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Pulling the Trigger for the Start of Irrigation in the Spring: Too Much Too Soon for Walnuts?

By
Ken Shackel | Plant Sciences Department, UC Davis, Allan Fulton | UCCE Soil and Water Advisor, Emeritus,, Kari Arnold | UCCE Orchard and Vineyard Advisor, Stanislaus County, Bruce Lampinen | UCCE Specialist, UC Davis and Luke Milliron | UCCE Orchard Advisor, Butte, Glenn and Tehama Counties
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Figure 1. An ailing tree at the Stanislaus site in 2018 showed signs of deterioration. Although the trunk was somewhat sunken at the soil line and necrosis was forming under the bark (center photo), samples were collected multiple times, but no Phytophthora spp. were isolated/found. This tree happened to be included in the delayed irrigation treatment and during the passing of three years appears to be recovering, specifically showing greater shade under the tree canopy at midday since the beginning of the trial in 2018 (photos by K. Arnold.)

Walnuts are generally regarded as very sensitive to water stress. Severe stress and defoliation can occur when irrigation is reduced in the summer or discontinued entirely for harvest. Since walnuts depend on stored soil moisture during this time, growers were historically advised to start irrigation early in the spring to save deep soil moisture ‘in the bank’ for use later in the season. However, research findings in a Red Bluff, Calif. walnut orchard have seriously challenged this conventional wisdom. In fact, trees that were given an early start of irrigation (late April) showed more water stress at harvest than trees that were given a delayed start of irrigation (late May/early June). Surprisingly, this occurred even though the delayed start trees received substantially less water (about 28 inches throughout the growing season) than the early start trees (about 38 inches). The Red Bluff orchard is on a deep silt-loam/fine sandy-loam soil. However, similar results are being found in one Stanislaus County orchard on heavier clay soil and one orchard in western Tehama County on stratified soils with gravelly subsoils and much lower water holding capacity.

Using the Right Tool
In many commercial orchards, in-season tree water stress is monitored by measuring midday stem water potential (SWP) using a pressure chamber (a.k.a. “pressure bomb,” see sacvalleyorchards.com/manuals). This same tool could be used, specifically, to decide when to start irrigation in the spring with the appropriate information on this subject. As a starting point, there is a reference level of SWP that is expected for a fully irrigated (non-stressed) walnut tree, which is called the “baseline” SWP. For more information about baseline SWP and how to obtain this value for a particular location, day and time, we suggest the following websites:

Baseline and advanced interpretation explained: sacvalleyorchards.com/manuals/stem-water-potential/using-baseline-swp-for-precise-interpretation/

Baseline values calculated for you at: informatics.plantsciences.ucdavis.edu/brooke_jacobs/index.php

Using the Tool to Trigger the Start of Irrigation
We began testing in 2014 in a nine-year-old commercial Chandler/Paradox orchard planted at 18 x 28 feet (86 trees per acre) on a deep, well-drained silt-loam/fine sandy-loam soil near Red Bluff. The test continued through 2019. The design of the experiment was simple: we compared control trees given 100% irrigation (see below) starting about 30 days after leaf-out to trees that were not irrigated until a trigger level of SWP was reached. We tested five trigger levels for the start of irrigation: a grower control (typically starting irrigation while the trees were still near baseline SWP), or 1, 2, 3 or 4 bars drier than baseline SWP.

We divided the field into 4 rows x 11 tree plots and had five individual plots for each trigger level. In total, the test consisted of 12.5 acres. Starting after leaf-out (about the third week of April), we measured the SWP of two middle trees in each plot every three or four days. When the average of those trees reached the trigger on two consecutive dates, we opened the sprinkler control valves to the tree rows in that plot. From then on, the plot was irrigated whenever the control plots and the rest of the orchard were irrigated.

Figure 2. Summary of average orchard water requirement (ET-rain) and applied irrigation for all delayed irrigation tests to date (2014-20). Daily CIMIS values for orchard water requirements were calculated beginning on April 1 based on current walnut crop coefficients, for each site and year, and averaged. Irrigation applied to all delayed treatments for each site and year were averaged for 10-day periods over the same seasons.

Initial Results in 2014
We expected that a 1- or 2-bar trigger might cause mild water stress with minimal effect on the trees, but the 3- or 4-bar triggers would show some detrimental effects. However, we were not sure how long of a delay would result from waiting to start irrigation using any of these trigger levels. We were also unsure if trees with late triggers would always be ‘behind’ in their water needs and experience severe water stress at harvest because we could not apply a ‘catch-up’ irrigation to any of the delayed trees.

In 2014, the 1-bar trigger occurred about the same time as the grower control, but much to our surprise, waiting for the 2-bar trigger gave one to two months of delay (depending on the plot), with the 3- and 4-bar triggers giving slightly longer delays (Table 1). Longer delays also resulted in less irrigation. In 2014, the control trees received 100% of calculated evapotranspiration (ET, see anrcatalog.ucanr.edu/ pdf/8533.pdf), whereas the 1- through 4-bar trees ranged from 89% to 66% of this value, respectively (Table 1).

Table 1. Irrigation start dates, seasonal irrigation applied (in inches and as the equivalent percent of irrigation requirement, calculated from ET minus in-season rainfall) and crop yield for each of the irrigation treatments imposed in the first year of the study (2014).

There were some negative effects on crop yield, with the 4-bar trigger reducing yield by about 10% (Table 1, see page 16), but there were also some positive signs. For instance, at harvest in October, the 2-, 3- and 4-bar triggers had a healthier canopy appearance than the controls. This matched our SWP measurements, which indicated that the delayed trees were less stressed than the controls (Table 2). This was the most surprising result from the first year of the study; during the delay period (May, June), the longer delays were associated with more stressed (more negative) SWP values as expected, with the controls being closest to the baseline. However, by harvest, the opposite was the case, with the controls being furthest from the baseline (Table 2).

Table 2. Average SWP measured in May and June 2014 when irrigation was being delayed in most of the treatments, and average SWP in October around harvest (October 17, 2014). Also shown are the baseline SWP values for the same time periods.

Trial results for 2015-18
Due to the overall improved appearance of trees in the delayed plots at harvest compared to the controls, the grower’s standard (control) irrigation start time in the entire orchard, including our control plots, was gradually delayed each year after 2014. Water applications in the orchard and the control plots became substantially less than 100% of the seasonal irrigation need (Table 3, see page 16). Yields also generally improved across treatments compared to 2014, even though canopy size as measured by midsummer ground shaded area has remained stable at 86%. Even with the changes over time that occurred in the control trees, delays associated with a 1- to 4-bar trigger showed small but consistent improvements in percent edible yield and relative value as well as substantial savings in water (Table 3). There were also indications of small but consistent increases in nut load. However, since nut load is determined by many factors, ongoing research in additional orchards is being conducted to determine if this effect is consistent.

Table 3. Average irrigation start date (and equivalent days after leaf-out), seasonal irrigation applied in inches (and equivalent percent of the seasonal irrigation requirement, as in Table 1), yield, percent edible yield, relative value and crop relative value (and equivalent percent of the control treatment.) Relative value is an index combining the two main economic drivers of walnut value (percent edible yield and kernel color), and crop relative value is Yield x Relative value.

Soil Moisture Storage and Possible Implication for Root Health
The soil in this location is a deep, well-drained silt-loam/fine sandy-loam, and soil moisture measurements have indicated that the trees in this orchard have access to at least 10 feet of stored soil moisture. In most years, rainfall is also sufficient to refill this soil profile. Hence, using the pressure chamber to determine when to start irrigating has enabled the grower to take maximum advantage of this soil moisture resource, potentially improving soil aeration and overall root health. This may be one of the reasons the delayed trees appeared healthier and less stressed around harvest compared to the controls. Answering this question with greater confidence will require more research focused on the root system.

Taking the Practice Beyond Red Bluff
It is also important to test the delayed irrigation approach on different soil types. Because this project was conducted in a relatively high rainfall area in the Sacramento Valley, extending these dramatic results to other areas within the state with differing rainfall and soils should be done with caution. We currently have two different trials underway to further test the merits of delaying the start of irrigation in walnut (a second site in Stanislaus County on heavier clay soil and a third trial in western Tehama County on stratified soils, with gravelly subsoils and much lower water holding capacity.) Both trials are a smaller-scale version of the Red Bluff trial.

In the Stanislaus County orchard consisting of Chandler on Vlach, results after three years suggest that similar benefits of delaying the first irrigation may be possible in this higher-clay-content soil site. Some ailing trees have shown partial recovery in the delay treatment, indicating the possibility of too much water being applied too early (Figure 1,see page 14). Yield at the Stanislaus site was not affected when irrigation was withheld until readings of 2 bars drier than baseline.

After two years, results from the western Tehama County test on soils with lower water holding capacity and soil layers that may restrict root depth suggest there may still be some benefit of delaying irrigation in terms of less tree stress at harvest, reduced water costs, and improved edible kernel. However, because of the lower water holding capacity of the soils, the delay may only be about one to two weeks with water savings of about 4 inches.

A key feature of using SWP to manage irrigation is that it provides growers with an orchard-specific measurement of tree water stress and hence allows them to safely take advantage of the existing soil moisture resource, regardless of soil depth, type and quantity of the stored soil moisture. Using SWP to delay the start of irrigation resulted in healthier-looking, less-water-stressed trees at harvest, challenging the conventional wisdom that an early start to irrigation is beneficial because it allows the saving of deep soil moisture ‘in the bank’ for use later in the season. Quite possibly, keeping this savings account too full in the spring may cause more problems than it solves.

The benefits of waiting to irrigate in spring until trees read 2 to 3 bars drier than the baseline despite the stark differences between these three sites is a powerful testament to the value of using the pressure chamber. Once growers use the pressure chamber to trigger the start of irrigation, they can continue to trigger irrigations throughout the season by waiting for SWP readings of 2 to 3 bars drier than the fully watered baseline.

Baseline and other information for interpreting SWP readings can be found at sacvalleyorchards.com/manuals/stem-water-potential/pressure-chamber-advanced-interpretation-in-walnut/.

These trials are also challenging the conventional wisdom that we must irrigate to keep up with ET to have healthy and high-yielding walnut orchards (Figure 2, see page 14). Stay tuned as these two new trials continue to add to our collection of experiences.

Research Updates on Integrated Pest Management for Citrus Mealybug in California Citrus

By
Sandipa Gautam, Ph.D., | Area Citrus IPM Advisor, Fresno, Madera, Tulare and Kern Counties
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Figure 1. Adult female citrus mealybug showing amber-colored eggs. Note the distinctly segmented body with a vertical line through the mid-section, a characteristic feature of citrus mealybug (photo by S. Gautam.) 2022-09-30T23:08:37Z

Citrus mealybug infestations continue to increase in the San Joaquin Valley (SJV) making this species an emerging concern for citrus growers. Although known to be present in California for more than 50 years, it was kept in check by natural enemies (Ebeling 1959) and was not a pest of concern until recently. As such, citrus mealybug is an understudied pest in California citrus systems, and little is known about its biology and field ecology, monitoring and threshold, and control options, making management challenging. In 2022, we initiated research studies to improve knowledge about field ecology, monitoring and management of mealybug in SJV citrus.

