- Author: Mike Hsu
Light irrigation before flooding stimulates microbes to remove nitrates from soil
With California enduring record-breaking rain and snow and Gov. Gavin Newsom recently easing restrictions on groundwater recharge, interest in “managed aquifer recharge” has never been higher. This process – by which floodwater is routed to sites such as farm fields so that it percolates into the aquifer – holds great promise as a tool to replenish depleted groundwater stores across the state.
But one concern, in the agricultural context, is how recharge might push nitrates from fertilizer into the groundwater supply. Consumption of well water contaminated with nitrates has been linked to increased risk of cancers, birth defects and other health impacts.
“Many growers want to provide farmland to help recharge groundwater, but they don't want to contribute to nitrate contamination of the groundwater, and they need to know how on-farm recharge practices might affect their crops,” said Matthew Fidelibus, a University of California Cooperative Extension specialist in the UC Davis Department of Viticulture and Enology.
A recently published study by UC scientists sheds new light on how nitrates move through an agricultural recharge site and how growers might reduce potential leaching. Researchers analyzed data from two grapevine vineyards at Kearney Agricultural Research and Extension Center in Fresno County – one flooded for two weeks, and other for four.
Understanding initial nitrate levels crucial
A key factor in mitigating contamination is understanding how much nitrate is in the soil at the outset, said study author Helen Dahlke, a UC Davis hydrologist and leader of UC Agriculture and Natural Resources' strategic initiative on water. In areas with little precipitation and cropping systems that require greater amounts of synthetic fertilizer, the accumulation of residual nitrate – resulting from nitrogen in the fertilizer not taken up by the plants – can be quite high.
“The percentage of nitrates in some soils can really increase over the years, particularly if you have many dry years in a row where you don't have access to irrigation water or natural precipitation flushing some of those nitrates out of the soil,” Dahlke said.
While intense rains in recent weeks have helped dilute nitrate concentrations naturally, farmers looking to participate in recharge during the dry years ahead should consider flooding their fields with greater volumes of water.
“If you're doing this for the first time – on-farm recharge in the winter – check your residual soil nitrate levels because if they're very high, you should apply a lot of water in order to make sure that the residual nitrate is diluted down,” said Dahlke, who also added that growers should check their soil properties for suitability of recharge projects.
She recommended using, as a “good first approximation,” the online Soil Agricultural Groundwater Banking Index map, a project led by Toby O'Geen, a UC Cooperative Extension soil resource specialist.
Researchers looking at other ways to reduce nitrates
Even before flooding the fields for recharge, there are several practices that can lower initial nitrate levels and risk of leaching. Cover crops such as alfalfa and triticale, for example, can help take up residual nitrates that accumulate from fertilizing a main crop over time.
Dahlke and Fidelibus – a co-author of the San Joaquin Valley vineyard study – both pointed to pre-flooding irrigation that encourages denitrification, a process in which soil microbes transform nitrates into gaseous forms of nitrogen.
“Those denitrifying microbes need to be stimulated to do the work,” said Dahlke. “What we have found is that if you do a little bit of irrigation before you start the flooding, increasing the soil moisture can get those microbes started and they can take out more nitrate from the soil.”
The timing and quantity of fertilizer applications are also major factors in reducing leaching. Although more growers are following high-frequency, low-concentration practices to maximize uptake by crops, Dahlke said there needs to be more emphasis on incorporating nitrogen transformation processes – such as denitrification – in the nutrient management guidelines that farmers follow.
“Implementing thoughtful nutrient management plans will play a particularly important role in participating farms,” Fidelibus added.
A more holistic view of groundwater recharge
In short, choices made during the growing season can affect those in the winter recharge season – and vice versa. For example, applying compost or other organic amendments to soil can give microbes the “fuel” they need for sustained denitrification.
“What we have found is that our denitrifying bacteria often run out of steam because they don't have enough carbon to do the work,” Dahlke said. “Like us, microbes need energy to do the work, and for microbes this energy comes from soil carbon.”
Then, adding moisture via recharge to that field with high organic content can stimulate mineralization and nitrification, processes in which microbes transform the organic nitrogen into ammonium – and subsequently nitrates – that the plants can then take up. Those naturally occurring nitrates would thus reduce the need for the grower to apply synthetic fertilizer.
“The winter on-farm recharge experiments have shown that altering the moisture regime in the winter has consequences for the nitrogen budget in the summer growing season,” Dahlke explained. “Theoretically, what we need to be doing is better integrating both seasons by keeping an eye on the soil-nitrogen balance across the whole year so that we can ensure, at the end of the growing season, the residual nitrate in the soil is minimized.”
The study, published in the journal Science of The Total Environment, was part of the post-doctoral work of former UC Davis researcher Elad Levintal. In addition to Fidelibus and Dahlke, other authors are Laibin Huang, Cristina Prieto García, Adolfo Coyotl, William Horwath and Jorge Rodrigues, all in the Department of Land, Air and Water Resources at UC Davis.
