Progressive Crop Consultant - July/August 2021

July / August 2021 Can AI Enhance the Profit and Environmental Sustainability of Agriculture?

New Peptide for Treatment and Prevention of HLB in Citrus A Spray Backstop System to Minimize Drift from Orchard Spray Application Cover Cropping to Achieve Management Goals

September 16-17, 2021 - Visalia, California Register at progressivecrop.com/conference SEE PAGE 18-19 FOR MORE INFORMATION

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PUBLISHER: Jason Scott Email: jason@jcsmarketinginc.com EDITOR: Marni Katz ASSOCIATE EDITOR: Cecilia Parsons Email: article@jcsmarketinginc.com PRODUCTION: design@jcsmarketinginc.com Phone: 559.352.4456 Fax: 559.472.3113 Web: www.progressivecrop.com

IN THIS ISSUE

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Can Artificial Intelligence Enhance the Profit and Environmental Sustainability of Agriculture?

Treating and Preventing Citrus Huanglongbing with a Stable Antimicrobial Peptide with Dual Function

CONTRIBUTING WRITERS & INDUSTRY SUPPORT

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A Spray Backstop System to Minimize Drift from Orchard Spray Application

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Cover Cropping to Achieve Management Goals

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Mitigating Tree Nut Stress and Disease Requires a Multi-Pronged Irrigation Approach

Ray G. Anderson Research Soil Scientist, USDA-ARS, U.S. Salinity Laboratory

Robert R. Krueger Horticulturist, USDA-ARS, National Clonal Germplasm Repository for Citrus and Dates

Karla Araujo Contained Research Facility, UC Davis

Jonatan Niño Sánchez Department of Microbiology and Plant Pathology, UC Riverside

Matt Comrey Technical Nutrition Agronomist, Wilbur-Ellis Agribusiness

Alireza Pourreza Director Digital Agriculture Lab, UC Davis

Ramesh Dhungel Research Hydrologist, USDA-ARS, U.S. Salinity Laboratory Kristine Elvin Godfreyb Contained Research Facility, UC Davis Gregory Kund Department of Entomology, UC Riverside

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Vineyard Water Management During Drought Years

Elia Scudiero Research Agronomist, UC Riverside Shulamit Shroder UCCE Community Education Specialist II, Kern County

Hailing Jin Department of Microbiology and Plant Pathology, Center for Plant Cell Biology, Institute for Integrative Genome Biology, UC Riverside

John Trumble Department of Entomology, UC Riverside Jeanette Warnert Communications Specialist, UC ANR George Zhuang UCCE Viticulture Farm Advisor, Fresno County

UC COOPERATIVE EXTENSION ADVISORY BOARD

Surendra Dara

UCCE Entomology and Biologicals Advisor, San Luis Obispo and Santa Barbara Counties

Kevin Day

Identifying the Potential and Impacts of On-Farm Groundwater Recharge

Caroline Roper Department of Microbiology and Plant Pathology Center for Plant Cell Biology, Institute for Integrative Genome Biology, UC Riverside

Chien-Yu Huang Department of Microbiology and Plant Pathology, UC Riverside

Jessie Kanter UCCE Small Farms and Specialty Crops Program, Fresno County

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Sonia Rios UCCE Area Subtropical Horticulture Advisor, Riverside County

Steven Koike Tri-Cal Diagnostics

Jhalendra Rijal

UCCE Integrated Pest Management Advisor, Stanislaus County

UCCE Pomology Farm Advisor, Tulare and Kings Counties Kris Tollerup UCCE Integrated Pest Management Advisor, Fresno, CA Elizabeth Fichtner UCCE Farm Advisor, Mohammad Yaghmour Kings and Tulare Counties UCCE Area Orchard Systems Advisor, Kern County Katherine Jarvis-Shean UCCE Orchard Systems Advisor, Sacramento, Solano and Yolo Counties

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The articles, research, industry updates, company profiles, and advertisements in this publication are the professional opinions of writers and advertisers. Progressive Crop Consultant does not assume any responsibility for the opinions given in the publication.

July / August 2021

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Can Artificial Intelligence Enhance the Profit and Environmental Sustainability of Agriculture?

Big data from ground and satellite measurements are being studied to improve production agriculture in the Southwestern U.S..

By ELIA SCUDIERO | Research Agronomist, UC Riverside RAY G. ANDERSON | Research Soil Scientist, USDA-ARS, U.S. Salinity Laboratory RAMESH DHUNGEL | Research Hydrologist, USDA-ARS, U.S. Salinity Laboratory SONIA RIOS | UCCE Area Subtropical Horticulture Advisor, Riverside County and ROBERT R. KRUEGER |Horticulturist, USDA-ARS, National Clonal Germplasm Repository for Citrus and Dates

T

he global population is projected by the United Nations to reach 9.7 billion people by 2050. To satisfy the needs for food and fiber for that many people, agricultural production should increase by >70%. Globally and here in the U.S., studies suggest that improved management of soil resources, irrigation water and nutrients through digital agriculture (See Sidebar Digital Agriculture on page 9.) tools can be the key to increasing agricultural production (Fig. 1). According to USDA, sustaining the economic viability of agriculture is only one of the goals that must be achieved to fully satisfy the long-term food and fiber needs of the American people. Other key goals are to reduce the environmental footprint of agriculture and to improve the quality of life for farm families and communities.

Improving Production Systems

Agriculture in the Southwestern U.S. is vital to the prosperity of rural communities and the national economy, both for internal consumption and for international export. Many crops are grown

in the Southwestern U.S., including field crops and many specialty fruits, vegetables and nuts. In particular, around $12 billion/year in income is generated by agriculture in California’s Salinas River Valley and in the Colorado River Basin, including regions in Southern California that use Colorado River water for irrigation. In these regions, agriculture employs more than 500,000 workers.

Sustaining and improving the agricultural productivity in the region in the long term is, however, under threat as growers face major challenges: climate change, disease and pest outbreaks, increasing salinity and diminished and/or degraded soil and water resources, to mention a few. The region has experienced prolonged and major droughts since 2000, with the possibility of streamflow reductions by more than 50% by 2100. Future requirements to preserve groundwater may remove 500,000 acres from production in California alone. Minimizing salt and nutrient loading is increasingly mandated for effective water reuse. Un-

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Figure 1: Economic feasibility analysis showing the relative value that agricultural technologies are expected to add to the current global crop production to achieve the United Nation’s 2050 food production goals. Data extracted from Goldman Sachs, July 2016, “Precision Farming: Cheating Malthus with Digital Agriculture”

®

IMAGINATION

INNOVATION

SCIENCE IN ACTION

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Figure 2: Left panel shows the map of evapotranspiration on November 10, 2019 for a selected region near Yuma, Ariz. using Landsat 8 imagery. This map has a red circle that is shown in the panel on the right illustrating evapotranspiration (ET) for the selected point for the season. Vegetation data from Landsat are combined with high-resolution meteorological data to estimate ET on an hourly time basis. The red bars illustrate rain events and the black bar illustrates the amount and timing of irrigation that would be needed to avoid unacceptable soil moisture depletion.

Continued from Page 4 certainty in the water supply in these regions is particularly troublesome, as agriculture there consumes 39% of the U.S. total irrigation water (almost 32 million acre-feet per year.) Additionally, climate change threatens to increase insect, pathogen and weed pressures and geographic distribution, while public interest and increasing on-farm costs push toward reducing pesticide use and organic farming.

Our current understanding of agricultural production systems indicates that Crop Yield is a complex function of Genetics × Environment × Management × Space × Time interactions. Understanding why yield deviates from optimal over space and time in different landscapes is key to adjusting management cost-effectively. When looking at the multi-year productivity of farmland, a rule of thumb might say that yield varies over time in response to climate as much as it changes across different spatial scales (within a field and across multiple fields) due to the variability in soil and landscape features.

Failure to adapt management to the

Sept.

16-17, 2021 Progressive Crop Consultant SEE PAGE618-19 FOR MORE INFORMATION

dynamic spatial and temporal variability of crop growth often results in crop loss or over-application of agronomic inputs, which can lead to economic loss and environmental degradation.

Real-time crop growth models that can leverage information from very high spatial and temporal resolution satellite imagery and ground networks of sensors, such as weather stations, are great candidates for guiding site-specific tailored agronomic management.

Moreover, when combined with artificial intelligence, crop growth models can help improve farm management while considering the tradeoffs between different sustainability aspects at different spatial scales. For example, how to increase field scale profitability while reducing regional-scale environmental impacts.

Current Research

With the overarching goal of increasing agricultural profitability by reducing and optimizing inputs to increase yield and curb losses from abiotic and biotic stressors in the Southwestern U.S., UC Riverside recently started a five-year project on the use of artificial intelligence and big data from high-resolution imagery and ground sensor networks to improve the management of irrigation, fertilization and soil salinity as well as to enable early detection of weeds and pests. The project is led by Elia Scudiero, a professional researcher in UC Riverside’s Department of Environmental Sciences, and includes several co-investigators at UC Riverside and UC ANR, USDA-ARS, July / August 2021

University of Arizona, Duke University, Kansas State University and University of Georgia. To accomplish its goal, the project relies on many collaborations with ag tech industry partners, including Planet Labs, Inc. Through the collaboration with Planet Labs, the project investigators will use daily high-resolution (around 12 feet) satellite imagery to monitor crop growth and soil properties.

