If the beginning of Nonpareil hullsplit coincides with egg laying time of the second flight of NOW, the risk of NOW damage increases. (all photos by Vicky Boyd.)
From the standpoint of navel orangeworm (NOW) management in almonds, hullsplit is the most critical fruit developmental stage. Almond fruits open at their suture during this period and become susceptible to NOW infestation. If the beginning of Nonpareil hullsplit coincides with egg laying time of the second flight of NOW, the risk of NOW damage increases. Therefore, understanding NOW activity and hullsplit status is vital for timely insecticide applications. The nuts on the southwest side of the canopy mature earlier, so the initiation of hullsplit in a block should be confirmed by regularly checking on the southwest side of the canopy. An orchard ladder or pruning tower can aid in reaching the fruits from the top of the representative trees to detect early split. Also, knowing the difference between true hullsplit and blank nut split is essential because blank nut hullsplit usually begins one to two weeks before the sound nut hullsplit begins. Nuts in edge trees, especially on the southern edge of an orchard, often start to split several days to a week ahead of trees within the orchard. Edge and blank splits signal the approach of hull split of sound nuts inside the orchard. Hullsplit timing varies among tree water status, varieties, geographic regions and weather conditions; however, prediction models are available to predict hullsplit timing for various locations. Research and field experience tell us that spraying earlier, rather than later, is the most effective strategy for reducing NOW damage.
NOW Traps and Spray Decision
NOW monitoring (egg, male and female) traps do not provide a specific treatment threshold; however, they can be highly beneficial for spray decision-making since they inform the grower about seasonal pest activities. Egg traps are helpful to set the egg-laying biofix for overwintering adults in the spring and track the heat units to determine the completion of that generation and the beginning of the second generation (i.e., 1050 degree-days). The second generation infests the early hullsplit nuts, and the timing for that generation is critically important to minimize the nut damage. The utility of egg traps for guiding spray timing would be less clear for the later generations because of the continuation of egg-laying due to overlapping adults among generations. However, the density of egg laying in egg traps and female moth activity (in Peterson traps or similar tools) are good indicators of potential NOW pressure and, possibly, nut damage.
Experience shows that unless an orchard is in an isolated location with a history of very low NOW damage and low NOW population, most almond orchards in the Northern Sacramento Valley warrant at least one spray at hullsplit. However, missing proper spray timing and not practicing other cultural practices such as winter sanitation may result in higher-than-acceptable levels of nut damage. Therefore, minimizing NOW damage requires practicing cultural techniques, such as winter sanitation and timely harvest, monitoring pest populations and making well-timed insecticide sprays. Once hulsplit begins and eggs are being laid in egg traps, spray to protect early hullsplit, watch traps tracking females (egg and/or Peterson traps) and check early split nuts for worms to make preharvest spray decisions and decide on harvest timing. If NOW pressure is high, it might be worth a talk with the processor to look at timely harvest and kernel (meats) production vs later harvest for inshell. If NOW pressure is high, damage can increase very rapidly, and every day the nuts are in the trees increases the danger of further damage.
Determining Spray Timing and Numbers
The number and timing of hullsplit sprays depend on orchard history, in-season pest monitoring information, use of other cultural practices such as winter sanitation and timely harvest, and overall risk of NOW immigration to the orchard. Typically, the first hullsplit spray for NOW is made when the eggs are being laid and the hullsplit begins. Commonly used, relatively reduced-risk insecticides only kill eggs and larvae. Some are effective against adults; however, insecticides must contact adults to be effective. Spraying during the night or early morning is more effective than spraying during the day. These insecticides can be effective for two to four weeks; a second application may be necessary if you continuously find high numbers of eggs, females and males in their respective traps and you cannot harvest the nuts before the third flight activity. Early hullsplit pollinizers split about two weeks after Nonpareil; the hullsplit typically starts when the NOW third flight begins. For many growers requiring a second hullsplit spray, a second application can help both Nonpareil and pollinizers. Late-season pollinizer varieties can be at risk from the latter portion of the third and fourth flights if not harvested on time.
Additional Considerations
Provide good spray coverage. Good spray coverage is essential for effective NOW control. Many years of study have indicated spray coverage is the function of tree height, sprayer speed and spray volume. In mature almond orchards, it has been shown that high-volume spray (150 to 200 GPA ) with a slower spraying speed (2 mph) provides better coverage and NOW control, though it is an unconventional practice for many growers.
Harvest on time. The goal of timely harvest is to get the almond harvested once 100% hull split occurs for all varieties, and if possible, before the beginning of the third generation for Nonpareil and before the fourth generation for other late varieties. Ensuring that nuts are at the proper maturity stage during the harvest is critical. Too early harvest can lead to mold development and chipping issues during the processing, reducing the nut quality.
Know your insecticides. Insecticides differ in their mode of action, efficacies to NOW and toxicity to beneficials (Table 1). Knowing those facts can help select the best insecticide for a particular situation. Most of the insecticides labeled for NOW target newly hatched larvae and have some impacts on eggs. Some have effects on all stages. Insecticides applied for NOW management at hullsplit must kill the egg or larva before feeding damage occurs. Another difference among NOW insecticide is their potential effects on spider mite predators such as predatory mites and sixspotted thrips. Non-selective insecticides, in general, are more toxic to those beneficial than selective insecticides. Therefore, the tradeoff of using broad-spectrum insecticide should always be considered.
Disclaimer: Products listed in this table do not constitute a recommendation, and many of the active ingredients presented in this article can be purchased under multiple trade names.
Understand seasonal changes in NOW biology and behavior. NOW development rate changes during the season. The first and second generation of NOW needs about 1050 degree-days to complete one generation. However, after that, NOW can complete a generation in only 750 degree-days due to better nutritional quality offered by the fresh nuts. Temperature plays a significant role, too, as higher temperatures during the summer accelerate egg hatching and larvae development.
Do Wider Beds in Fresh Market Onion Production Increase Total Water Productivity?
Imperial County, located in Southern California, has a large agricultural sector with close to 500,000 acres in production. The region’s major agricultural sectors are cattle feedlots, forages and vegetables. Imperial County and surrounding areas produce an estimated two-thirds of the nation’s vegetables in the winter. In the last decade, the use of pressurized irrigation systems (solid set sprinklers and drip irrigation) and wider beds in vegetable production has gained popularity in Imperial County. Imperial County is a top fresh market onion region with over 3,000 acres in annual production. Agriculture in Imperial County relies on just one water source: the Colorado River. As the Colorado River basin (CRB) is reaching more than 20 years of continuous drought, the US Bureau of Reclamation is working with basin users to develop strategies that may reduce water usage between 2 and 4 million acre-feet per year. Most of the projected water cuts are expected from agriculture due to its high share of water use. Agricultural users and water regulators in the CRB are discussing strategies to promote water conservation practices and improve water use efficiency while keeping a large and productive agricultural sector that feeds Americans with high-quality food all year long. The objective of this study was to assess the effect of two bed sizes and two irrigation amounts on onion yield and water productivity.
Do Wider Beds in Fresh Market Onion Production Increase Total Water Productivity?Figure 1. Onion sizes (L-R prepack, medium, jumbo, colossal and super colossal) per season. Terena in 2019-20 (top), and Hornet in 2020-21 (bottom).
