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

This report describes the 1993 results of continuing, large replicated trials in three San Joaquin Valley vineyards where we are studying the impact of three vineyard floor management systems on pests, weeds, and vine nutrient status. The experiments consisted of three main treatments: (1) an oat/vetch (two vineyards) or a merced rye/vetch mixture (one) vineyard planted in row middles, with row berms treated with preemergence herbicides, (2) the same as in 1, but the cover was cut and blown on berms for weed suppression (with only postemergence herbicides applied during late winter), and (3) cover crops were not planted, but row middles were clean-cultivated (control), and berms were treated with herbicides for weed suppression. An additional trial was established on a two-acre vineyard at the UC Kearney Agricultural Center. This vineyard included only treatments 2 and 3, and a treatment where we attempted to evaluate the impact of berm mulching on weed suppression without the use of preemergence and postemergence herbicides. During the 1993 season, the presence of cover crops did not affect the number of leafhoppers on vines in any of the four vineyards, but there were large differences between vineyards in the size of leafhopper infestations, levels of leafhopper-egg parasitism, and abundance and species composition of spiders. In general, two vineyards (7 and 10 years old) supported larger numbers of leafhoppers (11-15 nymphs per leaf) compared to the other two vineyards (more than 30 years old) where leafhopper numbers were lower than one nymph per leaf during the peak of the second generation. We do not really know the cause of these differences, but it appears that factors associated with vine age may have a significant impact on leafhopper abundance. The proportion of leafhopper eggs parasitized by Anagrus were also higher in the ‘young’ compared to ‘old’ vineyards, probably resulting from higher densities of leafhopper eggs in these vineyards. Parasitism in cover plots was similar to control plots, but there was a trend toward higher egg parasitism in cover plots during the second leafhopper generation. There were also large differences between vineyards, and between cover treatments within vineyards, in the abundance and species composition of spiders occurring on grapevines. In the ‘older’ vineyards, there were significantly higher populations of spiders on vines in cover crop and mulch plots compared to vines in control plots. Leafhoppers were too low in these vineyards for any effect to be observed during the 1993 season. The differences in spider abundance between cover and control treatments were only obtained in the old’ vineyards where spiders were predominantly of the web-building species. In the two young’ vineyards, where spiders were predominantly of the non-web-building (or hunting) species, we did not observe a significant effect of cover crops on spider abundance on vines. Initial 16 results from marking studies indicated that the cover crops are increasing the spider’s food abundance, and are providing an alternative foraging habitat for the spiders, but it is possible that not all spider species are similarly affected in this manner by the cover crops. Further results of these marking are pending. Results from experiments on weed suppression with dry mulch are variable. However, our studies and those conducted by C. Elmore in north coast vineyards indicated that yearly accumulation of biomass in vine rows should provide sufficient weed suppression to minimize the use of herbicides. The data on the nutritional status of vines showed that petiole tissue analysis for nitrate-nitrogen and potassium varied considerably between vineyards. In this first year of comparing fertilized and unfertilized plots (in the two ‘old’ vineyards) in the presence and absence of cover crops, we did not observe any significant impact of cover crops on nitrate-nitrogen status; but there was a trend toward higher potassium levels during veraision in the cover plots compared to control plots in the two ‘old’ vineyards. In the two ‘young’ vineyards, nitrate-nitrogen levels were reduced in the presence of cover crops, but no changes occurred in potassium levels. These effects were probably caused by factors related to the cover crop stand and weeds growing on berms. It is also possible that young’ vineyards are less competitive with cover crops than are ‘old’ vineyards. We are continuing to monitor changes in vine nutrient status which may not be reliably detected until the following year. Our initial budget for the three floor management systems indicates that the lowest cost is incurred in the absence of cover crops and when the commonly applied herbicides are used to control weed in the vine rows. We expect, however, that the overall benefit of using cover crops in vineyard production will be increased the cost of insecticides and fertilizers are incorporated into the cost/benefit analysis. Our findings are being disseminated to the scientific community and farmers in meetings and field days.

