vines, and 161-49 and 1616 had the highest pruning weights, indicating highly vigorous vines. Despite vigor differences noted during pruning, there were few to no differences in growth stage advancement at bud break, bloom, or fruit set. By the start of véraison Riparia Gloire and SO4 had the most advanced color development while 101- 14, 3309C, and own-rooted were the least advanced. However, within a 6 d window, the rootstocks became less different in percent of berries colored, and 3309C had the highest rate of color change. There were no differences in rootstock yield except for Riparia Gloire and SO4, which had the lowest and highest yields, respectively. Berry ripeness did not differ for most rootstocks. However, Schawarzmann had higher Brix than 420A, 5BB, 125AA, own-rooted, 5CTE and 99R. There were few differences in pH and variable differences in titratable acidity. We anticipated that variations in canopy size created by rootstock vigor may impact berry phenolics through vine stress and/or differences in canopy microclimate. However, there were no rootstock differences in total anthocyanin or phenolic content. There were minor differences in total tannins. We also anticipated that vine vigor conferred by rootstock may affect berry nitrogen, but there were few differences in juice primary amino N except for 1616 and 5BB that had more than double the primary amino N than 44-53 and own-rooted vines. This first year of data analysis suggests that rootstock has the greatest impact on vegetative growth and yield, thereby causing some differences in vine balance. There is less impact on Pinot noir phenological advancement, fruit ripeness, berry N or phenolics at harvest.

Characterizing Willamette Valley Soil Moisture and Grapevine Response under Drying Seasonal Conditions

Soil moisture, weather data, and vine growth response were measured in 2020 in one vineyard location that had Pinot noir of the same vine age, clone and rootstock growing in three soil types, including volcanic soils (Saum), sedimentary soils (Dupee), and marine sediment soils (Willamette Woodburn). Soil sensors measured soil moisture, soil temperature, and electrical conductivity for each soil type. Soil probes were installed to a depth of 18 and 36 inches under[1]vine and in the middle of the alley between rows. Soil moisture remained relatively consistent through much of spring, with the start of soil moisture decline beginning in mid-June. This occurred shortly after bloom, and continued throughout the rest of summer, when there was little to no precipitation. Vine growth measures of leaf area and lateral count did not vary in-season, but dormant season pruning weights show that the most vigorous vines grew in the Willamette[1]Woodburn soil. Vine vegetative vigor was similar for Dupee and Saum. Soil moisture decline was greatest at the 18” depth and varied less at the 36” depth, and the greatest decline occurred with Willamette-Woodburn, suggesting that the higher vigor vines required more water from the soil profile than vines in the other two soil types. Leaf water potential did not show clear differences in vine water stress of the three soil types. Berry weight lagged slightly for Willamette-Woodburn, but there were no differences in the overall growth curve through development. By harvest, yields were similar from each soil type. However, the Willamette Woodburn had lower Brix and sugar per berry compared to the other two soil types. Data analysis from the 2020 season continues as of this reporting. This research will continue through two additional growing seasons (2021 and 2022).

Evaluation of Seven Rootstocks Grown under Saline Condition in the San Joaquin Valley of California

Field trial, consisting of seven rootstocks, replicated in 3 times with 5 vines per experimental unit, was planted in 2015 at west side of Fresno County, and the scion variety of Pinot gris was field grafted in the spring of 2016. Data including soil/irrigation water salinity, tissue nutrients, yield components, harvest berry composition, and pruning weight were collected for the first (2017), the second (2018) and the third harvest (2019). Preliminary results from the three harvests have shown some interesting results among the rootstocks. Previously regarded salt-tolerant rootstocks for Cl, e.g., 1103P, 140Ru, Ramsey, performed as expected and our data were largely in line with previous results.

So far, rootstocks in our study had significant impact on plant nutrition, yield component and pruning weight, however, minimal effect on harvest fruit chemistry. Instead, vintage, especially irrigation water sources, played a crucial role in it. Interestingly, GRN 2 and 3 rootstocks which accumulated the most Cl, had the highest accumulative yields and pruning weight, although petiole Cl didn’t exceed the critical value. and this trial is still ongoing to collect more data to confirm the long-term impact. As for boron tolerance, GRN 3 rootstock showed the least amount of B uptake across three years. Correlation between juice Na and pH, berry size and B, have been found and it further validated the importance of rootstock selection, since rootstock might largely affect the fruit/wine chemical composition and yield.