Methods
This study was performed during two growing seasons (2019-20 and 2020-21) at the UC Desert Research and Extension Center (DREC) in Holtville, Calif. The major soil unit in testing areas is a Holtville clay with clay, silt and sand proportions of approximately 42%, 19% and 39%, respectively.
Table 1. Growth stage onion crop coefficient values used in this study (Montazar 2019).
Two irrigation levels were evaluated: 100% and 130% of crop evapotranspiration (ETc). ETc was computed using potential evapotranspiration from the California Irrigation Management Information Systems (CIMIS) station at DREC (Meloland station #87) and adjusted by stage-specific crop coefficients (Table 1) developed by Ali Montazar (2019) for Imperial County onion production. This study used sprinkler irrigation for crop emergence and until the beginning of bulbing to ensure an adequate establishment. This is a normal practice in our region. Irrigation treatments using drip systems were established in January of every season. Water amounts delivered through the drip irrigation system were computed using a daily water balance approach (irrigation needs = ETc – rainfall). Drip irrigation treatments started at bulb initiation (about eight leaves and a bulb diameter that was twice that of the neck.) Onion yields and sized distribution were measured at harvest. A two-sample t-test was performed to compare irrigation levels per bed size with SAS software. Total water productivity (TWP) was computed for each irrigation treatment. The TWP included rain and irrigation amounts. The TWP was computed as the total fresh onion yield divided by the total water use.
Table 2. Onion cultivars, seasonal conditions and irrigation amounts per season.
The drip tape was installed between 2- and 4-inches depth. One drip tape line was installed near the center of the 40-inch bed. Three drip tape lines were buried in the 80-inch bed plots. 4 and 12 lines of onions were planted in the 40-inch and 80-inch bed rows, respectively. Research plots were 50 feet long and comprised four rows on 40-inch beds and three rows on 80-inch beds.
Results and Discussion
Table 2 shows onion cultivars, seasonal conditions and irrigation amounts per season. Terena and Hornet varieties are short-day, yellow hybrid onions (Figure 1). Terena variety has a globe shape and Hornet is a grano-shaped onion.
Plant density used in this study is in the range used by growers in Imperial County (Table 3). Plant density on 80-inch beds was about 50% higher than onions planted on 40-inch beds.
Table 3. Plant density by cultivar and bed size.
Total yields responded to irrigation amounts except in the trial with Hornet (2020-21) on 80-inch beds (Table 4). In the Hornet trial (2020-21) on 80-inch beds, we noticed the irrigation treatment using 130% ETc yielded a higher proportion of pre-pack sizes compared to other treatments. We also noticed Hornet grown on 80-inch beds did not yield super colossal sizes. These results indicate Hornet seeds were planted too close in the 80-inch beds, reducing bulb size in the higher-irrigation treatment.
Table 4. Effect of irrigation rates on fresh market onion size distribution and total yield.
Terena treatments in 2019-20 produced a larger proportion (63% to 90%) of high-value bulbs (jumbo, colossal and super colossal) than Hornet in 2020-21(49% to 78%). High-value bulbs produced by irrigation treatments at 130% ETc were consistently higher than in the 100% ETc treatments. When onions were sorted by sizes and per-unit yields were computed, Terena’s yields were approximately 20% higher than Hornet’s yields.
Irrigation levels did not affect total water productivity (TWP) for Terena (2019-20) grown on 80-inch beds and Hornet (2020-2021) grown on 40-inch beds (Table 5). Total water productivity decreased as irrigation increased in the following trials: Terena (2019-20) on 40-inch beds and Hornet (2020-21) on 80-inch beds. Total yields on 80-inch beds increased between 10% and 31% compared to total yields on 40-inch beds (Table 5). Total water productivity was higher when onions were harvested on 80-inch beds than onions produced on 40-inch bed systems. Total water productivity of onions using 80-inch beds increased between 8% and 32% compared to onions grown on 40-inch beds.
Table 5. Total yields and water productivity by cultivar.
Highlights
Terena variety produced higher yields than Hornet variety.
High-value bulbs (jumbo, colossal and super colossal) produced by irrigation treatments at 130% ETc were consistently higher than with 100% ETc.
Total water productivity did not increase as irrigation levels increased regardless of bed size.
Fresh market onions on 80-inch beds were more water efficient than onions produced using 40-inch beds.
References
Montazar, A. 2019. Preliminary estimation of dehydrator onion crop water needs in the Imperial Valley. Agricultural Briefs 22(7):131-135. University of California Cooperative Extension – Imperial County.
This research included funds from the California Department of Food and Agriculture’s Fertilizer Research and Education Program.
For more information, contact Jairo Diaz at jdiazr@ucanr.edu or 760-791-0521.
Citrus leaf senescence caused by magnesium deficiency manifests when cytokinin and chlorophyll are oxidized out of leaves (all photos courtesy K. Van Leuven.)
Biostimulant products come from things existing in nature which are useful as solutions to agricultural problems. They are seen as ways to meet challenges in nutrient use efficiency, managing crop stresses and arriving at the end goal of improving crop yield and quality.
These products have created some tension in the agricultural community, particularly around regulatory requirements because they don’t fit squarely into either of the two traditional crop input categories. They’re not fertilizers and they’re not pest control chemicals. The crop input industry is organized around these historical categories. So where do biostimulants fit in and how do we begin to understand how to work with them?
Admittedly, overreliance on fertilizers and chemicals creates several problems for agriculture. With yield and quality being the goal of everyone involved, biostimulants can supplement vigor and crop resilience and improve yield and quality while reducing the environmental load of potentially harmful materials.
The newer science of molecular biology helps us to understand what goes on in the machinery of a plant as it reacts to its environment. Depending on conditions and the availability of nutrients and other essentials, the plant’s genetic code conducts its commitment to growth, using polarity and gravity as directional forces. Von Liebig understood 200 years ago how nutritional shortages could limit yield. What we understand now is that nutrition is only part of the equation. We can have soil nutrient levels at optimum and still have growth stalled out if other requirements are not met. At the center of it all is the genetic code. The rate of growth depends on signaling by hormones and the activation of genes inside the plant as it senses conditions around it. Gene activity is increased, or suppressed, in response to nutrient status and other factors like hormone concentration and environmental conditions. It has been estimated that over 30% of all the genes in higher plants have something to do with hormones.
Almonds treated with foliar cytokinin on the right are consistently larger and heavier.
Cytokinin in the Plant
One category of biostimulants is products which contain natural hormones like plant extracts and seaweed. The growth hormone cytokinin (CK) is an active component in some of these products. In the plant, the level of available CK can be increased in two different ways: stimulating synthesis of larger amounts of natural CK or adding exogenous applications of CK-containing materials. Applications of CK-containing seaweed products have become mainstream over the last 40 years. This can help with several plant processes at various stages of growth.
Natural systemic CK is produced primarily in the meristematic zones of the roots and apical growth points and is needed for cell division and other processes throughout the growth cycle. It is needed for ongoing formation of all tissue and controls the size and shape of leaves, as well as the how the leaves are arranged on a stem. Whether plants grow from a crown or from a stock or trunk they benefit from higher CK levels. CK determines the number of cells and potential size of a leaf. For crops like leafy vegetables, the size, quality and shelf life of the crop rely on a constant supply of CK during growth. CK is the hormone associated with holding back leaf senescence and is known as the juvenility hormone.
