PCC June July 2021 e

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 

Volume 6: Issue 4

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4 

10 

16 

20 

IN THIS ISSUE 

Can Artificial Intelligence 

Enhance the Profit 

and Environmental 

Sustainability of 

Agriculture? 

Treating and Preventing 

Citrus Huanglongbing 

with a Stable 

Antimicrobial Peptide 

with Dual Function 

A Spray Backstop 

System to Minimize Drift 

from Orchard Spray 

Application 

Cover Cropping to 

Achieve Management 

Goals 

4 

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 

CONTRIBUTING WRITERS & INDUSTRY SUPPORT 

Ray G. Anderson 

Research Soil Scientist, USDA-ARS, 

U.S. Salinity Laboratory 

Karla Araujo 

Contained Research Facility, UC 

Davis 

Matt Comrey 

Technical Nutrition Agronomist, 

Wilbur-Ellis Agribusiness 

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 

Chien-Yu Huang 

Department of Microbiology and Plant 

Pathology, UC Riverside 

Hailing Jin 


Pathology, Center for Plant Cell Biology, 

Institute for Integrative Genome Biology, 

UC Riverside 

Jessie Kanter 

UCCE Small Farms and Specialty Crops Program, 

Fresno County 

Robert R. Krueger 

Horticulturist, USDA-ARS, National Clonal 

Germplasm Repository for Citrus and Dates 

Jonatan Niño Sánchez 


Pathology, UC Riverside 

Alireza Pourreza 

Director Digital Agriculture Lab, UC Davis 

Sonia Rios 

UCCE Area Subtropical Horticulture Advisor, 

Riverside County 

Caroline Roper 


Pathology Center for Plant Cell Biology, 

Institute for Integrative Genome Biology, 

UC Riverside 

Elia Scudiero 

Research Agronomist, UC Riverside 

Shulamit Shroder 

UCCE Community Education Specialist II, 

Kern County 

John Trumble 

Department of Entomology, UC Riverside 

Jeanette Warnert 

Communications Specialist, UC ANR 

George Zhuang 

UCCE Viticulture Farm Advisor, Fresno County 

26 

30 

34 

Mitigating Tree Nut Stress 

and Disease Requires a 

Multi-Pronged Irrigation 

Approach 

Identifying the Potential 

and Impacts of On-Farm 

Groundwater Recharge 

Vineyard Water 

Management During 

Drought Years 

16 

30 

UC COOPERATIVE EXTENSION 

ADVISORY BOARD 

Surendra Dara 

UCCE Entomology and 

Biologicals Advisor, San Luis 

Obispo and Santa Barbara 

Counties 

Kevin Day 

UCCE Pomology Farm Advisor, 

Tulare and Kings Counties 

Elizabeth Fichtner 

UCCE Farm Advisor, 

Kings and Tulare Counties 

Katherine Jarvis-Shean 

UCCE Orchard Systems 

Advisor, Sacramento, Solano 

and Yolo Counties 

Steven Koike 

Tri-Cal Diagnostics 

Jhalendra Rijal 

UCCE Integrated Pest 

Management Advisor, 

Stanislaus County 

Kris Tollerup 

UCCE Integrated Pest Management 

Advisor, Fresno, CA 

Mohammad Yaghmour 

UCCE Area Orchard Systems 

Advisor, Kern County 

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 www.progressivecrop.com 3

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 

The 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- 

Continued on Page 6 

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” 

4 Progressive Crop Consultant July / August 2021

® 

IMAGINATION 

INNOVATION 

SCIENCE IN ACTION 

 

 

 

 


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 

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, 

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. 

SEE PAGE 618-19 FOR MORE Progressive INFORMATION Crop Consultant July / August 2021

L 

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 


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 (~20x20- 

inch 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).) 


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“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. 

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 

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 

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. 


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 GODFREY b | 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 

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.) 

Citrus 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 

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., Microcitrus 

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 


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. 

We screened SAMPs from several 

citrus relatives using a C. Liberibacter 

Vineyards and orchards grow best 

in fungally dominant soil. 

Fungal networks of mycelia and 

mycorrhizae extend the reach of 

roots to access nutrients and water. 

Beneficial fungi also help suppress 

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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. 


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. 

SAMP suppresses CLas in HLB-positive 

citrus trees. 

To determine whether MaSAMP can 

also suppress CLas in citrus trees, 

we used the pneumatic trunk injection 

method to deliver the MaSAMP 

solution into the HLB-positive trees. 

