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Developing an Effective Management Program for Roof Rats in Citrus

By
Roger A. Baldwin | UCCE Human-Wildlife Conflict Resolution Specialist, Dept. of Wildlife, Fish and Conservation Biology, UC Davis and Ryan Meinerz | Staff Research Associate, Dept. of Wildlife, Fish and Conservation Biology, UC Davis
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Figure 1. Example of tracking tunnel used to monitor roof rat activity (all photos courtesy R. Baldwin.

Roof rats (Rattus rattus) can cause extensive damage in citrus orchards through direct consumption of fruit, feeding of the cambium layer leading to mortality of branches, chewing on irrigation infrastructure and by posing as a potential food safety risk. The UC IPM Pest Management Guidelines for citrus (ipm.ucanr.edu/agriculture/citrus/Roof-Rats/) only list three management tools for roof rats: 1) cultural control, 2) rodenticide baiting and 3) trapping. Cultural control primarily involves removing vegetative materials from orchards to help deter roof rats, but the practicality of this approach is substantially limited in citrus given that the trees themselves provide ample cover for rats. This leaves rodenticides and trapping as the two primary tools for managing roof rats in citrus, although effective practices for each technique are unknown in citrus. Effective management of vertebrate pests also relies on quick and easy monitoring strategies to know when additional actions are needed to maintain low rodent density. Therefore, we initiated a series of studies in 2020 to develop an integrated pest management (IPM) program to manage roof rats in an efficacious and cost-effective manner. We summarize the findings from these studies in the following sections, ultimately providing a roadmap for an effective IPM approach for this invasive rodent pest.

Figure 2. Example of tracking card with roof rat footprints.

Monitoring
Effective management of all pests requires quick and easy monitoring strategies. We tested a strategy that used systematically placed tracking tunnels (Black Trakka, Gotcha Traps, or traps.co.nz) that contained a tracking card and ink pad to detect roof rat presence throughout orchards (tunnels tied to board placed up in tree; Fig. 1). When a rat visits the tunnel, it leaves ink footprints on the tracking card (Fig. 2). We determined one tracking tunnel approximately every 230 feet yielded an accurate estimate of current roof rat activity. A lure helps to draw rats into the tracking tunnel. We tested several options, including peanut butter, Liphatech Rat and Mouse AttractantTM and Liphatech NoToxTM wax blocks, and found all were equally effective. Given the ready availability and cheaper cost associated with peanut butter, it may be preferred by some, although the pre-packaged nature of the other attractants could make them desirable by users as well.

Figure 3. Overlapping roof rat home ranges that show the rats did not move out of the orchards.

Movement Patterns
We knew little about how roof rats moved throughout citrus orchards. Such knowledge is important to determine where to target management strategies, to understand ideal spacing between traps and bait stations and to assess when roof rats were active in the orchards. To understand movement patterns in roof rats, we deployed a unique tracking system that used cellular technology to identify locations every few seconds. This allowed us to determine areas utilized by rats as well as how far they moved throughout the landscape. We determined roof rats exclusively used orchards (Fig. 3), indicating management efforts should be targeted within orchards rather than in adjacent habitats. We also determined roof rats had large home ranges that averaged 5.8 acres; minimum home range size was 1.8 acres. This equated to a radius of approximately 280 ft and 160 ft for average- and minimum-sized home ranges, respectively. This information is very valuable in determining ideal spacing between traps and bait stations to guarantee rat access to at least one of these management tools within their home range. We also used remote-triggered cameras to determine when roof rats were active within orchards. Based on photo data, roof rats were active exclusively at night, with activity often peaking around midnight. If necessary, roof rat removal efforts could be targeted exclusively at night to eliminate non-target effects to diurnal species (i.e., those active only during daytime), although such actions would likely be cost-prohibitive.

Test of Potential Management Tools
We focused our control efforts on the use of rodenticides and trapping as the only two techniques currently available that were likely to have a substantial impact on roof rat populations within citrus orchards. Previous research indicated the use of a 0.005% diphacinone-treated oat bait sold by many County Agricultural Commissioner’s offices in California (countyofkingsca.gov/home/showpublisheddocument/27503/637667104610630000) was effective at reducing roof rat populations when used in elevated bait stations (Fig. 4) within almond orchards. However, almond and citrus orchards are very different both in the cover provided by the trees as well as the food sources available.  As such, we needed to test this product in citrus to determine its utility.

Figure 4. Bait station secured to branch in orange tree.

Effective IPM programs rely on more than one technique to safely and effectively manage pests. As such, we were interested in using trapping as an additional tool to manage roof rats. Historically, trapping in tree crops has relied on snap trapping, but snap traps are often viewed as too labor-intensive for use in production ag systems. The recent advent of the Goodnature® A24 trap had the potential to substantially reduce the amount of labor required to operate a trapping grid due to the long-lasting lure and use of a CO2 cartridge that would allow for use for four to six months without having to relure or reset the traps. 

Regardless of the tool used, spacing between each subsequent bait station or trap was important to ensure success while minimizing cost. We originally established trapping and baiting grids where individual units were separated by approximately 250 feet. However, initial testing across three separate orchards indicated this spacing was not effective for bait stations (efficacy = 12%), so we reduced the spacing to 160 feet for the final orchard to mimic the minimum size of a roof rat home range. This spacing resulted in much higher efficacy (77%), and we planned to use that spacing moving forward. Likewise, we did not find the A24 trap to be effective at reducing roof rat activity across our first three study sites; in fact, we observed an increase in rat activity at these sites (efficacy = –70%). After consultation with staff from Goodnature®, we placed a platform underneath each trap to assist the rats in pushing far enough up into the trap to activate it (Fig. 5). This modification increased efficacy for our final site (50%), so we planned to add this adjustment in subsequent trials.

Figure 5. Goodnature® A24 trap with platform underneath.

Develop and Test IPM Strategies
Taking information already learned, we developed an IPM strategy that used elevated bait stations at 160-foot spacing that contained 0.005% diphacinone-treated oats to initially knock down populations. Baiting typically lasted four weeks. We followed this up with two weeks of snap trapping using trapping tunnels tied to boards and placed in trees (Tomcat® Tunnel™ Trapping System, Motomco; Fig. 6). The trapping tunnels were targeted in areas with remaining rat activity to further reduce the population. Following completion of snap trapping, we deployed A24 traps for the remainder of a six-month period in an attempt to maintain low rat densities. We compared these results to that of a bait station-only approach (hereafter bait station) to determine which was most effective. Initial bait applications substantially reduced roof rat activity (efficacy = 73%), but neither the IPM nor bait station approaches adequately slowed reinvasion of the study sites (two-month post bait application efficacy:  bait station  = –5%, IPM  = 13%; 5-month post bait application efficacy: bait station  = 24%, IPM  = 43%). As such, we developed a second IPM approach that again incorporated bait stations to knock down populations. We followed this up with a snap trapping program, again using trapping tunnels. For this approach, we spaced the trapping tunnels in a grid pattern with the traps 245 feet apart. These trapping tunnels were operated for the remainder of a six-month period. This approach was very effective, with rat activity decreasing over time (two-month post bait application efficacy: bait station  = 34%, IPM  = 88%; 5-month post-bait application efficacy: bait station  = 85%, IPM  = 93%). In total, we removed 97 rats via snap trapping in IPM plots during this part of the trial, again indicating the effectiveness of this approach. For bait station plots, rats quickly rebounded two months after the completion of the baiting program, indicating the IPM approach was more effective. Interestingly, rat populations again declined in the bait station plot for unknown reasons (no additional bait was used for the remainder of the study), although the IPM plots were always more effective. From an efficacy perspective, the IPM program was the better approach given the importance of using multiple tools to maintain long-term efficacy of management programs.

Figure 6. Roof rat captured in trapping tunnel containing two snap traps.

We also collected information on the cost of these management programs to better inform which were most practical. The IPM plots that used A24 traps were by far the most expensive ($48.41/acre), primarily given the substantial cost associated with each trap (a minimum of $152/trap). Given this high cost and the limited efficacy of this approach, management programs using A24 traps were deemed impractical for use in citrus orchards. 

As expected, the bait station plots were the least expensive ($11.10/acre), but they were also less effective. Conversely, the IPM plots that relied on bait stations and trapping tunnels were more efficacious but also were more expensive to operate ($19.71/acre). However, this cost was far more reasonable when compared to trapping programs that included A24 traps. Furthermore, the primary difference in cost between bait station plots and IPM plots that used bait stations plus snap trapping was due to the cost of the trapping tunnels.

Assuming trapping tunnels could be used for several years, the cost for all subsequent years of this IPM program would essentially be the same as the bait station plots (bait station = $3.72/acre, IPM = $4.01/acre).  Although we have no direct quantifiable data on crop losses associated with roof rats, this IPM cost seems justifiable. For example, assuming a price of $20 for a box of fancy lemons (115 lemons per box) or naval oranges (72 per box), then only around one half to one box of fruit would have to be saved per acre per year to justify management costs for the first year, and a minimal amount of fruit would need to be saved to justify expenditures for subsequent years. This price does not account for infrastructure damage associated with rats nor the potential food safety risks associated with their presence in orchards, further increasing the value of this management approach.