CK also regulates chlorophyll synthesis in the leaves, and increased CK is known to improve photosynthesis. When we think about the influence that cell division also has on the size of the leaves, we realize CK is essential to the overall photosynthetic capacity of the plant. Photosynthesis is also supported longer in the growing season with senescence delayed by higher CK levels. We will also discuss the role that CK has in the transport of sugars to other parts of the plant. CK is important for plant processes and growth throughout the plant’s life cycle.
Leaves and nuts on cytokinin treated almonds are uniformly larger and more photosynthetic with less senescence observed.
Besides the meristematic zones of the roots and apical tips, there is another place in the plant where CK is produced and plays very important roles. That is in the points where any new vascular tissue is being developed. Vascular tissue conducts the movement of water, nutrients and other solutes throughout the plant. CK is not only required for the formation and growth of new vascular tissue, it is also a regulator in the movement of photosynthates and other solutes through the phloem. Phloem tissue has associated companion cells which are responsible for metabolism and the regulation of phloem function including sugar transport. These companion cells are connected to the sieve tube elements and form bridges to exchange signaling for the transport of sugars from source to sinks. Simply stated, the presence of CK in younger tissue is what calls for the movement of energy and nutrition from the source to the sink.
Similarly, in flowering crops, CK is needed for the development of reproductive tissue and can increase bloom and flower retention. During these times in the growth cycle when new tissue is developing, a spike in the pool of available hormones is necessary. When flowering takes place in unfavorable conditions, the strength of the flowers and pollen is limited by the lack of CK. If adequate hormones are not available, the genetic potential of the plant is compromised. Pollen quality improves with higher levels of CK. Fertilization and the cell division which takes place immediately afterward require available CK. It’s well-established that the quality and size of fruits and nuts are improved by higher levels of CK for cell division at fruit set and the early growth stages afterward. A tighter cell density at this stage contributes to improved size and quality after cell enlargement and crop bulking take place.
Corn lower leaf senescence indicates shortages of cytokinin as growth hormone is translocated downward to support growth of roots. Brace roots also indicate a shortage of growth hormone.
In Research
Several crops have to flower during hotter weather. Summer heat can take a toll on crop yield. Molecular research into the effects of high temperatures on crops during pollination was conducted by Stoller Group, now a division of Corteva Agriscience. It was found that genes responsible for production of enzymes which break CK down by oxidation become upregulated in the plant during higher temperatures. Under better conditions, as previously mentioned, the amount of CK in the plant peaks at the time of flowering to assist in the process of fertilization. Lower CK during hot weather diminishes pollen strength and reduces the shedding of pollen, and the resulting weakened embryos have a higher potential to abort. Infertility of flowers can result from heat-induced male sterility.
Stoller found supplying exogenous CK provided significant improvement to pollination during heat events. This strategy has been used extensively to improve pollination and grain or fruit set across a wide variety of crops by Stoller agronomists since this discovery 10 years ago.
CK is a biostimulant with benefits that have been discovered and researched extensively. Because of the many natural and bioidentical products containing CK, it serves as a great example of how biostimulant products can enhance and improve plant performance at different stages of growth. Increased understanding of plant physiology, molecular biology and genetics has led to many validations of biostimulant technologies to improve crop production and quality. CK products are only one example of the biological and biostimulant products which are entering the market and emerging as solutions to the problems of water and nutrient use efficiency, stress management, weather resistance and other improvements to crop yield and quality.
Figure 1. Herbicide-resistant common chickweed grows unscathed to the top of the triticale canopy in this field after it was treated with ALS inhibitor herbicide (all photos by N. Clark.)
Small grain cereals (wheat, triticale, rye, oats and barley) are planted each year in California on about 550,000 acres from the northern border near Oregon to the southern border near Mexico. These versatile crops can be harvested as grain, straw, hay, green-chop or silage. Today, much of the small grain cereals grown in California are used for dairy cattle feed. Dairy sales topped $7.6 billion in 2021, 14.8% of the state’s crop cash receipts.
Crop yield and quality reduction from weeds is a major concern of small grain producers. From 2015-19, PCAs recommended herbicides on an average of 941,000 small grain acres/year. Many fields were treated more than once for weed control (de Souza Dias et al. 2021). During this period, the most common herbicide modes of action on applied acres were acetolactate synthase (ALS) inhibitors (37.8%), synthetic auxins (31.8%) and protoporphyrinogen oxidase (PPO) inhibitors (17.7%). Major concerning weeds in California small grain crops include littleseed canarygrass (Phalaris minor), Italian ryegrass (Lolium multiflorum), shepherd’s-purse (Capsella bursa-pastoris), London rocket (Sisymbrium irio), mustards (Brassica spp.), little mallow/cheeseweed (Malva parviflora), coast fiddleneck (Amsinckia menziesii var. intermedia), burning nettle (Urtica urens) and common chickweed (Stelleria media).
In 2021, concerns from PCAs in the southern San Joaquin Valley became urgent as they noticed a pattern of failures to control common chickweed (escapes) several years in a row. Some fields were becoming overgrown with the weed (Figure 1).
Figure 1. Herbicide-resistant common chickweed grows unscathed to the top of the triticale canopy in this field after it was treated with ALS inhibitor herbicide (all photos by N. Clark.)
An Emerging Problem
As PCAs and pesticide product representatives noticed some ALS inhibitor herbicides weren’t working as well as expected to control common chickweed in small grains, they conducted field trials and reached out to UCCE Farm Advisors Nick Clark and Jose Dias and Specialist Brand Hanson. In 2021 when it was too difficult to dismiss the repeated escapes as mistakes in applications or poor environmental conditions, several UCCE Farm Advisors gave more attention to fields with escapes and conducted early evaluations attempting to repeat results PCAs and product reps saw in their trials.
The PCAs were concerned they were seeing effects of herbicide resistance. This is a genetic phenomenon occurring naturally as weeds are challenged to live when exposed to herbicides. Genetic variation in the weed population gives some individuals natural resistance to the damaging effects of the herbicide. The problem arises when the same herbicide is used repeatedly, and those naturally resistant individuals continue to reproduce and become dominant in the weed population.
The diligence of these PCAs is the reason the broader ag community alerted early to this potentially serious problem.
Early Warning Grows into Full-Blown Research
Early testing in a field with high common chickweed pressure and documented escapes from ALS inhibitors showed Clark, Dias and Hanson that herbicide resistance could not be ruled out as a potential cause of the escapes. In that field, Clark and Dias applied tribenuron methyl (Dupont Express Herbicide with Totalsol Soluble Granules) and pyroxsulam (Simplicity CA), two frequently used ALS inhibitors in California, at two and four times the highest allowable label rate. Clark and Dias observed that tribenuron slightly reduced the common chickweed growth and pyroxsulam had virtually no effect when compared to an untreated control (Figure 2). This evidence warranted more intensive studies on common chickweed populations with ALS inhibitor escapes. Clark partnered with California State University, Fresno Professor Anil Shrestha to conduct controlled environment trials. These were designed to learn whether these weeds were resistant to ALS inhibitors.
