PCC March 2019

March/April 2019 

Redefining IPM for the 21st Century 

Kasugamycin for 

Managing Walnut Blight 

How does kasugamycin-copper or -mancozeb 

mixtures compare to copper-mancozeb? 

Moving Toward Alternatives to Chlorpyrifos for 

Managing Maggots in Onions 

The Carbohydrate Observatory: A 

Citizen Science Research Project 

Understanding seasonal trends of starch and sugar 

in walnut, pistachio and almond under varying 

climatic conditions. 

Volume 4 : Issue 2 


www.progressivecrop.com 

1

2 Progressive Crop Consultant March/April 2019

4 

IN THIS ISSUE 

Redefining IPM for the 

21st Century 

PUBLISHER: Jason Scott 

Email: jason@jcsmarketinginc.com 

EDITOR: Kathy Coatney 

ASSOCIATE EDITOR: Cecilia Parsons 

Email: article@jcsmarketinginc.com 

PRODUCTION: design@jcsmarketinginc.com 

Phone: 559.352.4456 

Fax: 559.472.3113 

Web: www.progressivecrop.com 

12 

16 

20 

26 

34 

Kasugamycin for 

Managing Walnut Blight- 

How does kasugamycincopper 

or -mancozeb 

mixtures compare to 

copper-mancozeb? 

Moving Toward 

Alternatives to Chlorpyrifos 

for Managing Maggots in 

Onions 

The Carbohydrate 

Observatory: A Citizen 

Science Research Project 

-Understanding seasonal 

trends of starch and sugar 

in walnut, pistachio and 

almond under varying 

climatic conditions. 

Climate Change and 

California Agriculture 

The Many Possible 

Causes of “Gummy 

Nuts” in Almonds 

12 

16 

34 

CONTRIBUTING WRITERS & 

INDUSTRY SUPPORT 

J. E. Adaskaveg 

Department of 

Microbiology and Plant 

Pathology, University of 

California, Riverside, CA, L. 

Wade, Arysta Life Science, 

Roseville, CA 

Surendra K. Dara 

CE Advisor—Entomology 

and Biologicals, University 

of California Cooperative 

Extension, San Luis Obispo 

and Santa Barbara counties 

Anna Davidson 

Postdoc, Manager of the 

Carbohydrate Observatory 

Maciej Zwieniecki, 

Professor, Founder of the 


Tapan B. Pathak 

Division of Agriculture 

and Natural Resources, 

University of California, 

Merced, CA, Mahesh 

L. Maskey, Division of 

Agriculture and Natural 

Kevin Day 

County Director and 

UCCE Pomology Farm 

Advisor, Tulare/Kings County 

Dr. Brent Holtz 

County Director and UCCE 

Pomology Farm Advisor, San 

Joaquin County 

Steven T. Koike, 

Director, TriCal Diagnostics 

Emily J. Symmes 

UCCE IPM Advisor, 

Sacramento Valley 

Resources, University of 

California, Merced, CA, 

Department of Land, Air 

and Water Resources, 


Davis, CA, A. Dahlberg & 

Khaled M. Bali, Division 

of Agriculture and Natural 

Resources—Kearney 

Agricultural Research 

and Extension Center, 


Parlier, CA, Daniele 

Zaccaria, Department 

of Land, Air and Water 

Resources, University of 

California, Davis, CA 

Emily J. Symmes 

Sacramento Valley Area 

IPM Advisor University of 

California Cooperative 

Extension and Statewide 

IPM Program 

Rob Wilson 

UC ANR Intermountain 

Research and Extension 

Center Director & Farm 

Advisor 

UC COOPERATIVE EXTENSION 

ADVISORY BOARD 

Kris Tollerup 

UCCE Integrated Pest 

Management Advisor, 

Parlier, CA 

Surendra K. Dara 

CE Advisor—Entomology 

and Biologicals, University 

of California Cooperative 

Extension, San Luis Obispo 

and Santa Barbara 

counties 

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. 



3

PRODUCER 

Within 

the 

group 

Staying 

informed 

ECONOMICAL VIABILITY 

Acons 

Among 

growers 

COMMUNICATION 

PLANNING & 

ORGANIZATION 

Host Plant 

Resistance 

IPM 

Cultural 

Biological 

PEST 

MANAGEMENT 

KNOWLEDGE & 

RESOURCES 

CONSUMER 

Behavioral 

Chemical 

Microbial 

ENVIRONMETAL SAFETY 

Physical/ 

Mechanical 

Pest 

Managing 

info 

Available 

opons 

Monitoring 

Tools & 

Technology 

SELLER 

SOCIAL ACCEPTABILITY 

Redefining IPM for the 21st Century 

By: Surendra K. Dara | CE Advisor—Entomology and Biologicals, University of 

California Cooperative Extension, San Luis Obispo and Santa Barbara counties 

Integrated pest management, commonly 

referred to as IPM, is a 

concept of managing pests that has 

been in use for several decades. The 

definition and interpretation of IPM 

vary depending on the source, such as a 

university, institute, or a researcher, and 

its application varies even more widely 

depending on the practitioner. Here are 

a few examples of its definitions and 

interpretations: 

“IPM is an ecosystem-based strategy 

that focuses on long-term prevention 

of pests or their damage through a 

combination of techniques such as 

biological control, habitat manipulation, 

modification of cultural practices, and 

use of resistant varieties. Pesticides are 

4 Progressive Crop Consultant March/April 2019 

used only after monitoring indicates 

they are needed according to established 

guidelines, and treatments are made 

with the goal of removing only the 

target organism. Pest control materials 

are selected and applied in a manner 

that minimizes risks to human health, 

beneficial and nontarget organisms, and 

the environment.” UC IPM 

“Integrated Pest Management, or IPM, 

is an approach to solving pest problems 

by applying our knowledge about 

pests to prevent them from damaging 

crops, harming animals, infesting 

buildings or otherwise interfering 

with our livelihood or enjoyment of 

life. IPM means responding to pest 

problems with the most effective, leastrisk 

option.” IPM Institute of North 

America 

“A well-defined Integrated Pest 

Management (IPM) is a program 

that should be based on prevention, 

monitoring, and control which offers 

the opportunity to eliminate or 

drastically reduce the use of pesticides, 

and to minimize the toxicity of and 

exposure to any products which are 

used. IPM does this by utilizing a 

variety of methods and techniques, 

including cultural, biological and 

structural strategies to control a 

multitude of pest problems.” Beyond 

Pesticides 

“IPM is rotating chemicals from 

Continued on Page 6

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5

Continued from Page 4 

different mode of action groups.” A 

grower 

These definitions and interpretations 

represent a variety of objectives and 

strategies for managing pests. IPM 

is not a principle that can/should be 

strictly and equally applied to every 

situation, but a philosophy that can 

guide the practitioner to use it as 

appropriate for the situation. For 

example, varieties that are resistant 

to arthropod pests and diseases are 

available for some crops, but not 

for others. Mating disruption with 

pheromones is widely practiced for 

certain lepidopteran and coleopteran 

pests, but not for several hemipteran 

pests. Biological control is more readily 

employed for greenhouse pests, but 

not to the same extent under field 

conditions. While chemical pesticides 

should be used as the last resort, in 

principle, sometimes they are the first 

line of defense to prevent damage to the 

transplants by certain pests or area-wide 

spread of certain endemic or invasive 

pests and diseases. 

Crop production is an art, science, and 

business, and by adding environmental 

and social factors, IPM—an approach 

used in agriculture—can also be 

influenced by a number of factors. 

Each grower has their own strategy for 

producing crops, minimizing losses, 

and making a profit in a manner that 

is acceptable to the society, safe for the 

consumers, and less disruptive to the 

environment. In other words, “IPM 

is an approach to manage pests in an 

economically viable, socially acceptable, 

and environmentally safe manner”. 

Keeping this simple, but loaded, 

definition in mind and considering 

recent advances in crop production and 

protection, communication technology, 

and globalization of agriculture and 

commerce, here is the new paradigm 

of IPM with its management, business, 

and sustainability aspects. 

I. Management Aspect 

There are four major 

components in the 

IPM model that 

address various pest 

management options, 

the knowledge and 

resources the grower has in order to 

address the pest issue, planning and 

organization of information to take 

appropriate actions, and maintaining 

good communication to acquire and 

disseminate knowledge about pests and 

their management. 

1. Pest Management 

The concept of 

pest control has 

changed to pest 

management over 

the years knowing 

that a balanced 

approach to managing 

pest populations 

to levels that do not cause economic 

losses is better than eliminating due to 

environmental and economic reasons. 

Although the term control is frequently 

used in literature and conversations, 

it generally refers to management. A 

thorough knowledge of general IPM 

principles and various management 

options for all possible pest problems is 

important as some are preventive and 

others are curative. It is also essential 

to understand inherent and potential 

interactions among these management 

options to achieve maximum control. 

The following are common control 

options that can be employed at different 

stages of crop production to prevent, 

reduce, or treat pest infestations. Each of 

them may provide only a certain level of 

control, but their additive effect can be 

significant in preventing yield losses. 

IPM strategies 

a. Host plant resistance: It involves the 

use of pest resistant and tolerant cultivars 

developed through traditional breeding 

or genetic engineering. These cultivars 

possess physical, morphological, or 

biochemical characters that reduce the 

plant’s attractiveness or suitability for 

the pest to feed, develop, or reproduce 

successfully. These cultivars resist or 

tolerate pest damage and thus reduce the 

yield losses. 

b. Cultural control: Modifying 

agronomic practices to avoid or 

reduce pest infestations and damage 

refers to cultural control. Adjusting 

planting dates can help escape pest 

occurrence or avoid most vulnerable 

stages of the crop to coincide with the 

pest occurrence. Modifying irrigation 

practices, fertilizer program, plant or 

row spacing, and other agronomic 

practices can create conditions that are 

less suitable for the pest. Destroying 

crop residue and thorough cultivation 

will eliminate breeding sites and control 

soil-inhabiting stages of the pest. Crop 

rotation with non-host or tolerant 

crops will break the pest cycles and 

reduce their buildup year after year. 

Choosing clean seed and plant material 

will avoid the chances of introducing 

pests right from the beginning of the 

crop production. Sanitation practices to 

remove infected/infested plant material, 

regular cleaning of field equipment, 

avoiding accidental contamination of 

healthy fields through human activity 

are also important to prevent the pest 

spread. Intercropping of non-host 

plants or those that deter pests or using 

trap crops to divert pests away from the 

main crop are some of the other cultural 

control strategies. 

c. Biological control: Natural enemies 

such as predatory arthropods and 

parasitic wasps can be very effective in 

causing significant reductions in pest 

populations in certain circumstances. 

Periodical releases of commercially 

available natural enemies or conserving 

natural enemy populations by providing 

refuges or avoiding practices that 

harm them are some of the common 

practices to control endemic pests. To 

address invasive pest issues, classical 

biological control approach is typically 

employed where natural enemies from 

the native region of the invasive pest are 

imported, multiplied, and released in 

the new habitat of the pest. The release 

of irradiated, sterile insects is another 

biological control technique that is 

successfully used against a number of 

pests. 

