PCC July-August 2020


July / August 2020

Weed Control in Lettuce: A Comparison of Various

Weed Management Strategies and Costs

Making Sense of Biostimulants for Improving your Soil

Detection of Marked Lettuce and Tomato by an

Intelligent Cultivator

Virus Pathogens: Challenges to the Health of

Vegetable Crops


Volume 5: Issue 4

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Weed Control in Lettuce:

A Comparison of Various

Weed Management

Strategies and Costs

PUBLISHER: Jason Scott

Email: jason@jcsmarketinginc.com

EDITOR: Marni Katz


Email: article@jcsmarketinginc.com

PRODUCTION: design@jcsmarketinginc.com

Phone: 559.352.4456

Fax: 559.472.3113

Web: www.progressivecrop.com







Making Sense of

Biostimulants for

Improving Your Soil

Detection of Marked

Lettuce and Tomato by an

Intelligent Cultivator

Virus Pathogens:

Challenges to the Health

of Vegetable Crops

Cover Crops in California

Agriculture: An Overview

of Current Research

Lettuce Dieback: New

Virus Found to be

Associated with Soilborne

Disease in Lettuce



Steven A. Fennimore

UCCE Extension Specialist,

UC Davis

David C. Slaughter

Biological and Agricultural

Engineering Dept., UC Davis

Steven Koike

Director, TriCal Diagnostics

Franz Niederholzer

UC Farm Advisor, Colusa and

Sutter/Yuba Counties

Rhonda Smith

UC Farm Advisor, Sonoma


Surendra Dara

UCCE Entomology and

Biologicals Advisor, San Luis

Obispo and Santa Barbara


Kevin Day

UCCE Pomology Farm Advisor,

Tulare and Kings Counties

Elizabeth Fichtner

UCCE Farm Advisor,

Tulare County

Katherine Jarvis-Sheen

UCCE Orchard Systems Advisor,

Sacramento, Solano and

Yolo Counties

Shulamit Shroder

Community Education

Specialist, UCCE Kern


Richard Smith

UCCE Vegetable Crop and

Weed Science Farm Advisor

William M. Wintermantel

USDA-ARS, Salinas

Dr. Karl A. Wyant

CA & CCA Director of Ag

Science, Heliae Agriculture



Steven Koike

Tri-Cal Diagnostics

Jhalendra Rijal

UCCE Integrated Pest Management

Advisor, Stanislaus


Kris Tollerup

UCCE Integrated Pest Management

Advisor, Fresno, CA

Mohammad Yaghmour

UCCE Area Orchard Systems

Advisor, Kern County


Choosing Activator Spray

Adjuvants for Permanent



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


July / August 2020 www.progressivecrop.com 3

Weed Control in Lettuce

A Comparison of Various Weed Management Strategies and Costs

By RICHARD SMITH | UCCE Vegetable Crop and Weed Science

Farm Advisor

Economical and successful weed

control in lettuce can be accomplished

by utilizing key cultural practices,

cultivation technologies and herbicides.

Planting configurations vary from

40-inch wide beds with two seedlines to

80-inch wide beds with 5 to 6 seedlines.

Recent studies of weeding costs for lettuce

ranged from $454 to $623/A for 80-

inch wide beds with 5 seedlines of head

and 6 seedlines of romaine hearts lettuces,

respectively (see coststudies.ucdavis.edu/


Weeding costs included the following:

Herbicide applied in 4-inch wide bands

over the seedlines, cultivation, auto thinning

using a fertilizer to kill unwanted

lettuce plants and hand weeding/double

removal. The costs for auto thinning also

include fertilizer costs, which can satisfy

the need for the first fertilizer application.

Significant weed control is accomplished

by practices that occur before the crop

is planted. For instance, weed pressure

is affected by prior crop rotations and

how much weed seed was produced in

them. The weeding costs given above are

rough averages. If weed pressure is light,

weeding costs can be lower, but if weed

pressure is high, weeding costs can be

much higher. In the Salinas Valley, good

management of weeds is possible with

rotational crops such as baby vegetables

(spinach, baby lettuce and spring mix)

because they mature in 25 to 35 days and

don’t allow weeds to set seed. Long-season

crops such as pepper and annual

artichokes allow multiple waves of weeds

to germinate which are difficult to see

and remove once the plants get bigger.

Preirrigation is standard practice to

prepare the beds for planting. It stimulates

germination of a percentage of

weed seeds in the seedbank, and they are

subsequently killed by tillage operations.

Studies have shown that preirrigation

followed by tillage lowers weed pressure

to the subsequent crop by about 50%. In

organic production, pregermination is

one of the most powerful practices for reducing

weed pressure, and if time allows,

it can be repeated to further reduce weed


Preemergence Herbicides

There are three pre-emergence herbicides

available for use in lettuce production:

Balan, Prefar and Kerb. Balan and Prefar

provide good control of key warm season

weeds such as lambsquarters, pigweed

and purslane, as well as grasses (Table 1,

see page 5). Kerb is better at controlling

mustard and nightshade family weeds

such as shepherd’s purse and nightshades.

Balan is mechanically incorporated

into the soil and Prefar and Kerb

are commonly applied at or post planting

and incorporated into the soil with germination


Kerb is more mobile in water than

Prefar which can lead to issues with its

efficacy. Often 1.5 to 2.0 inches of water

are applied with the first irrigation to

germinate the crop which can cause Kerb

to move below the zone of germinating

weed seeds, especially on sandy soils. For

instance, Kerb is capable of controlling

purslane however, its efficacy can be low

on sandy soils due to its movement below

the zone of germinating weed seeds with

the first germination water. Prefar does

not leach as readily as Kerb and that

is why these two herbicides are often

mixed in the summer to control purslane

(Figure 1).

In the desert, the use of delayed applications

of Kerb has been used for many

years. Due to the large amounts of water

that are applied to keep the seeds moist

Drip germination in lettuce has resulted in

fewer weeds than sprinkler irrigation (photo by

Marni Katz.)

and cool, Kerb is applied in the 2nd or

3rd germination water, approximately 3

to 5 days following the first water, just

prior to the emergence of the lettuce

seedlings. The amount of water applied

in the second and third irrigation is less

than the first application and therefore

does not push the Kerb as deep in the

soil. Although the Salinas Valley is

cooler than the desert, evaluations here

have also found delayed applications to

improve the efficacy of Kerb (Figure 2,

see page 6). These data illustrate the loss

of control of purslane by Kerb when applied

before the first germination water,

as well as the improvement in efficacy

that results when applied after the first

germination water. It also illustrates the

role that Prefar plays in the control of

purslane when the efficacy of Kerb is reduced

by being pushed too deep. Clearly,

there is benefit from applying the Kerb

in the 2nd or 3rd germination water because

it helps to keep it in the zone where

weed seeds are germinating.

Figure 1. On left: Kerb at 3.5 pints/A applied at

planting; On right Kerb at 3.5 pints/A

+ Prefar at 1.0 gallon/A applied at planting. The

main weed is common purslane which

was not controlled by Kerb because it was

pushed below the zone of germinating

weed seeds by the germination water (photo

courtesy R. Smith.)

4 Progressive Crop Consultant July / August 2020

Table 1. Weed susceptibility to registered preemergent herbicides.

The use of single use drip tape injected 3

inches deep in the soil has become popular

in the Salinas Valley. The uniformity

of using new tape with each crop has

allowed growers to consider using drip

irrigation to germinate lettuce stands.

Although the same amount of water may

be applied to germinate the stand with

drip irrigation as with sprinklers, the

water tends to move upward with drip

irrigation. In drip germinated lettuce,

Kerb is sprayed on the soil surface and is

solubilized by the upward movement of

the drip applied water which allows it to

move just deep enough in the soil to control

germinating weeds, but not too deep

to reduce its efficacy (Table 2, see page

6). Interestingly, drip germination alone

resulted in fewer weeds than sprinkler


Lettuce is typically planted with 4-5

times more seed than is needed in

order to assure a good stand. At about 3

weeks after the first irrigation, lettuce is

Continued on Page 6

Weed species Balan Prefar Kerb






































































C — Control; P —Partial Control; N — No Control
























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12.0 c

13. 1c

52.1 a

10.8 c

Continued from Page 5

10.1 c


thinned. Traditionally lettuce has been 14.1 c

thinned by hand, but increasingly growers

are using auto thinners which spray 0.0001

38.4 b

an herbicide (Shark) or concentrated liquid

fertilizer (e.g. AN 20, 28-0-0-5, and

others) to kill the unwanted plants and

achieve the desired plant spacing. In the

process of thinning by hand or by auto

thinning, a significant portion of weeds

in the seedline is also removed.

Automated Thinning and Weeding

About 10 to 14 days after thinning,

hand weeding is carried out to remove

weeds from the seedline and any double

lettuce plants that were not removed in

the thinning operation. An increasing

number of Salinas Valley growers are

using autoweeders prior to hand weeding.

There are several autoweeders available:

Robovator (Denmark), Steketee (Netherlands),

Ferrari (Italy), Garford (England)

and FarmWise(USA). These machines

use a camera to capture the image of the

seedline and a computer that processes

the image and activates a kill mechanism

(a split or spinning blade) to remove unwanted

plants. The machines were originally

designed for use with transplanted

vegetables. We tested auto weeders and

found that they remove about 50% of the

weeds in the seedline and reduced the

subsequent hand weeding times by 35%.