Figure 2. Early results from a replicated field trial could not rule out the possibility of ALS inhibitor herbicide genetic resistance in common chickweed found in triticale in the southern San Joaquin Valley. “UTC” means “untreated control.” Skinny bars represent one standard deviation. Treatments under similar lowercase letters are not significantly different from each other according to Tukey’s HSD test (a= 0.05).
Seeds were collected by Clark and technicians Ben Halleck (UCCE) and Walter Martinez (Tulare County) from common chickweed in ALS inhibitor escape fields and an organic field where there was no recent ALS inhibitor application. The organic seeds were ALS inhibitor susceptible controls because their population was not pressured to evolve resistance. The weed seeds were planted in pots in a greenhouse and grown under a shade structure at Fresno State until they reached the ideal growth stage for ALS inhibitor control. After several experiment repetitions, Clark, Shrestha and Fresno State students Paola Vidales, Kiera Searcy and Jennifer Vidales confirmed what PCAs worried about. The common chickweed identified as ALS inhibitor escapes was in fact genetically resistant to tribenuron methyl and pyroxsulam (Heap 2023).
The UCCE and Fresno State team observed that the ALS inhibitor resistant common chickweed responded to increasing doses of pyroxsulam but not to tribenuron methyl. When the dose of pyroxsulam was increased up to eight times the max label rate, the treated common chickweed became more stunted and more of the plants died. Because the plants were not controlled with the maximum allowable rate, they are considered resistant. When using tribenuron methyl, the common chickweed plants sprayed with eight times the max allowable label rate appeared the same and survived as much as plants sprayed with half the label rate. Many of the plants survived the herbicide applications and eventually flowered, suggesting they may be reproductive.
The UCCE and Fresno State team held public field days at Fresno State. Product reps, PCAs and students gathered to observe the results of the study (Figure 3). The value of the field day was two-fold: 1) ag professionals and students learned about the confirmation of common chickweed populations in California that are genetically resistant to ALS inhibitors, and 2) the research team gleaned important insights from the ag professionals’ feedback. That feedback has been critical in guiding ongoing research.
Figure 3. Fresno State Masters student Jennifer Valdez-Herrera (right) teaches field day attendees about the findings of ALS herbicide resistance in common chickweed populations found in the southern San Joaquin Valley with real-life demonstrations of these weeds surviving various ALS inhibitor herbicide applications.
Future Directions
The two different responses between the herbicides gave clues to the researchers about the ways the plants were expressing resistance to the herbicides. Fresno State professor Katherine Waselkov, specializing in genetics of herbicide resistance, joined the team to begin exploring the genetic basis of this problem in California. The major question Waselkov is researching is whether the Central Valley populations of chickweed showing resistance to ALS inhibitors have mutations in the ALS gene, which codes for the enzyme targeted by the herbicides. By extracting DNA, conducting PCR and sequencing the entire ALS gene, the lab can detect possible resistance mutations that occur in this enzyme. Changes in several amino acids that directly interact with the herbicides are likely to cause resistance. These changes have been detected in other countries’ chickweed infestations (Marshall et al. 2010; Laforest and Soufiane 2018). Waselkov’s different approach will also screen for other less common mutations that could cause resistance.
Pyroxsulam and tribenuron methyl are not the only ALS inhibitor herbicides used in small grains. Industry professionals pointed out mesosulfuron-methyl (Osprey Herbicide), although used less commonly, is also regularly applied to small grains. Additionally, it was pointed out that small grain fields where common chickweed was resistant to ALS inhibitors were recently rotated out of or into alfalfa, a common rotation partner, where multiple ALS inhibitor herbicides are applied to control several important weeds. For these reasons, the UCCE and Fresno State team continues to expand the controlled environment study to determine the presence of ALS inhibitor resistance in common chickweed with more herbicide products.
Fresno State Masters student Jennifer Valdez-Herrera is earning a degree in plant science through further study into ALS inhibitor resistance in common chickweed in California. Studying under Shrestha, Waselkov and Clark, Valdez-Herrera is addressing all the issues above in addition to studying the impact that uncontrolled common chickweed has on yield and quality of small grains.
Growers and PCAs are apt to point out ALS inhibitors are a very important tool for controlling multiple serious weeds in small grains and alfalfa. Further, there are still several other herbicide modes of action registered for California small grains and alfalfa which have activity against common chickweed. The PCAs are the first line of defense in the field and the first consultants for knowledge about how to remedy weed problems. Our team continues to consult PCAs to guide future research endeavors in this field to make sure the solutions we seek are relevant and applicable to the industry.
References
Dias, J. L. C. dS., Clark, N., Mathesius, K., Light, S., Hanson, B., Lundy, M. E., Shrestha, A., 2021. Poor control of common chickweed with ALS-inhibitor herbicides reported in multiple small grain fields in the southern San Joaquin Valley. Is it a new case of herbicide resistance in California? UC Weed Science Blog. Retrieved 7/28/23, https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=50181.
Heap, I., 2023. The International Herbicide-Resistant Weed Database. Retrieved 7/28/23, http://weedscience.org/Pages/Case.aspx?ResistID=24253.
Laforest, M., and B. Soufiane. 2018. Coevolution of two sulfonylurea-resistant common chickweed (Stellaria media) biotypes with different mutations in the acetolactate synthase gene. Weed Science 66: 439-445.
Marshall, R., R. Hull, and S. R. Moss. 2010. Target site resistance to ALS inhibiting herbicides in Papaver rhoeas and Stellaria media biotypes from the UK. Weed Research 50: 621-630.
On-farm groundwater recharge methods include winter flooding, skipped row flooding, subsurface reverse tile drainage, recharge ponds and unlined canals or irrigation ditches (photo courtesy Almond Board of California.)
California relies heavily on groundwater, with 40% of annual demand supplied by aquifers during non-drought years and upwards of 60% during dry years. Approximately 83% of people in California receive part of their annual supply from groundwater, and many communities are exclusively reliant on well water for both ag and domestic purposes(3). Chronic groundwater overdraft, estimated at 2 million acre-feet per year since the 1960s, leaves California communities and farmland vulnerable to water shortages and rising surface water and pumping costs(1).
Groundwater depletion also causes subsidence, or land sinkage, as dry aquifers collapse. Subsidence damages infrastructure, such as roads, powerlines, pipes and canals. The California Aqueduct’s flow capacity has reduced due to increasing subsidence rates, with some areas surrounding the infrastructure sinking almost 1.25 inches per month(4). Groundwater recharge efforts have begun to reverse subsidence in some regions, but aquifer degradation and sunken land is often irreversible, and recharge efforts fail. The collapsed aquifers are irreparably damaged and can no longer store water.
California experienced the worst drought on record from 2012 to 2015, exacerbating groundwater pumping overdraft, water shortages and subsidence. Recharge during rainy years does not replenish the loss, and between 2010 and 2020, roughly 28% of monitored wells in California declined by 5 to 25 feet(3,6,8). State officials responded by passing the Sustainable Groundwater Management Act (SGMA) in 2014 to bring pumping and recharge into balance by 2040. Local groundwater sustainability agencies, tasked with developing recharge implementation plans, will likely include Agricultural Managed Aquifer Recharge (AgMAR) to meet requirements and balance the groundwater pumping budget(7).
