Research Bulletin
Response of Thompson Seedless Grapevines to Sustainable Viticultural Practices
by
R. K. Striegler1, M. A. Mayse2, W. O'Keefe3
and D. R. Wineman4
CATI Publication #970102
© copyright January 1997, all rights reserved
ABSTRACT
Consumer concerns about pesticide residues and environmental degradation are having a significant impact on the California grape industry. Growers are using a variety of practices from integrated pest management to certified organic production to reduce the amount of pesticides and other synthetic inputs used in vineyards. This experiment was undertaken to test selected sustainable cultural practices in a mature Thompson Seedless vineyard. Treatments included in the experiment were management of row middles (cultivated vs. perennial legume cover crop) and nitrogen fertilization (compost vs. synthetic fertilizer). Vine nutritional status, yield, fruit composition, pruning weight, and population levels of Erythroneura leafhoppers were monitored each season (1992-1994). In addition, efforts were expanded during the 1994 season to include assessing densities of spiders, herbivorous mites, and beneficial insects.
Conventional cultural practices (cultivation and synthetic fertilizer) produced the highest yields during the 1992 and 1993 seasons. This may have been due to the nutritional status of vines, which was generally better for the cultivation and synthetic fertilizer treatment, especially in 1992. In 1994, significant treatment effects on yield were not observed, indicating that legume cover crop plots had become fully established.
Sustainable cultural practices had little impact on growth, fruit composition, or insect pest pressure. Thompson Seedless grapes were grown without the use of herbicides and with only one insecticide application during the study. Vine diseases were managed by cultural practices and application of sulfur.
INTRODUCTION
The California grape industry is experiencing a period of fundamental change. Regulatory agencies, policy-makers, growers, and consumers are increasingly concerned about the negative environmental consequences of farming (National Research Council 1989). A key concern is pesticide residues in food (Pastore and Bruhn 1991, Pimentel et al. 1992). Growers and processors are aware of consumer concerns about food safety and are taking steps to reduce the amount of pesticides used in vineyards. The result of these efforts will mean less dependence on synthetic chemical inputs in vineyards and a greater reliance on sustainable viticultural practices. This trend is also occurring in the European (Boller 1993) and Australian (Turkington and Boys 1994) grape industries.
Sustainable viticultural practices emphasize diversity in the vineyard ecosystem to encourage beneficial arthropods, soil improvement through regular additions of organic matter, and substantial reduction of synthetic pesticide use (Ingels 1992). Vineyard ecosystem stability, enhanced environmental quality, and reduced production costs for growers can result when sustainable viticultural practices are used.
Fertilization and use of cover crops are cultural practices which are considerably important to agricultural sustainability in vineyards. Vineyard pest management can clearly be influenced by the nutritional status of grapevines. Variegated leafhopper (Erythroneura variabilis) nymph density is closely related to the nitrogen (N) status of grapevines (Mayse et al. 1991). Also, leaf N levels can influence the population dynamics of the Pacific spider mite (Wilson et al. 1986).
Grape disease incidence can be influenced by vineyard fertilization practices. Excessive use of nitrogen fertilizer often results in overly vigorous canopy development and increased incidence and severity of the bunch rot complex (Pearson and Goheen 1988). Host plant nitrogen status influences powdery mildew and downy mildew levels on grapes (Baveresco and Eibach 1987). Furthermore, the rate of infection of Phomopsis viticola increases when grapevine fertilization rates increase (Kast 1991).
In general, insect pest and disease pressures increase when high levels of synthetic N fertilizer are applied to grapevines. This relationship also occurs in nectarines (Daane et al. 1995, Ramirez 1993). Other negative effects of excessive use of N fertilizer in vineyards may include groundwater pollution, reduced productivity, and reduced fruit quality (Pease et al. 1995, Schaller 1991, Spayd et al. 1991, Weinbaum et al. 1992). Some of the negative aspects of high levels of N fertilization may be due to indirect effects(shading) rather than direct N effects (Smart 1991).