Citrus mealybug, Planococcus citri, is a soft-bodied, oval, flat and distinctly segmented insect covered with white mealy wax (Figure 1). This small polyphagous sap-sucking insect prefers to live in between fruit clusters or under the thick leaf canopy, therefore avoiding detection when population is low. Infestation causes chlorosis, leaf drop and stunted growth, while direct feeding damage results in tissue discoloration around feeding spots, fruit deformity and contamination of foliage and fruit with honeydew and sooty mold (Figure 2), resulting in loss of yield and marketability of fruit.

Figure 2. Navel orange infested with citrus mealybug. Infestations are usually found protected under the thick canopy of leaves. Also note the sooty mold on leaves (photo by David Haviland, UCCE.)

Adult female citrus mealybugs are ~3 millimeters long and have distinctly segmented bodies covered with white mealy wax. A female can lay up to 600 eggs in a lifetime, in a group of 5 to 20, deposited and protected within cottony ovisacs (Figure 1). These eggs hatch in 3 to 10 days depending on temperature and crawlers disperse to different parts of the plant looking for feeding sites. All life stages are mobile, and they spread by crawling from tree to tree, carried by ants, blown by wind and picked up by birds, machinery or labor crews. Immatures and adult females feed on leaves and fruits. Males have wings, are short-lived and do not feed.

Field Ecology for Monitoring
Citrus mealybug overwinters as eggs or crawlers in protected areas of the tree including fruit. Greenhouse research has shown eight complete generations in one year (Myers 1932). To generate information on seasonal activity and the number of generations in the SJV, we monitored citrus mealybug in 13 trees in a block known to have mealybug infestations in 2021. Double-sided sticky tapes were wrapped around the trunk and four main inner branches. Traps were replaced weekly, and the number of mealybugs caught on the traps was counted every week. In addition, males were monitored using a pheromone trap.

We found that mealybugs were present within the tree canopy throughout the season, but the areas where they aggregated changed as the season progressed. Overwintering eggs/crawlers serve as an inoculum for a new season. The first-generation activity starts in March as eggs hatch and crawlers move from the overwintering sites (cracks and crevices on trunk, leaves protected under thick canopy, fruit or from the ground) to new leaves and then to fruit. When fruit is present, most of the infestation was found on fruit. Males caught on the trap card showed six distinct peaks that preceded the crawler emergence, suggesting six complete generations of mealybug in the valley (Figure 3). This seasonal occurrence pattern can be used to scout for mealybug.

Figure 3. Citrus mealybug population in a citrus block. Adult and crawlers on sticky tape trap count (left Y axis) overlayed on mealybug males/trap (right Y axis). Flight peaks at the arrowheads suggests a new generation.

January to March: fruits or leaves. Mealybugs like protected areas. Check inside the tree canopy, fruit (navels and under the calyces) or leaves (underside) for overwintering adults, egg sacs and crawlers. Presence of sooty mold and white mealy wax is an indicator (Figure 2).

April to June: crawlers emerge and start moving into new flush or fruit. Egg masses, crawlers on the trunk and inner branches.

July to December: on fruit, feed and multiply. Overlapping generations can be present.
Monitoring can also be done by using a pheromone lure and traps. Place lure and traps by March 15, before the first-generation activity starts and monitor throughout the season. If you catch males on trap, scout trees for infestation and make pesticide applications when needed.

Management Options
Citrus mealybug is naturally regulated by natural enemies and predaceous insects. But recent outbreaks suggest that this ecological balance is shifting. Although very little is known about mealybug management in citrus systems, incorporating cultural, biological and chemical tools is encouraged as researchers continue to study the seasonal occurrence, efficacy and timing of pesticide applications for improved management.

Cultural control
Female and nymphal mealybugs are wingless and spread to new areas by humans, equipment, wind or birds. Sanitize equipment before moving to new areas of the grove. Pruning to open tree canopy and hedging trees to prevent canopy overlap will help slow the crawler movement and increase natural enemy activities.

Biological control
Several natural enemies are identified as effective biocontrol agents for controlling mealybug (Grafton-Cardwell et al. 2021).

Mealybug destroyer, Cryptolemus montrouzieri, is known to be a highly effective predator and is most effective when population pressure is high. This predator does not survive cold winters. It is readily available from commercial suppliers and can be released in orchards where mealybug was a problem in the previous year.

Anagyrus spp. and Coccidoxenoides have been found in mealybugs collected from the SJV, suggesting natural infestations. Future research will investigate presence of natural enemies and their impact on mealybug.

Chemical control
Although UC IPM guidelines have only one product recommended for mealybug control in citrus systems, several insecticides are recommended for controlling mealybug in grapes, pistachios and greenhouses (Haviland et al. 2019). Different species of mealybug may respond differently to insecticide applications. Moreover, different seasonal phenology of pests in separate cropping systems warrants crop-specific pesticide efficacy trials. Two field trials conducted in summer 2022 showed that one application significantly reduced mealybug infestation but did not provide long-term solution (Gautam 2022). If your block was infested, plan on an early season control, targeting the first-generation crawlers in late March/early April. Monitor populations and make a second application using a different group of insecticides within two weeks of the first application, targeting the newly hatched crawlers. Continue monitoring and use mealybug-effective materials.

Pheromone lure is not available as a management choice but is available for monitoring.

Other Factors
Natural enemies are relatively more susceptible to broad-spectrum insecticides such as organophosphates, carbamates, pyrethroids and neonicotinoids in groves with mealybug infestations, so reduce/limit the use of these chemicals. Spirotetramat is relatively non-toxic to mealybug destroyers (Grafton-Cardwell 2021).

Ant control: ants help in mealybug dispersion within the tree canopy and between trees and groves. They also defend mealybug colonies from natural enemies (Figure 4). Use effective ant control methods.

Figure 4. Citrus mealybug infestation in mandarin. Note the colonies at the cluster, bleached feeding spots and deformed fruit surface. Also note ants attending mealybugs (photo by S. Gautam.)

Prevent the spread of mealybug by informing the pruning/harvesting crew.
Strip and dispose of the fruit infested with mealybug as fruits may harbor mealybug for next season’s infestation.

Talk to neighboring growers about this threat.

The University of California and Fresno State University researchers are working on a citrus research board-funded project to improve knowledge on biology, seasonal phenology and management of citrus mealybug. Stay informed on research outcomes by participating in grower seminars, newsletter publications and UC IPM guidelines updates.

References
Ebeling, W. 1959. Subtropical fruit pests: Biology and control of citrus pests. Pp. 135-229. University of California, Division of Agricultural Sciences
Gautam, S.G., 2023. Core IPM Research Update: Working toward citrus mealybug IPM. Citrograph, 14: 32-36.
Grafton-Cardwell E.E., Baldwin R.A., Becker J.O., Eskalen A., Lovatt C.J., Rios S., Adaskaveg J.E., Faber B.A., Haviland D.R., Hembree K.J., Morse J.G., Westerdahl B.B. 2021. Revised continuously. UC IPM Pest Management Guidelines: Citrus. UC ANR Publication 3441. Davis, CA.
Haviland D.R., Bettiga L.J., Varela L.G., Baldwin R.A., Roncoroni J.A., Smith R.J., Westerdahl B.B., Bentley W.J., Daane K.M., Ferris H., Gubler W.D., Hembree K.J., Ingels C.A., Wunderlich L.R., Zalom F.G., Zasada I. 2019. Revised continuously. UC IPM Pest Management Guidelines: Grape. UC ANR Publication 3448. Davis, CA.
Myers, L.E. Two Economic Greenhouse Mealybugs of Mississippi, Journal of Economic Entomology, Volume 25, Issue 4, 1 August 1932, Pages 891–896, doi.org/10.1093/jee/25.4.891

Sensor-Controlled Sprayers for Specialty Crop Production

By
Brent Warneke | Faculty Research Assistant, Department of Horticulture, Oregon State University
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Figure 1. A classic axial fan air-blast sprayer (photo by B. Warneke.)

Many of the pesticide application technologies used in specialty crop production today are based on axial fan air-blast sprayers (Figure 1). Air-blast sprayers were first developed in the 1950s when orchard trees were 20 feet tall or more; today orchard trees of many crops are 7 to 13 feet tall. Air-blast sprayers are versatile and reliable and can be modified to fit numerous types of crops. Despite their popularity, standard axial fan air-blast sprayers have long had a reputation for inefficient application characteristics. For example, spray landing on the ground from radial air-blast sprayers can be from 20% to 40% of total applied spray volume. To improve application efficiency, sensor-controlled spray systems were designed in the 1980s as a way to reduce labor costs and pesticide waste. Sensor-controlled spray systems are receiving renewed interest as their reliability has improved and more options have become available.

Sensor Sprayer Types
There are two main types of sensor sprayers:

  • On/off sensor sprayers operate by automatically turning on individual nozzles or sections of nozzles on the spray boom when plants are sensed. Likewise, when no object is sensed, the spray will be turned off (Figure 2a).
  • Crop-adapting sprayers are similar in that the sensor is used to turn the sprayer on and off as it passes by plants. However, they go a step further by adjusting application volume, air volume or a combination of those two (Figure 2b).

Sensor controlled sprayers typically have an override where the user can bypass the sensor system and spray as they would with a standard sprayer if, for example, the sensor components of the sprayer are not working.

Figure 2. Illustration of on/off sensor sprayers (a) and canopy-adapting sensor sprayers (b). Sensors are illustrated with small red ovals and sensor field of view illustrated with grey shaded areas (illustration by B. Warneke.)

Sensors
Crop sensing systems are the “eyes” of the sprayer and determine crop shape by emitting and receiving signals. There are four basic sensor types used in sensor sprayers: infrared, ultrasonic, plant fluorescence and LiDAR. Each one of these sensors emit their own specific signal aimed at the plants that then bounce off the plants and return to the sensor. Some sensor types require multiple sensors to resolve plant structure characteristics.

Infrared (IR) sensors detect IR signals reflected from the plants and can be used in on/off sensor sprayers. Humidity and temperature do not interfere with IR sensing accuracy. However, low light conditions such as during dawn and dusk can interfere with an IR sensor. IR sensors are unable to accurately resolve plant structure. IR sensors are usually used in canopy spraying to trigger the release of spray from the whole side of a boom when a plant canopy is detected. These systems can also be used for herbicide sprayers. In that case, sensors are aimed at the plant trunks and turn off the sprayer as they pass the trunk, or they can trigger spray directly at the trunk to specifically target suckers.

