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Evaluation of Equipment for Improving Management of Drip Systems in Row Crops

By
Michael Cahn | UCCE Irrigation and Water Resources Advisor, Monterey County
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Figure 1. Average application rate of a drip system where the main valve is manually adjusted by an irrigator at the beginning of the irrigation set.

With ever-stricter regulations, growers will need to implement best management practices that lessen water quality impairments to surface and groundwater. Also, implementation of the Sustainable Groundwater Management Act will require growers to use water as efficiently as possible to produce their crops. A well-designed irrigation system that is properly managed and maintained is key to using water efficiently and preventing nutrient and pesticide losses that degrade water quality. State and federal programs, such as the State Water Efficiency and Enhancement Program and NRCS Environmental Quality Incentives Program, can finance equipment that improves the efficiency of irrigation systems.

Previous research in row crops has shown many drip irrigation systems could be better optimized through improved pressure regulation and careful monitoring of applied water using flowmeters. When pressure in drip systems is not accurately regulated and monitored, the application rate will vary during and between irrigation sets. Figure 1 shows the average application rate of a drip system when the main valve of the block is adjusted manually by an irrigator to achieve a desired pressure. The fluctuation in pressure among irrigations causes the drip system to have 33% variation in the application rate during the crop cycle.

This uncertainty in the system application rate limits the ability to accurately schedule how long to irrigate. A drip system that applies 0.12 inches per hour would need 8.3 hours to apply an inch of water. Under higher pressure conditions, the same system may have an application rate of 0.15 inches per hour and would only need to be operated 6.6 hours to apply an equivalent volume of water.

During the past few years, we have been evaluating equipment that can help growers irrigate with drip more precisely. Some of these tests were conducted under controlled conditions and others were done by trialing equipment in commercial fields.

Field Evaluation of Two Types of Pressure-Reducing Valves
Pressure-reducing valves (PRV) are valves that can automatically adjust the cross-sectional area of a passageway through which water flows to maintain a desired pressure downstream. When the downstream pressure rises above a threshold, the valve will automatically reduce the size of the passageway to restrict flow, and when pressure drops below a threshold, the valve will open the passageway to increase flow. PRVs can also be used like a regular valve to turn on and off the flow of water.

Figure 2. Netafim (left) and Nelson (right) pressure-regulating valves that were tested on drip systems installed in a 7.5-acre vegetable field.

We tested two types of PRVs in drip systems installed in a commercial vegetable field: the Netafim 90 series and the Nelson 800 series valves (Figure 2). Most PRVs use a mechanism similar to the Netafim 90, which has a bonnet with an internal rubber diaphragm that expands and contracts to adjust the flow rate, thereby maintaining a desired downstream pressure. In contrast, the Nelson 800 series uses a flexible rubber sleeve that expands and contracts around an internal cage to regulate flow and achieve a desired downstream pressure. Both models use a pilot with a mechanical spring to adjust the downstream pressure or set point of the regulator. The pipe diameter of both models tested was 6 inches and rated for the range of flow rates expected in the drip system that was installed in a 7.5-acre vegetable field. Both PRV models are available in smaller and larger diameters than 6 inches. Nelson offers a nylon version of the 800 series in the 1000 series model, which is limited to pipe diameters of 4 inches or less. Our objective was to evaluate how well the PRVs maintained the pressure between 8 and 10 psi in the submain with minimal adjustments during an irrigation set.

The Netafim 90 PRV was found to respond slowly to sudden increases in upstream pressure, which sometimes caused drip tape to either detach from the submain or burst under the excessive pressure. Additionally, the pilot on the Netafim 90 had to be adjusted several times during the irrigation set to maintain the pressure in the desired range of 8 to 10 psi.

The Nelson 800 PRV was found to adjust faster to increases in upstream pressure compared to the Netafim PRV. Also, the Nelson PRV only occasionally needed to be adjusted during an irrigation set. Figure 3 shows the upstream and downstream pressure of a 6-inch-diameter Nelson 800 PRV for several irrigation dates. Average downstream pressure was maintained at 10 psi despite fluctuations in upstream pressure ranging from 30 to 50 psi.

Figure 3. Pressure measured upstream and downstream of the Nelson 800 pressure reducing valve in a drip irrigated lettuce field. Upstream and downstream pressure were monitored using pressure transducers. Average downstream pressure was maintained at 10 psi despite fluctuations in upstream pressure ranging from 30 to 50 psi.

An important tip an irrigator involved with the field testing learned was at the start of an irrigation, the main valve for the block should be partially opened to allow the drip system to slowly come up to pressure. After the PRV begins regulating the pressure of the drip system, the block valve can be fully opened.

Accuracy of Flowmeters
Flowmeters are quite useful for scheduling irrigations and diagnosing problems that may occur in an irrigation system. Flow rates greater or less than normal may indicate leaks, plugging of emitters or sprinkler nozzles, or if the system is operating at too high or low of a pressure. Scheduling irrigations based on estimates of crop evapotranspiration requires converting between inches of water depth to gallons of water. Gallons of water applied can be converted to inches using the equation:

Inches of water applied = Total gallons of water applied ÷ [27,154 × acres of irrigation block]

Also, the time needed to apply a desired depth of water can be determined from flow rate:

Irrigation set time (minutes) = [depth of water (inches) × 27,154 × acres of irrigation block] ÷ flow rate (gal/min)

The accuracy and reliability of several models of flowmeters suitable for measuring the volume of water applied to an irrigation block were evaluated. The models included Seametric AG3000, Netafim Octave, Seametric AG90, Bermad M10 and the Netafim WST (Fig. 4). Except for the Netafim WST, all models have no moving parts, and most minimize turbulence so that the required length of straight pipe before and after the meter ranges from two to five pipe diameters. The AG3000, AG90 and M10 use magnetic sensors to measure flow rate. The Octave monitors flow using an ultrasonic sensor. The Netafim WST uses an impeller to measure flow which causes minimal loss of pressure across the meter. All models have digital registers and have an output cable that can interface with dataloggers so flowrate can be monitored remotely. Two AG3000 meters were included in the testing to evaluate if they provide similar measurements of flow.