Chemical 

Microbial 

Host Plant 

Resistance 

PEST 

MANAGEMENT 

Mechanical/ 

Physical 

Continued on Page 8 

Behavioral 

Cultural 

Biological 




7


d. Behavioral control: Behavior of the 

pest can be exploited for its control 

through baits, traps, and mating 

disruption techniques. Baits containing 

poisonous material will attract and kill 

the pests when distributed in the field 

or placed in traps. Pests are attracted 

to certain colors, lights, odors of 

attractants or pheromones. Devices that 

use one or more of these can be used 

to attract, trap or kill pests. Pheromone 

lures confuse adult insects and disrupt 

their mating potential, and thus reduce 

their offspring. 

e. Physical or mechanical control: 

This approach refers to the use of 

a variety of physical or mechanical 

techniques for pest exclusion, trapping 

(in some cases similar to the behavioral 

control), removal, or destruction. Pest 

exclusion with netting, handpicking or 

vacuuming to remove pests, mechanical 

tools for weed control, traps for rodent 

pests, modifying environmental 

conditions such as heat or humidity 

in greenhouses, steam sterilization or 

solarization, visual or physical bird 

deterrents such as reflective material 

or sonic devices are some examples for 

physical or mechanical control. 

f. Microbial control: Using 

entomopathogenic bacteria, fungi, 

microsporidia, nematodes, and viruses, 

and fermentation byproducts of 

microbes against arthropod pests, fungi 

against plant parasitic nematodes, and 

bacterial and fungal antagonizers of 

plant pathogens generally come under 

microbial control. As repeated use of 

certain microbial control options can 

also lead to resistance development in 

KNOWLEDGE & 

RESOURCES 

Tools & 

Technology 

Pest 

Control 

opons 

pest populations, caution should be 

exercised while using them. 

g. Chemical control: Chemical 

control typically refers to the use of 

synthetic chemical pesticides, but 

to be technically accurate, it should 

include synthetic chemicals as well as 

chemicals of microbial or botanical 

origin. Although botanical extracts 

such as azadirachtin and pyrethrins, 

and microbe-derived toxic metabolites 

such as avermectin and spinosad are 

regarded as biologicals, they are still 

chemical molecules, similar to synthetic 

chemicals, and possess many of the 

human and environmental safety risks 

as chemical pesticides do. Chemical 

pesticides are categorized into different 

groups based on their mode of action 

and rotating chemicals from different 

groups is recommended to reduce 

the risk of resistance development. 

Government regulations restrict the 

time and amount of certain chemical 

pesticides and help mitigate the 

associated risks. 

The new RNAi (ribonucleic acid 

interference) technology where 

double-stranded RNA is applied to 

silence specific genes in the target 

insect is considered a biopesticide 

application. Certain biostimulants 

based on minerals, microbes, plant 

extracts, seaweed or algae impart 

induced systemic resistance to pests and 

diseases, but are applied as amendments 

without any claims for pest or disease 

control. These new products or 

technologies can fall into one or more 

abovementioned categories. 

As required by the crop and pest 

situation, one or more of these control 

options can be used throughout the 

production period for effective pest 

management. When used effectively, 

non-chemical control options delay, 

reduce, or eliminate the use of chemical 

pesticides. 

2. Knowledge and Resources 

The knowledge 

of various control 

options, pest biology 

and damage potential, 

and suitability of 

available resources 

enables the grower to make a decision 

appropriate for their situation. 

a. Pest: Identification of the pest, 

understanding its biology and seasonal 

Acons 

Managing 

Info 

PLANNING & 

ORGANIZATION 

Monitoring 

population trends, damaging life stages 

and their habitats, nature of damage and 

its economic significance, vulnerability 

of each life for one or more control 

options, host preference and alternate 

hosts, and all the related information 

is critical for identifying an effective 

control strategy. 

b. Available control options: Since not 

all control options can be used against 

every pest, the grower has to choose the 

ones that are ideal for the situation. For 

example, systemic insecticides are more 

effective against pests that mine or bore 

into the plant tissue. Pests that follow 

a particular seasonal pattern can be 

controlled by adjusting planting dates. 

Commercially available natural enemies 

can be released against some, while 

mating disruption works well against 

others. Entomopathogenic nematodes 

can be used against certain soil pests, 

bacteria and viruses against pests with 

chewing mouthparts such as lepidoptera 

and coleopteran, and fungi against 

sucking pests. 

c. Tools and technology: A particular 

pest can be controlled by certain 

options, but they may not all be 

available in a particular place, for a 

particular crop, or within the available 

financial means. For example, the 

release of natural enemies may be 

possible in high-value specialty crops, 

but not in large acreage field crops. A 

particular pesticide might be registered 

against a pest on some crops, but not on 

all. Use of netting or tractor-mounted 

vacuums can be effective, but very 

expensive limiting their availability to 

those who can afford. 

This is a critical component where 

diagnostic and preventive or curative 

decisions are made based on available 

and affordable control options. 


3. Planning and Organization 

This component 

deals with the 

management 

aspect of the new 

IPM model for 

data collection, 

organization, 

and actual actions against pest 

infestations. 

a. Pest monitoring: Regularly 

monitoring the fields for pest 

occurrence and spread is a basic step 

in crop protection. Early detection in 

many cases can help address the pest 

situation by low-cost spot treatment 

or removal of pests or infected/ 

infested plant material. When pest 

infestations continue to grow, regular 

monitoring is necessary to assess 

the damage and determine the 

time to initiate farm-wide control. 

Monitoring is also important to avoid 

calendar-based pesticide applications 

especially at lower pest populations 

that do not warrant treatments. 

b. Managing information: A good 

recordkeeping about pests, their 

damage, effective treatments, seasonal 

fluctuations, interactions with 

environmental factors, irrigation 

practices, plant nutrition, and all 

related information from year to year 

will build the institutional knowledge 

and prepares the grower to take 

preventive or curative actions. 

c. Corrective actions: Taking 

timely action is probably the most 

important aspect of IPM. Even with 

all the knowledge about the pest 

and availability of resources for its 

effective management, losses can 

be prevented only when corrective 

actions are taken at the right time. Good 

farm management will allow the grower 

to take timely actions. These actions are 

not only necessary to prevent damage 

on a particular farm, but also to prevent 

the spread to neighboring farms. 

When pest management is neglected, 

it leads to area-wide problems with 

larger regulatory, social, and economic 

implications. 

4. Communication 

Good communication to transfer 

the individual or 

collective knowledge 

for the benefit of 

everyone is the 

last component 

of the new IPM 

model. Modern 

and traditional 

communication tools can be used for 

outreach as university and private 

researchers develop information about 

endemic and invasive pests, emerging 

threats, and new control strategies. 


Within the 

group 

Within the 

community 

Staying upto-date 

COMMUNICATION 



9


a. Staying informed: Growers and 

pest control professionals should stay 

informed about existing and emerging 

pests and their management options. 

Science-based information can be 

obtained by attending extension 

meetings, webinars, or workshops, 

reading newsletter, trade, extension, 

or scientific journal articles, and 

keeping in touch with researchers and 

other professionals through various 

communication channels. Wellinformed 

growers can be well prepared 

to address pest issues. 

b. Communication within the group: 

Educating farm crew through periodical 

training or communication will help 

with all aspects of pest management, 

proper pesticide handling, ensuring 

worker safety, and preventing 

environmental contamination. A 

knowledgeable field crew will be 

beneficial for effective implementation 

of pest management strategies. 

c. Communication among growers: 

Although certain crop production and 

protection strategies are considered 

proprietary information, pests do 

not have boundaries and can spread 

to multiple fields when they are not 

effectively managed throughout 

the region. Sharing knowledge and 

resources with each other will improve 

pest control efficacy and benefit the 

entire grower community. 

In addition to these four components 

with an IPM model, factors that 

influence profitable, safe, and affordable 

food production at a larger scale and 

their implications for global food 

security should also be included. There 

are two layers surrounding these four 

components addressing the business 

and sustainable aspects of food 

production. 

II. Business Aspect 

Consumers want 

nutritious, healthy, 

and tasty produce 

that is free of pest 

damage at affordable prices. Growers 

try to meet this demand by producing 

food that meets all the consumer needs, 

while maintaining environmental and 

human safety and still being able to 

make a profit. Sellers evaluate the 

market demand and strategize their 

sales to satisfy consumers while 

making their own profit to stay in 

the business. In an ideal system, 

consumer, producer, and seller would 

maintain a harmonious balance of 

food production and sale. In such a 

system, food is safe and affordable 

to everyone, there will be food 

security all over the world, and both 

growers and sellers make a good 

profit with no or minimal risk to the 

environment in the process of food 

production. 

However, this balance is frequently 

disrupted due to 1) consumers’ 

misunderstanding of various food 

production systems, their demand 

for perfectly shaped fruits and 

vegetables at affordable prices or their 

willingness to pay a premium price 

for food items that are perceived 

to be safe, 2) growers trying to find 

economical ways of producing 

high quality food while facing with 

continuous pest problems and other 

challenges, and 3) sellers trying to 

market organic food at a higher price 

as a safer alternative to conventionally 

produced food. If growers implement 

good IPM strategies to produce safe 

food and consumers are aware of this 

practice and gain confidence in food 

produced in an IPM system, then 

sellers would be able to market what 

informed-consumers demand. 

III. Sustainability Aspect 

As mentioned earlier, 

IPM is an approach to 

ensure economic viability 

at both consumer and 

producer level (seller is 

always expected to make 

a profit), environmental 

safety through a balanced use of all 

available pest control options, and 

social acceptability as food is safe and 

affordable. 

While organic food production 

is generally perceived as safe and 

sustainable, the following examples 

can explain why it is not necessarily 

true. Organic food production 

is not pesticide-free and some of 

the pesticides used in an organic 

system are as harmful to humans 

and non-target organisms as some 

chemical pesticides. Certain organically 

accepted pesticides have toxins or 

natural chemical molecules that are very 

similar to those in synthetic pesticides. 

In fact, some synthetic pesticides are 

manufactured imitating the pesticidal 

molecules of natural origin. Mechanical 

pest control practices such as 

vacuuming or tilling utilize fossil fuels 

and indirectly have a negative impact 

on the environment. For example, 

diesel-powered tractors are operated for 

vacuuming western tarnished bug in 

strawberry 2-3 times or more each week 

compared to a pesticide application 

typically requires the use of tractor 

once every 7-14 days. To control certain 

pests, multiple applications of organic 

pesticides might be necessary with 

associated costs and risks, while similar 

pest populations could be controlled by 

fewer chemical pesticide applications. It 

is very difficult to manage certain plant 

diseases and arthropod pests through 

non-chemical means and inadequate 

control not only leads to crop losses, 

but can result in their spread to larger 

areas making their control even more 

difficult. Many growers prefer a good 

IPM-based production to an organic 

production for the ease of operation and 

profitability. However, they continue 

to produce organic food to stay in 

business. 

While middle and upper-class 

consumers may be willing to pay 

higher prices for organically produced 

food, many of the low-income groups 

in developed and underdeveloped 

countries cannot afford such food. 

Organic food production can lead to 

social inequality and a false sense of 

wellbeing for those that can afford it. 

Food security for the growing world 

population is necessary through 

optimizing input costs, minimizing 

wastage, grower adoption of safe and 

sustainable practices, and consumer 

confidence in food produced through 

such practices. IPM addresses all 

the economic, environmental, and 

social aspects and provides safe and 

affordable food to the consumers and 

profits to producers and sellers, while 

maintaining environmental health. 

Comments about this article? We want 

to hear from you. Feel free to email us 

at article@jcsmarketinginc.com 


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11

Kasugamycin for Managing Walnut Blight 

How do kasugamycin-copper or -mancozeb mixtures compare to copper-mancozeb? 

By: J. E. Adaskaveg | Department of Microbiology and Plant Pathology, University of 

California, Riverside, CA, L. Wade | Arysta Life Science, Roseville, CA 

Figure 1. Pistillate 

flowers developing 

into healthy walnut 

fruitlets (left) and 

showing a primary 

infection (center) 

at the blossom end. 

Developing walnuts 

(right) with primary 

(blossom end) and 

secondary (fruitside) 

infections. All photos 

courtesy of Jim 

Adaskaveg. 

