Progressive Crop Consultant - September/October 2017

September/October 2017 The Pressure Chamber

Does it Have a Place in Your Toolbox?

Pythium Wilt Disease of Lettuce causing Diagnostic Challenge Remote Sensors for Precision Ag at West Hills College

California Pistachio Salt Tolerance Update

PUBLICATION

Volume 2 : Issue 5

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PUBLISHER: Jason Scott Email: jason@jcsmarketinginc.com EDITOR: Kathy Coatney Email: article@jcsmarketinginc.com PRODUCTION: design@jcsmarketinginc.com Phone: 559.352.4456 Fax: 559.472.3113 Web: www.progressivecrop.com

CONTRIBUTING WRITERS & INDUSTRY SUPPORT Terry Brase

Precision Ag Instructor, West Hills College

Richard Buchner

UCCE Orchard Advisor for Tehama County, Emeritus

Louise Ferguson Pomology Extension Specialist, Department of Plant Sciences

Allan Fulton

UCCE Irrigation and Water Resources Advisor

Craig E. Kallsen

Farm Advisor, UCCE Kern County

Steven T. Koike

UCCE, Plant Pathology Advisor, Monterey County

IN THIS ISSUE 4

UCCE Orchard Systems Specialist Butte, Glenn and Tehama Counties

Blake L. Sanden

12

Pythium Wilt Disease of Lettuce causing Diagnostic Challenge

16

Fumigation Alternatives for Tree and Vine Crops

22

Pesticide Safety in the Orchard

24

California Pistachio Salt Tolerance Update

28

Remote Sensors for Precision Ag at West Hills College

Farm Advisor, UCCE Kern County

Mike Stanghellini

Director of Research & Regulatory Affairs, TriCal Inc., Hollister, CA

Andreas Westphal

Amy Wolfe MPPA, CFRE President & CEO, AgSafe

Steven Koike

David Doll

Emily J. Symmes

Dr. Brent Holtz

Kris Tollerup

County Director and UCCE Pomology Farm Advisor, San Joaquin County

UCCE Plant Pathology Farm Advisor, Monterey & Santa Cruz Counties UCCE IPM Advisor, Sacramento Valley UCCE Integrated Pest Management Advisor, Parlier, CA

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.

2

Are There Novel Materials for Soil Fumigation?

Nematology, UC Riverside, Kearney Agricultural Research and Extension center, Parlier, CA

Kevin Day

UCCE Farm Advisor, Merced County

Does it Have a Place in Your Toolbox?

Luke Milliron

UC Cooperative Extension Advisory Board County Director and UCCE Pomology Farm Advisor, Tulare/Kings County

The Pressure Chamber –

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September/October 2017

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3

The Pressure Chamber –

Does It Have a Place in Your Tool Box?

By: Allan Fulton, Luke Milliron, and Richard Buchner

A

mong the most basic observations that can be used to manage irrigation in orchard crops are visual cues of crop water stress. However, these cues can be somewhat subjective and are often expressed after plant stress is higher than desired. Measuring midday stem water potential (SWP) using a pressure chamber is a quantitative method for evaluating plant water status, and relationships have been established between pressure chamber measurements and tree growth and productivity. From these relationships, guidelines have been developed to assist growers in making irrigation scheduling decisions.

This is an abbreviated discussion of pressure chamber operation and interpretation of measurements based upon peer-reviewed UC ANR Publication 8503, Using the Pressure Chamber for Irrigation Management in Walnut, Almond, and Prune (2014).

Plant Water Status During plant transpiration, water moves from the soil into fine root tips, up through the vascular system, and out into the atmosphere (fig.1).

Water flows through the tree from high potential in the soil (about –0.1 bar) to low potential in the 4

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Figure 1. Illustration of how water moves from the soil through an irrigated tree and into the atmosphere, from both a whole-tree and cellular perspective.

SWP measures the water potential gradient that drives this movement of water through the tree. Source: Adapted from Pearson 2008. Upper Saddle River, NJ: Pearson Education Inc.

September/October 2017

atmosphere (less than –40 bars).

Low potential is created at the leaf surface through small openings called stomata that open and close to regulate photosynthesis, gas exchange, and plant water loss.

Simultaneously, water held in the soil enters root tissue and begins its journey to the leaves. This creates a vacuum, or continuous column of tension within the water-conducting system of the tree. The amount of tension depends on the balance between available soil moisture and the rate at which water is transpired from leaves.

Midday Stem Water Potential (SWP) Concept A pressure chamber measures plant water tension by applying pressure to a severed leaf and stem enclosed in an airtight chamber (fig. 2). The sample leaf is covered with a foil-laminate bag for at least ten minutes before it excised from the tree. The leaf remains in the bag while the measurement is taken.

requiring more pressure to force water out of the cut surface of the leaf stem.

Stem water potential is a direct measure of water tension (negative pressure) within the plant and is given in metric units of pressure, such as bars (1 bar is about 1 atmosphere of pressure, or 14.5 psi). Technically, SWP should always be shown as a negative value (e.g., –10 bars), but in conversation and because the gauge does not indicate negative values, we often omit mentioning “negative” before the value. A larger number on the gauge indicates more tree water stress.

A B

C D

Figure 3. Four examples of commercially available pressure chambers. Example A is a pump-up style by PMS Instruments in which a foot pump creates air pressure in the rectangular chamber. Example B, by Specialty Engineering, also has a rectangular chamber, employs a tripod to hold the pressure chamber, and uses compressed carbon dioxide (CO2) gas to supply the pressure. Example C is a suitcase-style pressure chamber. It has a cylindrical chamber that uses nitrogen gas, to pressurize the chamber. Both PMS Instruments and Soilmoisture Equipment Corp. offer this style of pressure chamber. Example D is a consoleor bench-style pressure chamber by Soilmoisture Equipment Corp. that also uses nitrogen gas to pressurize the chamber. Photos: Courtesy of Plant Moisture Stress (PMS) Instrument Company, Albany, OR (A, C); Specialty Engineering, Waterford, CA (B); and Soilmoisture Equipment Corp., Santa Barbara, CA (D).

bers for measuring stem water potential (SWP).

All have the same basic components and operate on the same concept. The choice of a pressure chamber depends largely on preferences. Several pressure chamber styles and designs are available, ranging from simple manual pump-ups to consoles with more advanced features (fig. 3). Compressed nitrogen gas is a convenient, relatively inexpensive, and inert (safe) gas to use as a source of pressure.

Carbon dioxide gas is also used to supply pressure with some pressure chambers. Compressed air is used with some pressure chamber models. The cost may range from about $1,000 to about $7,000, depending on the style, design, and whether the unit is new or used.

Figure 2. Schematic showing how water potential is measured in a severed leaf and stem (petiole) using a hand-held pump-up pressure chamber. Source: Adapted from Plant Moisture Stress (PMS) Instrument Company.

The pressure required to force water out of the stem of a severed leaf equals the water potential and is measured by a pressure gauge.

As soil moisture is depleted, more tension develops in the plant,

Pressure Chamber Selection A number of companies produce durable, portable, and relatively inexpensive pressure cham-

Costs to Monitor Plant Water Status Growers and consultants who already use the pressure chamber and midday stem water potential as one of their management tools indicate the cost to integrate it into their management ranges from about $10 to $20 per acre annually. This is a

September/October 2017

Continued on Page 6

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Continued from Page 5 relatively low cost compared to the cost of water and other production practices. This accounts for the labor to take the measurements, compressed gas (nitrogen or carbon dioxide), time to evaluate the data, and maintenance of the instrument. It does not reflect the cost of purchasing the pressure chamber.

Pressure chambers are generally durable if handled and maintained reasonably.

It’s not uncommon for a pressure chamber to last many years, so the cost of the instrument is generally nominal when amortized over several years and many acres.

Making Stem Water Potential Measurements Time of Day Stem water potential (SWP) is best measured midday from 12:00 to 4:00 p.m. The idea is to make measurements when the tree is experiencing relatively constant and maximum water demand.

From a practical standpoint, irrigation managers are interested in the highest stress that trees experience, which is also at midday. Thus, the guidelines for interpreting SWP measurements have been made by using midday measurements.

Time of Season SWP measurements should begin during the spring and continue through the summer and fall. This approach will help indicate when to apply the first irrigation, show how orchard water status responds to irrigation decisions, how irrigation scheduling might be improved and how water status shifts with seasonal weather changes. Postponing SWP measurements until early summer will miss the progressive changes in orchard water status leading up to the summer season (when water demand is highest).

Time Between Irrigations Orchard acreage, time availability, coordination with other tasks, previous experience, and specific interests (trou-

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September/October 2017

bleshooting) may influence measurement frequency. Not all orchards have to be measured at the same frequency.

Measuring SWP just prior to irrigation and one or two days after irrigation provides the most information about the ongoing water status of the orchard.

SWP measurements taken just before irrigation will indicate the orchard water status when orchard stress is potentially the highest, and provide insight into whether the interval between irrigations needs to be adjusted. Monitoring SWP one or two days after irrigation will indicate how well the tree water status recovered after irrigation and give insight into whether the duration of irrigation is on track.

Larger changes in SWP may be observed before and after irrigation when irrigation frequency is stretched out more with a flood or sprinkler system that applies more water in a single irrigation. Conversely, smaller, more gradual changes in SWP may be observed with drip and microsprinkler systems that apply water at higher frequency and lower volume. So, it is not necessary or usually feasible to measure SWP before and after every drip or microsprinkler irrigation. Measuring SWP just before a drip or microsprinkler irrigation every seven to 10 days should provide reasonable trends and insights into the orchard water status.

How many measurements to make in an orchard?

Determining how many trees to monitor must balance having enough trees to reliably represent the orchard and being able to cover the desired total acreage in a timely and efficient manner. Sampling fewer trees and accepting the possibility of less-representative measurements is better than not using SWP at all because it is perceived as too labor intensive. The number of trees to monitor in an orchard also depends on soil variability and irrigation uniformity.

Fewer trees are needed if the orchard is growing on one predominant soil type with uniform irrigation. More trees

Continued on Page 8

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Continued from Page 6 are needed if there is more than one soil type and non-uniform irrigation. A sampling strategy that can be completed in about 30 to 60 minutes per orchard is ideal, especially if several orchards must be monitored on the same afternoon.

Understanding of orchard water status will improve as the number of trees monitored is increased. A sample size ranging from 5 to 10 trees per orchard is probably optimal for achieving representative measurements and covering acreage efficiently. One strategy is to start with a greater number of measurement trees and then pare down over time, as the trends and patterns of orchard water status is more understood.

Selecting trees for measurement Trees selected for SWP monitoring should be representative of the orchard.

