Pistachio rootstock UCB1 D90 in May of the third growing season in microplots with sand. On left, no root lesion nematodes. On right, many root lesion nematodes.
Pistachio has enjoyed decades of limited reported damage from plant-parasitic nematodes. This contrasts with other tree nut crops as almond and walnut. While almond and stone fruits were long encumbered by root-knot nematodes, these crops have benefitted from protection against these culprits by the introduction of peach rootstock ‘Nemaguard.’ This fascinating rootstock carries sustainable resistance since the 1960s. Only more recently has the detection of the peach root-knot nematode (PRKN) illustrated the vulnerability of this and other rootstocks with resistance to southern root-knot nematodes other root-knot nematode species. The distribution of PRKN is not documented yet. Both walnut and almond are efficient hosts of the walnut root lesion nematode (RLN). These crops have long-term challenges with RLN, especially its primary host walnut that gets damaged by only one nematode per 250 cc of soil. No sustainable resistance and tolerance to RLN is currently commercially available in rootstocks of these crops.
The interaction of plant-parasitic nematodes and their hosts is two-fold. The one component is the response to nematode infection; some genotypes respond with no damage independent of the nematode reproductive potential on their roots (i.e., they are tolerant). Others may get hurt from only a few nematodes (i.e., they are sensitive/intolerant). As the second response, the roots may allow for nematode reproduction (susceptible) or they may prevent such (resistant). Both parameters are very important for the field performance of plants. Genotypes that are resistant but sensitive will cause tolerance without any resistance and can lead to build-up of nematode population densities that may cause problems for the following plantings.
Pistachio seemed free from these issues, likely based on a 1980s survey where no typical plant-parasitic nematodes at meaningful numbers were found on pistachio roots. In those days, the developing pistachio industry frequently used Pistacia atlantica as rootstock for the nut cultivars. Genotypes of this species were known to be on average resistant to RLN. In fact, some researchers used it as a resistant standard in their resistance determining studies. Growing P. atlantica rootstocks may have protected plantings from nematode infections.
Plant diameter of pistachio rootstocks UCB1 D71 and UCB1 D90 at the end of two growing cycles grown in microplots infested with increasing population densities of Pratylenchus vulnus. The threshold level is indicated by the red line. This is a modified reprint of a figure from Eur. J. Hortic. Sci. 89 (3).
While such rootstocks were useful regarding plant-parasitic nematodes, they were highly susceptible to Verticillium dahliae, the causal agent of Verticillium wilt that threatened the industry in the late 1980s. This crisis required major changes in the management of pistachio. While some optimization in cultivation of the crop could mitigate severe damage, ultimately, plants were required that would not be impacted by this soil-borne disease. Crosses of Pistacia atlantica and P. integerrima were identified as being resistant to Verticillium wilt.
Some hybrid rootstocks were used as seedlings (e.g., PG II or UCB1), but with the capacity of clonally propagating the genotypes in tissue culture, clonal rootstocks became more abundant. The name UCB1 is used for any clones out of that specific directed cross. Yet, there are distinct differences among UCB1 rootstocks. This is indicated by some nurseries by providing additional identification of specific clones. With this change in rootstocks, the genetic protection from plant-parasitic nematodes may be obsolete. In fact, screens of experimental UCB1 clones conducted by Dr. M. McKenry illustrated large variability of nematode susceptibility. In more recent screens with the most widespread nematode species at the Kearney Agricultural Research and Extension Center (KARE), susceptibility of different UCB1 clones was confirmed under field conditions.
In addition to the changes in rootstock genetics, pistachio is now frequently planted to fields that were used for the production of other perennial crops hosting plant-parasitic nematodes. In screens of pistachio, limited susceptibility to root-knot nematodes, nematodes feared after grape or cotton, was ascertained. In contrast, susceptibility to RLN was on a similarly high level as under other known tree nut crops. The build-up of these populations took long, but the principal susceptibility was confirmed. Such finding creates the potential challenge when pistachio is planted after almond and in particular after walnut. The latter crop is going through a phase of ample removal because of water restrictions, fumigant restrictions, and poor market conditions. Walnut rootstocks liable to have been used for decade-old walnut orchards are notoriously leaving behind concerning RLN population densities. This creates a potential risk for planting pistachio.
Current Threat of Root Lesion Nematode Considering this background information, the question remains: How damaging is RLN to today’s pistachio plantings? The classical way to examine the damage potential of a plant-parasitic nematode on a specific crop is to expose the plant to increasing levels of the target nematode and measure growth response of the plants. In microplot experiments at KARE, such differing population densities were created by fractional fumigation that left behind varying population densities of RLN. Such plots were then planted to two different UCB1 clones that had tentatively shown different susceptibilities to RLN, and different overall vigor.