On-farm groundwater recharge methods include winter flooding, skipped row flooding, subsurface reverse tile drainage, recharge ponds and unlined canals or irrigation ditches. Traditional recharge methods like drainage basins require land dedicated solely to water percolation, limiting groundwater banking with high infrastructure construction costs and spatial constraints. Winter surface flooding on irrigated agricultural land can be implemented widely at low cost and with few infrastructure or management changes at the field level. With 5 million acres of California farmland suitable for AgMAR, groundwater banking can scale up dramatically if even a small percentage of acreage can dually purpose for recharge during rainy years(3,6).
University research and early commercial implementation on orchards, vineyards and other crops indicate AgMAR can effectively recharge aquifers and benefit production in multiple ways(3,5,6,7). AgMAR helps growers and landowners secure irrigation water availability for dry years while preventing further aquifer degradation and subsidence. When properly managed, recharge efforts can also improve soil quality by leaching excess salinity below the root zone. Well water quality also improves when clean stormwater dilutes nitrate and total dissolved solids (TDS) accumulated in the basin after years of ag chemical leaching and pumping overdraft. Better soil and irrigation water quality improve crop health and fertilizer use efficiency, leading to lower production costs or increased yield and crop quality.
Implementation
Growers and landowners implementing AgMAR must consider soil suitability for recharge, crop tolerance, water application timing and field management practices to protect both crop health and groundwater quality. Research and field studies conducted over the last 10 years can guide site selection and implementation, but land managers must evaluate their own soils and crops to adjust AgMAR protocols to fit the unique conditions at each ranch.
Site Selection
The Soil Agricultural Groundwater Banking Index (SAGBI), developed by Toby ‘O Geen and collaborators at UC Davis, provides a scoring system to determine farmland’s suitability for AgMAR6. SAGBI assigns scores in five categories, including deep percolation, root zone residence time, topography, chemical limitations and soil surface condition. The weighted average of scores in all categories is used to classify ag land as Excellent, Good, Moderately Good, Moderately Poor, Poor or Very Poor for agricultural groundwater banking. The best ground for recharge is on flat land with sandy or sandy loam soils. Ideally, the soil should have fast water penetration and infiltration, and little to no chemical limitations such as high salinity, nitrates or pesticide residue that could contaminate groundwater if leached into the basin(6,8).
Over 17.5 million acres of farmland in California have been scored using data from the USDA-NRCS Soil Survey Database, and 5 million acres were rated as Excellent, Good or Moderately Good for AgMAR. Most of the land suitable for recharge is found in the eastern Central Valley as well as some locations in Santa Maria, Salinas and Napa(6).
Land managers can look up their ground’s SAGBI score on UC Davis’ web-based mapping app at casoilresource.lawr.ucdavis.edu/sagbi/. Recharge suitability indicated by SAGBI scores should be verified on the field level by soil testing and site evaluation prior to AgMAR implementation.
Recharge methods from left to right: surface application to orchards, subsurface application to orchards and basins/water conveyance structures (illustrations by Jennifer Natali, courtesy Almond Board of California.)
Crop Suitability
Crop suitability for AgMAR depends on tolerance to soil saturation, crop value and financial risk, and likelihood of nitrate leaching due to typical fertilization patterns. Groundwater banking on agricultural lands is safest during winter dormancy or when fields are fallow. Surface applications typically flood the field with 6 to 8 inches of water that drains in a week or less depending on soil permeability(2,3,5). Oxygen levels in the soil decline due to standing water, and if saturation persists for too long, the crop’s roots may be harmed. Stressed and damaged root systems cannot absorb water and nutrients effectively, resulting in yield decline later in the season. Damaged roots are also more vulnerable to soilborne diseases such as phytophthora and fusarium, so fields with known disease pressure may not be good candidates for recharge(5,7). Growers can avoid root damage and even improve soil health by choosing appropriate fields and planning surface application to match soil drainage rates and crop tolerance thresholds.
Crops suitable for AgMAR include alfalfa, wine grapes, tomatoes, almonds and other tree nuts. Annual cropping fields can also be used for recharge during the fallow period between plantings. Alfalfa presents a good candidate for recharge because its relatively low value poses less financial risk compared with specialty crops if recharge damages production. AgMAR field experiments with alfalfa found no decline in root health or yield after winter flooding on well-draining soils, demonstrating growers can safely carry out recharge programs if the crop is in its dormant stage. Since alfalfa is sensitive to soil saturation during the growing season, researchers suggest rotating recharge sites to older crops scheduled for replanting the following year, especially if flooding events are expected in late winter or early spring(3,7).
Areas of soil suitability for groundwater banking in California taken from UC Davis web-based application. The application uses the Soil Agricultural Groundwater Banking Index to score specific farmland suitability.
Trees and vines are excellent candidates for groundwater banking, if flooded during winter dormancy, well before budbreak. Crop sensitivity to water logging varies with rootstock, but growers are advised to limit standing water duration to two days to avoid root damage. Wine grapes are fairly tolerant to soil saturation, and since they typically receive less nitrogen fertilizer than other crops, residual nitrate levels during the winter are low. Tomatoes, almonds and other tree nuts generally receive higher nitrogen application rates, posing greater risk to groundwater quality if excess nitrate leaches during flooding events. Low nitrate levels and saturation tolerance position vineyards as the safest crop candidate for recharge, but groundwater banking will be more effective if implemented on the state’s vast almond acreage as well(5,8).
UC Davis research funded in part by the California Almond Board demonstrated AgMAR efficacy on two commercial almond orchards in the Central Valley from 2015 to 20175. Researchers applied a total of 24 inches of water per year, split into multiple flooding events during winter dormancy for two consecutive years. One of the orchards, located near Dehli, Calif., has highly permeable, sandy soil. The second orchard, located in Modesto, has moderately permeable, sandy loam soil. Flood water at both locations percolated below the root zone in less than a week, and researchers found no negative impacts on tree water status, root health or yield at either site. Winter flooding proved an efficient recharge method, with over 90% of applied water percolating below the root zone on the sandy soil and over 80% percolation on the sandy loam. Similar studies on pistachios, wine grapes and tomatoes also resulted in effective groundwater banking while maintaining root health and productivity(2,3,5).
California almond growers have begun implementing AgMAR on their own orchards with promising results. Mark McKean, a prominent grower in the Fresno area, began testing flood irrigation and recharge in 2010, and by 2020 he had experimented with groundwater banking on 350 acres. In 2016, McKean banked 2 ac-ft/ac and in 2019 1.4 ac-ft/ac on a three-year-old almond orchard. McKean found better percolation when flood water was applied for short durations rather than one long set, and he reduces his fall nitrogen application rate on fields slated for winter recharge to prevent excess nitrate leaching below the root zone(7).
When properly managed, recharge efforts can also improve soil quality by leaching excess salinity below the root zone (photo courtesy Almond Board of California.)
Other growers have installed subsurface groundwater banking systems to pump recharge water below the root zone and bypass the risk posed by surface flooding. Diverted stormwater or other source water is piped down to a reverse tile drain system at least 8 feet below the soil surface. Several subsurface systems have been installed in the Central Valley since they were introduced in 2017, and some water districts offer incentives programs to help cover the costs. Subsurface recharge systems are expensive to install, but they facilitate high recharge volumes without impacting roots and crop health(7).