Good measurement trees would be of the same variety and rootstock and similar in age, degree of pruning, and canopy size. Measurement trees should be irrigated in the same manner as the rest of the orchard and should be healthy.

The trees selected for SWP measurement should be at least 100 feet inside the orchard and have other healthy trees growing around them. The same trees should be used to measure SWP each time to reduce variation from one day to the next. The sample trees should be

marked with flagging, spray paint, GPS waypoints (or a combination thereof), so they are easily located each time SWP measurements are made. Going back to the same trees for each reading, reduces variability and may be more important than the number of trees measured.

When sampling small trees, particularly during the first year, excessive leaf removal may be an issue, especially if the trees are planted later and not growing vigorously. One solution is to identify three or four side-by-side rows of uniformly growing trees in representative areas of an orchard. Each row will have three or four trees identified for SWP measurement. Then, using a rotational schedule, measure SWP in a different row each reading, returning to the first row after trees have had time to grow additional leaves suitable for measurement (usually two or three weeks after the previous measurement). Typically, second-year and older trees should have sufficient canopy, that rotating between rows of sample trees is no longer necessary.

How many leaves per tree?

To ensure the reading is not compromised by operator error or poor leaf selection, it is important to make two measurements per tree, or a single leaf from two adjacent trees. Compare the readings; if they are more than 0.5

Figure 4. From left to right, healthy full grown leaves are selected in the lower interior canopy of an almond, prune, and walnut tree near larger branches of the trunk and covered with mylar foil bags for at least ten minutes. Walnut has a compound leaf, so the terminal leaflet is selected because it has a longer petiole which assists the measurement. (Photos: A. Fulton)

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September/October 2017

to 1.0 bar apart, another leaf should be measured and the average of the closest leaves recorded. If the difference between the two leaves is great, bag another leaf and return to make the measurement. Then, take the average of the closest measurements. An alternative strategy is to bag three leaves per tree then measure the first two, and if the measured SWP is in good agreement pass on the third measurement.

Good SWP field measurement technique Consistent technique helps improve the accuracy of SWP measurements.

On the same almond trees, as much as a 2-bar discrepancy, plus or minus, has been documented with different operators. Such errors can be due to differences in speed and method of handling the sample from the time the leaf is excised until the measurement is completed, or they can be due to differences among operators in recognizing the endpoint.

Relying on one operator to check the same orchards over the season, using the same pressure chamber and consistent technique is recommended.

Good SWP measurement technique begins with properly selecting sample leaves and bagging or covering them with a foil-laminate bag (fig. 4). Leaves should be healthy, full grown, and without apparent nutritional deficiencies, yellowing from excessive shading or older age, or physical damage from wind or hail. Lower interior, shaded leaves are selected nearer to larger branches of the tree trunk, where the bagged leaf equilibrates readily with the tree’s main water-conducting system. Leaves higher in the canopy or farther out on a limb can give significantly different levels of SWP due to more resistance to water flow through more of the vascular system. Reflective, water-impervious mylar foil bags are commonly used for bagging leaves. Bags are available from some pressure chamber manufacturers. Nylon button fasteners, re-sealable zip fasteners, paper clips, clothes pins and other creative approaches can be used to fasten the foil bags to the leaves.

The importance of bagging or covering the sample leaves on large perennial trees can’t be emphasized enough.

Ideally, the sample leaves are bagged for at least 10 minutes prior to excising the leaf from the tree. It is acceptable to bag leaves longer than 10 minutes. They may be covered several hours or even a day in advance if it adds convenience.

After the trees with the sample leaves have been bagged for at least 10 minutes, it is time to measure the SWP with a pressure chamber. The leaf must remain enclosed in the bag after it is excised from the tree and placed in the chamber.

If the bag is removed at any time, the SWP level will begin to change rapidly (within seconds) as the leaf is exposed to the often hot, windy environment around it. Only using bagged leaves is an important practice in acquiring stable, more-representative measurements that are easily interpreted. Resist the temptation to remove the leaf from the foil bag even if it makes inserting the sample leaf

into the chamber more convenient.

Measuring SWP (fig. 5) after the bagged leaf is excised from a tree involves inserting the stem of a bagged leaf upward through the top of the pressure chamber and securing the protruding stem in the pressure chamber cap (A).

Then, placing the bagged leaf in the pressure chamber (B). A long stem is shown protruding out of the top of the chamber (C). Some researchers emphasize that the stem should not be protruding out this much for the most accurate measurement of SWP. Other researchers have observed Figure 5. Basic process of measuring midday SWP with that practitioners have more a pressure chamber. Photos: A. Fulton and K. Shackel. difficulty seeing the endpoint without the longer stem, which can also introduce error in from the surface of the cut stem and will seeing the endpoint. Apply pressure appear to glisten (D). With the addition inside the chamber and determine the of a little more pressure, water will cover endpoint for a SWP measurement. Close to the endpoint, water begins to exude Continued on Page 10

A B

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Interpreting SWP measurements Baseline concept to distinguish weather from irrigation effects

Because SWP is affected by weather conditions at the time measurements are made, readings can vary from one day to the next as the weather changes, even if irrigation management and soil moisture are relatively stable. This has led to the development of the baseline concept for irrigation managers who want to improve upon their interpretation of SWP readings for irrigation scheduling.

Continued on Page 38

Figure 6. Photo of actual SWP measurement in walnut using a pressure chamber with digital gauge. Plant water status of this walnut tree was -5.67 bars tension.

Photo: C. Haynes.

Continued from Page 9 the entire cross-section of the stem and reach the endpoint.

Most pressure chambers (excluding handheld pump-up models) incorporate an adjustable metering valve to regulate how fast pressure builds inside the chamber. Metering valves should be set to increase pressure slowly to lessen the chance of missing the endpoint. Operators should set the rate the chamber pressurizes slow enough so that it is possible to both watch for the endpoint and read the pressure gauge at the same time (fig. 6). Experience suggests that a pressure increase of 0.25 to 0.5 bars per second is just about right. When tree water status is at low to mild levels, pressure increases might be lower. Whereas, when tree water status is at moderate and higher levels, pressure increases may need to be higher to expedite measurements.

In addition, less error occurs when pressure entering the chamber is quickly stopped at the endpoint by using the shut-off valve (NOT the regulator valve). The pressure regulator valve is a needle type that can be easily damaged from over-tightening to stop pressure into the chamber.

SWP measurements should be completed as rapidly as possible after bagged leaves are excised.

It usually takes an experienced operator 15 to 30 seconds to take a measurement depending on the plant water status. To minimize variability, readings should be made within 1 minute of being removed from the tree.

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Table 1. Baseline SWP (bars) to expect for fully irrigated almonds or prune trees under different conditions of air temperature and relative humidity.

Temperature (ºF)

Air relative humidity (RH, %)

10

20

30

40

50

60

70

75

-7.3

-7.0

-6.6

-6.2

-5.9

-5.5

-5.2

80

-7.9

-7.5

-7.0

-6.6

-6.2

-5.8

-5.4

85

-8.5

-8.1

-7.6

-7.1

-6.6

-6.1

-5.6

90

-9.3

-8.7

-8.2

-7.6

-7.0

-6.4

-5.8

95

-10.2

-9.5

-8.8

-8.2

-7.5

-6.8

-6.1

100

-11.2

-10.4

-9.6

-8.8

-8.0

-7.2

-6.5

105

-12.3

-11.4

-10.5

-9.6

-8.7

-7.8

-6.8

110

-13.6

-12.6

-11.5

-10.4 -9.4

-8.3

-7.3

115

-15.1

-13.9

-12.6

-11.4

-9.0

-7.8

-10.2

Table 2. Baseline SWP (bars) to expect for fully irrigated walnut trees under different conditions of air temperature and relative humidity.

Temperature (ºF)

Air relative humidity (RH, %)

10

20

30

40

50

60

70

75

-4.5

-4.3

-4.2

-4.0

-3.8

-3.6

-3.4

80

-4.8

-4.6

-4.3

-4.1

-3.9

-3.7

-3.5

85

-5.2

-5.0

-4.7

-4.4

-4.1

-3.9

-3.6

90

-5.6

-5.2

-4.9

-4.6

-4.3

-4.0

-3.7

95

-6.0

-5.7

-5.3

-5.0

-4.6

-4.3

-3.9

100

-6.5

-6.1

-5.7

-5.3

-4.9

-4.5

-4.0

105

-7.2

-6.7

-6.2

-5.7

-5.2

-4.8

-4.3

110

-7.8

-7.3

-6.7

-6.2

-5.6

-5.0

-4.5

115

-8.7

-8.0

-7.4

-6.7

-6.0

-5.4

-4.8

September/October 2017

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Pythium Wilt Disease of Lettuce Causing Diagnostic Challenge By: Steven T. Koike | UC Cooperative Extension Plant Pathology Advisor | Monterey County

T

he development of a new problem on coastal lettuce, Pythium wilt, caused confusion and concern for growers in California’s Monterey County because the symptoms resembled those of other lettuce diseases. Such a situation serves as a reminder of the challenge of identifying soilborne diseases in crops using only symptoms.

In contrast to Pythium diseases of spinach, tomato, cotton, and other crops, this lettuce pathogen does not cause damping-off or symptoms on newly emerged, young lettuce

Pythium Wilt in Coastal California In recent seasons in Monterey County’s Salinas Valley, significant crop losses were caused by Pythium wilt disease of lettuce (Photo 1). Caused

by Pythium uncinulatum, this disease appeared to be only recently introduced to coastal California, and prior to 2014 was of minor importance. However, during 2014 and 2015 seasons, the problem spread to a number of locations in the valley and caused perhaps 30 percent or more losses in some fields.

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September/October 2017

Photo 2. Stunted lettuce plants with Pythium wilt disease. A healthy plant is on the left. Photos courtesy: Steven Koike

seedlings. Instead, above-ground Pythium wilt symptoms do not show on lettuce until plants are at the rosette or older stage. Plants infected at this point of growth will be stunted and lag behind healthy lettuce. As disease progresses, outer leaves will start to wilt during the warmer times of the day and eventually turn yellow before becoming brown and dead. In advanced stages, Photo 1. Lettuce field severely affected with Pythium wilt disease.

the entire foliar canopy can collapse and die. Diseased plants that survive will be small and substandard in quality (Photo 2, pg. 12). In the soil, the pathogen first attacks the small feeder roots of the lettuce, making them soft and brown gray in color. Late in disease development the taproot will be darkly discolored and the entire root system can be rotted (Photo 3).