Kasugamycin (tradename Kasumin) 

was registered in 2018 

for managing walnut blight and 

bacterial canker and blast on sweet 

cherry. The bactericide was already federally 

registered for fire blight on pome 

fruit, but in 2018, registration for this 

disease was also approved in California. 

Kasugamycin is a unique bactericide because 

it is not used in animal or human 

medicine. Environmental monitoring 

studies have shown that it does not 

select for human bacterial pathogen 

resistance with uses in plant agriculture. 

Furthermore, kasugamycin has its own 

Fungicide Resistance Action Committee 

(FRAC) Code 24 or mode of action that 

is different from other registered plant 

agricultural bactericides like streptomycin 

(FRAC Code 25) and oxytetracycline 

(FRAC Code 41). Kasugamycin 

meets new toxicology standards for pollinating 

insects (e.g., honey bees), it has 

a low animal toxicity with a “Caution” 

rating and a 12 h re-entry time on the 

label. As with any cautionary pesticide, 

mixers and applicators need to have 

standard personal protective equipment 

(PPE) when handling the bactericide. 

Copper is classified as FRAC Code M1 

for the first element historically used 

for fungal and bacterial disease control. 

Copper affects many physiological 

pathways in plant pathogens and is 

classified as having a multi-site (M) 

mode of action. Not many bactericides 

have been developed for managing 

plant bacterial diseases, and fewer have 

been registered. Thus, there has been a 

great dependency on copper. Because 

of the multi-site classification, many 

agriculturalists thought that plant 

pathogens would not develop resistance 

to copper. Unfortunately, after many 

years of usage, bacterial pathogens 

such as the walnut blight pathogen, 

Xanthomonas arboricola pv. juglandis 

(Xaj), have developed resistance to 

copper. This is a direct result and lack 

of alternative bactericides available 

of overuse of one active ingredient 

(i.e., copper) and being limited with 

the lack of bactericides available to 

apply modern approaches to resistance 

management such as rotating between 

active ingredients with different modes 

of action and limiting the total number 

of applications of any one mode of 

action per season as part of following 

“RULES” (http://ipm.ucanr.edu/PDF/ 

PMG/fungicideefficacytiming.pdf). 

Over-usage of any one active ingredient, 

such as copper, can create other 

environmental issues only including soil 

contamination, orchard water-runoff, 

higher concentrations in watersheds, 

and potential crop and non-crop 

phytotoxicity especially in perennial 

crop systems. 

After the industry used copper 

exclusively for approximately 50 years 

(1930s to 1980s), copper-maneb 

(e.g., Manex) mixtures were first 

identified for use on walnut in 1992 

and emergency registrations ensued 

for 22 years before a full registration 

was obtained for the related compound 

mancozeb in 2014. The walnut industry 

and University of California (UC) 

researchers knew that more alternatives 

were needed, otherwise someday the 

pathogen would develop resistance 

to copper-mancozeb. Because copper 

resistance had already developed, this 

selection pressure is maintained and 

resistance levels are increasing even 

when mancozeb is used in the mixture, 

because copper has been the only 

tank mix option. In effect, resistance 

management is not being effectively 

practiced since copper-resistance 

already exists and the use of mancozeb 

(M3) is selecting for resistant strains of 

the bacterial pathogen to the mancozeb 

mode of action. In the presence of 

copper resistance, having only one 

treatment (i.e., mancozeb) available 

to manage a disease not only can limit 

crop production each season but could 

economically devastate the entire 

industry by making harvests sporadic 

and inconsistent, lowering crop quality, 

and preventing profitability. Growers 

and the entire walnut industry consider 

walnut blight a threat to the industry 

and their livelihood. 

Why do we need kasugamycin for 

managing walnut blight? There is a 

great need to develop other modes of 

action for managing bacterial diseases 

including walnut blight that can be 

integrated into management programs. 

Kasugamycin was identified, developed, 

and registered for the purpose of 

resistance management, reducing 

over-usage of any one mode of action, 

and sustaining the walnut industry 

of California. The aminoglycoside 

bactericide has a unique mode of action 

(FRAC Code 24) as stated above and 

can be used in combination with copper 

or mancozeb. When kasugamycin is 

used in combination with mancozeb, 


esistance management is being practiced 

since resistance has not been found in Xaj 

pathogen populations to either mode of 

action. 

Use on Walnuts 

Kasugamycin is labeled as Kasumin for 

managing walnut blight at 64 fl oz/A in a 

minimum of 100 gal water/A for ground 

application. The full 64 fl oz per acre labeled 

rate for kasugamycin should always be used. 

Adjuvants that are stickers may also be used, 

whereas spreaders and penetrants should 

be avoided. Reduced spray volumes may 

be utilized for small trees provided that the 

volume of water is sufficient to provide good 

coverage of treated foliage. Applications 

should be initiated when conditions favor 

disease development. This is the same timing 

as for copper-mancozeb. In orchards with a 

history of the disease and when high rainfall 

is forecasted, applications should be initiated 

at 20-40 percent catkin expansion. Under 

less favorable conditions for disease (i.e., 

low rainfall forecasts and minimal dews), 

applications should start at 20-40 percent 

pistillate flower expansion (also known as 

the “prayer stage”). The preharvest interval is 

100 days or approximately mid- to late June 

depending on the walnut cultivar harvest 

date. The minimal re-application interval 

is seven days. The current labeled use of 

Kasumin allows for two applications or 128 fl 

oz of product per season with a label change 

for up to four (256 fl oz) per season planned 

later this year. Still, only two consecutive 

applications will be allowed without rotating 

to other modes of action. Alternate row 

applications, applications in orchards that are 

being fertilized with animal waste/manure, 

or animal grazing in orchards treated with 

Kasumin are not allowed. The first restriction 

is to prevent selection of resistant isolates 

of the target pathogen, Xaj; whereas, the 

latter two restrictions are to ensure that the 

selection of non-target, human-pathogen 

bacteria is prevented. 

For walnut blight management, the best 

way to use the bactericide is in combination 

with mancozeb or copper. Application 

management strategies for a four- or fivespray 

mixture, rotation program include, but 

are not limited to, the following: 

A) Copper/mancozeb—kasugamycin/ 

mancozeb—kasugamycin/copper—copper/ 

mancozeb 

B) Copper/mancozeb—kasugamycin/ 

mancozeb—copper/mancozeb— 

kasugamycin/copper— copper/mancozeb 




13


How do kasugamycin treatments compare to coppermancozeb 

treatments in managing disease? The research 

used to develop kasugamycin was based on a 7- to 10-day 

re-application interval. The reason for this was that Kasumin 

is only locally systemic or translaminar and thus, is less likely 

to be re-distributed. With new growth increasing the canopy 

volume weekly in the spring as walnut trees come out of 

dormancy, multiple and frequent applications are necessary. 

Kasugamycin-mancozeb mixtures applied in our research 

trials were often the most effective of all treatments evaluated. 

In general, bactericides have a short residual life of a few 

days to a week or two. In toxicology in-vitro testing, Xaj is 

only moderately sensitive to kasugamycin with a mid-range 

to high minimum inhibitory concentration (MIC) value. 

When kasugamycin is mixed with mancozeb, the MIC of 

the mixture is approximately 5 parts per million (ppm). 

Figure 2. Radial streaks of 16 isolates of Xaj on each plate exposed to different toxicants. Right image: Copper 50 ppm (fixed concentration). Spiral 

gradient plates with highest concentration towards the center and lowest concentration at the edge of the plate. Middle image: Kasugamycin (gradient 

range 0.5 to 64.9 ppm); and Left image: Kasugamycin+mancozeb (concentration gradients). Lack of growth towards the center of each plate indicates 

inhibition. No inhibition for copper at 50 ppm whereas inhibition concentrations averaged 20 and 5 ppm for kasugamycin and the kasugamycinmancozeb 

mixture, respectively. 

PUBLICATION 

Radial streaks of 16 isolates of Xaj on 

each plate exposed to different 

toxicants. Top image: Copper 50 ppm 

(fixed concentration). Spiral gradient 

plates with highest concentration 

towrds the center and lowest 

concentration at the edge of the 

plate. Middle image: Kasugamycin 

(gradient range 0.5 to 64.9 ppm); and 

Bottom image: Kasugamycin + 

mancozeb (concentration gradients). 

Lack of growth towards the center of 

each plate indicates inhibition. No 

inhibition for copper at 50 ppm 

whereas inhibition concentrations 

averaged 20 and 5 ppm for 

kasugamycin and the kasugamycinmancozeb 

mixture, respectively. 

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Kasugamycin is applied at 64 fl oz per 

100 gal or 100 ppm. Thus, the labeled 

rate is kasugamycin-mancozeb mixtures 

are approximately 20X of the MIC value 

for Xaj. Because of the short residual 

activity and a moderate buffering residue 

(20X), the rotation needs of bactericide 

mixtures containing kasugamycin 

described above need to be applied in 7- 

to 10-day intervals (Figure 3). 

Kasugamycin and Resistance 

Resistance is a relative term indicating 

a change in sensitivity to an inhibitory 

compound. A moderately high MIC for 

a bactericide does not mean that the 

pathogen is resistant. We have conducted 

baseline studies with kasugamycin, 

kasugamycin-copper, and kasugamycinmancozeb 

for Xaj with MIC values of 

20, 8.3, 5.3 ppm, respectively (Figure 2, 

see page 14). This was done before the 

bactericide was registered in California 

to determine any change in sensitivity 

after registration and commercial usage. 

To date, resistance has not been found 

and isolates evaluated are all within 

the baseline distributions. Still, with a 

single site mode of action compound 

such as kasugamycin, there is a risk for 

selecting resistant sub-populations of 

the pathogen especially when resistance 

management strategies are not employed. 

This is the reason why we developed the 

mixture-rotation programs suggested 

above. 

Conclusions 

The integration of bactericides with 

different modes of action and application 

strategies of rotations of mixtures of 

bactericides with different modes of 

action with forecasting tools such as 

XanthoCast (http://www.agtelemetry. 

com/) should provide the stewardship 

necessary for having the tools available 

for managing walnut blight for years 

to come. The hope with the Kasumin 

registration is to provide resistance 

management and prevent or reduce the 

risk of resistance to copper-mancozeb 

while new approaches can be developed 

and integrated to protect all of these 

compounds. Walnut blight is the most 

serious disease impacting growers 

in California and multiple tools like 

kasugamycin, copper, and mancozeb 

need to be available to maintain a 

successful industry. 




Treatment (Rate of ai oz/A) 

Control 

ATD - 12.9 

Kasugamycin - 1.3 

Kasugamycin + ATD - 1.3 + 12.9 

Copper + Mancozeb - 24 + 28.4 

Kasugamycin +Copper - 1.3 + 24 

Kasugamycin + Mancozeb - 1.3 + 28.4 

Copper - 24 

0 

d 

d 

cd 

bc 

bc 

b 

bc 

2 4 6 8 10 

Disease incidence (%) 

a 

Figure 3. Efficacy of 

treatments for managing 

walnut blight. 

Treatments applied 

using an air-blast 

sprayer (100 gal/A). The 

walnut blight pathogen 

was sensitive to copper. 

Disease incidence is 

the number of diseased 

nuts per 100 nuts 

evaluated. ATD = amino 

thiadiazole, an experimental 

bactericide. Bars 

followed by the same 

letter are not significantly 

different. 

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15

Yellow sticky traps work well for determining the number of 

seed corn maggot flies and onion maggot flies in onion fields. 

All photos courtesy of Rob Wilson. 

Moving Toward Alternatives 

to Chlorpyrifos for Managing 

Maggots in Onions 

By: Rob Wilson | UC ANR Intermountain Research and Extension 

Center Director & Farm Advisor 

Chlorpyrifos, an organophosphate 

pesticide used to kill of number 

of insect pests, has been the go-to 

insecticide for managing maggots in 

California onions. For many years, 

chlorpyrifos applied in-furrow at planting 

provided effective control of maggots 

in fresh market and dehy onions. 

