A Cover Crop System for Vineyard Pest, Weed and Nutrition Management

Large scale replicated trials were initiated in the fall of 1991 on three farms in the San Joaquin Valley. The first year’s data was collected during the 1992 season. In general, we observed an increase in the activity of natural enemies, especially spiders which resulted in a suppression of leafhopper numbers in some vineyards. The numbers of leafhoppers during the 1992 season were too low to observe a strong effect of cover crops on their numbers. Whole-vine spider exclusion and spider caging with leafhoppers indicated that the most common spiders in vineyards are important predators of leafhoppers. Continued monitoring of our vineyards is necessary to determine the long term effect of cover crops on the numbers of pests and their biological control agents. Our results on weed suppression with dry mulch is variable. However, our studies and those conducted by C. Elmore in north coast vineyards indicate that yearly accumulation of biomass in vine rows should provide sufficient weed suppression to minimize the use of herbicides. In table and wine grape vineyards, cover crops left to dry in row middles can suppress weeds, conserve soil moisture, decrease mowing costs, and reduce dust problems. The data on the nutritional status of vines did not show any treatment differences. This is not surprising, however, since the effect of cover crops on mineral nutrition of vines is a delayed effect, often not detectable until the following year. Our initial budget for alternative floor management systems indicates that the use of cover crops for weed management may increase the cost of grape production, primarily due to the added cost of cover crop seeds. This increased cost, however, should turn into savings when insecticide and fertilizer costs are included in the enterprise budget. It is anticipated that our cover crop system will reduce insecticide, herbicide, and fertilizer inputs. In the long term, seed costs should also be reduced, since the cover crops used in our studies are self-seeding.

Impact of Fungicide Use on Wine Quality

The purpose of this continuing research project was to investigate the use of combinations of sulfur and commercial fungicides to determine, if control of powdery mildew could be achieved with minimum or no sulfur residues remaining on the grapes themselves or in wine produced from those grapes. The judging panel could distinguish all of the white wine lots and two of the red wine lots from the control indicating some “sulfur” problems. These results indicate that late season applications of sulfur dust, in conjunction with the use of a fermenting yeast known to produce larger amounts of hydrogen sulfide, have the potential to produce off aromas in both red and white wines.

Improving nitrogen fertilizer efficiency in winegrapes

This was a four-year study on the effects of N fertilizer timing and rate on vine N status, fruit composition and quality, and vine yield in four wine cultivars important to the San Joaquin Valley — Barbera, Grenache, French Colombard, and Chenin blanc. It was initiated as a follow-up to previous studies in raisin and table grape vineyards which demonstrated the influence of N fertilizer timing on N availability and utilization and possible improvements in N fertilizer efficiency. Similar studies are needed in wine grape vineyards to determine N timing response as well as effects on vine tissue N levels, fruit composition, and vine yields. N fertilizer treatments included budbreak (BB), berryset (BS), veraison (V), and postharvest (PH) timings at 50 lb. N /ac, BS at 100 lb. N/ac, and check, no N (CKO). N status, as determined by bloom and veraison petiole N03-N and NH4-N (inorganic N compounds) levels, varied among the cultivars and was influenced by rate (0, 50, and 100 lbs. N) and the proximity of fertilizer application prior to petiole sampling date. Grenache and Barbera were the high and low extremes, respectively, in vine N status and responsiveness to N fertilizer application; Chenin blanc and French Colombard were intermediate. Generally, CKO and BS100 were the high and low vine N status treatments through the experiment. PH50 resulted in bloom N levels equal to BB100 and sometimes higher than BB50, suggesting that postharvest timing was the most efficient in supplying N between budbreak and bloom. Otherwise, timing effects on vine N status were minor. Petiole N03-N was more sensitive than NH4-N to N fertilizer treatment differences. However, the sum of the two analyses (expressed as “total inorganic N”) provided the most clear separation among treatment differences and thus could be considered as a useful tool in grapevine N fertility research. Grape soluble solids was the most responsive fruit parameter to N treatment. Generally, N fertilizer tended to decrease soluble solids, irrespective of timing, and with the highest N rate of 100 lbs. resulting in the largest decrease. Titratable acidity differences were mostly inverse to soluble solids. Grenache was the only cultivar with significant yield differences due to N fertilization, suggesting a N-deficient status with this cultivar. All of the N treatments in Grenache, with the exception of V50, increased vine yield over CKO. This result, as well as other negative effects of the V50 treatment, ie.lower titratable acidity and higher pH in Grenache and more cluster rot in French Colombard and Chenin blanc, indicate that veraison may be the least desirable of the timings studied here. The detrimental effects of the high 100 lb.N treatment were also demonstrated, primarily through delayed fruit maturation and some increased cluster rot incidence. The bloom petiole N03-N levels reported here should provide supportive data toward establishing critical vine tissue levels in the wine cultivars studied here.