In order to safeguard the crop plants, the

auto weeders leave an uncultivated safe

zone around the crop plants where weeds

can survive. As a result, auto weeders do

not remove all the weeds in the seedline,

but they help to make subsequent hand

weeding operations more efficient and


Depending on the weed pressure, some

lettuce fields are hand weeded one more

time a week or so prior to harvest. Given

the practices just outlined, perennial

weeds are not problems in the typical

lettuce rotations in the Salinas Valley.

The rapid turnaround of the crop (55

to 70 days during the summer) and the

frequent use of cultivation does not

allow enough time for weeds like field

bind weed or yellow nutsedge to build up

root reserves or nutlets before they are

21.4 c

60.3 a

24.0 c

60.5 a

145.3 b 42.6 b


26.9 c

2. Effect




Kerb application (at 3 pints/A) method (surface applied, drip injected or

untreated) 24.4 c and 52.1 irrigation ab method (surface tape, buried tape or sprinkler) on weed densities,

lettuce 41.1c stand 59.1 and a visual injury.

46.1 bc

297.6 a




51.9 ab

42.3 b

Surface applied Surface drip

Surface applied Buried drip²

Surface applied Sprinkler

Drip injected Surface drip

Drip injected Buried drip²


Surface drip


Buried drip²



Treatment Prob (F) 0.0002

Figure 2. Efficacy of Kerb applied at 3.5 pints/A at planting or in the 3rd germination

water; crop was romaine. Note that applying the Kerb after the first heavy application of

germination water greatly improved its effectiveness.

No./30ft 2

Shepherd’s Purse

















1.9 b

4.1 b

86.4 b

8.3 b

5.8 b

16.6 b

16.5 b

243.4 a



3rd Water


cultivated or disced out. In the summer,

purslane is the biggest concern because

it can build up high populations in the

seedbank and, because of their fleshy

tissue, can set seed even after being cut

by the cultivator knives. As a result, if it

is not effectively controlled in prior rotations,

it can result in high hand weeding

costs. Growers address purslane issues

by making bedtop applications of the

combination of Prefar and Kerb, as well

as by a combination of other practices

outlined above.

Weed densities (no. / 10ft 2 )



3rd Water

Kerb + Prefar



12.0 c

13. 1c

52.1 a

10.8 c

10.1 c


14.1 c

38.4 b




21.4 c

24.0 c

145.3 b

26.9 c

24.4 c


46.1 bc

297.6 a



Number of seedlings in 10 feet of a single plant line


Drip tape buried 2-3 inches deep





60.3 a

60.5 a

42.6 b

56.6 a

52.1 ab

59.1 a

51.9 ab

42.3 b

Although there have been no new herbicides

registered for use on lettuce in

many years, there have been significant

technological developments that have

improved efficiency of weed control in

lettuce. The increasing use of single use

drip tape and new automated thinning

and weeding technology have recently

contributed greatly in this regard.

Comments about this article? We want

to hear from you. Feel free to email us at










6 Progressive Crop Consultant July / August 2020



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Making Sense of Biostimulants for

Improving your Soil


Biostimulants…bio what???

You may have heard or read this

phrase several times over the past

year as this products category gains

traction in the agricultural marketplace.

Confused about what exactly

constitutes a biostimulant? You are not

the only one! A biostimulant includes

“diverse substances and microorganisms

that enhance plant growth” or

helps “amend the soil structure, function,

or performance.” Got it? No? That

is ok, please read on for more information.

Market Confusion

The exact definition of what a biostimulant

is, and what it is not, can be confusing

and leave some folks scratching

their head on what to expect regarding

product performance (Figure 1, see

page 9). A biostimulant tends to be an

“environmentally friendly alternative

to synthetic products” and can have

multiple impacts on the crop or soil,

although the exact definition of the

category is vague and open-ended. This

uncertainty has received increased

attention by regulators, and we should

expect to see more precise definitions


As it stands, there are many active ingredients

in this arena, and some growers

have struggled to find the right fit

for their farm. This confusion is regrettable

given the increasing popularity

of the category and the forecasted sales

growth rates. For example, the global

market for biostimulants was valued at

$2.19 billion in 2018 and is projected to

have a compound annual growth rate

of 12.5% from 2019 to 2024.




Molasses, Fish



Soil Enzymes



(Bacteria and


Seaweeds and


Humic and

Fulvic Acids &


Microbial food sources can be applied to

promote the native soil microbiome

(bacteria and fungi) and help provide a food

source to inoculants. Microbial food

products have tremendous variance in their

quality as food source (e.g., C:N ratio) and

the diversity of microbes they help feed

(e.g., composition of food source).

These types of product promote very

specific reactions in the soil, depending on

the enzyme used. For example, you get

protein breakdown by protease, urea

breakdown by urease, cellulose breakdown

by β glucosidase, phosphorus liberation by

phosphatase, etc..

Inoculants consist of single or multi-species

mixtures of bacteria and fungi (e.g., Bacillus,

Trichoderma, Pseudomonas, etc.) that can

help mineralize nutrients in the soil or

promote nutrient uptake by the plant.

Seaweeds are generally sourced from a few

species belonging to the genera Ascophyllum

and the end use is strongly determined

by both the seaweed species used and how

it is processed. Generally, seaweeds can be

used to activate plant growth and defense

and have also been used to help relieve

plant stress. Some seaweeds can help feed

soil bacteria and fungi, which is why they

are included here.

There exists a large range in the size and

structural arrangement of this group of active

ingredients. Generally, the acids can help with

nutrient retention and chelation of ions in the

soil (e.g., nitrate and potassium.) Biochar can

help provide a physical structure for microbes

to colonize in the soil environment.

Look for a food source that is both

high quality (low C:N ratio) and

contains various macromolecules

(e.g., lipids, proteins, and carbohydrates)

for best effects. Some

product types can have offensive

odors (e.g., fish preparations) or

have a high viscosity (e.g., molasses)

unless handled properly.

You must pick the right enzyme for

the job you want performed. An

enzyme can also be degraded

rapidly in the soil unless it is applied

in a protected form.

Since these are living products,

inoculant survival rates can be

sensitive to product handling and

storage and the condition of the soil

they are applied to. Furthermore,

inoculant survivability can be

negatively impacted by the water

quality (e.g., pH, presence of sodium

and chloride, etc.) used to apply the

material and UV exposure if applied

to the soil surface.

Check product claims before

applying to make sure the seaweed

species and extraction process

match with what you are trying to

get done.

Continued on Page 9

There are concerns about the

quality and sustainability of the

source of these types of products.

Check that product is derived from

high quality sources and that the

extraction processing results in a

product that is easy to handle.

8 Progressive Crop Consultant July / August 2020

Table 1: Biostimulants are sorted by their active ingredient (left side), a description of how

they work (center) and some general handling notes (right side).

Figure 1: Biostimulants can impact a crop in many ways depending on the active ingredient applied (graphic courtesy Ute Albrecht, Southwest

Florida Research and Education Center.)

Matching Clear Goals

Biostimulants can be derived from a

laundry list of different materials, with

studies listing roughly eight major

classes of active ingredients or more,

each with unique properties and modes

of action. However, my experience in

the field suggests that many of us have

unfortunately lumped the various

products in this category into one large

“other” bucket for simplicity, regardless

of the difference in how the product

works or what outcome should be


Below I help clarify the role of several

active ingredients to allow you to better

understand and also mix and match

the desired characteristics you are

looking for (Table 1, see page 8). This

reference table will allow you to determine

which features you want to put

to work into your biostimulant blend

based on your crop production method,

application equipment, and comfort

level. The biostimulant categories listed

complement an agronomically sound

fertilizer and irrigation program and

should be included as a part of a comprehensive

crop management program.

Caveat: I do not have enough space to

list all possible modes of action, but

instead I limit the table to the materials

that have an impact on the soil.

Understanding the Nuances

The biostimulant category offers many

exciting opportunities to growers and

can deliver new functionality to common

fertilizers when used in a blend.

Continued on Page 10

Move Water and

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• ENCOURAGES Improved Rooting and

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July / August 2020 www.progressivecrop.com 9

Continued from Page 9

Before jumping into this ‘other’ category,

start with the following question

“What features am I looking for?” This

honest query will help you pick the correct

ingredient needed and bring clarity

to the nuances of the biostimulant

category. Getting your product blend

right from the get-go can help improve

the soil on your farm and help jumpstart

your 2020 yield and quality goals.

Please consult with your local sales

representatives to help pick the right

active ingredient for the job and be sure

to jar test any new blend ideas you have

prior to tank mixing for compatibility


Furthermore, running a pilot or test

study can be a great way to learn which

biostimulant product is right for your

crop and production system. Keeping

good records of your observations will

help jog your memory about product

performance as the season wears on

and will help you formulate the right

blend for the job. A good pilot or trial

plan can go a long way with helping you

keep track of important information on

how your biostimulant blend is impacting

your crop.

Hungry for more information about

biostimulants and what they can do for

you? Many trade publications, such as

the one you are reading now, have begun

to cover this category in more detail

and there are several good articles

out there that are worth reading. Below

I provided some recommended reading

to help get you started along with some

online resources that are worth a look.