Ultrasonic sensors emit high-frequency sound waves to measure objects. This process is similar to how bats and dolphins use echolocation to navigate and search for food. When arranged in an array (usually three sensors per side of the sprayer), ultrasonic sensors can detect objects with approximately 4-inch resolution. This allows for calculation of canopy volume with similar accuracy to taking manual measurements. Despite their ability to resolve plant structures, ultrasonic sensors are usually used in on/off sensor sprayers. The initial patents on ultrasonic sensors have expired, so continued off-patent development has improved their quality and capability while reducing costs. Comparatively, ultrasonic sensors are more expensive than IR sensors but less expensive than laser sensors.

Plant fluorescence sensors emit a beam of visible light and detect the light reflected back to the sensor from the plant. These sensors have a spatial resolution of about 4 square inches and can detect a plant area as small as 0.40 square inches. Plant fluorescence sensors can collect plant structure data while in the field that can be stored and used for further planning. These sensors are typically used on “weed-seeing” herbicide sprayers to detect green weed tissue contrasted with the soil surface. They have also been integrated into on/off sensor-controlled canopy sprayers to trigger the spray when a green canopy is sensed.

Laser sensors used to characterize plants are termed LiDAR (light detection and ranging) sensors. These sensors have a laser beam that is directed in an arc around the sensor by a spinning mirror inside the sensor housing. The laser beam is then reflected by any surface it hits, relaying distance and structure information to the sensor. Compared to other sensors, LiDAR most accurately measures crop structure, with resolution to several hundreths of an inch. This gives LiDAR sensors the capability of being used for other tasks while spraying, such as measuring canopy vigor. LiDAR sensors are typically used on crop-adapting sensor sprayers, where only one sensor is needed for each sprayer. These sensors are the most expensive of those listed.

One other sensor is needed for sensor-controlled canopy sprayers. Currently, retrofitted sensor spraying systems are not directly connected to the speedometer on the tractor, so a separate speed sensor is needed to convey the sprayer ground speed to the sensor system. A few examples of speed sensor types used are wheel bolt sensors and radar sensors that use Doppler technology. Maintaining accurate speed detection is critical to ensure spray is released on target.

Sensor Sprayer Efficacy and Efficiency
Insect and disease control with sensor-controlled sprayers has been shown to be similar to that of standard sprayers. Using a LiDAR-based variable rate sprayer has resulted in equivalent control of insect pests, such as codling moth, oriental fruit moth and spotted wing drosophila, to that of an air-blast sprayer. Diseases such as powdery mildew, apple scab, brown rot, anthracnose and mummy berry have also been controlled equal to that of an air-blast sprayer. Ultrasonic sensor-controlled systems have resulted in similar control to standard sprayers on difficult-to-control pests such as apple rust mite and pear psylla and diseases such as apple scab and apple powdery mildew. The amount of spray saved when using sensor sprayers varies, but generally, spray volume savings in the range of 20% to 70% can be seen depending on the sensor system used and the crop. Higher savings typically correspond to more precise sensors such as LiDAR in crops with more variable canopies, while lower-volume savings occur with less precise sensors and less variable crops.

There are a host of other benefits of sensor sprayers beyond spray volume savings. Each spray tank goes further so less time is spent spraying, decreasing the need to refill the sprayer and thus saving on labor. A reduction in the number of sprayer fill-ups required leads to less fuel costs and water savings in addition to a lower amount of wear on the tractor. When spray operations can be completed quicker, it can also make it easier to fit sprays into critical application windows.

Environmental Benefits of Using Sensor Sprayers
Spray drift is greatly reduced with sensor-controlled sprayers. Some studies have shown 23% to 45% of the applied pesticide volume can drift off target using standard air-blast sprayers in orchards. Ultrasonic sensor systems can reduce ground deposition by 70% compared to a standard air-blast sprayer in orchards. Canopy-adapting sprayers can be even more effective at reducing drift in orchards, in one study by 70% to 100% compared to a standard air-blast sprayer depending on the tree growth stage. Lower chemical load on non-target locations also helps decrease the rate of development of pesticide resistance. Other considerations include less pesticide contamination of surface and groundwater and lower chances of exposure on non-target organisms such as beneficial insect populations and livestock.

One Step Further: Autonomous Sensor-Controlled Air-Blast Sprayers
Sensor-controlled sprayers help increase spray application efficiency, but there still needs to be an educated applicator driving the tractor and operating the sprayer. Agricultural labor has become less reliable and more expensive, so some manufacturers are producing autonomous sprayers that can be monitored remotely during a spray process. Global Unmanned Spray Systems (GUSS), Jacto and Hol Spraying Systems, in partnership with AgXeed (Figure 3, see page 6), are three manufacturers that are making fully autonomous self-propelled sprayers for specialty crop production. All three manufacturers are integrating sensors onto the autonomous sprayers to do variable rate spraying. Autonomous sprayers offer lower labor requirements and more efficiency than manned sprayer applications and represent the future of spraying in specialty crops.

Figure 3. The Hol Spraying Systems/AgXeed autonomous sprayer and robot. This unit is equipped with plant fluorescence sensors (photo by Amanda Dooney.)

This work was provided in part by the USDA-ARS Integration of Intelligent Spray Technology into IPM Programs in Specialty Crop Production (Project Number 58-5082-2-010).

Resources
Warneke, B. W., Zhu, H., Nackley, L. L., Pscheidt, J. W.(2020) ‘Canopy spray application technology in specialty crops: a slowly evolving landscape’, Pest Management Science, 77(5), pp. 2157–2164. doi: 10.1002/ps.6167.
Nackley, L. L., Warneke, B., Fessler, L., Pscheidt, J. W., Lockwood, D., Wright, W. C., Sun, X., Fulcher, A. (2021) ‘Variable-rate Spray Technology Optimizes Pesticide Application by Adjusting for Seasonal Shifts in Deciduous Perennial Crops’, HortTechnology, 31(4), pp. 479–489. doi: 10.21273/horttech04794-21.
Giles, D. K., Klassen, P., Niederholzer, F., Downey, D. (2011) ‘“Smart” sprayer technology provides environmental and economic benefits in California orchards’, California Agriculture, 65(2), pp. 85–89. doi: 10.3733/ca.v065n02p85.
Chen, L., Wallhead, M., Reding, M., Horst, L., Zhu, H. (2020) ‘Control of Insect Pests and Diseases in an Ohio Fruit Farm with a Laser-guided Intelligent Sprayer’, HortTechnology, 30(2), pp. 168–175. doi: 10.21273/horttech04497-19.

Unwrapping the Possibility of Watermelon Grafting

By
Zheng Wang
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Figure 1. A grafted watermelon plant is a recombined physical hybrid from two different plants. The lower part of a grafted plant comes from a rootstock seedling, which was bred for superior root traits, while the upper part is the regular commercial watermelon plant for producing fruit.

You will probably not surprise to see the grafting of fruit and nut tree crops and well understand the needs to do so. But when I am going to discuss watermelon grafting here, I am sure many of you may pause and ask, “Can watermelon graft? What is that? Why graft watermelons? Is it still sweet after grafting? Is it still watermelon?” 

Why Graft Watermelons?

The main purpose of grafting watermelons is to deliver plant traits to farms much faster than the traditional breeding programs. It is NOT designed to replace breeding and other well-documented techniques of developing new cultivars; rather, it offers growers and consultants a quicker and unique way to solve production problems. In contrast to the traditional breeding system, a grafted watermelon seedling comes from the recombination of two different plant species into one physical (not genetic) hybrid (Figure 1). The phenotypic traits that are brought to farms after grafting include soilborne disease resistance, abiotic stress tolerance, greater nutrient and water use efficiency, higher fruit yield and better fruit quality. 

Do I Need Grafted Watermelons?

This is a question you should always ask before making a decision. The following are some of the benefits from using grafted watermelons. If you have the related issues but with limited solutions, you may consider grafting as your “icebreaker”. Again, please be mindful that these benefits may not be observed at each farm. Using grafting must be proceeded in a case-by-case protocol.

Grafted watermelon can have a stronger resistance to soilborne diseases (e.g., Fusarium wilt, Fusarium crown and root rot, Verticillium wilt, etc.)

Grafted watermelon can produce a higher fruit yield under soilborne pathogenic pressure than non-grafted plants.

Grafted watermelon can produce a higher fruit yield under low-disease or disease-free conditions. 

Grafted watermelon can use water and nutrients differently compared to non-grafted plants. 

Grafted watermelon may have better fruit quality than non-grafted plants.  

Where to Find Rootstock Information and How to Graft?  

A majority of the rootstock varieties can be found at vegetablegrafting.org/resources/rootstock-tables/cucurbit-rootstocks/. Please note that 1.) not all cucurbit rootstocks can be grafted onto watermelons; 2.) information about the disease resistance of each rootstock was collected from the seed company; and 3.) the rootstock table may be updated as new or old rootstocks emerge or disappear. In addition to the table, you can also seek help from rootstock seed suppliers, nurseries that work on grafting watermelons, extension advisors and your trusted growers. For information about grafting nurseries, feel free to give me a call or email me (209-525-6822, zzwwang@ucanr.edu) and I will give you a list. If you need to know the grafting methods, check out the grafting manual at vegetablegrafting.org/resources/grafting-manual/. 

Possible Challenges and Pitfalls

Using grafted watermelons can be challenging. Such challenges can be serious under some, though rare, circumstances. 

Cost

In the past, growers only dealt with scion, but now they must think about rootstocks. Paying extra money for the rootstock seeds and producing grafted transplants will definitely increase the production cost. Currently, the most effective way in balancing the cost is to plant grafted watermelons in a wider spacing while still managing them for a higher fruit yield. Nowadays in California, grafted watermelons should be planted in a 4- to 5-foot in-row spacing, which is equivalent to 1,200 to 1,500 plants per acre compared to 2,200 plants per acre for the non-grafted plants. 

Extended vegetative growth and delayed harvest

Due to their stronger rootability and growth vigor, grafted watermelons usually produce more and larger canopies than regular plants, possibly resulting in delayed fruit maturity. Also, the typical exterior changes that indicate fruit maturity (e.g., a dried tendril and leaflet) on non-grafted watermelons may not be true for grafted plants. An early harvest that causes a lower sugar content can occur frequently in grafted fields. From our past observations as well as research from other areas, a possible 7- to 10-day delay from non-grafted plants could be reasonable for harvesting grafted watermelons.

Figure 2. Cumulative fruit yield for the two grafted watermelon combinations (RS1 and RS2) compared to the non-grafted scion (SC). Note the similar yields in the firsAt harvest.

 

Yield advantage at later harvests 

If you don’t see a higher yield at your first harvest or even a lower yield, be patient and the longer harvest window of grafted plants will give you the subsequent melons. Figures 2 and 3 were from my previous trials in 2021 and 2022 and indicated the yield superiority in the subsequent harvests for grafted plants (Figure 2) and the duration for grafted plants to maintain canopy compared to non-grafted plants (Figure 3). 

Figure 3. Canopy coverage for watermelons grafted onto rootstocks Cobalt, Flexifort and RS841 compared to the non-grafted watermelons (NonG). Note: the field was transplanted on May 17, 2022. The first harvest was made on August 9, 2022.