Some of the artificial intelligence applications that the project will develop, for a variety of crops, include crop inventorying, soil mapping, estimations of crop water use and requirement, estimation of plant and soil nutrient status, analyzing the feasibility and cost-effectiveness of variable-rate fertigation across selected regions in the Southwestern U.S. and detecting weeds and pathogens. In the remainder of this article, we will provide a general overview and some preliminary results from some selected applications: Water use and water requirement estimations, mapping soil salinity with remote sensing and detecting biotic stressors.

Water Use and Water Requirement Estimations

A key element of this project will be to provide reliable estimates of crop water use and irrigation forecasting at a very high resolution daily. Current remote-sensing-based evapotranspiration models often suffer from infrequent satellite overpasses, which are generally available weekly or every two weeks at the 30- to 100-foot spatial resolution.

Such sporadic information is a limitation, especially for vegetable crops, which can have very fast-growing cycles. To overcome this and other limitations, the project is integrating daily meteorological information (from state and federal networks) and high-resolution satellite data from Planet Labs with BAITSSS, an evapotranspiration (ET) computer model developed by Dr. Ramesh Dhungel (a Research Scientist in the project) and colleagues. The current version of BAITSSS leverages information from the Landsat 8 satellite platform (NASA). An example application of the model is shown in Figure 1(see page 6), where BAITSSS is combined with soil moisture modeling to forecast when irrigation is needed to supply crop water demand.

The project’s lead on water use and requirement estimations, Dr. Ray Anderson (USDA-ARS U.S. Salinity Laboratory, Riverside, Calif.), says that

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Figure 3: 2013 remote sensing soil salinity (electrical conductivity of a saturated paste extract, ECe) for the zero to four feet soil profile in the western San Joaquin Valley. The map was generated using Landsat imagery with a resolution of about 900x900 ft. Current research is investigating the use of Planet imagery with a resolution of 12x12 ft to generate soil salinity maps across all irrigated farmland in the Southwestern U.S.

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Figure 4: Planet SkySat imagery from UC Riverside’s Coachella Valley Agricultural Research Station. In panel A, the high-resolution (~20x20inch resolution) is shown in natural colors and using the Normalized Difference Vegetation Index (NDVI). The different crops present at the site are highlighted in panel A. Panels B, C and D show pictures taken as a ground-truth for the machine-learning weed classifier under development in the project. The project is also collecting imagery from unmanned aerial vehicles (UAV) at selected test sites to detect biotic stress (weeds and pathogens), panel E (photos courtesy R. Krueger (B, C and D) and E. Scudiero (E).)

Continued from Page 7

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July / August 2021

“along with irrigation management, we believe the daily imagery from Planet Labs will allow growers and irrigation managers to see evapotranspiration anomalies within a field. These anomalies could indicate irrigation issues or decreases in plant health due to other abiotic stressors such as salinity or nutrient deficiency. Identification of these anomalies will ensure more efficient field scouting and earlier identification of issues before permanent yield loss occurs.”

Mapping Soil Properties

Accurate knowledge of spatial variability of soil properties, such as texture, hydraulic properties and salinity, is important to best understand the reasons of crop yield spatial variability. Scudiero’s Digital Agronomy Lab at UC Riverside is developing novel tools based on machine learning to automate near-ground sensing of soil properties and remote sensing of soil salinity in irrigated farmland. Soil salinity maps are very useful to inform field-scale irrigation practices (e.g., to calculate the amount of irrigation water needed to avoid harmful accumulation of salts in the soil profile.)

In this project, Scudiero and his team will be using fieldscale soil maps of soil salinity collected in the past (since the early 1980s) and throughout the next five years by UC Riverside, the USDA-ARS U.S. Salinity Laboratory, the University of Arizona and other collaborators to generate soil salinity maps for the entire Southwestern U.S. Figure 3 (see page 7) shows the soil salinity map produced for the western San Joaquin Valley by Scudiero and colleagues (see the additional resources section.) In particular, Scudiero’s

team will use the ground information to calibrate the Planet Labs time-series imagery to predict soil salinity in the root zone (e.g., the top four feet of the soil profile.) Satellite imagery alone is generally not sufficient to predict soil salinity. Other stressors (water stress, nutrient deficiency) have similar imagery properties to salinity. However, in short periods (two to five years), salinity remains fairly stable throughout the soil profile, contrary to other more transient stressors. Because of these differences in temporal variability between stressors, multi-year time series can be used to detect and map crop health reduction due to soil salinity.

Biotic Stress Detection

Within this project, UC Riverside scientists are using smartphone pictures, drone imagery and Planet Labs imagery (20 inch and 12 feet resolution) to develop artificial intelligence classifiers for early biotic stress detection. Figure 4 (see page 8) shows an example of a survey carried out by project collaborators Sonia Rios (UC ANR) and Robert Krueger (USDA-ARS) at UC Riverside’s Coachella Valley Agricultural Research Station in Thermal, Calif. Preliminary analyses show that 20x20-inch resolution imagery from the Planet SkySat satellites can be used to identify the presence of weeds against bare soil ground coverage in citrus and date palm. Single-date imagery cannot be used to successfully distinguish between different weed species. The project investigators hope that repeated satellite imagery, together with drone imagery, will be successful at identifying the emergence and extent of weeds and other biotic stress at the field and farm scales. To develop such tools, project investigators are carrying out controlled experiments at the UC Riverside research farms with controlled weed and pathogen pressures on a variety of vegetable crops.

Training to Growers and Consultants

From conversations with stakeholders in California and other states in the Southwestern U.S., we learned that most growers are committed to maintaining the quality and profitability of

soil, water and other natural resources in the long term. Additionally, many growers recognize the potential of integrating digital agriculture technologies in their daily decision-making. Nevertheless, investing in new technology always requires considerable commitment. Therefore, growers would like to have more information on the cost-effectiveness of state-of-the-art technologies and on the suitability of technology to their local agricultural system. To address these and other questions, the project team is establishing a multistate cooperative extension network to develop training programs for growers and consultants on the topics of precision agriculture, digital agriculture and the use of soil and plant sensors in agriculture. These training activities, scheduled to start in 2021, will include contributions from university personnel and industry members.

satellite overpasses: methodology and case study in agricultural dominant semi-arid areas: Time integration of evapotranspiration. Meteorol. Appl.

23(4): 714–730. doi: 10.1002/met.1596.

Scudiero, E., Corwin, D.L., Anderson, R.G., Yemoto, K., Clary, W., Wang, Z.L., Skaggs, T.H., 2017.

Remote sensing is a viable tool for mapping soil salinity in agricultural lands. California Agriculture 71, 231-238. doi: 10.3733/ca.2017a0009

Comments about this article? We want to hear from you. Feel free to email us at article@jcsmarketinginc.com

This article provided an overview and some preliminary results for the University of California Riverside, or UCR, -led project on “Artificial Intelligence for Sustainable Water, Nutrient, Salinity, and Pest Management in the Western US”.

The project is funded by U.S. Department of Agriculture’s National Institute of Food and Agriculture (Grant Number: 2020-69012-31914). Through September 2025, the project will investigate the use of daily high-resolution satellite imagery and data science to identify inefficiencies in agronomic management practices and to support improved irrigation, fertilization and pest control in the irrigated farmland across the Colorado River Basin and Central and Southern California.

Additional information on the project and the content of this article can be requested from Elia Scudiero (elia.scudiero@ucr.edu). Information about the project’s cooperative extension events can be requested from ai4sa@ucr.edu.

Further information about research on remote sensing of soil salinity and evapotranspiration can be found in the additional resources section.

Additional Resources: Dhungel, R., R.G. Allen, R. Trezza, and C.W. Robison. 2016. Evapotranspiration between

July / August 2021

Digital Agriculture As part of the ongoing Fourth Industrial Revolution we are experiencing a rapid expansion of: 1) Agricultural technologies, such as novel and inexpensive ground plant and soil sensors, robots, and aerial and remote sensors which allow measuring agricultural processes with high accuracy; 2) Artificial intelligence and other data science methods that can help distill field and remote measurements into relevant information for ag decision-making; and 3) Agricultural models and decision-support tools that can suggest management actions to growers and consultants in real time through the Internet of Things.

The discipline of Digital Agriculture develops knowledge and tools in the intersection between high-performance computing and big environmental data (from ground and remote sensors.)

Digital Agriculture tools promise to monitor agricultural processes at very high spatial and temporal resolution so that growers will be able to tailor management to address local needs dynamically. Implementing such tools will gradually shift agricultural systems from generalized management of farm resources to highly optimized, site & time-specific, and real-time management via automated systems. In this context, agricultural automation does not refer only to self-operating machinery, but also measurement-to-decision-support tools that will not require much human input.