About the Author

Dr. Karl Wyant currently serves as the

Director of Ag Science at Heliae® Agriculture

where he oversees the internal

and external PhycoTerra® trials, assists

with building regenerative agriculture

implementation, and oversees agronomy

training. Prior to Heliae® Agriculture,

Dr. Wyant worked as a field

agronomist for a major ag retailer serving

the California and Arizona growing

regions. To learn more about the future

of soil health and regenerative agriculture,

you can follow his webinar and

blog series at PhycoTerra.com.

Further Resources

• Soil Health Partnership Blog -



• Soil Health Institute Blog - https://


• PhycoTerra® Blog - https://phycoterra.com/blog/


Albrecht, Ute. (2019). Plant biostimulants:

definition and overview of categories

and effects. IFAS Extension HS1330.

Specialists in Biological Control Since 1975

We have developed Timing and

Release Rates for most crops

Drone Services Available For Releasing

Beneficial Insects

Calvo Velez, Pamela & Nelson, Louise

& Kloepper, Joseph. (2014). Agricultural

uses of plant biostimulants. Plant and

Soil. 383. 10.1007/s11104-014-2131-8.

Drobek, Magdalena & Frąc, Magdalena

& Cybulska, Justyna. (2019). Plant Biostimulants:

Importance of the Quality

and Yield of Horticultural Crops and

the Improvement of Plant Tolerance to

Abiotic Stress—A Review. Agronomy. 9.

335. 10.3390/agronomy9060335.

Rouphael, Y., Colla, G., eds. (2020). Biostimulants

in Agriculture. Lausanne:

Frontiers Media SA. doi: 10.3389/978-2-


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10 Progressive Crop Consultant July / August 2020


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Science-Driven Nutrition

Detection of Marked Lettuce and

Tomato by an Intelligent Cultivator

By STEVEN A. FENNIMORE | UCCE Extension Specialist, UC Davis and

DAVID C. SLAUGHTER | Biological and Agricultural Engineering Dept., UC Davis

All photos courtesy of S. Fennimore.

Weeds are difficult to control in lettuce and tomato

due to labor shortages, increasing costs of hand

weeding and limited herbicide options. Lettuce is

very sensitive to weed competition, plus there is no tolerance

for contamination of bagged lettuce salad mixes with weeds;

therefore, weeds must be controlled if lettuce is to be harvested.

Consequently, mechanical weed control is an important part

of an integrated weed management program in conventional

and organic vegetable crops. Traditional inter-row cultivation,

however, only removes weeds between crop rows and leaves

the weeds within the crop row. The removal of in-row weeds

requires hand weeding, a time-consuming and expensive


Additional Environmental Stress Conditions that the product is useful for:

What is

Anti-Stress 550®?

When to apply

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Vegetable Weed Control Costs

Weed control costs for conventional head lettuce production

in California are estimated at $216 to $319 per acre, while

weed control costs in organic leaf lettuce are $489 per acre,

on average, at current labor rates. In conventional processing

tomatoes, weed control costs are about $225 per acre or

12% of production costs. Additionally, hand weeding costs

have increased due to labor shortages, changes in California

overtime regulations and increasing minimum wages as well

as decreased labor immigration from Mexico. The result is

greater vulnerability of growers to crop losses due to weeds.

Automation of weed removal may be a method to contain

or reduce weed control costs in vegetable crops. Intelligent

intra-row cultivators (IC) provide an alternate weed management

option to standard inter-row cultivation. Previous

results have shown that IC can reduce the need for hand

weeding compared to standard cultivators and may reduce

weed control costs.

The Robovator® cultivator evaluated by Lati et al. (2016) relied

on pattern recognition of the rows and crop plants within

the rows based on the expected crop spacing within the rows.

When these spatial cues are unavailable, as can occur in an

organic field with a high weed density, this approach cannot

differentiate between crops and weeds, and thus it relies on a

size difference between crops and weeds, as well as a low to

moderate weed population to function accurately.

Intelligent Cultivation. Intelligent intra-row cultivation

requires three technologies; a machine-vision system that detects

crop plants and weeds, image classification and decision

algorithm that differentiates between crop plants and weeds,

and an automated weed removal mechanism that controls the

weed while protecting the crop. Precision guidance systems,

decision algorithms, and precision in-row weed control

devices are commercially available or are at an advanced level

of development. Accurate crop detection and differentiation

from weeds, at normal cultivation speeds, would allow for

greatly improved intra-row cultivators.

Weed/Crop Differentiation. The main challenge for intelligent

intra-row cultivation is to differentiate between crops and

weeds using digital imagery and processing at field operation

speeds of at least 1 mph in high weed density fields with travel

speeds above 2 mph required for economic acceptability for

low to moderate weed loads.

12 Progressive Crop Consultant July / August 2020

Figure 3. (a) Topical marker on lettuce plants.

A new method of crop and weed differentiation

called “crop signaling” is presented

in the research “Crop Signaling

for Automated Weed/Crop Differentiation

and Mechanized Weed Control in

Vegetable Crops” by Raja et al. 2019 out

of UC Davis. It is based on the idea that

the identity of the crop is known with

certainty when it is planted, whether

transplanted or seeded. Thus, if the crop

has a marker or signal that an IC can

reliably detect, then the IC would recognize

the signal and protect the crop.

Plants without the signal, i.e., weeds,

would not be protected and would be

removed by the IC. The objective of this

work was to test a crop signaling system

for crop detection accuracy and weed

control efficacy by an IC in lettuce and


Marking System Descriptions. Two

methods of plant signaling were tested,

physical plant markers and topical

markers. Biodegradable straws coated

with a fluorescent marker were used as

the plant markers in this study (Figure

1). The straws were then placed next to

tomato seedlings in the planting trays

and then transplanted together (Figure


The topical marker used on plant foliage

was green or orange fluorescent water-based

paint (Figure 3a,b). A paint

sprayer was used to apply the topical

marker to lettuce foliage and tomato

seedlings prior to planting, while they

were in trays. Another method was to

spray the marker onto tomato stems as

they were transplanted (Figure 4).

Intelligent Cultivator. The IC used

in this research was developed at the

University of California, Davis. It uses

a machine vision system specifically

Figure 3. (b) Spray application of topical marker on crop plants.

designed to detect the physical labels

and topical markers on the crop (Figures

5&6, see page 14). Weed control

was done by mechanical knives, which

the IC opens to avoid the marked crop

plants and closes (Figure 6, see page 14)

to uproot weeds in the intra-row space.

Field Trials

Eight field trials in tomato at Davis, Calif.,

and six in lettuce at Salinas, Calif.,

were conducted during 2016-2018.

Tomato. Field trials in processing

tomatoes were located on a silt loam soil

on the UC Davis vegetable field crops

research station near Davis. The tomatoes

were seeded in trays and kept in a

greenhouse for 45 to 60 days until they

were about 10 inches tall. Tomatoes

were transplanted into 60-inch beds at

15-inch spacing in a single center row.

Two tomato trials were carried to yield.

Plant labels were added to seedling

trays prior to transplanting (Figure 1)

or the topical marker was applied to

trays of tomato seedlings as described

above (Figure 4). Tomato transplants

were marked with paint 4 inches above

the soil line. About three weeks after

planting, all plots were cultivated with

a standard mechanical cultivator which

only removed weeds outside the plant

line. The standard cultivator left a

7-inch non-cultivated band centered on

the crop row.

Weed densities by species were measured

before and after cultivation in

four 7-inch-wide (centered on crop row)

by 20-foot-long sample areas randomly

placed along the length of the plots. The

time required by a laborer to hand weed

the 20-foot areas was recorded. Two

tomato trials were maintained until

Continued on Page 14

Figure 1. Plant labels in tray of tomato

seedlings prior to transplanting. The

labels and tomato plants were

transplanted together in the field.

Figure 2. Holland transplanter with

butterfly transfer fingers used for

transplanting plant labels and tomatoes


Figure 4. Topical marker sprayed

on tomato transplants by applicator

mounted on the transplanter during

the process of transplanting.

July / August 2020 www.progressivecrop.com 13

Continued from Page 13

harvest so that marketable yield data

could be collected.

Lettuce. Field trials using Romaine

lettuce were conducted in a sandy loam

soil at the USDA research station in

Salinas, Calif. Four weeks after seeding,

the whole experiment was cultivated

with a standard mechanical cultivator.

The standard cultivator left a 6-inch

non-cultivated band centered on the

crop row (Figure 7). The IC operated

within .75 inches of the lettuce plants

on all sides. Pre-cultivation weed

counts were measured the day before

cultivation and post-cultivation weed

counts were taken the day after cultivation.

Weed densities were measured in a

6-inch band centered on the crop row in

each of two 20 -foot-long samples in the

field. Weeds that were uprooted were

considered dead. After cultivation, hand

weeding was performed and timed as

described for the tomato trials. The

time spent by a laborer to hand weed

with a hoe was recorded.

The 2017 lettuce trials were maintained

until commercial maturity and number

of marketable heads and weight of marketable

heads were recorded. The 2018

trial was conducted at a commercial

lettuce field near Salinas, Calif.

Figure 5. Image of a tomato plant with a green label taken (a) under normal light plus UV

light, and (b) under UV light only. Note the reflections of the green label in the six mirrors,

and the actual label in the center of the image.

