California Agriculture
California Agriculture
California Agriculture
University of California
California Agriculture

Current issue and featured articles

July-September 2020
Volume 74, Number 3

Peer-reviewed research and review articles

Surveys of 12 California crops for phytoseiid predatory mites show changes compared to earlier studies
by Elizabeth E. Grafton-Cardwell, Walter Bentley, Mary Bianchi, Frances E. Cave, Rachel Elkins, Larry Godfrey, Ping Gu, David Haviland, David Headrick, Mark Hoddle, James McMurtry, Maria Murrietta, Nicholas Mills, Yuling Ouyang, Carolyn Pickel, Stephanie Rill, Menelaos C. Stavrinides, Lucia G. Varela
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In phytoseiid samples from 25 counties from 2000 to 2018, the western predatory mite, long recognized as an important biological control agent, was found in relatively low numbers.
Phytoseiid mites are key predators in agricultural crops. However, not all species regulate pest populations below economic thresholds, and therefore knowledge of which species are associated with particular crops aids pest control recommendations. Surveys of 12 crops across six geographical regions of California demonstrated that phytoseiid species varied by crop and geographical location, with subtropical crops exhibiting the lowest species diversity and grape the greatest. The western predatory mite, Galendromus occidentalis, long cited as a dominant species in California crops, was not found to be the major species in most situations. Euseius stipulatus, a species introduced in the 1970s, was found in the surveyed crops in many areas of the state and appears to be displacing E. hibisci along the south coast.
Strawberry growers are unlikely to forgo soil fumigation with disease-resistant cultivars alone
by Julie Guthman
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A UC survey found that disease resistant cultivars have not yet become a priority for strawberry growers, mainly because of economic pressures.
A major collaborative project launched in 2017 to accelerate the development of disease-resistant strawberry cultivars is responding urgently to two developments: increasing restrictions on fumigant use and the appearance of two novel pathogens not evidently manageable with allowed fumigants. As part of that project, I sought to understand the factors that guide growers' cultivar choice and assess their willingness to choose a pathogen-resistant cultivar to reduce or potentially replace fumigation. From a survey completed by 33 strawberry growers and in-depth interviews with 20 growers, I found that most growers prioritize yield in choosing cultivars, despite the industrywide problem with low prices. Few growers said they would be willing to substitute disease-resistant cultivars for fumigation without fail-safe disease control methods. Many growers, even those with existing organic programs, would opt for soilless systems in a tighter regulatory environment. This study thus suggests that disease resistance breeding must be coupled with support for other disease management techniques, and the economic situation that makes growers feel that they cannot forgo yield also needs attention.
Agricultural managed aquifer recharge — water quality factors to consider
by Hannah Waterhouse, Sandra Bachand, Daniel Mountjoy, Joseph Choperena, Philip A.M. Bachand, Helen E. Dahlke, William R. Horwath
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AgMAR could counteract groundwater overdraft, yet impacts to water quality must be considered — current growing season N management and historical legacy nitrate in the subsurface need to be taken into account.
The resilience and productivity of California's agriculture is threatened by groundwater overdraft, reduction in aquifer water quality, increased land subsidence damage to infrastructure and an irreversible reduction in groundwater storage capacity. Intentionally flooding agricultural fields during winter — a practice referred to as agricultural managed aquifer recharge (AgMAR) — can help counteract overdraft. However, the potential for AgMAR to exacerbate nitrate/salt leaching and contamination of at-risk aquifers remains a critical concern. To quantify the risk of groundwater contamination with AgMAR, we took 30-foot-long soil cores in 12 almond orchards, processing tomato fields and wine grape vineyards on low- and high-permeability soils, measured nitrate and total dissolved solids concentrations and calculated stored nitrate-N. Wine grape vineyards on permeable soils had the least nitrate leaching risk observed. However, almond orchards and tomato fields could be leveraged for AgMAR if dedicated recharge sites were established and clean surface water used for recharge. Historical land use, current nitrogen management and soil permeability class are the main factors to consider before implementing AgMAR.
Terminated marketing order provided resources to California peach and nectarine growers
by Zoë Plakias, Rachael Goodhue, Jeffrey Williams
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The authors found that industry information provided via marketing orders was a significant factor for respondents who voted to continue the orders.
Marketing orders allow farmers to collectively fund industry-wide services that may be difficult to provide through a voluntary approach. But not all farmers support collective approaches. We employed ballot data from U.S. Department of Agriculture and survey data we collected to explore why farmers in California voted to terminate the federal fresh peach and nectarine marketing orders in 2011 and the implications of this termination. Even after controlling for other factors, we found that farmers who produced more were significantly less likely to vote for continuation. We also found that detailed industry information provided via the marketing orders was significantly more important to respondents voting for continuation, and respondents with more organic production were significantly more likely to vote for continuation. These results suggest farmers may have lost important production and marketing resources due to termination of the orders, with evidence that smaller farms were more affected. This termination may thus have accelerated the exit of farmers from this industry.
Brown spot in table grape Redglobe controlled in study with sulfur dioxide and temperature treatments
by Cassandra A. Young, Robin A. Choudhury, Carlos H. Crisosto, W. Douglas Gubler
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A popular export table grape variety showed no disease development for 32 days at 2°C after being treated with 200 ppm-h SO2.
Brown spot is a postharvest disease of grapes caused by Cladosporium species in the San Joaquin Valley of California. It spreads during cold storage and transport, resulting in severe economic losses to late table grape cultivars, which are grown mainly for export to countries such as China and Mexico. We examined the effect of temperature and sulfur dioxide (SO2) treatments on fungal growth and infection of Redglobe berries by three Cladosporium species: Cladosporium ramotenellum, C. cladosporioides and C. limoniforme. Redglobe is especially popular for export. Fungal colonies growing on potato dextrose agar in petri plates stored at -2°C grew slower than those stored at 2°C, and an 400 ppm-h SO2 treatment significantly reduced fungal growth of all three species and at all temperatures tested. Redglobe berries inoculated with the Cladosporium species, treated with SO2 concentrations of 100 ppm-h, 200 ppm-h and 400 ppm-h and incubated in high relative humidity chambers for 28 to 32 days at 2°C, showed little incidence of disease. The development of brown spot on berries was entirely prevented with the treatment of 200 ppm-h SO2 for all Cladosporium species tested.
Monterey pine forest made a remarkable recovery from pitch canker
by Thomas R. Gordon, Gregory J. Reynolds, Sharon C. Kirkpatrick, Andrew J. Storer, David L. Wood, Daniel M. Fernandez, Brice A. McPherson
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For 3 years pitch canker progressed rapidly through native stands on the Monterey Peninsula, then changed little over 14 years, before steadily declining.
Monterey pine (Pinus radiata) is a species of limited distribution, with three native populations in California. In 1986, a disease known as pitch canker, caused by the fungus Fusarium circinatum, was identified as the cause of extensive mortality in planted Monterey pines in Santa Cruz County. Monitoring studies on the Monterey Peninsula documented rapid progression of the disease in the native forest during the 1990s, with most trees sustaining some level of infection. However, between 1999 and 2013, the severity of pitch canker stabilized, with many previously diseased trees then free of symptoms, and plots monitored between 2011 and 2015 documented a steady decline in the occurrence of new infections. Consequently, whereas pitch canker was once a conspicuous visual blight in the forest, by the end of the observation period, symptomatic trees had become a rarity. The arrested development of pitch canker is suggestive of a reduction in the frequency and duration of fog near the coast, which provides conditions necessary for the pathogen to establish infections.