Management Recommendations
We recommend the following IPM strategy for managing roof rats in citrus:

1. Conduct initial monitoring using tracking tunnels separated by 230 feet to determine an uptick in roof rat abundance.

2. When roof rat activity is high (based on grower-defined thresholds (no official threshold yet established)), implement a baiting program (0.005% diphacinone-treated oats) using elevated bait stations separated by 160 feet. Operate bait stations until bait consumption is minimal.

3. Place trapping tunnels in a grid pattern, with tunnels spaced approximately 245 feet apart. Check traps approximately every three weeks to rebait and reset as needed.

4. Operate tracking tunnels every three months to determine the status of the roof rat population.  Additional bait applications can be used as necessary.

Please note not all diphacinone products have the same label specifications. To our knowledge, only the CDFA product tested in our studies is allowable for use within orchards during the bearing season. Also, regulations surrounding rodenticide use often change. Be sure to check up-to-date regulations surrounding the use of any rodenticide before using.

Plant Stress Defense and Management: How to Leverage Stress Response Mechanisms with Biostimulants

By
Eryn Wingate | Executive Board Member, Western Region Certified Crop Advisers, Lead Agronomist, Tri-Tech Ag Products, Inc.
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Every season, crops must overcome environmental stressors from a range of biotic and abiotic conditions, including pest and pathogen pressure, drought, extreme weather or soil salinity. Abiotic stress alone can cause over 50% growth loss in most plant species, and disease pressure can cause even greater damage (Rejeb et al. 2014). Careful attention to crop protection and nutrition usually keeps crops healthy through periods of moderate stress, but with increasing production costs and tight regulations on ag chemical use, growers and consultants may benefit from additional materials to improve crop stress tolerance. Biostimulant products derived from natural plant compounds initiate defense mechanisms to control pests and pathogen pressure and protect against abiotic stressors (Shiade et al. 2024). All plants have evolved protective mechanisms to prevent damage from environmental stress, and by leveraging those innate processes with biostimulant applications, we can improve crop health and minimize stress symptoms. Understanding plant defense mechanisms and the biochemicals involved will help managers differentiate between biostimulants on the market and determine how to integrate them into standard crop management practices effectively.

The Four Stages of Plant Defense Mechanisms
Exposure to biotic or abiotic stress initiates defensive pathways that plants carry out in four stages: stress sensing, signal transduction, gene expression regulation and physiological adaptations to stress.

Table 1. Metabolites and their plant stress response roles. Primary and secondary metabolites are produced by plants in response to biotic or abiotic threats.

Stress detection
Immediately upon exposure to an environmental stressor, the plant must perceive the threat quickly to launch an effective defense. Plants identify stressors with molecular signal receptors, or by detecting other cellular changes caused by environmental conditions. Plants are equipped with many types of signal receptors to differentiate between individual threats in their environment. Stress is perceived when the sensory receptor binds with the threat signal molecule, causing a change in the receptor’s shape. Photoreceptors detect ultraviolet radiation and can trigger changes in growth according to light quality, intensity and duration. Hormone receptors and other types of molecular sensors can detect pathogens and pests or receive warning signals from soil microorganisms that initiate defense mechanisms (Shiade et al. 2024; Lal et al. 2023).

Plants also perceive stress when environmental conditions cause cellular changes, such as irregular ion flux or membrane fluidity. Increased calcium concentration and changes in osmolyte levels can indicate stress from drought, salinity or other adverse soil conditions. Cellular membrane fluidity fluctuates according to temperature, alerting cells to high heat or severe cold. These initial stress alerts trigger a defensive cascade that ultimately leads to genetic and physiological adaptations to protect the plant from the threat (Shiade et al. 2024).

Signal transduction
After the stressor is detected at the molecular level, the threat warning must be communicated within the cell, between cells and throughout the plant. Signal transduction pathways transmit the stress warning through a series of chemical reactions that result in genetic regulation and physiological changes. Several signal transduction pathways have been observed following stress exposure, and the type of response launched depends on the particular stressor perceived (Rejeb et al. 2014). Initial threat detection increases the concentration of signaling molecules and phytohormones such as reactive oxygen species (ROS), abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) that facilitate the defense response cascade. ROS rapidly increases in response to stress but serves different purposes for abiotic versus biotic threats. Pest and pathogen defense cascades are usually mediated by SA, JA and ET, while ABA directs abiotic stress defense (Shiade et al. 2024; Rejeb et al. 2014).

Gene expression regulation
Stress signal transduction often leads to changes in genetic expression that result in defensive biochemical or physiological changes within the plant. Stress signals are received by transcription factors (TFs) that can activate or suppress certain genes to adapt to environmental stress. Researchers have identified several pathogen-related (PR) genes and proteins that induce resistance to fungal and bacterial infection. Some PR genes, such as the Botrytis Susceptible (BOS1) gene, offer both biotic and abiotic defense. Upon activation. BOS1 induces resistance to necrotrophic pathogens and provides protection from osmotic stress (Rejeb et al. 2014). PR genes are normally activated by pathogen attack perception, but sometimes abiotic stressors can upregulate the pathogen defense genes. Cold stress triggers an accumulation of TFs that upregulate expression of certain PR genes in addition to genes involved in cold protection (Rejeb et al. 2014).

Physiological change
Stress signaling and genetic regulation result in physiological changes that defend the plant against the stressor. Stress response signaling leads to biopesticide production, cell wall reinforcements and other measures to prevent damage from environmental stress (Rejeb et al. 2014). For example, plants respond to drought stress by closing stomata and producing osmoprotectant compounds to conserve water. Rice plants prevent damage from soil salinity by increasing proline and other compounds that relieve osmotic pressure and toxicity (Shiade et al. 2024). Many other plants have similar adaptations to mitigate salinity and other adverse soil conditions. Plants confronted with pathogen pressure produce antimicrobial compounds and strengthen cell walls to deter attackers and prevent further infection (Shiade et al. 2024). 

Leveraging Plant Stress Responses with Biostimulants
Plant stress defense researchers have identified many biochemicals that are critically responsible for protective mechanisms. These biochemicals include primary and secondary metabolites produced by plants in response to biotic or abiotic threats. Studies show that many metabolites offer stress protection when applied to crops as biostimulants or biopesticides. Integrating biologically based materials into standard crop management practices can improve pest control efficacy and maintain plant health under stressful environmental conditions (Lal et al. 2023; Shiade et al. 2024; Rejeb et al. 2014).

Primary metabolites
Primary metabolites (PMs), critical to plant growth and development, include carbohydrates, amino acids, proteins, and lipids. They provide the plant’s energy source and act as building blocks for macromolecules and cellular structures. PMs can also aid in stress defense by detecting threats, serving as signaling molecules, regulating osmotic potential and more. Carbohydrates including oligosaccharides, disaccharides and fructans accumulate in response to drought or salinity and alleviate stress through osmotic regulation. The protein proline also offers drought protection in addition to other functions, including pH buffering, protein structure stabilization and ROS scavenging. Lipids play important roles in stress perception and signaling, such as detecting changes in membrane fluidity due to extreme shifts in temperature. Many lipids also serve as the precursors necessary to build secondary metabolites critical to plant defense (Shiade et al. 2024).

Secondary metabolites
Secondary metabolites (SMs) include tens of thousands of biochemicals responsible for a wide range of functions, including enzyme regulation, signaling within and between cells, communication with soil microorganisms and more. SMs are categorized into four groups: terpenoids, phenolics, sulfur-containing compounds and nitrogen-containing compounds. SMs play crucial roles in plant defense mechanisms, and many of these compounds effectively reduce stress symptoms when applied as biostimulants (Shiade et al. 2024).

Terpenoids are the most abundant group of SMs, carrying out many functions, such as growth regulation, pollinator attraction and plant defense. The terpenoid JA is a critical signaling molecule that activates defense genes to protect plants again pathogens (Rejeb et al. 2014). Many other terpenes are precursor molecules for phytohormones required to deter pests or protect against other stressors (Shiade et al. 2024).

Many phenolic compounds, such as courmarins, tannins, salicylic acid (SA) and lignin, have been identified as effective biostimulants for both biotic and abiotic stress prevention (Lal et al. 2023). Coumarins help provide protection against pathogenic fungi and herbivorous pests. Another phenolic, lignin, toughens cell walls and provides physical protection against biotic and abiotic stress. Several types of tannins serve as natural pesticides by killing or repelling insects and pathogens. SA primarily serves in pathogen and pest defense signaling but can also mitigate drought and other abiotic stressors (Banothu and Uma 2021). SA treatments provide resistance against sheath blight and other fungal pathogens in several crops. SA has also been shown to prevent drought stress in corn and mitigate toxicity from the heavy metal cadmium in Triticum aestivum (Lal et al. 2023.)

The S- and N-containing compounds comprise a smaller set of secondary metabolites, but research shows several promising candidates for agricultural use. The S-containing compounds, including glucosinolates and related compounds, are found in brassicas and certain flowering plants. Foliar application of glucosinolate has been shown to control aphids on tomatoes and Spanish broom plants and may offer protection against other pests and pathogens on a variety of crops (Shiade et al. 2024).