Unfortunately, serious environmental 

concerns associated with chlorpyrifos 

and the potential for insecticide resistance 

is forcing the onion industry to 

find alternative insecticides. Starting on 

January 1, 2019, the California Department 

of Pesticide Regulation (DPR) 

has recommended County Agricultural 

Commissioners implement interim re- 

strictions on chlorpyrifos use. Detailed 

information on the interim restrictions 

and regulations can be found on 

the DPR website at https://www.cdpr. 

ca.gov/docs/pressrls/2018/111518.htm. 

DPR is completing a formal regulatory 

process to list chlorpyrifos as a toxic air 

contaminant with permanent restrictions, 

but until the regulatory process 

is complete, interim restrictions are in 

place. 

Onion and Seed Corn Maggot 

Both onion maggot, Delia antiqua, and 

seed corn maggot, Delia platura, are 

problem pests in California onions. 

Larvae of both species feed on young 

onion plants, often resulting in 

seedling mortality and onion stand 

reduction by more than 50 percent 

of desired seeding rate. Seed corn 

maggot flies lay their eggs in fields in 

spring and larvae live in the soil and 

feed on seeds and developing plants 

of several crops. Onion maggot flies 

lay their eggs in young onion fields 

and larvae live in the soil feeding on 

onions belowground. Onion maggots 

are specific to onions and other 

allium crops. Preventative farming 

practices such as late plantings, 

increasing seeding rates, avoiding 

tilling manures and crop residues 

shortly before planting, and removing 

onion culls can prevent damage from 

maggots, but preventative measures 

alone are not always effective, 

especially in fields with heavy maggot 

pressure. 

Insecticide applied at planting 

provides the most consistent 

control of maggots. The key to 

effective insecticide use for maggot 

control is applying the insecticide 

prophylactically at the time of 

planting. Growers shouldn’t wait to 

apply insecticides until maggot larvae 

are found as maggot larvae feed on 

seeds, germinating plants, and young 

onions. This feeding results in rapid 

plant mortality, thus insecticide 

application after planting is rarely 

effective. 

Research 

Research studies at the University of 

California (UC) UC Intermountain 


Differences in onion stand caused by maggot feeding for various insecticide treatments in 

IREC research plots. The plot with few onions is the untreated control. 




17


Research and Extension Center (IREC) 

in Tulelake, California are investigating 

insecticides and insecticide application 

methods to find alternatives to 

chlorpyrifos. This research effort has 

received admirable support from the 

California Garlic and Onion Research 

Advisory Board with the hopes of 

pro-actively finding alternatives before 

chlorpyrifos restrictions. Insecticide 

applications at planting can be made 

in-furrow, broadcast, or as a seed 

treatment. IREC research has shown 

the efficacy of different application 

methods is dependent on insecticide 

choice. For example, spinosad was 

only effective when applied as a 

seed treatment (Regard) as spinosad 

(Entrust) applied in-furrow at planting 

had similar onion stands compared to 

the untreated control. 

Insecticide Alternatives to 

Chlorpyrifos 

Seed treatment with spinosad (FarMore 

OI100 and FI500) or clothianidin 

(Sepresto) provided similar or better 

suppression of maggot compared to 

chlorpyrifos at the maximum labeled 

rate in-furrow. This trend was true 

multiple years in multiple studies 

conducted at IREC. Outside the 

research world, multiple Tulelake onion 

growers had success using spinosad or 

clothianidin seed treatments instead 

of chlorpyrifos in 2017 and 2018. 

Applying chlorpyrifos in combination 

with spinosad and clothianidin seed 

treatment did not increase onion 

stands compared to using either 

seed treatment alone. Cyromazine 

(Trigard) seed treatment is another 

alternative. Trigard provided similar 

maggot suppression compared to 

FarMore FI500 and Sepresto in 2018, 

and it is seed treatment used in other 

parts of the United States for maggot 

control. Bifenthrin was the only tested 

insecticide applied in-furrow that 

provided similar efficacy compared to 

chlorpyrifos. Unfortunately, bifenthrin 

is not currently labeled for use in 

California onions. 

Other Considerations 

Fungicides used in combination 

with insecticide seed treatment may 

influence onion stands. In 2018, 

Sepresto (insecticide) + FarMore F300 

(fungicide) + pro-gro (fungicide) 

resulted in the highest onion stands in a 

field with a combination of maggots and 

smut. In fields without smut, thiram 

and FarMore F300 fungicide package 

gave good early season disease control 

in IREC studies. 

Yellow sticky traps placed along field 

edges can offer growers an early 

warning for potential maggot problems 

(see picture). Seed corn maggot and 

onion maggot flies are readily captured 

on sticky traps and the traps (changed 

once a week) provide growers an 

indication of the number of flies during 

onion establishment. Tillage of green 

plants, plant residues, and manures 

attract thousands of egg-laying female 

flies and crop damage is often severe 

when crops are planted within the first 

Early season onion stand differences 

caused by maggot feeding. 

Seed corn maggot larvae feeding on 

onion seedling. 

few weeks of tillage. Cool, wet weather 

and delayed plant emergence are other 

factors that promote crop damage from 

seed corn maggot. First generation 

maggots are most problematic as their 

feeding kills seedling plants, but later 

generation maggots can feed on plants 

and bulbs in the summer. Damage from 

later generation onion maggot is rarely 

economically important in California 

except for fields with diseased, decaying 

onion bulbs making mid-season and 

late season disease control important to 

prevent late season maggot problems. 




Hand-harvesting onions to determine 

yield differences. 

Late season onion maggot feeding on an 

onion bulb. 




19

Santa Rosa 

Petaluma 

Redding 

Napa 

Yuba City 

Sacramento 

Vallejo 

Antioch Stockton 

San Francisco 

Hayward 

San Jose 

Tracy 

Santa Cruz 

Watsonville 

Salinas 

The Carbohydrate 

Observatory: A Citizen 

Science Research Project 

Reno 

Fresno 

St. George 

Salt Lake City 

Understanding seasonal trends of starch and 

sugar in walnut, pistachio and almond under 

varying climatic conditions. 

By: Anna Davidson | Postdoc, Manager of the Carbohydrate 

Observatory and Maciej Zwieniecki | Professor, Founder of the 


Provo 

Las Vegas 

Topo 

Satellite 

Monterey Soledad 

Paso Robles 

Streets 

San Luis 

Obispo 

Santa Maria 

Lompoc 

Santa Barbara 

rsfield 

Lancaster 

Palmdale 

Victorville 

Kingman 

Lake Havasu 

Almond Orchards 

Walnut Orchards 

Pistachio Orchards 

Pecan Orchards 

Figure 1. Map of the California Central Valley with all participating almond (red), pistachio (green) and walnut (blue) orchards. 

Background 

One can consider that the currency 

of nut trees are non-structural 

carbohydrates (NSCs), meaning 

sugar and starch. Carbohydrates provide 

the energy for growth, defense, and 

healthy flowering, ultimately resulting 

in yield. Soluble carbohydrates or sugar, 

can be considered as the cash that flows 

around the tree assuring growth and 

paying for services like defense from 

pests, frost, or mineral uptake. Starch is 

the form of currency that can be considered 

as the savings account, which is 

stored in the wood and roots of the tree 

during dormancy to be used in the following 

spring. Seasonal trends of sugars 

and starches are highly dynamic and 

can fluctuate with species, variety, tree 

age, temperature, climate, and management 

practices. 

The physiological changes in terms 

of carbohydrate dynamics a tree 

undergoes in preparation for dormancy 

especially in warmer climates, is still 

poorly understood. This is especially 

true due to the negative effects resulting 

from climate variability including 

the decrease in winter fog, chilling 

hours, winter drought, and an increase 

in annual temperatures. To better 

understand the seasonal fluctuations 

of carbohydrates, we decided to take 

an accelerated approach to do research 

in trees. Instead of having a single 

study with few variables, we use the 

entire Central Valley as our research 

laboratory. To accomplish such a task, 

we take a citizen scientist approach with 

the help from growers, farm advisors, 

and commodity boards. From ~450 

sites around the state, walnut, almond, 

and pistachio growers send us monthly 

samples of twigs and bark based on a 

very simple and fast protocol. Monthly 

samples allow us to track the seasonal 

trends over several years so that we may 

make well-informed decisions on the 

timing and nature of our management 

practices. 

Protocol 

Growers simply clip one twig from 

three representative trees, about four 

inches at the base of the current season’s 

growth, remove the bark and drop the 

three sticks and bark in an envelope and 

mail it to us with information including 

the name of the site, date, species and 

variety, orchard age, and latitude and 

longitude. Once the samples reach us 

through the mail, we dry them, grind 

them, weigh them, and perform a 

chemical analysis in the laboratory 

to determine the amount of sugar 

and starch of each sample. We then 

upload all results to our web-based 

interactive map (Figure 1) (https:// 

mzwienie.shinyapps.io/Shiny_test/) and 

data analysis tool (https://zlab-carbobservatory.herokuapp.com/) 

where 

growers can access their data in real 

time and follow their own trends of 

starch and sugar in each orchard they 

sample. One can also compare multiple 

orchards at a time. All analyses are free 

of charge to participants. 

Results 

Figure 2 (see page 22) shows 

the 2016/17 seasonal trends of 

carbohydrates in in walnuts, almonds 

and pistachios. Higher levels of 





21

NSC_wood 

300 

200 

100 

2016 winter level 

2017 boom 

Carbohydrate recovery 

2017 pre dormancy level 

species 

almond 

pistachio 

walnut 

Figure 2. Seasonal 

patterns of soluble 

sugars and starch 

concentration in 

three major tree 

crop species walnut, 

almond and 

pistachio during 

the 2016-2017 

seasons. Each 

data point represents 

a single 

orchard. Lines are 

running average 

content of the 

total carbohydrate 

concentration in 

wood. 

0 

300 

400 

Julian_Day 

500 

600 

Figure 3. Season 

dynamics and 

relative content of 

soluble sugars and 

starch in wood, 

bark, and total in 

walnut twigs. Despite 

high variation 

in total content 

ration between 

sugars and starch 

remains relatively 

constant throughout 

the year. 

Concentration [mg/g DW] 

Relative Content 

300 

200 

150 

100 

50 

0 

1.0 

0.8 

0.6 

0.4 

Wood 

NSCs 

Starch 

2016 2017 

Soluble carbohydrates 

Starch 

Bark 

Twig 

2016 2017 2016 2017 

0.2 

0.0 

2016 2017 

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct 

2016 2017 2016 2017 

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct 


carbohydrate in the winters of 2016 

and 2017 provide the carbohydrates 

or energy to support spring bloom. 

During summer, all species reduced 

reserves to low levels reflecting a 

demand for carbohydrates to support 

yield and tree growth that exceeds or 

is equal to photosynthetic supply. In 

the fall, carbohydrate levels recover 

and accumulate reserves going into 

dormancy and ultimately for the 

following spring. Interestingly, walnut 

shows symptoms of very late recovery 

underlying the need for the postharvest 

management even in October. 

Pistachio (green) accumulated almost 

twice as much carbohydrate in the fall 

of 2017 compared to 2016 potentially 

reflecting its strong alternating crop 

behavior—2016 was considered an OFF 

year, 2017 was an ON year, and 2018 

was an OFF year, potentially supported 

by an increased accumulation of NSCs. 

We also found that from preliminary 

analyses of data from the Carbohydrate 

Observatory that starch to soluble 

sugars ratio is relatively constant during 

a year (Figure 3). 