New technology for determining vine water status and controlling vineyard

We confirmed that ultrasonic acoustic emissions occur in field-grown grapevines subjected to water deficits. The appropriate sensor and frequency for adequate detection was determined. Although we can record acoustic emission rates in the field, we are not satisfied with the present sensor attachment. For Cabernet Sauvignon vines in the Lodi area, differences in the rate of acoustic emissions at the onset of veraison were easily resolved between vines receiving irrigation every other day and vines from which water was withheld up to that point. The rate of acoustic emissions from the stressed vines was two to five times greater than from the irrigated vines at midday. When water was withheld (at 50%veraison) from some vines for only six days, there were again large differences (up to five-fold) in the rate of acoustic emissions at midday. The results suggest that the water status of a vineyard may be estimated daily and automatically without grower labor input.

Influence of nitrogen fertilization and other vineyard cultural practices on the

Sauvignon blanc and Chardonnay grapevines grown at the North Oakville Experimental Vineyard (OEV) generally responded favorably to increased rate of nitrogen fenilization up to 90 lbs/acre as demonstrated by higher yield, shoot growth, pruning weight and total leaf area per vine. Differences in fruit composition between low and high nitrogen fertilization were relatively small. However, high N generally increased the pH, TA, malate, K, and arginine levels of must of both cultivars. The field fertilization did certainly improve the ability of the juices to ferment. Without the nitrogen additions, the juices took twice as long to ferment. Otherwise, the routine must and wine analyses show little differences.

Shoot positioning as a means of improving fruit composition, wine quality

Two replicated shoot positioning field trials were conducted from 1987 to 1990; one with Chardonnay, Sauvignon blanc and Cabernet Sauvignon vines at the Oakville Experimental Vineyard and the other with Chenin blanc vines at Davis. Chardonnay. Sauvignon blanc and Cabernet Sauvignon trial: The main conclusions obtained from the Chardonnay, Sauvignon blanc and Cabernet Sauvignon shoot positioning-hedging trial conducted at the Oakville Experimental Vineyard over a period of five years (1987 to 1990) may be summarized as follows. Conclusions are based on five-year averages. 1. Shoot positioning/hedging (SP) of Chardonnay and Sauvignon blanc increased crop yield by 4 to 5%or 0.4 tons/acre compared to no shoot positioning (NSP) and hedging (Tables 1 and 2). Shoot positioning and hedging did not effect the crop yield of Cabernet Sauvignon. 2. SP reduced the amount of direct light and increased the amount of indirect light (diffuse light) in the fruiting zone.

The Regulation of Fruit Growth, Fruit Composition, and Wine Composition by Water Management