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Treating and Preventing Citrus Huanglongbing with a Stable Antimicrobial Peptide with Dual Functions

By CHIEN-YU HUANG | Department of Microbiology and Plant Pathology, UC Riverside KARLA ARAUJO | Contained Research Facility, UC Davis JONATAN NIÑO SÁNCHEZ | Department of Microbiology and Plant Pathology, UC Riverside SOGREGORY KUND | Department of Entomology, UC Riverside JOHN TRUMBLE | Department of Entomology, UC Riverside CAROLINE ROPER | Department of Microbiology and Plant Pathology, Center for Plant Cell Biology, Institute for Integrative Genome Biology, UC Riverside KRISTINE ELVIN GODFREYb | Department of Entomology, UC Riverside and HAILING JIN |Department of Microbiology and Plant Pathology, Center for Plant Cell Biology, Institute for Integrative Genome Biology, UC Riverside

C

itrus Huanglongbing (HLB, citrus greening disease) is caused by the vector-transmitted phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas). It is the most destructive disease which infects all commercial citrus varieties and threatens citrus industries worldwide (Bove 2006; Graham et al. 2020). Current management strategies include insecticide application to control the transmission vector Asian citrus psyllids (ACP) and antibiotics treatment to inhibit CLas (Barnett et al. 2019), but neither could control HLB effectively. Since the first report of HLB in Florida in 2005, citrus acreage and production in Florida decreased by 38% and 74%, respectively (Graham et al. 2020; Stokstad 2012). The disease has spread to citrus-producing states, including Texas and California. In severely affected areas, such as Florida, effective therapy is demanded because disease eradication is impractical. In recently impacted areas, such as California, preventing new infections is most urgent. Hence, innovative therapeutic and preventive strategies to combat 10

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HLB are urgently needed to ensure the survival of the citrus industry.

One of the most effective and eco-friendly strategies for disease management is to utilize plant innate immunity-related genes from disease-resistant or tolerant varieties for plant protection. Upon pathogen infection, plant defense response genes undergo expression reprogramming to trigger plant innate immunity. Plant endogenous small RNAs play a pivotal role in this regulatory process (Huang et al. 2019; Zhao et al. 2013b), including phytohormone- and chemical-induced systemic acquired resistance or defense priming, which can promote robust host immune responses upon subsequent pathogen challenges (Brigitte et al. 2017; Zheng Qing and Xinnian 2013).

Our Approaches

HLB tolerance has been observed in some hybrids (e.g., US-942 and Sydney hybrid 72 (Albrecht and Bowman 2011, 2012a)) or citrus relatives (e.g., MicrocJuly / August 2021

Damaged citrus leaves due to Asian Citrus Psyllid feeding. Researchers have identified a list of candidate plant immune regulators that may contribute to HLB-tolerance (photo courtesy USDA-ARS.)

itrus australiasica, Eremocitrus glauca and Poncirus trifoliata) (Ramadugu et al. 2016). By comparative analysis of small RNA profiles and the target gene expression between HLB-sensitive cultivars and HLB-tolerant citrus hybrids and relatives (Albrecht and Bowman 2011, 2012a), we identified a list of candidate natural defense genes potentially responsible for HLB tolerance (Huang et al. 2020). One of the candidate regulators is a novel anti-microbial peptide (AMP), which we named “stable antimicrobial peptide” (SAMP).

Here, we demonstrate that SAMP not only has antimicrobial activity but also has priming activity and can induce citrus systemic defense responses. This dual-functional SAMP can reduce CLas titer, suppress disease symptoms in HLB-positive trees and activate plant systemic defense responses against new infection.

Results

Through the comparative expression analysis of small RNAs and the target genes between HLB-sensitive culti-

solanacearum (CLso)/potato psyllid/ Nicotiana benthamiana interaction system to mimic the natural transmission and infection circuit of the HLB complex. We found that the SAMP from Ma Australian finger lime (MaSAMP) had the strongest effect on suppressing CLso disease and inhibiting bacterial growth in plants. To directly determine the bactericidal activity of MaSAMP on Liberbacter spp, we developed a viability/cytotoxicity assay of Lcr, a close culturable

relative of the CLas and CLso (Fagen et al. 2014; Leonard et al. 2012; Merfa et al.

2019). Using this assay, we found that MaSAMP can rapidly kill the bacterial cells within five hours, which is more efficient than the bactericidal antibiotic, Streptomycin. While the heat sensitivity of antibiotics is a major drawback for controlling CLas in citrus fields, we found that SAMPs are surprisingly heat stable. A prolonged exposure to

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Crop Resilience vars and HLB-tolerant citrus US-942 (Poncirus trifoliata x Citrus reticulata) and microcitrus Sydney hybrid 72 (Microcitrus virgate from M. australis × M. australasica) (Huang et al. 2020), we identified a list of candidate plant immune regulators that are potentially contributable to HLB-tolerance. One candidate regulator is a 67-amino acid (aa) peptide, SAMP, that was predicted with antimicrobial activity (Park et al.

2007). SAMP has significantly higher expression levels in both HLB-tolerant hybrids US-942 and Syd 72 than the HLB-susceptible control trees. We further cloned SAMP genes from HLB-tolerant citrus relatives. We found that SAMP transcripts are closely related and have a significantly higher expression level in HLB-tolerant varieties. We further detected the 6.7kD SAMP in the phloem-rich tissue; bark peels of HLB-tolerant Ma and Pt but not in the susceptible Cs. These results support that the SAMPs are likely associated with the HLB-tolerance trait. According to our functional analysis of SAMP, we list the advantages of using SAMP to manage citrus HLB as the following: SAMP has bactericidal activity and is heat stable.

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Figure 1: New growth leaves form buffer- (set as mock, left) or SAMP-treated (right) HLB-positive ‘Madam Vinous’ sweet oranges trees.

Continued from Page 11 extreme temperatures of 60 degrees C for 20 hours had minimal effect on MaSAMP, which still retained most of its bactericidal activity, whereas Streptomycin lost its antibacterial activity following the same temperature incubation. Thus, SAMP is a heat-stable, plant-derived antimicrobial peptide that can directly kill Lcr and suppress CLso in plants.

trees. Taken together, these results demonstrate across three trials that SAMP injection can suppress CLas titer in three different HLB-susceptible citrus varieties and can cause trees in declining health to recover.

SAMP treatment safeguards healthy citrus trees from CLas infection.

Protecting healthy trees from CLas infection is critical for managing HLB. The establishment of defense priming SAMP suppresses CLas in HLB-positive in plants can promote faster and/or citrus trees. stronger host immune responses upon pathogen challenges (Brigitte et al.

To determine whether MaSAMP can 2017; Zheng Qing and Xinnian 2013). also suppress CLas in citrus trees, To determine whether MaSAMP has we used the pneumatic trunk injecpriming activity, we applied it by foliar tion method to deliver the MaSAMP spray to citrus plants. We found that solution into the HLB-positive trees.

MaSAMP applications triggered proWe first tested with eight CLas-posilonged induction of defense response tive Citrus macrophylla with similar genes. Thus, SAMP can potentially bacterial titer and disease symptoms for “vaccinate” uninfected citrus trees and the treatment. At eight weeks following induce defense responses to combat two doses injected, separated by two against HLB. To test the protection months of MaSAMP injection, the disability of SAMP on citrus trees, we ease symptoms and the bacterial titer applied the MaSAMP solution or buffer in all six treated trees were drastically as mock treatment by foliar spray onto reduced compared to the mock-treated young, healthy ‘Madam Vinous’ sweet (buffer only) plants and one tree with orange trees. Five days after treatment, no CLas detected. We further tested the trees were exposed to ACP carrywith HLB-positive ‘Madam Vinous’ ing CLas under the “no choice feeding” sweet oranges and Lisbon Lemon condition for 21 days. We treated trees trees; those appeared to have declining with MaSAMP solution by foliar spray symptoms and similar CLas titer. After every two months subsequently. The MaSAMP treatments, the trees had result indicates that MaSAMP-treated developed symptomless new flushes, trees have a lower infection rate. while mock trees exhibited symptomatic flushes (Fig. 1). The CLas titer was SAMP disrupts the outer membrane reduced in MaSAMP-treated trees, and causes cell lysis of the bacterial cell. while it increased in the mock-treated 12

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To understand the mechanism of MaSAMP bactericidal activity, morphological changes of Lcr post-MaSAMP treatment were observed using transmission electron microscopy. Application of 10 μM MaSAMP to Lcr caused cytosol leakage and the release of small extracellular vesicles after 30 minutes of incubation. The Lcr cells were lysed within two hours of incubation. We isolated the membrane fraction from the MaSAMP treated Lcr and detected the enrichment of MaSAMP in the outer membrane fraction compared with the inner membrane fraction. Thus, MaSAMP likely disrupts mainly the outer membrane of Lcr and breaks the bacterial cells, which leads to cell lysis.

SAMP has Low toxicity.

Because SAMP is internalized by citrus, it is important to test its phytotoxicity.

We injected different concentrations of MaSAMP solution directly into citrus leaves and found that MaSAMP has little phytotoxicity. Furthermore, we found that MaSAMP can be detected in fruit tissue of both Australian finger lime and trifoliate orange by Western blot analysis and is very sensitive to human endopeptidase Pepsin, a major gastric enzyme produced by stomach chief cells. Thus, MaSAMP in Australian finger lime has already been consumed by humans for hundreds of years and can be easily digested (Figure 2, see page 13). These results suggest a low possibility of toxicity of SAMP on citrus and humans, although additional safety assessment tests are necessary for regulatory approval.

Conclusion

Current methods for HLB management include insecticidal control of the vector (Stansly et al. 2014), antibacterial treatments (Blaustein et al. 2018; Gottwald 2010; Hu et al. 2018; Zhang et al. 2014) and nutrient supplements (Rouse 2013; Zhao et al. 2013a). The overuse of insecticides and antibiotics is known to pose threats to human and animal health and select for resistance in the target insect population (Tiwari et al. 2011). Further, current bactericidal or bacteriostatic treatments mostly involve sprays of antibiotics, such as streptomycin and oxytetracycline, which are likely to select for antibiotic-resistant bacteria strains and disrupt the citrus microbiome and ecosystem and may further affect the effectiveness of these antibiotics for medical antibacterial treatment in humans and animals.