Table 2. Studies show many metabolites offer stress protection when applied to crops as biostimulants or biopesticides.

Integrating Biostimulants into Standard Management Practices
Independent research at universities and biotech labs around the world corroborate biostimulant efficacy in crop protection and stress prevention. Many metabolites offer protection against both biotic and abiotic threats, while others offer protection against specific stressors. Plant species differ in their defense capabilities, so some metabolites may work well on some crops, but not others. Similarly, some antimicrobial compounds control a specific group of fungal or bacterial pathogens, while others offer control over a broad spectrum of organisms. Further research will improve our understanding of the circumstances best suited to various metabolites, but learning about the active ingredients in biostimulants already on the market will help growers and consultants determine the crops and field conditions most likely to benefit from application. Although biostimulants might not replace conventional crop protection products, they offer a valuable tool to enhance crop stress tolerance and improve fertilizer and pesticide efficiency.

References
Lal, M. K., Tiwari, R. K., Altaf, M. A., et al., 2023. Editorial: Abiotic and biotic stress in horticultural crops: insight into recent advances in the underlying tolerance mechanism. Frontiers in Plant Science. 14:1212982.

Rejeb, I.N., Pastor, V., Mauch-Mani, B., 2014. Plant Responses to Simultaneous Biotic and Abiotic Stress: Molecular Mechanisms. Plants. 3, 458-475.

Shiade, S. R. G., Zand-Silakhoor, A., Fathi, A., et al., 2024. Plant metabolites and signaling pathways in response to biotic and abiotic stress: Exploring bio stimulant applications. Plant Stress. 12, 100454.

Banothu, V., Uma, A., 2021. Chapter: Effect of Biotic and Abiotic Stresses on Plant Metabolic Pathways. Intech Open.

Success of Pollinator Habitat Establishment is Affected by Weed Management Decisions

By
Ryan J. Hill | UCCE Agronomy and Weed Sciences Advisor, Tehama County and Marcelo L. Moretti | Associate Professor, Dept. of Horticulture, Oregon State University
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Figure 1. Herbicides applied just after planting improved the establishment of California poppy.

Pollinator health has been a concern for many growers in the western U.S. in recent years. Pollinator insects are essential to produce many economically and nutritionally important crops grown in this region. These crops include blueberries, almonds, sunflowers, cucurbits and many others. Notably, almond pollination in California plays a vital role in the apiary industry, driving beekeepers to haul huge numbers of bee colonies to California for the few weeks in early spring when almonds bloom. Bees are selective of the pollen and nectar they forage, and diverse floral resources can allow bees to forage according to their nutritional needs. As pollinator health has grown as a concern, managing farmlands to promote pollinator health is often a goal for many land managers.

A common practice in many California orchards is to allow resident vegetation (weeds) to grow in row middles. This can reduce soil compaction and erosion, and sometimes, these resident weeds can also provide habitat for pollinators (if not mowed). However, these weedy species are often not of high nutritional quality for hungry pollinators, and species composition varies widely. As weedy species set seed, they can become a weed management headache. Resident weeds are resident for a reason, and it is often wise to keep them closely mowed to discourage seed production. An alternate option is to manage non-crop vegetation actively.

Active management of non-crop vegetation can involve cover cropping, conservation hedgerow plantings in field margins and establishing wildflower meadows in regions adjacent to crop fields. For any of these options, species selection and weed management are two of the most important factors affecting success. Small-seeded wildflower species are especially sensitive to competition from annual and perennial weeds. This article summarizes our research on the interaction of weed control methods and species selection in fall-seeded pollinator habitats.

Locations and Treatments
Three locations in Oregon’s Willamette Valley were selected for studies. Two were drip-irrigated hazelnut orchards, and one was a field with sprinkler irrigation. Each location received different soil preparation. The first orchard location (Corvallis) was not tilled, and soil compaction was an issue. The second orchard location (Amity) was power-harrowed, so the top two inches of soil were loosened. The third location (Lewis-Brown Research Farm) was plowed and disked.

All three locations were seeded in the fall with a set of flowering species with potential for pollinator habitat. These included hairy vetch (Vicia villosa) at 60 lb/A; lacy phacelia (Phacelia tanacetifolia) at 12 lb/A; California poppy (Eschscholzia californica) at 8 lb/A; and farewell-to-spring (Clarkia amoena), globe gilia (Gilia capitata) and sweet alyssum (Lobularia maritima) at 2 lb/A.

Table 1. Trade name, active ingredient and rate of herbicides applied to pollinator habitat species. Eight herbicides were applied at planting, and four herbicides with post-emergent activity were applied 30 days after crop emergence.

These species were planted in rows, and herbicide treatments were applied over the top perpendicular to planting rows (Table 1). Four herbicides were applied after crop emergence, and the rest were applied one day after planting. Glyphosate treatments were only included in the orchard trials, and all other herbicides were selected because they exhibit some level of soil residual activity. Experimental plots were replicated four times at each location, and each species was treated as a separate experiment. A crop oil concentrate at 1% v/v was included for Motif (mesotrione) and Basagran (bentazon), while a nonionic surfactant at 0.25% was included for Matrix (rimsulfuron) and Quinstar 4L (quinclorac). All post-emergent treatments (and glyphosate) included ammonium sulfate equivalent to 8.5 lb/100 gal.

In Amity, competition from perennial grasses resulted in inconsistent stand establishment. A grass-selective herbicide (clethodim) was used, and the site was reseeded six months after the initial planting when soil conditions were appropriate.

Site Differences
Drastic differences were seen between sites. Crop coverage at the Corvallis site was below 28% for all species except hairy vetch, which had 89% coverage. Winter annual weeds can compete very strongly with fall-seeded wildflowers, so some of our untreated control plots were devoid of the planted species.

Several species did well at the Amity location. Phacelia in the glyphosate plots had the best establishment at this site (81% coverage) due to glyphosate’s good control of perennial grasses that were not killed by the power harrow. Phacelia is also very competitive with annual weeds, so preemergent treatments were unnecessary. One drawback of phacelia is that it can out-compete other planted species when included in cover crops or wildflower mixes.

Lewis-Brown (LB) plots initially had the best crop establishment (75% to 100% coverage for all species) due to more extensive site preparation. However, this location had intense pressure from perennial weeds, so our good initial crop establishment did not translate to a long-term pollinator habitat. The plots at LB where indaziflam (Alion) was applied produced a good stand of Canada thistle (Cirsium arvense) by the end of the trial, which the bees loved.

Treatments Applied After Crop Emergence
Treatments at this application timing were challenging to evaluate for crop safety. Weed control efficacy was inadequate. So, crop establishment was often not good enough to assess crop injury confidently.

One exception was hairy vetch. This species exhibited good tolerance to a post-emergent application of bentazon, a result seen at all three locations. The results from two trials also suggest farewell-to-spring tolerated post applications of quinclorac. Not enough data were collected to reach conclusions for the other four species.

Treatments Applied Prior to Crop Emergence
Preemergent herbicides often had inconsistent crop safety; however, several combinations seemed safe. Napropamide was safe for use with lacy phacelia, globe gilia, farewell-to-spring and sweet alyssum, while flumioxazin and pendimethalin were safe with poppy (Fig. 1). All five species only had adequate crop establishment at two of the three locations. Hairy vetch establishment was improved by simazine applications at all three locations, but crop coverage was not significantly different from the untreated control for this species. Hairy vetch was the only species where it seemed herbicide treatments or tillage added little benefit to its competitiveness. Figure 2 shows the treatment by species combinations that were sometimes safe versus the combinations that were consistently safe for the planted species.

Figure 2. Crop coverage pictures from two months after planting the Lewis-Brown research farm show the planted species (rows) tolerated several preemergent herbicides (columns). A black outline surrounds successful combinations seen in at least one of the other two trials. Combinations that were never seen to be successful again are surrounded by a red outline.

The glyphosate application improved gilia, phacelia and poppy establishment. For phacelia, glyphosate was by far the best treatment, while for gilia and poppy, glyphosate was of similar efficacy as the non-injurious preemergent treatments (napropamide for gilia and flumioxazin/pendimethalin for poppy).

All three trials were conducted on fine soils with organic matter content ranging from 2% to 7% (USDA-NCSS soil survey, websoilsurvey.sc.egov.usda.gov/). The safety of preemergent herbicides for pollinator species establishment may vary depending on soil characteristics.

This research broadly demonstrates something likely well understood already: that weed control prior to planting (whether through tillage or herbicide) should not be skipped. Pollinator habitat is not something that is usually intensively managed, and it can be tempting to cut costs. I have seen too often when corners are cut at establishment, you can end up exactly where you started: with a field full of weeds (and a monetary loss for the time and herbicides invested).

Soil compaction and perennial weeds must be addressed to have a successful pollinator habitat planting. Our research also shows certain preemergent herbicides can improve habitat establishment, but crop safety must be adequately established. This is especially true of different soil types and environments. In California’s Central Valley, pendimethalin has been seen to occasionally cause injury in poppy plantings, which is in contrast with this study. This may be due to different soil characteristics affecting toxicity to the emerging seedlings. Preemergent herbicides like pendimethalin can also be used at a delayed preemergent timing, waiting until just after seedlings emerge to apply the herbicide. This is possible only if the herbicide has no post-emergent activity on the treated crop.