Pythium uncinulatum, like most Pythium species, produces swimming spores (zoospores) that are released and move within the water film in the soil. Water flow from irrigation, flooding, and movement of soil via tractors and equipment can spread this pathogen. In addition to zoospores, the pathogen also produces a type of spore (oospore) that is encased within a spiny outer covering (oogonium) (Photo 4). It is the oospore that allows the pathogen to survive in the soil in the absence of susceptible plants. Pythium uncinulatum is host specific to lettuce and apparently does not infect other vegetable crops such as broccoli, cabbage, carrot, onion, pepper, radish, spinach, or tomato.

eases. Stunting of plants, wilting and yellowing of leaves, and collapse of plant foliage can also be caused by lettuce drop (pathogen: Sclerotinia minor) and Botrytis crown rot (pathogen: Botrytis cinerea) (Table 1).

Differentiating between these three diseases therefore requires careful examination of all plant tissues (leaves, crown, roots) as well as lab-based analysis.

Photo 4. Microscopic, spherical, spiny structures (oospores within oogonia) help Pythium uncinulatum survive in the soil between lettuce crops. Photo Courtesy: Steven Koike

ing and analysis.

Note that there is one more confounding factor: two

In approaching this diagnostic challenge, first note if the lettuce plant has the general Photo 3. Dark, rotted root system of lettuce infected with Pythifoliar symptom profile of stunting, wilting, yel- um uncinulatum. Healthy lettuce roots are on the right. lowing, and collapsing.

Secondly, the crown lettuce infecting virus pathogens, ImSymptoms Pythium Sclerotinia Botrytis should be carefulpatiens necrotic spot virus (INSV) and ly examined. If the Lettuce necrotic stunt virus (LNSV), can crown is discolored, Stunted plants Yes Yes Yes cause foliar symptoms that may resemble soft, decayed, and sup- those caused by the early stages of Pythiporting visible fungal Wilted leaves Yes Yes Yes um wilt. INSV and LNSV both infect letgrowth, then the tuce and result in declining plant foliage Yellowed Yes Yes Yes disease is likely Sclethat is yellowed and necrotic. The field rotinia lettuce drop leaves professional must therefore also account or Botrytis crown rot; for the possibility of these viruses during Collapsed Yes Yes Yes if the crown is solid the diagnostic investigation. plants and intact and there are no signs of fungal Decayed No Yes Yes The Challenge of Diagnosing structures, then PythSoilborne Diseases crowns ium wilt is a possibility. Finally the root Rotted root Yes No No One reason why soilborne pathosystem, taproot plus gens are problematic is that they are system attached feeder roots, difficult to diagnose in the field. Besides must be inspected in the Pythium wilt example, there are Table 1. Similarity of disease symptoms for three soilborne detail. Carefully dig many other situations in which disease pathogens of lettuce. up the root system identification based only on symptoms is and wash off the soil. problematic. Lettuce growers in coastal If the roots are dark and rotted, Pythium California face two damaging vascular Pythium Wilt Confused wilt must be suspected. Sclerotinia and wilt diseases: Fusarium wilt and VertiWith Other Diseases Botrytis pathogens generally do not infect cillium wilt. These two diseases cause very similar symptoms of overall plant roots at all. These three steps can assist in Because Pythium wilt causes arriving at a tentative diagnosis in the field. decline, wilting, collapsing, and discolabove-ground, foliar symptoms that oration of the internal vascular tissue of As for most plant diseases, confirmation of are not distinctive or unique, this Pythium wilt and other problems can only problem was initially misdiagnosed Continued on Page 14 be accomplished following laboratory testand confused with other lettuce dis-

September/October 2017

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13

Continued from Page 13

Symptoms

Fusarium

Verticillium

Ammonium

Yellowed leaves

Yes

Yes

Yes

Wilted leaves

Yes

Yes

Yes

Collapsed plants

Yes

Yes

Yes

Vascular discoloration

Yes

Yes

Yes

Decayed crowns

No

No

No

Rotted roots

No

No

No

Symptomatic young plants

Yes

No

Yes

Symptomatic mature plants

Yes

Yes

No

taproots (Table 2, Pg. 14). The two wilts may be differentiated in that Verticillium wilt symptoms usually show on mature plants that are close to harvest, while Fusarium wilt symptoms can occur on both young and mature plants. However, the buildup of ammonium in soils can damage the central cores of lettuce taproots. Lettuce roots exposed to high levels of ammonium therefore may look like a Fusarium wilt case.

Again, to know precisely the cause of declining lettuce plants that have vascular discoloration in taproots, one must have the plants tested by a pathology lab.

Table 2. Similarity of disease symptoms for lettuce affected by Fusarium wilt, Verticillium wilt, and ammonium toxicity.

Strawberry growers face a similarly daunting task of trying to identify soilborne diseases.

Three significant diseases have

virtually identical symptoms of off-color, gray green foliage, wilting of leaves, collapsing of plants, dark internal discoloration of strawberry crowns, and eventual death of the plant (Table

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3, pg. 15). Because Macrophomina crown rot, Fusarium wilt, and Phytophthora crown rot share all of these symptom features, it is not possible to know which one is present in the field. Two of these problems, Macrophomina crown rot and Fusarium wilt (Photo 5), are very damaging to strawberry plantings and in addition have been steadily spreading; both pathogens are now found in all of the major coastal strawberry production regions in California.

Why Accurate Diagnoses are Essential in Production Agriculture Some might feel that “a dead plant is a dead plant” and that knowing the exact name of the responsible pathogen or factor is not that important. As an extension plant pathologist, I would suggest that it is absolutely essential in today’s competitive agricultural world to know precisely the cause of a problem. Accurate pathogen identification always has been the first required step in fashioning an integrated pest management (IPM) program for dealing with plant diseases. Pathogen identification enables the grower to know which crops are susceptible to the agent, which crop cultivars might be advisable to plant due to genetic resistance, how the pathogen gets around and is spread, and other

Symptoms

Macrophomina

Fusarium

Phytophthora

Older leaves affected first

Yes

Yes

Yes

Gray green foliage

Yes

Yes

Yes

Wilted leaves

Yes

Yes

Yes

Collapsed plants

Yes

Yes

Yes

Discolored crowns

Yes

Yes

Yes

Death of entire plant

Yes

Yes

Yes

Table 3. Strawberry plants affected by Macrophomina, Fusarium, and Phytophthora all show very similar symptoms.

pertinent biological information.

Since most plant pathogens cannot be confirmed without lab analyses, accurate disease identification will require close collaboration between growers, field personnel, and extension or private laboratories.

Managing Pythium Wilt and Other Soilborne Pathogens The following management strategies should be considered when dealing with Pythium wilt disease. These measures also illustrate general principles that can be adopted for controlling other soilborne diseases. (1) Avoid planting lettuce into fields with a known history of the problem. (2) For infested fields, rotate to non-lettuce crops. Various research studies demonstrated that Pythium uncinulatum is host-specific to lettuce and will not infect other vegetable crops.

(3) Implement field sanitation practices to minimize the movement of contaminated soil from infested to clean fields. (4) Be aware that surface water runoff from infested fields may be contaminated with the pathogen.

Flooding events may also spread the pathogen to previously clean fields. (5) Prepare beds so that drainage of water is enhanced, since all Pythium species are favored by wet soil conditions. (6)

A

B

C

D

Photo 5. Strawberry plants infected with Macrophomina (A and C) or Fusarium (B and D) have identical symptoms consisting of wilting and collapse of leaves, discoloration of internal crown tissue, and death of the entire plant. Photos Courtesy: Steven Koike

Manage the irrigation so that excessive soil moisture is avoided. (7) The effectiveness of fungicides for controlling Pythium wilt in California is currently unknown and will require field trials. (8) The planting of lettuce cultivars that are resistant to P. uncinulatum would be the preferred IPM method for control; however, such cultivars are not available at this time. (9) The application of pre-plant treatments such as soil fumigants

and biological control agents are not relevant at the moment for Pythium wilt of lettuce; however, such measures should certainly be considered for strawberry and other crops and their respective soilborne pathogens.

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FUMIGATION ALTERNATIVES FOR TREE AND VINE CROPS

Are there novel materials for

soil fumigation?

Orchard fumigation rig (background) with soil sealing rig (foreground) Photo Courtesy: Andrea Westphal

By: Andreas Westphal | Department of Nematology, University of California, Riverside, Kearney Agricultural Research and Extension center, Parlier, CA By: Mike Stanghellini | Director of Research & Regulatory Affairs, TriCal Inc., Hollister, CA

S

ustainability of many crops is put at risk by the build-up of soil-borne plant pathogens. While yield-reducing effects of air-borne maladies can often be effectively mitigated by pesticide inputs, soil-borne problems are much more challenging. By definition, pathogens causing these diseases persist in the complex environment of the soil matrix. On the one hand, this complexity increases the time that it takes for these culprits to spread. On the other hand, soil complexity renders suppression of these pathogens and pests generally more challenging than air-borne diseases and pests. In fact, soil-borne maladies are often the key components that determine how frequently a crop or crop group can be grown in specific fields.

These restrictions lay the foundation for crop rotation programs. For example, in nematode management, the use of non-susceptible host plants of specific nematode pests may be prescribed to avoid the build-up of injurious nema16

Progressive Crop Consultant

tode populations. Such strategies can be very effective when crops of similar value and equipment requirements are used. The situation becomes challenging when high-value crops are produced in highly specialized systems where a diversion to less profitable crops is not feasible, such as when high land rental costs introduce additional economic factors that come into play. In some cases, limited crop rotation alternatives exist because of the wide host range of pathogens or pests. For example, root-knot nematodes have very wide host ranges, and a rotation based on various susceptible vegetable crops would be unfit to reduce infestation levels in fields.

Methyl Bromide For decades, soil fumigation has been used to reduce these important crop-limiting pests by rendering pestfree (or effectively managed) soil environments. Since its registration in the

September/October 2017

US in 1961, methyl bromide had been an effective tool to reduce soil-borne maladies. One main strength of methyl bromide is its versatility under different application conditions. Its high vapor pressure coupled with a greater-than-air density allowed methyl bromide to effectively penetrate complex soil matrices that differed considerably in temperature, moisture level, and texture. Concerns about methyl bromide’s ozone-depleting potential in the stratosphere led to its phase-out under the Montreal Protocol. After its phase-out from general use in the US in 2005, Critical Use Exemptions (CUE) were granted for selected applications where technical alternatives were not available. This helped protect against market disruptions in strawberry and other high-value crop industries in the US and other countries.

The 2016 growing season marked the end of the CUE process in the United States. For 2017 and future years, the

Continued on Page 18

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Progressive Crop Consultant

September/October 2017

TRE-1034, 1/17

With the phase-out of non-QPS methyl bromide, the proven mainstays of soil fumigation have become more important; namely, 1,3-dichloropropene (1,3-D) and chloropicrin-containing products. The 1,3-D formulations, e.g., Telone products, are experiencing a resurgent popularity.