Cover crops can provide multiple benefits in vineyard management (Bugg 1995,1996; Miller et al. 1989). The well-managed cover crop acts as an insectary for increasing the numbers of beneficial predatory insects and spiders (Bugg 1991, Hanna et al. 1995, Hofmann 1993, Mayse and O'Keefe 1993, O'Keefe 1993). Reductions in leafhopper populations are associated with the use of cover crops, although the mechanisms by which control is achieved are not fully understood (Garcia 1993). The incidence of soil disease can also be influenced by cover crop use. Phytophthora crown and root rot did not occur on apple trees in grass and crown vetch plots during the course of a four-year study (Merwin et al. 1992).
Use of cover crops provides the grower with another tool for management of vine canopy development and nutritional status (Winkler et al. 1974). Some beneficial aspects of cover crops include reduced soil erosion, improved soil structure, suppression of weed growth, increased water infiltration, reduced groundwater pollution, reduced dust, and reduced sunburn of fruit (Blake 1991, Folorunso et al. 1992, Gaffney and van der Grinten 1991, Gulick et al. 1994, Louw and Bennie 1991, Meisinger et al. 1991, Miller et al. 1989, Smith 1993). In addition, cover crops can provide N and increase availability of other nutrients for the crop (Miller et al. 1989).
Potential disadvantages for cover crop use include increased competition for water and nutrients, the possible harboring of insect or vertebrate pests, allelopathy, and the cost of establishment and maintenance (Esteve 1992, Kliewer 1991, Mayse and O'Keefe 1993, Perez-Munoz 1993, Van Huyssteen and Weber 1980, Wolpert et al. 1993). Selection of cover crop should be based on vineyard site characteristics and management objectives; there is obviously no "perfect" cover crop.
The move toward sustainable viticultural practices in grape vineyards will not occur without some difficulty. The process of converting from conventional practices to sustainable practices does not merely involve low-input farming (Ingels 1992). In fact, considerable management and non-chemical inputs are required for successful grape production in a sustainable system. The development of sound technical information is required so that growers can effectively make the transition to sustainable viticulture practices. More research on biological, cultural, and chemical alternatives to currently available synthetic pesticides is needed to assure that productivity is maintained during the transition period (Jacobsen and Backman 1993, Stimmann and Ferguson 1990, Zalom and Strand 1990).
A comprehensive long-term, multi-disciplinary effort is needed to evaluate sustainable viticulture practices. More information in this area would benefit growers, consumers and the environment. Therefore, the objectives of this experiment were to determine the effects of N fertilization practices and cover crop on growth, yield, fruit composition, vine nutritional status, soil nutrient composition, and populations of selected arthropods on Thompson Seedless grapevines.
STUDY AREA AND METHODS
This experiment was conducted in a mature, own-rooted Thompson Seedless (Delano clone) vineyard located on the California State University, Fresno Agricultural Laboratory. The vineyard was planted in 1980 on a Ramona loam soil. Row orientation was east to west and vine spacing was 7 ft X 12 ft (vine x row). Vines were cane-pruned and irrigated by sprinklers. The trellis system consisted of a 6 ft (5 ft height above soil surface) grape stake with an 18 inch crossarm at the top of the stake. Canes were tied on a wire located at 4 ft.
The cultural variables under investigation were nitrogen fertilization and use of cover crops. Vines received compost (2 ton/acre of 100% compost in 1992; 4 ton/acre of 50% compost, 25% gypsum, and 25% limestone in 1993 and 1994)as a broadcast application in the spring (March-April), or 50 lbs/acre synthetic N fertilizer (ammonium sulfate in 1992, Triple 15 (15N-15K-15P) in 1993, and ammonium sulfate in 1994) as a band application next to the berm during the postharvest period (September-October). Compost in cultivated plots was incorporated during normal row middle tillage operations, while compost in cover crop plots was simply broadcast and not incorporated. Synthetic N fertilizer was placed approximately 4 inches below the soil surface in both cultivated and cover crop plots. All treatments were irrigated with canal water exclusively during the experiment.