Figure 4. Flowmeter models tested included A) Seametric AG90, B) Bermad M10, C) Netafim WST, D) Netafim Octave and E) Seametric AG3000.

The accuracy of the various flowmeters was evaluated by assembling each model in series along a 6-inch-diameter main line separated by at least 3 feet of straight pipe. The average flow rate evaluated was ~330 gallons per minute, and each evaluation lasted about half an hour. Approximately 10,000 gallons of water were pumped through the main line, and the total volume recorded by each meter was noted at the end of the irrigation set. The evaluation process was repeated five times so the average and standard deviation of the total volumes could be calculated for each flowmeter model. Because the volume of water pumped through the meters was large and could not be physically measured, values recorded from each meter were compared relative to the mean volume of the group of meters tested.

Results of Flowmeter Testing
Most flowmeters had less than ±1.5% difference from the mean volume calculated for the group of meters, demonstrating they had similar estimates of flow (Table 1). The Netafim Ultrasonic and Seametric AG3000 meters were the closest to the mean volume. The Netafim WST, which has an impeller, was consistently less than the mean volume. The Seametric AG90, which inserts into an irrigation pipe, provided the next lowest estimates of flow volume, and the Bermad M10 gave the highest estimate of flow volume, averaging 1.7% higher than the mean volume. Note the two AG3000 meters measured very similar volumes.   

Table 1. Flowmeter estimates of water volume in gallons and percentage difference from the mean volume of all meters evaluated.

The accuracy and reliability of several flowmeters and PRVs were evaluated through field testing. These tests were done on a limited number of models, and other options are likely available that have equal or better accuracy and reliability than the equipment tested in this study. Nevertheless, these evaluations identified several models of flowmeters and PRVs that appeared to be accurate and reliable and could potentially greatly improve the operation of drip irrigation systems installed in row crops.    

This project was supported by the California Leafy Green Research Board.

An Overview of Nitrogen Mineralization in Organic Agriculture

By
Christopher Hight | CCA, Soil Laboratory Manager, Betteravia Farms
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Figure 1. Changes in soil mineral nitrogen concentrations associated with organic amendments incubated at 23 degrees C for 84 days, with reference to unamended control soils. Values are averages of amendments incubated in conventional and organic soils (Lazicki, Geisseler et al. 2020).

Organic agriculture has increased in popularity in recent decades due to its reputation as an environmentally friendly alternative to conventional agriculture. However, significant improvements to organic practices must be implemented to ensure adequate, sustainable productivity in both organic and conventional systems. Nutrient management in organic agriculture requires further investigation to provide information on nitrogen availability to organic growers for agronomic reasons as well as prevent environmental consequences from nutrient loss in agricultural fields. In conventional agriculture, and in limited instances in organic agriculture, the nitrate quick test can be used before the exponential growth phase to determine available nitrate (Bundy and Andraski 1995). However, in contrast to conventional N-containing fertilizers, N management in organic vegetable systems is a challenge because complex organic forms of N, which originate from crop residues, compost, manures and other organic materials, require mineralization by microbes to become plant-available (Drinkwater and Snapp 2007). In addition to environmental and agronomic concerns, legislation has been passed to limit the application of N to agriculturally managed fields in California through the California Regional Water Quality Control Board. With all this in mind, it is important to understand what affects N availability to plants as well as what may sequester N in the soil.

Process of Nitrogen Mineralization
In organic agriculture, N is applied in the form of proteins, DNA and other N-containing organic molecules, which must go through the process of microbial decomposition to become available to plants (Drinkwater and Snapp 2007). Organic N is first converted to ammonium (NH4-N) by microbial populations and, in aerobic conditions, is subsequently oxidized to nitrate (NO3-N). Manure must go through a similar process for the majority of its N-containing molecules but may also contain some mineral N. Microbial decomposition and release of inorganic N to the soil solution, or N mineralization (N-min), depends on many factors, such as soil temperature, moisture and texture as well as carbon to nitrogen (C:N) ratio of the microbial substrate (Figure 1) (Lazicki, Geisseler et al. 2020). The wide variety of soil orders in combination with seasonal temperatures and environmental conditions make predicting N-min difficult. Knowing the textural class and parent material of the soil being farmed can save a lot of time with nutrient and irrigation efficiencies when acquiring a new piece of ground. By knowing the textural class (Figure 2) and parent material of the soil, key physical and chemical characteristics can be estimated, such as water holding capacity, permanent wilting point, the ability to maintain and sequester soil organic matter (SOM) and cation exchange capacity. This gives the grower a bit more insight into the field’s characteristics and its capacity to retain and provide nutrients to plants.

Figure 2. Soil Textural Class Triangle uses the percentage of sand, silt and clay to determine textural class of the soil. By knowing the textural class and parent material of the soil, key physical and chemical characteristics can be estimated, such as water holding capacity, permanent wilting point, the ability to maintain and sequester soil organic matter and cation exchange capacity.

Nitrogen Mineralization Studies on Soil Organic Matter and Amendments
Studies have been performed in conventional agricultural fields to determine temporal patterns of N-min from SOM, however studies of N-min from SOM in agricultural fields on the central coast of California are limited (Miller and Geisseler 2018). Agricultural crops require more N than can typically be mineralized from SOM during a vegetable crop’s growing season (Figure 3). In addition, the temporal pattern of N released from SOM may not line up with patterns of crop uptake, which leads to the need for applications of commercial N sources. Studies have been done looking into contributions of N from SOM and an organic amendment of composition 4-4-2 (4% N, 4% P2O5 and 2% K2O) in vegetables on the central coast; however, the investigation of longer-season vegetables in a multitude of soil textures is limited (Smith, Cahn et al. 2022). Longer-term temporal mineralization of N from SOM and organic amendments in different soil textures is warranted. It is likely the N dynamics of SOM and organic amendments vary between crops with short or long growing seasons, such as spinach and butter lettuce, which are in the ground for 30 to 45 days, compared to longer-term crops, such as broccoli and cauliflower, which are in the ground closer to 90 days.