This summary will constitute a brief overview of what we have learned about water stress in winegrape production in California. This project was designed to extend our initial study with hillside Cabernet franc and Sauvignon blanc (1984-1988) by extending our understanding of the role of vine water status in determining yield, fruit and wine composition, and wine sensory attributes. Experiments and sites were designed to test several questions including: whether vine water status can be readily controlled at different sites and soils (Carneros and Lodi areas); whether other important red winegrape varieties, Pinot noir and Cabernet Sauvignon, respond similarly to seasonal water deficits; whether water stress at veraison is particularly critical in determining the composition of harvested fruit; whether the changes in fruit composition caused by water stress are due directly to water stress or to indirect effects of changes in the cluster microclimate; and which specific flavor and aroma compounds are responsible for the sensory differences caused by water stress. We report that soil and vine water status is readily controlled in drip-irrigated vineyards of Pinot noir (Carneros) and Cabernet Sauvignon (Lodi). For the Pinot noir site, differences in water status of vines receiving the standard or a supplemented rate of irrigation were greater than we obtained in the hillside Cabernet franc and Sauvignon blanc we used in our earlier study. This shows that our earlier results can be extrapolated to valley floors and are not indicative only of hillside vineyards with shallow soils and high exposures. The results also show the utility of drip irrigation for control of vine water status and the prevalence of water deficits in winegrape production in the North Coast. Thus, many growers can control vineyard water status; we are working to identify how much stress should occur and when to meet grower and winemaker objectives. The results from three sites and four varieties over several seasons indicate some clear generalizations. Regulation of vine water status is effective in controlling fruitfulness, and many vineyards may be under-irrigated for maximum fruitfulness. Early season stress is more effective in decreasing berry size and fruitfulness in the following season. Several aspects of fruit ripening are to some extent controlled by vine water status. Properly timed water stress increases the color of red winegrape juice, the concentration of phenolics (tannins), and the concentration of amino acids in juice of red and white varieties. As long as the water stress is moderate, these changes occur without significant effects on sugar accumulation and titratable acidity. The timing of water stress is important. For color and phenolics, stress early in the season (before veraison) is more important. However, early season stress also leads to significant decreases in the concentration of malate (increasing the tartrate:malate ratio) which may be perceived as positive or negative, and leads to inhibited yields in the following season. Water stress after veraison (a common practice), is more important than early stress in increasing the concentration of amino acids in the harvested juice and also decreases the relative amount of proline in the total amino acids. This is important because proline is largely unavailable for yeast during fermentation. Postveraison stress is less effective in increasing color, phenolics, and the tartratermalate ratio. If late stress is severe enough, delayed ripening and inhibited yield in the following season occur. These differences in fruit composition lead to significant differences in the appearance, aroma, and taste of wines. The results of sensory analyses indicates that all stresses are not equal. That is, the appearance, aroma, and taste of a wine made from vines that experienced and early stress are different from those attributes of a wine made from vines that experienced water stress only after veraison. We are now testing whether these changes in fruitfulness, fruit composition, and wine sensory attributes are due to water stress directly or are due to the effects of water stress on the canopy microclimate. If the latter is true, water stress may not be necessary to obtain these differences. Early season water deficits increased light and air temperature in cluster zone throughout much of the season. Postveraison water deficits can also cause an increase in light in the cluster zone, apparently due to leaf abscission which we intend to quantify in 1992. Another approach was to include treatments that counteracted the effects of the irrigation treatments on microclimate. In some control vines, leaves were removed to open the canopy and make conditions similar to those experienced under the Early deficit treatment. Likewise, in some Early deficit vines clusters were bagged with shade cloth to decrease light penetration. The results suggest that changes in fruit growth and composition that occur in response to water stress are unlikely to be due to changes in the canopy; rather fruit responses appear to be due to changes in the physiology of the berry caused by water stress conditions. However, these treatments, partial defoliation of control vines to create “Early deficit-like” canopies and shading of Early deficit clusters were only partially successful in accurately recreating the alternative cluster environments. The plan for 1992 includes modifications in these treatments to more accurately reproduce their alternative cluster environment goals and sensory analysis of Pinot noir and Cabernet Sauvignon wines from different water stress treatments.