The citizen science approach allows us 

to look across multiple variables like 

climate, tree age, rootstock, yield, etc. 

Initial looks at the accumulation of 

starch and sugar versus tree age revealed 

that older trees tend to accumulate 

much higher levels of carbohydrates in 

twigs potentially reflecting their reduced 

relative growth in relation to leaf 

biomass and increase of yield potential 

(tree investment in reproduction). This 

information also allows for assessing 

the goal of carbohydrate accumulation 

during post-harvest management while 

preparing trees for dormancy within 

each age group. 



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rely on 

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It supports overall plant health, yet is easily lost. N-Sure ® is a slow 

release liquid fertilizer that can be applied as a foliar spray and 

delivers nitrogen for up to 10 weeks in the soil. The slow release 

formulation makes N-Sure ® a convenient and safe 

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Crop Vitality Specialists can provide assistance regarding 

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Learn more about N-Sure ® on cropvitality.com 

Start a Conversation today with Your Crop Vitality Specialist 

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23

Wood 

Bark 

Total 

NSCs concentration 

300 

250 

200 

150 

100 

50 

0 

2016 

2016 

2016 

Figure 4. Relationship 

between tree 

age (planting time) 

and 2017 carbohydrate 

content in 

twigs during winter 

months and during 

summer. Winter 

content of NSCs 

was much higher 

in older trees then 

in young trees 

however this difference 

was not as 

pronounced during 

summer. 

Year of planting Year of planting Year of planting 


Our newest data analysis tool (http:// 

zlab-carb-observatory.herokuapp.com; 

Figure 5) allows anyone to compare 

different orchards of any of the three 

nut species with any combination of 

differences. For example, Figure 5 

compares two different walnut orchards, 

one old dry farmed-dying orchard 

(in green) located in Paso Robles and 

one young organic irrigated orchard 

in Winters. The old dry farm has no 

reserves in the summer and is using 

everything it possibly can to survive. It 

likely puts little effort into making new 

vegetative growth. 

Conclusions 

In the future, we hope to look further 

into how starch and sugar content relate 

to variety, climate variations, yield, and 

management practices. We need more 

data and more participation by growers 

to find the answers to these questions. 

Please consider joining our long-term 

study! 

Carbohydrates [mg/g DW] 

400 

350 

300 

250 

200 

150 

100 

50 

0 

Jul 2016 Jan 2017 Jul 2017 Jan 2018 

Date 

Jul 2018 

All farms 

Old Dry Farmed Walnuts 

Young Irrigated Walnuts 

Figure 5. Comparison 

of two 

different walnut 

orchards, one 

old dying orchard 

(in green) 

located in Paso 

Robles and one 

young organic 

irrigated orchard 

in Winters. 

Interested in Participating? 

Please contact Anna Davidson by email 

adavidson@ucdavis.edu or by phone 

(815) 212-4409. 

Please go to our website to access more 

information, our protocol, map and 

data analysis tools. 

http://www.plantsciences.ucdavis.edu/ 

plantsciences_faculty/zwieniecki/CR/ 

cr.html 


to hear from you. Feel free to email us at 

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www.wrtag.com25

Climate Change and 

California Agriculture 

By: Tapan B. Pathak | Division of Agriculture and 

Natural Resources, University of California, Merced, 

CA, Mahesh L. Maskey, Jeffery A. Dahlberg, Khaled M. 

Bali, Daniele Zaccaria | University of California, Division 

of Agriculture and Natural Resources (UCANR), Faith 

Kearns | Division of Agriculture and Natural Resources 

California Institute for Water Resources, University of 

California, Oakland, CA 

Introduction 

California is the largest and most 

diverse agricultural state that 

produces over a third of the 

country's vegetables and two-thirds of 

fruits and nuts. Among 400 different 

commodities, California is a leading 

producer of many of those commodities 

[1]. At the same time, changing climate 

signals such as temperatures, precipitation 

patterns, extreme calamities and 

water availability poses many challenges 

to the state’s agricultural sector, which 

would not only translate into national 

food security issues but also economic 

impacts that could disrupt state and 

national commodities systems. 

Recent publication by Pathak et al. 

[2] outlined a detailed literature 

review to document the most current 

understanding on California´s climate 

trends in terms of temperature, 

precipitation, snowpack, and extreme 

events such as heat waves, drought, and 

flooding, and their relative impacts on 

the state’s highly productive and diverse 

agricultural sector. For instance, both 

average and extreme temperatures 

and precipitation patterns influence 

crop yields, pest, and the length of the 

growing season. Because of climate 

change, extreme events, such as heat 

waves, floods, and droughts, may lead 

to larger production losses, earlier 

spring arrival, and warmer winters 

due to temperature increases allow 

disease and pest pressure increase, 

shrinking snowpack leading to a greater 

risk for agriculture water availability. 

This paper summarizes key findings 

from that paper in the context of: (a) 

historic trends and projected changes 

in temperature, precipitation and heat 

waves; (b) current situation and future 

expectations of drought and flood; and 

(c) consequent impacts on agriculture. 

Climate change trends in 

California 

The climate of California varies from 

hot desert to subarctic environments 

lying in the proximity to the Pacific 

Coast with Mediterranean climate 

influenced by the cold ocean currents 

and offshore winds. This section reviews 

historical and projected trends of key 

climatic parameters: temperature, 

precipitation, and extreme events like 

heat waves, drought and floods. 

Temperature 

Climate change in California is often 

exemplified by increased temperature in 

both space and time. While temperature 

has globally increased by 1.4°F since 

Degrees (F) 

2.0 

0.0 

-2.0 

1880, the rate of increase in minimum 

temperature is greater than that of 

mean or maximum temperatures. 

Figure 1 shows that observed mean 

temperature in the state has been 

increasing from 1.3 to 2.3 °F in the past 

century [3]. The recent severe drought 

in the state has been exaggerated by 

increased temperature and insignificant 

precipitation. As seen in Table 1 (see 

page 27), summer after spring happens 

with the largest warming trends and 

least warming is experienced during 

winter and spring after implying 

inter-annual variability for the period 

from 1895 to 2018 [4]. However, least 

warming during fall and winter has 

been reported for the period 1895-2010. 

Spatial variation of temperature also 

reveals that Southern part of the state 

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 

Figure 1: California statewide mean temperature departure in (°F), October through September 

[3]. Black line denotes 11-year running mean. 


experiences greater warming than 

compared to Northern California. 

Outcome of different climate models 

reveals that there will be more 

pronounced increment in summer 

temperature than in winter resulting 

into more warming in inland areas 

than in coastal regions. The increasing 

temperature trends will decrease the 

snow-to-precipitation ratio [5]. The 

state’s Sierra snowpack starts to melt 

earlier and faster than usual due to 

frequent heat spells [6]. For instance, 

future trends for far north and far 

south of the state are presented in 

Figure 2 (see page 28) which suggests 

how temperatures will continue to 

increase until 2100 under different 

emission scenarios from different 

climate models. Precipitation is a 

crucial parameter to state’s water 

supply for the agricultural purposes 

and such parameter exhibits 

significant inter-annual variability 

[3]. The variability of precipitation in 

California is a unique phenomenon 

implying that such unpredictability 

is more notable in California than 

Table 1: California temperature departures, 1895-2018, for mean, max and min linear trends in °C 

within the period shown [4]. 

Linear departures Annual Winter Spring Summer Fall 

A. Mean 

Temperature 

1895-2018 +1.88 

1949-2018 

1975-2018 

B. Max temperature 

1895-2018 

1949-2018 

1975-2018 

C. Min temperature 

1895-2018 

1949-2018 

1975-2018 

+2.03 

+1.32 

+1.58 

+1.18 

+0.79 

+2.18 

+2.87 

+1.85 

+1.55 

+4.74 

+2.12 

+1.68 

+4.74 

+2.39 

+1.42 

+4.74 

+1.85 

+1.20 

+0.46 

+0.89 

+0.80 

+0.29 

+2.39 

+1.60 

+1.21 

+1.72 

+3.07 

+2.02 

+1.59 

+2.82 

+0.86 

+0.75 

+3.31 

+3.18 

+2.43 

+1.70 

+0.89 

+0.68 

+1.02 

+0.59 

+0.04 

+2.39 

+2.36 

+1.40 

other parts of the country. Having Mediterranean climate, most of the rainfall 

happens during cool season (October to April). Despite having unnoticeable 

temporal pattern, study shows total annual precipitation has increased at an average 

rate of 5 millimeter (mm) per decade for the contiguous 48 states. In addition, 

there is increase in extreme single day precipitation events and increased temporal 


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Maximum Temperature ( F) 

Maximum Temperature ( F) 

instance, future trends for far north and far south of the state are presented in Figure 2 which 

suggests how temperatures will continue to increase until 2100 under different emission scenarios 

from different climate models. 

86 

84 

82 

80 

78 

76 

74 

72 

86 

84 

82 

80 

78 

76 

74 

72 

1960 1980 2000 2020 2040 2060 2080 

94 

92 

90 

88 

86 

84 

82 

80 

78 

76 

74 

72 

1960 1980 2000 2020 2040 2060 2080 

94 

92 

90 

88 

86 

84 

82 

80 

78 

76 

74 

72 

1960 1980 2000 2020 2040 2060 2080 1960 1980 2000 2020 2040 2060 2080 

Figure 2: California 

historical 

and projected 

maximum temperature 

for the 

period 1950-2099 

for Shasta (top), 

and Los Angeles 

County (bottom). 

Left: projection 

based on RCP4.5 

and Right: RCP8.5 

[7]. These annual 

values are from 

four climate models: 

HadGEM2-ES 

(red), CNRM-CM5 

(cyan), CanESM2 

(dark yellow) 

and MIROC5 

(magenta). 

Figure 2: California historical and projected maximum temperature for the period 1950-2099 for 

Shasta (top), and Los Angeles County (bottom). Left: projection based on RCP4.5 and Right: RCP8.5 

[7]. These annual values from four climate models: HadGEM2-ES (red), CNRM-CM5 (cyan), 

CanESM2 (dark yellow) and MIROC5 (magenta). 

Year 

Region Name 

3.2. Precipitation 1900 1925 1950 1975 2000 2018 

North Precipitation Coast is a crucial 1664 parameter 1693 to state’s water 2168 supply for 1925 the agricultural 1509purposes 1427 and 

such North parameter Central exhibits 1132 significant inter-annual 1114 variability 1525 [3]. The 1294 variability 1156 of precipitation 912in 

California Northeastis a unique phenomenon 492 implying 478 that such 655 unpredictability 519 is more 491 notable in California 493 

than other parts of the country. Having Mediterranean climate, most of the rainfall happens during 

Sierra 

1049 956 1667 1140 1159 958 

cool season (October to April). Despite having unnoticeable temporal pattern, study shows total 

Sacramento-Delta 438 420 626 448 567 459 

annual precipitation has increased at an average rate of 5 millimeter (mm) per decade for the 

Central Coast 531 522 750 526 753 549 

contiguous 48 states. In addition, there is with increase in extreme single day precipitation events 

and San increased Joaquin Valley temporal 293 precipitation 256 variability as 296 observed in Table 246 2. 354 282 

South Coast 333 290 246 293 376 297 

South Table Interior 2: California annual 383 (calendar 376 year) precipitation 271 every 25 years 345except 2018383 

starting from 345 1900 

in mm in 11 climatological regions [4] 

Mohave Desert 127 158 95 101 116 

Sonoran Desert 61 Year 115 38 53 51 

Region Name 

1900 1925 1950 1975 2000 2018 

North Coast 1664 1693 2168 1925 1509 1427 

North Central 1132 1114 1525 1294 1156 912 

Northeast 492 478 655 519 491 493 


Sierra 1049 956 1667 1140 1159 958 

Sacramento-Delta 438 420 626 448 567 459 

precipitation variability as observed in 

Table 2. 