Figure 3. Relative nitrogen mineralization rates in different regions of California as affected by soil temperature (Miller and Geisseler 2018).

Influence of Soil Texture on Nitrogen Mineralization
Soil texture influences the level of mineralization with varying quantities of pore space, clay content, cation exchange capacity, surface area of particles, water holding capacity, plant available water and potential for organic matter accumulation as well as microbial activity and substrate protection (Feller and Beare 1997). In soils with a high clay content, SOM may be protected by soil aggregates and thus present a lower N-min rate compared to a sandy soil with SOM that is labile and unprotected (Hassink, Bouwman et al. 1993). Ros (2012) found N-min was primarily related to the size of total and extractable organic matter fractions, whereas variables reflecting soil texture were less important. However, other studies have found texture to be an important part of regression models predicting N-min from SOM (Miller, Aegerter et al. 2019). N-min from SOM depends on many factors, with two of the most important shown to be soil temperature and soil texture. Moisture is also an important variable but is held constant in laboratory incubations (Miller, Aegerter et al. 2019). Temperature sensitivity of N-min from SOM in agricultural soils has been investigated by Miller et al. (2018) and was found to be significant, especially in cropping systems where crops are grown year-round (Miller and Geisseler 2018). As both soil texture and temperature have been shown to affect N-min in SOM, we expect to see both characteristics influence N-min in organic amendments. With contrasting evidence of the importance of texture in N-min from SOM, many organic fields with varying soil textures are needed to provide an optimal research environment.

Particulate and Mineral Associated Organic Matter
Particulate organic matter (POM) and mineral-associated organic matter (MAOM) are two types of organic matter generally associated with carbon sequestration and N cycling. The MAOM fraction forms through associations between the organic matter and the soil minerals helping to determine the chemical properties and stability of the interactions (Daly, Jilling et al. 2021). Particulate organic matter is primarily composed of N-containing polymers from decomposing plant matter, bacterial waste and microorganism byproducts. The balance between MAOM and POM depends on the sorption and desorption potential of the soil minerals. Emerging frameworks suggest while depolymerization is a rate-limiting step to N-min, bioavailable organic N may be driven by other factors (Figure 4). At this point, more investigation into how POM and MAOM effect N-min needs to occur.

Figure 4. The emerging model emphasizes three major compartments: 1) depolymerization and solubilization, in grey; 2) interactions between bioavailable organic nitrogen and minerals, in orange; and 3) microbial assimilation, recycling and mineralization of organic nitrogen, in blue. Black arrows represent the direction of nitrogen flow between pools. Green arrows indicate the direction of plant root exudate carbon flow. This model does not attempt to capture all steps in the process (Daly, Jilling et al. 2021).

Soil Incubations to Determine Mineral Nitrogen
While laboratory incubations of organic amendments have been performed to determine the timeline of mineralization or immobilization of N under controlled conditions, investigation to determine if in situ field studies match predicted mineralization rates is required (Lazicki, Geisseler et al. 2020). Several models have been used to fit N-min dynamics, yet a reliable model that provides a high R2 value across different environments is elusive (Miller, Aegerter et al. 2019, Morvan, Beff et al. 2022). Further work on in-field incubations using undisturbed soil cores in organic broccoli fields are being performed on the Central Coast using data collected from 2023 to 2025.

With the increasing demand for organic production, a firm understanding of N-min dynamics will be a key piece of information for farms to remain profitable as well as limit environmental pollution. While a fool-proof estimation of N-min is yet to be discovered promising physical, chemical and biological characteristics of the soil, such as soil texture, SOM fraction and quantity, microbial respiration and the percent of N within the soil are all promising measures.

References
Bundy, L. G. and T. W. Andraski (1995). “Soil yield potential effects on performance of soil nitrate tests.” Journal of Production Agriculture 8(4): 561-568.

Daly, A. B., et al. (2021). “A holistic framework integrating plant-microbe-mineral regulation of soil bioavailable nitrogen.” Biogeochemistry 154(2): 211-229.

Drinkwater, L. E. and S. S. Snapp (2007). Nutrients in Agroecosystems: Rethinking the Management Paradigm, Elsevier: 163-186.

Feller, C. and M. H. Beare (1997). “Physical control of soil organic matter dynamics in the tropics.” Geoderma 79(1-4): 69-116.

Hassink, J., et al. (1993). “RELATIONSHIPS BETWEEN SOIL TEXTURE, PHYSICAL PROTECTION OF ORGANIC-MATTER, SOIL BIOTA, AND C-MINERALIZATION AND N-MINERALIZATION IN GRASSLAND SOILS.” Geoderma 57(1-2): 105-128.

Lazicki, P., et al. (2020). “Nitrogen mineralization from organic amendments is variable but predictable.” Journal of Environmental Quality 49(2): 483-495.

Miller, K., et al. (2019). “Relationship Between Soil Properties and Nitrogen Mineralization in Undisturbed Soil Cores from California Agroecosystems.” Communications in Soil Science and Plant Analysis 50(1): 77-92.

Miller, K. S. and D. Geisseler (2018). “Temperature sensitivity of nitrogen mineralization in agricultural soils.” Biology and Fertility of Soils 54(7): 853-860.

Morvan, T., et al. (2022). “An Original Experimental Design to Quantify and Model Net Mineralization of Organic Nitrogen in the Field.” Nitrogen 3(2): 197-212.

Ros, G. H. (2012). “Predicting soil N mineralization using organic matter fractions and soil properties: A re-analysis of literature data.” Soil Biology & Biochemistry 45: 132-135.

