California Agriculture

Observational trials at the UC Kearney Research and Extension Center indicate that new southern highbush blueberry cultivars, which require fewer “chill hours” to produce fruit, are well adapted to the San Joaquin Valley climate. In a replicated cultivar evaluation, we quantified yields and identified several productive and flavorful varieties. These initial trials and ongoing studies on irrigation, plant spacing, mulches and pruning will improve the likelihood of establishing this promising new crop in the semiarid valley. Because blueberries are acid-loving, the soil must be extensively treated before planting, at considerable expense. Growers considering planting or expanding blueberry acreage should develop sound business plans, accounting for lower future prices and improved growing, harvesting and packing efficiencies.

Scientists at the UC Kearney Research and Extension Center have developed a new method to produce dried-on-vine (DOV) raisins. Prior DOV systems required costly trellising and harvesting equipment, putting DOV out of reach for most growers. Our new, within-row-alternate-bearing DOV (WRAB DOV) method can be used with the existing trellis and no retrofitting. DOV raisins are machine harvested, reducing human contact and production costs, and improving profitability. Drying raisins on the vine eliminates the need for intensive cultivation to prepare terraces down row middles. This method also removes the problem of using and disposing of paper trays, solving an important air-quality issue for raisin growers.

Simply put, a fruit tree can be viewed as a solar collector that converts sunlight into fruit. The more efficiently this is done, the greater the potential yield and profit. Consequently, growers face an important question when planting an orchard — what planting system to use? While varieties can be changed rather easily through grafting, the spacing, rootstock and conformation aspects of an orchard are typically permanent until that orchard is removed entirely, usually only after 15 to 20 years. These aspects can have profound effects on orchard productivity. Research conducted at the UC Kearney Research and Extension Center on orchard systems — including higher-density plantings and pruning techniques that enhance light interception — has allowed growers to make better-informed decisions when planting new orchards.

Production costs in peaches are highly dependent on the cost of labor to prune, thin and harvest trees — costs that would drop if growers had rootstocks that decreased tree size. Collaborating researchers from UC Davis and the U.S. Department of Agriculture’s (USDA) Horticulture Crops Research Laboratory in Parlier screened several promising clonal, size-controlling rootstocks for California peach production. In field tests, two peach scion cultivars (‘Flavorcrest’ and ‘Loadel’) on five interspecific hybrid rootstocks yielded positive results. After 8 years in the orchard, they performed well compared with trees on ‘Nemaguard’ rootstock (the California standard for peaches), with reduced trunk circumferences (60% to 95%), reduced dormant (22% to 80%) and summer (40% to 80%) pruning weights, and acceptable fruit size and crop yields (54% to 98%), since smaller trees will be planted at higher densities. This project has identified three new dwarfing rootstocks with commercial potential for California peach production. One rootstock (‘Hiawatha’) is already available through commercial nurseries and UC and USDA have jointly licensed the two others for commercial use.

Organic farmers and limited-resource growers in the San Joaquin Valley and other agricultural areas in California — many of whom are ethnic minorities — encounter limited options and environmental constraints when seeking economically viable pest management methods. Over the past 8 years, we have conducted weed research and implementation projects on soil solarization at the UC Kearney Research and Extension Center and on farms in the surrounding San Joaquin Valley. In the Kearney studies, small-scale solarization in parsley reduced weed biomass 94% to 99% over the untreated control. Furthermore, in an on-farm study, solarization provided effective weed control for strawberries at a much lower cost than methyl bromide, with comparable yields. This research has provided guidelines and technical support for growers wishing to implement solarization and related techniques for nonchemical soil disinfestation in a wide variety of specialty crops.

We compared reflective plastic and wheat straw mulches with conventional bare soil for managing aphid-borne virus diseases and silverleaf whitefly in cantaloupe. The occurrence of aphid-borne virus diseases was significantly reduced with both mulches as opposed to bare soil, and reflective plastic performed better than wheat straw. Silverleaf whitefly numbers, both adults and nymphs, were reduced equally by plastic mulch and wheat straw, and were significantly lower than with bare soil. Reflective plastic produced mature melons sooner and more cartons per acre than the other production systems. The reflective plastic system also produced a greater number of large-size melons, which are favored in the late-season market. Plants grown over straw mulch produced higher overall yields, including large-size melons, than those grown over bare soil.

”Large bug” damage to pistachio nuts varies by season, as well as among insect species and development stages, with larger bugs typically causing more damage than smaller bugs. We investigated pistachio damage by three large bug species (leaffooted bug, redshouldered stink bug and flat green stink bug) at different development stages and throughout the season, using field surveys and cage studies. Before fruit set occurs in June, most damaged nuts are dropped from the cluster without reducing fruit load. The midseason period (June to July) is the most critical because the damaged nuts remain in the cluster. After shell hardening, the kernel is largely protected from bug feeding.