Nationwide, 1,3-D-containing products have been used since the 1950s, but its use in California was temporarily suspended in1990 after ambient air monitoring programs detected high concentrations in the San Joaquin Valley air basin. Products containing 1,3-D were reintroduced for use in California in 1995, with the addition of several strict control measures. Beginning January 2017, the application of 1, 3-D is limited to 136,000 pounds per township (=36 square mile area) per year in order to maintain ambient air concentrations below the California Department of Pesticide Regulation’s threshold. Previous regulations that allowed for a “carry-over” of unused amounts of the annual township allotment to the following year are no longer in effect.

These restrictions make grower access to 1,3-D in some California counties challenging when multiple crops potentially benefit from the application of the material. Recent research has demonstrated that the use of totally impermeable film (TIF) significantly reduces fugitive emissions and can allow for reductions of application rates without loss of efficacy in pathogen and pest suppression. However, regulatory constraints remain a limiting factor of 1,3-D use in California. Grower demand exceeds regulatory permitted use.

Chloropicrin use has also increased as the availability of methyl bromide decreased. For decades, most chloropicrin was used in combination with methyl bromide, but it also has a long history of use as a sole active ingredient because of its excellent fungicidal properties. Because of its “warning agent” (tear-gas-like) properties, it is used in structural fumigation when otherwise odorless gases are used. For example, it was used in small doses to warn against inadvertent exposure to methyl bromide, and now sulfuryl fluoride, both of which are

Table: Active ingredients and trade names of potential and actual soil fumigant and biocide materials at various levels of developmenta

Active ingredient

Productb

Application ratesc Pest controlledd

Signal word(s)

1,3-D (1,3-dichloropropene)

Telone II, Telone EC, mixes w/ chloropicrin

9-33.7 gal/Ae

N, F, (W)f

Warning

Chloropicrin

Chloropicrin 100 Tri-Clor, mixes with 1,3-D

7-25 gal/A

F, (N)f

Danger, Poison

MITC generators (methyl-isothiocyanate)

Vapam HL K-PAM HL

50-100 gal/A

N, F, W

Danger, Poison

AITC (allyl-isothiocyanate)

Dominus

10-40 gal/A

N, F, W

Danger

DMDS (dimethyl disulfide)

Paladin

37-54.2 gal/A

N, F, W

Danger

EDN (ethanedinitrile)

EDN

N/Dg

N, F, W

Danger

Cyanamide 50%

N/Df

N/Dg

N, F, W

Danger, corrosive

Data are for informational purposes only. Always consult the current safety data sheet (SDS) and pesticide label before using chemical compounds. b These are examples of trade-named products. There may be other products with the same active ingredients that may have similar properties as the ones named here. c Application rates are based on currently available label information. d Efficacy according to currently published information and experience; additional information may become available: N = nematodes, F= fungal pathogens, W= weeds; parentheses indicate partial activity. e Maximum label rate in California. f When applying by shanks, improper soil moisture content may reduce movement of material. g N/D: Not determined; information not yet available. a

odorless, when conducting residential fumigations for termite control.

In the soil, chloropicrin is known to have synergistic, or complimentary, pesticidal activity when combined with methyl bromide or 1,3-D. More and more frequently, due to the loss of methyl bromide and current restrictions on 1,3-D, chloropicrin is used as a sole active ingredient for soil fumigation. It can be effective against soilborne fungal and bacterial pathogens, as well as some arthropod pests, but is generally not suitable as a stand-alone nematicide due to its inability to penetrate woody tissues (like old roots which may harbor nematodes), and is not effective on weed seed banks in the soil. Recent studies have demonstrated that chloropicrin is a valuable material when orchard growers are confronted with “replant disorders”; conditions which may be encountered when planting the same or similar crop back-to-back, e.g., almonds

following almonds.

Another long-known chemistry are the MITC (methyl isothiocyanate) generators: metam sodium (e.g., Vapam® HL), metam potassium (e.g., K-PAM® HL), and dazomet (e.g., Temozad). These materials quickly break down after application to form MITC, which is active against weed seeds, soil-borne pathogens and other pests. These materials can be applied via shank application or chemigation through the drip irrigation system (metam sodium and metam potassium) or as a granule (Temozad) applied using scoops, shakers, shanks, drop-type fertilizer spreaders, or other suitable equipment. Research on perennials has demonstrated that in these high value crops, the active ingredient needs to be delivered to the site of

Continued on Page 20

Unfumigated (Foreground) Chloropicrin fumigated (Background)

September/October 2017

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19

Continued from Page 19 action in the soil. Typically, this is done with soil drenches using irrigation water, whereby the biocidal material is dissolved in the irrigation water, which is then applied at or near the soil surface to percolate the soil matrix. Depending on the soil depth targeted, this could be up to 6 inch/A of water for a five-foot depth of application. The key to ensure efficacy of this material is delivery to the target site. Some air quality issues limit the use of this material. The water-drench logistics for application as pre-plant material in deep-rooting perennials somewhat hinders its’ widespread adoption.

Allyl isothiocyanate (AITC) is the new biofumigant active ingredient

20

formulated as Dominus®.

This synthetically-produced, but chemically identical, version of the naturally-occurring compound present in many Brassica species is used for biocidal treatments.

Although general knowledge of plant-derived AITC is robust, its development as a pure-form soil fumigation product is recent. Currently, AITC can be applied by shank application or drip irrigation (“chemigation”) for use in annual crops, but has less demanding labels than the traditional soil fumigants. For example, Dominus has no, or very small, buffer zones and its use currently does not require a written Fumigant Management Plan (a highly detailed,

Progressive Crop Consultant

September/October 2017

Almond trees planted at the same time in fumigated soil (lower image) and non-fumigated soil (upper image).

site-specific, pre-application record of intended use). Like MITC generators, AITC may exhibit slow movement in the soil, suggesting that its use for perennial crops may need more research to optimize its utility in scenarios having larger soil volumes that need to be treated. Like other materials, AITC could be used in crop termination, whereby application is done after final harvest to ‘burn down’ the roots and reduce pathogen pressure before the crop residue is tilled under for decomposition. Although AITC product labels have generally less restrictive requirements than the traditional fumigants, its application does entail the use of proper PPE due to corrosiveness and skin irritation potential. This should not deter from use if high efficacy rates can be shown. Registration of this material was granted federally several years ago, and is currently registered in 27 different states. Registration in California is pending.

Another material used commercially in some US states is dimethyl disulfide (DMDS), formulated as Paladin or Paladin EC. Outside of agriculture, DMDS is primarily used as catalyst in hydrocarbon processing. Its acute toxicity and other exposure effects are such that it is considered reasonably safe to use. In fact, low doses of this material are used as food additives to impart a pungent garlic odor. However, at elevated concentrations, this same odor can be highly objectionable, and so it is regulated based on its potential to be an odor nuisance. DMDS soil fumigation product labels currently mandate the use of low-permeability tarps for mitigation of the odor of this effective nematicide, Aqueous cyanamide formulations at 10 and 50 percent a.i., fall into Pesticide Categories III or II, respectively, and constitute a candidate for biocidal treatments. For tree and vine crops, this non-fuming, non-odorous, highly water soluble biocide will be tree site or strip applied and further diluted 250 to 500-fold to reach 99 percent of the target sites. The active ingredient metabolizes in a cascade of nitrogenous materials within soil but is also adequately systemic to kill live roots, endoparasitic life stages of nematodes within and plant tissues above. It does not penetrate well

into dead plant tissues. Application rates and introduction of nitrogen amounts still need to be worked out and will likely vary for different crops and soil textures.

This liquid is currently available globally as a plant growth regulator under several brand names. It is also commercialized for its lethal actions upon enzymes, which may explain plant responses that are visibly more complex than just an overdose of nitrogen. This solution is well-suited for use in replant problem sites for the so-called “Starve and Switch” strategy, meaning the reduction of pathogenic organisms of a particular crop and the following change to different crop genetics. The commercial development of such formulations is at its beginning.

A quite different material currently under experimental exploration is ethanedinitrile (EDN). This material has a lower boiling point and a higher vapor pressure than methyl bromide.

At ambient temperature it is a gas, and thus requires pressurized handling and potentially novel application equipment. EDN has a short half-life in soil.

Current research is focused on safe and uniform application, and improving its residence time in soil. Crop trials have shown its potential to be a broad-spectrum fumigant.

In summary, in the “post methyl bromide era”, the older, and proven, chemistries like 1,3-D, chloropicrin, and the MITC generators are in high demand, and are often used in various combinations; while newer chemistries undergo optimization as methyl bromide alternatives. Work on these newer materials is ongoing, where adjustments and technical advancements may be required before some of them can be widely used. Overall, the concept of a “drop-in replacement” for methyl bromide very likely needs to be shelved, as agricultural production converts to the concept of Integrated Pest Management, i.e., the use of multiple pest control strategies and tactics for the management of soil-borne maladies for high-value crop production.

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21

Pesticide Safety in the

Orchard

By: Amy Wolfe | MPPA, CFRE | President and CEO, AgSafe

A

s we spray our orchard floors in preparation for the harvest that is just around the corner it is important that we consider pesticide safety. This is such a busy season for growers that employee safety and training often do not get the time and energy needed. To help tackle these important pesticide safety elements it helps to put them into five categories: • • • • •

Personal protective equipment Decontamination Employee training Emergency medical care Label requirements

must wear a long-sleeved shirt, long pants and shoes plus socks. As a good rule of thumb, keep this as the minimum PPE that should always be worn when applying. In addition to the minimum requirements, Rely 280 calls for chemical resistant gloves and specifies what material the glove is made from. The Department of Pesticide Regulation (DPR) has a glove category selection key card that helps you navigate glove material requirements.

Personal Protective Equipment Personal protective equipment (PPE) serves as a protective barrier between the applicator and the pesticide. While pesticide use in orchards is common practice, it is important to remember that pesticides are used to kill pests, which means it has the potential to harm the applicator if not used safely.

Prior to spraying read the label PPE requirements—the label is the law and both owners and employees must follow the PPE section. Consider the Rely 280 label. The PPE section is in bold and breaks out the PPE requirements for handlers versus mixer/loaders. Handlers 22

Progressive Crop Consultant

employer must provide all necessary PPE. When an employee is going to make a pesticide application, under 3CCR Section 3738, he or she must also wear chemical resistant gloves and protective eyewear, even when not required by the label. Another important element to note, any employee handling a “DANGER” or “WARNING” product must wear coveralls. These are just a few of the highlights on PPE regulations. Be sure to review 3CCR 3738 for the full regulation: http:// www.cdpr.ca.gov/ docs/legbills/ calcode/030302. htm#a6738.