Wine Grape Canopy Management Practices in the San Joaquin Valley

Recent wine grape vine training and trellising studies in the San Joaquin Valley have demonstrated the influence of cordon height, foliage support wires, and quadrilateral training on vine yield, fruit composition, and wine quality. Concurrent studies in other districts have also demonstrated the beneficial effects of cluster exposure through leaf removal and other canopy management practices on grape composition and reduced fungal disease incidence. The purpose of this study was to determine the influence of several leaf removal techniques and shoot hedging on wine grape cultivars and trellis systems suitable to the San Joaquin Valley. Three leaf removal treatments — Hand, Window, and Hedge — and a check, no leaf removal were compared with French Colombard and Barbera. Leaf removal increased fruit zone light exposure in French Colombard temporarily due to its continued vigorous growth. This resulted in no fruit zone light environment differences at veraison and harvest. There were no significant fruit composition or bunch rot effects from any of the treatments in this cultivar. Hand leaf removal reduced vine yields, presumably due to berry loss from sun exposure soon after the treatment was imposed. Greater treatment effects were measured with Barbera. This could be explained by the more extreme and persistent effects of leaf removal in this less vigorous cultivar. Window leaf removal increased sunlight exposure through veraison while hand leaf removal was effective through harvest. However, this improved fruit exposure was not accompanied by any benefits in fruit composition, including soluble solids, pH, or anthocyanin content. The only treatment effects on fruit composition were reduced berry weight and titratable acidity in the Window and Hand treatments, respectively. The only positive effect was a reduction in bunch rot incidence from all of the leaf removal treatments as compared to Check. This included Hedge which did not improve fruit zone light environment at any time. This would suggest that hedging may provide adequate air circulation to reduce bunch rot while not changing the fruit zone light environment. Overall, the results further demonstrate that canopy management leaf removal has the potential to reduce bunch rot in the San Joaquin Valley. However, the rapid regrowth of vigorous French Colombard may reduce or eliminate this effect unless practiced at a later date during fruit ripening. The mechanical methods of Window and Hedge were as effective as Hand, cluster-region leaf removal in reducing rot. Thus, they would be preferable under San Joaquin Valley conditions due to economics. Leaf removal did not improve fruit composition as has been reported in cooler, coastal region studies. In fact, some leaf removal methods reduced berry weight, and titratable acidity in Barbera and yield in French Colombard. This was especially true of the more severe Hand leaf removal treatment where fruit exposure in this hot region caused berry burn. Thus, leaf removal canopy management practices in the San Joaquin Valley can be recommended for bunch rot control but are of questionable, if not detrimental, benefit to fruit composition and yield.

Water Requirements During and After Vineyard Establishment for Chardonnay Grapevines Grown in The Carneros District

A study is being conducted to determine the water use or vineyard evapotranspiration (ET) of Chardonnay grapevines during vineyard establishment. ET is the combined loss of water by evaporation from the soil and transpiration by the vine. The experimental vineyard is located in the Carneros District of Napa Valley. In addition, the vines were grafted onto two different rootstocks (110R and 5C) to determine if there were differences in water use between the two. The rootstocks were planted in June of 1990 and Fall chip budded with the Chardonnay scion. Vineyard water use was determined by measuring soil water depletion and monitoring the addition of water via irrigation or measured rainfall. Soil water content was measured with a neutron probe (also called a hydroprobe or soil moisture gauge). Access tubes,. needed to measure soil water content, were placed at eight sites throughout the vineyard (four sites per rootstock). At each site six access tubes were places such that the soil water content in one quarter of an individual vine’s root volume could be quantified down to a depth of 3 m (approximately 10 ft) . The tubes were placed equidistant from one another: two tubes in the vine row, two tubes midway between rows and two tubes midway between the four tubes mentioned previously. The tubes also extended midway between v\Lnes within a row. The year 1992 represented the third growing season for this vineyard. Soil water content decreased rapidly early on in the growing season until the end of May. After this time soil water content remained constant until the end of October. The depth of water extraction from the soil extended down 1.3 m (4.25 ft). The constancy of the soil’s water content from June until October indicated that the water applied via irrigation equaled the amount of water used by the vineyard. The average seasonal water use of both rootstocks was 236 mm (9.3 inches) of water. This is equivalent to approximately 750 1 (200 gallons) of water per vine. The amount of water used by the vines supplied from the soil resevoir was 42%of the total amount used. Midday leaf water potentials were measured every two weeks during the season. Leaf water potential is a measure of a vine’s water status and would give an indication as to whether the vine experienced stress at any time. Of the eleven times measurements were taken, leaf water potential readings on nine dates indicated that the vines were not under stress (values were less negative than – 0.8 MPa or -8 bars) . However, on two of the dates leaf water potentials became more negative than -1.0 MPa. Values more negative than -1.0 MPa are associated with vine water stress in mature vines. It should be pointed out that on both of those dates ambient temperatures were greater than 32°C (90°F) and relative humidities were quite low. This information would indicate that irrigation amounts should have been increased during those periods when evaporative demand was greater than normal. One of the primary objectives of this study is to determine crop coefficients for vines grown in a cool climate during vineyard establishment. The crop coefficient for this vineyard started out at 0.5 and decreased thereafter until day of year 220 (approximately the first week in August) when the crop coefficient reached a minimum of 0.2. Subsequent to this date the crop coefficient increased and reached a maximum of 0.4 at the end of the growing season. The relatively high crop coefficient early in the season could have been due to the percolation of water below the depth of the access tubes (which would have been measured as vineyard water use) and the fact that a natural cover crop had been established during the winter months. The cover crop would have used a considerable amount of water until the vine rows started drying out. The increase in the crop coefficient from August until the end of the growing season was due to the continued growth of the vines up to November. An increase in the crop coefficient up to the end of the season has previously been reported for vines grown in the San Joaquin Valley during the second year of vineyard establishment.