Projections depicted from global 

climate model surmise that the nature 

of state’s precipitation will probably 

change with more intense atmospheric 

rivers and longer dry spells between 

them. The simulations from general 

circulation models (GCM) suggest 

that California will maintain its 

Mediterranean climate with relatively 

cool and wet winters and hot dry 

summers. As expected, insignificant 

linear trends at a 95 percent confidence 


interval is observed in Figure 3 (see 

page 29) implying that that variability 

of annual precipitation will continue 

during the next century and that the 

state will be vulnerable to both drought 

and flood. 

Extreme Heat Waves 

California is expected to have increased 

intensity and frequency of heat waves. 

As an illustration, Figure 4 (see 

page 30) shows how heat events are 

increasing and will continue under 

different emission scenarios for Kern 

County [7]. As seen, the number of 

such events will be increased more in 

129 

95 

Table 2: California 

annual (calendar 

year) precipitation 

every 25 years except 

2018 starting 

from 1900 in mm 

in 11 climatological 

regions [4]. 

magnitude under RCP 8.5 than under 

RCP 4.5. Several studies show that if 

the night temperature sustained above 

normal, the plants keeps metabolizing 

starch as photosynthetic sugars may 

be used in the rapid plant growth, 

consequently resulting into reduced dry 

matter. Such may cause considerable 

damage to crop and yields depending 

on whether heat waves occur during the 

critical crop growth stage [8]. 

Drought and Flood 

The effect of ongoing drought is not 

only limited to agricultural crop 

production but also risk of wild fires

Mediterranean climate with relatively cool and wet winters and hot dry summers. As expected, 

insignificant linear trends at a 95 percent confidence interval is observed in Figure 3 implying that 

that variability of annual precipitation will continue during the next century and that the state will 

Figure 3: Historical and projected precipitation trend for the Sacramento. Left: projection based on RCP4.5 and Right: RCP8.5 [7]. These annual 

values from be vulnerable four climate to models: both HadGEM2-ES drought and (red), flood. CNRM-CM5 (cyan), CanESM2 (dark yellow) and MIROC5 (magenta). 

Precipitation (inches/year) 

Emissions peak around 2040, then decline (RCP 4.5) Emissions continue to rise strongly through 2050 

and plateau around 2100(RCP 8.5) 

Range of annual average values from all 32 Modeled Data (2006-2099) Modeled Data (2006-2099) 

LOCA downscaled climate models HadGEM2-ES 

Range of annual average values from all 32 

Modeled Variability Envelope CNRM-CM5 

LOCA downscaled climate models HadGEM2-ES 

Observed Data (1950-2005) CanESM2 

Modeled Variability Envelope CNRM-CM5 

MIROC5 

Observed Data (1950-2005) 

CanESM2 

MIROC5 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

Precipitation (inches/year) 

5 

1960 1980 2000 2020 2040 2060 2080 1960 1980 2000 2020 2040 2060 2080 

that dries out vegetation Figure 3: Historical due declining and projected ground precipitation water trend more for flooding the Sacramento. events Left: and projection further influence based on spring streamflow 

levels. Decision RCP4.5 makers and are Right: aware RCP8.5 of such [7]. These measure annual values [10]. from For four instance, climate models: in the HadGEM2-ES Sacramento River (red), basin experience 

referring popular CNRM-CM5 drought index (cyan), called CanESM2 the (dark Palmer yellow) and MIROC5 earlier peak (magenta). runoff time after last 40 decades of last century[11] 

Drought Severity Index (PDSI) [9]. The historical reducing the ability to refill reservoirs after the flood season is 

variability of the Extreme PDSI tells Heat us Waves the state’s most severe over and indicating a flood-drought pattern within the state 

drought conditions that happened during the 2013-14 which may potentially invite frequent flood events in California. 

winter over the last California 122 years of is expected record. to have increased intensity Figure 5 and frequency Table 3 (see of heat page waves. 31) illustrate As an how the monthly 

illustration, Figure 4 shows how heat events are stream increasing flow peaks and will continue be shifted under by the different end of the century for 

Warmer environments emission also scenarios possibly for increase Kern County both [7]. winter As seen, the number of such events will be increased more in 

flooding and summer magnitude water under deficits RCP and 8.5 there than under will be 


RCP 4.5. Several studies show that if the night temperature 

sustained above normal, the plants keeps metabolizing starch as photosynthetic sugars may be used 

in the rapid plant growth, consequently resulting into reduced dry matter. Such may cause 

Control Pocket Gophers in Tree Fruit & Nuts 

considerable damage to crop and yields depending on whether heat waves occur during the critical 

crop growth stage [8]. 

55 

50 

45 

40 

35 

30 

25 

20 

15 

10 

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29


American River. As observed, while the peak happened in 

the month of May over the last 90 years, such will be shifted 

as earlier as January. 

Key impact on California Agriculture 

Global crop production needs to be double by 2050 to meet 

the projected demand for food from rising population, 

diet shifts, and increasing biofuels consumption [12]. 

Increased temperature due to climate change can intensify 

this challenge. As defined in Table 4 (see page 31), 

individual crops have specific optimum temperature ranges 

for optimum production and exposure to extremely high 

temperatures during these growth stages can affect growth 

and yield. 

Figure 6 (see page 32) reveals the impacts of climate 

change on crop yields for different field crops such as 

alfalfa, cotton, maize, winter wheat, tomato, rice, and 

sunflower in Yolo County and throughout the Central 

Valley assessed through a process-based crop model 

[14-15]. Alfalfa yields were predicted to increase under 

climate change, while yields from tomato and rice remain 

unaffected. Overall, 4°C increase in temperature may 

reduce yields from most fruits by more than 5 percent, 

which may reach up to 40 percent in some important 

regions [16]. However, these modeling projections did not 

take into account technological trends, water stress, and 

other management practices. Therefore, the yield numbers 

55 

50 

45 

40 

35 

30 

25 

20 

15 

10 

80 

70 

60 

50 

Warm Days 

< Historical (1950-2005) Future (2006-2099) > 

5 

0 1960 1980 2000 

Historical (1950-2005) Future (2006-2099) 

Observed historical and Modeled future projections 

modeled historical data 

Warm Nights 


Figure 4: Historical and projected Heat days and Warm nights for K 

based on RCP4.5 and Bottom: RCP8.5 [7]. 


15 

The effect of ongoing drought is not only limited to agricultu 

10 

wild 

5 

fires that dries out vegetation due to declining ground water 

0 

1960 1980 2000 2020 2040 2060 2080 2100 

of such measure referring popular drought index called the Palm 

[9]. 90 The historical variability of the PDSI tells us the state’s most s 


80 



happened during the 2013-14 winter over the last 122 years of rec 

40 

Warmer environments also possibly increase both winter flo 

30 

40 

and 30 

20 

there will be more flooding events and further influence spri 

20 

10 

the 10 Sacramento River basin experience earlier peak runoff time af 

0 

0 

1960 1980 2000 2020 2040 2060 2080 2100 [11] 1960 reducing 1980 the 2000 ability 2020to refill 2040 reservoirs 2060 2080after 2100 the flood season i 

Year 

Year 

flood-drought pattern within the state which may potentially inv 

Observed (1950-2005) HadGEM2-ES (Warm/Drier) CNRM-CM5 (Cooler/Wetter) CanESM2 (Average) MIROC5 (Complement) Observed (1950-2005) HadGEM2-ES (Warm/Drier) CNRM-CM5 (Cooler/Wetter) CanESM2 (Average) MIROC5 (Complement) 

Figure California. 4: Historical Figure and projected 5 and Heat Table days 3 illustrate and Warm nights how the for Kern monthly stream 

Figure 4: Historical and projected Heat days and County. Warm end Top: of nights projection the century for Kern based County. for on American RCP4.5 Top: and projection River. Bottom: As RCP8.5 observed, [7]. while the pe 

based on RCP4.5 and Bottom: RCP8.5 [7]. 

over the last 90 year, such will be shifted as earlier as January. 



55 

50 

45 

40 

35 

30 

25 

20 

15 

10 

80 

70 

60 

50 

40 

30 

20 

10 

0 

55 

50 

45 

40 

35 

30 

25 

20 

2020 2040 2060 2080 2100 

100 

can swing, depending on the type of stress and adaptation 

measures implemented. 

Many fruit and nut crops require cold temperatures in 

winter to break dormancy, which defines a location’s 

suitability for the production of many tree crops. These 

fruit and nut species adapted to temperate or cool 

subtropical climates where chilling each winter is needed 

to achieve homogeneous and simultaneous flowering and 

steady crop yields. The lack of adequate chilling hours 

as projected by climate change can delay pollination and 

foliation that reduces fruit yield and quality. Climate 

change may have impact on the incidence and severity 

of plant pests and disease and influence the further coevolution 

of plants and their pathogens. Moreover, plant 

diseases could be used as indicators of climate change in 

which the environment can move from disease-suppressive 

to disease-conducive or vice versa. 

Conclusions and Future Recommendations 

A range of studies on trends and impacts of climate change 

on California agriculture presented in this paper justifies 

70 

60 

50 

Warm Days 


5 

0 1960 1980 2000 

1960 




1980 2000 2020 

Year 


55 < Historical (195 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

2020 2040 2060 2080 2100 1960 1980 

100 

90 

80 

Observed 

70 

60 

50 

40 

30 

20 

10 

2040 2060 2080 5 of 2100 8 

0 

1960 1980 

Observed (1950-2005) HadGEM2-ES (Warm/Drier) CNRM-CM5 (Cooler/Wetter) CanESM2 (Average) MIROC5 (Complement) Observed (1950-2005) HadGEM2-ES 

Observed HadGEM2-ES (Warmer/Drier) CNRM-CM5 (Cooler/Wetter) 

CanESM2 (Average) CanESM2 (Average) 

9,000 10,000 

Historical 

modeled h

e last 122 years of record. 

over the last 90 year, such will be shifted as earlier as January. 

rease both winter flooding and summer water deficits 

Observed HadGEM2-ES (Warmer/Drier) CNRM-CM5 (Cooler/Wetter) 

CanESM2 (Average) CanESM2 (Average) 

urther influence spring streamflow [10]. For instance, in 

9,000 

10,000 

r peak runoff time after last 40 decades of last century Observed Data 

8,000 

1922-2014 9,000 

ter the flood 7,000 

8,000 

season is over and indicating a Model Projections 

6,000 

1950-2099 7,000 

may potentially invite frequent flood events in 

6,000 

5,000 

5,000 

w the monthly 4,000 stream flow peaks will be shifted by the 4,000 

3,000 

3,000 

served, while the peak happened in the month of May 

2,000 

2,000 

arlier as 1,000 January. 

1,000 

Oct Nov Dec Jan Feb Mar 

• Fire Blight 

r/Wetter) 

10,000 

ed Data 

14 9,000 

8,000 

rojections 

99 7,000 

6,000 

5,000 

4,000 

3,000 

2,000 

1,000 

Figure 5: Historical and projected monthly stream flow (cubic • feet Gummosis per second) • Bot for Diseases American Riv 

Observed Data 

Fair Oaks. Left: projection based on RCP4.5 1922-2014 and Right: RCP8.5 [7]. 

Model Projections 

1950-2099 

ENHANCES FRUIT COLOR 

Table 3: Magnitude of monthly stream flow (cubic feet per second) at midpoint of the hydrograp 

shown in Figure 5 [7] 

Use for, Tree Nuts, Stone 

Fruits, Apples, Pears, Citrus, 

Emission Scenario Observed HadGEM2-ES CNRM-CM5 Avocados, CanESM2 MIRO 

RCP 4.5 21,635 26,613 36,031 and Blueberries. 33,496 25,794 

Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep 

Month 

Figure 5: Historical and projected monthly stream flow (cubic feet per second) for American 

River RCP at Fair 8.5 Oaks. Left: projection based 21,635 on RCP4.5 and Right: RCP8.5 27,290 [7]. 