We first tested with eight CLas-positive 

Citrus macrophylla with similar 

bacterial titer and disease symptoms for 

the treatment. At eight weeks following 

two doses injected, separated by two 

months of MaSAMP injection, the disease 

symptoms and the bacterial titer 

in all six treated trees were drastically 

reduced compared to the mock-treated 

(buffer only) plants and one tree with 

no CLas detected. We further tested 

with HLB-positive ‘Madam Vinous’ 

sweet oranges and Lisbon Lemon 

trees; those appeared to have declining 

symptoms and similar CLas titer. After 

MaSAMP treatments, the trees had 

developed symptomless new flushes, 

while mock trees exhibited symptomatic 

flushes (Fig. 1). The CLas titer was 

reduced in MaSAMP-treated trees, 

while it increased in the mock-treated 

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 

in plants can promote faster and/or 

stronger host immune responses upon 

pathogen challenges (Brigitte et al. 

2017; Zheng Qing and Xinnian 2013). 

To determine whether MaSAMP has 

priming activity, we applied it by foliar 

spray to citrus plants. We found that 

MaSAMP applications triggered prolonged 

induction of defense response 

genes. Thus, SAMP can potentially 

“vaccinate” uninfected citrus trees and 

induce defense responses to combat 

against HLB. To test the protection 

ability of SAMP on citrus trees, we 

applied the MaSAMP solution or buffer 

as mock treatment by foliar spray onto 

young, healthy ‘Madam Vinous’ sweet 

orange trees. Five days after treatment, 

the trees were exposed to ACP carrying 

CLas under the “no choice feeding” 

condition for 21 days. We treated trees 

with MaSAMP solution by foliar spray 

every two months subsequently. The 

result indicates that MaSAMP-treated 

trees have a lower infection rate. 

SAMP disrupts the outer membrane 

and causes cell lysis of the bacterial cell. 

To understand the mechanism of Ma- 

SAMP 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 antibiotics, 

reducing bacterial generations 

and further lowering the possibility of 

evolved resistance (Fantner et al. 2010). 

Most importantly, the heat stability of 


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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. 

Contact Hailing Jin at hailingj@ucr.edu 

for more information. 

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 genus 

Liberibacter. International journal of systematic 

and evolutionary microbiology 64, 2461-2466. 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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, 1092- 

1101. 

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. 

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. 

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. 

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. 






Minimizing Drift 

from Orchard 

Spray Application 

by Spray Backstop 

System 

By ALIREZA POURREZA | Director, Digital Agriculture 

Lab, UC Davis 

A sprayer working in a young almond orchard in Northern California (all 

photos courtesy Digital Agriculture Lab.) 

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

A prototype of the spray backstop system developed at the Digital 

Agriculture Lab at UC Davis. 

Cotton ribbon stretched around two rows of trees for continuous 

loop sampling. 

Spray backstop blocking the spray cloud from moving upwards. 

The 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 

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 

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, 


educing 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 

Progressive Crop Consultant Magazine’s 

popular two-day Crop 

Consultant Conference will return 

this year as a live conference and trade 

show, featuring seminars worth 10 hours 

of CCA and 8 hours of DPR continuing 

education credits, a live trade show, and 

the presentation of Western Region CCA 

Association’s popular CCA of the Year 

Award and honorariums and scholarships. 

The Crop Consultant Conference 

will be held on Sept. 16 and 17 at the 

Visalia Convention Center. 

The Crop Consultant Conference has 

become a premier event held in the San 

Joaquin Valley each September for Pest 

Control Advisors and Certified Crop 

Advisers. Co-hosted by JCS Marketing, 

the publisher of Progressive Crop Consultant 

Magazine, and Western Region 

Certified Crop Advisers Association, 

the event brings industry experts and 

suppliers, researchers and crop consultants 

together for two days of education, 

networking and entertainment. 

“We are excited to be back to doing our 

events in person, and expect another 

sell-out event for crop consultants in the 

Western United States,” said JCS Marketing 

Publisher and CEO Jason Scott. 

“Agriculture is a relationship-driven 

business and there is no substitute for 

live events.” 

Topics for the two days of seminars 

include: Various seminars on managing 

pests and diseases in high-value 

specialty crops, tank mix safety and 

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. 