Smith, R., et al. (2022). “Fine-tuning fertilizer applications in organic cool-season leafy green crops can increase soil quality and yields.” California Agriculture 76(2-3): 77-84.

Field-Scale Ultraviolet-C Light Applications to Manage Grapevine Powdery Mildew

By
Alexa McDaniel | Ph.D., Research Scholar, Viticulture Research and Extension, Dept. of Horticultural Science, North Carolina State University
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Over-the-row ultraviolet-C light (UV-C) array applying a UV-C dose of 200 J/m2 to Vitis vinifera ‘Chardonnay’ grapevines for management of grapevine powdery mildew. Tractor driver is wearing proper personnel protective equipment (clothes and gloves that cover all skin with ANSI Z87.1 rated eyeglasses and face shield). Shown are applications made at early shoot growth (left) and applications made at post-bloom (right) (all photos courtesy A. McDaniel.)

Ultraviolet-C light (UV-C) for crop protection is not a new concept, but how we integrate the technology into pest and disease management programs is. Field application of UV-C has successfully reduced powdery mildews of strawberry (Onofre et al. 2021), cantaloupe (Lopes et al. 2023) and grapevine (Ledermann et al. 2021, Gadoury et al. 2023). This was possible due to key findings that enhanced efficacy by applying UV-C during a dark period that continued for at least four hours after application (Janisiewicz et al. 2016a, Suthaparan et al. 2016a, Onofre et al. 2021). This dark period allows the UV-C damage to be permanent by bypassing the robust fungal photolyase repair mechanism that is driven by the blue and UV-A components of sunlight (Beggs 2002). Applying UV-C followed by a dark period (i.e., a lower, non-phytotoxic dose) can be used to suppress pathogens effectively (Suthaparan et al. 2014, 2016b, Janisiewicz et al. 2016b, Onofre et al. 2021). Pioneers in UV-C application for crop protection (David Gadoury with Cornell University and USDA scientists Fumiomi Takeda and Wojciech Janisiewicz) have previously written articles in Progressive Crop Consultant. These articles further describe past UV-C research that has led to the successful application of UV-C.

Our research conducted at Washington State University, led by Michelle Moyer, was in collaboration with Gadoury and Walt Mahaffee with USDA to expand UV-C applications for field-scale pest and disease management in grapevines. Our goal was to expand our knowledge of how UV-C can be used as a non-pesticidal alternative to suppress grapevine powdery mildew (Fig. 1) in Eastern Washington State Vitis vinifera vineyards through testing different timing and intervals for UV-C application. We additionally explored UV-C effects on basic fruit chemistry. The results of our research (McDaniel et al. 2024) are now published open access at American Journal for Viticulture and Enology.

Figure 1. Grapevine powdery mildew (Erysiphe necator) colonizes the surface of any green tissue, making it susceptible to environmental conditions. Powdery mildew appears as white, powdery mass on the surface of the plant. Infection can reduce photosynthesis of leaves (left) and renders fruit (right) unusable due to negative effects on pH, soluble sugars and flavor components.

Experiment Methods
Our field-scale UV-C (Fig. 2) array was based on designs from Gadoury and Mahaffee. It was built as an over-the-row triangular arch with 12 ballasts to power 24 UV-C lamps backed by polished aluminum reflectors. It was supported by a metal tower attached to the three-point hitch system of a tractor. Thick PVC strips were mounted on each end of the array like curtains to contain UV-C within the apparatus. The framework was built by VineTech Equipment in Prosser, Wash. A dose of 200 J/m2 was achieved by adjusting ground speed based upon the array length and mean irradiance at the approximate height of the fruiting zone in the vineyard (Gadoury et al. 2023). UV-C dose is based on how long the vine is exposed; a longer array, slower speed or more bulbs would equal a higher dosage. UV-C treatments were applied 30 minutes post-sunset to allow for an optimal dark period.

Our trials conducted at the WSU research vineyard in Prosser tested UV-C intervals of weekly or twice-weekly in two different timing strategies to manage grapevine powdery mildew from 2020-22. Early season timing trial consisted of UV-C treatments made from early shoot growth to pre-bloom followed by a fungicide spray program post-bloom. Season-long timing trial had UV-C treatments that replaced all fungicides from early shoot growth to three weeks post-fruit set. Early season UV-C treatments were compared against three controls and season-long UV-C treatments were compared against two controls.

Figure 2. Washington State University over-the-row ultraviolet-C light (UV-C) array at the WSU research vineyard in Prosser, Wash.

Treatments for the early season trial:
1) Early weekly UV-C
2) Early twice-weekly UV-C
3) Early unsprayed, no fungicide treatments from early shoot growth till pre-bloom
4) Unsprayed, no fungicide treatments for the season
5) Fungicide program, based off typical spray programs in Eastern Washington Vineyards

Treatments for the Season-long Trial:
1) Weekly UV-C
2) Twice-weekly UV-C
3) Unsprayed, no fungicide treatments for the season
4) Fungicide program, based off typical spray programs in Eastern Washington Vineyards

Grapevine powdery mildew was visually rated as a disease severity percentage on leaves and clusters from bloom and continued until harvest. Severity ratings were converted into an area under disease progress curve (AUDPC), which quantifies disease intensity over time, providing an understanding of how disease accumulates throughout the season. The following berry harvest metrics were measured: yield, soluble solids, titratable acidity and pH. Berry skin tannin and phenolic concentrations were additionally measured from season-long treatments following the Adam-Harbertson Methods (Harbertson et al. 2002, 2003) as phenolics are shown to increase with sunlight exposure.

Figure 3. Results based on our research published in American Journal for Viticulture and Enology (McDaniel et al. 2024). Foliar and cluster disease severity ratings represented as accumulated area under disease progress curve (AUDPC) for early season treatments. A) and C) 2020 and 2022 foliar disease AUDPC, respectively. B) and D) 2020 and 2022 cluster disease AUDPC, respectively. 2021 foliar and cluster disease is not represented as there was low to no recorded disease. Error bars are standard error (n = 4). Different letters denote significant differences among treatment means at α = 0.05 using Tukey’s honest significant difference test.