 

Different responses to irrigation and fertilization 

With the changes of growth characteristics, you may need to consider adjusting your irrigation and fertilization to meet the needs of grafted watermelons. Figure 4 was also from a field trial in 2022 and showed the different canopy regrowth after supplying with irrigation after each harvest between grafted and non-grafted watermelons. It is obvious to see that the canopy of non-grafted watermelons was consistently lower than the grafted plants since the start of fruit harvest on mid-July 2022 (Figure 4). In the meantime, grafted watermelons may take up nitrogen differently from the non-grafted watermelons as well. Figure 5 was a trial I conducted in 2021 which demonstrated the different patterns of nitrogen uptake between grafted and non-grafted watermelons, especially at the last part of the growth cycle.

Figure 4. Seasonal canopy coverage between grafted watermelons grafted onto Camelforce and Cobalt rootstocks and the nongrafted control. Note the fluctuation of canopy regrowth curves due to continuous irrigation and fertilization after each harvest since mid-July 2022.

 

Scion-rootstock incompatibility and change of fruit exterior and interior quality 

This is a complex question. In many cases, you will not be able to detect the incompatibility until the plants are transplanted or even when fruits are harvested and cut open. Symptoms of incompatibility early in the season could have 1.) poor early growth compared to other combinations and the non-grafted control; 2.) death after a few days of transplanting; 3.) incomplete removal of rootstock shoot part at grafting (Figure 6); and 4.) deformed plants. Of course, some of the symptoms may look similar to pathogen infestations. Symptoms of grafting incompatibility at a later stage of the growth cycle usually include a much lower final yield compared to others and a significant reduction in fruit quality (e.g., smell, flesh color, sweetness and hollow heart). For fruit quality, most noticeable changes after grafting that you may see or taste include thicker rinds, bigger fruit and firmer flesh. Therefore, preparing accordingly to meet your needs and your customer’s preference is important.

Figure 5. Cumulative N uptake for the two rootstock-scion combinations (RS1 and RS2) and the non-grafted control (SC). Note the timing of the first (93 DAT), second (104 DAT), third (114 DAT) and fourth harvests (166 DAT, unmarked).

Figure 6. Rootstock shoot protruded from the graft union, indicating an incomplete removal of rootstock shoot at grafting.

The Five Rs of Nutrition in the Vineyard

By
Sean Jacobs | Senior Agronomist, Technical Sales and Marketing Representative, Agro-K
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Maximizing nutrient uptake and energy conversion of vines begins at budbreak and leads to larger, higher-quality crops (all photos by S. Jacobs.)

Whether your grapes are destined for wine, raisins, juice or the table, the job of the grapevine is to capture energy from the sun and use it to convert CO2 and H2O to carbohydrates and O2. Maximizing this process begins at budbreak and leads to larger, higher-quality crops. To have the greatest influence, a wholistic understanding of plant nutrition and crop production need to be engaged to ensure that every dime of fertilizer applied best serves its intended purpose. This is where the Five Rs of Plant Nutrition enters the decision-making process. 

By combining knowledge from plant physiology, soil science, microbiology and chemistry, core principles of plant nutrition arise that affect fertilizer use efficiency, yield and quality. The Five Rs provide this science-driven approach in a memorable way that helps to confirm or guide nutritional decisions. A series of checks, the Five Rs ensure that the applied nutrients get into the plant when and where they are needed with minimal unintended nutrient interactions or losses. When framed as a question, the Five Rs can be stated as:

For my application, is this the:

  • Right Nutrient?
  • Right Time/Crop Stage?
  • Right Form?
  • Right Nutrient Mix?
  • Right Place in the Plant?

The order of the Five Rs is easily re-organized to fit a given scenario, and when focusing on a specific growth period, considering the right time/crop stage first has the most value.

A combination of both foliar and soil-based applications for most nutrients will be necessary to meet the quantities and timing required for high-level production.

Right Time/Crop Stage

Early spring is a critical period for grapevine growth and development as the foundational support components of the plant and crop (xylem, phloem, initial leaves and new roots) are created. This timing is also very challenging because the ability of the plant to obtain and move the needed nutrients from the soil is hindered by low soil temperature and low atmospheric evaporative draw.

Cold soils inhibit microbial growth and function, preventing the cycling of nutrients from plant-unavailable to plant-available forms. Similarly, mineral solubility in the soil-water solution is also reduced as soil temperatures decline. 

Evapotranspiration is the primary mechanism for the movement of nutrients between the soil and the plant. When air temperatures decrease, so does the amount of moisture a given volume of air can hold as does the rate that the air can cause evaporation. Coupled with the small surface area of new growth, very low levels of evapotranspiration result. 

The classical thought that soil nutrient stores alone can support optimal plant growth and development early on is brought into doubt, and other fertility decisions need to be considered. Other non-evapotranspirational mechanisms exist within the plant to mobilize and move stored nutrients but should be viewed purely as supplemental.

Plant nutrient demands fluctuate over the season, and nutrient-to-nutrient ratios shift subsequently.

Right Place in the Plant

Just as important as what nutrient to apply and when to apply it is identifying the most efficient or appropriate place in the plant for the application. As addressed above, early spring conditions result in the soil being a poor nutrient source with low delivery efficiency. Yet, nearly all nutrients found in or used by the plant over a growing season were sourced from the soil via the roots. So then why is “Right place in the plant” a core principle? When nutrient demand timing and nutrient delivery limitations meet, the end goal of maximizing economic yield focuses less on ‘where is the nutrient found?’ and more on ‘where are the nutrients needed and what is the best way to deliver them?’ Hint: The answer isn’t always via soil application.

For most nutrients, what is found in or applied to the soil meets the volume requirements of the plant, but often soil conditions during the demand period or interactions with other nutrients limit their availability. Even in soils with perfect nutrient conditions, periods still exist where only foliar nutrient applications can meet the limited window of nutrient demand. Post-budbreak is such a time and pre-bloom, bud differentiation, set and berry development, post-veraison and postharvest periods all see benefits from timely foliar applications. During these periods, foliar nutrient applications can achieve results that the soil cannot deliver. And to go a step further, foliarly applied nutrients can uniquely alter nutrient ratios or balances within the plant in ways that cannot be achieved economically, or potentially at all, through the soil. 

However, just as the soil cannot deliver all nutrients at the right time, the volume of macronutrients needed cannot be delivered solely through foliar feeding. In the end, some combination of both foliar and soil-based applications for most nutrients will be necessary to meet the quantities and timing required for high-level production.

Right Form(ulation)

The soil and leaf environments to which nutrients are applied are extremely harsh, albeit in almost opposite ways. This article will not go into specifics, but generally, foliar applications dry quickly, are exposed to comparatively high oxygen concentrations and wide-ranging temperatures, and are bombarded with solar radiation. The soil, on the other hand, is very chemically and physically active at the molecular scale and is teaming with life that needs many of the nutrients for its life cycle that our plants require.

Nutrient products are available as various compounds that can be organized roughly into four formulation groups: insoluble salts, soluble salts, chelates and complexes. Ideal use scenarios can generally be defined for each formulation group, and nutrient stability and uptake performance rely heavily upon where (right place) they are applied. 

The main points to understand are 1.) there is no “magic bullet” formulation possessing very high performance as a nutrient delivery vehicle in both soil and foliar applications; and 2.) something else is always competing with the plant to acquire applied nutrients or acting against the formulation of the nutrient, decreasing its availability to the plant. 

Insoluble salts: carbonates, oxides, hydroxides 

Nearly or completely insoluble in aqueous solutions but micronize well. Foliarly, they coat tissues like paint and are highly effective reflectors/blockers of sunlight. As such, they are often used to prevent sunburn or sunscald of plant tissues. Foliar nutrient delivery performance is very poor. Slow conversion to plant-available forms in the soil results in poor performance.

Soluble salts: sulfates, nitrates, acetates, chlorides 

Soil applications usually perform very poorly, but low cost can offset the inefficiency. Foliar performance is poor as uptake is slow, and excess accumulation of the companion anions (SO4, NO3, C2H3O2, Cl) elicit stress responses in the plant or are otherwise problematic.

Chelates: EDTA, EDDHA, EDDHSA, citric acid, amino acids 

Synthetic, EDTA-type and similar are generally large, highly water-soluble materials that perform exceptionally well in soil applications. They are toxic to plants and soil organisms, however, and can solubilize heavy metals in the soil, causing accumulation in plant tissues. They are not great foliars.

Conversely, citric and amino acid based chelates perform well when applied foliarly and are less toxic. Stability is an issue in soil applications compared to EDTA-type chelates. 

Complexes: dextrose-lactose, mannitol, glucoheptonate, lignosulfonate 

Poor performers in soil applications, these naturally derived materials make average to exceptional foliar delivery vehicles. Molecular weight and size of the complex affects performance. The complexing compounds of some offer carbon skeletons that are easily assimilated by the plant once the nutrient is removed. 

Right Nutrient/Nutrient Mix

While not always grouped together, the right nutrient and the right nutrient mix are closely related. Plant nutrient demands fluctuate over the season, and nutrient-to-nutrient ratios shift subsequently. Nutrient-to-nutrient inhibitions, synergies, antagonisms and stimulations exist and must be accounted for as not all nutrients work well together in the plant at the same time.

In our budbreak to pre-bloom vineyard, demand for all nutrients except potassium are high, and calcium and phosphorus are both needed early on. But Ca and P antagonize each other, decreasing application and assimilation efficiency. So, which do we apply? If we assess the right time and right place components, foliar application of Ca makes sense since it moves exclusively with the transpiration stream and as is needed in the leaves to initiate cell division and develop cell walls. P on the other hand provides the energy needed for cell division and other growth functions to occur. Under ideal circumstances, we could apply P foliarly today and Ca foliarly in three to seven days and see the greatest benefit. Practicality doesn’t often allow for this type of application situation, and in our attempts to reduce the number of passes through the field, we must apply both simultaneously.

Tie-ups and antagonisms in real-world agriculture are inevitable and will occur in the spray tank and in the plant. But synergies and gains in efficiency and yield will occur from a little time, effort and application of the Five Rs into your nutritional program.

The Agronomy of Water

By
Karl Wyant
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A water sample can help quantify your risk of negative fertilizer interactions, which can help you choose the right kinds of formulations based on your on-farm water quality (photo by Vicky Boyd.)

Water ties all aspects of crop production together and is the largest input on most farm operations on a per-acre basis (both by volume and weight.) Aside from recent challenges with water quantity, we are also experiencing issues with quality, or the chemical and physical properties that characterize a water source.  In this article, I will describe how water quality is important to five categories of farm management: nitrogen management plans, fertilizer performance, predicting salinity issues, irrigation efficiency and distribution uniformity, and interactions with crop protection products (Figure 1). 