These trees were grown for two years. At the end of the second year, plant growth was measured and related to the initial population densities of RLN. With higher numbers of RLN, plants were weaker at this evaluation time, this was proportionally true for both rootstocks. In statistical analysis with the so-called “Seinhorst function,” the threshold level was determined where nematode numbers started causing damage to the trees. This level was around 13.3 vermiform nematodes per 250 cc of soil. Similar confirmatory results were found in a second microplot experiment (a more technical report of this study can be accessed atpubhort.org/ejhs/ahi/1420/1420.pdf.)
Such population densities can be frequently found after a walnut orchard. So, care needs to be taken when pistachio follows walnut. Similar numbers can also be found after almond. Sampling is mandatory when replacing a tree nut crop with another. RLN infects the nut crops and the presumed benefits of “crop rotation,” or better crop change, do not encompass this nematode pest. It infects all three nut crops with differing damage potential. The recommendation is to sample fields for nematode detection holds (more soil sampling information and tips are found at youtube.com/watch?v=U7x0xHoKqC8.)
When diagnosing nematode problems in the field, those may often be overlooked. Pistachio orchards are cultivated under a rigorous pruning and training regimen. Some of the variability from tree-to-tree that would be a typical first indication of tree health issues may go unnoticed because of that. In addition, other soil differences may overlay areas where nematode infections are damaging and may lead to unwarranted conclusion of their responsibility for the differences detected. To bring more clarity to this topic, more field work is necessary. Especially the threshold level experiments require confirmation under commercial conditions. If readers were interested in participating in such studies, they are encouraged to reach out to the author Dr. Andreas Westphal at andreas.westphal@ucr.edu or 559-646-6555 to learn more on what is involved in such studies.
In summary, pistachio is not home-free from the risk for nematode damage. Vigilance and care need to be exercised when planning a new orchard planting. Soil sampling for nematode detection should be an instrumental procedure in this process.
Accumulation of mineralized nitrogen in the ammonium pool at lower temperatures has important implications for soil testing and strategic fertilizer placement (photo by Mark Bolda, UCCE.)
In organic crop production, fertilizers need to be mineralized by soil organisms before nitrogen becomes plant-available. Many factors can impact this mineralization process, including fertilizer chemical composition, physical fertilizer properties, soil properties and application method1–3. Moreover, microbial activity, including N mineralization, is affected by temperature4,5. Typically, an increase in N mineralization with increasing temperature is expected, but data is lacking to inform management decisions. Anecdotal evidence suggests organic fertilizer products perform differently based on the season in which the product is applied. This begs the question: Do organic fertilizer products show different sensitivities to soil temperature? We put this question to the test in a laboratory incubation study.
Laboratory Incubation Study We set up a laboratory incubation experiment in which soil was incubated with 12 organic fertilizers at 41, 50, 59, 69 and 77 degrees F for five weeks. Soil was collected from an organic vegetable field near Bakersfield, Calif. The soil was mapped at Milagro (coarse-loamy, mixed, superactive, nonacid, thermic Typic Torrifluvents), with soil analyses showing 64% sand, 18% clay and 0.6% soil organic carbon. The soil was sieved, homogenized and air-dried prior to incubation. Fertilizers were obtained from an organic vegetable grower and representative of commercially available products in the region. Fertilizers were categorized as dairy manure compost, chicken manure compost, bone meal dominated pellets, poultry litter dominated pellets and mixed feedstock pellets. The experiment was set up as a full factorial design, including 5 temperatures and 13 fertility treatments (12 fertilizer products and an unamended control), each replicated 4 times, for a total of 260 experimental units. For each experimental unit, we placed 250g of air-dried soil in plastic containers, rewetted to 60% water holding capacity and pre-incubated at the treatment temperature for seven days. Subsequently, fertilizers were added at a rate of 50 mg N/kg soil, roughly equivalent to incorporating 100 lbs N/acre in the top 6 inches of soil. We collected 5 g of soil from each experimental unit on days 0, 7, 14, 21 and 35 and analyzed for ammonium and nitrate concentration. The percentage of N mineralized was calculated as the difference in N concentration in the treatments receiving fertilizer compared to an unamended control divided by the total N applied.