Decontamination Even in the best of situations sometimes PPE or equipment fails and employees become contaminated with pesticides. In The Department of Pesticide Regulation has a glove category selecthese scenarios, a tion key card that helps you navigate glove material requirements. properly stocked decontamination kit can help prevent significant injuries.

In addition to the label, employees Under 3CCR 6734, employers are rehave extra PPE requirements per Title quired to provide decontamination facil3 of the California Code of Regulations ities for handlers. Theses kits or facilities (3CCR). This regulation states that the

September/October 2017

need to have the following items: • • • • •

Sufficient water, at least 3 gallons per handler Soap Single use towels One clean pair of coveralls One pint of eyewash immediately available if the pesticide label requires protective eyewear, although best practice is to provide eyewash in the kit at all times

The decontamination kit can be at the mix/load site as long as it is not more than quarter mile from the handler. In addition to the items listed above, at the mix/load site there needs to be an emergency eye flushing system. The regulation has very specific requirements as to the amount of water and flow. See 3CCR 6734 for details: http://www.cdpr.ca.gov/ docs/legbills/calcode/030302.htm.

to be taken to emergency care. This is why 3CCR 6726 has requirements about emergency medical postings. Emergency medical care must be planned for employees in advance. Employees shall be informed of the name and location of the facility where care is available.

This information needs to be posted in a prominent place in the workplace.

This information also needs to be filled out on the Pesticide Safety Information Series A8 and A9 forms.

For the entire Pesticide Safety In-

Employee Training Training serves as the backbone to any safety program and with pesticide training, 3CCR 6724 specifically states what is required. Employees need to be trained prior to handling pesticides, on every pesticide label they will handle, anytime a new pesticide is introduced to the operation and annually thereafter.

The elements covered during pesticide training are robust. Recently DPR adopted the Environmental Protection Agency’s (EPA) revised Worker Protections Standard (WPS) which increased the number of training elements. These elements include: • • • • • •

Routes by which pesticides can enter the body Signs and symptoms of pesticide exposure Emergency first aid procedures Heat illness prevention Warnings about taking pesticides home Proper decontamination

Training must be given in such a way that the employee can understand, correlate with the grower’s written pesticide program and be documented.

Emergency Medical Care In severe cases employees may need

It is vitally important that this information is readily available to employees. An easy fix is to attach a key chain to the equipment keys used for application with the required information.

formation Series visit; http://www.cdpr. ca.gov/docs/whs/psisenglish.htm. It is vitally important that this information is readily available to employees. An easy fix is to attach a key chain to the equipment keys used for application with the required information.

Label Requirements As previously noted, the label is the law. The pesticide label covers everything from application rate and PPE to first aid and application restrictions. Among application restrictions are the restricted entry interval and pre-harvest interval.

Restricted entry interval (REI) as defined by DPR, is the period of time after a field is treated with a pesticide during which restrictions on entry are in effect to protect persons from potential

exposure to hazardous levels of pesticide residues. Basically, the REI is the amount of time that has to elapse before anyone can enter the field without proper PPE. In regards to employee safety, handlers should always check the REI time on a label prior to spraying and follow notification regulations so that other employees know not to enter that treated field.

Pre-harvest intervals (PHI) are different than REIs in that this is the amount of time that has to elapse before you can harvest your orchard and have the nuts be safe to eat. Pre-harvest intervals are intended to provide a period between the application of a pesticide and harvest so the crop will meet the established pesticide residue tolerance and protect the public from possible exposure to excessive residues. For example, the Goal 2XL label states “do not apply within 15 to 30 days of harvest of almonds,” and “within 7 days of harvest of walnuts.”

Unfortunately, this information on the label is not on the front page, it is buried on page 31 of 33, emphasizing the need to review the entire label prior to application.

As harvest rapidly approaches, be sure to consider these five essential operational areas to ensure pesticide safety for employees during this busy time of year: personal protective equipment, decontamination, employee training, emergency medical care and label requirements.

For more information about pesticide safety, or any worker safety, health, human resources, labor relations, or food safety issues, please visit www.agsafe.org, call us at (209) 526-4400 or via email at safeinfo@agsafe.org.

AgSafe is a 501c3 nonprofit providing training, education, outreach and tools in the areas of safety, labor relations, food safety and human resources for the food and farming industries. Since 1991, AgSafe has educated nearly 75,000 employers, supervisors, and workers about these critical issues.

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

September/October 2017

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23

California Pistachio Salt Tolerance Update by: Blake Sanden, Louise Ferguson and Beau Antongiovanni

T

he classic concept of crop salt tolerance is over simplified. Focusing only on rootzone salinity this calculation consists of a single soil saturation extract salinity threshold (ECe) with a “% relative yield” decline function (Figure 1).

Research This approach minimizes the real world variability in actual tree growth and yield due to additional specific ion toxicities, nutritional problems often associated with high pH soils and soil textural problems causing water logging/anoxia and possible disease. The current salinity tolerance function for California pistachios is essentially the same as cotton. It was developed from a small plot study during the eight to 13th leaf years in a commercial orchard in northwestern Kern County from 1997-2002 to give a final threshold of 9.4 dS/m ECe and an 8.4 perent relative yield decline above that level. These values were confirmed for seedling growth in saline sand-tank studies at the United States Department of Agriculture (USDA) Salinity Lab in Riverside, California.

Figure 1. Currently accepted pistachio salt tolerance curve compared to cotton, alfalfa and almond (Sanden and Ferguson, 2004)

A second large scale field study applied fresh and

Continued on Page 26 24

Progressive Crop Consultant

September/October 2017

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25

Figure 2. Field locations of Kern County expanded 2014-16 pistachio salinity study.

and water content increase, so does the conductance, which is often called the EMa (apparent electro-magnetic conductivity), and subsequently correlated to a conventional laboratory ECe from a soil sample to give an ECa (apparent electroconductivity).

Continued from Page 24

possible within each orchard.

saline irrigation treatments (0.5 to 5.2 dS/m EC) from planting through 10th leaf. Trees were commercially harvested starting in 2011. Average 2011-14 root zone salinity ranged from 2.5 to 13.2 dS/m and caused a significant edible inshell yield reduction of 96 to 235 lb/ ac (depending on rootstock) in the combined 4 year yield: a one to three percent decline for every unit EC (dS/m) increase over 6 ds/m.

We selected three to four areas in each field where Area 1 appeared least saline and had the biggest trees and Area 4 appearing most saline with smaller trees (Figure 3). In addition, we used repeated measurements of electromagnetic conductance at the base of monitoring trees using a hand-held Geonics EM-38 DD (Figure 3).

This is a rapid assay device that is non-intrusive—simply emitting a magnetic signal of known strength at one end of the device and recording the strength of the ‘conductance’ Figure 3. Area 1 with average rootzone ECe of 12.9 dS/m received at the (‘summed’ EMa = 241 mS/cm) compared to Area 4 of the same field other end. As with an average rootzone ECe of 9.0 dS/m (‘summed’ EMa = 877 the salinity mS/cm).

Expanded Salinity Survey In 2014, a greatly expanded salinity survey including 10 commercial fields (9th – 15th leaf) was initiated in western Kern County with 136 individual tree data points ranging from a threeyear average root zone (five foot depth) salinity of 1.6 to 20.5 dS/m (Figure 2).

The study sites, spread across 200 square miles with a total field size of 1330 acres) were specifically selected to incorporate a wide range of soil textural and chemical differences such as water-logging, high pH and varying concentrations of sodium, chloride and boron as much as

26

Progressive Crop Consultant

September/October 2017

Figure 3 (pg. 26) perfectly illustrates the insufficiency of using the classic ECe threshold concept as the main driver for tree growth and yield decline. The Area 1 tree has almost double the canopy volume of Area 4, but the average rootzone ECe from 2014-16 is actually higher for Area 1 (12.9 dS/m) compared to Area 4 of the same field with an average rootzone ECe of 9.0 dS/m. Whoa—that makes no sense—the tree with less salt is smaller?! What the lab ECe salinity does not take into account is the high pH, bad sodic silty clay structure that makes Area 4 prone to water-logging.

We used a somewhat novel approach taking EM38 measurements right at the single-line drip hose and 30 inches away from the hose to get a larger snapshot of the rootzone. We then summed the resulting four horizontal and vertical EMa readings for a “summed” conductance. Doing this we find Area 1 has a summed EMa of 241 mS/cm and Area 4 was 877 mS/cm—fully 360 percent higher conductance for Area 4 compared to Area 1, and higher conductance means more salt and water logging. Thus, the EM38 provided a superior integration of the combined impact of salinity and soil structure on final tree growth.

Figure 4 illustrates the three-year summed yield decline as a function of rootzone ECe to a depth of five feet. The trend is highly significant, and similar to the 10 year field trial preceding this survey: resulting in a yield reduction of 144 to 351 lb/ac (three year cumulative inshell yield) for every unit ECe > 6.5 dS/m. Definitely a lower yield loss salinity “threshold” then we published 13 years ago, but the scatter under the curve is large (R2 = 0.107)—indicating the multiple other factors affecting tree growth and yield. Figure 5 uses the “summed” EM38 conductivity (EMa) as an estimator of yield decline and improves the R2 to 0.207 (Regressing the natural log transformed EM38 readings with the lab ECe rootzone salinity gave an R2 = 0.695, data not shown.)

hundreds of measurements per hour as opposed to three to four soil cores—and it’s integration of excessive water logging problems.

Blake L Sanden, Farm Advisor UCCE Kern County 1031 South Mount Vernon Avenue Bakersfield, CA 93307, USA (661) 868-6218 blsanden@ucdavis.edu Louise Ferguson, Pomology Extension Specialist Department of Plant Sciences

3045 Wickson Hall Davis, CA 95616 530-752-0507 lferguson@ucdavis.edu Craig E. Kallsen, Farm Advisor UCCE Kern County 1031 South Mount Vernon Avenue Bakersfield, CA 93307, USA (661) 868-6221 blsanden@ucdavis.edu Comments about this article? We want to hear from you. Feel free to email us at article@jcsmarketinginc.com

Figure 4. Three year cumulative pistachio yield as a function of rootzone salinity.

No Perfect Tool So there is no perfect tool for indexing soil and salinity impacts on pistachio yield, but the use of the EM38 shows promise both in it’s ease of use—

Figure 5. Three year cumulative pistachio yield as a function of EM38 “summed” EMa readings.