Given the long list of interactions that farm water quality can impact, I would like to make the case for the explicit consideration of what I call “the agronomy of water”, in which we are managing this input similarly to how we take soil and plant tissue tests. After reading this, my hope is to convince you of the utility derived from water sample data to help manage this important natural resource. Remember, a water sample can help inform the management strategy of the five aforementioned categories, often in the same analysis. Thus, a water sample can allow you to better manage five aspects of crop production for the price of one analysis. 

Figure 1. Crop yield is strongly influenced by the agronomy of water and the quality of applied irrigation and sprays. Water quality can be managed to influence several different areas of crop protection, as shown above.

 

Nitrogen Management Plans

All water sources contain dissolved ions in various quantities. Most often, we think of the ions that cause salinity issues and toxicities, which include sodium and chloride. However, due to recent changes in regulatory policy, we now must think about the nutrient content of water applied to field to help mitigate groundwater pollution risk. In this case, nitrate (NO3–) values can be measured in your irrigation water, converted to lbs/acre, and applied against your annual crop nitrogen budget. The benefit of a water quality analysis is that you can keep track of other important nutrients, which can help inform your overall management program. A water sample can help estimate the presence of nitrates in your irrigation water and contribute to an improved understanding regarding in-field nitrogen management compliance. 

Predictive Salinity Management 

Soil salinity is often caused by the application of irrigation water that is high in dissolved salts, particularly sodium, chlorine and boron. These ions can cause a loss in crop yield due to specific ion toxicities (e.g., boron toxicity) and, in the case of sodium, can drive a loss of soil structure, impairing drainage and impeding water penetration and infiltration. A water sample can help quantify your risk of salt damage to your fields and crops and help you formulate a proactive reclamation plan. 

Negative Fertilizer Interactions 

Ok, so you have the water, now you just apply it to the crop, right? Not so fast! Ions in the water can interact with each other and form solid precipitates (chalky or cottage cheese like substances). For example, water that is high in calcium and/or has a high pH can negatively interact with the phosphate in your fertilizer program. When calcium and phosphate come together, they form a mineral called apatite that is highly insoluble. This can have a severe impact on the liquid blends you are applying to your crops. I have seen cases where one ranch has tremendous success running a certain blend but the ranch down the road cannot use the same formulation due to interactions with local water quality. Along the same lines, when phosphate and calcium form precipitates, the nutrients are no longer available for your crop to drive yield, which is a waste of input dollars. A water sample can help quantify your risk of negative fertilizer interactions, which can help you choose the right kinds of formulations based on your on-farm water quality. 

Irrigation Efficiency and Distribution Uniformity 

Continuing with the clogging theme, fertilizer precipitates, and other minerals, can also lodge themselves in drip emitters, sprinkler heads and other types of nozzles. Once these particles clog your system, you lose irrigation system efficiency. When your irrigation system is clogged, you now must use more water and power to deliver the same irrigation set to your thirsty crops than you would if your system had higher efficiency. Furthermore, the clogging of lines can impact some parts of the field more than others, causing an issue with the distribution uniformity of your applied water. A water sample can help you understand drip system clogging risk and formulate a plan to deal with it. Further fieldwork is required to quantify changes in distribution uniformity across a field, but it can be beneficial for improving overall irrigation efficiency.

Ag Chem Efficacy 

Now let’s turn to your crop protection program. An essential question to ask here is, “How good is your spray water quality?” The answer demonstrates the connection between your ag chem programs (e.g., pesticides, herbicides and fungicides) and the most common mechanism for conveyance (e.g., spray water). However, the constituents of the water can have severe impacts on the pest control capacity of your ag chem spray program (e.g., efficacy). A pesticide spray that controls a pest to a high degree per application has high efficacy. An ag chem spray that doesn’t quite do the job has low efficacy. Efficacy is strongly influenced by the interaction between the active ingredient in your applied pesticide and the water it is mixed with (e.g., spray tank, chemigation, etc.)  

Five main physical components of water can have a negative influence on efficacy: pH, bicarbonates, hardness, total dissolved solids and turbidity. For example, spray water that is characterized by alkaline pH (>7) can cause an issue called alkaline hydrolysis, a scenario where the pH causes the active ingredient in the crop protection product to lose its efficacy due to physical and chemical deterioration. In another example, certain ions in the spray water, called hardness, can tie up the active ingredient in your ag chem and render it unusable for pest control.    

Some pesticides are more strongly influenced by these components than others, and management programs should work to improve efficacy on a case-by-case basis. A water sample can help you determine how your ag chem sprays interact with water quality and put together a water conditioning and adjuvant plan to improve the overall activity and control of your pesticide programs. 

Take a Water Sample

The importance of understanding your water quality cannot be understated, and a water sampling program will help you form a solid foundation of “water” agronomy and will produce tangible benefits for your farm. Ironically, while water is often the largest farm input by volume and weight, a water sample can be one of the cheapest inputs (e.g., $/acre) around as one water source can serve many acres on a given ranch. However, many folks do not have a regular water sampling program in place to monitor changes in water quality. Talk to a Certified Crop Advisor today about starting a water sampling program and to improve the agronomy of water on your farm operation across the five categories described above. 

Resources

Irrigation Water Salinity and Crop Production, anrcatalog.ucanr.edu/pdf/8066.pdf 

The Impact of Water Quality on Pesticide Performance, extension.purdue.edu/extmedia/ppp/ppp-86.pdf 

Water Quality for Crop Production, ag.umass.edu/sites/ag.umass.edu/files/book/pdf/ghbmpwaterqualityforcropprod.pdf 

Agronomy of Water FieldLink (originally created by author), helenaagri.com/fieldlink/the-agronomy-of-water/

The Agronomy of Water Series (2 CCA credits available: Soil and Water), wrcca.org/continuinged

Five Tools For The Price Of One: The Case For Farm Water Testing https://nutrien-ekonomics.com/news/five-tools-for-the-price-of-one-the-case-for-farm-water-testing/

Adapting Solutions: A Novel Product for Soil Salinity Management

By
Raquel Gomez | M.Sc., Research Agronomist, AgroPlantae and Davide Ciceri | Ph.D., Senior R&D Scientist, AgroPlantae
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Figure 1. Almond (left) and pistachio (right) foliage yellowing and leaf margins with symptoms of salt toxicity (photos courtesy R. Gomez).

It is estimated that approximately 78 million acres in the western San Joaquin Valley are affected by soil salinity, with 30% of that acreage categorized as strongly or extremely saline (Scudiero et al. 2017). In soil, salinity refers to the presence of dissolvable ions like sodium, potassium, magnesium, calcium, chloride and nitrate. Salinity stress in crops occurs either from high concentration in the soil solution of specific ions like Na and Cl or mixes of several soluble salts. Geological processes in the San Joaquin Valley (accumulation of marine coastal alluvium) and low-quality irrigation water are major contributors to soil salinity (Corwin 2003). Additionally, as crops absorb water, and water evaporates from the soil surface, salts are left behind in the rootzone. Crops growing in saline conditions experience osmotic stress, diminished growth, yield and shortened lifespan (Figure 1). With little access to high-quality irrigation water and ongoing drought, there are few solutions to alleviate salinity. Updated management practices are required to face the situation at hand. 

Figure 1. Almond (top) and pistachio (bottom) foliage yellowing and leaf margins with symptoms of salt toxicity (photos courtesy R. Gomez).

The most traditional method to manage soil salinity is by application of calcium in the form of gypsum (CaSO4 ·2H2O). As CaSO4 breaks down it yields calcium (Ca) and sulfate (SO42-) ions. Calcium works to desorb Na from the soil particle surface while the remaining sulfate binds the loose Na to yield sodium sulfate (Na2SO4). In turn, the newly formed sodium sulphate can be leached and moved down the soil profile and out of the rootzone. Additionally, replacing Na with Ca flocculates the soil for improved water infiltration and pore space. However, the application of gypsum as a sole salinity manager has challenges. Gypsum may require specialized machinery and equipment to facilitate applications in water-saving drip line, and attention should be paid to water quality or Ca may precipitate and plug the irrigation system. Finally, it is not uncommon to see Ca stratification in the soil from years of broadcast applications that have not fully dissolved or incorporated.

The standard practice for salinity management is a 0.5- to 2-ton/ac application of gypsum in the fall in anticipation of fall/winter rain. This practice has become the feel-good practice of many growers to date. However, updating such a practice becomes necessary when considering the current effects of climate change and economic circumstances. 2021 marked the driest winter months in 100 years with record lows of snow and rainfall as well as the third year of extreme drought. No rainfall and low access to irrigation water means little of the surface-applied gypsum is dissolving in the soil. With less access to surface water, more reliance on ground water and decreasing crop prices, growers now more than ever should reassess their management practices. 

Modern methods for salinity management focus on high solubility products, ease of application and efficiency. CATION-EX5 PLUS is a salinity management product formulated by AgroPlantae. It is fully miscible with water and better suited for fertigation systems than traditional gypsum and contains other important elements to help rebalance soil, improve fertility and invigorate soil microbes. CATION-EX5 PLUS is 0-0-5, Ca 10%, S 8%, Co 0.10% and Mo 0.10%.

The following are results of a three-year study on soil salinity management in California’s Tulare Basin.  Treatments were designated Grower’s Standard or CATION-EX5 PLUS. The Grower’s Standard (GS) consisted of a yearly postharvest application of broadcast gypsum 95% at a rate of 1 ton/ac in an 80-acre block and multiple in-season solution-grade gypsum applications through the irrigation system. AgroPlantae’s CATION-EX5 PLUS (CTX) was applied at 11 gal/ac over 80 acres, split into five applications over the growing season. Soil samples were taken in a predetermined area of each treatment block in 12-inch increments until 5 ft depth was achieved. Soil samples were taken in the spring before the first irrigation occurred and again in the fall after the last in-season irrigation in each year of the study. The spring sample acted as a baseline for the year while the fall soil sample showed the changes that occurred in the soil during the span of the crop growing months.                                                                                                

CATION-EX5 PLUS Reduces Salinity Buildup

Soil Electrical Conductivity (EC) is the measure of soil salinity. EC is highly influenced by soil texture, i.e., sand, silt and clay. For example, clay-like soils attract and retain more positive charges (cations) and have higher EC values than other soil textures. Based on saturation percentages, the soil in both treatment blocks was determined to be clay-loam. Aside from soil texture, soil EC is also highly affected by inputs like irrigation water and fertilizer. 

In this study, a greater degree of variability was seen in GS treatment when compared to CATION-EX5 PLUS (Figure 2). In 2020, EC values were comparable amongst treatments. However, in 2021 and 2022, large increases in soil EC were seen in the GS treatment. EC increased 24%, 742% and 573% in the GS treatment for 2020, 2021 and 2022, respectively. The red dotted line represents the average EC for the soil profile. The large increase in values suggests the GS treatment is neither remediating nor alleviating water penetration or infiltration. In the CTX treated area, EC decreased on average by 14%, increased by 145% and 117% in 2020, 2021 and 2022, respectively. With the application of CATION-EX5 PLUS, soil is maintaining a manageable EC level and showing greater performance for managing soil salinity when compared to conventional gypsum (GS). The increases seen in the GS area call for an update to salinity management practices. 