Lastly, be careful using herbicides around pollinator habitats to protect the pollinator species from injury. Herbicides and surfactants can be toxic to insects and should not be used near flowering plants while bees are active.

Crop Performance with Biostimulants

By
Kirk Van Leuven | PCA, CCA, Biologicals Innovation Specialist, Corteva
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Almond trees where hormone signaling is weak (left) due to chronic stress compared to trees with good hormonal balance (right) manifesting in larger leaves at the bottom and inside (photos courtesy K. Van Leuven.)

How can we grow great crops when conditions are not great? Crop performance is a combination of both managing stress and supporting plant growth. Plant systemic resistance to injury and stress has been studied for 100 years. Recently, gene mapping and molecular chemistry are helping us to understand these defenses and explain how signaling and feedback mechanisms conduct plant health. In some cropping years, environmental factors have a more negative impact on the crop than the typical pests and diseases we are more prepared to manage. The question in a lot of growers’ minds is how to harden crops in the field to be more resilient when it comes to problems like drought, heat, cold, salinity or other stress.

Response of hormone synthesis, translocation or perception to developmental, environmental or biotic factors (source: Mineral Nutrition of Higher Plants, pg. 126).

Biostimulants are defined as crop inputs for improved nutrient use efficiency and tolerance to environmental stress. These materials work systemically to keep the metabolic machinery in the plant functioning even when conditions in the field may not be favorable. By definition, biostimulants can improve crop nutrition regardless of the small amounts of essential nutrients in them and the low rate per acre at which they are applied. They help the crop pick up and assimilate nutrition. Some definitions of biostimulants include phrases like “botanically active substances” and refer to improved “crop quality traits.” These benefits provide economic value to the grower.

The genetic code of plants is set up to respond to the environment. In the planted seed, depending on conditions, when water is imbibed and hormone degrades and the hormone balance changes, the embryo activates and biochemical processes start up the machinery of growth, and the seed sprouts. Growth is affected when hormone balance changes. More hormones are endogenously synthesized when the cells of new plant tissues are divided and grow. These hormones translocate and accumulate in the plant, or they can also degrade or become diluted. As the hormone balance and concentration changes, plant growth can speed up, slow down, transition to a different physiological stage, or the plant can set up defenses to create a response to conditions in the field.

Nutrients are not picked up and used for growth unless they are called for by signals inside the plant. A soil test can tell us all the essential minerals are available and ready to be taken up, but the uptake of nutrients will not happen without hormone signaling to activate metabolism and growth processes. Biostimulant products can help create signaling for stress management or growth processes when hormones are in short supply or not naturally present.

Source-Sink Relationships in Plant Growth

New leaves and other organs are sinks for energy and nutrition, which need to be supplied by more mature parts of the plant. A new leaf will size according to available growth hormones. It must develop its own organelles, pigments and vascular system. When fully developed it changes from being a sink to becoming a source for sugars and other solutes, which can be transported to new sinks calling for nutrition. Sinks can be leaves, buds, fruits, shoots or roots, and they can be strong sinks or relatively weak.

In the textbook Mineral Nutrition of Higher Plants, first published in 1986 by Horst Marschner, the authors thoroughly discuss the role of plant hormones in source-sink relationships in the fifth chapter. They discuss yield as a result of nutrition, light and dark cycles in metabolism, phloem loading and unloading, and the initiation and development of reproductive organs. It discusses how these source-sink relationships depend on the concentration and balance of hormones. And it discusses the role of nutrition and plant stress in affecting endogenous hormone concentrations. Knowledge of these principles predates any use of the term ‘biostimulant’ by 10 to 20 years. It was also long before gene mapping and molecular chemistry were able to tell us these processes are controlled by gene activity. We are now at a time when biostimulant products are the primary tools being studied and used to manage source-sink relationships in commercial agriculture. 

Growth hormones cytokinin and auxin are synthesized in one part of plant and transported to other tissues where they interact to make a response.

Small Leaves and Weak Sinks

Small leaves and weak sinks make average crops. The first factor predetermining the size of a leaf or of a fruit is the number of cells in the organ. Cell division and cell growth depend on hormone balance and concentration. When conditions or vigor at the time of flowering and fruit set are limiting the hormones available for cell division, the potential size and quality of the leaves and fruit will also be limited.

For permanent crops, especially some deciduous crops like almonds and tree fruit, the flowering and fruit set happens right after bud-break, before the leaf canopy develops. The tree starts out operating on reserves that were stored before dormancy. Temperatures and weather conditions during bloom make flowering and fruit set unpredictable and affect the development of the leaf canopy. It is no wonder growers have adopted the use of seaweed and other biostimulant products to help improve their crops in these initial stages.

As leaves emerge, the photosynthetic capacity of the tree has to catch up after bloom. The demand for hormones, photosynthates and nutrients is high. Where hormone concentration is low, cell division will be reduced. Cell division determines cell density and sink strength for attracting nutrient resources for growth.  Cell division precedes cell growth and cell density in the fruit and seeds and is one of the main factors in the quality of the crop.

Signaling hormones salicylate and jasmonate move through the plant to protect it in systemic acquired resistance and induced systemic resistance responses to stress.

Systemic Signaling

Simply stated, hormones are produced in one part of the plant and transported to other tissues where they interact to make a response. Cytokinin produced in the root tips travels up with the water column to the tissues in the top where it contributes to cell division. Auxin produced at apical growth points is translocated downward and influences and initiates growth along the way. When auxin makes it all the way down to the roots, it causes branching and initiates more root tips, which in turn makes more cytokinin to flow up again to the top. Under good conditions, this feedback loop continues, and the crop progresses through each stage of growth as driven by the genetic code.

When stress impacts plant growth, other signals raise the defenses, and imbalances occur. Growth is slowed or interrupted. Stress signaling molecules, including ethylene, ABA, salicylic acid, jasmonates, terpenes and others, work to modulate growth processes and harden the plant for survival. Some stressors act to reduce photosynthesis, respiration and growth. They can damage cells with stress hormones, biproducts of incomplete respiration, oxidation and cellular breakdown. Wilting and senescence occur.

Restoring and Maintaining Balance

Healthy plants have evolved with multiple protective stress responses built in. Stress can strike aboveground or belowground. If either the roots or the canopy suffers, to the degree that it suffers, biofeedback signaling will be sent to the other parts of the plant, and the plant makes adaptations. Studies on acquired and induced resistance to stress have explained the systemic mechanisms of plant defenses. Like other processes in the life of the plant, defense against stress is also coded in the genes and uses hormone signaling.    

Jasmonic acid and salicylic acid are hormones which act chemically in their own pathways and in crosstalk with other biochemicals to defend the plant against stress in a variety of ways. They regulate proteins and enzymes used in both defense and recovery from stress. Long ago, the stress responses involving these signaling compounds were given the names systemic acquired resistance and induced systemic resistance.

ABA is probably the main defense hormone, but it works by putting on the brakes and trying to preserve plant resources until conditions are better for the plant. Like cytokinin, it is produced in the roots and travels up the water stream to all parts of the plant. It works with calcium and potassium as a regulator of transpiration in the stomata. During stress and at the end of the life cycle, it reduces the growth hormones and terminates growth.    

Stress comes to crops in all forms these days. Understanding the signaling part of the growth processes and the signaling that occurs with stress at different crop stages helps us to know how to build in some resilience to yield- and quality-limiting events. We have tools when setbacks occur. Biostimulant products work as natural partners to proactively preserve yield, or countermeasures in case of stress events, to enhance or preserve the genetic potential of the crop. A single application of a biostimulant product may or may not be a game-changer (seed treatment can be a tremendous advantage) but when some of these products are layered together in season-long programs with good grower-standard practices, the resulting crop performace can be significantly better.

California Sweet Cherry Water Use: Evapotranspiration, Postharvest Deficit Irrigation and Sweet Cherry Yield

By
Kosana Suvočarev | Assistant Professor of Cooperative Extension, Dept. of Land, Air and Water Resources, UC Davis, Giulia Marino | Associate Professor of Cooperative Extension, Dept. of Plant Sciences, UC Davis and Jarin Tasnim Anika | Graduate Student, Horticulture and Agronomy Graduate Group, UC Davis
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Figure 1. Eddy covariance biometeorological station for evapotranspiration measurements.

California fruit growers are concerned with water resource limitations due to increased drought frequency. UCCE and UC researchers are collaborating to address these concerns with science-based methodology to irrigation management. Many growers rely on pressure bomb measurements or crop coefficients from local UCCE offices to decide on irrigation demands of their crop. New technologies for modeling or remote sensing of evapotranspiration estimation are appearing to help navigate careful irrigation management for optimal yields.