29,274 ALSO HELPS TO CONTROL 36,665 GROUND 25,556 

tream flow (cubic feet per second) for American River at 

SQUIRRELS, GOPHERS, MICE, AND VOLES. 

and Right: RCP8.5 [7]. 

(cubic feet per second) at midpoint of the hydrographs 

ORGANIC TREE WASH 

ELIMINATES FOOD SOURCES THAT CAN CAUSE: 

• Canker 

Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Ju 

Month 

Month 

Emission Scenario Observed HadGEM2-ES CNRM-CM5 CanESM2 MIROC5 

RCP 4.5 21,635 26,613 36,031 33,496 25,794 

RCP 8.5 

21,635 27,290 29,274 36,665 25,556 

Begin Spraying at bud break or 

2 to 4 inch shoot growth, two 

quarts per 100 GPA. 

Table 3: Magnitude of monthly stream flow (cubic feet per second) at midpoint of the hydrographs 

shown in Figure 5 [7]. 

M2-ES CNRM-CM5 CanESM2 MIROC5 

36,031 33,496 25,794 

Climatic 

Classification 

29,274 Crop 36,665 Temperature for 25,556 

Hot 

Warm 

Cool-Warm 

Cool 

Watermelon 

Melon 

Sweet 

potato 

Cucumber 

Pepper 

Sweet corn 

21-35 

21-32 

16-35 

16-35 

16-35 

Snap beans 16-30 

Tomato 16-30 

Onion 

Garlic 

10-30 

7-25 

20-25 

Turnip 10-35 

18-25 

Pea 10-30 

Potato 7-26 

Lettuce 5-26 

16-25 

Cabbage 

Broccoli 

Spinach 

Acceptable 

O 

Germination ( C) 

10-30 

10-30 

4-16 

Optimal 

Temperature 

O 

for yield( C) 

25-27 18-35 

20-25 12-30(35) 

16-18(25) 

Acceptable 

Temperature 

Growth Range 

O 

( C) 

7-30 

5-25 

5-25(30) 

5-25 

Repeat every 14 days, mixes 

well with fertilizers, we 

recommend using, 

PURE PROTEIN DRY 15-1-1 

Table 4: Temperature thresholds for selected vegetable crops [13]. 



31


the importance of enhancing adaptive 

capacity of agriculture to reduce 

vulnerability to climate change and 

gain substantial benefits. The observed 

changes in climate added and will 

continue to add increasing pressure 

on agricultural production systems in 

California. 

Reduced chill, increased pest pressures, 

increased water demand and waterinduced 

stress, as well as variable and 

unreliable water supply are examples of 

factors that are projected to adversely 

impacting yield and quality of various 

crops grown in California. Such impacts 

may further intensify the challenges to 

meet the local and global food demands. 

For this reason, there is urgent need 

to address such issues with resource 

limitations and food security challenges. 

There is a clear need for localized 

agricultural adaptation research that 

could alleviate agricultural risks due to 

increased temperatures and extreme 

heat. Water being a central issue for 

California, there needs to be a priority 

on agricultural adaptation to water 

shortages. To help growers manage 

risks, it is important to develop locally 

relevant, need-based decision support 

tools that can effectively integrate 

various stressors to help growers make 

informed choices. Research only has 

value if it leads to informed decisionmaking, 

so stakeholder engagement 

should be the central component of 

climate and agricultural research. 

References 

1.CDFA (California Department of 

Food and Agriculture). California 

Agricultural Statistics Review 

2015-2016. Available online: 

https://www.cdfa.ca.gov/statistics/ 

PDFs/2016Report.pdf (accessed on 16 

June 2017). 

2.Pathak, T.; Maskey, M.; Dahlberg, 

J.; Kearns, F.; Bali, K.; Zaccaria, D. 

Climate change trends and impacts on 

California agriculture: a detailed review. 

Agronomy, 2018, 8(3), 25. 

3.DWR (California Department 

of Water Resources). Hydroclimate 

Report. Water Year 2017. 

4.California Climate Tracker. Generate 

Products Available online: https://wrcc. 

dri.edu/Climate/Tracker-/CA/ (accessed 

on 19 January 2019) 

5.DWR (California Department of 

Water Resources). California Climate 

Science and Data for Water Resources 

Percent Change 

10 

0 

-10 

-20 

-30 

10 

0 

-10 

-20 

-30 

10 

0 

-10 

-20 

-30 

Alfalfa 

2020 2040 2060 2080 2100 2020 2040 2060 2080 2100 2020 2040 2060 2080 2100 

Tomato 

2020 2040 2060 2080 2100 2020 2040 2060 2080 2100 

Cotton 

Safflower 

Rice 

Sunflower 

2020 2040 2060 2080 2100 2020 2040 2060 

2080 2100 

Maize 

2020 2040 2060 

Figure 6: Crop Yield response to warming Year in California’s Central Valley [14]. 

Management. Available online: 

http://www.water.ca.gov/climatechange/ 

docs/CA_-Climate_Science_and_Data_ 

Final_Release_June_2015. pdf (accessed 

on 19 June 2017). 

6.Reid, C.E.; O’Neill, M.S.; Gronlund, 

C.J.; Brines, S.J.; Brown, D.G.; Diez- 

Roux, A.V.; Schwartz, J. Mapping 

community determinants of heat 

vulnerability. Environ. Health Perspect, 

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7.Cal-adapt. Exploring California’s 

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annual-averages/#climatevar-=tasm 

ax&scenario=rcp45&lat=38.59375&l 

ng=-121. 

8.Wreford, A.; Adger, W.N. Adaptation 

in agriculture: historic effects of heat 

waves and droughts on UK agriculture. 

International Journal of Agricultural 

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9.Zargar, A.; Sadiq, R.; Naser, B.; 

Khan, F.I. A review of drought indices. 

Environmental Reviews, 2011, 19, 333- 

349. 

10.Sommer, L. With Climate 

Change, California Is Likely To 

See More Extreme Flooding. 

Available online: http://www.npr. 

org/2017/02/28/517495739/withclimate-change-california-is-likelyto-see-mo-re-extreme-f 

looding, 2017 

(accessed on 19 June 2017). 

11.DWR (California Department of 

Water Resources). California Climate 

Science and Data for Water Resources 

Management. Available online: http:// 

www.water.ca.gov/climatechange/docs/ 

CA_-Climate_Science_and_Data_ 

Final_Release_June_2015. pdf 

Wheat 

2080 2100 

A2 Higher Emissions Scenario 

B1 Lower Emissions Scenario 

12.Ray, D.K.; Mueller, N.D.; West, 

P.C.; Foley, J.A. Yield trends are 

insufficient to double crop production 

by 2050. PLoS One, 2013, 8, 1-7. 

13.Hatfield, J.; Boote. K.; Fay, P.; 

Hahn, L.; Izaurralde, C.; Kimball, 

B.A.; Mader, T.; Morgan, J.; Ort, D.; 

Polley, W.; Thomson, A.; Wolfe, D; 

Agriculture, In: The effects of climate 

change on agriculture, land resources, 

water resources, and biodiversity. A 

report by the U.S. Climate Change 

Science Program and the Subcommittee 

on Global Change Research, 

Washington DC., USA, pp 21-74, 2008 

14.Lee, J.; De Gryze, S; Six, J. Effect of 

climate change on field crop production 

in California’s Central Valley. Climatic 

Change, 2011, 109(1), 335-353. 

15.Jackson, L.E.; Wheeler, S.M.; 

Hollander, A.D.; O’Geen, A.T.; Orlove, 

B.S.; Six, J.; Sumner, D.A.; 

Santos-Martin, F.; Kramer, J.B.; 

Horwath, W.R.; Howitt, R.E.; Tomich, 

T.P. Case study on potential agricultural 

responses to climate change in a 

California landscape, Climatic Change, 

2011, 109(1), S407-S427. 

16.Zilberman, D.; Kaplan, S. 

Giannini Foundation of Agricultural 

Economics, University of California. 

An Overview of California’s 

Agricultural Adaptation to Climate 

Change. Available online: https://s. 

giannini.ucop.edu/uploads/giannini_ 

public/73/c8/73c82d70-b296-4424- 

82f6-2c04c7859aa4/-v18n1 

_6.pdf (accessed on 26 June 2017). 





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33

Photo 1. Anthracnose infection symptoms on 

almond hull. Photos courteys of B. Holtz, University 

of California Cooperative Extension. 

The Many Possible 

Causes of 

“Gummy Nuts” 

in Almonds 

By: Emily J. Symmes | Sacramento Valley 

Area IPM Advisor University of California 

Cooperative Extension and Statewide IPM 

Program 

Beginning in March, there are a 

number of biological factors and 

non-biological conditions that 

may cause “gumming” in almonds. 

When we refer to “gummy nuts”, we are 

actually referring to a suite of different 

symptoms depending on which part of 

the fruit is affected. To distinguish those 

in this article, the term “hull gummosis” 

refers to the exudates (typically clear to 

amber in color) that are visible on the 

outside of the fruit. Depending on the 

cause and extent of fruit damage, we 

may also observe gumming evident on 

the shell and in the kernel, in addition 

to other symptoms. 

In order to develop the best approach 

to maintaining crop health and kernel 

quality, it is important to be able to 

distinguish among the various causes 

of hull, shell, and kernel gumming. 

In general, dissection of the fruit, as 

well as a holistic approach to assessing 

the orchard is necessary to determine 

the cause of symptom expression and 

potential for significant crop damage. 

Often, once hull gummosis and possible 

underlying kernel damage are observed, 

the window of opportunity or ideal 

treatment timing to prevent or mitigate 

the cause of already-damaged fruit has 

passed. However, understanding how 

to best identify the cause of symptoms, 

relationships to other orchard 

factors that lead to the development 

of symptoms and damage, and the 

timing of initial onset will allow you to 

develop improved monitoring and crop 

protection strategies. 

Diseases 

With most hull gummosis arising from 

disease infection, there will typically 

be other signs and disease symptoms 

apparent on other parts of the plant. 

Additional factors such as orchard 

history of disease, timing of symptom 

development, cultivars affected, 

and weather patterns conducive to 

particular disease development can help 

distinguish the cause(s) of symptoms 

being observed in the orchard. The most 

common pathogen-induced infections 

that can lead to hull gummosis include 

anthracnose and bacterial spot. 

Anthracnose 

When small nuts are infected, they 

shrivel and turn a rusty orange color. 

When larger nuts are infected, they 

exhibit round, sunken, orangish lesions 

and profuse hull gummosis as the 

infection progresses into the kernel. 

Anthracnose gummosis is typically 

amber in color with multiple exudate 

sites on the hull (Photo 1). Eventually, 

infected nuts die and remain attached to 

the spur as mummies. 

Other signs and symptoms of 

anthracnose infection may be evident 

on flowers, foliage, spurs, shoots, 

limbs, and branches. Blossom blight 

of infected flowers looks similar to 

brown rot strikes. Infected leaves tend 

to develop water-soaked lesions that 

eventually fade in color, developing into 

marginal necrosis. Leaves die but often 

remain attached to branches. Dieback 

often occurs on shoots and branches 

that bear infected nuts. 