Figure 2. CATION-EX5 PLUS effectively manages soil EC compared to the Grower’s Standard (1 ton/ac gypsum). Red dotted line shows the average EC of all depths.

 

Managing Soil Sodium and Chloride

Sodium chloride (NaCl) is the most common and problematic salt in irrigation water. Sodium in soil antagonizes uptake of other beneficial elements like K, disperses soil and can create chemical compaction.  

At the start of the experiment in 2020, soil sodium was comparable amongst treatments (Figure 3). In the GS treatment, a clear Na increase is seen from spring to fall in each experimental year. The red dotted line represents the average Na for the soil profile. Overall, the GS increased 53%, 654% and 429% soil Na concentration in 2020, 2021 and 2022, respectively. Comparatively, in the CATION-EX5 PLUS treated field, soil sodium decreased by 4%, increased by 162% and 82% in 2020, 2021 and 2022, respectively. An increase in soil Na cannot be avoided but it can be managed to reduce toxicity, osmotic stress and mitigate growth/yield. Additionally, in 2022 CTX treated soil is beginning to show an effect carry over rate. In 2022 CTX treated soils showed little fluctuation in soil Na. This alludes to the lasting effects of proper soil flocculation.   

Figure 3. Soil sodium is effectively reduced by CATION-EX5 PLUS when compared to standard management practices like gypsum. Red dotted line shows the average sodium of all depths.

Soil Cl is an essential element needed by plants for growth, however when too much Cl is available in the soil, toxicity occurs. Cl toxicity is common in poorly drained soil and areas being irrigated with ground water. Cl is negatively charged, so it may be leached through the soil profile with irrigation water. When irrigation water is the largest contributor of Cl, and soil Na creates an environment for poor soil drainage, management becomes more critical. When soil Cl exceeds 5 meq/L, a management protocol should be activated. 

In this trial, soil Cl began the experiment below the action threshold in both treatments (Figure 4). The red dotted line represents the average Cl for the soil profile. As the experiment progressed, Cl concentration built in the soil profile. In 2021, the GS increased the soil concentration about 2257% from spring to fall and about 697% in 2022. The CTX treatment had lower increases of 378% and 112% from spring to fall in 2021 and 2022, respectively. 

Figure 4. Soil chloride is effectively reduced by CATION-EX5 PLUS when compared to standard management practices like gypsum. Red dotted line shows the average chloride of all depths.

No soil infiltration measurements were collected for this study; however, the data suggest an increase in water movement and wetted area in the soil profile. This is corroborated with the reduction in Na and Cl as well as EC. CTX is improving soil structure and remediating soil salinity. 

The increases in soil EC and Na seen in the GS treatment call for an update to salinity management practices. Experimental data from this long-term and large-scale trial show CATION-EX5 PLUS effectively managing soil salinity, especially when compared to the standard yearly gypsum application. Modern agriculture puts great emphasis on efficiency, precise solutions and ease of use while still demanding results. CATION-EX5 PLUS offers results, efficiency, precision and ease of use in fertigation systems like those used in California’s Central Valley. 

Properly Timed Foliar-Applied Urea and Phosphite Increase Citrus Yield and Fruit Size

By
PCC
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The goal of properly timed foliar-applied fertilizer is to increase the economic benefit derived from the grower’s fertilization program. In this strategy, fertilizer is applied at key stages in citrus tree phenology (the series of developmental events that result in fruit production and tree growth) (Figure 1). Key stages of tree phenology are associated with important physiological and developmental processes. The fertilizer application is timed to stimulate a specific physiological process and achieve a plant growth regulator effect that increases flowering, fruit set, yield, fruit size or fruit quality, even when the tree is not deficient in the applied nutrient based on standard leaf or other tissue analyses. Properly timing the fertilizer application is important because the developmental stage of the target organ determines the result obtained. 

Figure 1. Key stages of citrus tree phenology from left to right: flowering, fruit set, fruit development (exponential fruit growth), fruit maturation and flushes of shoot and root growth.

 

Winter Applications Increase Yield and Fruit Size 

The key stage of citrus tree phenology being targeted is bud determinacy, the irreversible commitment of buds that transitioned from vegetative shoot development to floral development in the fall to produce inflorescences and flowers. When a bud is determined, it can no longer revert to vegetative shoot growth. At the microscopic level, bud determinacy is identified by the initiation of sepals, the green outer leaves that cover and protect the developing floral bud (Lord and Eckard 1987). Due to annual variations in temperatures and the fact that not all shoots and buds are the same age or at the same stage of development, January 1 through February 15 is an effective window within which to make foliar applications of urea or potassium phosphite to increase flowering in citrus. 

Low-biuret urea (46% N, 0.25% biuret, 25 lb N/acre or a 50 lb bag of low-biuret urea per acre) applied in mid-January to target irreversible commitment to flowering in ‘Washington’ navel orange trees resulted in a net increase in three-year cumulative yield of 17,355 lb/96 trees/acre compared to control trees receiving five-fold more urea applied to the soil. Delaying the foliar-application of low-biuret urea to mid-February resulted in a net increase in three-year cumulative yield of 13,757 lb/96 trees/acre. In both cases, yield of commercially valuable size fruit of packing carton sizes 88+72+56 (2.7 to 3.5 inches in transverse diameter) increased as total yield increased (Ali and Lovatt 1994). Similarly, potassium phosphite as Nutri-Phite (0-28-26; Verdesian Life Sciences), which supplies P as PO3, not PO4, applied at 0.64 gal/acre in January to target irreversible commitment to flowering of Valencia orange trees in Florida resulted in an average net increase of 900 inflorescences per 12-inch square frame, a 166% increase in inflorescence number compared to untreated control trees, and produced a net increase in yield of 6,853 lb/acre (Albrigo 1991). In a second experiment, foliar-applied Nutri-Phite (0-28-26, 0.64 gal/acre) in January resulted in a net increase in four-year cumulative Valencia orange yield of 25,247 lb/acre, an average net increase of 6,311 lb/acre/year for the four years of the experiment, with an average net increase of more than 400 lb total soluble solids per acre per year (Albrigo 1991).

Summer Applications Increase Yield 

The key stage of phenology being targeted is maximum peel thickness, which marks the end of Stage I of fruit development in citrus in which fruit growth is predominantly by cell division, and the beginning of Stage II of citrus fruit development, the period of exponential fruit growth by cell expansion. At maximum peel thickness, all the cells that make up the mature fruit are present. Subsequent fruit growth is by the uptake of water into the juice sacs. The cells of the albedo, the white layer of the peel, simply stretch to accommodate the increasing size of the juice sacs; only the flavedo, the outer colored layer of the peel, continues cell division through harvest. The goal is to stimulate additional cell divisions just prior to maximum peel thickness. One additional cell division would double the number of cells in the fruit, a second cell division would quadruple the number of cells, etc. When these additional cells enlarge, fruit size is increased. Field research with navel orange, Valencia and mandarin cultivars documented the efficacy of foliar applications of low-biuret urea or potassium phosphite made between the last week of June and mid-July.

Foliar-applied low-biuret urea (46% N, 0.25% biuret, 25 lb N/acre) in early July to target maximum peel thickness in ‘Washington’ navel orange trees resulted in a net increase in yield of commercially valuable size fruit of packing carton sizes 88+72 (2.7-3.15 inches in transverse diameter) of 4,927 lb/109 trees/acre/year and a net increase in yield of larger fruit of packing carton size 56 (3.2 to 3.5 inches in transverse diameter) of 3,374 lb/109 trees/acre/year compared to untreated control trees (Lovatt 1999). 

To stimulate cell division prior to maximum peel thickness, Nutri-Phite was applied at 0.49 gal/acre in mid-May and again in mid-July. Nutri-Phite produced a net increase in fruit of packing carton sizes 88+72 of 5,078 lb/109 trees/acre/year with a net increase in yield of larger fruit of packing carton size 56 of 4,158 lb/109 trees/acre/year. The Nutri-Phite treatment also increased fruit total soluble solids (TSS) by early November compared to the untreated control (P < 0.001), achieving a TSS:acid ratio of 8.1 by early November compared to 7.2 for fruit from untreated control trees (P < 0.01) (Lovatt 1999). 

Application and Flowering Physiologies

It is well known that flowering in citrus is induced by low temperature (LT ≤59 degrees F day/ ≤50 degrees F, but >7 degrees F night) and by water-deficit stress (WD ≤-2.4 MPa stem water potential) for eight weeks. Inflorescence and flower number both increase as the duration of the LT or WD period increases. Citrus is unique in the plant world. Not only does flowering increase with the increased duration of WD, but also with the increasing severity of WD (Figure 2). 

Figure 2. The figure on the left illustrates that flowering in lemon trees increases as the severity of water-deficit stress increases (the numbers on the x-axis become more negative.) Water-deficit stress is measured as predawn xylem pressure potential (PDXPP) in MegaPascals (MPa). The figure on the right demonstrates that flowering increases with the increased number of days lemon trees are maintained at a moderate water-deficit stress (-2.4 MPa PDXPP). After Hake (1995)

 

In the late 80s, my lab discovered that leaf ammonium concentrations increased with the increased duration of both LT and WD and that leaf ammonium concentrations increased in parallel with the increase in both inflorescence number and flower number in response to the duration of LT or WD (Lovatt et al. 1988a, b). Since urea applied to leaves is catabolized by the plant enzyme urease to ammonia and carbon dioxide (NH3 and CO2), my lab tested the capacity of foliar-applied urea to supplement ammonium accumulation during LT and WD and increase citrus flowering. For navel orange trees exposed to a 50% or 25% shorter period of low temperature, foliar-applied low-biuret urea increased flowering 194% and 230%, respectively. Foliar-application of low-biuret urea to lemon trees maintained at a moderate WD (-2.4 MPa predawn xylem pressure potential) at the end of the 50-day stress period increased flowering 260% compared to WD-treated trees not receiving foliar-applied low-biuret urea (Lovatt et al. 1988a, b). At the level of gene transcription, new evidence suggests that LT and WD initiate flowering through overlapping genetic pathways (Tang and Lovatt 2022). Thus, WD provides a tool to increase citrus flowering in growing areas experiencing warmer, dry winters, with foliar-applied low-biuret urea or potassium phosphite able to supplement both LT and WD to increase citrus flowering. 

These Different Molecules Share the Same Benefits, but How?

Recent evidence suggests that both urea and phosphite increase tree nitrogen status and cytokinin biosynthesis. In both roots and leaves, nitrate and ammonium upregulate the expression of the key gene regulating cytokinin biosynthesis, IPT, the gene encoding isopentenyl transferase, which catalyzes the rate-limiting step in cytokinin biosynthesis (Sakakibara 2006; Sakakibara et al. 2006). Nitrate taken up by roots and cytokinin synthesized in the roots move in the xylem to the shoots and leaves, where nitrate upregulates the genes for N assimilation and the IPT gene for cytokinin biosynthesis. Cytokinin and metabolites synthesized in the leaves are then transported in the phloem to the roots. Thus, N and cytokinins work together to promote and coordinate root and shoot growth, bud break and flowering. Ammonium derived from foliar-applied urea would stimulate N assimilation and upregulate cytokinin biosynthesis. Cytokinins are known to promote flowering and fruit growth.