Evapotranspiration and Crop Water Status

The losses of water from soil evaporation and plant transpiration combine into a process called evapotranspiration, or crop water demand. These water losses from liquid to gas need to be supplied through irrigation for healthy crops and optimal yield quantity and quality. We have methods that measure in air how much water is being used in agricultural fields. When we use these measurements (called biometeorological measurements) to develop irrigation recommendations, we call it ET-based irrigation. This might be somewhat misleading, because measured ET is not the straightforward quantity to be used. It needs to be increased to compensate for water losses along the irrigation network and make up for sections of the plots under the lower water distribution. If there was a rain event that supplied substantial water to the soil profile, irrigation should be lowered to account for that natural water supply. This makes ET-based irrigation complex to approach when it comes to orchards and vineyards. What makes irrigation decisions even more challenging are many additional factors that increase the uncertainty in developing universal information even within the same perennial crop species. Some of the factors that influence variability in orchard/vineyard ET are crop density, fruit load, trellis system, soil type, crop age, row orientation, regulated deficit irrigation, salinity in soil or irrigation water, floor management, etc. To determine more closely perennial crop water demand, it is recommended in addition to biometeorological measurements we do also plant-based measurements to determine their water status. This helps us inform our measurements with the response of the plant to the meteorological, soil and irrigation conditions. Biometeorological measurements consider the biology of plants and provide information on water use at a landscape scale. Therefore, there is a benefit of independent checking of plant water status with more physiological-based methods like stem water potential (SWP) measurements. In our project, we tried to combine biometeorological approach to ET measurements and add soil moisture and SWP to better interpret the feedback between the soil, plant and atmosphere.

Figure 2. Stem water potential measurements being taken.           

New Crop Coefficients from California Sweet Cherry Orchards 

In response to one of the priorities of the sweet cherry industry, and thanks to the support from California Cherry Board, in 2019 we started a project in three sweet cherry orchards in San Joaquin County near Linden, Calif. to determine local irrigation needs for cherries. Our experiment included biometeorological measurements of ET (Fig. 1), pressure chamber measurements of SWP (Fig. 2) and shallow soil moisture. We also reached out to irrigation managers to use their data from neutron probe readings on root zone soil moisture taken manually across several measurement points in each of the orchards. The experiment was hosted by the same grower, and the mature sweet cherry orchards were near each other, with the only differences in the three orchards being tree row orientation, tree density (from the densest at 16 ft x 16 ft, the medium sparse at 20 ft x 20 ft and most sparse at 22 ft x 20 ft), rootstock and the irrigation system (the most sparse orchard had sprinklers while the other two were drip irrigated.) Our main goal was to use these measurements to develop crop coefficients (Kc), the universal correction from the reference grass evapotranspiration (ETo) to the crop evapotranspiration (ETc). For that reason, part of the funding was used to install a new weather station over the reference grass near Linden as part of the California Irrigation Management Information System network (cimis.water.ca.gov/) of freely available weather and ETo data. We collected five years of data across the three orchards and used this rich dataset to develop the Kc curve of values as they change over the season and between different hydrological years and orchards.

Figure 3. Harvest sampling and lab analyses by graduate student Jarin Tasnim Anika.

Regulated Deficit Irrigation in California Sweet Cherry

Continued support from the  sweet cherry industry helped us establish a new project in 2021 that builds upon the one we started in 2019. Whereas our initial goal was to develop the irrigation recommendations for the fully irrigated orchards, the new project was focused on regulated deficit irrigation. Within the same three orchards where our measurements were already established, we started deficit irrigation in the postharvest period with closing the valves in one of the two driplines. Our monitoring again included SWP to check the water status between the trees that were fully irrigated (control) and those under postharvest deficit irrigation (PDI). To fully evaluate what PDI means for the sweet cherry industry, we collected samples of harvested fruits (Fig. 3) from PDI and control trees and compared the quality and quantity difference in two consecutive years.

Results

We developed a crop coefficient curve for sweet cherry in local California conditions. It relies on daily ETa data for five years and three orchards that are managed for full irrigation. By averaging these 14 curves (we reduced the number of orchards from three to two during the first year of the COVID-19 pandemic) into one, we expected to represent different orchards and hydrological years for universal values to serve as outreach to the local sweet cherry industry. When we model ETc based on Kc values as an outcome of this measurement period, we noticed there was quite a bit of variability between different years and orchards. Therefore, our crop coefficient curve might be close to irrigation needs of sweet cherry but can also under or overestimate the (actual) ETa on a particular day.   

Table 1. Potential for water savings in postharvest sweet cherry irrigation

Our exploratory study with PDI shows there is potential for water savings in the postharvest period in sweet cherry (Table 1), between 26% and 50%. Our harvest data analyses show although there were some differences in yield, either increased or decreased yield quantity with the deficit, there was no significant difference between the control and PDI trees in yield and in most of the fruit quality parameters. Some of the differences we observed in fruit quality were not consistent between orchards, or the treatment.

We are developing outreach material for sweet cherry growers to both consider using our newly developed crop coefficient curve and to manage their PDI based on pressure chamber SWP measurements. We think using SWP helps carefully manage the amount of deficit crops are experiencing to reduce potential negative effects on the yield quality or quantity.

Study Limitations

Although we collected large amount of data to develop universal crop coefficient curve for local conditions, it only approximates the particular water use, due to complexity in orchard system water management.

Our results on PDI were very limited in sample size and duration (only two years), and the long-term effects cannot be evaluated based on such a short period of PDI. The sample size we collected was very laborious for the yield quality and quantity evaluation, but the large differences between the trees within the same treatment might mask the significance of the statistical differences.

Please use the information from our website on crop irrigation scheduling, frost protection and similar biometeorological materials at biomet.ucdavis.edu/.

First Detection of Red Leaf Blotch: A New Disease of Almond in California

By
Florent Trouillas | Associate Professor of Cooperative Extension, Dept. of Plant Pathology, UC Davis, Alejandro Hernandez-Rosas | Graduate Student, Dept. of Plant Pathology, UC Davis, Rosa Frias | Laboratory Assistant, Dept. of Plant Pathology, UC Davis, Tawanda Maguvu | Postdoctoral Researcher, Dept. of Plant Pathology, UC Davis, Cameron Zuber | UCCE Orchard Crops Advisor, Merced and Madera Counties and Phoebe Gordon | UCCE Orchard Systems Advisor, Madera and Merced Counties
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Figure 1. Early symptoms of red leaf blotch include small, pale yellowish spots or blotches that affect both sides of the leaves (all photos by A. Hernandez and F. Trouillas.)

Red leaf blotch (RLB), caused by the fungal pathogen Polystigma amygdalinum, is one of the most important leaf diseases currently affecting almond trees in the Mediterranean basin, particularly in Spain, and regions of the Middle East. In late May 2024, unusual symptoms on leaves, including yellow spots and orange to dark red-brown blotches, were detected in an almond orchard (Nonpareil, Monterey and Fritz) on the border of Merced and Madera counties. The disease has since been observed in Madera, Merced, San Joaquin and Stanislaus Counties, suggesting it is somewhat widespread in the Northern San Joaquin Valley. Following field sampling as well as morphological and DNA/PCR analyses, our laboratory confirmed the detection of P. amygdalinum from symptomatic leaves. This is the first detection of P. amygdalinum from California almond, and the pest has formally been confirmed as being present in the state by both CDFA and USDA. Growers and PCAs should be on the lookout for RLB as it is new to California and a serious disease of almond.

Disease Symptoms and Biology

Symptoms of RLB initiate as small, pale yellowish spots or blotches that affect both sides of the leaves (Fig. 1). As the disease progresses, the blotches grow larger (1 to 2 cm) and turn yellow-orange with a reddish-brown center (Fig. 2). At advanced stages of disease development, leaves become necrotic, curl and drop prematurely. Mainly the leaves are affected, and premature defoliation of trees can occur, thus decreasing the photosynthetic capacity of the tree during the current and following growing season, leading to a general decrease in yield.

Figure 2. Advanced symptoms of red leaf blotch include larger, yellow-orange blotches (1 to 2 cm) that turn reddish-brown in their center.

The disease is monocyclic, with only one primary infection cycle. The primary inoculum are ascospores that form in perithecia (sexual fruiting bodies) on fallen infected leaves from the previous growing season. Infection occurs after petal fall when young leaves emerge and spring rains occur. Rain is essential for the release and dispersion of ascospores from perithecia. The disease may not be noticed before late April to mid-May as infection remains latent for approximately 35 to 40 days. Infected leaves develop small yellow blotches that expand and become orangish to reddish-brown, with variable shapes and sizes, as the fungus colonizes more leaf tissue. During spring/summer, leaves contain the pycnidia (asexual fruiting bodies) of the fungus, which produce filiform conidia. These asexual spores do not cause new infection on leaves. Infection of leaves decrease drastically after June and with high summer temperatures. Rain combined with mild temperatures in spring and early summer generally lead to higher disease incidence.

Disease Management

Research and experience in Spain where RLB is more common have shown one preventive fungicide application at petal fall and two additional applications at two and five weeks after petal fall if rains persist are effective at controlling the disease (this exact timing is not critical but depends on the occurrence of rainfall.) This means fungicide applications and timings to control other common diseases of almond in California, such as shot hole or anthracnose, will likely also control this pathogen. Researchers in Spain also have shown FRAC groups 7, 11, M3, M4 and some FRAC3 chemistries are most effective. Cultural practices focused on eliminating the primary inoculum of infected fallen leaves also can help mitigate the disease. These consist of removing leaf litter or applying urea to accelerate its decomposition. However, such strategies are only effective when applied over a wide area. Fungicides applied during bloom and after symptoms are visible are not effective.