Table grapes commonly suffer from tissue browning during harvest, packing, storage and shelf life, resulting in lower prices and reduced access to markets. We evaluated the development of browning symptoms in ‘Princess’ table grapes. The berries had high skin browning but very low flesh browning incidences. The most skin browning was found in highly mature grapes and appeared after 3 weeks of cold storage. Skin browning was directly related to fruit maturity, but vineyard location had a greater impact on the incidence of skin browning than maturity. In all locations, the skin browning susceptibility of ‘Princess’ table grapes rapidly increased when the berries reached a titratable acidity of less than or equal to 0.60% and/or a soluble solids concentration greater than or equal to 18.0%. Based on this work, we recommend harvesting ‘Princess’ at a soluble solids concentration between 16.0% and 18.0%.

New reduced-risk fungicides are highly effective in managing both pre- and postharvest diseases of stone fruit crops in California, and they have replaced most previously registered materials. These fungicides have a low impact on the environment, high specificity to target organisms, and low potential for groundwater contamination and human health risks. In stone fruit, they can be successfully used to manage brown rot blossom blight in a new delayed-bloom fungicide application program for low-precipitation years. In our studies, we found that fungicide treatments applied 1 to 14 days before harvest were also effective against preharvest brown rot and protected fruit from fungal decays initiated during harvest. Practices such as mixtures and rotations can be implemented to prevent resistance from developing and to ensure the lasting efficacy of these reduced-risk fungicides.

For the past decade, we have been developing techniques for monitoring pre- and postharvest diseases of tree fruit, nuts and vines at the UC Kearney Research and Extension Center (KREC). We have also advanced new methods to monitor pathogen resistance to fungicides, which growers can now use to make decisions on disease management. Although accurate, the conventional techniques are time-consuming and only provide results after 5 to 21 days. Molecular methods offer the possibility of faster, more reliable and efficient tests. We have developed such methods to monitor, diagnose and quantify crop pathogens. We have also used these new molecular techniques to answer complex questions on the biology of tree fruit and nut pathogens, the origin of their inoculum sources, changes in their population structures and the epidemiology of the diseases they cause. We are now working on molecular techniques that could supplement the conventional ones we have developed for vine diseases. Private diagnostic laboratories have been using a number of these conventional techniques and plan to adopt some of the molecular ones in the near future.

The sustainability of water resources is key to continued prosperity in the San Joaquin Valley and California. The vadose zone is an often-ignored layer of wet but unsaturated sediments between the land surface and the water table. It plays an important role in groundwater recharge and in controlling the flux and attenuation of nitrate and other potential groundwater contaminants. In a former orchard at the UC Kearney Research and Extension Center, we investigated the processes that control the movement of water, nitrate and other contaminants through the deep vadose zone. These processes were found to be controlled by the alluvial sedimentary geology of the vadose zone, which is highly heterogeneous. This heterogeneity should be considered when interpreting soil and deep vadose zone monitoring data and assessing of the leaching potential of agricultural chemicals. The transport of contaminants through the vadose zone may be significantly faster than previously assumed, while denitrification is likely limited or insignificant in the oxic, alluvial vadose zone of the eastern San Joaquin Valley.

In 1986, two large weighing lysimeters, — one in a peach orchard and the other in a grape vineyard — were constructed at the UC Kearney Research and Extension Center as a joint effort between UC and the U.S. Department of Agriculture’s Water Management Laboratory. Hourly weight changes in the lysimeters have been used to measure the daily and seasonal water use of trees and vines for nearly 20 years. Peaches and grapes exhibit similar seasonal crop-coefficient patterns that start as low as 0.1 in March, increase linearly until early July and then remain constant between 1.1 and 1.2 for the remainder of the season (provided the plants remain disease-, pest- and stress-free). The linear increase phase is proportional to the increase in canopy light-interception and leaf area. These relationships have facilitated modeling to predict crop evapotranspiration under various conditions. The peach lysimeter has also been useful for studying the effects of water stress on tree water use and for evaluating other methods of estimating water use.

In recent decades, air quality has improved near most cities but not in rural areas such as the San Joaquin Valley. Many studies using diverse exposure techniques have shown that ground-level ozone air pollution reduces plant growth and yield, from negligible impacts in some species to over 30% losses in others. We studied the interaction of ozone with weed competition from yellow nutsedge in Pima cotton and tomato in open-top field-exposure chambers at the UC Kearney Research and Extension Center in Fresno County. Ozone impacts on cotton (which is relatively sensitive) were compounded by weed competition, whereas tomato (which is less sensitive) competed well at all ozone concentrations. Our data suggests that crop-loss estimates obtained in single-factor experiments accurately reflect the serious risk of ozone to agriculture, but that more accurate yield predictions will require the consideration of interactions between the components of complex crop production systems, including weed competition.