Water Quality Considerations
Groundwater banking on ag lands has the potential to significantly improve water resource security in California, but AgMAR poses a risk to groundwater quality if excess salts, nitrates and other residual contaminants are leached down to aquifers. Preliminary soil testing can assess field suitability for recharge before each rainy season, and sites with excess nitrate, salinity or pesticide residue may be passed up in favor of sites with optimum chemical characteristics. Fields with higher residual contamination may still be good candidates for recharge if enough clean stormwater is available to dilute nitrate and salinity down to safe drinking water quality standards. Growers can also prepare for winter recharge by reducing the proportion of annual nitrogen fertilizer applied in the fall.
Residual contaminants in the vadose zone, the unsaturated area between the soil and the groundwater table, may also pose a risk to groundwater quality. Salts, nitrate and ag chemicals accumulated in the vadose zone after years of commercial agriculture may be mobilized by high-volume recharge events and leach down to the groundwater basin. Hannah Waterhouse and colleagues at UC Davis analyzed soil core data down to 30 feet on 12 fields in the Kings groundwater basin to quantify potential risk of nitrate and salt contamination to aquifers8. The study compared the effects of soil permeability, crop type and fertilizer management on nitrate and salt accumulation in topsoil and below. Fields with lower water infiltration rates had higher nitrate and salinity levels compared with more permeable ground. Soils with slower water infiltration rates stored on average 732 lbs N/ac while lighter, well-draining soils stored 542 lbs N/ac within the 30-foot profile(8). Information gleaned from this study and other research can help determine the source and volume of water required for recharge at each site to ensure that leached contaminants are sufficiently diluted to protect well water quality.
Crop type and grower management also strongly affected nitrate and salinity levels. High nitrogen application rates on tomatoes and almonds were reflected in the soil profile, while wine grapes with lower N applications and deep root systems almost always contained the lowest nitrate levels. Elevated residual nitrate found on one outlier vineyard was explained by the grower’s fertilizer management. While other wine grapes received split N applications, the field with unusually high residual nitrate received the entire year’s N supply in one shot at the beginning of the season, demonstrating management’s strong impact on nitrate leaching8. Regardless of crop type, growers implementing AgMAR can protect underlying groundwater by testing the soil’s N level in fall and adjusting fertilizer management to prevent nitrate leaching.
Groundwater quality monitoring and collaboration between growers, researchers and water agencies will help to safely implement AgMAR and improve recommendations to meet differing requirements at each ranch. Further research is required to understand how the vadose zone’s characteristics will impact groundwater quality in response to AgMAR, but initial studies indicate that the benefits of recharging our groundwater basins outweigh the potential risks when appropriate sites and field management strategies are implemented.
Average groundwater overdraft in California is estimated at about 2 million acre-feet per year, and from 2005 to 2010, the Central Valley alone overdrafted an estimated 1.1 to 2.6 million acre-feet(1,6). Pumping restrictions required by SGMA may cause between 750,000 and 1 million acres of agricultural lands to go fallow without new supply mitigation measures(7). Agricultural lands rated as Excellent or Good by SAGBI can percolate an estimated 1 foot of water per day, and if AgMAR were implemented on suitable wine grape acreage in the Central Valley, growers could bank 460 million acre-feet of water per day(6,8). AgMAR implementation at scale will require supply rights and infrastructure to divert excess stormwater to agricultural fields, but SGMA funding and compliance deadlines will likely motivate stakeholders to facilitate on-farm groundwater recharge efforts. Thousands of acres of wine grapes, almonds, alfalfa and tomatoes are planted on land suitable for groundwater banking, giving growers an opportunity to secure water resources for future crop production and their communities.
References
1. [CDWR]California Department of Water Resources. 2009. Bulletin 160–09: California water plan update. Sacramento (CA): California Department of Water Resources. http://www.waterplan.water. ca.gov/cwpu2009/.
2. Levintal E, Kniffin M, Ganot Y, Marwaha N, Murphy N, Dahlke H (2022): Agricultural managed aquifer recharge (Ag-MAR)—a method for sustainable groundwater management: A review, Critical Reviews in Environmental Science and Technology, DOI: 10.1080/10643389.2022.2050160
3. Dahlke H, LaHue G, Mautner M, Murphy N, Patterson N, Waterhouse H, Yang F, Foglia L. 2018. Chapter Eight – Managed Aquifer Recharge as a Tool to Enhance Sustainable Groundwater Management in California: Examples From Field and Modeling Studies. Editor(s): Jan Friesen, Leonor Rodríguez-Sinobas. Advances in Chemical Pollution, Environmental Management and Protection. Elsevier, Volume 3: 215-275. ISSN 2468-9289. ISBN 9780128142998. https://doi.org/10.1016/bs.apmp.2018.07.003.
4. Lopes et al. 2017. California Aqueduct Subsidence Study. California Department of Water Resources, Division of Engineering, San Luis and San Joaquin Field Divisions. https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Engineering-And-Construction/Files/Subsidence/Aqueduct_Subsidence_Study-Accessibility_Compatibility.pdf
5. Ma X, Dahlke H, Duncan R, Doll D, Martinez P, Lampinen B, Volder A. 2022. Winter flooding recharges groundwater in almond orchards with limited effects on root dynamics and yield. Calif Agr 76(2):70-76. https://doi.org/10.3733/ca.2022a0008.
6. O’Geen AT, Saal M, Dahlke H, et al. 2015. Soil suitability index identifies potential areas for groundwater banking on agricultural lands. Calif Agr 69:75– 84. https://doi.org/10.3733/ ca.v069n02p75
7. Roseman J, Lee E, Asgil L, Mountjoy D. 2021. Almond Board of California, Document #2021R0060. https://www.almonds.com/sites/default/files/2021-12/WO-6177_ABC_GroundwaterRecharge_Web_SinglePage.pdf
8. Waterhouse H, Bachand S, Mountjoy D, Choperena J, Bachand P, Dahlke H, Horwath W. 2020. Agricultural managed aquifer recharge — water quality factors to consider. Calif Agr 74(3):144-154. https://doi.org/10.3733/ca.2020a0020.
Advancements in irrigation systems, such as drip, micro and pivot systems, have improved water distribution and incorporated fertigation (photo by Taylor Chalstrom.)
As agriculture faces increasing pressure to produce more food with less resources, the role of irrigation and nutrient management has become ever more critical. Efficient irrigation and nutrient management practices are essential not only for maximizing crop yield and quality, but also for promoting sustainability and minimizing environmental impacts. In this article, we will explore the interconnected role of irrigation and nutrient management in agriculture and how growers and advisors can implement strategies to improve their efficiency and effectiveness. By understanding the relationship between these two critical factors, we can promote sustainable agriculture while ensuring food security for generations to come.
Years ago, as I was interviewing many farm managers and their advisors to better understand their irrigation practices, I kept hearing one common statement: “The fastest way to compromise a great nutrition plan is to irrigate improperly.” Efficient irrigation management is crucial to minimize water losses, optimize nutrient use efficiency, improve soil health and increase grower profitability. Their goal is to manage irrigation by applying it at the proper time and rate for the specific crop demand and soil conditions. Excessive watering can cause waterlogging, nutrient leaching, soil erosion, disease and decreased crop yields. Conversely, insufficient watering can result in stunted growth and reduced harvest.