If you suspect that you have this new disease in your almond orchard, please contact your local UCCE farm advisor.

Mentioning of any active ingredients or products is not an endorsement or recommendation. All chemicals must be applied following the chemical label, local and federal regulations. Please check with your PCA to confirm rates and site-specific restrictions. The authors are not liable for any damage from use or misuse.

References
López-Moral, A., Agustí-Brisach, C., Ruiz-Prados, M.D., Lovera, M., Luque, F., Arquero, O. and Trapero, A., 2023. Biological and urea treatments reduce the primary inoculum of red leaf blotch of almond caused by Polystigma amygdalinum. Plant Disease, 107(7), pp.2088-2095.

Torguet, L. 2022. El inicio del fin de la mancha ocre (Polystigma amygdalinum) como enfermedad clave del almendro. XIV Jornada Del Almendro. Les Borges Blanques, 22 septiembre 2022. IRTA.

Torguet, L., Maldonado, M., Miarnau, X. 2019. Importancia y control de las enfermedades en el cultivo del almendro. Agricultura 1026: 72-77.

Torguet, L., Zazurca, L., Martinez, G., Pons-Solé, G., Luque, J., Miarnau, X. 2022. Evaluation of fungicides and application strategies for the management of the red leaf blotch disease of almond. Horticulture 8, 50.

Crop Consultant Conference: A Premier Event for Agricultural Professionals

By
Jason Scott, MS | Publisher/CEO, Progressive Crop Consultant, JCS Marketing Inc.
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The Crop Consultant Conference will take place on September 25 and 26 at the Visalia Convention Center in Visalia, Calif. (all photos by K. Platts.)

As the agriculture industry continues to evolve, staying informed and connected is more critical than ever. The Crop Consultant Conference, hosted by JCS Marketing Inc., is rapidly becoming a must-attend event for professionals in the field. With growing attendance, an increasing number of exhibits, and a surge in sponsorships, this year’s conference promises to be the most impactful yet.

Growing Attendance and Exhibits
Year after year, the Crop Consultant Conference has seen a significant increase in the number of attendees. This surge reflects the conference’s reputation as a leading platform for learning, networking and showcasing the latest advancements in crop consulting. This year, we expect record-breaking attendance as more professionals recognize the value of participating in this pivotal event.

The exhibit hall will feature an expanded array of booths from top industry players, providing attendees with firsthand access to innovative products, services and technologies that are shaping the future of agriculture. From cutting-edge equipment to the latest in crop management solutions, the exhibit hall will be a hub of activity and discovery.

Enhanced CE Management with Online Courses
Understanding the importance of continuing education (CE) for crop consultants, JCS Marketing Inc. has developed an online CE manager to streamline the process for attendees. This user-friendly platform allows participants to manage their course schedules, track credits and ensure they meet their professional development requirements seamlessly.

To provide comprehensive educational opportunities, JCS Marketing Inc. is offering 11 online credits before the conference, 9 live credits during the event and an additional 20+ credits available online after the conference. This extensive offering ensures attendees can fulfill all their CE requirements in one place, maximizing both their time and investment.

Record Attendance and Sponsorships Expected
The growing success of the Crop Consultant Conference is mirrored by the increasing interest from sponsors. This year’s event is set to feature more sponsors than ever before, each eager to connect with the highly engaged audience the conference attracts. Sponsors recognize the value of aligning with a conference that draws the best and brightest in the industry, providing unparalleled opportunities for brand exposure and relationship building.

Unmatched Networking Opportunities
Networking is a cornerstone of the Crop Consultant Conference, and this year, we’ve expanded our offerings to include more networking parties than ever before. These events provide attendees with invaluable opportunities to connect with peers, share insights and forge lasting professional relationships. Whether you’re looking to collaborate on new projects, exchange ideas or simply catch up with old friends, the networking parties are not to be missed.

The exhibit hall will feature an expanded array of booths from top industry players.

Don’t Miss Out
With its growing attendance, enhanced educational offerings and increased sponsorship, the Crop Consultant Conference is poised to be the premier event of the year for agricultural professionals. Whether you’re a seasoned consultant or new to the field, this conference offers a wealth of opportunities to learn, connect and grow.

Join us at the Crop Consultant Conference on September 25 and 26 at the Visalia Convention Center in Visalia, Calif. and be part of a vibrant community dedicated to advancing the future of crop consulting. Secure your spot today and ensure you don’t miss out on the premier event of the year.

Barn Owls: Nature’s Gopher Killers

By
Nick Davis | PCA, CCA, MBA
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Barn owls (Tyto alba) are invaluable allies for growers seeking effective, sustainable and natural pest management against gophers, voles and field mice (photo courtesy N. Davis.)

Barn owls (Tyto alba) are birds of prey known for their heart-shaped faces and eerie, yet captivating, screeches in the night. These nocturnal hunters play a crucial role in ecosystems worldwide by helping to control rodent populations, making them invaluable allies for growers seeking effective, sustainable and natural pest management against gophers, voles and field mice.

One of the most striking features of barn owls is their exceptional hearing and vision, which allow them to hunt effectively in low light conditions. Their keen sense of hearing is aided by facial discs that funnel sound to their asymmetrical ears, enabling them to pinpoint the slightest rustle of prey in the darkness. Additionally, their feathers are uniquely designed to muffle the sound of their flight, allowing them to approach prey silently, making them highly efficient hunters.

The Gopher Problem: A Closer Look
Gophers, along with voles and field mice, are notorious for their extensive tunneling activities, which can cause significant damage to crops. One of the problematic issues gophers create in row crop farming (i.e., tomatoes) is chewing through subsurface irrigation hoses. These hoses are vital for delivering precise amounts of water to crops, ensuring they receive the moisture needed for optimal growth. When gophers chew through these hoses, it can lead to uneven watering, water wastage and even crop failure. The cost of repairing or replacing damaged irrigation systems can be substantial, adding to the financial burden on growers.

In addition to damaging irrigation systems, gophers pose a severe threat to young almond and pistachio trees. These trees are particularly vulnerable in their early stages of growth when their root systems are not fully developed. Gophers chew through the roots and bark, disrupting the flow of nutrients and water, ultimately leading to the death of the trees. This not only affects the current crop yield but also has long-term implications for the orchard’s productivity. The loss of young trees means additional expenses for replanting and lost time waiting for new trees to mature and start producing.

Traditional Pest Control Methods: Challenges and Drawbacks
Traditionally, growers have relied on methods such as traps and poisons to control gopher populations. While these methods can be somewhat effective, they come with significant drawbacks. The most significant drawback to trapping is the cost of this activity. Trapping requires constant monitoring and maintenance, making them labor-intensive and costly over time. Poisons, well, California doesn’t like us to use them, so the formulations are becoming softer and softer. Moreover, these methods do not address the root of the problem and can lead to a cycle of constant management without achieving a sustainable solution.

Barn Owls: A Sustainable and Effective Solution
Barn owls primarily feed on small mammals such as gophers, voles and field mice, which coincidentally make them ideal for controlling agricultural pests. A single barn owl family can consume up to 60 pounds of rodents in a year. Consultants should consider recommending growers to consider installing barn owl boxes in their fields immediately for several reasons.

Firstly, Barn owls simply kill to survive. No other gopher management method can compete with a barn owl family’s need to eat and properly nourish the two to six owlets in the nesting box. This unique need is what makes barn owls stand out from all other gopher management methods; it’s where their magic lies. Secondly, they save money on irrigation repair and reduce payroll. Thirdly, managing barn owl nesting boxes is easy. It’s like cooking with a crockpot; ‘set it and forget it!’ They require minimal time and money to maintain.

To get barn owls working ASAP, simply install specially designed nesting boxes. After installation, these nesting boxes will attract male barn owls first. After some necessary courting, he will attract a female, and then we’re off to the races. These nesting boxes mimic the natural hollows of trees where barn owls typically nest, providing them with a safe and secure place to raise their young. By installing these boxes, we’re encouraging them to take up residence on the farm and build a community of barn owls that will prey on rodents night after night.

It’s essential to ensure boxes are placed at a height of 10 to 12 feet off the ground (photo courtesy Vineyard Team.)

Installing Barn Owl Boxes
To effectively utilize barn owls for pest control, growers need to install suitable nesting boxes in strategic locations. These boxes should be placed on poles (stick with steel, it’ll last as long as the box, or longer.) Wood poles will disappoint you in the coming years, I promise. It’s essential to ensure boxes are placed at a height of 10 to 12 feet off the ground. Growers should also make sure the entrance hole is facing away from prevailing winds to keep the interior dry and comfortable for the owls.

Maintaining the boxes is relatively straightforward. Growers should check them periodically (annually, or at least every other year) to ensure the owl pellets from the previous year are cleaned out. Cleaning the boxes once a year, typically before the breeding season occurs in late December to early January, helps encourage owls to return and use the boxes for nesting.