On the contrary, SAMPs have a distinct mode of action and tend to interact with the bacterial cell membrane

Figure 2: Fruits of Australian finger lime contain MaSAMP, which has already been consumed by humans for hundreds of years and is easily digested.

through nonspecific mechanisms, making the emergence of resistant bacteria less likely (Jochumsen et al. 2016; Rodriguez-Rojas et al. 2014). Moreover, SAMP kills bacteria faster than anti-

biotics, reducing bacterial generations and further lowering the possibility of evolved resistance (Fantner et al. 2010).

Most importantly, the heat stability of

Continued on Page 14

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Continued from Page 13

nus Liberibacter. International journal of systematic and evolutionary microbiology 64, 2461-2466.

SAMP can provide a prolonged and durable effect in the field compared to heat-sensitive antibiotics. SAMP not only kills bacteria cells but can also prime plant immune responses to prevent/reduce infection. In our greenhouse trials, SAMP has been shown to treat HLB-positive trees and inhibit the emergence of new HLB-infection in healthy trees. Field trials, which can take several years, are currently being initiated in Florida to confirm the efficacy of SAMP in controlling HLB. Field trials also include testing multiple peptide application methods for citrus growers to prevent and treat HLB.

Fantner, G.E., Barbero, R.J., Gray, D.S., and Belcher, A.M. (2010). Kinetics of antimicrobial peptide activity measured on individual bacterial cells using high-speed atomic force microscopy. Nature nanotechnology 5, 280-285.

Gottwald, T.R. (2010). Current epidemiological understanding of citrus Huanglongbing. Annu Rev Phytopathol 48, 119-139.

Graham, J., Gottwald, T., and Setamou, M. (2020).

Status of Huanglongbing (HLB) outbreaks in Florida, California and Texas. Trop Plant Pathol.

Contact Hailing Jin at hailingj@ucr.edu for more information.

References: Albrecht, U., and Bowman, K.D. (2011). Tolerance of the trifoliate citrus hybrid US-897 (Citrus reticulata x Poncirus trifoliata) to huanglongbing.

HortScience 46, 16-22.

Albrecht, U., and Bowman, K.D. (2012a). Tolerance of trifoliate citrus hybrids to Candidatus liberibacter asiaticus. . Sc Horticulturae 147, 71-80.

Barnett, M.J., Solow-Cordero, D.E., and Long, S.R. (2019). A high-throughput system to identify inhibitors of Candidatus Liberibacter asiaticus transcription regulators. Proceedings of the National Academy of Sciences of the United States of America 116, 18009-18014.

Blaustein, R.A., Lorca, G.L., and Teplitski, M. (2018). Challenges for Managing Candidatus Liberibacter spp. (Huanglongbing Disease Pathogen): Current Control Measures and Future Directions.

Phytopathology 108, 424-435.

Bove, J.M. (2006). Huanglongbing: a destructive, newly-emerging, century-old disease of citrus. . J Plant Pathol 88, 7-37.

Brigitte, M.-M., Ivan, B., Estrella, L., and Victor, F. (2017). Defense Priming: An Adaptive Part of Induced Resistance. Annual review of plant biology 68, 485-512.

Fagen, J.R., Leonard, M.T., Coyle, J.F., McCullough, C.M., Davis-Richardson, A.G., Davis, M.J., and Triplett, E.W. (2014). Liberibacter crescens gen. nov., sp. nov., the first cultured member of the ge-

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Ramadugu, C., Keremane, M.L., Halbert, S.E., Duan, Y.P., Roose, M.L., Stover, E., and Lee, R.F. (2016). Long-Term Field Evaluation Reveals Huanglongbing Resistance in Citrus Relatives. Plant Dis 100, 1858-1869.

Rodriguez-Rojas, A., Makarova, O., and Rolff, J. (2014). Antimicrobials, stress and mutagenesis.

PLoS pathogens 10, e1004445.

Rouse, B.B. (2013). Rehabilitation of HLB Infected Citrus Trees using Severe Pruning and Nutritional Sprays. Proc Fla State Hort Soc 126, 51-54.

Hu, J., Jiang, J., and Wang, N. (2018). Control of Citrus Huanglongbing via Trunk Injection of Plant Defense Activators and Antibiotics. Phytopathology 108, 186-195.

Stansly, P.A., Arevalo, H.A., Qureshi, J.A., Jones, M.M., Hendricks, K., Roberts, P.D., and Roka, F.M. (2014). Vector control and foliar nutrition to maintain economic sustainability of bearing citrus in Florida groves affected by huanglongbing. Pest Manag Sci 70, 415-426.

Huang, C., Niu, D., Kund, G., Jones, M., Albrecht, U., Nguyen, L., Bui, C., Ramadugu, C., Bowman, K., Trumble, J., et al. (2020). Identification of citrus defense regulators against citrus Huanglongbing disease and establishment of an innovative rapid functional screening system. Plant Biotechnology Journal.

Stokstad, E. (2012). Agriculture. Dread citrus disease turns up in California, Texas. Science 336, 283-284.

Tiwari, S., Mann, R.S., Rogers, M.E., and Stelinski, L.L. (2011). Insecticide resistance in field populations of Asian citrus psyllid in Florida. Pest management science 67, 1258-1268.

Huang, C.Y., Wang, H., Hu, P., Hamby, R., and Jin, H. (2019). Small RNAs - Big Players in Plant-Microbe Interactions. Cell host & microbe 26, 173-182.

Zhang, M., Guo, Y., Powell, C.A., Doud, M.S., Yang, C., and Duan, Y. (2014). Effective antibiotics against ‘Candidatus Liberibacter asiaticus’ in HLB-affected citrus plants identified via the graftbased evaluation. PloS one 9, e111032.

Jochumsen, N., Marvig, R.L., Damkiaer, S., Jensen, R.L., Paulander, W., Molin, S., Jelsbak, L., and Folkesson, A. (2016). The evolution of antimicrobial peptide resistance in Pseudomonas aeruginosa is shaped by strong epistatic interactions. Nature communications 7, 13002.

Leonard, M.T., Fagen, J.R., Davis-Richardson, A.G., Davis, M.J., and Triplett, E.W. (2012). Complete genome sequence of Liberibacter crescens BT-1.

Standards in genomic sciences 7, 271-283.

Merfa, M.V., Perez-Lopez, E., Naranjo, E., Jain, M., Gabriel, D.W., and De La Fuente, L. (2019).

Progress and Obstacles in Culturing ‘Candidatus Liberibacter asiaticus’, the Bacterium Associated with Huanglongbing. Phytopathology 109, 10921101.

Park, S.C., Lee, J.R., Shin, S.O., Park, Y., Lee, S.Y., and Hahm, K.S. (2007). Characterization of a heat-stable protein with antimicrobial activity from Arabidopsis thaliana. Biochemical and biophysical research communications 362, 562-567.

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Zhao, H., Sun, R., Albrecht, U., Padmanabhan, C., Wang, A., Coffey, M.D., Girke, T., Wang, Z., Close, T.J., Roose, M., et al. (2013a). Small RNA profiling reveals phosphorus deficiency as a contributing factor in symptom expression for citrus huanglongbing disease. Mol Plant 6, 301-310.

Zhao, H., Sun, R., Albrecht, U., Padmanabhan, C., Wang, A., Coffey, M.D., Girke, T., Wang, Z., Close, T.J., Roose, M., et al. (2013b). Small RNA Profiling Reveals Phosphorus Deficiency as a Contributing Factor in Symptom Expression for Citrus Huanglongbing Disease. Molecular Plant 6, 301-310.

Zheng Qing, F., and Xinnian, D. (2013). Systemic Acquired Resistance: Turning Local Infection into Global Defense. Annual review of plant biology 64, 839-863.

Comments about this article? We want to hear from you. Feel free to email us at article@jcsmarketinginc.com

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15

Minimizing Drift from Orchard Spray Application

by Spray Backstop System

By ALIREZA POURREZA | Director, Digital Agriculture

A sprayer working in a young almond orchard in Northern California (all photos courtesy Digital Agriculture Lab.)

Lab, UC Davis

A prototype of the spray backstop system developed at the Digital Agriculture Lab at UC Davis.

Thermal view of spray cloud escaping the canopy from the top.

Cotton ribbon stretched around two rows of trees for continuous loop sampling.

T

he use of pesticides can be very effective in protecting trees from pests and diseases. However, many times this is also accompanied by negative impacts on humans and the environment. Off-target movement of chemical spray has always been a challenge for growers because it can contaminate the environment, reduce spray efficacy and impose liabilities.

California has stringent pesticide laws

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Spray backstop blocking the spray cloud from moving upwards.

and regulations and orchard spray application is considered a high-risk operation. California law establishes a buffer between schools and any pesticide spraying location. Growers are required to notify the public when they spray pesticides. This includes schools, daycare facilities, and county agricultural commissions.

Spray drift can be reduced by choosing the right type of nozzle, adjusting and

July / August 2021

calibrating sprayer settings, defining shelter zones and specifically modified practices in the downwind rows. Reducing the movement of spray droplets to sensitive areas might be accomplished by these methods, but they are in clear contrast with strategies that lead to a uniform on-target deposition.

For instance, spraying with larger droplets can reduce the amount of drift, but it also decreases the effectiveness of the spray at higher parts of a tree. Likewise,

reducing airflow results in reduced drift; however, it also diminishes the spray efficacy in zones farther from the sprayer, for example, on treetops. Drift can also be reduced when we use slower ground speed and higher liquid flow rate, but the impact is not significant.