Co-hosted by: 

Hosted by: 

Register today at: 

progressivecrop.com/conference 

18 Progressive Crop Consultant July / August 2021 

Includes: 

2 Days of Continuing 

Education Seminars 

Mixer (2 drinks included) 

Trade Show 

2 Breakfasts 

2 Lunches 

$150 Per Person

7:00AM - 8:00AM 

8:00AM - 9:30AM 

9:30AM - 10:30AM 

10:30AM - 12:00PM 

12:00PM - 1:00PM 

1:00PM - 2:30PM 

2:30PM - 3:30PM 

3:30PM - 4:30PM 

4:30PM - 5:30PM 

7:00AM - 8:00AM 

8:00AM - 9:30AM 

9:30AM - 10:30AM 

10:30AM - 12:00PM 

12:00PM - 1:00PM 

1:00PM - 3:00PM 

*Specific topics, subject to change 

**all CE hours pending approval 

A G E N D A 

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

SEPTEMBER 16, 2021 

Breakfast and Trade Show 

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**) 

Break and Trade Show 

Seminars*: Topics Include: Using Big Data to Improve Production Agriculture, 

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

Lunch: Announcement of WRCCA Scholarship Winners 

Seminars*: Topics Include: Grape Yield Prediction with Machine Learning, 

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


Seminars*: Panel Discussion: New and Upcoming Nut Varieties and Rootstocks 

and the Future of Pest Management (1.0 DPR and 1.0 CCA hours**) 

Mixer 

SEPTEMBER 17, 2021 

Breakfast and Trade Show 

Seminars*: Topics Include: New Findings on Walnut Mold, 

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


Seminars*: Topics include: A Brief Tutorial on Chemistry in the Tank Mix, 

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

Lunch: Announcement of WRCCA CCA of the Year and Honorarium Winners 

Seminars*: Topics Include: Panel Discussion on Soil Reports for Nitrogen 

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**) 


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 

Honey bee on brassica flower in Shafter (all photos courtesy 

S. Shroder.) 

Cover crops can provide many 

benefits to growers, like improving 

water infiltration and reducing 

nutrient loss. However, growers in 

California’s southern San Joaquin Valley 

worry that the lack of consistent winter 

rainfall and high cost of water in the 

area make cover cropping impractical 

(Mitchell et al. 2017). 

In this article, we’ll discuss how well 

different cover crop mixes suppressed 

weeds, provided resources for beneficial 

insects and improved water infiltration. 

We’ll also delve into water requirements 

and effects on soil nitrate. 

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 

both locations. We 

worked with Kamprath 

Seeds to select cover 

crop species based on 

mixes that had done well in a demonstration 

trial in Shafter the previous 

year. Two beginner-friendly simple 

mixes were also evaluated. 

Based on the results of our trial, we 

have outlined some suggestions for trying 

out cover crops on your farm. Our 

suggestions depend on your goals for 

planting cover crops and your concerns 

about fitting them into your existing 

cropping system. 

Goal: Weed Suppression 

Based on the trial results, if your goal 

is weed suppression, you may consider 

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 


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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. 

Shafter: Comparing Irrigated VS Non-Irrigated 

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

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. 

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. 

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. 

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. 

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. 

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- 


Irrigated (left) and non-irrigated (right) soil 

builder mix in Shafter on March 11, 2021. 

Irrigated (left) and non-irrigated (right) 

barley & common vetch in Shafter on March 

11, 2021. 

Irrigated (left) and non-irrigated (right) 

brassica pollinator mix in Shafter on March 

11, 2021. 

Irrigated (left) and non-irrigated (right) soil 

health mix in Shafter on March 11, 2021. 

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Kearney: Comparing Irrigated VS Non-Irrigated 

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


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 

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. 

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 


INSECT CONTROL FOR LEAFY GREENS 

& 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.) 

Nut growers are essentially paid 

in two ways: they can either 

produce higher yields or reduce 

the number of deductions from the 

processor. When nut trees aren’t 

receiving enough water, you’ll see an 

increased number of blanks, shriveled 

kernels and pinched kernels. It’s also 

common to see more disease and mite 

activity in the orchard. All of these 

issues can hurt final nut grade and yield, 

which ultimately affects grower payouts 

from the processor. 

During this tight water year, it’s more 

important than ever to help growers 

implement a science-based approach 

to reduce the risk of disease and 

maximize orchard yields. In years of 

drought, salinity levels in the soil are 

going to rise, making it critical to keep 

orchards at proper field capacity so that 

trees do not suffer from physiological 

drought stress. 

This is not as simple, however, as ensuring 

the right amount of water is applied 

to the orchard. Growers are going to 

be pressured to match water deliveries 

to crop stage and demands throughout 

the season, rather than honing in on 

evapotranspiration (ET) rates. 

So the question for CCAs and PCAs is, 

“How do we help growers implement a 

strategic irrigation schedule?” My answer 

starts and ends with measurement. 

When you ask growers how much water 

they’re putting out, they often come 

back with a number of hours irrigated. 

But this number does not tell you how 

much water is being applied to an orchard; 

that can be drastically different 

based on application rates. 

The first step in making the most of 

irrigation is working with your growers 

to understand how many inches 

of water per acre of soil the irrigation 

system puts out per hour. This piece 

of information is often overlooked but 

is essentially the backbone of a good 

irrigation schedule. Everything else 

should be calculated based on that 

application rate. When water is limited, 

it’s also helpful to know the exact depth 

of soil that needs to be wetted to avoid 

overirrigating. 