Row middles were either cultivated or planted with a perennial legume cover crop (Germain's Seeds, Inc. insectary mix: trefoil, buckwheat, little burnet, baby blue eyes, California poppy, allysum, and white yarrow). Cover crops were planted in October 1990 and replanted in October 1991 to achieve a more consistent stand. Cultivated plots were tilled approximately 4-6 times during the growing season. The cover crop plots were mowed infrequently. Cover crops were mowed 6-8 inches above the soil surface to have minimal impact on beneficial arthropods and to allow for reseeding of annual flowering plants in the cover crop mix.
Weeds within the vine row were controlled by cultivation. Sulfur applications were used to control fungal pathogens. An important goal of this study was to manage insect pests using biological and cultural control. Therefore, synthetic insecticides were not applied in this experiment except in 1994, when a combination of insecticidal soap, pyrethrin, and foliar oils was applied two weeks prior to harvest for control of leafhopper adults. This "soft insecticide" combination is acceptable for organically grown fruit (Bentley et al. 1995). All other cultural practices were standard for the San Joaquin Valley (Bentley et al. 1995, Elmore et al. 1995, Flaherty et al. 1992, Gubler et al. 1995, Kodira and Westerdahl 1995).
A completely randomized experimental design was used. There were four replications of treatments and plots were 120 ft x 147 ft (width x length). Viticultural data were collected from ten vines in the center of each plot. Insect population densities were sampled near these ten vines.
Data collection was extensive due to the complexity of the vineyard agroecosystem. Vine leaf petiole samples were collected at bloom and analyzed for nitrate nitrogen (NO3-N), phosphorus (P), potassium (K), zinc (Zn), sodium (Na), manganese (Mn), boron (B), and magnesium (Mg) (Christensen et al. 1978). Berry samples (100-berries) were collected the day before harvest for measurement of fruit composition. The percent soluble solids, pH and titratable acidity of these samples were determined by standard procedures (Zoecklein et al. 1989).
At harvest, yield and yield components of ten-vine plots were determined. Dormant season pruning weight was determined in December of each season. Soil samples were collected mid-row on 17-20 September 1994. Three subsamples were collected from each ten vine plot at two depths: 1 and 2 feet. Samples were then submitted to a laboratory for analysis of pH, exchange capacity (ECe), and nutrient levels.
Seasonal population levels of the variegated leafhopper (Erythroneura variabilis) (VLH) and the western grape leafhopper (E. elegantula) (WGLH) were monitored during the 1992-1994 growing seasons (Flaherty et al. 1992). During 1994, sampling was expanded to include spider, beneficial insect, lacewing egg, and herbivorous mite densities.
Data were subjected to a factorial analysis of variance, and means were separated by Duncan's multiple range test (Steel and Torrie 1980).
RESULTS AND DISCUSSION
Cultural practices had a significant effect on yield during the 1992 season (Table 1). Yield was significantly higher in the cultivated + synthetic fertilizer treatment than in the cover crop + synthetic fertilizer treatment. Treatments receiving compost did not differ significantly in yield from the synthetic fertilizer treatments. Cluster weight was substantially lower for vines with cover crop + synthetic fertilizer than for vines receiving the other treatments. The cultivated + synthetic fertilizer treatment produced significantly higher berry weight than the other treatments. Clusters/vine, berries/cluster, and pruning weight were not significantly affected by treatment. Furthermore, treatments had no significant effect on fruit composition in 1992 (Table 2).
Nutrient status of vines was influenced to a limited degree by treatment in 1992 (Table 3). The level of phosphorus in petioles was higher for cultivated + synthetic fertilizer treated vines than for vines receiving the other treatments. Boron was lowest in petioles from vines in the cover crop + synthetic fertilizer treatment. None of the other nutrient levels differed significantly.
In 1993, yield was again influenced by cultural practices (Table 4). Yield was higher in plots which were cultivated than in plots with cover crop. Cluster weight and berries/cluster were greater in the cultivated + synthetic fertilizer treatment than in the two cover crop treatments. Clusters/vine, berry weight, and pruning weight were not significantly affected by treatment. No significant differences were observed for fruit composition or vine nutritional status in 1993 (Tables 5 and 6).