Success Stories and Benefits
Growers report significant benefits from using barn owls for gopher control. For example, a vineyard in California saw a dramatic reduction in gopher damage to their vines after installing several barn owl boxes. The owls quickly took up residence and began hunting the gophers, leading to healthier vines. In another case, an almond orchard experienced fewer young tree losses and reduced irrigation system damage thanks to the presence of barn owls. Other growers report having less gopher mounds in their almond orchards, reducing their need to replace mower cups and blades and saving them money.

Beyond pest control, the use of barn owls also contributes to a farm’s sustainability goals. By reducing the need for chemical poisons and traps, growers can lower their environmental impact and promote a more natural balance within their agricultural systems. This approach aligns with the principles of regenerative farming, which emphasize working with nature to create resilient and productive landscapes.

Barn owls are remarkable birds, playing a vital role in controlling rodent populations and maintaining the health of agricultural ecosystems. By using barn owls for pest control, growers can reduce their reliance on chemical control, protect their crops and help to conserve these beautiful birds for future generations.

Gophers, along with voles and field mice, are notorious for their extensive tunneling activities, which can cause significant damage to crops (photo by Vicky Boyd.)

As growers and consultants seek cost-effective and sustainable ways to manage pests, the use of barn owls offers a promising solution. By leveraging the natural hunting abilities of these birds, growers can reduce gopher populations and mitigate the associated damage to crops and irrigation systems. This approach not only addresses an immediate problem but also supports the broader goals of sustainability and ecological health.

Encouraging the presence of barn owls on farms is a testament to the power of nature in solving agricultural challenges. It highlights the importance of working with, rather than against, natural processes to create productive and sustainable farming systems. As more growers adopt this method, it has the potential to transform pest management practices and contribute to a more resilient and thriving agricultural landscape.

P.S. Install one barn owl box per 10 acres for managing light to medium populations of gophers.

Nick Davis, MBA, is a PCA and CCA as well as owner of The Owl Box Company. He can be contacted at 559-352-8067 for questions on managing gophers easily and inexpensively.

Application of CropManage Irrigation Online Decision-Support Tool for Processing Tomato and Watermelon Production in the Northern San Joaquin Valley

By
Zheng Wang | UCCE Vegetable Crops and Irrigation Advisor, Stanislaus County, Michael Cahn | UCCE Irrigation and Water Resources Advisor, Monterey County and Fayrouz Buojaylah | UCCE Assistant Specialist, Stanislaus County
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Figure 1. The flow meter is installed in the main pump system of a processing tomato field. With this setup, the flow meter monitors irrigation information for the whole field (photo by Z. Wang.)

Intensified drought in California has limited groundwater supply for crop production. Therefore, vegetable growers need to implement more efficient and crop-oriented irrigation management. Leveraging online decision-support tools can help growers with real-time, effective monitoring of their irrigation and provide updated irrigation recommendations across the crop cycle. CropManage (CM), developed and operated by UCCE, is a weather-based online decision-support tool that provides recommendations for efficient and sustainable irrigation and fertilization applications. CM combines a wide variety of data inputs including past and future weather, evapotranspiration (ET), satellite imagery, soil physical and chemical properties, irrigation system efficiency and other related variables to generate accurate and timely irrigation and fertilization recommendations based on crop-specific models. Growers can compare their actual irrigation amount and timing to the recommendations made by CM and make the proper adjustment to their crops.

Since 2011, CM has been used by growers, farm advisors and research scientists. CM (https://cropmanage.ucanr.edu/) is a credible, free-access, long-term tool for irrigation scheduling and has been adapted for a variety of vegetable crops (carrots, cabbage, broccoli, lettuce, spinach, etc.), berry crops (raspberry and strawberry), tree nut crops (almond, walnut and pistachio) and agronomic crops (alfalfa and corn). Most of the currently adopted vegetables, however, are mainly produced in the Central Coast, so we have been experimenting with Solanaceous and Cucurbits (processing tomato and watermelon) that dominate the Central Valley to examine the feasibility of adapting CM into these warm-season vegetables.

Table 1. Comparisons of watermelon grower’s actual total application of irrigation (inches per acre) and split between water applied via sprinkler and subsurface drip with the recommendations made by CropManage.

What Did We Do?

Since 2021, we have conducted numerous trials in the northern San Joaquin Valley to compare growers’ irrigation scheduling with the recommendations made by CM to determine if CM is potentially adaptable to processing tomato and watermelon irrigation management.

Figure 2. The flow meter is installed in the main blue flat in a watermelon field. With this setup, the flow meter records irrigation information only for the monitored area (photo by Z. Wang.)

Each field trial began with taking pre-plant soil samples and then was followed by setting up the flow meter, moisture sensors and communication devices to access real-time irrigation information remotely. The location where a flow meter is set highly depends on the setup of the field irrigation system. For processing tomato trials, the flow meter is usually connected to the main pump to monitor the whole field irrigation because all irrigation lines and sub-lines are underground, whereas the flow meter is typically connected to a main line in the watermelon trials to monitor the downstream acreage (Figures 1 and 2). The communication devices include a datalogger that saves the information of each irrigation event (duration, flow rate and total volume) and a cellular modem that transfers the data collected by the datalogger to CM (Figure 3).

Figure 3. The configuration and wiring of the communication devices. The flow meter in Figures 1 and 2 was connected to the CR300 datalogger (top) to store flow rate, duration and total volume of each irrigation event. The collected data communicates with the Cell210 cellular modem (bottom) and becomes available simultaneously in CropManage. A 12V battery (right) and a solar panel shown in Figure 2 were used to supply energy to the devices. (photo by Z. Wang.)

As an ET-based irrigation tool, CM provides irrigation recommendations based on the following equations:

Equation 1: Total ETcrop = Avg. ETref × Avg. Kcrop × days since last irrigation

Equation 2: Recommended irrigation amount = Total ETcrop × 100 ÷ (IDU × (1 – leaching requirement)) – total precipitation

In Equation 1, Total ETcrop is the actual evapotranspiration of processing tomato or watermelon in our cases; ETref is the reference ET near the field, which is accessed through the California Irrigation Management Information System (CIMIS), and Kcrop is the crop coefficient for processing tomato or watermelon, which directly relates to the crop canopy development over the crop cycle. By taking the in-field measurement of percent canopy coverage or accessing the coverage data through NASA’s Satellite Irrigation Management Support (SIMS), CM will provide daily crop coefficients (Figure 4).

Figure 4. Left: Watermelon canopy development curve generated by CropManage over the growing season. We collected the user data (green square) weekly in the field using an infrared camera. The SIMS canopy data (dark diamond) were obtained through NASA SIMS System. Right: Seasonal watermelon crop coefficient generated based on the ETref and crop canopy development.

In Equation 2, IDU stands for irrigation distribution uniformity, which is typically between 85% to 95% for subsurface drip irrigation for processing tomato and watermelon production. If using sprinkler or furrow irrigation, the IDU could be less than 75%. Leaching requirement is the fraction of applied water, including rainfall, that drains below the root zone. Growers can set the leaching requirement to zero when subsurface drip irrigation is used to ease the process of calculating irrigation recommendation. Typically, as no precipitation occurs during the growth of most warm-season vegetables in the Central Valley, Equation 2 can be simplified to Equation 3. Figure 5 shows an example of how much irrigation is recommended by CM. Growers can then adjust the actual irrigation amount based on the CM recommendation (Figure 6).        

Equation 3: Recommended irrigation amount = Total ETcrop ÷ IDU (expressed as %)

Figure 5. An example showing the process of calculating the recommended irrigation in CropManage.
Figure 6. The comparison of CropManage’s recommended irrigation (0.64 inches) vs grower’s applied irrigation amount (0.60 inches). Since we have the flow meter and communication devices connected, the actual water applied is autopopulated from the flow meter to CropManage without the need to download from datalogger or check the flow meter reading in person.

Adaptability in Processing Tomatoes and Watermelons.

For watermelon, the total amount of water applied was dramatically different among years (2021-23). The biggest difference was contributed by the amount of sprinkler irrigation for transplant establishment. Due to drought conditions in 2021 and 2022, 19 and 14.5 inches of sprinkler irrigation, respectively, were made to transplants prior to shifting to the subsurface drip, whereas only 2 inches of sprinkler irrigation was applied in 2023 (Table 1). Because our flow meter only connected to the lines of the drip system, the information of total amount and schedule of sprinkler irrigation was provided by growers. In Table 1, if we only focus on the CM recommendations for subsurface drip system, it is clear the recommended total amounts were close to the grower’s actual applications in all years. For processing tomato, growers took a well adoption of CM irrigation recommendations with differences of less than 4 inches per acre in 2022 and only 0.3 inches per acre in 2023 between the actual total application and CM recommendations (Figure 7).

Table 1. Comparisons of watermelon grower’s actual total application of irrigation (inches per acre) and split between water applied via sprinkler and subsurface drip with the recommendations made by CropManage.
Figure 7. Comparisons between the actual cumulative applied irrigation to processing tomatoes and recommendations made by CropManage in 2022 (top) and 2023 (bottom).

FAQs from Users

Attending CM training workshops is another way to get familiar with this decision-support tool. Each year, we host over 10 in-person workshops statewide as well as virtual trainings. Below are the most frequently asked questions from the workshop attendees and grower-collaborators.

