California Agriculture is a quarterly peer-reviewed journal reporting research, reviews and news on California’s agricultural, natural and human resources.
Volume 74, Issue 3, 2020
Research and Review Articles
Surveys of 12 California crops for phytoseiid predatory mites show changes compared to earlier studies
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.
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.
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.
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
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 (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
An interview with UC Cooperative Extension experts about the effect of the coronavirus pandemic on food systems, youth development and nutrition.
Recent articles from the Agricultural Experiment Station campuses and UC ANR's county offices, institutes and research and extension centers.
Surveys show that agritourism operators in California need increased support from their local governments and the state regarding regulatory requirements.