Cultural practices did not significantly affect yield, growth, fruit composition, or vine nutritional status during the 1994 season (Tables 7,8, and 9). In general, yields were low during the 1992-1994 seasons. This result may be due in part to clonal selection. The vineyard used in this experiment has the "Delano" clone of Thompson Seedless. This clone is characterized by low numbers of clusters/vine and a tendency towards alternate bearing (Vincent E. Petrucci, personal communication 1993). These characteristics may result from mild leafroll virus infection (Peter Christensen, personal communication 1993).
Treatment effects on soil composition were limited, and those that occurred were near the soil surface (one foot depth) (Tables 10 and 11). Phosphorus was higher in cultivated + compost plots than in cultivated + synthetic fertilizer plots, and boron was lowest in the cultivated + synthetic fertilizer plots at the one-foot depth (Table 10). Although not statistically significant, it should be noted that NO3-N at the two foot depth was numerically higher for the cultivated + synthetic fertilizer treatment than for the other treatments (Table 11).
Arthropod population densities were not significantly influenced by cultural practices during 1992 and 1993 (data not presented). However, in 1994 when data collection efforts were expanded, significant population differences for various arthropods were observed (Table 12). Spider densities were higher in the cultivated + compost plots. Vines with the cover crop + compost treatment demonstrated not only higher lacewing egg counts, but also lower densities of western grape leafhopper nymphs. Densities of beneficial insects, variegated leafhopper, and herbivorous mite populations were not affected by treatment in 1994. Overall, population densities of arthropods were relatively low during this three-year study. Sprinkler irrigation may have played a role in these general results, since this method can reduce insect numbers by physically washing insects from the vine canopy (Flaherty et al. 1992).
CONCLUSIONS
Thompson Seedless grapevines were grown for three seasons using sustainable viticulture practices. Limited insecticides and no herbicides were applied during this period. Disease and insect pressures were minimal, and plots receiving compost fertilizer would have qualified for certified organic production.
Conventional cultural practices (cultivation and synthetic N fertilizer) produced higher yields during the 1992 and 1993 seasons. This result may have been due to vine nutritional status which was generally best for the cultivation + synthetic fertilizer treatment, especially in 1992. In 1994, when cover crop treatments had become fully established, there were no significant treatment effects on yield or vine nutritional status.
Treatments in this study had very limited effects on vine growth, fruit composition, or arthropod population densities. However, the sustainable treatment of cover crop + compost demonstrated both higher predatory lacewing eggs and lower western grape leafhopper nymph densities in 1994. Measurement of soil composition after three seasons of treatment indicated a potential negative impact of the cultivated + synthetic N fertilizer on nutrient levels, and may suggest downward movement of NO3-N through the soil profile. Grape growers in the San Joaquin Valley of California currently have alternative production practices at their disposal which can help reduce pesticide use and ameliorate other environmental concerns.
ACKNOWLEDGEMENTS
Funding for this project was provided by the California Agricultural Technology Institute. We gratefully acknowledge the donation of cover crop seed by Germain's Seeds, Inc. and the donation of compost by New Era Farm Services, Inc. In addition, we wish to thank T.G. Schmeiser Co. Inc. for use of a grain drill to establish cover crops.
The authors also thank Greg Berg, Fidel Garcia, Lalith Nissanga, Debra Dexter-Mendez, Vidal Perez-Munoz, and Mark Salwasser for technical assistance with portions of this project.
ABOUT THE AUTHORS
1 Research Scientist
2 Professor
3 Research Associate; Present Position & Address: Teacher, Thoreau Middle School P.O. Box 787 Thoreau, NM 87323
4 Research Technician; Present Position & Address: Research Director, California Table Grape Commission 2975 N. Maroa Ave. Fresno, CA 93755
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CALIFORNIA AGRICULTURAL TECHNOLOGY INSTITUTE - CATI
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