California Agriculture

The California wine-grape sector has invested considerable time, money and effort in collective enterprises to reach fellow growers and assess the industry as a whole on sustainability. At the same time, California wine-grape production has become increasingly branded by particular geographic regions. Premium wine grapes are grown in regions with high population growth, high land values and often, charged environmental politics. Growers and their institutions have developed several agro-environmental partnerships to assess, improve and publicly represent their environmental stewardship and farming practices. We review trends in several regional and statewide indicators of sustainability, including crush prices, grape acreage, population growth and pesticide use. This review is based on 2 years of field research with participants in wine-grape partnerships, a review of documentary evidence, technical advisory work with the programs and summary assessment of case-study data, as well as an analysis of 10 years of Pesticide Use Report data for California wine-grape growers.

The widespread adoption of sustainable winegrowing practices depends not only on rigorous science, but also on its effective delivery to growers. The Lodi Winegrape Commission (LWC) created a unique self-assessment workbook and implementation program for increasing the adoption of sustainable winegrowing practices. This project was based on results from published research projects — many generated by UC scientists — and on-farm demonstration projects carried out by LWC growers and vineyard consultants. Data from two grower surveys shows that the program led to the increased adoption of specific sustainable winegrowing practices in the Lodi region. It has also served as a model for programs in other wine regions, including in California and New York.

In many parts of coastal California, agricultural water needs during the summer are met by tapping riparian and groundwater resources, which has led to documented decreases in stream flow during the dry season. This has consequences for salmon, including sudden drying of habitat, higher water temperatures and changes in the invertebrate prey base. We developed a new, spatially explicit analytical tool to quantify and map human and environmental needs, model daily stream-flow rates, and estimate regulatory flow requirements and cumulative impacts of reservoirs. This tool is part of a decision support system that can be integrated in a Geographic Information System (GIS) with other restoration considerations. This research provides a basis for placing additional reservoir storage where projects are not likely to affect adult salmon passage, while reducing water demand from surface and subsurface flows during spring and summer, ultimately improving both habitat for salmonids and water supply for growers.

In the 1930s and 1940s, little was known about viruses, and information on plant diseases caused by viruses was just beginning to appear in the scientific literature. Problems with grapevines in California, first referred to as “red leaf,” were initially attributed to inexperience in viticultural techniques and poor growing conditions. However, the problem was later identified as leafroll disease, which causes red leaves, and poor yields and fruit quality. We evaluated its rate of spread for 5 years in a Napa Valley vineyard, and found an average rate of more than 10% per year. Leafroll disease can be vectored by low-level populations of grape mealybugs, and is now spreading rapidly in at least one Napa Valley vineyard for unknown reasons. Using stock for planting vines that is certified as virus-free is a key strategy in preventing the spread of grapevine leafroll disease.

Dieback, or “dead arm,” in noncoastal California grapevines is most commonly caused by Botryosphaeria spp. Using Koch’s postulates, we demonstrated that isolates of B. obtusa are pathogenic on grapevines. We initiated studies to investigate the life cycle of B. obtusa and ways to control it with cultural practices. Fungal spores disseminated by rainstorms were collected in traps in an Arbuckle vineyard from December 2006 through spring 2007. The data suggests that B. obtusa was rain-disseminated throughout winter and spring, and that pycnidia on deadwood in the vines is a major source of inoculum for new infections. Transmission may also be possible via vegetative propagation, pruning shears and insects. Durable latex paints were investigated for protecting pruning and surgical wounds; a self-priming latex paint was shown to be an effective barrier and was nonphytotoxic.

Mealybugs have become increasingly important vineyard pests — a result of their direct damage to the vine, their role in transmitting grapevine leafroll viruses, and the costs for their control. Numerous mealybug species are found in vineyards, and each has different biological traits that affect sustainable control options. We review the mealybug pests and their natural enemies to provide some clarification about current trends in biological control tactics and needed directions for future work.