Key Findings Lowest N mineralization for dairy manure compost, highest for chicken litter compost
Nitrogen release varied drastically across fertilizer products and temperatures (Figure 1). Most products showed a net release of N, with up to 85% of fertilizer N released within the five-week incubation period. In some cases, we observed net negative mineralization rates up to -59%, indicating fertilizer addition caused immobilization of mineral N from sources other than the fertilizer. Dairy manure compost exhibited notably low N mineralization rates, ranging from -29% to 22%, with a mean of -1.4%. Other studies have found similarly low rates, with values mostly below 10%6–10. The carbon to nitrogen (C:N) ratios of dairy manure composts in our study were 11 and 13, within the range of ratios between 8 and 16 reported for other studies6,8. Mineralization rates for dairy manure compost trend low compared to rates expected based on C:N ratios2. In contrast, chicken manure compost showed higher mineralization rates (between 15% and 77%) with an average of 41%. Mineralization rates of chicken manure composts vary across studies. Rates of 25% and 54%11, 28% and 35%2, 100%12 and 0.4% to 6%13 have been reported. Variation in chicken manure compost mineralization may stem from differences in feedstock quality, including bulking agent and particle size as well as composting practices (e.g., turning and watering)14,15. Our findings indicate chicken manure compost performed similarly or slightly better than poultry litter pellets, though other studies reported higher rates for pellets2,6. High variability between studies underscores the need for case-specific evaluations of fertilizer effectiveness in agriculture.
Figure 1. Percent fertilizer nitrogen mineralized over the five-week incubation period by fertilizer category and temperature treatment. Different lowercase letters indicate significant differences in percent fertilizer nitrogen mineralized between fertilizer categories within a temperature treatment, averaged across the incubation period, based on the Dunn multiple comparison test with Holm adjustment.
The initial flush of soil organic N mineralization increased with temperature, but not fertilizer N release
The study found increasing temperature did not significantly increase the percent of N mineralized from fertilizers (Figure 1). In contrast, previous studies show varying responses. For instance, temperature moderately influenced mineralization in one study, increasing N mineralization from 56% to 66% as temperatures rose from 50 degrees F to 77 degrees F16. Another study observed a strong temperature effect on dairy manure N mineralization, with 24% occurring in summer (77 degrees F to 86 degrees F) and only 2% in winter (50 degrees F)10. The difference might be linked to the much higher application rates in those studies compared to this one. Further research is needed on the impact of application rates on N mineralization, especially considering the relevance for optimizing fertilizer timing (single vs split application) and placement (e.g., banding leads to more concentrated fertilizer placement than broadcasting). Like fertilizer mineralization, soil organic N mineralization in our study did not increase with increasing temperature during the 35-day incubation period, though soil mineral N content at the end of the pre-incubation did increase linearly from 42 mg N/kg at 41 degrees F to 49 mg N/kg at 77 degrees F (Figure 2). Other research has shown exponential increases in soil organic N mineralization with temperature beyond the pre-incubation period 5,17. Compared to other studies, our soil had a high sand content (>60%) and very low soil organic carbon content (ca. 0.6%), suggesting the characteristics of the soil and environmental factors like rewetting may play a role in temperature sensitivity of mineralization, which has implications for irrigation management.
Relative performance of fertilizer products varied by temperature
The ranking of mineralization rates among fertilizers varied by temperature (Figure 1). Bonemeal-dominated pellets ranked among the best products in terms of N release at temperatures of 59 degrees F and above, while chicken litter-dominated pellets ranked relatively high at 50 degrees F and 59 degrees F but performed worse above 68 degrees F (Figure 1). Other studies have found differences in the relative performance of organic fertilizers at different temperatures16,18, but clear insights on the fertilizer properties that lead to better performance at low vs high temperature are missing. Studies on decomposition of plant litter suggest decomposition slows down more severely with decreasing temperatures for ‘low-quality’ litter, such as roots, than ‘higher quality’ litter, such as shoots, but findings vary across studies4,19. With respect to N availability of organic fertilizers, not only temperature effects on N mineralization, but also temperature effects on loss pathways need to be considered. Ammonia volatilization is known to increase with increasing temperature and can constitute a substantial loss of N from organic fertilizers, especially in sandy and alkaline soils, but subsurface application and management of soil pH can help reduce loss20,21. Likewise, gaseous N loss through denitrification may be enhanced at higher temperatures22. Thus, organic fertilizer properties can affect both the temperature sensitivity of fertilizer N mineralization and loss pathways for released N. Growers may benefit from fine-tuning organic fertility management by season.