Figure 6. The actuator device used in this project: (a) Weed knives closed - uprooting weeds

in crop row, (b) Weed knives open avoiding tomato plant.

Statistical Analysis. RStudio Version

1.1.383 was used for statistical analysis.

Differences between pre- and post-cultivation

weed counts determined weed

removal effectiveness. The most efficacious

treatments removed the greatest

proportion of weeds.

The difference in weed densities between

pre and post cultivation were

analyzed using analysis of co-variance,

to measure the effect of cultivator type

on weed density. Analysis of variance

(ANOVA) was performed on the

hand-weeding time data to measure the

effect of the cultivators. Weights were

determined for both lettuce and tomato

yields, and in lettuce, the number of

heads was also determined.

Weed Control. The IC was more

effective than the standard cultivator

Figure 7. The plant layout used in the lettuce plantings: (a) Single crop row of lettuce on

1 m beds. The control rows are with no crop signal visible, (b) physical labels in lettuce row

two weeks after transplanting.

at removing weeds from the inter-row

space. The data were pooled separately

for tomato and lettuce. In tomato seed

lines, 1 weed per square foot remained

after IC while 10.5 weeds per square

foot remained after standard cultivation.

This is a 90% reduction in the number

of weeds remaining after cultivation


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(“3 Bugs. 1 Jug.”) And university trials in Arizona and California show that Radiant

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Table 1




Weeds remaining

after cultivation

Time to

hand weed

Marketable yield

Tomato b


Lettuce c






No./ft 2

1.0 a

10.5 b

1.7 a

5.0 b


7.8 a

14.9 b

16.0 a

29.1 b


44,045 a

50,217 a

40,830 a

33,887 a

No. lettuce

heads/ A



15,851 a

14,945 a


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Since 1982


Continued from Page 14

reduction in weeds remaining after cultivation

(see Table 1).

Handweeding in the tomato trials

required 7.8 hours/A following the IC

while the standard cultivator required

14.9 hours/A which is a 48% reduction






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Continued from Page 16

marketable fruit yields in the IC and

standard cultivator treatments of 44,045

and 50,217 lbs./A, respectively (P>0.05).

Similarly, there were no differences

between the cultivators in their effect on

lettuce yields (P>0.05) (Table 1). Yield

data were analyzed both as the number

of marketable lettuce heads per acre and

fresh weights.

Weed/Crop Differentiation. One of

the biggest challenges for automated

intra-row cultivation is to enable a computer

and vision system to differentiate

between crops and weeds at normal field

travel speeds. The commercially available

IC ‘Robovator®’ uses pattern recognition

to recognize the crop row and can

perform intra-row weeding at speeds of

1 mph (Lati et al. 2016). However, this requires

a distinct crop pattern best found

such as in a transplanted field where

the crop is much larger than the weeds

and the crop stand is consistent. Further,

when high weed densities obscure the

2-dimensional crop row pattern, the intra-row

weeding program does not work.

Two types of crop signals were tested,

Solutions for the Earth

physical plant labels and topical markers.

The methods have very low false positive

error rates and the classification accuracy

achieved for both techniques approaches

100%. The crop signaling technique appears

to be effective in creating a reliable

method for automatic detection of crop

plants in vegetable fields with high weed

densities. Crop signaling technology

could facilitate development of automated

weed control robots that are as

accurate in crop/weed differentiation as

human workers are.

A recommendation for future work is to

develop a commercially viable marking

method that is machine readable, yet

does not contaminate harvested produce

or the field soil and subsequent rotational

crops. For transplanted stem crops like

tomato, a biodegradable machine-readable

tag attached to each stem as the

transplanter sets the plants should be

explored for commercial potential.

Lettuce will probably require a machine-readable

label attached to the first

true leaves or a machine-readable label

on the fiber-coated plant plug as it is set

in the soil as is done with the Plant Tape®

(www.planttape.com) system of vegetable



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Regardless of the technology used for

crop weed differentiation, development

of intelligent weed removal technology

has improved weed control programs for

horticultural crops that continue to rely

on a limited number of herbicides and

hand weeding. However, there is much

more to do to improve vegetable weed


Acknowledgments. Thanks to the

USDA Institute of Food and Agriculture

Specialty Crop Research Initiative (US-

DA-NIFA-SCRI-004530), the California

Tomato Research Institute and the California

Leafy Greens Research Program

for financial support.


Lati, R.N., M.C. Siemens, J.S. Rachuy, and S.A.

Fennimore. 2016. Intra-row Weed Removal in

Broccoli and Transplanted Lettuce with an Intelligent

Cultivator. Weed Technology 30:655-663




Comments about this article? We want

to hear from you. Feel free to email us at


18 Progressive Crop Consultant July / August 2020

Virus Pathogens: Challenges to

the Health of Vegetable Crops

By STEVEN KOIKE | Director, TriCal Diagnostics

Farmers and other field

professionals producing vegetable

crops face a bewildering array

of challenges. Insects and mites feed

on, disfigure, and eat away at produce

quality. Weeds compete with the vegetables

for precious resources and can

require extensive labor to be removed.

Fertilizer and water inputs can be

costly. The economic cycle of planting,

growing, harvesting, and marketing

can be a “black hole” that engulfs

company resources while offering few

guarantees of profits. Another group

of challenges is embodied by the many

plant pathogens that cause diseases of

vegetable crops. One particular group

of pathogens of interest are the viruses

that infect plants.

Virus Pathogens of Plants

Viruses that infect plants are similar,

in shape and constitution, to the

viruses that infect insects, animals,

and yes, people. A virus consists of a

piece or two of genetic material (either

DNA or RNA) that is surrounded and

protected by a protein coat or covering.

In the grand scheme of biology, such

a nucleic acid + protein structure is

extremely simple and basic. This entity

is also extremely tiny. Since a virus is

composed of two types of chemicals, it

is much smaller than a plant cell and

cannot be observed with a regular microscope.

Only with the use of electron

microscopes can the body of the virus

be observed. The outer protein coat

gives the virus a distinctive shape, and

plant viruses can look like long flexible

threads, short rigid rods, or spherical,

geometric polyhedrals (See photos 1

and 2.)

Different viruses have different shapes and appear as long threads, rigid rods, or geometric

spheres. Photo 1: Left, tomato chlorosis virus (photo courtesy K. Schlueter, USDA) and,

Photo 2: right, cucumber mosaic virus (photo courtesy M. Kim, USDA.)

Plant viruses, like all viruses, do not

function or operate outside of their

hosts. To become active the virus must

be introduced into a living plant cell,

after which the virus mechanism activates

and highjacks the cell’s processes,

forcing the host cell to produce more

virus RNA or DNA and virus proteins.

These components are assembled into

new viruses which are then translocated

throughout the plant by being

carried in plant fluids that stream into

stems, leaves, flowers, and fruits.

Diseases Caused by Viruses

As with viruses that infect people and

animals, plant pathogenic viruses at

first show no evidence of their initial

incursion into the host. There is a latent

period or lag-time during which the virus

is steadily orchestrating the manufacture

of additional virus nucleic acids

and proteins. At a certain critical point,

the virus population causes enough

physiological and metabolic disruption

so as to cause visible symptoms, which

collectively we call the disease.

Disease symptoms caused by viruses

can vary greatly and are influenced by

the vegetable variety, age of plant when

first infected, the strain of the virus,

and environmental conditions under

which the crop is grown. In general,

vegetable crops infected with viruses

will show one or more types of foliar

symptoms. Leaf color changes with the

development of yellow or brown spots,

light and dark green patterns (mosaic,

mottling), concentric ring patterns

(ringspot), and yellow or white blotches

and streaks. In some cases, the entire

foliage of the plant turns yellow, orange,

or red. Some viruses cause a curious

reaction where only the veins of the leaf

become yellow or brown. Leaves can

be misshapen in various ways, from

simple curling, to unusual elongation

(strap leaf), to severe twisting and

deformation. Internodes along the stem

become abnormally shortened, resulting

in tight bunching of leaves. Flowers

also change appearance with streaks

of color in the petals (color break). For

fruiting vegetables, the fruit may show

only subtle color breaks and patterns,

20 Progressive Crop Consultant July / August 2020

or alternatively become grossly deformed.

Overall plant growth can be

stunted and crop development can be


All vegetable crops suffer from at least

one virus pathogen, while some crops

are subject to a dozen different ones.

Table 1(see page 22) lists selected

vegetable crops and some of the viruses

affecting these crops in the U.S. Like

fungal and bacterial pathogens, virus

pathogen occurrence and importance

vary with geographic region. A virus

that is important on California lettuce

may be incidental or lacking on lettuce

in Florida. Likewise, the set of viruses

that North American tomato growers

must deal with will be different than

tomato viruses occurring in South

America or Asia.

The economic impact of a particular

crop-virus interaction depends on the

inherent aggressiveness of the virus, incidence

of the disease, and the susceptibility

of the crop. Regarding the crop,

a critically important factor is the type

of harvested commodity. For example,

leafy commodities such as lettuce and

spinach will be especially vulnerable

to viruses that cause leaf symptoms.