Evaluation of interactive effects of mechanical leafing and deficit irrigation on berry composition and wine chemistry of Vitis vinifera cv. Cabernet Sauvignon Grown in the San Joaquin Valley of California

Mechanical leafing at bloom and berry set either on one side or both sides of the canopy does not affect the final yield. Leafing, either on bloom or berry set, improves the anthocyanins accumulation during the ripening and increases the harvest berry anthocyanins. Light exposure resulted from bloom leafing only lasts for approximately 2 weeks, and such a short period of light exposure during/after bloom is enough to increase the anthocyanins accumulation and final berry anthocyanins. Overexposure from berry set leafing might promote the anthocyanins degradation at the end of berry ripening.

Water deficit during cell growth stage I, from berry set to veraison, reduces berry size and ultimate yield, although the decline of berry size and yield depends on the severity of water deficit during the stage I. Water deficit increases the harvest berry anthocyanins, although its increase mainly results from the high skin/pulp ratio associated with small berry size.

From research wine micro-fermentation, resulted wine color follows the similar pattern of harvest berry color.

Characterizing Willamette Valley Soil Moisture and Grapevine Response under Drying Seasonal Conditions

A commercial vineyard in the north Willamette Valley was selected for this research based on the availability of three soil classes of interest that contain vineyards of the same age, cultivar and rootstock. The soil types include volcanic soils, sedimentary soils, and marine sediment soils. Soil moisture sensors were purchased in fall 2019 and installed in January 2020 into two monitoring sites within each soil classification. Soil probes were installed to a depth of 18 and 36 inches under-vine and in the middle of the alleyway between rows. Sensors are currently measuring soil volumetric soil moisture and temperature continuously and will be monitored for three growing seasons. The first vineyard measurements began in 2020. Since funding was received four months prior to this reporting, there is no data to report at this time.

Developing solar-induced chlorophyll fluorescence as a ground-based and remotely-sensed physiological indicator of grapevine stress

Upon having access to the funds, we initiated work in June 2019 and plan to build off of these findings to continue efforts and analysis in 2020-2021. We conducted a dry down experiment on potted grapevines to evaluate the relationship between active and passive (i.e. ground based equivalent of SIF) fluorescence and used this experiment system to test modification to a gas exchange system for comparing active and passive fluorescence while simultaneously measuring gas exchange. We also utilized existing field installations, specifically at the Ripperdan GRAPEX-Grape Remote sensing and Atmospheric Profiling and Evapotranspiration eXperiment site, which contains a variable rate irrigation system where water can be delivered down to 30m by 30m pixels and contains 4 treatments (two stressed and two well-watered) for comparison of active and passive fluorescence responses of grapevines under stress conditions.

Interaction of red blotch virus (GRBV) and deficit irrigation on grapevine water relations, disease development, and vine productivity

The second year of a field experiment with two irrigation treatments – wet (W) and dry (D) – and two vine disease statuses – healthy (GRBV-) and infected (GRBV+) – was continued in a commercial vineyard to understand the interaction between GRBV infection and deficit irrigation on disease development, vine productivity, and fruit quality. W vines were irrigated at 100% of crop evapotranspiration (ETc), while D vines received water at 50% ETc. Within each irrigation treatment, GRBV- and GRBV+ vines (split-plot) that were previously identified in early 2017 based on symptomology data from 2016. The identified vines were confirmed as GRBV+ and GRBV- by PCR-based assays. GRBV- vines from 2017 were re-tested in early 2018 to confirm disease status.

In both years, measurements of vine water status (midday stem water potential; Ψstem) were made at regular intervals throughout the growing season beginning just after berry set until just before harvest. Similarly, disease severity was recorded every week after the first symptom appearance was observed on GRBV+ vines. At harvest, berry samples were collected for berry size and compositional analyses; and vine yield and yield components were determined.