September/October 2017

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27

Remote Sensors for Precision Ag at West Hills College

By Terry Brase | Precision Ag instructor at West Hills College

I

n the previous article of this series, Unmanned Aerial Systems (UAS) were discussed. For all of the attention that UAS get, it is just a piece of the puzzle. First of all the unmanned aerial vehicles are only one type of platform for capturing imagery; there are also satellites, manned flights, and ground based. Second, platforms are not even the most important part of the puzzle.

The imagery that shows how a crop is doing or provides information is the important part. That leads us this month to the sensors or cameras that are used to capture the imagery.

Sensors in agriculture should be a bigger deal than UAS, but they aren’t as cool so they don’t get all the headlines.

They are like the guy that is in the background doing all the work, but doesn’t get the credit.

Sensors are a big topic, so this is actually going to be a two part article.

This month will be “remote sensors”;

28

Progressive Crop Consultant

next month will be “contact sensors”.

Remote sensors do not actually touch or “contact” the object or materials they are sensing, therefore they are remote.

The difference between a sensor and a camera also needs to be established, though the terms are commonly used interchangeably. I once took a group of students to visit an aerial imagery service company that had just purchased a sensor for approximately $1 million.

Before the tour, I cautioned the students that when this sensor was discussed, it should not be called a camera. Of course upon arrival, the tour guide referred to the sensor... as a camera. He corrected himself when he saw the student’s reaction.

A camera most properly uses film and a shutter to allow light to briefly “expose” the film. There is a light sensitive chemical on the film that reacts to the various types of light and turns color.

The resulting photograph is a chemical

September/October 2017

process.

Instead of film, a sensor has a “detector” that captures the amount of light when a shutter is used to expose it. This is an electronic process and results in a digital value…thus the term digital camera. Digital cameras use sensors to create imagery. One problem with sensors in digital cameras is that the detectors only capture the amount or strength of the light and not the color. To solve this, most digital cameras separate the light (using a type of prism) and then have filters that only allow one color to expose each of three different sets of detectors; one for the “red”, one for the “green”, and one for the “blue”. This is known as RGB image and when all three are combined and displayed in their proper colors, a natural color image is the result.

Higher quality sensors don’t separate the light to individual filters and then re-

Continued on Page 30

A DV E R TO R I A L

N E M AT O D E S :

ROOT HEALTH & TREE LONGEVITY THREAT GROWERS CAN’T SEE

Nematodes, microscopic roundworms barely visible to the naked eye, pose a serious problem for walnut and almond growers. Even with proper sanitation and fumigation practices, nematodes can still become an issue after setting new trees. Nematode populations can build up in the soil, attack tree roots and impact overall tree health.

UNSEEN BUT FIERCE ON

ROOT HEALTH NEMATODE THREATS TO ORCHARD HEALTH AND LONGEVITY

NEMATODES CAUSED

ROOT DAMAGE

REDUCED WATER & NUTRIENT UPTAKE

LOW TREE VIGOR

DISEASE TRANSMISSION

-345 Lb./A REDUCED CROP YIELD1

BEST PRACTICES FOR TREATING NEMATODES3

OF YIELD LOSS

IN YOUNG TREES

%

SUPPRESSION OF ROOT LESION NEMATODES

SUPPRESSION OF RING NEMATODES

Two-year trials show

MOVENTO® SUPPRESSES RING NEMATODES BY 85% NUMBER OF NEMATODES

YRS

59

85%

3. Applications of Movento® in established orchards resulted in a reduction of nematode populations.

Movento does offer a nematode management tool that can easily be incorporated into a tree nut grower’s cultural practices.

Trial conducted by Gary Braness, Bayer CropScience, Kerman, CA, 2009–2011.

MOVENTO ® 9 oz.

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According to Tim Weststeyn, a pest control advisor (PCA) with Crop Production Services in Vernalis, CA. He consults on 4,000 to 5,000 acres of tree nuts and is in his third year of treating established almond trees with Movento for nematode management.4

800 400

25

Applications of Movento® in established orchards helped result in:

2. If possible, fumigate the soil prior to planting new trees. This will reduce the number of nematodes initially, but will offer only a temporary solution.

UNTREATED

IN YIELD LOSS WORLDWIDE2

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A NEMATODE-CAUSED TREE DEATH CAN CREATE

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Ring nematodes/500g sample in almonds (2009–2011) (Butte & Padre pooled, n=24 trees) Trial conducted by Gary Braness, Bayer CropScience, Kerman, CA.

Average yield loss in lbs. per acre is based on California Agricultural Statistics Review, 2014–2015. California Department of Food and Agriculture.

“Nematodes: A Threat to Sustainability of Agriculture,” Satyandra Singh, Bijendra Singh and A.P. Singh.

3 University of California – Cooperative Extension. Department of Agriculture and Resource Economics. UC Davis, 2012.

4 “The Dangers of Nematodes,” Growing Produce – 2012.

1

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2

© 2017 Bayer CropScience LP, 2 TW Alexander Drive, Research Triangle Park, NC 27709. Bayer, the Bayer Cross, and Movento are registered trademarks of Bayer. Always read and follow label instructions. Not all products are registered for use in every state. For additional product information, call toll-free 1-866-99-BAYER (1-866-992-2937) or visit our website at www.CropScience.Bayer.us.

September/October 2017

www.progressivecrop.com

29

Continued from Page 28 combine. They have one set of detectors within a sensor which allows the user to capture a very specific color. This allows more control and thus better analysis.

It is also important to understand what light and color are. There is a wide range of energy coming from our sun, with a very small part of it as visible light. This light is in the form of waves of energy that are differentiated by the lengths of the waves. Each wavelength reacts differently when it strikes an object; it is reflected or absorbed by an object1 at different degrees. When a wavelength is reflected by an object, our eyes see the reflectance as a color associated with that wavelength.

A piece of red cloth can be used as a further example. It will absorb most of the light that hits it, but because of the dye that was used it reflects the red wavelength that we see. Variations of red color are slightly different wavelengths or a combination of blue and green wavelengths that we see. A black shirt

30

Progressive Crop Consultant

absorbs all of the light wavelengths and does not reflect anything (thus why a black shirt is hotter on a sunny day); a white shirt reflects all light wavelengths (why a white shirt is cooler).

A digital camera will separate the colors into groups of wavelengths (also called “bands”) and a detector senses how much of each color has been reflected from the object, and thus an image is captured.

The resolution of the image is what determines how detailed that image is. Resolution in cameras is commonly measured as “megapixels”. This is basically the number of detectors that the sensor has. Each detector captures the reflectance for one piece of the image (also known as a pixel). Higher quality sensor’s resolution is measured as “Ground Sample Distance” (GSD) which is a distance of ground that represented by one pixel. A GSD of 1 ft means the detector has captured the reflectance from a 1 ft X 1 ft area of the ground and is represented by one pixel.

Digital cameras used in the majority

September/October 2017

of smaller drones are RGB cameras that result in the natural color imagery. This works for many users to see aerial imagery of a house or property, construction site, accident scene, crime scene, or utility lines. West Hills has several UAS with RGB cameras that are used for documenting a crop, field, or irrigation on the Farm of the Future. The resolution of 1–2 inches allow us to see individual plants, irrigation lines, and branch structure within our orchard trees. This is valuable for documenting what is in the field and some simple determination for management. But it doesn’t give us detailed data about the health or vigor of a plant for further analysis.

Besides visible light wavelengths, there are also light wavelengths that are not visible to the naked human eye.

The most common of these invisible wavelengths are: NIR (Near Infrared that is nearest to the red visible band); Mid-range IR (this is a wavelength that reflects thermal waves); and Far IR (this is infrared that is farther from the red visible band). They have their own reflectance properties, which means that

even if the human eye cannot see them, a digital sensor can!

Just like capturing red, green, or blue bands, a filter can be used to separate the infrared band and then captured by the detector. The value of capturing an NIR is the difference in its reflectance properties from plant tissue.

• • •

Green wavelengths reflect at a relatively high percent from healthy plant tissue, which why we see plants as green Red wavelengths reflect at a relatively low percent and absorbed at a high percent from healthy plant tissue, be cause photosynthesis relies heavily on red light Infrared wavelengths reflect at a significantly high percent from healthy plant tissue

From this we can also say that red is reflected at a high percent from stressed or dead plant tissue, (which is why tree leaves turn red or other colors in the fall). Infrared will be absorbed by stressed or unhealthy plant tissue, meaning that less will be reflected. Capturing infrared reflectance allows the user to determine the stress level of plant tissue. The higher the reflectance, the less stress the plant is experiencing.

There is a small problem; if infrared is invisible how do we see it? When we take a RGB image, the image is displayed using its respective color. How do we display an image that has no color? Most analysts will display the infrared band as red

Continued on Page 32

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Continued from Page 31 which creates what is known as a false color image. All green vegetation is now displayed as red. And not just red, but the differences in shades of red are based on the stress level of the plant. Bright red is a healthy plant; pink or dirty red is a stressed plant.

Where do we get this multispec imagery from? If you have an unmanned aerial systems with a multispectral sensor you can capture your own imagery.

However there are two other common and valuable platforms: satellite and manned flight. I taught a digital imagery course for years using Landsat satellite multispectral imagery. This imagery is FREE if you know how to find it, download it and process it. (This is beyond the scope of this article; come to West Hills College and take my Digital Imagery class!)

There are seven Landsat satellites taking images of every part of the earth.

The imagery is available for download in the form of seven separate bands of Blue, Green, Red, NIR, Mid-Range IR (Thermal), Far-Range IR, and depending on the Landsat another IR and or panchrometric (higher resolution black and white). Sensors used vary for each Landsat satellite and included: MultiSpec Scanners; Thematic Mapper; Enhanced Thematic Mappers; and Operational Land Imager. Each captures a slightly 32

Progressive Crop Consultant

different set of wavelength bands. Users need to decide which bands are most appropriate for them.

These are all high quality sensors, but since they are 400-500 miles above the earth, they are fairly low resolution.

The GSD of most Landsat imagery is 10 meters, much lower resolution than the two to three inches from a UAS. This

provides wide imagery over a field, but does not allow images of individual trees or plants. For large fields or farms, satellite imagery is good infrared data that can be converted to NDVI (Normalized Differential Vegetative Index) or other vegetative indices.

Another problem with satellite imagery is that many times clouds are covering the earth surface. Most wavelengths that we want, do not pass through clouds and therefore the image is restricted.

There are very short radar wavelengths that will pass through clouds, but they provide limited useful data.

These concerns are addressed by the use of manned aerial flights. Missions can be flown at 1200 ft to 28,000 ft AGL (Above Ground Level) and at almost any time. Clouds can be avoided and a large area covered very quickly. Considering the area covered, the cost on a per acre basis is very reasonable.