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. 


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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. 


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 (CI- 

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 

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). 

• Flush complete system after fertilizer 

injections. 

• Flush from larger to smaller lines; mains 

and submains, then laterals. 

• When flushing lateral lines, ensure 

proper velocity and volume to purge 

contaminants. 

• Never open more than five to eight laterals 

at a time, as additional open lines will 

reduce the velocity of flow, which reduces 

the effectiveness of the flush (dependent 

upon the total number of laterals per 

block). 

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. 

By JEANETTE WARNERT | Communications Specialist, UC ANR 

On-farm recharge has the potential to clean up groundwater that has been contaminated with nitrogen and/or pesticides (photo by H. 

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The drying time of Anti-Stress is 

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Aquifers 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. 



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“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. 

“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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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.) 

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.” 

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. 

“For some of the drought years, overdraft 

was estimated to be as high as nine million 

acre-feet a year,” Dahlke said. 

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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 


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 2020- 

21 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. 

The Dahlke Lab’s groundwater 

banking project has planned more 

studies in groundwater basins across 

the state to close knowledge gaps on 

suitable locations, technical implementation 

and long-term operation. 

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.) 

This past winter was a dry year 

with a total of

Textural Class 

Available Water (inches) a 

Root Zone Depth (feet) 

Allowable Depletion 

Percentage b Inches c 

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. 

c 

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. 

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 

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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. 

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. 


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 

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 

Grapevine Canopy 

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 

SEE PAGE 36 18-19 FOR MORE Progressive INFORMATION Crop Consultant July / August 2021

elated to the amount of sunlight captured 

by the canopy, and a larger canopy 

needs more water than a smaller 

canopy due to receiving more light. 

Therefore, as the shoot grows and the 

canopy expands, grapevines generally 

use more water. Most grapevine 

irrigation recommendations are 

based on the canopy size and climatic 

condition. Canopy management (e.g., 

leafing, shoot tucking/thinning and 

hedging) has the potential to influence 

the canopy size and amount of 

light received, and ultimately water use. 

Midday Leaf Water Potential (Bar)* 

> -10 

-10 to -12 

-12 to -14 

< -14 

Grapevine Water Stress 

No stress 

Mild stress 

Medium stress 

Severe stress 

*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. 

Table 2: Midday leaf water potential readings and related grapevine water stress. 

Grapevines at different growth stages 

have different sensitivity and tolerance 

to water stress. From budbreak 

to bloom, the grapevine is very sensitive 

to water stress, and even a mild 

stress will hinder growth and cause 

irreversible yield loss. Generally, water 

stored in the soil profile after winter 

precipitation is enough to support vine 

growth. However, in years of drought 

such as this year, soil moisture may 

be inadequate to support growth, and 

irrigation is needed to replenish soil 

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 

active canopy still produces the 

carbon to refill the reserve of 

trunk and roots, and the reserve will be 

used to support the vine growth of the 

following season. Postharvest irrigation 

in abundant quantities can also help to 

leach the salts and alleviate the concern 

of salinity. 

Weather Condition 

As was previously mentioned, grapevine 

water use depends on two main 

factors: canopy size and weather. Most 

growers know crop evapotranspiration 

(ET) and can calculate grapevine water 

demand over a given period through 

the calculation: grapevine evapotranspiration 

(ETc) = reference evapotranspiration 

(ETo) × crop coefficient (Kc). 

Growers can simply use ETc to irrigate 

the grapevines weekly by using gallons/ 

vine/week or hours/vine/week after 

factoring in the application rate. Kc is 

related to canopy size (Figure 2, see 

page 38) and ETo is related to weather 


Upward growing shoot tip and tendril (left), before pressure bomb, bagging leaf before cutting the 

petiole (top right), during pressure bomb, using magnified glass to observe popping water from the 

petiole (bottom right). 


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. 

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

Figure is elaborated in Williams and Trout 2005. 

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: 

98 

0.4 

• Check soil moisture at all levels. 

Maximum daily air temperature (F 

96 

94 

92 

90 

88 

86 

84 

82 

80 

78 

Max daily temperature (r 2 =0.81) 

Daily ETo (r 2 =0.97) 

76 

0.0 

300 350 400 450 500 550 600 650 700 

0.3 

0.2 

0.1 

Daily ETo (in) 

• Assess canopy, vine water status and 

weather condition. 

• Spend water when it is needed the 

most. 

• Salinity becomes an important 

factor in determining water 

management. 

• Mow cover crop or middle row 

vegetation early to preserve soil 

moisture. 

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. 


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 

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 

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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