With respect to vine water relations and gas exchange, there were no significant interactions among experimental treatments. Irrigation treatment and disease status both impacted these aspects of vine physiology, but they acted independently, with water deficits consistently reducing vine water status, and GRBV infection consistently increasing it. In other words, GRBV infection had the same effect on vine water status (stem water potential) under both well-watered and deficit conditions. However, the significant impact of GRBV infection on vine water status only arose post-veraison – at the same time that foliar symptoms became visible. The increase in post-veraison water status under GRBV+ conditions was likely a consequence of reduced stomatal conductance, which in turn reduced net photosynthesis.

Berry development was similarly impacted by the treatments independently, with consistently larger berries in W and GRBV+ vines. This was observed at nearly every sample date in each year, but differences between vines of different disease status only became significant post-veraison. TSS were also only significantly different between GRBV- and GRBV+ vines post-veraison, and there was a limited impact of irrigation treatment. In contrast, pH and TA were more variable among treatments and years, suggesting that GRBV has a limited effect on organic acid metabolism.

Irrigation treatment and disease status impacted skin and seed phenolic concentrations to varying degrees over two seasons. Whereas irrigation treatment and disease status impacted skin phenolic concentration together, disease status alone impacted seed phenolic concentration. In both years, skin anthocyanin concentration was increased with deficit irrigation – in both GRBV- and GRBV+ vines – but only increases in GRBV- vines were statistically significant. Conversely, skin tannin and iron-reactive phenolic concentrations were variably affected by treatments, and results were not consistent between years. In seeds, there were no effects of the irrigation treatments, but disease status significantly reduced both tannin and iron-reactive phenolic concentrations. This effect was consistent between years. All together, these results suggest that the genetic control of phenolic metabolism by GRBV infection is stronger than the environmental control due to vine water deficits. Furthermore, experimental results suggest that keeping vines well-watered may mitigate some of the negative effects of GRBV infection, but ultimate changes in secondary metabolism due to GRBV infection may necessitate using infected fruit for different wine programs (e.g. rosé and/or sparkling) or blending with lots from healthy vineyards.

In contrast to 2017 data on disease severity, significant differences on rate of disease progression as well as disease severity were observed in 2018 between the wet and dry treatments. The vines in wet irrigation treatment showed significantly low disease severity at harvest and two weeks prior to harvest. The differences were observed as significant increase in vine canopy in irrigation treatment compared to vines in dry treatment; as a result percent of symptomatic leaves in wet treatment vines were less compared to dry treatment vines. Even though, the virus expression remained same (symptomatic) within wet and dry treatments, it would be informative to assess the status of virus (quantity) within each treatment. Furthermore, the carry over effect of less severe vines on vine health as well as fruit qualities would be an additional information on long-term management of GRBV infected vines.

The removal of the experimental vineyard site between 2018 and 2019 precluded some of the confirmatory and deeper data collection that was originally planned. Unfortunately, this is an all too common occurance with respect to recent research efforts on GRBV. The new site was on a heavier soil, thus irrigation differences were not manifested until just prior to harvest. However, several of the vine physiological responses to GRBV infection that were observed at the first site in 2017 and 2018 were also observed at the new site in 2019. These included: (1) higher water status, (2) lower photosynthetic rate and stomatal conductance, and (3) lower sugar and color in fruit. Additionally, there were no effects of GRBV on yield or yield components, as in 2017/2018. These consistencies across the two sites underscore the main effects of GRBV on grapevines and future research efforts should be targeted at exploring the underlying mechanisms behind them.

Assessing Rootstock Biology and Water Uptake through Proximal Sensing under Different Wetting/Drying Conditions

Considering the importance of the water status as a driver of the whole plant physiology at the vineyard scale, developing new technologies for the space-time measurement of roots distribution and water uptake is crucial for the development of efficient precision viticulture practices. This project (2017-2022) is devoted to the development of a non-invasive technique (electrical resistivity tomography, ERT), as a tool to compare dynamic changes in root growth and water uptake patterns, and to apply this technique to the study of several commercially available rootstocks under varied irrigation delivery methods (drippers, micro-sprinklers) and water regimes (sustained deficit irrigation, rain fed, fully watered). The project was conducted in a vineyard at UC Davis specifically planted for the study of the response of rootstocks to watering systems. According to the timetable of the funded proposal the first year consisted of two parallel objectives: i) the calibration of ERT to soil water and roots distribution iii) the physiological monitoring of different rootstocks under different water amounts and delivery methods.