Results
The hot and dry weather conditions in Eastern Washington State for 2020 and 2021 did not favor grapevine powdery mildew infection. In 2021, the daytime high temperatures from bloom to veraison exceeded 35 degrees C for 17 consecutive days. The 2022 vintage was dramatically different than the previous years, creating a favorable climate for grapevine powdery mildew infection with below-average temperatures and above-average precipitation. The low disease pressure experienced in 2020 and 2021 influenced our ability to evaluate the potential reduction of disease. It is hard to separate disease ratings when low to no disease is present.

Early season UV-C treatments significantly reduced foliar and cluster disease severity relative to the unsprayed control in 2020 and 2022 (Fig. 3). In 2021, due to extended high temperatures throughout the growing season, there was low foliar and no cluster disease in the vineyard, resulting in no separation of UV-C treatments, the fungicide program and untreated vines. Overall, early season UV-C, either weekly or twice-weekly, was as effective as the fungicide program for reducing disease. This indicates UV-C could replace standard fungicides early season in a grapevine powdery mildew management program for Eastern Washington.

Season-long UV-C treatments (Fig. 4) in 2020 did not significantly reduce foliar or cluster AUDPC relative to the unsprayed treatments. In 2021, due to high temperatures and low precipitation, there were no differences between any treatments in foliar AUDPC, and there was no observable disease on clusters regardless of treatment. In 2022, weekly and twice-weekly UV-C applications significantly reduced foliar AUDPC relative to the season-long unsprayed control. Cluster AUDPC in 2022 was significantly reduced with weekly and twice-weekly UV-C applications with twice-weekly UV-C performing the best. In 2022, both UV-C treatments did not significantly reduce foliar, or cluster disease compared to the fungicide program. This leads us to believe that improvements to UV-C application treatments can be improved. Season-long UV-C did not affect the yield, pH, titratable acidity or Brix of the grapes. The effects of season-long UV-C on tannins and phenolics were inconsistent year-to-year, suggesting factors other than UV-C were more influential on these measures. Nonetheless, season-long or exclusive UV-C for grapevine powdery mildew management requires further evaluation under Washington State conditions.

While not always statically significant, the consistent trend in reduction in powdery mildew disease as a result of weekly or twice-weekly UV-C treatment indicates the incorporation of UV-C into a vineyard IPM program could be an effective alternative for powdery mildew management without compromising fruit quality. To improve UV-C efficacy we believe that canopy management could play a critical role. For this experiment, we did not implement any canopy management practices as we were trying to induce grapevine powdery mildew disease. As with many foliar pesticide applications, coverage of the desired target is required for efficacy. A dense and complex canopy architecture (which our canopies were in this experiment) makes access to the fruiting zone a challenge. This might explain the more-consistent disease suppression when UV-C was applied only in the early season as canopies were less dense and allowed greater UV-C penetration. This also suggests UV-C efficacy may respond favorably to pruning and training systems that open canopies and expose the fruiting zone. To allow light (or spray) penetration to the fruiting zone, the following practices are important: shoot training, shoot thinning and leaf removal.

Figure 4. Results based on our research published in American Journal for Viticulture and Enology (McDaniel et al. 2024). Foliar and cluster disease severity ratings represented as accumulated area under disease progress curve (AUDPC) for season-long treatments, including an unsprayed control, a full fungicide program or weekly/twice-weekly ultraviolet-C light (UV-C) treatments. A) and C) 2020 and 2022 foliar disease AUDPC, respectively. B) and D) 2020 and 2022 cluster disease AUDPC, respectively. 2021 foliar and cluster disease is not represented as there was low to no recorded disease. Error bars are standard error (n = 4). Different letters denote significant differences among treatment means for season-long AUDPC (final date) at α = 0.05 using Tukey’s honest significant difference test.

Future Studies
Though we are pleased with these results, considering the benefits and potential negative impacts must be addressed for grower adoption. One main benefit of UV-C is it can be used in rain or wind, making it less environmentally dependent compared to typical pesticide sprays. Another plus, there is immediate reentry after treatment. This technology provides a residue free option, uses no water, and can be used when fungicide resistance is present in populations. All these examples make UV-C a potential tool for a sustainability forward pest management program. The cons to this technology are application is labor-intensive and requires nighttime applications. Management of grapevine powdery mildew with UV-C would demand at least weekly applications, most likely twice-weekly for improved efficacy, which could be a substantial amount of tractor hours. As it is a new tool, it requires either you build it yourself or wait for commercial units. Lastly, the effects to beneficial insects are still unknown. Studies that explore these challenges will be important for grower adoption.

UV-C research is still being continued at WSU within the Moyer lab by graduate student Jesse Stevens based on the results from these findings. They are exploring how canopy management practices, such as shoot thinning and fruit zone leaf removal, can increase the efficacy of UV-C applications for managing grapevine powdery mildew.   

Alexa McDaniel is now a Viticulture Extension and Research Scholar at North Carolina State University where her program aims to provide extension education materials for best viticulture practices and field-applied research focuses on pest and disease management. Additional information on UV-C can be requested from McDaniel (almcdan2@ncsu.edu) or Michelle Moyer (michelle.moyer@wsu.edu).

This work was funded by the Washington State Grape and Wine Research Program. The author would like to thank Bernadette Gagnier, Jake Shrader, Maria Mireles, Charlotte Oliver and Margaret McCoy for their help in this project.