Liquid ant baits are an alternative to broad-spectrum insecticide sprays conventionally used to control Argentine ants. We review the development of liquid ant baits, which capitalize on the ants’ sugar-feeding requirements and social structure to deliver small doses of toxicant throughout the colony. The ant bait program described here, developed for commercial vineyards, also has the potential to facilitate the use of biological controls for mealybug and scale pests. The implementation of an Argentine ant bait program will enable grape growers to target other pests more selectively with insecticides, further contributing to their sustainable viticulture practices.

Management of the vineyard floor affects soil and crop productivity, as well as runoff and sediment that leave the vineyard. In Monterey County, weed control is typically conducted in a 4-foot-wide area under the vines, while cover crops are planted in the middles between vine rows. This 5-year multidisciplinary study in a low rainfall vineyard evaluated the impact of weed control strategies (cultivation, pre-emergence and post-emergence herbicides) in the vine rows, factorially arranged with three cover-crop treatments in the middles. We studied soil compaction, moisture and runoff; vine and soil nutrition; soil microbial biomass and mycorrhizae; and grape yield and quality. The late-maturing ‘Trios 102’ triticale used more water during the vine growing season than the earlier maturing ‘Merced’ rye. Cover crops increased organic matter and microbial biomass in the middles and reduced sediment loss. Weed control treatments did not affect crop yield or soil nutritional and microbiological parameters, but cultivation increased soil compaction at 4 to 7 inches deep. Weed control strategies and cover crops must be chosen carefully to maximize benefits and minimize negative environmental impacts.

Self-reseeding annual cover crops can regenerate in subsequent years without tilling the seedbed and can be part of a strategy to protect vineyard soil from erosion. We compared 22 such cultivars in a 1-year-old ‘Syrah’ wine-grape vineyard located at 1,400 feet in Lake County. We found significant differences between species in the amount of biomass produced in 2004 and 2005. All of the species studied were relatively low statured and fit well in vineyard middles. Pressure bomb readings taken after the cover crops stopped growing showed that with a dry spring (2004), vines with cover crops were modestly more stressed than those under tillage prior to July irrigations, but after irrigation the cover-cropped vines were slightly less stressed. In 2005, which had rainfall in late spring, there were no differences in vine water status throughout the season. We conclude that water use by the cover crop must have been relatively low and did not result in excessive vine water stress.

The Lodi Winegrape District is one of the largest in California and encompasses a wide diversity of wine-grape varieties, production systems and soils, which complicates grape nutrient management. To identify regions within this district that have similar nutrient-management needs, we are developing a soil-landscape model based on soil survey information. Our current model identifies five regions within the Lodi district with presumed relationships between soil properties and potassium-supplying ability. Region 1 has weakly developed, clay-rich soils in basin alluvium; region 2 has weakly developed, coarser-textured soils on recent alluvial fans, flood plains and stream terraces; region 3 has moderately developed soils on low terraces derived from granitic alluvium; region 4 has highly developed soils on high terraces derived from mixed alluvium; and region 5 has weakly developed soils formed on undulating volcanic terrain. Field and lab studies of soils in these regions show that our model is reasonable in concept, but that it must be fine-tuned to account for differing degrees of soil variability within each region in order to make realistic nutrient-management predictions.

Sustainable vineyard fertilization can lead to cost savings while protecting the environment. However, appropriate fertilization conditions depend on the rootstocks, which differ in their uptake of macro- and micronutrients, as well as on the vineyard soils’ physical and chemical characteristics, which affect the soil nutrient reservoir. We studied identical sets of 14 rootstocks on three different soils. Rootstocks had a significant impact on petiole levels of nitrogen and potassium throughout the growing season. Pruning weight and fruit yield also varied considerably by rootstock and site. However, rootstock performance was not consistent among sites, nor was the seasonal pattern of change in nitrogen and potassium consistent among sites. The observed differences emphasize the impact of soil texture and nutrient availability on plant growth. Further studies will help guide the development of site-specific sustainable fertilization regimens.