A professional aerial imagery service does not just stick a camera out the

September/October 2017

window. The sensor is a high quality multispectral camera mounted in an opening of the belly of the aircraft. The deliverables from the service could be the raw images if the customer has the means to do the processing or a final ortho-rectified and georeferenced image ready for interpretation. West Hills relies on GeoG2 Solutions, an aerial imagery service that provides us with monthly in-

frared images of our Farm of the Future for use in class.

Though I introduced this topic saying that imagery is the important step, there is actually a lot more to remote sensing.

There is also imagery processing software, spectral signatures, hypersprectral, radiance, and thermal bands that have importance to agriculture. Most of it is still being researched. Hopefully the background in this article has been helpful to continue forward when it becomes available.

1. Actually there are four things that happen to light when it hits an object: transmitted, reflected, absorbed, refracted. For simplicity, reflected and absorbed are the two things we are most concerned about.

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

Please Visit us at the CAPCA Conference, Booth 322!

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The Burden of Compliance—Dealing with the Skyrocketing Costs of Compliance Proactively; Panel Speakers—Roger Isom, Emily Rooney, Scott Mueller, Dan Errotabere, and Robert Gulack (Almond, Walnut and Pistachio Combined Session To Be Held in Almond Building)

8:30 AM

How to Maximize Biological Control for Spider Mites in Almonds; David Haviland; UC Davis Entomology & Pest Management Farm Advisor; PCA-CE Credits: 30 minutes; Other

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Management of Soilborne Diseases in Replanted Almond Orchards; Mohammad Yaghmour, UCCE Farm Advisor; PCA-CE Credits: 30 minutes; Other FSMA Produce Safety 101 for Almond Growers; Tim Birmingham, Director, Quality Assurance & Industry Services, Almond Board of California

Advances in Spray Application Technology for Precision Chemical Applications; Alireza Pourreza, UCCE Extension Advisor;

Anthracnose; Is it a Threat to California Pistachios?

Themis Michailides; Professor and Plant Pathologist at UC Davis;

Monitoring and Management of Walnuts Pests—Codling Moth, Husk Fly, and NOW; Charles Burks, USDA/ARS Research Entomologist;

Best Treatment Timings and Management of Large and Small Bugs in Pistachios; Kris Tollerup, UC Statewide IPM;

State of the Industry Update; Jennifer Williams and Claire Lee, California Walnut Board

State of the Industry Update; American Pistachio Growers; Richard Matoian, Executive Director

PCA-CE Credits: 30 minutes; Other

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10:00 AM Break/ Trade Show 10:30 AM

Identification, Control, and Management of Ant Populations; Kris Tollerup, UC statewide IPM; PCA-CE Credits: 30 minutes; Other

The Latest in Spray Applications for Controlling Botryosphaeria; Themis Michailides, Professor and Plant Pathologist at UC Davis;

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Soilborne Diseases from Chemical Controls to Best Management Practices for Pistachios; Phoebe Gordon, UCCE Farm Advisor;

11:00 AM

Self-Compatibility: Is It Right for Your Orchard?

Craig Ledbetter, Research Geneticist USDA/ARS

Preplant Site Selection, Planning and Designing your Walnut Orchard; Mae Culumber, UCCE Farm Advisor

Pistachio Rootstock Production and Selection; Elizabeth Fichtner, UCCE Farm Advisor

11:30 AM

Controlling Fungal and Bacterial Diseases in Almonds; Brent Holtz, UCCE Farm Advisor; PCA-CE Credits: 30 minutes; Other

New Chemistries for Weed Control Pre- and Post-Emergent; Kurt Hembree, UCCE Farm Advisor;

The Latest on Winter Chill and How it Impacts Crop Prediction; Craig Kallsen, UCCE Farm Advisor;

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The Burden of Compliance—Dealing with the Skyrocketing Costs of Compliance Proactively; Panel Speakers—Roger Isom, Emily Rooney, Scott Mueller, Dan Errotabere, and Robert Gulack (Almond and Walnut Combined Session To Be Held in Workshop Area)

8:30 AM

Advances in Spray Application Technology for Precision Chemical Applications; Alireza Pourreza, UCCE Extension Advisor; CCA-CE Credits- Integrated Pest Mgmt: 30 Minutes PCA-CE Credits: 30 minutes; Other

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Early Beehive Removal and Spray Timing Prevention and Management of Soilborne Diseases; Can Impact Pollination and Yield; Dani Lightle, UCCE Farm Advisor; Wes Asai, Pomology Consultant; CCA-CE Credits - Soil & Water Mgmt: CCA-CE Credits - Integrated Pest 30 AM Minutes Mgmt: 30 Minutes 10:00 Break/ Trade Show PCA-CE Credits: 30 minutes; Other PCA-CE Credits: 30 minutes; Other

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FSMA Produce Safety 101 for Almond Growers; Tim Birmingham, Director, Quality Assurance & Industry Services, Almond Board of California

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Management Zone Based Precision Irrigation That Uses Leaf Monitors to Sense Plant Water Status and Improve Irrigation Efficiency; Shrinivasa Upadhyaya, Professor, UC Davis

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New Regulations for Spraying Near Schools and Day Cares; Marline Azevedo, Deputy Agricultural Commissioner—Pesticide Division, Stanislaus Agriculture Department; CCA-CE Credits - Integrated Pest Mgmt: 30 Minutes

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New Research is Developing a Soil App to Evaluate Nitrogen Leaching; Toby O’Geen, UCCE Extension Specialist

11:30 AM

Managing Fungal and Bacterial Diseases in Almonds; Emily Symmes, UCCE Area IPM Advisor; CCA-CE Credits - Integrated Pest Mgmt: 30 Minutes PCA-CE Credits: 30 minutes; Other

Advances in Spray Application Technology for Precision Chemical Applications; Alireza Pourreza, Assistant CE Advisor; CCA-CE Credits Integrated Pest Mgmt: 30 Minutes PCA-CE Credits: 30 minutes; Other

Turning Ag Waste into Profit; Bill Ort, Research Leader USDA/ARS

12:00 PM - Free BBQ Tri-Tip Lunch 12:15 PM

Navel Orangeworm Management—Current and Future Prospects; Brad Higbee, Field Research & Development Manager for Trécé Inc.; CCA-CE Credits - Integrated Pest Mgmt: 30 Mintues; PCA-CE Credits: 30 minutes (Almond and Walnut Combined Session To Be Held in Workshop Area)

1:00 PM

Trade Show PCA-CE Credits: 15 minutes; Other

1:30 PM

Adjourn

September/October 2017

www.progressivecrop.com

37

Continued from Page 10 The fully irrigated baseline SWP for any given day and time is defined as the SWP that is expected if soil moisture is abundant and not limiting transpiration under the prevailing temperature and humidity conditions. Baseline may or may not vary with crop species. For example, baseline conditions for almond and prune are very similar but are different for walnut. Baseline values are derived from mathematical models and have been validated in field experiments in almond, prune, and walnut.

Estimates of fully irrigated baseline SWP for almond and prune over a range of air temperatures and relative humidity are provided in Table 1 (pg. 10). Estimates of a fully irrigated baseline SWP for walnut are provided in Table 2 (pg. 10). During cool weather, estimates of baseline SWP may be –5 to –8 bars in almond and prune. However, under extremely hot and dry conditions, baseline SWP for almond and prune may be –11 to nearly -14 bars. In walnut, estimates of baseline SWP may be -3 to -5 bars during cool weather and -6.5 to near -8 bars during extremely hot, dry weather.

Estimating baseline conditions requires access to public or private weather databases (e.g., CIMIS) that provide hourly temperature and relative humidity data, or an inexpensive, simple handheld instrument that can measure temperature and relative humidity in the orchard at the same time that SWP measurements are taken.

An online calculator of baseline SWP is also available through the Fruit and Nut Research and Information Center: informatics. plantsciences.ucdavis.edu/Brooke_Jacobs/ index.php. Alternatively, go to the Fruit and Nut Center website (fruitsandnuts.ucdavis.edu), go to the “Weather Related Models” tab and select “Stem Water Potential”.

Comparing orchard SWP measurements to fully irrigated baseline SWP estimates allows you to express your orchard’s SWP in bars below baseline. An example would be a day when weather is normal, the almond baseline is –10 bars and the orchard SWP measurements average –14 bars,

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or 4 bars below baseline. This suggests that tree water stress is occurring and that the need for irrigation is approaching. In contrast, on a different but very hot, windy day in the same almond orchard, when the baseline is –13 bars SWP and the orchard measurement averages -14 bars or just 1 bar below baseline, it suggests these readings are more associated with hot, dry weather conditions than with irrigation management. Once the hot, dry weather passes, both the estimate of baseline SWP and the orchard SWP measurements may recover to levels indicating lower tree stress. This information could guard against irrigating too soon or too much and help manage incidences of root and foliar diseases and improve access into the orchard for other activities.

Other potential benefits of comparing fully irrigated baseline to the actual SWP measurements include identifying orchards that are steadily near or above the fully irrigated baseline the entire season. These orchards may be at more risk of eventual tree loss from excess water and resulting diseases. Comparisons between baseline and actual SWP measurements can also be helpful to dial-in regulated deficit irrigation strategies to better manage uniformity of crop development and maturation. Examples include more uniform hull split and harvest timing in almond and higher sugar content and lower “dry-away” in prune. Using baseline estimates may also enable earlier use of a pressure chamber and SWP in the spring when weather is more variable.

Interpretive Guidelines for Almond, Prune, and Walnut Additional interpretive guidelines are provided in Tables 3, 4 (pg. 40) , and 5 (pg. 42), for almond, prune, and walnut, respectively. They show that the ranges in SWP for almond and prune are similar up to a point. Almond may be able to survive more extreme levels of water stress than prune. However, research has not been conducted in prune to quantify how extreme water stress can become before mature trees no longer survive. Research has shown that walnut has a distinctly different range in water stress than almond or prune.

In general, UC production research has shown the optimum range in crop water stress levels of about -6 to -18 bars for almond, -6 to -20 bars for prune, and -4 to -8 bars for walnut. Some production research indicates that exceptional production may be possible when irrigation is managed to maintain tree water status to minimize or completely avoid SWP stress levels for the entire season. However, university experiments and production experience supporting intensive irrigation management that sustains orchards near baseline and minimal tree stress throughout the season is not conclusive. Concern exists about higher incidence of root and foliar diseases, lower limb shading and dieback, and shortened orchard life. Orchards, where SWP fluctuates more within these optimum ranges may still yield competitively but incur less tree loss and produce longer.