Water Conservation in Wine Grape Production

An infrared thermometer (IRT) was used to schedule irrigation and impose selected levels of stress in two vineyards during the 1992 season. Cabernet Sauvignon vines in Paso Robles, CA and French Colombard vines in Ripperdan, 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 berry set to harvest. Water use was reduced during the berry set to harvest period by up to 50.3% and 39.0%, respectively for Cabernet Sauvignon and French Colombard vines. Treatments which exposed vines to moderate levels of stress for the entire berry set to harvest period produced the greatest reductions in water use. Irrigation scheduling treatments had no significant effect on yield or dormant pruning weight of Cabernet Sauvignon vines. However, growth and yield of French Colombard vines were significantly reduced by all programmed water stress treatments. Fruit maturity was generally advanced by increasing water stress but differences were often not statistically significant. These results are preliminary and several more years of data collection are required before equilibrium results are obtained. The Wade Manufacturing Pulsator microsprayer was subjected to further developmental testing in the Center for Irrigation Technology sprinkler testing laboratory to refine its application pattern. A commercial plot was established in a Chardonnay vineyard near Los Alamos, CA and field testing of the Pulsator occurred this spring. Analysis of data is not complete but it appears that the Pulsator microsprayer was as effective as overhead sprinklers for frost protection.

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

This project quantified the role of cluster microclimate in water stress responses, tested the importance of stress at veraison, and extended what we learned about seasonal water stress in hillside Cabernet franc and Sauvignon blanc production in Napa Valley to additional sites and varieties. Soil and vine water status is readily controlled in drip-irrigated vineyards of Pinot noir (Carneros) and Cabernet Sauvignon (Lodi). Changes in vineyard water status, yield, and fruit and wine composition caused by pre- (Early Deficit) and post-(Late Deficit) veraison stress were similar to our earlier results. Thus, those results, showing control of yield and fruit composition, can largely 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 and, because of this, color, phenolics, malate, and amino acids, and other compounds in the fruit. Light penetration into the canopy increased in Early Deficit vines early in the season compared to other treatments. After veraison light penetration increased slowly in C vines and rapidly in Late Deficit vines, in part due to differences in the rate of leaf drop. The role of microclimate in the stress responses of fruit development was investigated with reciprocal treatments that, e.g., created in some well-irrigated vines a canopy that mimicked the canopy of Early Deficit vines and vice versa. Temperature in the cluster zone did not differ significantly among any treatments, and, therefore, was unlikely to explain the differences that we observed in fruit and wine composition. However, differences in the light environments were important in establishing part (less than 50%) of the decreased pH and and increased color of Early Deficit wines. This shows that water deficits can be used to improve canopy structure. Water stress at veraison was not found to be critical for controlling fruit composition. However, the timing of the water stress was important in determining sensory attributes because judges could easily discriminate between wines made from Early Deficit and Late Deficit vines.