The viruses of pepper that cause fruit

malformations are more important

than the pepper viruses that only cause

mild mosaics in the foliage. For celery

grown in California, cucumber mosaic

virus (CMV) causes some leaf mosaic

and mottling but rarely causes any

symptoms on the celery petioles and,

therefore, is of little concern. However,

a different virus, Apium Virus Y, can

cause celery petioles to turn brown,

making the celery unmarketable.

Detecting and Diagnosing Viruses

Confirmation of a virus requires testing.

We acknowledge that experienced

growers and field personnel, who have

looked at virus diseases of a particular

Spinach leaves are greatly deformed and

discolored by Tobacco rattle virus (photo

by S. Koike, TriCal Diagnostics.)

crop for many years, can develop a

good diagnostic sense for such problems.

However, to be scientifically

sound and accurate, diagnosing virus

diseases cannot be achieved without

clinical testing. Virus disease symptoms

pose particular challenges to diagnosticians

because the wide range of

virus-like symptoms can also be caused

by other factors.

Symptoms caused by viruses can also

Continued on Page 22




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Continued from Page 21

be caused by genetic disorders, nutritional imbalances,

environmental extremes, phytotoxicity

from pesticides and fertilizers, and other factors

(see Table 2 on page 24.) Fortunately, diagnostic

labs have the tools that can identify most of the

commonly occurring viruses in vegetables. Such

tests rely on either serology (using antibodies

that detect the antigens of virus proteins) or

molecular biology (using probes that recognize

nucleic acid sequences of the virus.)

Epidemiology of Virus Diseases

Development of virus diseases of plants involves

several factors. In contrast to some human

viruses, plant viruses are not moved around in

the air or deposited on surfaces waiting to come

into contact with a plant. Rather, plant pathogenic

viruses typically originate from a living

source or “reservoir.”

The reservoir is often an infected weed that is

near the site where the vegetable crop will be

planted, or the reservoir can be an infected volunteer

crop plant in the field (Factor 1.) Vectors

(Factor 2) are the insects, mites, and nematodes

that have fed on a virus-infected plant, ingested

virus particles, and now are capable of injecting

the viruses into the next plant that is fed upon.

For the great majority of viruses that infect

vegetables, the viruses are moved by vectors

from reservoir hosts to healthy crops (Factor 3).

Aphids are the most common vectors (See Table

1.) Other insects (thrips, leafhoppers, beetles)

also carry viruses, as do a few soilborne nematodes

and one soilborne fungus.

The epidemiology, or progress of disease spread,

Continued on Page 24


Crucifers Cauliflower mosaic virus aphid

Cucumber mosaic virus


Turnip mosaic virus


Carrot Carrot mottle virus aphid

Carrot redleaf virus


Carrot thin leaf virus


Celery Apium virus Y aphid

Celery mosaic virus


Cucumber mosaic virus


Cucurbits Beet curly top virus leafhopper

Cucumber mosaic virus


Cucurbit yellow stunting disorder virus whitefly

Papaya ringspot virus


Squash mosaic virus

cucumber beetle, seed

Watermelon mosaic virus


Zucchini yellow mosaic virus


Lettuce Alfalfa mosaic virus aphid

Beet western yellows virus


Cucumber mosaic virus


Impatiens necrotic spot virus


Lettuce mosaic virus

aphid, seed

Mirafiori lettuce virus

soilborne fungus

Tomato spotted wilt virus


Turnip mosaic virus


Onion Iris yellow spot virus thrips

Onion yellow dwarf virus


Pepper Alfalfa mosaic virus aphid

Beet curly top virus


Cucumber mosaic virus


Impatiens necrotic spot virus


Pepper mottle virus


Potato virus Y


Tobacco etch virus


Tomato spotted wilt virus


Spinach Cucumber mosaic virus aphid

Impatiens necrotic spot virus


Tobacco rattle virus

soilborne nematode

Tomato spotted wilt virus


Tomato Alfalfa mosaic virus aphid

Beet curly top virus


Cucumber mosaic virus


Potato virus Y


Tobacco etch virus


Tobacco mosaic virus

seed, mechanical transmission

Tobacco streak virus

thrips, tomato pollen

Tomato infectious chlorosis virus whitefly

Tomato spotted wilt virus


Tomato yellow leaf curl virus


In lettuce, Impatiens necrotic spot virus results in

distorted plants and brown leaf lesions (photo by S.


Table 1. Selected vegetable crops, virus pathogens, and means of virus dispersal.

22 Progressive Crop Consultant July / August 2020

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Symptoms caused

by viruses

by viruses

leaf spots

leaf mosaic

leaf mottling

leaf ringspot

leaf chlorotic streaking

leaf curling

leaf elongation/strap leaf

leaf vein yellowing

young leaf distortion

foliage bunching

foliage yellowing

foliage reddening

flower color break

fruit distortions

plant stunting

yield reduction

Similar symptons caused by non-virus factors

Similar symptoms caused by non-virus factors

fungal or bacterial pathogens, pesticide damage

plant nutrition, genetic disorders

plant nutrition, genetic disorders

physiological factors, sensitivity to leaf wetness

genetic disorders

environmental extremes, pesticide damage

pesticide damage

plant nutrition, pesticide damage

pesticide damage, insect toxins, environmental extremes

pesticide damage, insect toxins, environmental extremes

plant nutrition, genetic disorders, pesticide damage, irrigation issues

plant nutrition, genetic disorders, environmental extremes

genetic disorders

genetic disorders

soilborne pathogens, compacted soils, irrigation issues

other pathogens, pests, field conditions, environmental extremes

Table 2. Symptoms caused by viruses and other factors that can create similar symptoms

Continued from Page 22

depends on the complex interaction of

the three factors mentioned above.

Factor 1 Reservoir: What is the nature

of the virus reservoir? Which weed

species are present? Are there high

numbers of virus-infected weeds or volunteer

plants in the area? A virus with

a broad host range, such as Tomato

spotted wilt virus (TSWV), may be

present in dozens of weeds and numerous

volunteer plants on a particular


Factor 2 Vector: Which vectors are in

the vicinity? What are their populations

and dispersal patterns? How do

wind patterns and geographic features

influence dispersal? What is the

extent of vector increase within the

crop, which can result in plant-to-plant

spread within that planting?

Factor 3 Vegetable Crop: What is the

crop diversity in the area being considered

and which viruses affect these

crops? For example, could CMV, which

has a broad host range, spread between

different vegetables? If the region is

widely planted to one crop, such as lettuce,

will a particular virus affect many

lettuce plantings? Too much of the

same crop, densely cropped in one region,

could result in rapid virus spread

and disease epidemics. In contrast, if a

region has only one onion field among

many non-allium crops, a narrow hostrange

pathogen such as Iris yellow spot

virus will infect only the onions. The

answers to these and other questions

have significant bearing on the management

of virus diseases.

Managing Virus Diseases

Diagnosis: The first step in disease

management is accurately identifying

the precise pathogen involved. Molecular

and serological assays are available

for most of the major virus pathogens

affecting vegetables. Knowing which

virus is involved enables one to know

the reservoir plants harboring the virus,

the vectors involved, and the potential

target crops.

Exclusion: Prevent the virus from entering

the production system. For lettuce,

Carrot fields severely infected with viruses

become noticeably yellow to orange in color

(photo by S. Koike.)

cucurbits and tomato, some viruses are

carried in the seed; therefore, use seed

that has been tested or certified to not

harbor the pathogen. For crops started

as transplants, employ IPM practices

to prevent infection at the transplant

stage. Note that for the few vegetable

crops propagated by cuttings or plant

divisions (example: artichoke), viruses

will be readily spread if infected

propagative material is used to plant

new fields.

Reservoir host eradication: Remove

the initial sources of the virus, which

are infected weeds and volunteer crop

24 Progressive Crop Consultant July / August 2020

Apium virus Y causes disfiguring brown

lesions on celery petioles (photo by S. Koike.)

A number of virus pathogens cause damage

to the fruits of some vegetable crops (photo

by S. Koike.)

plants. Plant viruses are present mostly

in living plants and generally not in soil,

water, equipment surfaces, or the air.

Controlling weeds and other reservoir

plants is therefore a critical part of virus


Manage the vectors: Use IPM practices to

control the virus vectors. The great majority

of vegetable-infecting viruses only

reach a crop via an insect vector. Complete

control of an insect pest is rarely

possible, so strategies should attempt

to manage the insects as best as possible.

Keep in mind that the vectors are

also present on the reservoir weeds and

plants outside of the field. Once a virus

is introduced into the crop, intra-field,

plant-to-plant spread will be achieved

only through movement of the vector.

Destruction of the old crop: Once a crop

has been harvested, the passed-over

plants and shoots growing from remaining

crop roots can serve as virus reservoirs

if they are infected. Old vegetable

fields should, therefore, be disked and

plowed under in a timely manner.

Resistant cultivars: If available, growers

should select cultivars that are bred

to be resistant to the virus pathogens.

Note, however, that the usefulness of

such genetic plant resistance may not

last. Researchers found that the use of

tomato and pepper cultivars resistant to

TSWV has allowed for the development

of “resistance breaking” (RB) strains

of the virus. Through mutation and selection,

these new strains of TSWV can

cause disease in the previously resistant


Chemicals or pesticides: Currently there

are no chemical treatments that can be

applied to plants that would prevent

infection from viruses or prevent development

of virus disease.