Spray Backstop

At the Digital Agriculture Lab at UC Davis, a sprayer attachment system called Spray Backstop was developed to minimize drift potential and possibly improve spray coverage on the treetops.

The backstop system is a screen structure that can be raised above the trees using a foldable mast.

A test was conducted in a mature almond orchard to determine how much drifting could be reduced with the backstop system. A cotton ribbon loop was stretched around the trees to quantify the droplets scaping the tree canopy. The ribbon could capture all spray droplets that did not deposit on-target and were not blocked by the

backstop system.

The orchard was sprayed with a mix of water and fluorescent dye. The ribbon was cut into sub-samples and analyzed with the fluorometry method. Comparing the ribbon samples from the test with and without the backstop showed that the Spray Backstop system could effectively block the spray droplets escaping the canopy from treetops or sides and reduce drift potential by 78%.

Leaf samples were also collected from trees in both spray application conditions and analyzed by the fluorometry technique. Unlike the conventional drift control methods, using the spray backstop system does not change overall canopy deposition and could also help improve deposition on treetops.

Adopting the spray backstop system into the orchard spray application practice will reduce the environmental degradation while protecting residential areas and schools from exposure to chemicals. On the other side, growers

can adjust their sprayer for more air and finer droplets (that will improve spray coverage and efficacy in the upper canopy area) without being concerned about drifting because the backstop system can stop spray droplet movement above the trees. A uniformly applied treatment will significantly reduce the risk of crop failure due to pests and diseases.

The spray backstop system is simple and could be easily implemented without significant modifications to the grower’s spray rigs. This system can help growers to be compliant with pesticide regulations and maintain good environmental stewardship. You can find more information about the spray backstop project at the digital Agriculture lab website: digitalaglab.ucdavis.edu.

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Visalia, California

Returns as In-Person Event Over Two Days P

rogressive Crop Consultant Mag- sultant Magazine, and Western Region azine’s popular two-day Crop Certified Crop Advisers Association, Consultant Conference will return the event brings industry experts and this year as a live conference and trade suppliers, researchers and crop consulshow, featuring seminars worth 10 hours tants together for two days of education, of CCA and 8 hours of DPR continuing networking and entertainment. education credits, a live trade show, and the presentation of Western Region CCA “We are excited to be back to doing our Association’s popular CCA of the Year events in person, and expect another Award and honorariums and scholarsell-out event for crop consultants in the ships. The Crop Consultant Conference Western United States,” said JCS Marwill be held on Sept. 16 and 17 at the keting Publisher and CEO Jason Scott.

Visalia Convention Center.

“Agriculture is a relationship-driven business and there is no substitute for The Crop Consultant Conference has live events.” become a premier event held in the San Joaquin Valley each September for Pest Topics for the two days of seminars Control Advisors and Certified Crop include: Various seminars on managAdvisers. Co-hosted by JCS Marketing, ing pests and diseases in high-value the publisher of Progressive Crop Conspecialty crops, tank mix safety and

Hosted by:

Co-hosted by:

regulations, fertilizer management, soil health, new technology, new varieties and rootstocks and their impact on tree nut pest management.

The conference will conclude with two one-hour panels offering hard-to-get CCA credits and moderated by Western Region CCA related to nitrogen monitoring, use, application and management as well as the various regulatory requirements around irrigated nitrogen management.

Registration fees for the two-day event are $150, or less than $15 per CE unit.

Pre-registration is required and can be done at progressivecrop.com/conference.

Includes: 2 Days of Continuing Education Seminars Mixer (2 drinks included) Trade Show

Register today at:

progressivecrop.com/conference 18 Progressive Crop Consultant July / August 2021

2 Breakfasts 2 Lunches $150 Per Person

AGENDA

Total CEU: 8.0 hr. DPR, 10.0 hr. CCA, 2.0 hr. INMP**

SEPTEMBER 16, 2021

7:00AM - 8:00AM Breakfast and Trade Show 8:00AM - 9:30AM Seminars*: Topics Include: Managing Nut Pests in a Down Market,

Integrated Weed Management in Citrus and Grape Pest Management (1.5 DPR and 1.5 CCA hours**)

9:30AM - 10:30AM Break and Trade Show 10:30AM - 12:00PM Seminars*: Topics Include: Using Big Data to Improve Production Agriculture,

Biostimulants in an IPM Program, TBD (0.5 DPR and 1.5 CCA hours**)

12:00PM - 1:00PM Lunch: Announcement of WRCCA Scholarship Winners 1:00PM - 2:30PM Seminars*: Topics Include: Grape Yield Prediction with Machine Learning,

Simple Ways to Drive Soil Health, TBD (1.5 CCA hours**)

2:30PM - 3:30PM Break and Trade Show 3:30PM - 4:30PM Seminars*: Panel Discussion: New and Upcoming Nut Varieties and Rootstocks

and the Future of Pest Management (1.0 DPR and 1.0 CCA hours**) 4:30PM - 5:30PM Mixer

SEPTEMBER 17, 2021 7:00AM - 8:00AM Breakfast and Trade Show 8:00AM - 9:30AM Seminars*: Topics Include: New Findings on Walnut Mold,

Grapevine Disease Management, TBD (1.5 DPR and 1.5 CCA hours**)

9:30AM - 10:30AM Break and Trade Show 10:30AM - 12:00PM Seminars*: Topics include: A Brief Tutorial on Chemistry in the Tank Mix,

Cover Crops in California, TBD (.5 DPR and 1.5 CCA hours**)

12:00PM - 1:00PM Lunch: Announcement of WRCCA CCA of the Year and Honorarium Winners 1:00PM - 3:00PM Seminars*: Topics Include: Panel Discussion on Soil Reports for Nitrogen *Specific topics, subject to change **all CE hours pending approval

Management Plans, and A Conversation about Nitrogen Utilization and Application to Improve Use Efficiency and Meet INMP Requirements (2.0 CCA hours, 2.0 INMP hours**) July / August 2021

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19

Cover Cropping to Achieve Management Goals

Lessons Learned from Cover Crop Trials in the San Joaquin Valley

By SHULAMIT SHRODER | UCCE Community Education Specialist II, Kern County and JESSIE KANTER | UCCE Small Farms and Specialty Crops Program, Fresno County Location of Trials & Cover Crop Species Selection

These trials took place at UC-managed research farms in Shafter (Kern County) and Parlier (Fresno County). The Parlier research farm is the Kearney Agricultural Research and Extension Center and will be referred to as “Kearney” for the remainder of this article.

Four common cover crop mixes were planted at Kearney and five mixes were planted at Shafter (Table 1, see page 22). Three of the mixes were similar in Honey bee on brassica flower in Shafter (all photos courtesy both locations. We S. Shroder.) worked with Kamprath Seeds to select cover crop species based on over crops can provide many mixes that had done well in a demonbenefits to growers, like improving stration trial in Shafter the previous water infiltration and reducing year. Two beginner-friendly simple nutrient loss. However, growers in mixes were also evaluated.

California’s southern San Joaquin Valley worry that the lack of consistent winter Based on the results of our trial, we rainfall and high cost of water in the have outlined some suggestions for tryarea make cover cropping impractical ing out cover crops on your farm. Our (Mitchell et al. 2017). suggestions depend on your goals for planting cover crops and your concerns In this article, we’ll discuss how well about fitting them into your existing different cover crop mixes suppressed cropping system. weeds, provided resources for beneficial insects and improved water infiltration. Goal: Weed Suppression Based on the trial results, if your goal We’ll also delve into water requireis weed suppression, you may consider ments and effects on soil nitrate.

C

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July / August 2021

the following.

Grasses appeared to be most effective at suppressing weeds, especially Merced rye which grew vigorously in both irrigated and non-irrigated plots at both locations. Brassicas also contributed to weed suppression, while legumes were least competitive with weeds, especially at Kearney.

If your goal is weed suppression, consider higher percentages of grasses and brassicas in your seed mix.

Higher seeding rates led to greater weed suppression. If weed suppression is your goal, consider seeding at rates higher than recommended.

If it is not realistic to purchase enough seed for higher seeding rates, pre-irrigating before the first rain to allow weeds to germinate, followed by cultivation prior to planting, can help knock back weeds that may compete with cover crops at germination.

If pre-irrigation is not possible, consider waiting for weed seeds to germinate after the first decent rain, then cultivating your field before planting your cover crop.

However, try to not wait too long, as colder temperatures may inhibit germination. This may have been another reason why cover crop establishment was not as vigorous at Kearney; we planted on December 18 when the high temperature was 53 degrees F and the low was 40 degrees F. Compare this to the planting date of November 5 in Shafter where the high was 20 degrees

F warmer (high of 73 degrees F, low of 52 degrees F).

With legume species, in particular, less initial germination correlated with a low percentage of legume species in the final stand. Grasses, however, germinated well in both locations and dominated the final stand.

Therefore, if you cannot plant until later in the winter, consider seeding grass seeds in higher proportions, especially if weed suppression is your goal.

Irrigating directly after seeding may help cover crops establish more vigorously and compete with weeds. If you are not able to irrigate your cover crop, planting right before a rain event can help your cover crops establish a better stand.

Do not assume that there is enough moisture in the soil to help your cover crops properly germinate if you are

Figure 1: Infiltration rates in both fields at Kearney before the cover crops were planted (blue) and while the cover crops were fully established (orange).

planting after a rain event.

Goal: Provide Resources for Beneficial Insects

The legumes in both locations did not perform well, so they did not offer much sustenance for pollinators.