References
Beggs CB. 2002. A quantitative method for evaluating the photo-reactivation of ultraviolet damaged microorganisms. Photochem Photobiol Sci 1:431-437. DOI: 10.1039/B202801H

Harbertson JF, Kennedy JA and Adams DO. 2002. Tannin in skins and seeds of Cabernet Sauvignon, Syrah, and Pinot noir berries during ripening. Am J Enol Vitic 53:54-59. DOI: 10.5344/ajev.2002.53.1.54

Harbertson JF, Picciotto EA and Adams DO. 2003. Measurement of polymeric pigments in grape berry extract sand wines using a protein precipitation assay combined with bisulfite bleaching. Am J Enol Vitic 54:301-306. DOI: 10.5344/ajev.2003.54.4.301

Gadoury DM, Sapkota S, Cadle-Davidson L, Underhill A, McCann T, Gold KM et al. 2023. Effects of nighttime applications of germicidal ultraviolet light upon powdery mildew (Erysiphe necator), downy mildew (Plasmopara viticola), and sour rot of grapevine. Plant Dis 107:1452-1462. DOI: 10.1094/PDIS-04-22-0984-RE

Janisiewicz WJ, Takeda F, Nichols B, Glenn DM, Jurick II WM and Camp MJ. 2016a. Use of low-dose UV-C irradiation to control powdery mildew caused by Podosphaera aphanis on strawberry plants. Can J Plant Pathol 38:430-439. DOI: 10.1080/07060661.2016.1263807

Janisiewicz WJ, Takeda F, Glenn DM, Camp MJ and Jurick II WM. 2016b. Dark period following UV-C treatment enhances killing of Botrytis cinerea conidia and controls gray mold of strawberries. Phytopathology 106:386-394. DOI: 10.1094/PHYTO-09-15-0240-R

Ledermann L, Daouda S, Gouttesoulard C, Aarrouf J and Urban L. 2021. Flashes of UV-C light stimulate defenses of Vitis vinifera L. ‘Chardonnay’ against Erysiphe necator in greenhouse and vineyard conditions. Plant Dis 105:2106-2113. DOI: 10.1094/PDIS-10-20-2229-RE

Lopes UP, Alonzo G, Onofre RB, Melo PP, Vallad GE, Gadoury DM et al. 2023. Effective management of powdery mildew in cantaloupe plants using nighttime applications of UV light. Plant Dis 107:2483-2489. DOI: 10.1094/PDIS-08-22-1941-RE

McDaniel AL, Mireles M, Gadoury D, Collins T and Moyer MM. 2024. Effects of ultraviolet-C light on grapevine powdery mildew and fruit quality in Vitis vinifera Chardonnay. Am J Enol Vitic 75:0750014. DOI: 10.5344/ajev.2024.23071

Onofre RB, Gadoury DM, Stensvand A, Bierman A, Rea M and Peres NA. 2021. Use of ultraviolet light to suppress powdery mildew in strawberry fruit production fields. Plant Dis 105:2402-2409. DOI: 10.1094/PDIS-04-20-0781-RE

Suthaparan A, Stensvand A, Solhaug KA, Torre S, Telfer KH, Ruud AK et al. 2014. Suppression of cucumber powdery mildew (Podosphaera xanthii) by supplemental UV-B radiation in greenhouses can be augmented or reduced by background radiation quality. Plant Dis 98:1349-1357. DOI: 10.1094/PDIS-03-13-0222-RE

Suthaparan A, Solhaug KA, Stensvand A and Gislerød HR. 2016a. Determination of U V action spectra affecting the infection process of Oidium neolycopersici, the cause of tomato powdery mildew. J Photochem Photobiol B 156:41-49. DOI: 10.1016/j.jphotobiol.2016.01.009

Suthaparan A, Solhaug KA, Bjugstad N, Gislerød HR, Gadoury DM and Stensvand A. 2016b. Suppression of powdery mildews by UV-B: Application frequency and timing, dose, reflectance, and automation. Plant Dis 100:1643-1650. DOI: 10.1094/PDIS-12-15-1440-RE

2024 Crop Consultant Conference: Final Reminder Improved Continuing Education, Priceless Networking Available to All

By
Taylor Chalstrom | Editor
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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.)

It’s looking to be another banner year for the Crop Consultant Conference, with attendance filling up and a record number of exhibitors already confirmed for the trade show floor. Hosted by JCS Marketing Inc. in collaboration with Western Region Certified Crop Advisers, the conference will offer improved continuing education units (CEU) with the new CEU Manager, a place to organize all earned units from the Crop Consultant Conference and other JCS events, and networking opportunities for PCAs/CCAs, grower-applicators and industry stakeholders/researchers not found at similar events.

We spoke with JCS Marketing Inc. CEO and Progressive Crop Consultant publisher Jason Scott about the abundance of unique opportunities at this event and why it’s never one to miss.

New CEU Manager
“We’ve built a new CEU Manager where you can track all your CEUs that you’ve attended at JCS Marketing,” Scott said. “There’s 60.5 CEUs available before, during and after the conference. So, if you attend the conference, ideally you can get your hours for both years right in one shot, so more than enough hours available.”

Before the conference, all registered attendees have access to 11.0 hours of online courses covering essential topics. The conference will provide 12.5 hours of in-person CEUs through a variety of expert-led sessions and hands-on presentations. Additionally, post-conference, registered attendees will have access to an extensive range of online courses, bringing the total to over 60 CEU hours, enough to meet all annual requirements. All online CEU courses will remain open until Dec. 31, 2024.

The CEU Manager is part of the MyAgLife app, available for both IOS and Android.

“We couldn’t make it any easier. Get all your hours and then manage all your hours. You’re able to print out your certificates and track it all, and we turn it into the governing bodies for both DPR and for Western Region CCA,” Scott said. “If there’s any deficits or overages or anything you need to address with the governing bodies, you can print it out, you can look at what you’ve completed, and it makes it really easy.”

The exhibit hall is set to feature a record-breaking number of sponsors and exhibitors.

Record-Breaking Exhibitors, Networking Opportunities
Sponsor and exhibitor numbers are at an all-time high for the 2024 Crop Consultant Conference, highlighting the impact of this event and industry’s commitment to being available for consultants and grower-applicators.