Comments about this article? We want

to hear from you. Feel free to email us

at article@jcsmarketinginc.com

July / August 2020 www.progressivecrop.com 25

By planting time, the cover crop residue

has already decomposed. This method

reduces runoff and erosion but does not

reduce nitrate leaching, so this is best for

fields with runoff problems but without

high nitrate levels. However, this method

makes controlling weeds during a wet

winter difficult and costs more than simply

leaving the field bare (Brennan, 2017.)





By SHULAMIT SHRODER | Community Education Specialist,

UCCE Kern County

Almond Orchard Cover Crop study in Kern County, March 2019 (photo by S. Shroder.)

Growers throughout the

country and around the world

plant a wide range of cover crops

for a variety of reasons. Cover crops can

reduce soil compaction, improve water

infiltration, improve soil structure, and

feed soil microbes: they encourage a

healthier and more diverse soil ecosystem.

Researchers in California are analyzing

the best ways to incorporate cover

cropping into the state’s diverse agricultural

systems, from high-value vegetable

production on the central coast to the

cotton, tomato, and almond fields of the

central valley.

Cover Crops on the Central Coast

Researchers working with central coast

vegetable growers have devised innovative

ways to use cover crops to reduce

nitrate leaching and agricultural runoff,

thereby improving both local ecosystems

and soil health.

Eric Brennan and his team at the USDA

Agricultural Research Service started the

Salinas Organic Cropping Systems trial

in the Salinas Valley in 2003 to understand

the long-term impacts of various

cropping systems and soil amendments.

This trial focuses on organic lettuce and

broccoli, two of the high-value crops

grown in the area known as the nation’s

salad bowl.

To maintain soil organic matter and

provide nutrients to their crops, organic

vegetable growers in this area prefer applying

compost instead of planting cover

crops. The amount of time that cover

crops require for incorporation and

decomposition can shorten the growing

season for these high-value crops (Brennan

& Boyd, 2012.) To make this practice

more feasible for growers in the area, this

group of researchers has developed three

strategies for integrating cover crops into

the vegetable cropping systems of the

Central Coast.

Option 1: Plant the cover crops only

in furrow bottoms, not the entire field.

After 50 to 60 days of growth, the grower

can spray the cover crops and then do

the usual tillage necessary to prepare

the ground for planting the cash crops.

Option 2: Plant non-legume cover crops

on the vegetable beds and mow the cover

crops repeatedly throughout the growing

season. This maximizes nitrate scavenging

while minimizing the amount of

residue that needs to decompose right

before planting. The ideal cover crop for

this practice would be a grass, like cereal

rye. Repeated mowing would reduce the

amount of water lost to evapotranspiration

from the cover crop but still enable

the rye to scavenge nutrients that could

otherwise be lost to leaching (Brennan,


Option 3: Turn the cover crop residues

into a highly nutritious juice and

compost. To do this practice, a grower

would plant a non-leguminous cover

crop in October and allow it to grow

until mid-December, at which point it

will have scavenged most of the nitrogen

that it will use. The grower then harvests

the cover crop, leaving as little residue

behind as possible. They can then feed

the residue into a screw press, which will

separate the liquids and solids. The liquid

component has a relatively low nitrogen

concentration and can be applied to the

vegetable crop to fulfill some of the crop’s

nutrient needs. The solid residues can be

composted and applied at a convenient

time, to provide organic matter to the

soil (Brennan, 2017.)

Researchers are still working on refining

these strategies, but they could allow

central coast vegetable growers to reap

the rewards associated with cover crops

while maintaining a profitable enterprise.

Annual Systems in the Central Valley

For the past 20 years, Jeff Mitchell and

his team at UC Cooperative Extension

have studied the effects of reduced tillage

and cover crops on a tomato-cotton

rotation at the UC’s West Side Research

and Extension Center. This study mea-

Continued on Page 28

26 Progressive Crop Consultant July / August 2020


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Continued from Page 26

sures the efficacy of these practices in reducing air pollution

and increasing soil organic matter. Reduced tillage and cover

cropping have resulted in less dust emissions compared to conventionally

managed fields (Mitchell et al., 2017.) They found

that cover cropping increased soil organic matter more than

conservation tillage alone did (Veenstra et al., 2006.) Overall,

these practices have improved soil health by increasing aggregate

stability, water infiltration, and soil organic matter while

maintaining similar yields to the conventional system (Mitchell

et al., 2017.) This study has allowed researchers to see the

long-term effects of conservation tillage and cover cropping on

tomato and cotton systems in the San Joaquin Valley.

Field day at the West Side REC in 2010, discussing cover cropping and

conservation tillage (photo courtesy Jeff Mitchell, UCCE.)

Another UC research team in the Central Valley, led by Kate

Scow at the Russell Ranch near UC Davis, examined the longterm

effects of cover cropping on organic tomatoes and corn.

These researchers found that cover cropping encouraged the

proliferation of diverse types of beneficial fungi known as arbuscular

mycorrhizal fungi (Bender & Bowles, 2018). Under optimal

environmental conditions, cover cropping was correlated

with higher tomato yields. In contrast, corn did not enjoy the

same benefits from organic management that the tomatoes did

and had lower yields compared to fields without cover crops

(Bender & Bowles, 2018). These studies have found important

benefits to including cover crops in annual systems, but growers

will need to further refine the practice to fit their needs.


If you could climb inside your plant, and take a

look at the cell walls, you would see they are

made of calcium and a pectin. The pectin acts like

a glue forming Calcium Pectase, to help keep the

cell walls strong and tight. When calcium is available

to the cells the walls become as strong as

concrete. When calcium is limited the walls are as

weak as paper.

When the cell walls are strong the plant is

strong, this includes the roots. Calcium is a major

player in the construction of some hormone and

enzyme systems that can help protect the plant

from insect and disease attack.

Check with your soil advisor and make sure

you have enough calcium in your soil to protect

your plant and your crop.

Lack of Calcium

leaves plant cell walls

open to invaders.

Calcium makes

cell walls resistant

to invaders.

Ask for it by name

Blue Mountain Minerals

Naturally the Best!

For more information 209-533-0127x12

Mustard cover crops in a table grape vineyard, March 2020 (photo by S.


Perennial Systems in the Central Valley

Amélie Gaudin and her team from UC Davis and UC Cooperative

Extension are quantifying and communicating the

benefits and tradeoffs of planting winter cover crops in almond

orchards. They established trials throughout the Central Valley.

Planting cover crops in almonds increases bee forage, improves

soil health, and encourages resiliency. The researchers have

found that cover crops resulted in increased water infiltration.

Despite the common concern that cover crops would increase

frost risk, they found that cover cropping did not affect ambient

air temperatures 3 and 5 feet above the ground. Moreover, the

ground cover worked as a buffer, keeping temperatures more

stable than bare ground did (Gaudin, 2020.)

Other benefits included a decrease in sodicity, improved

trafficability in the wintertime, and an increase in aggregation.

The soil microbial ecosystem showed increased biomass. Bees

enjoyed a more diverse, varied diet, contributing to better bee

health. Finally, cover crops reduced weed diversity and growth.

They did not reduce germination since both the cover crops and

the weeds emerged at the same time. All these benefits start

to outweigh the costs of implementation after about 10 years

(Gaudin, 2020). Many of these soil and ecosystem benefits are

not unique to almond orchards, and could also benefit other

28 Progressive Crop Consultant July / August 2020

perennial cropping systems in the Central


Funding Options

UC and USDA researchers have found

benefits to cover cropping in diverse

agricultural systems throughout California,

from almond orchards to lettuce and

tomato fields. These include reducing

erosion, compaction, and nutrient leaching,

along with improving soil aggregation

and providing habitat for beneficial

insects. Cover crops may improve the

soils upon which your crops depend and

increase your operation’s resiliency in

the face of a changing climate.

The California Department of Food and

Agriculture’s Healthy Soils Program and

the USDA NRCS EQIP provide incentives

for planting cover crops. Check out


Program to learn more about the CDFA’s

program. There are 10 technical assistance

providers working throughout the

state who can help you select your cover

crop species, apply for the program, and

implement your practices. Go to


to find your closest climate smart specialist.

Works Cited

(2010). [Field day at West Side Research and

Extension Center] [Photograph]. California

Agriculture. http://calag.ucanr.edu/Archive/?article=ca.v070n02p53

Bender, S.F & Bowles, T.M. (2018). Effects of AMF

diversity and community composition on nutrient

cycling as shaped by long-term agricultural

management. Russell Ranch 2018 Annual Report.



Brennan, E. B. (2017). Can we grow organic or

conventional vegetables sustainably without

cover crops? HortTechnology, 27(2), 151-161.

Brennan, E. B., & Boyd, N. S. (2012). Winter cover

crop seeding rate and variety affects during

eight years of organic vegetables: I. Cover crop

biomass production. Agronomy Journal, 104(3),


Gaudin, A. (2020, February 4). What do cover

crops have to offer? [PowerPoint slides].

University of California Agriculture and Natural

Resources. https://ucanr.edu/sites/calasa/


Mitchell, J. P., Shrestha, A., Mathesius, K., Scow, K.

M., Southard, R. J., Haney, R. L., ... & Horwath, W.