Surprisingly, phosphite was recently demonstrated to upregulate key genes for N assimilation, resulting in increased nitrate uptake (Vereet et al. 2021) and, as predicted by the information presented above, phosphite increased cytokinin biosynthesis (Swarup et al. 2020). Phosphite supplied to roots resulted in significantly greater cytokinin concentrations one day after phosphite treatment (P ≤0.05). Root cytokinin concentrations continued to increase during the week-long experiment to a level greater than untreated control plants (P ≤0.05).

The Window for Increasing Flowering is Now

 Citrus flowering is induced by LT and WD stress. WD stress of approximately -2.4 MPa stem water potential can be maintained by deficit-irrigation, and thus WD can be used to supplement LT in citrus areas with warmer, dryer winters due to global climate change. Winter prebloom foliar-applied urea and Nutri-Phite can be used to supplement LT and WD stress to increase citrus flowering and yield. In addition, summer foliar-applied low-biuret urea or Nutri-Phite can be used at maximum peel thickness to further increase fruit size and yield of commercially valuable size fruit. In light of recent research results, foliar-applied urea and Nutri-Phite likely achieve a plant growth regulator effect by increasing N assimilation and cytokinin biosynthesis. 

Foliar fertilizers should be applied in sufficient water for good canopy coverage (at a final pH of 5.5 +/- 0.5), but not to run off, which is a waste of product. Pooling of fertilizers at the leaf tip can result in tip burn. The application should be like a pesticide spray with good canopy mixing and coverage of the upper and under surfaces of the leaves on the exterior and interior of the canopy and the target organ. Always follow the product label. Results cited for properly timed foliar-applied potassium phosphite reported herein are for Nutri-Phite, Verdesian Life Sciences, the only commercial product for which results appear in peer-reviewed journals.

References

Albrigo, L.G. 1991. Effects of foliar applications of urea or Nutri-Phite on flowering and yields of Valencia orange trees. Florida State Hort. Soc. 112:1-4.

Ali, A. and Lovatt, C.J. 1994. Winter application of low-biuret urea to the foliage of ‘Washington’ navel orange increased yield J. Amer. Soc. Hort. Sci. 119:1144-1150. 

Hake, K.D. 1995. Regulation of Flowering in Citrus limon by water-deficit stress and nitrogen compounds. PhD Dissertation, University of California, Riverside. 149p.

Lord, E.M. and Eckard, K.J. 1987. Shoot development in Citrus sinensis L. (Washington navel orange). II. Alteration of developmental fate of flowering shoots after GA3 treatment. Bot. Gaz. 148:17–22

Lovatt, C.J. 1999. Timing citrus and avocado foliar nutrient applications to increase fruit set and size. HortTechnology 9:607-612. 

Lovatt, C.J., Zheng, Y. and Hake, K.D. 1988a. A new look at the Kraus Kraybill hypothesis and flowering in Citrus. 6th. Intl. Citrus Congr. 1:475-483.

Lovatt, C.J., Zheng, Y. and Hake, K.D. 1988b. Demonstration of a change in nitrogen metabolism essential to floral induction in Citrus. Israel J. Bot. 37:181-188.

Sakakibara, H. 2006. Cytokinins: Activity, biosynthesis, and translocation. Annu. Rev. Plant Biol. 57:431-449.

Sakakibara, H. Takei, K. and Hirose, N. 2006. Interactions between nitrogen and cytokinin in the regulation of metabolism and development. Trends in Plant Science 11: 40-448.

Swarup, R., Mohammed, U., Davis, J. and Rossall, S. 2020. Role of phosphite in plant growth and development. White paper, School of Biosciences, Univ. Nottingham. 

Tang, L. and Lovatt, C.J. 2022. Effects of water-deficit stress and gibberellic acid on floral gene expression and floral determinacy in ‘Washington’ navel orange. J. Amer. Soc. Hort. Sci. 147(4):183-195.

Verreet, J.-A., Prahl, K.C., Loof, S. Birr, T. Klink, H., Cai, D. Xu, S. 2021. Phosphite plant biostimulant mode of action: gene expression, phytohormone levels, enzyme activity. Electronic Biological Inorganic Chemistry PO3 Workshop. (Christian-Albrechts-University of Kiel, Germany).

Irrigation Tools and Information for Efficient Water Management in California Avocado Production Systems

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Figure 1. a) An aerial view of the flux tower from a distance;

In California, avocados are primarily grown in southern and central parts of the state along the coast. These regions face uncertain water supplies, mandatory reductions of water use and the rising cost of water, while efficient use of irrigation water is one of the highest conservation priorities. Data on water use by avocado orchards and optimal irrigation strategies needs to be updated in light of increasing water pressure in order to achieve efficient water and fertilizer management. Moreover, due to increasing salinity in water sources, effective irrigation is more critical to ensure optimal yield and high-quality fruit as avocados are one of the most salt-sensitive crops.

An ongoing irrigation study aims to acquire relevant information on crop water consumption and develop more accurate crop coefficient curves over the season for avocados under different environment and cropping systems in southern California. Extensive data collection is being conducted in nine mature avocado sites in San Diego, Temecula, Orange and Ventura counties using combined cutting-edge ground- and remote-sensing technologies. A combination of surface renewal and eddy covariance equipment (flux tower, Figure 1) are utilized to measure actual crop evapotranspiration to develop crop coefficient curves at each site. Several other sensors and equipment are being used to monitor soil and plant water status and soil salinity, and high-resolution images are being captured by unmanned aerial systems to evaluate canopy features. This article provides some avocado water management tips based on the preliminary findings of this study. 

Figure 1. a) An aerial view of the flux tower from a distance;
Figure 1. b) a ground view of the tower;
Figure 1. c) a close look from the top of the flux tower demonstrates net radiometer sensor and two fine thermocouple sensors at one of the avocado experimental sites.

 

Variable Spatial and Temporal Crop Water Needs

The results from the avocado experimental sites demonstrate considerable variability in avocado consumptive water use, both spatially and temporally. The cumulative avocado consumptive water use (actual evapotranspiration or ET) across two sites in Temecula (site 1 with an elevation of 1,490 ft. above sea level) and in the San Pasqual Valley, Escondido (site 2 with an elevation of 720 ft. above sea level) varied from 28.1 in. to 38.5 in. over a 235-day period (Figure 2). The average daily actual ET was 0.18 in d-1 in June/July at site 2, while the amount was 0.14 in d-1 at site 1 for the same period. This is a notable difference of actual ET between these two sites. More uniform daily crop water consumption over the summer period occurred at site 2 when compared with site 1 located at a higher elevation.   

Figure 2. Daily actual evapotranspiration at avocado site 1 and over 235-day period (April 14, 2022 through December 4, 2022). Considering daily actual ET measured and tree spacings, the average crop water consumption during this period was determined to be 40.8 gallons per day per tree at site 2 and 20.1 gallons per day per tree at site 1. Tree spacings were different at the two sites, so per-tree use varies more than the per-area ET as mentioned above.

 

Crop Coefficient Values a Valuable Tool 

To estimate crop water requirements, various crop coefficient (Kc) values of 0.64 (Grismer et al. 2000), 0.72 (Gardiazabal et al. 2003) and 0.86 (Oster et al. 2007) were reported for “Hass” avocado. Kc value is greatly impacted by differences in climatic conditions, canopy features (size of crop canopy and shaded area), row orientation, soil and irrigation water salinities, and amounts and frequencies of water applied. The question is, can we use a single crop coefficient the entire season that is based upon data developed decades ago that does not consider the impacts of plant density, row orientation and microirrigation? Can this value be used for different avocado regions in California? 

Figure 3, see page 10 demonstrates the trend of daily actual crop coefficient values over a 235-day period at avocado site 2. More variations in water use were found during fall months when compared with spring and summer months. Average crop coefficient values of 0.77, 0.72 and 0.81 were determined for the periods of April to May, June to mid-October, and mid-October to early December, respectively. Lower crop coefficient values were obtained at site 1. It needs to be noted that these sites haven’t been under water stress and/or salinity stress most of the study period while due to the two heat waves in late June and early September, trees could experience heat stress for a while. The continuous measurements across the experimental sites will provide a comprehensive data set to update and develop more accurate crop coefficient curves for avocados at each site. These crop coefficient curves could be considered as an effective tool for irrigation management in California avocado production systems.  

Figure 3. Actual crop coefficient values determined at the avocado experimental site 2 in Escondido. The orchard has a south-facing slope and the dominant soil texture is sandy loam.

 

Soil Moisture Sensing 

Understanding the effects of irrigation events on soil moisture provides critical insight for growers about the present growing environment for crops. While experienced growers have learned over seasons of observations how their soils and water interact, utilizing a soil moisture measuring device of some sort enables them to put a number on their observations and more accurately track trends over time.

The sensors allow growers to better understand the frequency and duration of irrigation events needed and to maintain adequate moisture based on the crops being grown. There are instances where irrigation cycles are occurring too often and for far longer periods than needed to achieve field capacity of the rooted volume of soil. There are also instances where the use of sensors revealed malfunctioning irrigation system components by reporting unusually dry soil in areas that should have received ample irrigation. Soil moisture sensing is contingent on having a well-maintained irrigation system with a good distribution uniformity. Soil moisture sensors should be used as a useful tool to answer the following critical questions:

What is the water status of the soil early in the irrigation season?

When is the right time for the first and subsequent irrigation events? 

Is the soil profile full after each irrigation event? 

What is the length of irrigation time?

Should the irrigation practice be changed?

An example of the use of soil moisture monitoring at site 1 over a six-month period is shown in Figure 4. Half-hourly soil water tension was plotted for multiple depths (6”, 12” and 18”). The data shows that soil water was maintained at a desired level within the crop root zone at this site due to the frequent drip irrigation events. Although the average soil water tension varied over time in the top 18 in. of the soil, it never declined below 5 centibars and never exceeded 56 centibars over the period. The average values at 6”, 12” and 18” deep over the period were 20.1, 11.5 and 9.0 centibars, respectively. The soil moisture data at this site indicates that the irrigation frequency was scheduled properly while shorter irrigation runs could be considered in each irrigation event to improve irrigation efficiency. 

 

Figure 4. Half-hourly soil water tension (centibar) measured at depths of 6”, 12” and 18” at avocado site 1 over a six-month period (March 14, 2022 through October 13, 2022).

Soil Types and Conditions, Canopy Features and Row Orientations

Avocado is one of the most salinity sensitive crops produced in California but is commonly grown in areas where poor quality is common. In recent years, salinity problems in California avocado have become increasingly common as the cost of irrigation water has risen and the availability of low salinity water for agriculture has diminished. 