Trellises for Wine Grape Canopy Management in the San Joaquin Valley

A wine grape trellis trial was designed and planted in 1986 at the U.C. Kearney Agricultural’ Center in the San Joaquin Valley. The purpose was to determine the most suitable trellis system for optimum fruit production and composition in cultivars of major importance to the area; ease and economics of vine management and mechanization were also considerations. Two cultivars, French Colombard and Barbera, were tested on 8 trellis designs suited to mechanization over 3 years, 1989-91. Data collection involved measurements of vine growth and light environment and fruit characteristics and composition; wine making, must and wine composition, and wine sensory analysis were also performed. Contrast analysis was conducted on major comparisons of vine training and trellis configuration for individual years and all 3 years (overall effects). The addition of one foliage support wire above bilateral cordons (11″ distance) increased pruning brush weights; vine yield was also increased through increased berry and cluster weights and with little or no effect on fruit composition in both cultivars. Similar effects were also obtained by increasing the bilateral cordon height by 11″ in both cultivars and with some advanced fruit maturation in Barbera.

Vineyard canopy management practices for improving grapevine

The main conclusions from a four-year study of trellising, row spacing and pruning level of Cabernet Sauvignon at the Oakville Experimental Vineyard are as follows: 1. Reducing row spacing from 12 feet to 8 feet increased crop yield by 35%or 2.8 tons/acre with little or no significant difference in fruit composition. 2. At each of the three row spacings, quadrilateral cordon (QC) trellised vines produced approximately two tons/acre higher yield than bilateral cordon (BC) trellised vines averaged over a period of four years. 3. At the same level of “Brix at harvest, QC fruit had lower pH and higher levels of anthocyanins than BC fruit. 4. The higher level of anthocyanins in QC fruits than BC fruits was correlated to greater amount of photosynthetic active radiation in the fruiting region of the former treatment. 5. Increasing the pruning level from 24 to 60 buds per vine increased crop yield from 6.6 tons/acre to 11.0 tons/acre. 6. With increase in the number of buds per vine from 36 to 60 there was an average of 7 to 10 days delay in ripening. 7. At harvest, fruits from vines pruned to 48 and 60 buds/vine had lower pH, TA, malic acid and K and higher anthocyanin than vines pruned to 24 and 36 buds per vine. 8. Dividing the canopy, reducing distance between rows, and increasing the number of buds per vine all reduced shoot length, shoot weight, pruning weight per vine, and primary and lateral leaf area per shoot and increased the cropping efficiency. 9. The canopy density of QC trellised vines was significantly less than BC trellised vines. 10. Sensory analysis showed that BC wines could be distinguished from QC wines. 11. Sensory analysis could not distinguish between low crop wines (24 buds/vine) and high crop wines (60 buds/vine).

Water Conservation in Wine Grape Production

An infrared thermometer (IRT) was used to schedule irrigation in five vineyards. Chardonnay vines in Madera and Santa Maria, CA; Chenin Blanc vines in Madera, CA; and Cabernet Sauvignon vines in Madera and Paso Robles, CA were used in this project. Water was applied to drip irrigated vines only when IRT measurements indicated a certain level of water stress. If vines were below that stress level, water was withheld until stress increased to the selected level. Treatments were imposed from veraison to harvest at the Madera and Santa Maria locations; and from berry set to harvest at the Paso Robles location. Water use for irrigation was reduced by up to 3 0%in this study. The amount of water saved depended on environmental conditions and the irrigation scheduling practices of the grower-cooperator. Irrigation scheduling treatments had no significant effect on vine yield or dormant pruning weight. Fruit composition displayed only a small response to the treatment. In general, %soluble solids of fruit were increased by increasing water stress. The effect of treatment on titratable acidity and pH was not consistent. These results are preliminary and several more years of data collection are required before equilibrium results can be obtained. Commercially available microsprayers were tested in the Center for Irrigation Technology Sprinkler Testing Laboratory on the CSU, Fresno campus. All microsprayers tested were not suitable for targeted frost protection of vines. Prototype microsprayers from two manufacturers were also evaluated. The Wade Manufacturing Pulsator was identified as being capable of providing adequate targeted frost protection with a flow rate of approximately 11 gpm/acre. Thus, the commercial application of this technology would substantially reduce water use for frost protection. Additional testing is needed to evaluate the Pulsator microsprayer under field conditions using differential application rates.