Agronomists have accepted and are committed to the 4Rs of Nutrient Management. While traditionally our 4Rs focus has been on the nutrients delivered with fertilizers, we can use the same paradigm to manage the equally essential, and in some crop systems more limiting, nutrients of hydrogen and oxygen delivered in the form of H2O.
The same paradigm of nutrient management can be used for irrigation management when thinking about right source, right place, right time and right rate. Efficient irrigation management is crucial to optimize nutrient use efficiency.
Right Source
Choosing the right source of water for irrigation is crucial. Water quality can vary significantly, and it is essential to consider factors such as salinity, alkalinity and potential contaminants. Testing the water source and ensuring it meets the required quality standards will help prevent adverse effects on soil health and plant growth. Growers and advisors should consider the following factors regarding water quality:
Salinity
High salt concentration can harm plants, reduce crop yield and quality, and affect soil health. Use electrical conductivity (EC) or total dissolved solids (TDS) meters to measure salinity and manage it through leaching, salt-tolerant crops or water treatment.
pH
Water acidity or alkalinity affects nutrient availability, uptake and soil health. Maintain a pH range of 6.0 to 7.5 through pH-adjusting chemicals or selecting pH-tolerant crops.
Nutrient content
Nitrogen, phosphorus, and potassium levels in water impact plant growth and nutrient management. Adjust fertilizer rates or choose crops suitable for specific nutrient levels.
Pathogens and contaminants
Water may contain harmful bacteria, viruses and heavy metals that affect plant and human health. Implement water treatment, testing and monitoring practices.
Water availability
Consider the source, quantity and timing of water for irrigation. Implement water management practices to ensure availability throughout the growing season.
Water quality is vital for agricultural irrigation. Though growers cannot control the quality of their water source, they can monitor and adjust it as needed. Consider all relevant factors to ensure suitable water for crop growth without posing risks to plants or human health.
Right Place
The “right place” in irrigation management involves effectively delivering water and nutrients to the plant’s effective root zone. Advancements in irrigation systems, such as drip, micro and pivot systems, have improved water distribution and incorporated fertigation (applying fertilizers through irrigation.) Fertigation increases nutrient efficiency, reduces waste and promotes soil health. Regular maintenance ensures high distribution uniformity, avoiding uneven irrigation. To evaluate distribution uniformity, contact your local Natural Resources Conservation Department or refer to this resource: ucanr.edu/sites/farmwaterquality/files/156399.pdf. Proper installation, maintenance and monitoring optimize the right place for uniform water and nutrient distribution, maximizing crop yield and sustainability.
Right Time
Knowing how much and when to turn on irrigation is crucial for maximizing water efficiency, promoting healthy plant growth, and optimizing crop yield. Consider the following factors:
Crop water needs
Understand the specific water requirements of each crop, considering different growth stages and their corresponding water demands. This knowledge helps determine when irrigation is necessary for optimal crop development.
Soil moisture monitoring
Regularly monitor soil moisture levels using sensors or visual inspection techniques. This information identifies when the soil has dried sufficiently to require irrigation, avoiding both overirrigation and underirrigation.
Weather conditions
Monitor weather forecasts and local climatic patterns. Factors like temperature, humidity, wind and solar radiation influence evapotranspiration rates, affecting water loss from the soil and plants. Adjust irrigation timing based on anticipated water loss.
Plant stress indicators
Observe signs of water stress, such as wilting, leaf rolling and changes in leaf color, to determine irrigation needs. Providing water at the right time prevents water stress, promotes optimal plant growth and minimizes crop yield losses.
Remote plant stress monitoring
Innovative technologies using sensors, aerial imagery or satellite data enable real-time monitoring of plant stress levels. Adjust irrigation timing based on these insights, improving water efficiency and crop performance.
Irrigation scheduling techniques
Utilize techniques like soil moisture-based scheduling, crop evapotranspiration (ET) data or plant water demand. These tools guide when to irrigate, considering crop needs and environmental conditions.
Water conservation considerations
In water-limited regions, time irrigation to maximize water use efficiency. Avoid peak water demand periods, applying water during cooler, less evaporative periods to minimize water loss and optimize utilization.
By considering these factors, growers can determine the appropriate timing for irrigation, ensuring crops receive adequate water when needed the most. This approach maximizes water efficiency, conserves resources and promotes healthy plant growth and optimal crop yield.
Right Rate
Once we know the amount of water the plant needs and when, we need to determine how frequently and how long to apply the water so that we do not have runoff or infiltration below the effective root zone. This might be an area for most improvement. By determining the appropriate rate, we can ensure that water and nutrients remain within the effective root zone, where plants can efficiently utilize them. This minimizes leaching and evaporation, reducing loss and waste.
To determine the right rate of irrigation:
Understand soil characteristics, including type, infiltration rate and water holding capacity
Determine the irrigation application rate specific to your system
Consider the water demand of the crop
Several tools can aid in developing an effective irrigation schedule. These include evapotranspiration models, soil moisture monitoring and plant-based sensors that track water and nutrient uptake. By utilizing these approaches, farmers can align irrigation events with actual plant and soil water needs, maximizing water use efficiency.
As agriculture strives to meet the growing global food demand while conserving resources, the proper management of irrigation and nutrients has emerged as a critical aspect. This article has emphasized the importance of efficient irrigation and nutrient management practices for achieving optimal crop production, maintaining high-quality harvests and reducing environmental harm. By adopting the 4Rs of Irrigation Management that improve the efficiency and efficacy of these practices, growers and advisors can contribute to sustainable agriculture. Through a comprehensive understanding of the interplay between irrigation and nutrient management, we can pave the way for a future where agriculture meets the needs of the present while safeguarding the needs of future generations.
The concept of well-balanced plant nutrition in citrus.
Citrus huanglongbing (HLB), previously called citrus greening disease, has been covered in thousands of articles. New technology and management tools continue to be discovered. Yet we still have no definitive cure and it could be a while before we see one. New rootstocks are surfacing, and treatment techniques are having some effect on slowing this devastating disease that has destroyed huge amounts of the world’s citrus trees.
As a Certified Professional Agronomist and a Certified Crop Advisor, I must prepare myself for HLB and its arrival here in the rest of California. It is currently here in one county. Many qualified scientists are approaching this menace with specific technologies, genetic modifications to trees, finding new resistant root stocks and of course trying to find something that will destroy the infections as well as a chemical or biological spray or injection that can combat and even cure the infection.
Others are addressing the psyllid vector, Diaphorina citri, that spreads the disease. Spray timings, mating disruptions, netting to cover the trees and keep the insect off them are being addressed.
Well balanced nutrition programs for citrus may improve yield, tree health, fruit quality and additional years of production in HLB-infected trees.
Nutrition for Management
One approach I have used over the years mainly in Florida and Texas is based on years of repeated trials. It calls for using balanced and adjusted plant nutrition. It is most certainly not a cure, but I have witnessed improved yield, tree health, fruit quality and additional years of production from infected trees. It takes management and a deep knowledge of your citrus trees. When are the critical times of a developing tree, flower, bud development and fruit set, sizing, brix production and even color?
We need to study the amounts and most effective types of nutrients to apply. Balancing your crop nutrition is always critical, but a crop infected with the HLB needs special attention and application changes from a healthy crop nutrition program. A well-balanced program may handle these demands.