What information do we need to enter in CM when setting up the monitored field?

To get more accurate recommendations, we ask users to put in detailed information about the commodity (crop type, acres of the monitored area, coordinates of the field and planting and harvest dates), irrigation settings (water source, nitrogen concentration in the water, irrigation application rate [typically expressed as inches/hour] and distribution uniformity), soil type (autopopulate with entering the field coordinates) and crop settings (previous crop, crop total nitrogen uptake and water stress setting for crops requiring water reduction or cutoff).

If I don’t have the communication devices, can I still use CM to manage my irrigation application?

Yes. As shown in Figure 6, with the flow meter connecting with communication devices, the actual water applied is autopopulated from the flow meter to CM without the need to download from datalogger or check the flow meter reading in person. Without these devices, you can still have your irrigator keep the irrigation duration and enter the hours of irrigation to CM manually. Then, CM will transfer hours of application to inches applied based on the irrigation settings entered earlier.

I don’t have time to measure the crop canopy coverage. Can I only rely on NASA’s measurements?

Yes, but you need to make sure the field is clean from weeds, especially in the furrows (Figure 8). The percent canopy coverage generated by NASA’s satellite imagery system cannot separate weeded areas from the true coverage of cash crop. 

Figure 8. Weed infestation in the furrows of the watermelon field due to sprinkler irrigation and possibly ineffective weed control. In this case, watermelon canopy coverage collected by NASA SIMS will be greater than the actual coverage, potentially resulting in inaccurate irrigation recommendations (photo by Z. Wang.)

I have many fields to manage. Can I assign other farm crew for the fields that rely on CM for irrigation?

Yes. In the Ranch Settings, you can assign as many ranch members as you like and give them different levels of access permissions (e.g., view only vs permission to edit settings).

Can I export the field settings and irrigation information?

Yes. If you have full permission, you can export your field settings and cumulative irrigation charts, which can be viewed as an Excel or PDF file.

What are some resources that can help with my use of CM?

CropManage has a knowledge base including tutorial guidance and more FAQs (help.cropmanage.ucanr.edu/tutorials/). Also, keep an eye on the Veg Views Newsletter for future CropManage hands-on trainings (cestanislaus.ucanr.edu/news_102/Veg_Views/).

If I begin using CM in my field, will you help us co-manage the irrigation?

Yes. If you need, I will be working with growers to set up CM and deliver recommendations. We will work together to make sure CM provides timely and accurate irrigation recommendations, which will serve as an important reference for your irrigation decisions.

Investigating the Link Between Glyphosate Application, Cold Temperature and Bacterial Stem Blight in Alfalfa: Insights to Date

By
Giuliano Galdi | UCCE Agronomy and Crops Advisor, Merced and Stanislaus Counties
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Figure 1. Dead stems showing symptoms of shepherd’s crook in alfalfa (photo by S. Orloff.)

Roundup Ready technology builds genetic resistance to glyphosate into crops, providing an excellent tool for weed management. Initial screening in the early 2000s found good crop safety in alfalfa, leading many growers to rely on glyphosate as the only herbicide. Although using the same chemical control season after season is not a good idea because it may accelerate herbicide resistance in weeds, Roundup Ready alfalfa has been successfully used with few to no concerns. However, the combination of glyphosate and cold weather may cause crop injury, especially in regions where frost events typically follow the herbicide application in spring.

It All Started in Siskiyou County
The issue was first observed in 2014 by Steve Orloff, former UCCE farm advisor in Siskiyou County. A Roundup Ready alfalfa field showed injury after glyphosate application followed by freezing temperatures. The main symptoms were plant stunting, chlorosis and “shepherd’s crook,” in which individual alfalfa stems curl over and die (Figure 1). Yield reductions were also observed for the first cutting. Steve noticed the injury could be related to the glyphosate application because a section of the field where an irrigation wheel line prevented the herbicide application looked perfectly normal.

Injury Symptoms Were Like a Known Disease
Interestingly, the injury seen was very similar to symptoms caused by frost and/or bacterial stem blight (BSB) caused by Pseudomona syringae, a waterborne bacteria present everywhere. The bacteria can exacerbate frost damage due to its protein that mimics a crystalline structure and provides a starting point for ice formation, damaging the plant tissue and serving as an entrance port into the leaves and stems. Once into the plant tissue, colonization leads to infection and symptoms about 7 to 10 days after the frost event. Symptoms on stems start as water-soaked lesions that extend down one side. Leaves become water-soaked and often are twisted and deformed. Currently, there are no resistant alfalfa varieties nor effective control methods besides harvesting the crop earlier.

Let the Research Begin
Steve replicated the symptoms in field trials conducted in 2015-17. The field trials showed yield reductions of up to 0.7 tons/acre in the first cutting in Scott Valley. Crop injury was not observed in a similar field trial conducted in 2014, probably because there was no frost event after the glyphosate application. Similar impacts were observed in a trial at the UC Intermountain Research and Extension Center, near Tulelake, Calif.; additional yield reductions were observed with higher glyphosate rates (Table 1).

Table 1. Yields of first and second cutting in 2015 in the Intermountain Region of California. Trial conducted by Steve Orloff.

Broadening the Scope of the Research
Based on this work, a multi-year project started to investigate the effects of glyphosate rate and application timing at 24 sites over five years, measuring the impact on alfalfa crop height and biomass yield. Results were published in the Agronomy Journal in 2023 (Loveland et al. 2023). All locations in this study were in the Intermountain West (California and Utah), and results showed while summer glyphosate application did not injure alfalfa, spring applications reduced crop height at 76% of the sites and biomass at 62% of the sites.

In sites where glyphosate application resulted in crop injury, low (22 oz/acre) and high (44 oz/acre) rates of glyphosate reduced yields by 0.24 tons/acre and 0.47 tons/acre, respectively (Figure 2). 

Figure 2. Effects of low and high rates of glyphosate on crop height and yield when compared to untreated check (UTC). Only sites that showed statistical significance were depicted in the graph.

Data also showed the crop height at glyphosate application influenced the degree of injury (Figure 3), with greater yield reductions at 30 to 40 cm (12 to 16 in) than at 5 to 10 cm (2 to 4 in).

Figure 3. Alfalfa yield response (Mg ha -1) to glyphosate applied at six alfalfa heights up to 40 cm (16 in) at four sites in California and Utah in 2019 (Loveland et al. 2023).

Do Alfalfa Growers Need to Panic?
As alarming as the possibility of injury might sound, its occurrence and degree are widely variable, and the crop resumes normal growth and yields after first cutting. Figure 4 illustrates this complexity and variability throughout experimental sites where harvest yield and crop height were assessed. Note the locations represented in the following graph are colder than the San Joaquin Valley and the injury happened after glyphosate applications in spring.

As of 2024, this type of injury has mostly been reported in the Intermountain West due to its high altitude and cooler weather. However, one field I visited in early February 2024 in Firebaugh, Calif. brought my attention back to the issue. The field was planted in fall 2023 and had many of the symptoms previously mentioned: plant stunting, typical shepherd’s crook, chlorosis and dead stems. While all these symptoms could be exclusively due to bacterial blight infections or frost, parts of the field where glyphosate application was accidentally skipped looked better.

Figure 4. The percentage of sites (17) where spring-applied glyphosate reduced alfalfa first cutting yield, height at harvest, both or neither (Loveland et al. 2023).

Considerations and Recommendations
The exact role of glyphosate, temperature, and P. syringae in the injury is unclear. Although most of the sites in this research showed some level of injury followed by glyphosate application in the spring, the degree of the damage was widely variable. Interestingly, injury symptoms worsened with increasing glyphosate rate and crop height up to when alfalfa was 8 inches tall, but the crop somehow seemed less susceptible when glyphosate was applied at 8 to 16 inches tall.

While the initial cases of BSB of alfalfa were reported in 1904 in Colorado, relatively few cases were reported during the second half of the 20th century. However, the disease has become increasingly problematic in past decades, especially in areas where frost events are favorable. Would the re-emerging BSB in alfalfa have something to do with the extensive use of Roundup Ready technology and the overreliance on glyphosate?  Future research is needed to answer this question.

Current UC IPM weed management guidelines for Roundup Ready alfalfa recommend rotating herbicides with different modes of action to reduce the development of herbicide-resistant weeds and avoid glyphosate overuse during the colder winter months. Second, spray glyphosate when the alfalfa is short (<2 in) when using the higher rate (44 oz/acre) or 4 inches when spraying at the lower rate (22 oz/acre). Third, use the lowest glyphosate rate possible according to the weeds present and their stage of development; a soil residual herbicide tank mixed with early glyphosate application should provide adequate late-emerged weed control. Finally, pay attention to the weather forecast; applying glyphosate before frost events increases the likelihood of crop injury, especially in old stands. 

References

Loveland, L.C., Orloff, S.B., Yost, M.A., Bohle, M., Galdi, G.C., Getts, T., Putnam, D.H., Ransom, C.V., Samac, D. A., Wilson, R., and Creech, J E. (2023). Glyphosate-resistant alfalfa can exhibit injury after glyphosate application in the Intermountain West. Agronomy Journal, 115, 1827-1841. https://doi.org/10.1002/agj2.21352

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