Spiders in Vineyard Agro-Ecosystems

The overall goal of this research project is to continue to elucidate the ecological roles, along with the potential economic value, of spiders in vineyard agro-ecosystems. Key objectives include determining which spider species may be directly associated with vineyard cover crops, and further delineating the patterns of abundance and distribution of important spider species in vineyards. One clear pattern beginning to emerge is an inverse relationship between spider and leafhopper densities in vineyards. Our findings in this and other studies agree with several other vineyard spider researchers in demonstrating that when spiders are abundant, leafhoppers generally tend to stay below economically damaging levels. The two most abundant spiders sampled from mid-June to the end of November in 1992 belonged to the family Clubionidae (two-clawed hunting spiders): Trachelas pacificus and Chiracanthium inclusum. Juveniles for these two species were roughly three times as abundant as adults in samples taken throughout the season. The next most abundant spider was Theridion (family Theridiidae), which was also of particular interest by being detected only in the grapevine canopy (i.e., almost never from the cover crops between vine rows). It should be noted that the most abundant spiders commonly detected in both cover crops and canopy were Trachelas and the micryphantids. Another particularly noteworthy discovery during the 1992 season involves an apparent correlation between western grapeleaf skeletonizer (WGLS) mortality and Trachelas pacificus occurrence. Corrugated cardboard bands wrapped around the base of vines are very effective in concentrating WGLS larvae seeking pupation sites. Bands which individually contained up to 70 WGLS pupae typically were found to be free of spiders. However, it was also not uncommon to find bands with only 5 to 10 WGLS pupae; in virtualy all these cases a large number of Trachelas juveniles were also found residing in those bands. Additional data collected in “round-the-clock” sampling trials during 1992 indicated that in estimating population densities for several important spider species, the actual time of day when samples are taken can be of considerable importance.

Influence of Windbreaks on the Vegetative and Reproductive Growth

Spring and summer months in the Salinas Valley are characterized by strong daily winds. It is widely held that vine growth and productivity in this region are reduced due to the presence of excessive wind. Chardonnay, the major wine grape cultivar of this region, appears to be particularly sensitive to excessive winds. Salinas Valley wine grape growers have recently expressed interest in the use of windbreaks to increase vegetative and reproductive growth. The long-term effects of windbreaks on vine vegetative growth, yield components, fruit composition, and wine quality have not been adequately investigated. A study was initiated in spring of 1991 to determine the effects of windbreaks on the vegetative and reproductive growth of Chardonnay grapevines in the Salinas Valley. Grapevines grown in artificial wind shelters were compared to grapevines exposed to ambient wind (control). Wind speed was reduced by up to 50%within the shelters, depending upon sensor distance above ground. Marked differences in the vegetative and reproductive growth of sheltered and non-sheltered vines were observed in both seasons. Sheltered vines had significantly larger primary and lateral leaves, and greater primary and total leaf areas compared to the control vines. The specific weight (mg dry weight-cm’2 leaf area) of both primary and lateral leaves was greater for the control than for sheltered vines. The number of nodes per shoot was similar for both treatments, however, the internode length of sheltered vines was significantly greater than the internode length of non-sheltered vines. The rate of shoot growth was also significantly greater for sheltered vines than for non-sheltered vines. Stomatal conductance and carbon assimilation rate were slightly greater for sheltered vines than for unsheltered vines, while no difference was found in leaf water potential between the treatments. Significant differences in vine yield components were not observed between the treatments in 1991. However, in 1992 cluster number, cluster weight, and fruit yield of sheltered vines was greater than for non-sheltered vines. Fruit and must composition were similar for both treatments. The results indicate that sheltered vines produced higher yields than non-sheltered vines due to their greater vegetative growth and vine capacity. Sensory evaluations of wines produced in 1991 and 1992 will be performed in the upcoming season.

Influence of Potassium Deficiency and Temporary

Executive Summary The aim of the proposed research was to study several sites with a history of potassium deficiency using a rapid screening procedure for putrescine levels that we developed in previous work on this project. The objective was to validate our screening procedure and provide recommendations regarding sampling time so that putrescine screening can be used routinely for managing potassium deficient vineyards. During the 1992 season we analyzed leaf samples for putrescine by HPLC and a recently developed TLC screening procedure that increased sample throughput by an order of magnitude. Results show that our screening procedure can certainly be used for Cabernet sauvignon and Chardonnay and can probably be used with other varieties. The screening procedure can be used to detect elevated putrescine in leaves at bloom whether or not leaf symptoms are present. The screening procedure is equally useful at veraison and after harvest. In all cases when samples were scored by TLC as having increased putrescine, the result was confirmed by HPLC. False positives and false negatives were not observed. Putrescine in leaves as estimated by the screening procedure correlated well with a subjective evaluation of the degree of potassium deficiency suffered by the vine. Results obtained during the 92/93 season served to validate of our screening procedure. We can now provide specific recommendations regarding sampling time so that putrescine screening can be effectively used for managing potassium deficient sites. In addition we have identified several “ideal” locations to apply the procedures we have developed. Interestingly, we found one site in our study that did not fit the typical potassium deficiency syndrome. This site provides an opportunity to characterize other disorders besides potassium deficiency that can lead to increased putrescine levels in the leaves.