/h3>/h3>/h3>/h3>- Author: Mike Hsu
Serving Amador, Calaveras, El Dorado and Tuolumne counties, UC Cooperative Extension advisor Lynn Wunderlich was formally assigned to focus on viticulture and integrated pest management in the region. But her innate curiosity – as well as her dedication to meeting the wide-ranging needs of local communities – led her to develop expertise in a remarkable array of topics.
“That was both the challenge and the opportunity of being a foothill farm advisor – lots of small farms, lots of diverse agriculture, so I got to do some cool things,” said Wunderlich, who is set to retire on July 1. “To serve the needs of the clientele up here was very gratifying and interesting.”
Wunderlich earned her bachelor's degree in bacteriology and plant pathology from University of Wisconsin-Madison and her master's in plant protection and pest management from UC Davis. After several years as a UCCE staff research associate in Ventura and Monterey counties, Wunderlich began as a UCCE farm advisor in 2000 for El Dorado and Amador counties.
Although initially tasked with supporting tree fruit and specialty crop growers in topics such as researching alternative methods for managing codling moths, Wunderlich soon found herself studying organizational dynamics and bylaws to help the Placerville Fruit Growers Association cooperative transition to become a Limited Liability Company.
“It was really different than anything I'd been trained in before,” Wunderlich said.
That early experience set the tone for the rest of her career, as she continued to seek out – and share – knowledge across the expansive breadth of her work. In 2007, Wunderlich took on the viticulture role in Amador and El Dorado counties, where grape growers sought counsel on controlling a newly discovered pest.
“Every farm advisor has some quintessential moments of their career, and Gill's mealybug was one of mine,” Wunderlich recalled. “It's really unique; it's not found in very many places in California and it had never been described as a pest on wine grapes.”
In addition to developing effective management tactics for Gill's mealybug, Wunderlich worked with growers and the late Doug Gubler, UCCE specialist emeritus, to set up seven powdery mildew stations and rain gauges across the foothills. The stations filled a great need in the region by providing accessible, applicable pest and disease forecasting and precipitation data.
Crediting her colleagues' tutelage, Wunderlich also deepened her understanding of the diverse soils in the foothills and the latest research on evapotranspiration on wine grapes – all in the name of delivering the most current and useful information to growers.
When Christmas tree growers in the foothills found their white firs decimated by a phytophthora pathogen, Wunderlich helped them switch to Nordmann and Turkish firs, which were naturally resistant. She became one of only a few experts in the UC system on these conifers, and, in one of her last accomplishments as farm advisor, organized the International Christmas Tree Research and Extension Conference in California earlier this month.
Another late-career highlight for Wunderlich was developing training materials on the proper calibration and use of air blast sprayers. Alongside Franz Niederholzer, UCCE farm advisor for Sutter, Yuba and Colusa counties, and UC IPM colleagues Lisa Blecker, Petr Kosina and Cheryl Reynolds, Wunderlich developed, delivered and evaluated a curriculum that included both in-person classes and online components. Their efforts were recognized with an IPM Achievement Award from the California Department of Pesticide Regulation, and the online course continues to be used today.
“It's nice to be able to leave something like that behind; its principles are still valid, no matter what type of sprayer you're using,” said Wunderlich, citing it as one of her enduring legacies.
In retirement, Wunderlich plans to continue her lifelong learning and also spend more time with friends and family – especially on camping trips on the east side of the Sierra.
And, as for growers such as Chuck Mansfield, they hope Wunderlich will stay connected.
“While we are all very happy for Lynn, her presence will be sorely missed,” Mansfield said. “We hope Lynn remains a regular fixture and friend in our community.”
- Author: Emily C. Dooley, UC Davis
Project designed to prevent red blotch and other grapevine diseases
A new, $5.25 million greenhouse is being built on the University of California, Davis, campus to safeguard an important grapevine collection from red blotch disease and other pathogens.
The 14,400-square-foot greenhouse will have a vestibuled entry, be insect-proof and provide another level of disease protection. It is being spearheaded by Foundation Plant Services, or FPS, which provides the U.S. grape industry with high-quality, virus-tested grapevine plant material.
The program serves as the primary source for grapevine plant material distributed to nurseries under the California Department of Agriculture's Grapevine Registration and Certification Program, which provides the majority of grapevines planted in the United States. For the grape industry, it is essential to protect this material from disease-carrying insects and guarantee fast access to clean plant material.
“The program is considered the largest quarantine center for the grapevine industry in the United States,” said Maher Al Rwahnih, a plant pathologist and FPS director. “This is kind of a game changer for us.”