The brassicas in Shafter started blooming at the beginning of February, while the radishes at Kearney started blooming in mid-March. This provided forage and diversity for the pollinating bees

Continued on Page 22

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21

Continued from Page 21 from the surrounding almond orchards.

Butterflies were also observed on these flowers.

Ladybugs were seen on the grasses in both locations, which can serve to keep aphid populations low on surrounding or subsequent crops.

Goal: Improve Water Infiltration

At Kearney, water infiltration was measured using a mini disk infiltrometer before planting cover crops and right before termination of cover crops.

Infiltration rates were higher when cover crops were in the ground compared to the bare soil prior to planting cover crops (Figure 1, see page 21).

There were no significant differences in infiltration across seed mixes. Therefore, based on our results, there is not one particular mix we would recommend to improve water infiltration.

Having roots in the ground improves the ability of water to enter the soil surface.

Roots create channels for water to enter so that more water can be stored in the soil.

With lower infiltration rates, water is more likely to run off the surface. This is why avoiding bare soil is important if your goal is to increase the amount of water that can be stored in your soil.

Shafter: Comparing Irrigated VS Non-Irrigated

Concern: Will the Cover Crops Tie Up Soil Nitrogen?

In Shafter, we took soil samples zero to six inches deep on April 21 in the irrigated plots five weeks after termination. The fallow samples had an average soil nitrate level of 45.8 mg/kg, which was similar to the soil nitrate level that we measured before planting the cover crops.

Figure 2: Aboveground dry biomass was collected in Shafter on March 17, 2021.

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July / August 2021

Two of the mixes had soil nitrate levels that were very close to that of the fallow area: the brassica pollinator mix and the soil builder mix. Those mixes were mostly or entirely brassicas, so they decomposed quickly enough that the nitrate they had taken up had returned to the soil. The other three mixes (soil health, rye and

peas, and barley and vetch) led to significantly lower soil nitrate levels.

Grasses have higher C:N ratios, which meant that soil microbes took longer to decompose their residue and needed to mine soil nitrogen to do so.

Concern: Can I Really Grow Cover Crops Without Irrigation?

Shafter All of the non-irrigated plots contributed some amount of biomass. This means that even if you cannot irrigate your cover crops, you can still reap some soil health benefits.

Irrigated (left) and non-irrigated (right) soil builder mix in Shafter on March 11, 2021.

For four of the mixes, the irrigated plots produced at least twice as much biomass as the non-irrigated plots. In contrast, the non-irrigated brassica plots produced almost as much biomass as the irrigated brassica plots (Figure 2, see page 22).

Kearney The differences in biomass between the irrigated and non-irrigated plots were not significant, except for the irrigated Merced rye and peas and the irrigated barley and vetch. These two irrigated mixes yielded nearly twice as much biomass as their non-irrigated counterparts (Figure 3, see page 24). This biomass was mostly attributable to the grasses in both mixes.

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Irrigated (left) and non-irrigated (right) barley & common vetch in Shafter on March 11, 2021.

The non-irrigated side produced an impressive amount of biomass given that it only received 3.71 inches of water.

The mixes that performed best without irrigation were the Merced rye and peas and the soil builder mix. The daikon radish performed well in the soil builder mix without supplemental irrigation, and the Merced rye performed well in the rye and peas mix without supplemental irrigation. Therefore, Merced rye and radish would be good options if you are unable to irrigate your cover crops.

Irrigated (left) and non-irrigated (right) brassica pollinator mix in Shafter on March 11, 2021.

Scan the code with your camera app or Google Lens

Conclusion

Based on the results of our trial, cover crops can still produce biomass without irrigation in the San Joaquin Valley, even during a drier winter. While they may not produce huge amounts of biomass without irrigation, having roots in the ground is still beneficial for suppressing weeds, providing beneficial insect habitat, increasing water infiltration and feed-

Continued on Page 24

redoxgrows.com @redoxgrows Irrigated (left) and non-irrigated (right) soil health mix in Shafter on March 11, 2021.

July / August 2021

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Kearney: Comparing Irrigated VS Non-Irrigated

Figure 3: Aboveground dry biomass was collected in Kearney on March 22, 2021.

Continued from Page 23 ing your soil biology.

Cover crops turn sunlight into carbon that feeds your soil microbes through their root system. Therefore, by simply maintaining living ground cover, you are feeding your soil. In addition, when

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you return cover crop residues to your soil, microbes will decompose them and slowly release available nutrients that can be used by your cash crop.

If you are growing cover crops in a limited water environment, consider planting Merced rye and brassica species, such as daikon radish or mustards.

July / August 2021

Irrigated (left) and non-irrigated (right) rye These are also good options if you plant your cover crops later in the winter.

Triticale also grows well with limited water and decomposes more quickly than Merced rye, which is something to consider if you are disking in your cover crop and planting an annual crop

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& peas in Shafter on March 11, 2021 shortly after.

In both Shafter and Kearney, the legume species did not perform well because of the high soil nitrate levels and low water conditions. If your field is in a similar situation, it might be better to save money and not buy legume seeds.

You should also think about the cash crop that will follow the cover crop. For example, you do not want to plant a brassica cash crop after a brassica cover crop. This might increase pest pressure for your brassica cash crop.

Interested in trying out cover crops? The USDA and the California Department of Food and Agriculture have programs to help you pay for it. For more information about the USDA resources, reach out to your local USDA Natural Resource Conservation Service office. For more information about the CDFA Healthy Soils Program or getting started with cover crops, reach out to Shulamit Shroder at sashroder@ucanr.edu or Jessie Kanter at jakanter@ucanr.edu.

Resources

Mitchell, J. P., Shrestha, A., Mathesius, K., Scow, K. M., Southard, R. J., Haney, R. L., Schmidt, R., Munk, D.S. & Horwath, W. R. (2017). Cover cropping and no-tillage improve soil health in an arid irrigated cropping system in California’s San Joaquin Valley, USA. Soil and Tillage Research, 165, 325-335.

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Mitigating Tree Nut Stress and Disease Requires a Multi-Pronged Irrigation Approach

By MATT COMREY | Technical Nutrition Agronomist, Wilbur-Ellis Agribusiness

Flush lines regularly to prevent clogging, particularly after fertilizer injections (all photos courtesy Wilbur-Ellis Agribusiness.)

N

ut growers are essentially paid the season, rather than honing in on in two ways: they can either evapotranspiration (ET) rates. produce higher yields or reduce the number of deductions from the So the question for CCAs and PCAs is, processor. When nut trees aren’t “How do we help growers implement a receiving enough water, you’ll see an strategic irrigation schedule?” My anincreased number of blanks, shriveled swer starts and ends with measurement. kernels and pinched kernels. It’s also When you ask growers how much water common to see more disease and mite they’re putting out, they often come activity in the orchard. All of these back with a number of hours irrigated. issues can hurt final nut grade and yield, But this number does not tell you how which ultimately affects grower payouts much water is being applied to an orfrom the processor. chard; that can be drastically different based on application rates.

During this tight water year, it’s more important than ever to help growers The first step in making the most of implement a science-based approach irrigation is working with your growto reduce the risk of disease and ers to understand how many inches maximize orchard yields. In years of of water per acre of soil the irrigation drought, salinity levels in the soil are system puts out per hour. This piece going to rise, making it critical to keep of information is often overlooked but orchards at proper field capacity so that is essentially the backbone of a good trees do not suffer from physiological irrigation schedule. Everything else drought stress. should be calculated based on that application rate. When water is limited, This is not as simple, however, as ensur- it’s also helpful to know the exact depth ing the right amount of water is applied of soil that needs to be wetted to avoid to the orchard. Growers are going to overirrigating. be pressured to match water deliveries to crop stage and demands throughout 26

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Measuring Orchard Demands

After the exact application rate is determined, growers can start designing an irrigation schedule that meets the needs of their orchards.

Irrigation scheduling maximizes the use of available water by applying the exact amount of water needed to replenish soil moisture. This practice offers growers numerous potential benefits: • Possibly reduces the grower’s cost of water through more efficiently timed irrigations.

• Lowers fertilizer costs by limiting surface runoff.

• Maximizes net returns by increasing crop yields and crop quality.

• Aids in controlling root zone salinity accumulation through controlled leaching.

• Decreases common disease pressures.

I once worked directly with a walnut grower who was able to increase edible yield by 4% just from having a better understanding of his irrigation output rates and adopting a more scientific irrigation approach. By measuring crop-water demand and optimizing water usage, the grower saw dramatic benefits in nut quality across the board, including both color and size advantages.

There are essentially three approaches to water management, and growers should use at least two of the following to design an irrigation schedule: Soil-Based Approach Soil-based methods are simply measuring how much water is being stored in the soil. If there is less water in the soil, and the water is held at a greater tension, it will be more difficult for the trees’ roots to take up that water, resulting in tree stress.

An individualized irrigation plan and schedule takes into account a grower’s water quality and goals.

While irrigating based on the feel or appearance of the soil is a commonly used method, more precise measuring tools such as tensiometers, probes and electrical resistance blocks will provide more specific information that growers can utilize to produce healthier, more consistent nut crops.

Like the other approaches mentioned below, soil moisture readings can be used by themselves to schedule irrigations, but they are most beneficial when used in combination with other methods of irrigation scheduling.

Continued on Page 28

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Figure 1: Soil monitoring reports can help growers and crop advisors understand what is happening with soil moisture in real time.