Evaluation of Winegrape Clones

Thirteen clones or selections of Pinot noir were harvested for sparkling wine. Large differences were seen in maturity. Plots picked at 19 Brix were harvested over a 3-week period from August 21 through September 10. Yield was correlated more with cluster wt and cluster number than with berries/cluster. Among the clones, large differences were seen in virtually all measured parameters, including pruning wt, the several yield components, and in juice yield/ton of fruit. Duplicate wine lots were made from each clone or selection. Three clonal wines were rejected (both lots): one clone was defective in its aroma and two clones were lost because they underwent malo-lactic fermentation. The remaining 10 clones or selections are undergoing duo-trio analysis and further evaluation by cooperating winemakers. Further analysis of previous years’ data from two Chardonnay plots and one Cabernet Sauvignon plot reveals a close correlation of yield with cluster wt, more so than cluster number. For both varieties, there was a slight negative trend of pruning wt with yield, indicating that some yield increases were obtained at the expense of growth or that the yield:prunings ratio could have been altered by formula pruning, i.e. adjusting the buds retained at pruning based on the pruning wt. The Zinfandel clonal trial suffered heat damage and fruit shriveling prior to harvest skewing the data and resulting few reliable differences. Other trials with Merlot, Cabernet Sauvignon and Chardonnay are maturing and will be available in the near future.

Clonal Testing of Winegrapes in the San Joaquin Valley

An on-going San Joaquin Valley wine cultivar clonal evaluation trial was initiated in 1986 and planted into the first trial block in 1987. Location is the University of California Kearney Agricultural Center, Parlier where cultural conditions and practices can be closely monitored. All of the selections are indexed FPMS sources, most of which are registered. None have ever been compared in clonal studies in California. Thus, industry would benefit from performance information on available selections. This study utilizes 15 single-vine replicates in randomized complete blocks for each cultivar. 1992 was the fourth year of comparison for three selections each of French Colombard and Chenin blanc. Each cultivar compares two different selections which are registered (indexed as virus free) but not heat treated. Additionally, each cultivar includes a heat treated selection. Thus, we are studying the possible influence of heat treatment on virus-free material of French Colombard and Chenin blanc. Barbera was in its second year of data taking. This compares an Italian selection, Rauscedo 6 (FPMS Clone 2), with Marshall (FPMS Clone 1). Barbera Clone 1 presently involves much of the present commercial acreage but was later found to contain mild leafroll. French Colombard. Clone 5 was the only selection to show significant differences in 1992, with higher yields due to heavier clusters. The heavier clusters were due to more berries per cluster. This higher yield also contributed to lower fruit soluble solids of about l°Brix as compared to Clones 1 and 2. These results were different from those of past years when Clone 1 and Clone 5 were highest and lowest in yield, respectively. Clone 2 was the best overall selection in previous years but showed no benefits in its performance in 1992. Therefore, two more years of data taking are anticipated in order to better determine long-term differences. Chenin blanc. Clone 5 again produced the smallest berries and clusters of earliest maturity. However, rot also continues to be highest with this selection. Clone 4 continues to appear to be the best selection as it has had the highest yield and lowest rot incidence in the past. It was also more fruitful than Clone 1 in 1992. Clone 4 is a heat treatment of Clone 1. This is an interesting comparison, as the heat treated selection (Clone 4) has more favorable vine yield characteristics than Clone 1. 10-1 Barbera. These two selections were included to compare the widely planted Clone 1 (Marshall) with the only registered and recently introduced Clone 2 (Rauscedo). Because of its virus-free status, Clone 2 would appear to be the recommended choice for future planting. However, its larger berry size and higher yields contribute to later fruit maturity and lower fruit anthocyanin content. Thus, harvests in future years will be made at the same stage of fruit maturation rather than calendar date. This will enable us to more accurately compare fruit composition effects on wine quality. Wines were made from all of the selections in 1992. These results will be reported at a later date when sensory analyses have been completed. Zinfandel was eliminated in 1991 due to inclusion of a misnamed selection. Grenache, Sanqiovese, and Muscat blanc trials were established in 1993 with 3 selections of each cultivar.