In a vineyard  planted to Chardonnay on different rootstocks representing the most common parentage classes in a randomized complete block design, six soil pits were dug for the description of the soil profile, the sampling of the soil for chemical-physical analysis and the installation of TDR probes for the monitoring of soil moisture. The pits were then refilled, and the soil allowed to resettle for several months before the measurements commenced. The section of the field used for calibration purposes was not irrigated throughout the season in order to cover a full range of moisture levels for that soil (from 24% to 8% vol.). In close correspondence of these pits, at the soil surface 300 stainless steel electrodes were installed at a distance of 0.62 m (2 feet) for the ERT monitoring. A single ERT measurement was able to cover 2 plants on 2 different rootstock, which represented an experimental unit. The water status of the plants within the experimental units was monitored using infrared thermography and pressure chamber measurements. Photosynthesis and stomatal conductance were monitored through a pressure bomb. The electrical resistivity was calibrated to the soil water content obtained by TDR measurements, and two different pedoelectrical models were fitted to the data: Archie, in a canonical and linearized form, and Waxman and Smits models. Their performances were tested on 20% of the data from the whole dataset that were excluded from the model fitting and then used as a test set of unobserved data. The three modeling approaches gave similar results on the test set, with a slight decrease in performance from the log-log linearization (RMSE = 1.22% vol., R2 = 0.73) and the Archie law (1.22 %vol., R2 = 0.73), to the Waxman-Smits model (RMSE = 1.23% vol., R2 = 0.73). The Archie law was then chosen, and confirmed to be well adapted to predominantly sandy soils such as the experiment site. This calibration was used to transform the ERT images from soil resistivity to soil moisture maps under different rootstocks in 2D and 3D. This is the first report in the world that 3D images of soil moisture were developed in a vineyard, and the first time that this technique was used to compare rootstock physiology in general. Great differences were found between contiguous rootstocks in their spatial use of water. The presented data show how lateral heterogeneity in soil moisture could reduce the efficiency of spot measurements, as those obtained with soil probes, and soil moisture sensors in determining irrigation needs. In the experimental region, despite not irrigating for the whole summer, grapevines did not suffer from water stress  until it reached the harvest date where the midday stem water potential were at -0.7 MPa across all four rootstocks and no significant differences were found. Chardonnay/110R had lowest assimilation rate (P<0.0017) and stomatal conductance (P<0.05) but had greater berry weight, cluster weight and yield per vine, whereas Chardonnay grafted on 140Ru had the lowest yield among four rootstocks. At harvest Chardonnay /110R also had the greatest total soluble solids and lowest titratable acidity. The results of Ravaz Index (kg yield/kg of pruning weight) indicated that Chardonnay /110R and 140Ru achieved vine balance where the Ravaz Index were around 5. However, Chardonnay /101-14Mgt and 420A had  Ravaz Index lower than 3, indicating that excessive vegetative growth due to no water stress resulting in the unbalance of the grapevine. Our results suggested that although Chardonnay /110R had lower vigor, it had higher productivity and enhanced ripening of grapes, related to better vine balance under non-limiting water supply. Further information is needed to test the interaction between rootstocks and different irrigation regimes and their drought resistance. At the end of the season, the soil was sampled with an auger at different locations in the middle between contiguous ERT electrodes. The soil was sampled every 0.1 m, brought to the laboratory and oven-dried. Roots were physically separated from the soil, and their presence was assessed in a gravimetrically (mg of roots per g of soil). The presence of roots was negatively correlated to soil moisture obtained through ERT (r = 0.45), i.e. greater the amount of roots, lower the amount of water at the end of a dry period. This relations was then used to map the roots using ERT as ancillary variable for the spatialization, therefore enabling us to use the electrical images of the roots assess plant water use.