Continued from Page 27 Probes, for example, are very good at giving the user a snapshot of what the soil moisture looks like at varying depths. It will determine how deep in the soil profile water is infiltrating and ensure calculated water applications are not over- or under-irrigating trees.

However, these probes do not tell you how often or how long to irrigate. Combining the data from the probes with ET measurements on a regular basis will provide enough information to make a strategic irrigation schedule.

Climate-Based Approach This approach requires a deeper understanding and balancing of irrigation application rates and ET. This is the combination of transpiration, or water evaporation from plant leaves, and evaporation from the soil surface.

Many factors affect ET, including air temperature, humidity and wind speed; soil factors such as texture, structure and density; and plant factors such as plant type, root depth and canopy density, height and stage of growth.

Reference ET information is available from a variety of sources, but most well-known is the California Irrigation Management Information System (CI28

Progressive Crop Consultant

MIS) network of nearly 100 California weather stations that provide daily ET values.

Knowing how closely the amount of irrigation water plus rainfall matches estimates of real-time orchard ET can help make irrigation scheduling decisions, especially if this information is teamed with other measurement approaches.

Plant-Based Approach Plants respond in different ways to keep their water supply and demand in balance, and most plant-based methods for irrigation management are based on the measurement of one or more of these responses.

The pressure chamber method for measuring the tension of water within the plant has been shown to be a reliable and commonly used measurement of stress in orchards.

One drawback of using a pressure chamber is that the tool doesn’t distinguish between types of stress. If you are utilizing a pressure chamber in a particularly weak or diseased portion of the orchard, it will not decipher between stress due to poor water supply, salinity and/or pests. This is why it’s important to measure crop-water demands with a climate- or soil-based

July / August 2021

approach in addition to the pressure chamber approach.

By using ET and a pressure chamber, we found that one almond grower was putting out the right amount of water in the orchard, but the watering interval was too wide. Once the water interval was shortened, trees experienced less stress and were able to produce a much better crop.

I always encourage growers to utilize at least two of the three water management approaches because in the orchard, data or feedback you get from each approach doesn’t always line up perfectly. There’s always some subjectivity. Carefully evaluating the information provided by multiple sources can help determine the best irrigation schedule for your growers’ orchards this year.

Maximizing Irrigation Scheduling ROI

There is not a one-size-fits-all irrigation approach or schedule, and choosing which tools to use depends on water quality available to the grower and operation goals.

Combining a plant-based approach with a climate-based approach has been the most effective in my experience, but

there are certainly benefits to monitoring soil moisture status in combination with either of the other two methods .

Companies like Wilbur-Ellis Agribusiness are beginning to offer irrigation scheduling services (Figure 1, see page 28), that include weekly plant stress readings, soil moisture conditions and watering interval recommendations based on replacement ET. These programs have been shown to reduce drastic swings in irrigation protocols, which in turn reduces pest and disease pressures on orchards. These programs also allow growers to focus on other aspects of irrigation and orchard management rather than worrying about interpreting results.

Another critical piece of irrigation management to keep in mind is system maintenance, which includes in-season flushing. Keeping a clean system with good distribution uniformity is essential when growers are trying to get the most out of orchard irrigation scheduling (Figure 2).

Key In-Season Flushing Protocols • Regularly flush laterals (monthly, biweekly if needed).

Figure 2: System maintenance is key to more efficient water delivery.

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Identifying the Potential and Impacts of On-Farm Groundwater Recharge Scientists explore agronomic impacts and best scenarios for success of winter-time flooding to recharge depleted groundwater tables.

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quifers have become depleted from decades of overuse. Drilling deeper is an option for farmers, but prohibitively expensive for low-income residents in disadvantaged communities in the San Joaquin Valley.

A UC scientist believes managed aquifer recharge on agricultural lands close to populations with parched wells is a hopeful solution.

Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater. If relatively clean mountain runoff is used, the water filtering down to the aquifer will address another major groundwater concern: nitrogen and pesticide contamination.

“The recharge has the potential to clean up groundwater,” she said.

Five years ago, UCCE Specialist Toby O’Geen developed an interactive map (casoilresource.lawr.ucdavis.edu/sagbi/) that identifies 3.6 million acres of California farmland with the best potential for replenishing the aquifer based on soil type, land use, topography and other factors.

Dahlke and her colleagues analyzed the map and identified nearly 3,000 locations where flooding suitable ag land will recharge water for 288 rural communities, half of which rely mainly on groundwater for drinking water. The research was published by Advancing Earth and Space Science in February 2021.

Continued on Page 32 30

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July / August 2021

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Continued from Page 30 “If we have the choice to pick a location where recharge could happen, choose those upstream from these communities,” Dahlke said. “Recharge will create a groundwater mound which is like a bubble of water floating in the subsurface. It takes time to reach the groundwater table. That bubble floating higher above the groundwater table might just be enough to provide for a community’s water needs.”

Filling Reservoirs Under the Ground

Many climate models for California suggest long-term precipitation amounts will not change; however, the winter rainy season will be shorter and more intense.

Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater (photo by Joe Proudman, UC Davis.)

“That puts us in a difficult spot,” Dahlke said. “Our reservoirs are built to buffer some rain storms, but are mainly built to store the slowly melting snowpack in the spring. In the coming years, all the water will come down earlier, snowmelt likely in March and April and more water in winter from rainfall events.”

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She is working with water districts and farmers to consider a change in managing water in reservoirs.

“We want to think about drawing reservoirs empty and putting the water underground during the fall and early winter. Then you have a lot of room to handle the enormous amounts of runoff we expect when we have a warm atmospheric river rain event on snow in the spring,” she said.

“However, farmers are hesitant. They like to see water behind the dams.”

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Interest in groundwater banking has been lifted with the implementation of the 2014 Sustainable Groundwater Management Act (SGMA). The law requires governments and water agencies to stop overdraft and bring groundwater basins into balanced levels of pumping and recharge by 2040. Before SGMA, there were no statewide laws governing groundwater pumping, and groundwater was used widely to irrigate farms when surface supplies were cut due to drought.

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Dahlke believes wintertime flooding for groundwater recharge can help water districts meet SGMA rules. “We have to do anything we can to store any surplus water that becomes available to save it for drier times, and our aquifers provide a

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huge storage for that,” she said.

Farming Impacts

The Dahlke Lab is collaborating with UC ANR farm advisors and specialists as well as scientists at other UC campuses to learn about agronomic impacts of flooding a variety of agricultural crops, including almonds, alfalfa and grapes.

In the San Joaquin Valley, UCCE Irrigation Specialist Khaled Bali led an intermittent groundwater recharge trial on alfalfa. The researchers applied up to 16 inches per week with no significant impact on alfalfa yield.

“You could do groundwater recharge in winter and then turn the water off completely and still get a cutting or two of alfalfa before summer,” he said.

This past winter, Dahlke was prepared to flood 1,000 acres of land with water from the Consumnes River. Even though winter 202021 was another drought year, the research will go on. Her team was able to flood a 400-acre vineyard and, in collaboration with scientists from UC Santa Cruz, deploy sensors in the field to measure infiltration rates to better understand whether sediment in the flood water could clog pores in the soil. Her team also collaborates with Ate Visser of Lawrence Livermore National Laboratory in using isotope and noble gas data to determine the groundwater age and flow.

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They also plan to address operational, economic and legal feasibility of groundwater banking on agricultural land.

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Vineyard Water Management During Drought Years

By GEORGE ZHUANG | UCCE Viticulture Farm Advisor, Fresno County

From top left to top right: poor and uneven budbreak; stunted shoot growth (Front: Petite Verdot on 5BB rootstock; Back: Cabernet Sauvignon on 1103P rootstock). From bottom left to bottom right: stunted canopy growth on Pinot Gris on Freedom rootstock with suckers pushing vigorously at the base; poor fruit set with excessive berry abscission (all photos courtesy G. Zhuang.)

T

his past winter was a dry year with a total of <8.5 inches of precipitation (November 2020 to March 2021) and that was significantly lower than the historical average of 11 inches for the San Joaquin Valley (SJV). Winter drought and freeze temperatures last November and December (minimal temperature was as low as 20 degrees F) caused some severe damages on grapevines, and the delayed spring growth (DSG) was mostly reported by growers.

DSG includes erratic bud break, stunted shoot development, dead shoot tips and excessive berry abscission with suckers pushing vigorously at the base of the vine. Some DSG symptoms were very similar to the symptoms of drought or 34

Progressive Crop Consultant

grapevine water stress. Generally, most DSG vines recover after an irrigation event starts; however, the impact of DSG on yield might persist (poor fruit set and small bunch size), and growers likely suffer from the economic loss.

In the face of upcoming drought challenges, growers need to focus on three main areas to improve irrigation efficiency and maximize yield and quality: soil, grapevine canopy and weather condition.

Soil

Most grape roots are in the top three feet of soil, and this is the main area where our irrigation works. Due to the

July / August 2021

lack of winter precipitation, dormant or early season irrigation is necessary to restore the soil moisture, which supports grapevine budbreak as well as early season shoot elongation. Given that the severe drought condition persists in most of SJV, growers need to assess the soil dryness and moisture content at individual sites, since different soil types can hold moisture quite differently (Table 1). Sandier soil holds water poorly in comparison to clay soil. Conversely, water percolates more rapidly in sandy soil than clay soil. Therefore, with the same amount of precipitation or irrigation, clay soil can hold more than sandy soil and grapevine/cover crop can deplete the

Textural Class

Available Water (inches)a

Root Zone Depth (feet)

Allowable Depletion Percentageb

Inchesc

Sandy

0.6

4.5

50

1.4

Loam

1.5

3.5

50

2.6

Clay

2.5

3.5

50

4.4

a b The percentage allowable depletion represents the amount of available water that can be extracted before the next irrigation. To avoid stress, irrigation should occur when 30% to 50% of available water is depleted throughout the root zone. 50% depletion is used in this example.