The source of water across the six avocado experimental sites in San Diego, Temecula and Orange counties have an ECe greater than 1.0 dS m-1 and chloride >100 ppm. Across the sites, the maximum soil ECe of the top 1 foot was measured (3.4 dS m-1) at a site with 28-year-old trees and a silty loam soil texture in Orange County. A high chloride content (311.3 ppm) was also measured in the top 1 foot. A leaf chloride percentage of 0.465 was observed in early September at this orchard. Under such circumstances, yield improvement could be gained for the avocado orchard with increasing amounts of applied water to leach salt and particularly chloride from the effective crop root zone. Excess irrigation can be considered as beneficial water use for salinity management in avocado groves while the optimal leaching strategy could be different from site to site depending on soil types and salinity status, quality of irrigation water and irrigation system.     

Ground shading percentage or canopy cover that provides a good estimation of canopy size/volume (Figure 5) and the amount of light that it can intercept is likely the most important driver influencing crop water needs. At the experimental sites, canopy vegetation cover percentage for each tree derived from drone-based multispectral imagery ranged from 0% (missing trees) to 100%. For instance, the canopy cover varied from 33.5% to 98.9% with tree spacings of 15 ft. × 18 ft. at site 1 versus 40.3% to 94.5% at site 2 with tree spacings of 20 ft. × 20 ft. The average canopy cover was 71.6% and 85.4% around the flux towers at site 1 and site 2, respectively. This clearly indicates that site 2 has a greater light interception, and as a result, greater crop water needs are expected when compared with site 1 (Figure 2). 

Figure 5. Polygons RGB Mosaic of avocado trees at site 2 (top) and avocado tree centers and polygons at site 1 (bottom). Trees at both experimental sites (around monitoring stations) have a south-facing slope orientation.

 

Both sites 1 and 2 have a south-facing slope orientation which means there is unlikely to be a notable impact from slope differences on the crop water use between the two sites. Avocado sites with north- and east-facing slope orientations are expected to have lower crop water needs even in a single orchard, and accordingly sometimes a different irrigation schedule is required for different zones under different orientations in an avocado orchard.  

The Key to Building a Soil that Can Suppress Pathogens Naturally

By
Cassidy Million | Ph.D., Director of Ag Science, Heliae Agriculture
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Figure 1. Disease suppressive soils are defined as soils that naturally defend against pathogenic diseases before the disease attacks a growing crop, or the pathogen can infect the crop but the disease declines with successive cropping.

When it comes to managing soilborne plant diseases, methods for reducing or eliminating the impact of pathogens have heavily relied on selecting a resistant plant variety and the use of pesticides for protection. However, the soil, and specifically the biological component consisting of microbes, can be another tool growers can leverage to help protect their crops from soilborne diseases.

Healthy soil and a healthy crop can lead to reduced disease incidence, an idea that is becoming more widely accepted across the agriculture industry. Many questions arise with this concept: How are soil health and soilborne plant diseases related? Can soil health result in less plant disease increase pressure? What are the mechanisms behind this phenomenon? In this article, we will discuss and address these questions and take a deep dive into a phenomenon known as disease suppressive soil (DSS). 

Microbes Are the Key 

The activity of soil microbes or communities of microbes is what gives rise to a soil’s disease suppressive properties. As you can imagine, the soil is a complex system, and understanding the mechanisms that contribute to DSS is extremely intricate. Imagine the interactions between beneficial and pathogenic microbes as a battlefield belowground. In the case of DSS, the beneficial microbes win. Just like on a battlefield, many tactics, strategies and tools are needed to overcome your opponent; and the same is true for soil microbes. Multiple mechanisms from increasing soil and plant health to directly impacting pathogenic microbes are used by soil microbes in the battle between beneficial and pathogenic. 

Soil microbes play a key role in soil health and soil quality, which in return affect the degree of disease suppression in the soil. Soil microbes have multiple ways in which they suppress disease in the soil, including the improvement of plant health, inducing natural defense response in the host plant, secreting enzymes and antibiotics, and through microbial competition.

Defining Suppressive Soils and Why They Matter

DSS are defined as soils that naturally defend against pathogenic diseases before the disease attacks a growing crop, or the pathogen can infect the crop but the disease declines with successive cropping (Figure 1). Soils can do this through reducing the establishment or growth of pathogenic fungi or bacteria due to the makeup of their microbiome (the communities of fungi and bacteria that are living in the soil.) Soils can keep disease development at a minimum even in the presence of a susceptible host and disease-causing pathogen. 

Figure 1. Disease suppressive soils are defined as soils that naturally defend against pathogenic diseases before the disease attacks a growing crop, or the pathogen can infect the crop but the disease declines with successive cropping.

DSS can be naturally occurring but may also be developed over time through cropping practices. To help clarify, we can divide suppressive soils into two categories: general suppression and specific suppression (Figure 2). 

Figure 2. Disease suppressive soils can be naturally occurring but may also be developed over time through cropping practices. They can be categorized by general suppression and specific suppression.

General suppression gives protection towards multiple pathogenic microbes. The increase in the abundance (number of microbes) and diversity (which microbes are present) of soil microbes is key in general suppression. These soil microbes out-compete pathogenic microbes, which in turn creates general suppression. Agricultural practices can impact this type of suppression with practices such as soil sterilization reducing it and good soil health practices enhancing it. 

Specific suppression can occur naturally (long-standing) or be induced. Long-standing specific suppression is associated with a specific species of microbes or group of microbes and is naturally found in the soil without the presence of a plant. Induced specific suppression can be produced through monoculture of a crop, growing susceptible crops or by mixing small amounts of a suppressive soil into a conducive soil. 

So, why should we care about DSS? As we move forward in modern agriculture, restriction of fumigants and pesticides as well as the lack of available resistant cultivars will only make these soilborne diseases harder to manage. DSS are another tool growers can use to help manage difficult-to-control soilborne diseases.

Soil Microbial Battleground

Beneficial soil microbes have a direct impact on their pathogenic counterparts, but on the battlefield, the beneficial soil microbes can outcompete their pathogenic counterparts, leading to greater numbers of beneficial soil microbes. Underground, beneficial soil microbes use up the exudates and nutrients created and released by the host plant, blocking the pathogenic microbes from accessing and utilizing the resources needed to survive. 

During the battle, the soil microbes can display hyperparasitism, where they infect pathogenic microbes present in the soil that pose a threat to the crops around them. Plus, the microbes can cause additional impact on their pathogenic microbial counterparts by secreting antibiotic-like enzymes and toxins in a process called antibiosis. Antibiosis is a competitive tactic that kills other pathogenic microbes and increases their ability to battle and defend themselves and the host plant from pathogens. 

The health of the host plant above the ground is impacted by the soil health under the ground. The structure of the soil plays a direct role in plant health in terms of water infiltration, retention, soil compaction and access to nutrients. Soil microbes are to key to improving soil structure through increased soil aggregation, and aid in the release of mineral nutrients through a conversion process called nutrient mineralization. Soil microbes secrete exudates that can trigger disease-resistant responses for host plants. All these factors aid in improved plant health and performance. 

Soil Health Impact 

In the soil microbe-pathogen battleground, what exactly are these microbes battling for? For the pathogenic microbes, they are battling to infect the host plant. To understand the complexity and how soil health and microbes play a role in plant disease and DSS, we need to review the disease triangle (Figure 3). For disease to occur, there must be three components (representative of the sides of a triangle):

Susceptible host

Conducive environment

Virulent pathogen

If one of these sides is removed, plant disease is not possible. The environment side of the triangle is where we need to focus as this is the side that includes soil health and holds the key to creating disease suppressive soils.

Figure 3. Disease triangle. For disease to occur, there must be three components, representative of the sides of a triangle. The environment side of the triangle shows the impact of soil health and microbes on the ability for disease to occur.

 

The physical and chemical properties of the soil, including pH, soil organic carbon and nutrients, provide the habitat for microbial activity. Crop management practices such as tillage, irrigation, fertilization, the addition of green manures and weed management can directly impact the soil environment and microbial activity in the soil. Many of the factors mentioned here directly impact microbial activity and communities and are the key to soil health. Many of the factors mentioned here directly impact microbial activity and communities and are the key to soil health. As we know, there are tools and inputs to help improve soil health, meaning that if we manipulate the environment (the soil) and remove one side of the disease triangle, we can reduce plant disease infection. Therefore, improving soil health can lead to general DSS (Figure 4). 

Figure 4, How soil health is related to general disease suppression and the agricultural practices that impact soil health leading to general disease suppression.

 

What about specific disease suppression? If we recall, long-term monocropping can be used to achieve specific suppression in some systems, which is contradictive of soil health practices. The literature has noted the ability to achieve specific disease suppression by using compost and green manures. However, increasing soil health and increasing microbial communities is the best practice to achieve and improve general suppression and increase crop productivity. 

Managing the Soil to Achieve Disease Suppressive Soil 

Soil microbes are key to DSS, and soil health directly impacts the makeup and activity of soil microbes. DSS can be negatively or positively impacted by cropping systems and management practices. Soil management practices that positively influence DSS include crop rotation, intercropping, minimum tillage practices, fertility or organic inputs such as manures and composts. In addition, adding liable carbon sources as a food source for microbes will increase the abundance and diversity of soil microbes, which is critical in general suppression. Crop rotation is an important factor; rotating to a non-host can aid in reducing soilborne diseases and positively impact soil microbial diversity. What we know to date is that suppressive soils are usually mediated through soil microbial community shifts overtime, so adopting practices and amendments that increase soil health and organic matter and increase the diversity and abundance of microbial activity in your soils will aid in suppressing soilborne diseases.

Battle for a Sustainable Solution to Soilborne Diseases   

DSS can be a tool for soilborne disease management. Adapting soil health practices and increasing the soil microbial activity is a sustainable agricultural practice that will be key in the coming years. Building and maintaining suppressive soils can provide a solid foundation for crops to thrive. We’ve discussed the key to DSS (microbes) and contribution to a soil’s disease suppressive properties. Focusing on the improvement of soil health and natural defense response belowground is a good first step toward the achievement of healthy crops that are protected against soilborne disease.

Resources

Disease Suppressive Soils: New Insights from the Soil Microbiome- https://doi.org/10.1094/PHYTO-03-17-0111-RVW

Disease-Suppressive Soils—Beyond Food Production: a Critical Review- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7953945/ 

Current Insights into the Role of Rhizosphere Bacteria in Disease Suppressive Soils- https://doi.org/10.3389/fmicb.2017.02529 

Plant Health Management: Pathogen Suppressive Soils- https://doi.org/10.1016/B978-0-444-52512-3.00182-0

Fine-Tuning with Soil Health; Soilborne Disease?- https://csanr.wsu.edu/fine-tuning-soilborne-disease/

Developing Disease-Suppressive Soil Through Agronomic Management- https://www.researchgate.net/publication/292011599_Developing_Disease-Suppressive_Soil_Through_Agronomic_Management 

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