News and opinion

Q-and-A: COVID-19's effects on food systems, youth development programs and nutrition
by Lucien Crowder
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An interview with UC Cooperative Extension experts about the effect of the coronavirus pandemic on food systems, youth development and nutrition.
Research highlights
by Lucien Crowder
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Recent articles from the Agricultural Experiment Station campuses and UC ANR's county offices, institutes and research and extension centers.
California's agritourism operations expand despite facing regulatory challenges
by Shermain D. Hardesty, Penny Leff
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Surveys show that agritourism operators in California need increased support from their local governments and the state regarding regulatory requirements.
Complexity in 4-H youth enrollment: A response to Davy et al. (2020)
by Steven M. Worker
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Early view articles

Growers follow the label: An analysis of bee-toxic pesticide use in almond orchards during bloom
by Jennie L. Durant, Brittney K. Goodrich, Kelly T. Chang, Evan Yoshimoto
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Pesticide use data indicate that almond growers have reduced labeled bee-toxic pesticide use, but unlabeled bee-toxic agrochemicals are still applied during bloom.
California almond orchards are most U.S. beekeepers' first stop on their pollination and honey production circuit, so the agrochemicals bees are exposed to in almonds can shape the vitality of their colony for the rest of the year. We explored the potential for honey bee exposure to bee-toxic agrochemicals during almond bloom by utilizing the California Department of Pesticide Regulations' Pesticide Use Report database from 1990 to 2016. We found that overall, growers are observing the pesticide labels and reducing their use of labeled bee-toxic pesticides during almond bloom. However, we also found that insect growth regulators, fungicides and organosilicone surfactants — agrochemicals often not labeled as toxic to bees — are commonly applied during almond bloom. These agrochemicals can be sublethally or synergistically toxic to adult honey bees and bee larvae, presenting potential harm to colonies during almond pollination. Our findings demonstrate the need for a shift in the U.S. Environmental Protection Agency's labeling requirements, as well as continued communication between almond growers, pesticide applicators and beekeepers to keep colonies at a low risk of bee-toxic agrochemical exposure.


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