Values obtained by multiplying available water × root zone depth × percentage allowable depletion. To avoid stress, irrigation must take place after the vineyard has used this amount of water.

c

Table 1: Representative values for available water content, rooting depth and allowable depletions for different soil types. Table is elaborated in Raisin Production Manual (L.P. Christensen 2000).

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soil moisture much faster at sandier sites. Soil type can be accessed through UC Davis SoilWeb (casoilresource.lawr. ucdavis.edu/gmap/), or can be analyzed through commercial laboratories by collecting representative soil samples at a site. Cover crop and middle row vegetation should be mowed earlier under drought conditions to preserve soil moisture.

After understanding the different soil types and their water-holding characteristics, growers need to make the decision for first irrigation based on the soil moisture content, and that can be quite important during the current drought condition. Numerous tools are available for growers to use to measure soil moisture content, the most commonly used method being ‘feel and appearance’, where growers use a shovel or auger to dig out soil samples at different depths and feel the moisture by squeezing the soil in their hand.

Currently, growers have access to many inexpensive soil moisture sensors which can help to measure soil moisture in real time. There are mainly two types of soil moisture sensors: one measures soil water tension (e.g., tensiometer or WaterMark™) and the other measures soil volumetric water content (e.g., Neutron probe and capacitance

Continued on Page 36

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Soil water tension sensor (left) and soil volumetric water content sensor (right).

depleted or the available water content. Soil volumetric water content, on the other hand, offers the percentage of water volume versus the total soil volume and tells you the amount of water stored in the soil and how much you need to irrigate to maintain the desired water content. Growers should irrigate the vineyard when 30% to 50% of allowable water is depleted throughout the root zone (Table 1, see page 35).

The soil volumetric water content is also well correlated to plant water stress measured by midday leaf water potential, and growers can potentially use soil volumetric water content to assess grapevine water stress (Figure 1).

Overall, growers can use both soil-water tension and soil volumetric water content mentioned above to monitor the soil moisture either indirectly or directly and schedule irrigation.

Grapevine Canopy

Figure 1: Soil volumetric water content is correlated with midday leaf water potential. Data were collected from various irrigation treatments: 0.2 ETc, 0.6 ETc, 1.0 ETc and 1.4 ETc. Figure is elaborated in Williams and Trout 2005.

Continued from Page 35 sensor). Soil-water tension tells you how hard it is for the grapevine roots to pull water from the soil particles, and

Sept.

16-17, 2021 Progressive Crop Consultant SEE PAGE36 18-19 FOR MORE INFORMATION

the reading is typically negative with units of centibar. The more negative the reading, the harder it is for the grapevine roots to absorb water. Growers can set up the pre-determined value of soil-water tension (e.g., -30 centibar) at a certain soil depth {e.g., two feet).

Once the soil-water tension reaches the value, the irrigation should start. The pre-determined soil-water tension is usually between -30 and -40 centibar in SJV, varying across different soil types. However, soil-water tension does not tell you how much water has been July / August 2021

Grapevine canopy growth (e.g., budbreak and shoot elongation) depends on the availability of water and nutrients in the soil. Lack of soil moisture during the dormant season increases the risk of freeze damage and hinders the start of budbreak and early season shoot elongation, causing DSG. DSG usually occurs when the grapevine suffers water stress at the beginning of the growing season. If the drought condition persists, shoot elongation might be hampered, and the shoot tip might die off due to lack of water. Under severe drought stress, inflorescences will die off and cause significant yield loss.

To prevent early season vine water stress, soil moisture is the key measurement to decide when to irrigate as was previously mentioned. However, canopy appearance and visual assessment can help to confirm the success of an irrigation program. First, upward-growing shoot tips and tendrils are the most obvious signs of a healthy canopy. Second, a pressure chamber to measure midday leaf water potential might offer a powerful tool to validate the irrigation program (Table 2, see page 37).

Grapevine water demand is directly

Midday Leaf Water Potential (Bar)*

Grapevine Water Stress

related to the amount of sunlight captured by the canopy, and a larger canNo stress > -10 opy needs more water than a smaller canopy due to receiving more light.

Mild stress -10 to -12 Therefore, as the shoot grows and the canopy expands, grapevines generMedium stress ally use more water. Most grapevine -12 to -14 irrigation recommendations are based on the canopy size and climatic Severe stress < -14 condition. Canopy management (e.g., leafing, shoot tucking/thinning and *Midday leaf water potential is measured using sun-exposed newly mature leaf during the solar noon from 12:00 p.m. to 3:00 p.m. in SJV. hedging) has the potential to influence the canopy size and amount of light received, and ultimately water use. Table 2: Midday leaf water potential readings and related grapevine water stress.

Grapevines at different growth stages have different sensitivity and tolertrunk and roots, and the reserve will be (ET) and can calculate grapevine water demand over a given period through ance to water stress. From budbreak used to support the vine growth of the to bloom, the grapevine is very senfollowing season. Postharvest irrigation the calculation: grapevine evapotranspiration (ETc) = reference evapotranssitive to water stress, and even a mild in abundant quantities can also help to stress will hinder growth and cause leach the salts and alleviate the concern piration (ETo) × crop coefficient (Kc).

Growers can simply use ETc to irrigate irreversible yield loss. Generally, water of salinity. the grapevines weekly by using gallons/ stored in the soil profile after winter Weather Condition vine/week or hours/vine/week after precipitation is enough to support vine As was previously mentioned, grapefactoring in the application rate. Kc is growth. However, in years of drought vine water use depends on two main related to canopy size (Figure 2, see such as this year, soil moisture may factors: canopy size and weather. Most page 38) and ETo is related to weather be inadequate to support growth, and growers know crop evapotranspiration irrigation is needed to replenish soil Continued on Page 38 moisture. From bloom to fruit set, the grapevine is also sensitive to water stress, and severe stress causes poor set and yield loss. Fruit set to veraison is a good time to apply some stress if growers are looking to reduce berry size (e.g., smaller berry) and improve berry quality (e.g., color). However, the benefit of improved berry quality might come with the sacrifice of yield.

Typically, in SJV, mild stress is recommended at this period to balance quality and yield. From veraison to harvest may be the best time to apply some stress to advance berry ripening and reduce disease pressure (e.g., bunch rot). However, growers need to avoid severe stress which results in excessive defoliation, since a healthy canopy is required for photosynthesis.

Postharvest, it is generally recommended to replenish the soil profile, since the photosynthetic Upward growing shoot tip and tendril (left), before pressure bomb, bagging leaf before cutting the active canopy still produces the petiole (top right), during pressure bomb, using magnified glass to observe popping water from the petiole (bottom right). carbon to refill the reserve of July / August 2021

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37

loss and maturation delay.

Currently, there are different ways growers can adjust irrigation based on weather condition: 1) National Weather Service (digital.weather.gov/) provides the weather forecast as well as forecasted ETo. Growers can adjust the irrigation amount based on forecasted weather and ET. 2) UCCE is launching weekly crop ET reports (ucanr.edu/sites/viticulture-fresno/ Irrigation_Scheduling/), so growers do not need to calculate weekly grapevine ET or gallons/vine/week themselves.

Irrigation can be simply followed on the ET reports.

Finally, growers need to put economic consideration into water management.

Water might be better used for younger blocks than a vineyard which is near the end of its lifespan, and it also makes more economic sense to use water for the cultivar which has a better price when the water is scarce. The take-home message on vineyard water management is:

Figure 2: Crop coefficient (Kc) is correlated with canopy size measured by leaf area.

Figure is elaborated in Williams and Trout 2005.

98

•

0.4

0.3

92 90 88

84 82

0.1

80

Max daily temperature (r2=0.81) Daily ETo (r2=0.97)

78 76

0.0 300

350

400

450

500

550

600

650

700

Solar radiation (Ly/day)

Figure 3: Sunlight is strongly correlated with ETo and ambient temperature. Data points are extracted from last 10 years’ average during months of August, September and October at CIMIS station #56 in Los Banos.

Continued from Page 37 condition (Figure 3). ETo is strongly correlated with sunlight and the ambient temperature. A sunny and cloudless day will drive more grapevine water use than a cloudy and foggy day or a 38

•

Spend water when it is needed the most.

•

Salinity becomes an important factor in determining water management.

0.2

86

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smokey day as occurred last year due to the wildfires. Similarly, a forecasted heat wave will cause severe water stress if there is lack of irrigation. Water stress coupled with temperatures >100 degrees F disrupts berry growth and sugar accumulation and causes yield July / August 2021

Check soil moisture at all levels.

• Assess canopy, vine water status and weather condition.

94

Daily ETo (in)

Maximum daily air temperature (F

96

• Mow cover crop or middle row vegetation early to preserve soil moisture.

References Williams, L. and Trout, T. 2005. Relationships among Vine- and Soil-Based Measures of Water Status in a Thompson Seedless Vineyard in Response to High-Frequency Drip Irrigation. Am J Enol Vitic. 56: 357-366.

L. P. Christensen. 2000. Raisin Production Manual.

University of California Agriculture and Natural Resources Publication 3393.

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