“It’s incredible what’s happening,” Scott said. “There are so many sponsors this year, more exhibitors than we’ve ever had.

Scott attributes the growth of the event to three things: low cost per CEU, location and unique networking opportunities.

With the cost of registration at $395, that brings the per-CEU cost to about $6.60, well below the industry standard. “It’s super affordable,” Scott said.

The conference is also strategically positioned at the Visalia Convention Center in the heart of the Central Valley. The high-density surrounding areas of Fresno and Tulare counties are home to a high percentage of the state’s consultants, making the conference convenient for travel.

“Easy traveling for the locals, they can be in their own bed,” Scott said. “If you do have to travel, if you’re coming from Northern California or Southern California, hotel rates are very inexpensive and affordable, and it’s just an easy way to do business.”

While there are multiple opportunities for fun at the Crop Consultant Conference, Scott iterated that it’s not a “party conference.”

“That’s not to say this isn’t a really positive and fun experience,” he said. “I think there’s a big difference between a party conference and having fun. There’s a lot of fun. There’s a lot of networking. There’s a lot of after-hours gatherings… But it does have a more serious undertone to it, and the group of individuals are here to really grow their businesses.

“This is really the place to do big business, and if you’re not here, you’re missing out on some of the biggest opportunities to network with independent [consultants], researchers and ag retailers,” Scott added.

The 2024 Crop Consultant Conference will take place on September 25 and 26. Register today at progressivecrop.com/conference.

Outsmarting Your Fertilizer “Competition” to Improve Uptake Efficiency

By
design@jcsmarketinginc.com
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Figure 1. Estimates of soil-applied fertilizer efficiencies (amount used by crop relative to how much was applied total to the soil.) Phosphorus and potassium have a relatively lower use efficiency compared to nitrogen due to some of the factors listed in this article. Anticipating and counteracting soil “competition” can help push up fertilizer use efficiencies and drive a higher return on investment for the grower (Fixen et al. 2014).

One fortunate aspect of working in agriculture is that hot summer days are soon followed by harvest season and the onset of nicer weather. Now is the time to start thinking about postharvest soil fertility maintenance. Dry, granular phosphorus and potassium fertilizer products (e.g., monoammonium phosphorus (MAP:11-52-0); sulfate of potash (SOP: 0-0-60); and muriate of potash (MOP: 0-0-50)) are popular choices for postharvest fertility work and a cornerstone of annual nutrient budgets for many crops.

These fertilizer formulations are popular with growers as they can apply bulk nutrients to help maintain soil fertility and get a jump start on supporting the next crop cycle. A goal for growers and their advisor network is to manage the applied nutrients so that much of it ends up in the crop and fertilizer uptake efficiency is optimized. With that in mind, there are various sources of “competition” that prevent the uptake of the nutrient by the plant, which leads to inefficiency in fertilizer use (Fig. 1). 

In a perfect world, 100% of your fertilizer application would be used by the crop to produce yield and fertilizer losses would be minimized. However, as Figure 1 shows, use efficiency of fertilizers vary greatly by nutrient category. In this article, we will name the various efficiency loss pathways, or “competition”, for your phosphorus and potassium fertilizer applications so you can work with a crop advisor to try and work around them. Before we jump in, let’s start with some phosphorus and potassium 101.

Why Crops Need Phosphorus and Potassium
Phosphorus and potassium are plant macronutrients that are required in large quantities by the crop to produce a high-quality yield. However, phosphorus and potassium play different roles in the plant. Phosphorus is a crucial component for converting solar energy into food, fiber and other plant products. Furthermore, phosphorus also plays a key role in the metabolism of sugars, energy storage and transfer, cell growth and the transfer of genetic information. An interesting bit about phosphorus is that one can point out the element embedded in the actual chemical structures the plant is using to create yield. For example, you can point to the actual phosphorus that is used to build the biological currency plants use to “pay” for work (ATP) and the phosphorus backbone that all crops use to construct their genes (DNA).

On the other hand, potassium is much more difficult to point out in the crops’ molecular portfolio. However, we do know that potassium is still needed by the plant to support high yields. Briefly, potassium increases crop yield, improves crop quality, reduces disease and decreases risk of lodging and branch breakage, etc. Except for nitrogen, plants often require more potassium than any other nutrient as it plays vital roles in crop growth and development.

Understanding the Competition
With the basics out of the way, we can now turn our attention to the factors that reduce your fertilizer uptake efficiency by the crop. I will refer to the factors as “competition” because these factors interfere with your best laid fertilizer plans and will reduce your crops’ ability to use the nutrient. This will impact the return on your fertilizer investment when harvest comes around.
  
Phosphorus Competition
Phosphorus can be a tricky nutrient to manage because it interacts with other soil elements that will reduce overall fertilizer use efficiency (Fig. 2). Due to these interactions, the fertilizer use efficiency range can be quite low. For example, only 5% to 30% of the fertilizer might get used by the crop during the growing season (Fig. 1). The rest is lost to the competition.

Slow Diffusion Rates
Diffusion refers to the movement of phosphorus from the bulk soil toward the roots by way of a concentration gradient. Other plant uptake pathways exist but diffusion is the major pathway for phosphorus. For example, research shows that corn acquires ~93% of its phosphorus needs for the season via diffusion (Barber 1995; Fernandez 2016). What’s the catch? The rate of diffusion is sensitive to several factors that reduce the speed of how fast the process, or rate, can occur. These soil factors include cold temperatures, compaction, saturated or dry conditions. When these factors reduce the diffusion rate, they can compete with your fertilizer application by not allowing the crop to be adequately supplied with phosphate. 

Mineral Antagonisms
In some cases, the over application of other nutrients can prevent plant uptake of phosphorus, which will reduce fertilizer use efficiency. For example, research shows that excessive applications of calcium and magnesium can interfere with the crop’s ability to use phosphorus (Better Crop 1999).