Low temperatures limit nitrification rates
Across fertilizer treatments, nitrate concentrations increased and ammonium concentrations decreased with increasing temperature (Figure 2), indicating an inhibitory effect of low temperature on nitrification as documented by previous studies16,23,24. Soil tests may underestimate plant-available N from organic fertilizers at colder temperatures since they typically report only nitrate. In our experiment, ammonium concentrations reached up to 35 ppm at lower temperatures, equivalent to approximately 70 lbs N/acre in the top 6 inches of soil. While both ammonium and nitrate are plant-available forms of N, the preferred ratio of ammonium to nitrate differs between crops and is affected by soil properties like pH25. Therefore, impacts of reduced nitrification due to low temperature on yield are expected to be context-dependent. In general, ammonium is much less mobile in the soil compared to nitrate26. This suggests placement of organic fertilizer closer to the plant roots (e.g., through banding) may be beneficial to increase N availability during colder periods as long as application rates are low enough to keep ammonium concentrations below toxicity levels27.
Figure 2. Soil nitrate and ammonium concentrations (in ppm N or mg N/kg soil) by temperature over the five-week incubation period for all 12 fertilizer treatments and the non-fertilized control. The Kruskal-Wallis rank sum test showed a significant effect on temperature and fertilizer treatment on nitrate and ammonium concentrations averaged across time. Dark blue = dairy manure compost, light blue = chicken manure compost, red = bonemeal dominated pellets, yellow = poultry litter dominated pellets, brown = mixed feedstock pellets and grey = unamended control soil.
Implications for Nitrogen Management in Organic Production We wanted to find out if different organic fertilizers react differently to changes in soil temperature. Our research showed bonemeal-dominated pellets work better in warmer conditions, while poultry litter-dominated pellets perform better at moderate temperatures. Other studies also highlight temperature and soil type can impact how well fertilizers work. In general, our results caution against assuming fertilizers release N faster at higher temperatures. Mineralization of dairy manure compost was very low, even when considering the relatively high C:N ratio of 11 to 13, making it more useful for improving soil quality rather than as a primary fertilizer. Lower temperatures inhibited nitrification, which has been well documented in other studies. Accumulation of mineralized N in the ammonium pool at lower temperatures has important implications for soil testing and strategic fertilizer placement. Implications for yield are expected to be crop- and soil-dependent. Future research directions include how soil types, the impact of temperatures above 77 degrees F and higher rates of fertilizer affect N mineralization rates.
References 1. Cassity‐Duffey, K., et. al. Soil Science Society of America Journal 84, 534–542 (2020).
2. Lazicki, P., Geisseler, D. & Lloyd, M. Journal of Environmental Quality 49, 483–495 (2020).
3. Smith, R., et. al. California Agriculture 76, 77–84 (2022).
4. Fierer, N., et. al. Ecology 86, 320–326 (2005).
5. Miller, K., et al. Communications in Soil Science and Plant Analysis 50, 77–92 (2019).
6. Hartz, T. K., Mitchell, J. P. & Giannini, C., HortScience 35, 209–212 (2000).
7. Hurisso, T. T., et al. Organic Agriculture 2, 219–232 (2012).
8. Hurisso, T. T., et al. Agrosystems, Geosciences & Environment 7,
e20555 (2024).
9. Lehrsch, G. A., et al. Nutrient Cycling in Agroecosystems 104, 125–142 (2016).
10. Watts, D. B., Torbert, H. A. & Prior, S. A. Soil Science 175, 27 (2010).
11. Geisseler, D., et. al. Journal of Environmental Quality 50,
1325–1338 (2021).
12. Johnson, H. J., et al. HortTechnology 22, 37-43 (2012).
13. Tyson, S. C. & Cabrera, M. L. Communications in Soil Science and Plant Analysis 24, 2361–2374 (1993).
14. Leconte, M. C., et al. Biology and Fertility of Soils 47, 897–906 (2011).
15. Shi, W., et al. Applied Soil Ecology 11, 17–28 (1999).
16. Hartz, T. K. & Johnstone, P. R. HortTechnology 16, 39-42 (2006)
17. Dessureault-Rompré, J. et al. Geoderma 157, 97–108 (2010).
18. Thangarajan, R., et al. Environmental Science and Pollution Research 22, 8843–8854 (2015).
19. Conant, R.T., et al. Global Change Biology 17, 3392–3404 (2011).
20. Terman, L. Advances in Agronomy 31, 189–220 (1979).
21. Wester-Larsen, L., et al., Journal of Environmental Management 323, 116249 (2022).
22. Dai, Z. et al. Global Change Biology 26, 5267–5276 (2020).
23. Sims, J. T. Journal of Environmental Quality 15, 59–63 (1986).
24. Tisdale, S. L. Soil Fertility and Fertilizers (1993).
25. Chen, J. et al. Field Crops Research 307, 109240 (2024).
26. Norton, J. & Ouyang, Y. Front Microbiol 10, 1931 (2019).
27. Esteban, R., et al. Plant Science 248, 92–101 (2016).
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.
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.
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 anew 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
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.
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
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.
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.
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.