A history of serving the grapevine industry
FPS has maintained healthy grapevine planting stock on the UC Davis campus for more than 70 years in open fields at the Classic and Russell Ranch foundation vineyards. FPS scientists first detected grapevine red blotch virus at Russell Ranch in 2017. By 2021, an estimated 51.6% of the crop there was infected. Material from that vineyard is not being sold, and the site is now part of an epidemiological study to try to pinpoint how the disease is transmitted.
FPS pathologists have detected red blotch on less than 1% of the Classic vineyard crop. But it may not always be that way in the future.
“We don't know how long the Classic vineyard will remain clean,” Al Rwahnih said. “Every testing season, this is what keeps me up at night. We're not sure why it's happening in Russell Ranch and not the Classic vineyard.”
Once the greenhouse is operating, grapevines propagated from plant material from the Classic vineyard will be moved into the greenhouse, tested and verified as clean from disease. From there it will be sold to nurseries, which will grow additional plants to sell to growers.
Two greenhouses part of plan
Normally the foundation has 4,000 vines available, but the greenhouse will only house 2,000 vines, so inventory will be cut in half.
“This phase is just a starting phase,” Al Rwahnih said. “It's not sufficient for our needs.”
FPS plans to build another greenhouse in the next two to three years to increase capacity.
Industry groups and FPS identified greenhouses as the best way to protect the plants from red blotch and other pathogens transmitted by insects. They are also consulting with those same people on the grape varieties to include in the greenhouse.
“We have a large selection, and we need to make sure all the varieties that are important to industry are contained,” he said.
The first greenhouse is expected to be finished by the end of 2023.
Funding for the first greenhouse is coming from a variety of sources. The California Fruit Tree, Nut Tree and Grapevine Improvement Advisory Board, managed by the California Department of Food and Agriculture, contributed $4 million to the project. The California Grape Rootstock Research Foundation gave $500,000, Foundation Plant Services with UC Davis is funding $450,000, and the California Grape Rootstock Commission gave $100,000.
“This is crucial for the grapevine industry, and we are very grateful for the support,” Al Rwahnih said.
/h3>/h3>/h3>- Author: Karen Giovannini
UCCE is looking for growers interested in collaborating on a study of vineyard irrigation water use to help provide more accurate usage rates to the Groundwater Sustainable Agencies (GSAs) whose current modeled volumes overestimate actual use based on site variability and deficit irrigation practices.
- Author: Lucia G. Varela
Want to know what bug is making holes in the leaves of you shrub or eating your fruit? Or what is the pesky weed you cannot get rid off? The University of California Division of Agriculture and Natural Resources publications have four sets of Pest Identification cards for you. These pocket-size, sturdy, laminated cards can be easily carried with you as a quick reference wherever you need them. The sets are also available as electronic publications formatted for iOS and Kindle compatible devices.
Vineyard Pest Identification and Monitoring
These cards are also available as a separate card set, publication #3538, in Spanish. You can purchase each card set alone or in bundles for a price break. The bundles are perfect for vineyard managers and their crews.
Tree Fruit Pest Identification
The Tree Fruit Pest Identification Card Set is available only as an electronic publication. The set covers major insect and mite pests and several important diseases in California deciduous tree fruits and nuts. Each pest is identified by a description and close-up photographs of important life stages to help you know how and when to look for these pests -- in both growing and dormant seasons. The cards also include descriptions of natural enemies.
Landscape Pest Identification
The Landscape Pest Identification Card Set will help landscape maintenance professionals and home gardeners identify and manage most major common pest problems in the landscape. The 43 cards cover 80 common insect pests and mites and 40 diseases of flowers, shrubs and trees.
Weed Identification and Monitoring
The Weed Identification and Monitoring Card Set is based on the bestselling Weeds of California and Other Western States; this is the perfect pocket-sized companion for anyone working in the field.
Each weed is identified by a description and excellent close-up color photographs of various growth stages with 187 photos in all. On the reverse of each card is a description of growth stages, habitat, distribution and management tips. It also includes handy inch and metric measurement scales. A sturdy rivet keeps the set together so individual cards don't go astray.
Pests of the Garden and Small Farm
A new set Pests of the Garden and Small Farm Card Set is coming out soon. Stay tuned for its release. Currently, there is a beautiful book, publication 3332.
To purchase the card sets or electronic versions, visit the University of California Division of Agriculture and Natural Resources publication catalog. Refer to the table below for the publication and click on the version you want:
- Card set - a deck of cards on a spindle
- E-pub - electronic version for iOS
- Kindle - electronic version for Kindle
Publication name |
Card set |
E-pub |
Kindle |
Landscape Pest ID Cards |
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Vineyard Pest ID and Monitoring Cards |
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Tarjetas de identification de plagas de la vid (Spanish) |
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Weed Pest ID and Monitoring Cards |
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Tree Fruit ID Cards |
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Backyard gardeners, if you still cannot identify that weed, bug or problem with your plant, you can always bring a sample to our Master Gardeners desk. There is a drop box available to leave samples after hours.
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