Figure 2. Five competitors that reduce your phosphorus fertilizer use efficiency by the crop. A crop advisor can help you understand how to best work around the competition and get more phosphorus into the crop (Fixen et al. 2014).

Soil pH
Competition for your phosphorus fertilizer efficiency can be influenced by soil pH and the conditions that result. For example, when soil calcium and phosphorus fertilizer come together under alkaline conditions (pH >7), they form a hard mineral called apatite, which is the same mineral that your bones and teeth are made of. Once this mineral fixation happens, the bound phosphorus is now unavailable for plant uptake, with reduces use efficiency. Figure 3 shows just how much soil phosphorus is made unavailable by the calcium complexes (up to 80% to 90% reduction in availability).

Figure 3. Most soil phosphorus is tied up by calcium in the Western U.S. due to elevated soil pH (>7 or alkaline soil). Note the small fraction of phosphorus that is available for plant uptake in alkaline soils. Source: Soils – Part 6: Phosphorus and Potassium in the Soil – University of Nebraska Extension Service.

Runoff/Erosion
Phosphorus can bind to soil particles and move off the field during rainfall or during snowmelt. When soil leaves the field (erosion), it is more than likely carrying some phosphorus with it, reducing use efficiency. This impact is more pronounced on sloped fields or in areas where heavy rainfall occurs.

Poor Recommendations
This might not be an obvious source of competition, but how your local ag lab analyzes the pool of soil phosphorus matters. Briefly, soil phosphorus can be measured by three different methods: Bray, Mehlich and Olson. Your local soil pH should determine the extraction method as each works best within a certain pH range. A mismatch in method will overestimate or underestimate the phosphorus supply. The wrong test will lead to a bad fertilizer recommendation, which can be inefficient. In this case, your competition is bad information.

Potassium Competition
Now let’s turn our attention to the competition for potassium. Crops are generally able to use more of the potassium provided by a fertilizer application relative to phosphorus. For example, 30% to 60% of potassium fertilizer might get used by the crop (Fig. 1). The rest is lost to the competition (Fig. 4).

Figure 4. Five competitors that reduce your potassium fertilizer use efficiency by the crop. A crop advisor can help you understand how to work around the competition to ensure the potassium gets into the plant (Fixen et al. 2014).

Potassium ‘Tie Up’ by Clays
Soils differ in their relative amounts of sand, silt and clay particles (e.g., texture), and this distribution impacts several important soil properties. For example, clay-dominated soils tend to have a high cation exchange capacity (CEC), which is the relative ability of a soil to store positively charged nutrients.  However, nutrient release rates are inversely related to the CEC value. In this sense, high clay soil will release stored potassium back to soil solution at a slower rate relative to sandy soil. Thus, clays can temporarily ‘tie up’ potassium fertilizer applications by storing it and then only slowly releasing it back to the soil for plant use, which can decrease efficiency.   

Potassium Fixation
This next competitor occurs under unique circumstances and has a longer lasting impact on your fertilizer program. Certain clay minerals can scavenge potassium from the soil, including your potassium fertilizers, and render the nutrient unavailable for plant use. This process is called potassium fixation. Strong potassium fixation capacity is typically associated with the presence of vermiculite and mica-based materials in soil, in a specific age class, and from soils derived from granite parent material (Pettygrove and Southard 2003). Fixed potassium, once tied up, is no longer available for plant use during the growing season and can have a serious impact on fertilizer use efficiency. 

Leaching
Sandy soils are poor at storing nutrients (e.g., low CEC). Furthermore, they also have a high nutrient release rate back to the soil. Under these conditions, potassium fertilizers can leach out of the root zone, which reduces the fertilizer use efficiency. Thus, excessive rainfall or irrigation sets on sandy, low-organic-matter ground can compete with your crop for potassium.

Dry Soils
Unlike phosphorus, ~20% of potassium is moved to the plant through the soil moisture in a process called mass flow (Barber 1995; Fernandez 2016). Excessively dry soils at any depth throughout the rooting zone will prevent potassium from reaching the plant and reduce the fertilizer use efficiency.

Antagonisms with Sodium
This section applies to those struggling with excessive sodium in the irrigation water or soil. In this case, the sodium can compete with your potassium fertilizer plan by interfering with the crops’ ability to acquire the nutrient. Wakeel (2013) recently summarized the antagonistic effect of sodium on potassium uptake in detail. Briefly, excess sodium makes it difficult for potassium to be taken up by the crop and sodium also increases potassium leakage from the crop, which reduces use efficiency.

Outsmarting the Competition
Now that we have named the competition, we can work to manage them to minimize their impact and get more of your applied nutrients into the crop where it belongs. Where to begin? Much of the competition mentioned above can be identified through routine soil testing and from sound advice from a local crop advisor. By using an updated soil report, you and your advisory team can be on the lookout for conditions that might give your fertilizer competition a leg up in the field so you can manage them appropriately. For some other situations, a call to the local lab can confirm they are using the right extraction method and generating the correct fertilizer recommendation. Local geological and soil series reports, if you suspect potassium fixation, can be invaluable for understanding conditions in your field that could be competing for your potassium. As reviewed here, several competitors are working to reduce the use efficiency of your expensive fall fertilizer applications. If you are concerned about improving nutrient uptake in the crop and keeping the phosphorus and potassium out of the hands of the competition, consider contacting a local crop advisor to help develop a field-specific management plan. Once your competition is formally identified, using this article as a guide, it can be better managed on your farm and fertilizer use efficiency for phosphorus and potassium can be increased.

Dr. Karl Wyant serves as the Director of Agronomy at Nutrien. Please visit www.nutrien-ekonomics.com for the latest in nutrient management topics and other free resources.

References
Fixen et al. 2014. Nutrient/Fertilizer Use Efficiency: Measurement, Current Situation and Trends

Better Crops/Vol. 83 (1999, No. 1). Phosphorus Interactions with Other Nutrients

Wakeel 2013. Potassium–sodium interactions in soil and plant under saline-sodic conditions

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/.

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