R. (2017). Cover cropping and no-tillage improve

soil health in an arid irrigated cropping system

Community Education Specialist Alli Fish and

a daikon radish cover crop in December 2019

(photo by Rose Hayden-Smith.)

in California’s San Joaquin Valley, USA. Soil and

Tillage Research, 165, 325-335.

Veenstra, J., Horwath, W., Mitchell, J., & Munk, D.

(2006). Conservation tillage and cover cropping

influence soil properties in San Joaquin Valley

cotton-tomato crop. California Agriculture, 60(3),


Comments about this article? We want

to hear from you. Feel free to email us at


Helping Farmers Grow NATURALLY Since 1974


Office: 559-686-3833 Fax: 559-686-1453

2904 E. Oakdale Ave. | Tulare, CA 93274


July / August 2020 www.progressivecrop.com 29

Lettuce Dieback: New Virus

Found to be Associated with

Soilborne Disease in Lettuce


Lettuce dieback is a soilborne virus

disease known to cause significant

losses for lettuce production throughout

all western growing regions. The

disease was originally described in the

Salinas Valley in the late 1990s following

severe flooding along the Salinas River

but has now been found throughout

coastal and inland lettuce production

regions of California, the winter production

region in southwestern Arizona and

Imperial Valley, California.

The disease is most prevalent on romaine

lettuce but is known to occur on all

non-crisphead (iceberg) lettuce types.

Most modern crisphead lettuces are

resistant, and an increasing number of

romaine cultivars now carry resistance

as well. Symptoms of lettuce dieback

include yellowing and necrosis of outer

leaves, stunted growth and death of

affected plants (Figure 1). Plants infected

young may fail to develop beyond the 8

to 10 leaf stage, but symptoms can develop

at any point in the growing season,

and fields often exhibit a range of plant

sizes with some plants appearing healthy

and maturing normally, while others

became stunted and never fully develop

(Figure 2, see page 31).

Initial symptoms begin with yellowing

and necrosis (death) of small veins in

outer leaves, with the necrosis expanding

into larger areas within and between

veins. Inner leaves of the head usually

retain their color, but some romaine

varieties may also exhibit bright chlorotic

flecks within veins of leaves at the center

of the head that resembles tiny stars.

These are most visible when affected

leaves are held up to a light source (Figure

3, see page 31).

This vein-flecking symptom is not always

present on infected romaine, but when

observed it is an excellent diagnostic

indicator. The vein flecking symptom is

less common on other types of lettuce

and is more difficult to observe on red

lettuce. Losses resulting from lettuce

dieback can range from a few plants to

complete loss of crop. In most severely

affected fields lettuce heads are not

harvested because the plants will not

meet quality standards. Symptoms of the

disease are frequently found in low lying

areas with poor drainage, in areas near

rivers, on recently flooded land, and in

areas where soil has been dredged from

a river or ditch and spread onto adjacent


Figure 1. Romaine lettuce plant near maturity showing

classic symptoms of outer leaf yellowing and necrosis.

Symptoms may develop at any growth stage (all photos

courtesy W.M. Wintermantel.)

Symptoms of lettuce dieback can be

mistaken for those of other diseases, particularly

lettuce drop, a disease caused by

a fungus, and symptoms of two viruses

transmitted by thrips. It is fairly easy to

differentiate lettuce drop from lettuce

dieback because lettuce drop, caused by

fungi in the genus Sclerotinia, results in a

soft rot, outer leaves often flatten against

the ground, and heads easily separate

from the root, whereas with lettuce

dieback the root remains firmly attached

to the head. The two thrips-transmitted

viruses, impatiens necrotic spot virus

(INSV) and tomato spotted wilt virus

(TSWV), also cause necrotic (dead)

patches on leaves of infected lettuce

plants that resemble symptoms of lettuce

dieback, and therefore it can be difficult

to differentiate the two diseases. Diagnostic

tests can be used to differentiate

lettuce plants infected with these viruses

from those with lettuce dieback disease.

Serological detection methods including

commercially available immunostrips

that can be used in the field to determine

infection with INSV or TSWV, but

immunostrips are not available for the

viruses associated with lettuce dieback

disease. Therefore, confirmation of lettuce

dieback requires laboratory testing,

which can include both molecular biology

and serological methods. In some

cases, lettuce plants may be infected by

multiple pathogens simultaneously and

this may complicate diagnosis.

Lettuce dieback is probably a very old

disease of crisphead (iceberg) lettuce

that disappeared for many years before

reemerging with a new name as a disease

of other lettuce types. In the 1930s a

disease known as brown blight devastated

lettuce production in California with

symptoms that closely resembled those

of lettuce dieback based on descriptions

and illustrations at the time.

Iceberg lettuce was the main type of lettuce

grown in the 1930s, and it suffered

severe losses from brown blight for many

years until a source of resistance was

identified by a USDA scientist, Ivan Jagger.

This source of resistance was eventually

bred into all subsequent iceberg

lettuce types, beginning with the variety

Imperial, and this eliminated the threat

from brown blight. In the early 2000s,

after the appearance of lettuce dieback,

USDA scientists identified a source of

resistance to lettuce dieback from the

crisphead lettuce variety Salinas, and

through genetic studies found that the

source of resistance to lettuce dieback is

also present in the brown blight-resistant

lettuces developed by Jagger over 70

years earlier, but was not in earlier susceptible

lettuce varieties. In other words,

only crisphead lettuce varieties that

predate the variety Imperial could develop

symptoms of lettuce dieback. This

suggests the two diseases may actually

be the same. The resistance to lettuce dieback

has been incorporated into several

romaine lettuce varieties, as well as some

leaf and butter lettuce varieties, but there

remain many lettuces that are susceptible

to lettuce dieback disease.

30 Progressive Crop Consultant July / August 2020

Figure 2. Romaine lettuce plants in a field

showing variation in severity typical of lettuce

dieback including stunted growth, as well as

yellowing and necrosis of outer leaves.

Since the late 1990s, lettuce dieback has

been believed to be caused by infection of

lettuce plants with either of two viruses

from the genus Tombusvirus; tomato

bushy stunt virus (TBSV) and Moroccan

pepper virus (MPV). These viruses are

absent from healthy lettuce but have

been found regularly in association with

lettuce dieback disease. However, there

have been numerous situations in which

neither virus was found in association

with obvious disease symptoms. Furthermore,

it has not been possible to

consistently and easily reproduce disease

symptoms when lettuce is inoculated

with either virus in a laboratory setting,

raising the possibility that an additional

virus may contribute to causing lettuce

dieback disease.

In an attempt to identify a possible

additional virus contributing to lettuce

dieback disease, high throughput

sequencing (HTS) was used on several

lettuce plants exhibiting dieback symptoms,

which led to the identification of

a new virus consistently associated with

diseased plants but not with healthy

lettuce plants. This novel virus was most

closely related to a recently identified

and poorly characterized virus from watermelon

in China, watermelon crinkle

leaf associated virus, which was found

using the same HTS approach.

The newly identified lettuce virus, tentatively

named lettuce dieback associated

virus (LDaV) shares an extremely low

genetic relationship with the watermelon

virus, which suggests that although the

two viruses are related, they are very

distantly related to one another. Using

a combination of HTS and traditional

DNA sequencing the genome of the

new virus, LDaV, was assembled and

Figure 3. Romaine lettuce leaf from the inner

portion of a head showing star-shaped chlorotic

flecking in veins characteristic of lettuce dieback

disease on romaine.

methods were developed to allow rapid

detection of the virus from lettuce leaf

extracts using RT-PCR, a routine laboratory

diagnostic method. LDaV has now

been found not only in lettuce showing

dieback symptoms collected recently, but

it has also been found in older archived

samples of lettuce nucleic acid collected

from plants showing dieback symptoms

over the past 20 years, including many

that also contained MPV or TBSV. To

date, LDaV has not been found in healthy

lettuce plants. Interestingly, genetic

comparison showed that LDaV isolates

collected from coastal California production

regions are closely related to one

another, and desert isolates from Arizona

and Imperial Valley, California also are

closely related to one another. However,

coastal and desert isolates differ genetically

from one another, suggesting

perhaps some regional adaptation of the

virus to plants grown under the different

climatic conditions.

Further research will clarify the role of

LDaV in lettuce dieback disease and how

it relates to the two tombusviruses, MPV

and TBSV, that have long been linked

to the disease. Studies to date, however,

strongly suggest a role for LDaV in

lettuce dieback disease development, and

research is in progress to clarify the ability

of LDaV to produce lettuce dieback

symptoms when inoculated to lettuce

plants, as well as whether or not the new

virus can infect lettuce plants carrying a

gene for resistance to lettuce dieback.

Comments about this article? We want

to hear from you. Feel free to email us at


July / August 2020 www.progressivecrop.com 31

Choosing Activator Spray Adjuvants for Permanent Crops

By FRANZ NIEDERHOLZER | UC Farm Advisor, Colusa and Sutter/Yuba Counties and

RHONDA SMITH | UC Farm Advisor, Sonoma County

Agricultural spray adjuvants are

materials added to the spray tank

when loading the sprayer. They

include products classified as activator adjuvants

and marketed as wetters/spreaders,

stickers, humectants, and/or penetrators.

Activator adjuvants are marketed to improve

the performance of pesticides and

foliar fertilizers.