SWP is a field diagnostic tool that can help growers and consultants understand short and long term trends in orchard water status. With this knowledge they can tailor their management to achieve specific objectives and goals which may be different between orchards due to different growing environments and orchard designs (i.e. cultivar and rootstock choices, planting configurations, and pruning practices). The pressure chamber

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Progressive Crop Consultant

September/October 2017

and SWP may also assist with minimizing the impacts of short water supplies during drought and balance water and energy costs.

Using SWP in other crops

A grower or consultant who invests in a pressure chamber may be interested in its application in other crops. Research and development of the pressure chamber to evaluate crop water stress and help with on-farm water management has been undertaken in several crops for many decades. It’s readiness for on-farm use varies among crops. Cotton and wine grapes are examples of crops where an abundance of research has been conducted and information extend-

ed so the pressure chamber is available as a water management tool. However, in both of these crops the measurement technique differs from SWP. In these crops with smaller canopies, the sample leaf is not covered with a bag and leaf water potential is measured not SWP. In crops such as olive, pistachio, peach, pecan, seed alfalfa, and others there is limited information available.

The information may originate both from California and other areas of the U.S. and internationally and may involve different sampling techniques other than SWP such as leaf water potential and shaded leaf water potential. If interested in any of these crops, one suggested source to begin a literature search is:

https://www.pmsinstrument.com/research/.

A Stand-Alone or Complementary Irrigation Management Tool

As a direct measure of tree response to irrigation management and the soil, climate, and orchard environment, SWP is unique compared with other methods that assess tree water status indirectly.

As such, through trial and error, SWP can be used—and has been successfully adopted by many growers—as a standalone technique for irrigation schedul-

Continued on Page 40

Table 3. SWP levels in almond, consideration of how SWP might compare to baseline values under typical weather conditions, and the corresponding water stress symptoms to expect.

SWP range General Baseline (bars) Stress Level consideration

Water stress symptoms in almond

-2 to -6

None

(Likely above typical baseline)

(Not commonly observed)

-6 to -10

Minimal

At or within 2 bars below baseline under normal conditions

Typical from leaf-out through mid-June. Stimulates shoot growth, especially in developing orchards. Higher yield potential may be possible if these levels are sustained (if the only limiting factor). Sustaining these levels may result in higher incidence of disease and reduced life span.

-10 to -12

Mild

Near baseline under hot, These levels of stress may be appropriate during the phase dry conditions, but 2 to 4 of growth just before the onset of hull split (late June). bars below baseline under normal or cooler weather

-12 to -14

Mild to Moderate

Within 2 bars below baseline under hot, dry conditions, and 4 to 6 bars below baseline under normal or cool conditions

Reduced growth in young trees and shoot extension in mature trees. Suitable in late June up to the onset of hull split (July). Still produce competitively. Recommended level after harvest. May reduce energy costs or help cope with drought conditions.

-14 to -18

Moderate to High

Likely 4 to 5 bars below baseline under hot, dry conditions and 6 to 8 bars below baseline in normal or cooler weather.

Stops shoot growth in young orchards. Mature almonds can tolerate this level during hull split (July) and still yield competitively. May help control diseases such as hull rot and alternaria leaf spot, if present. May expedite hull split and lead to more uniform nut maturity. Also may help reduce energy costs and cope with drought conditions.

-18 to -22

High

Within 8 to 14 bars below baseline

Slow to no growth in mature orchards. Interior leaf yellowing with some leaf drop. Should be avoided for extended periods. Likely to reduce yield potential.

-22 to -30

Very High

Within 14 to 24 bars below baseline

Wilting observed. Stomatal conductance of CO2 and photosynthesis declines as much as 50% and impacts yield potential. Some limb dieback.

-30 to -60

Severe

Within 24 to 50 bars below baseline

Extensive or complete defoliation is common. Trees may survive despite severe defoliation. Severely reduced or no bloom and very low yield expected following one to two seasons until trees are rejuvenated.

Below -60 bars

Extreme

(Substantially below baseline under all conditions)

(Trees are probably dead or dying)

September/October 2017

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39

Continued from Page 39 ing. Using SWP alone, along with the interpretive guidelines presented in this article, gives relatively straightforward insights to the question of when to irrigate and with experience, perhaps how long to irrigate.

However, trial and error may not suffice in some situations, and SWP measurements alone may not provide enough information. For example, pressure chamber measurements can show low crop stress in April, May, and early June in orchards, even though irrigation has been postponed or reduced.

The trees consume water stored in the root zone from winter rainfall or winter irrigation during this period to satisfy

their water requirements. Without some monitoring of soil moisture or tracking of ET, it is possible to experience a sudden and rapid increase in tree water stress in July, August, and September.

Meanwhile, a drip or microsprinkler irrigation system will not have the capacity to apply enough water to recharge the soil moisture depletion and sufficiently relieve tree stress. This can be a problem particularly in orchards where soils have slow water infiltration. Irrigation should be initiated before the stored water is depleted excessively.

Using SWP in conjunction with soil moisture monitoring or a water budget (or both) can also help overcome some common limitations of soil moisture monitoring and irrigating according to

ET. SWP readings that indicate low tree stress, even though soil moisture sensors indicate dry soil, might suggest that trees are getting water from greater depths in the root zone that are not being monitored by soil moisture sensors. This situation could indicate a need for deeper soil moisture monitoring or placing soil moisture sensors in more representative locations. Similarly, SWP readings that indicate desirable levels of tree stress, even when a water budget indicates under-irrigation, suggest the need to reexamine assumptions about ET rates, rooting depth, and available moisture reserves.

In a different situation, SWP mea-

Continued on Page 42

SWP range (bars)

General Baseline Stress Level consideration

Water stress symptoms in almond

-2 to -6

None

(Likely above typical baseline)

(Not commonly observed)

-6 to -8

Minimal

At or near baseline under normal or cool weather conditions

Typical in March and April. Indicates soil moisture is not limiting. If sustained, higher incidence of root and foliar diseases and tree loss may occur.

-8 to -12

Minimal to Mild

At or near baseline under hot, dry conditions, but may be 2 to 4 bars below baseline under normal or cool conditions

Favors rapid shoot growth and fruit sizing in orchards when minimal crop stress is sustained from April through mid-June.

-12 to -16

Mild to Moderate

May be 2 to 4 bars beSuggested mild levels of stress during late June, July, and low baseline under hot, early August. Shoot growth slowed but fruit sizing unaffectdry conditions, but may ed. May help manage energy and irrigation costs. be 4 to 6 bars below baseline under normal or cooler weather

-16 to -20

Moderate to High

Within 6 to 8 bars below baseline

Should be avoided until fruit sizing is completed. Appropriate for late August after fruit sizing is completed. Imposing moderate to high levels of crop stress by reducing irrigation about two weeks before harvest may increase sugar content in fruit and reduce moisture content or “dry-away” (fruit drying costs).

-20 to -30

High to Severe

Within 8 to 20 bars below baseline

More likely to occur in late August and early September during and after harvest. Extended periods of high crop stress before harvest will result in defoliation and exposure of limbs and fruit to sunburn. Extended periods of high stress after harvest may also negatively affect the condition of trees going into dormancy.

Below -30

Extreme

(Substantially below baseline under all conditions)

Extended periods of severe crop stress should be avoided.

Trees will defoliate and be exposed to sunburn, increasing the risk of canker diseases. Potentially shorten productive life of orchard.

Table 4. SWP levels in prune, consideration of how SWP might compare to baseline values under various weather conditions, and the corresponding water stress symptoms to expect.

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Progressive Crop Consultant

September/October 2017

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SWP range General Baseline (bars) Stress Level consideration

Water stress symptoms in almond

Higher than –2

None

(Likely above typical baseline)

(Not commonly observed)

-2 to –4

None

At or above typical baseline

Fully irrigated. Commonly observed when orchards are irrigated according to estimates of real-time evapotranspiration (ETc). If sustained, long term root and tree health may be a concern, especially on California Black rootstock

-4 to –6

Minimal

May equal or be as much High rate of shoot growth visible, suggested level from leafas 2 bars below typical out until mid-June when nut sizing is completed baseline

-6 to –8

Mild

May equal baseline Shoot growth in non-bearing and bearing trees has been under hot, dry conditions, observed to decline. These levels do not appear to affect but may be 2 to 4 bars kernel development or quality. below baseline under

-8 to –10

Moderate

May be 1 to 2 bars below baseline under hot, dry conditions but 4 to 6 bars below baseline under normal or cooler weather

Shoot growth in non-bearing trees may stop, nut sizing may be reduced in bearing trees and bud development for next season may be negatively affected.

-10 to –12

High

Likely 3 to 4 bars below baseline under hot, dry conditions and 6 to 8 bars below baseline in normal or cooler weather.

Temporary wilting of leaves and shrivel of hulls has been observed. New shoot growth may be sparse or absent and some defoliation may be evident. If sustained, nut size will likely be reduced with darker kernel color.

-12 to –14

Very High

Likely 5 to 6 bars below Results in moderate defoliation. Should be avoided. baseline under hot, dry conditions and 8 to 10 bars below baseline in normal or cooler weather.

-14 to –18

Severe

Likely 7 to 8 bars below Severe defoliation, trees are likely dying. baseline under hot, dry conditions and 10 to 12 bars below baseline in normal or cooler weather.

Below -18

Extreme

(Substantially below base- (Trees are probably dead or dying) line under all conditions)

Table 5. SWP levels in walnut, consideration of how SWP might compare to baseline values under various weather conditions, and the corresponding water stress symptoms to expect.

Continued from Page 40

Acknowledgements

surements might indicate moderate to high tree stress even when soil moisture sensors placed within the wetted pattern of the drip emitter show high soil moisture content. In this case, additional investigation is necessary to determine if roots are healthy and growing, and if the soil moisture sensors accurately represent the root zone and predominant soil types or if the sensors are defective.

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Progressive Crop Consultant

Numerous (too many to mention all) University of California faculty and students, extension specialists and farm advisors have invested years of field research to develop the pressure chamber and midday stem water potential as a viable on-farm water management tool for almond, prune, and walnut. This article would not be possible without their contributions and seeks to capture all of the experience as accurately and

September/October 2017

concisely as possible. Special thanks to Ken Shackel, Bruce Lampinen, and Sam Metcalf of UC Davis Department of Plant Sciences for their career long commitment to seeing the pressure chamber and midday stem water developed into a valuable water management tool.

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

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info@callnrg.com Tel: 559.564.1236 34284-B Road 196 www.callnrg.com Fax: 559.564.1238 Woodlake, CA 93286 September/October 2017 www.progressivecrop.com 43

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