Prolonged warm (>59°F), rainy weather, 

especially extending well into spring, 

are most conducive to disease spread 

and development. Summer infections 

can occur if irrigation contacts the tree 

canopy. All varieties are susceptible, 

with Butte, Fritz, Monterey, Peerless, 

Price, and Winters among the most 

susceptible, and Nonpareil considered 

less susceptible to anthracnose 

infection. Symptom development 

on the outside of the fruit (i.e., hull 

gummosis) is dependent on the timing 

of initial infection and rate of disease 

progression. In most years, it is evident 

by mid- to late-April. 

Bacterial Spot 

The most obvious symptoms of 

infection occur on nuts. Typically, hull 

lesions begin as small water-soaked 

circular spots. Infections enlarge and 

become necrotic, with obvious lesions 





35

Photo 2A. Necrotic lesions under almond hull caused by bacterial spot infection. 



developing inward into the hull, shell, 

and kernel (Photo 2A). Infection sites 

on the hull exhibit profuse amber 

colored gumming, often from multiple 

exudate sites (Photo 2B). Early-season 

infections can cause fruit drop. 

Leaf and shoot symptoms may occur, 

but are less common than fruit 

symptoms. Small, water-soaked, 

circular leaf lesions develop along 

the midrib and toward the tip and 

Photo 2B. Bacterial spot infection symptoms 

on almond hull. 

margin of the leaf, eventually becoming 

chlorotic then necrotic, leaving 

irregular shaped holes (Photo 2C). If 

the disease is severe, defoliation can 

occur. Twig lesions may develop on 

green shoots. 

High moisture conditions and warm 

temperatures (> 68°F) favor infection. 

Severe infections are most common 

with frequent periods of rainfall or 

irrigation during fruit development. 

Fritz is highly susceptible to bacterial 

spot infection and symptom 

development. Other varieties such as 

Butte, Carmel, Monterey, Nonpareil, 

Padre, and Price are also susceptible 

but generally exhibit less disease 

severity. Hull gummosis symptoms 

are first visible one to three weeks 

after initial infection, therefore 

appearance will be related to 

when the favorable conditions 

for infection occurred that 

season. In most years, bacterial 

spot hull gummosis symptoms 

become apparent by mid- to late- 

April. 

Insects 

A number of insects of different 

types can cause hull gummosis. We 

Photo 2C. Foliar symptoms of bacterial spot 

infection. 

tend to most commonly associate its 

appearance with Hemipteran insects, 

or “true bugs” (i.e., those with piercingsucking 

mouthparts), particularly 

leaffooted bugs and stink bugs. In 

addition to these, a number of other 

“bugs” may be present in orchards and 

cause some extent of hull gummosis 

(e.g., box elder bugs, Phytocoris species, 

Calocoris species) but often without 

significant kernel or crop damage. 

Entries by Lepidopteran (worm) pests 

may also result in hull gummosis 

(i.e., peach twig borer and oriental 

fruit moth). What follows focuses on 

distinguishing leaffooted and stink bug 

feeding in almond. 

Hull gummosis expression and the 

extent of shell and kernel damage 

caused by bug feeding differs depending 

upon the insect species and life stage 

(adults or nymphs), the growth size and 



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varieties are more susceptible to kernel 

damage for a longer period during the 

season. 

Photo 3. Leaffooted bug egg mass on almond. Photo courtesy of 

Integral Ag, Inc. 


stage of the fruit being fed on (therefore, 

seasonal timing of damage), and the 

variety. The key factors for differential 

fruit symptom expression involve the 

length of the insect mouthpart and 

what part of the fruit tissue it can reach 

(differs based on species, and among 

adults and nymphs of a species); hull 

thickness and shell hardness (which 

both vary in time and among varieties); 

and stage of kernel development when 

the feeding occurred. In addition, 

feeding wounds caused by bugs can 

provide openings for secondary 

pathogens to invade the fruit (e.g., 

bacteria, yeast, fungi) and some plantfeeding 

bugs are known to inject 

salivary enzymes which may exacerbate 

damage symptoms. 

In general, when evaluating insectcaused 

hull gummosis, shell, and 

kernel damage, darkened feeding 

channels are evident upon dissection 

into the hull, and may extend into the 

shell and kernel (based on the factors 

noted above). The most accurate 

way to distinguish which bug species 

are causing the damage is by visual 

observation of the presence of the 

insects themselves (adults, nymphs, 

or egg masses), but other factors such 

as timing, extent of damage, and 

landscape characteristics can assist in 

the evaluation. 

Leaffooted Bugs (Family 

Coreidae) 

Leaffooted bug (LFB) feeding on young 

fruit prior to shell hardening can cause 

the kernel to wither and abort, and may 

lead to nut drop. LFB feeding impacts 

lessen as the season progresses, with 

kernel damage decreasing as shells 

harden. After shells begin to harden, 

feeding may still cause dark spots and 

gumming of the kernel, or wrinkled and 

misshaped kernels. 

Hull gummosis caused by LFB feeding 

is clear to light-yellow, originating 

from each feeding site. Therefore, 

there can be single or multiple hull 

gummosis sites, but we often observe 

multiple exudates on individual nuts 

arising from LFB infestation. The extent 

of kernel damage depends on if the 

feeding entry made it through the shell 

and into the kernel. Softer shell almond 

LFB damage typically occurs in a fairly 

distinct window of time, usually during 

March and April. Hull gummosis 

caused by LFB usually manifests 3 to 10 

days after the initial feeding occurrence. 

As season progresses into May and 

June, the LFB feeding decreases while 

stink bug feeding by species commonly 

encountered in almond orchards can 

increase. Monitor for visual evidence 

of LFB adults, nymphs, and egg masses 

(Photo 3) from March to early May. 

Basing treatments on the appearance of 

significant hull gummosis or aborted 

nuts on the ground may be ineffective, 

as there can be more than a one-week 

lag time between feeding and evidence 

of symptoms, and dispersing insects 

may have moved out of the orchard by 

that time. 

Stink Bugs (Family 

Pentatomidae) 

There are a number of species of stink 

bugs that can be found in almond 

orchards. These include green stink 

bug (most commonly encountered), 

redshouldered stink bug, Uhler stink 

bug, and rough stink bug. Be aware that 

rough stink bugs are not plant pests; 

they are predators of other insects. 

More recently, the invasive brown 

marmorated stink bug (BMSB) has been 

reported in a few almond orchards in 

the northern San Joaquin Valley. BMSB 

will be addressed in a separate section 

in this article. 

Feeding by the more common stink 

bugs in almond orchards typically 

occurs from May through July. 

Movement into orchards in spring 

occurs when other hosts (weeds, other 

plants, crops) dry down, so this can 

be earlier in years with dry winters/ 

springs. The appearance of hull 

gummosis is similar to LFB (clear to 

light-colored gumming, often exuding 

from multiple puncture holes), but 

the timing of incidence is a key factor 

in separating the two. Because stink 

bug feeding occurs later in the season 

than LFB feeding, it typically only 

extends into the hull, and nut abortion 

and kernel damage are rare. However, 

severe feeding may impact quality 



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39


due to misshapen or wrinkled kernels, 

perhaps due to increases in secondary 

pathogens. Stink bugs are generally 

less mobile than LFB, so feeding tends 

to be more localized (clustered) in the 

orchard. Additionally, monitoring or 

observing visual evidence of stink bug 

adults (Photo 4), nymphs, and barrelshaped 

egg masses, can help indicate 

whether they were the likely cause of 

fruit symptoms. 

Brown Marmorated Stink Bug 

BMSB has been present in California 

since 2008, but has only recently 

begun to move into some agricultural 

commodities (commercial kiwifruit 

detection beginning in 2014, peach in 

2016, and almond in 2017). Because this 

is an invasive species with a very wide 

host range, minimal natural controls, 

and highly aggregative behavior, there 

is considerable concern as to the 

potential impacts should it become 

more widespread and established in 

California agricultural crops. 

To date in almond, reports of serious 

infestation and damage have been 

limited to a few orchards in Stanislaus 

and Merced counties, but it is important 

to be aware of how to determine the 

possible presence of this pest in almond 

orchards, and how to distinguish 

damage caused by BMSB versus other 

bug feeding. Evidence of feeding (hull 

gummosis from multiple feeding 

sites) and shell and kernel damage by 

BMSB seems to be similar to LFB (if 

occurring earlier in the season, before 

shell hardening) or other stink bugs 

(if occurring later in the season, after 

shell hardening), although the severity 

may appear greater for BMSB due to 

its aggregative nature (infest in larger 

numbers in clustered areas of orchards). 

Recent research by University of 

California Cooperative Extension 

(UCCE) Area IPM Advisor Jhalendra 

Rijal in the impacted orchards indicates 

that BMSB may move into almonds 

and begin feeding as early as mid- 

March, and may continue well into 

summer. Therefore, it may be difficult to 

distinguish whether hull gummosis, nut 

drop, and/or kernel damage observed 

in March and April was caused by LFB 

or BMSB feeding. If hull gummosis 

is occurring later in the summer, 

it may be difficult to distinguish 

between BSMB or other stink bugs. 

Researchers are actively working on 

traps and lures for LFB, but to date, 

none are commercially available. 

Fortunately, there are traps and lures 

available for BMSB. In addition to 

trapping and visual observations of 

the pests themselves (adults, nymphs, 

egg masses), taking into account 

landscape-scale characteristics and 

location of the damage in the orchard 

may help indicate the source of damage 

if occurring during the same time 

period as either LFB or other stink bugs. 

For BMSB, pay particular attention 

to border rows next to overwintering 


Photo 4. Adult green stink bug on almond. Photo courtesy of 

Integral Ag, Inc. 




41


aggregation shelters (e.g., structures, 

wood piles), other known hosts (e.g., 

tree of heaven is known to harbor high 

populations of BMSB), and riparian/ 

natural environment interfaces. 

Other Causes of Hull Gummosis 

A number of other factors may result in 

the expression of hull gummosis. These 

include physical causes (such as hail 

or tractor strikes to the fruit), nutrient 

deficiency (i.e., boron), herbicide 

damage, and physiological responses. 

Boron deficiency will typically manifest 

as clear gummosis on the sides of the 

hull or at the suture line. Copious 

internal gumming and discoloration 

the of developing kernel will be visible 

upon dissection (Photo 5). Fruit drop 

may occur, or, if nuts remain in the 

tree, internal gumming will harden and 

cause misshapen kernels. Boron status 

can be evaluated by analyzing hull 

samples. 

Physiological processes, not relating 

to any of the aforementioned sources, 

may result in hull gummosis. This is 

thought to be due to rapid expansion 

of the growing kernel, causing 

increased pressure on the shell and 

hull. Dissection of the fruit will show 

a water-soaked gummy area on the 

outside of the shell, clear gummosis 

exuding from the suture line or side of 

hull, and no apparent feeding channel 

(that would indicate bug as the culprit). 

Kernels appear healthy and affected 

nuts often remain on the tree with no 

negative impacts. 

In summary, hull gummosis in almonds 

can be caused by a number of different 

biological and non-biological factors. 

Appearance of hull gummosis may be 

indicative of more severe crop damage 

if kernels are also impacted, but this 

is not always the case. Understanding 

how to best identify the sources of 

such symptoms is a key component for 

effective integrated pest management 

and crop production programs. 

More detail on identification and 

management of all of the potential 

causes of hull gummosis, shell, and 

kernel gumming damage can be found 

on the UC Statewide IPM Program 

online Pest management Guidelines 

for Almonds (ipm.ucanr.edu/PMG/ 

selectnewpest.almonds.html), and in 

multiple posts at the Sacramento Valley 

Orchard Source Website (http://www. 

sacvalleyorcards.com/) and the Almond 

Doctor Blog (http://thealmonddoctor. 

com/). 

Photo 5. Symptoms of boron deficiency 

in almond kernel. Photo courtesy 

of D. Lightle, University of California 

Cooperative Extension. 




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