Activator adjuvants can have a place in

tree (and vine) crop sprays, but matching

the material to the job can be tricky. A

bad match can lead to minor or major

losses to the grower. Minor losses can result

from excess spreading and pesticide

runoff from the target plant. Phytotoxicity

can cause major damage.

This article describes ingredients and

functions of activator adjuvants commonly

sprayed on tree and vine crops.

Suggestions regarding activator adjuvant

selection are offered. Growers must

make their own activator adjuvant use

decisions based on experience, particular

needs, and risk tolerance.

Should You Use an Adjuvant?

Read and follow the specific instructions

on the label. If the pesticide or foliar fertilizer

label indicates the product should

be used with certain types or brand of adjuvant(s),

that’s what you need to use. For

example, the Bravo Weather Stik® label

cautions against using certain specific adjuvants

and puts the responsibility in the

PCA or grower court regarding adjuvant

use. If the label includes phrases such as

"use of an adjuvant may improve results"

or “complete coverage is needed for best

results” then you may want to look into

selecting and using an appropriate activator


Before proceeding with use of an activator

adjuvant, first look at your existing

spray program. Are you already doing

the best spray job you can? Good spray

coverage begins with proper sprayer

calibration and set up. Is your sprayer

calibration dialed in for different stages of

canopy development? Optimum sprayer

set up—gallons of spray per acre, ground

speed, fan output, and nozzle selection/

arrangement—changes from dormant to

bloom to early growing season to preharvest

sprays. Adjusting your sprayer to best

match orchard and vineyard conditions

at each general stage in canopy development

is the foundation of an effective,

efficient spray program. An activator

adjuvant will not make up for excessive

tractor speed, poor nozzle arrangement

and/or worn nozzles. Your money is best

spent first dialing in your sprayer(s) for

the whole season, before considering an

extra material in the tank (that is not

required on the label).

If you have your sprayer(s) dialed in for

each orchard and stage of growth, now

is the time to say “OK, I want to think

about a little extra boost to my spray job.”

Which Activator Adjuvant to Choose?

First, know the properties of the pesticide

you will use. Does it work on the

plant surface or inside the plant? This is

a key point in selecting adjuvants. Here

is a quick review of the main classifications

and characteristics of activator

adjuvants as they currently appear in the

field. Note: Certain products can provide

more than one adjuvant property that can

be beneficial in the field. For example,

non-ionic surfactants can work as surfactants

and penetrators, depending on use


Wetters/spreaders: These materials contain

surfactants that decrease the contact

angle and increase the spreading of the

spray droplet on the target. High rates

of wetters/spreaders may also increase

penetration of pesticide into the target

tissue (leaves or fruit), potentially causing

phytotoxicity. Excessive spreading of

pesticide spray solution and runoff from

the target may result when using a new or

higher rate of spreader—especially when

using silicon “super-spreaders”. Test new

combinations of spreader material(s) and

spray volume before regular use. Spray

volume per acre or adjuvant use rate will

probably have to be reduced if a labeled rate

of adjuvant provides excessive spreading.

To check for excessive spreading, place

a length of black plastic sheeting under

several trees or vines in a row. Secure

the plastic with spikes, wire staples, and/

or weights. Spray the new adjuvant and

pesticide combination using your current

sprayer set up. Reenter the field right

after spraying, wearing appropriate

PPE, and evaluate coverage. If material

is pooling at the lower portion of leaves

and/or fruit, excessive spreading is occurring.

Check to see if pooling is occurring

only in a certain area(s) of the canopy

or throughout the canopy. If more spray

solution is landing on the black plastic

tarp under the trees/vines than between

them, then runoff is occurring. [Some

ground deposit should be expected from

standard airblast sprayer use.]

Compare the results of your adjuvant test

with a similar application of your current

pesticide/adjuvant combination on

another portion of the row. If there is no

pooling or runoff with the new adjuvant

in the tank, you can use the adjuvant

with confidence. A lack of pooling or

run off with the new adjuvant also might

mean that your old sprayer setup and

tank mix didn’t deliver adequate coverage.

If the test with the new adjuvant showed

pooling on leaves and/or runoff on the

ground, you have several choices: 1) You

can reduce spray volume per acre by

replacing some or all nozzles with smaller

nozzle sizes on the sprayer in an effort to

reduce overspreading. If you saw overspreading

on some portions of the canopy,

but not others, reduce nozzle size only

on the part of the spray boom that targets

the over-sprayed part of the canopy. Recheck

spray coverage if nozzling changes

were made. 2) Reduce the adjuvant rate

and recheck coverage/spreading. 3) You

can just go back to your established program

without the new adjuvant.

Continued on Page 34

32 Progressive Crop Consultant July / August 2020

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What’s the “best” course of action?

That depends on your farming operation.

Reducing spray volume per acre

means more ground covered per full

spray tank – a potential time and cost

savings. If spraying is done during the

heat of the day in hot, dry climate, spray

water evaporation is a major issue and

it may be best to keep the higher spray

volume and reduce the spreader rate or

eliminate it entirely. Checking coverage

and overspreading allows you to make

the best decision possible, avoid damage

and, hopefully, save money. All farming

operations are different. Make the choice

that best fits your farm.

Stickers: These adjuvants can increase

the retention time of the pesticide on the

leaf and reduce rain wash off. They may

limit movement of systemic pesticides

into the plant, and are probably most

beneficial when used with protectant

materials (cover sprays). Do you overhead

irrigate? Is there rain on the horizon?

If you answer yes to either one of these

questions, you may benefit from using a


Humectants: Under low humidity

conditions humectants can help reduce

spray droplet evaporation before and

after deposition on the plant. This is

especially valuable when small droplets

and/or materials that must be absorbed

into the plant (systemic pesticides, PGRs,

nutrients, etc.) are used in the summer

under high temperature and low relative

humidity conditions.

Penetrators: Frequently used with herbicides,

these products include oils (petroleum,

vegetable, or modified vegetable

oils) and non-ionic surfactants used at

higher rates. In crop sprays, penetrators

can be used to increase absorption of

systemic pesticides (for example, oil with

Agri-Mek) as well as translaminar materials.

Penetrator adjuvants should be used

with caution or avoided entirely with surface

active pesticides such as cover sprays

or else phyto may result. Finally, some

penetrators can increase the rain-fastness

of some pesticides.

What Adjuvant Material to Choose?

Use a product intended for crop spraying.

Many activator adjuvants were developed

and intended for use with herbicides.

Products that are advertised for use with

plant growth regulators should have a

higher chance of crop safety compared

with those that don't. This is still no guarantee

of a phyto-free application.

Ask for help from the adjuvant manufacturer’s

sales rep if needed. How much do

they know about the particular activator

adjuvant in the spray mix you are planning?

Will the Adjuvant Work?

If you choose to use an adjuvant that is

not specifically listed on the pesticide or

foliar fertilizer label, jar test the planned

spray solution first. Use the same spray

water source. Include all leaf feeds, other

adjuvants, and pesticide(s) that you plan

to put in the spray tank. Do this before

tank mixing these materials.

A lot of time and money rides on effective

pesticide application. Do your homework

before the spray tank is filled and you will

be well on your way to solid results.

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34 Progressive Crop Consultant July / August 2020







NEW! Peelable


NEW! Patent Pending, High-Release

Microporous Gel Multi-Gender

NOW Attractant System:



NOW L 2 High

Pheromone Lure

NOW L 2 Low

Pheromone Lure

















NOW PPO - HR L 2 + NOW L 2 - L = Multi-gender,

greater attraction in mating disrupted almonds

Pheromone PPO + Pheromone PPO PPO + Pheromone PPO PPO + Pheromone

Wing Trap, Modified PHEROCON® VI DELTA Trap PHEROCON® VI DELTA Modified

Source: SOURCE: Dr. Dr. Chuck Chuck Burks, Burks, Research Research Entomologist, USDA-ARS, USDA-ARS, Parlier, Parlier, CA, N=8 CA Mating Disrupted Almonds, N=8

NOTE: Do not use in organic orchards, or orchards

that are being prepared for organic approval.

Key Features: PHEROCON ® NOW PPO-HR L 2

• Multi-Gender NOW Attractant

• Duplicates Standard USDA vial release rate

• 12 weeks field longevity

• Easy to use; ready-to-use barrier pack

• Best Practice: Combine NOW PPO-HR L 2 + NOW L 2 -L =

Multi-gender, greater attraction in mating disrupted

almonds, pistachios, and walnuts

• Change NOW PPO-HR L 2 lure at 12 weeks and change

NOW L 2 -L lure at 4-6 weeks

Trap Options:

• PHEROCON ® 1CD QUICK-CHANGE, with expanded

SNUG-FIT ® spacer

• PHEROCON ® VI DELTA Modified, with cut outs






Your Edge – And Ours – Is Knowledge.

Contact your local supplier and order now!

Visit our website: www.trece.com or call: 1- 866-785-1313.







© 2020, Trécé Inc., Adair, OK USA • TRECE, PHEROCON and CIDETRAK are registered trademarks of Trece, Inc., Adair, OK USA • TRE-1770, 06/20

Improve nitrogen

metabolism during


diKaP (0-31-50) focuses on

abiotic stress defense by improving

nitrogen metabolism that can lead

to reduced incidence of hull rot.


Call to learn more:

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Find us online: http://redoxchem.com

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