We must do more than a normal citrus nutrient program. One should understand the beneficial use of foliar applications of fertilizer nutrients and SAR (Systemic Acquired Resistance) products to maintain the health of HLB-infected trees. Several groves have maintained tree health and production by producing 7 to 10 years of profitable crops. The cultural production programs consist of a foliar spray cocktail of nutrients and SAR products applied three or more times per year to coincide with the initiation of vegetative growth flushes. The application of the nutrient/SAR foliage spray program can reduce and ameliorate HLB leaf symptoms and includes a good soil-applied dry fertilizer.
The greatest gain in fruit retention can be made during fruit set.
We know the movement of the bacteria inside the roots and leaves severely blocks the phloem tissue of these two areas. The interruption and restrictions to the movement of nutrients and sugars results in leaves dropping and remaining leaves being smaller. By using products that can stimulate or increase the efficiency of these affected areas, we should be able to improve movement of nutrients and sugars.
Research showed potassium nitrate resulted in the highest net income increase, $1925/ha/year in HLB-infected Florida citrus.
Research has shown that by applying potassium foliarly, we can improve the health and production of infected trees. Polyamines have also been shown to be effective.
Other Possibilities
If simply changing application methods and fertilizer sources can make major changes, we can start to imagine other possibilities.
Potassium nitrate sprays show less greening in Valencia oranges.
Verdesian Life Science has proven the use of phosphites can improve sugar and size in citrus fruit when compared to citrus trees not using this technology. Phosphites also trigger a plant’s own ability to increase its SAR. Plants have this ability to help them fight off infections.
Verdesian also uses other proven biostimulant products such as Primacy Alpha that through foliar applications increase nitrogen assimilation. The increased effect on the glutamine/glutamate pathway increases production of amino acids, proteins, lipids and other essential building blocks in the plant. We have seen a consistent increase in new root growth. This could lead to a healthier pathway for water and nutrient uptake. Water uptake is essential to carry nutrients and provides much of the fruit weight.
Increased leaf size, chlorophyll production and CO2 fixation were documented in citrus trials using Primacy Alpha.
With Primacy Alpha, we documented increased leaf size, chlorophyll production, and CO2 fixation which could offset the HLB effects on leaves. With the documented increased flowering, we might improve fruit counts. With consistent nitrogen uptake improvement along with whole plant biomass gains, we continue to successfully improve the health and production on healthy and stressed crops. Primacy Alpha also contains a cytokinin precursor. It triggers the plant to produce more natural cytokinin. Cytokinins play a role in cell division. More cells mean bigger fruit.
If plant growth, smaller fruit, leaf development, reduced chlorophyl production and activity plus phloem blockage are results of HLB, it stands to reason that stimulating and improving these restricted systems could benefit the affected crop.
Seeing the effect HLB has on fruit size and fruit color, we must seek ways to offset these things that reduce marketability of fruit.
Products such as Cyto-Red+, a unique blend of patented technologies, help support plant performance through chelation/complexation.
Products such as Cyto-Red+, a unique blend of patented technologies, help support plant performance through chelation/complexation. MAC Trigger upregulates genes involved in the shikimic acid secondary metabolite pathway, which leads to production of flavonoids and anthocyanins, which are the main color components of the fruits.
I could spend days and thousands of additional words and examples to show how using plant nutrition and biostimulant could help at least extend the quality production on HLB infected citrus. It is not a cure but simply a temporary solution to continue producing fruit while we find a permanent solution.
In mature orchards, 65% to 80% of the spray flow should be applied through the top half of open nozzles (all photos by F. Niederholzer.)
The goal of airblast spraying is a uniform pesticide deposition of a known, prescribed pesticide rate throughout the entire target (tree canopy). Done right the first time, a good spray job saves the time and money of a second spray plus income lost due to crop damage in the case of a poor first spray (in tough economic times, a second spray for the same problem may not be in the budget.)
There are several steps to achieving this goal. Skipping any step will reduce spray efficacy and efficiency.
Step 1: The sprayer should travel at an appropriate speed to allow spray to reach the treetops. Too slow sprayer speed wastes time, too fast means poor coverage in the treetops and the risk of income loss due to crop damage.
Step 2: Point larger nozzles at thicker canopy (more leaves and nuts). For most orchard crops, this means 65% to 80% of the spray flow (gallons per minute) should be applied through the top half of open nozzles.
Step 3: Measure gallons per acre sprayed and, using total spray tank volume, determine the amount of pesticide product to add to each tank, to match your PCA’s recommendation.
Step 4: Check coverage with water-sensitive paper (WSP) placed in the canopy.
Details Ground speed
Airblast spraying uses air from the sprayer’s fan(s) to move the pesticide throughout the tree. If the fan’s air doesn’t reach the treetops, the pesticide won’t either. Ground speed is a simple and effective way to adjust air movement through the canopy, especially between bloom and harvest when spray coverage is most challenging.
The sprayer should travel fast enough so air from the sprayer’s fan reaches up through the tree to just above the tops of the tallest trees. To check this, at a time of day with little to no wind, tie a short (18-inch) length of surveyor’s ribbon to a section of PVC pipe or conduit and run the tubing up through the middle of a tree to a height just above the tallest trees in a planting. With the sprayer fan “on,” drive the sprayer past the tree with the flagging at tractor and sprayer settings you think is appropriate (e.g., full sprayer air delivery and 2.25 MPH sprayer speed).
Tying flagging to the nozzle ports and running the fan can help show you which ports point where in the tree. If the fan’s air doesn’t reach the treetops, the pesticide won’t either.
If the flagging flutters out to 45 degrees from the vertical as the sprayer passes the tree, the speed is appropriate for that planting at that time of the season. If the flagging just barely moves or doesn’t move at all, repeat the process with slower tractor speed. If the flagging kicks up to the vertical (180 degrees from dead hang), repeat the process at a faster tractor speed. Record the tractor and sprayer settings that deliver air movement from the sprayer fan to just above the canopy. Calculate the acres per minute sprayed at that ground speed by multiplying ground speed (feet per minute) by the row width. Note: If spraying on a day with slight winds, drive slower, delivering more fan air to compete with the wind and better cover the upper canopy.
Nozzle selection
With a gallons per acre (GPA) target from your PCA and the appropriate sprayer speed measured with the aforementioned “flagging on a pole” process, calculate the sprayer output (gallons sprayed per minute; GPM) needed.
Gallons per minute = (Gallons per acre) x (Acres per minute)
Now select nozzles to deliver the GPM you just calculated (on paper). More spray should be applied to areas of the tree with more leaf area. Upper-canopy locations often hold more crop than the rest of the tree and are the toughest to cover. Extra spray volume with larger nozzle size targeted there will deliver more uniform coverage.
Step 1: Park the sprayer in the orchard and look where the different nozzle ports are located. Tying flagging to the nozzle ports and running the fan can help show you which ports point where in the tree.
Step 2: Using the manufacturer’s catalog and desired system pressure (for example, 150 psi), select nozzle sizes to locate on different nozzle ports. The goal for mature trees is 65% to 80% of the total GPM going out the top half of the open nozzles. That is, if there are 16 nozzles per side of a sprayer that should be open in a particular orchard based on the sprayer and tree size, the top 8 should have most of the total GPM. Using the same nozzle size at every nozzle port will, at best, overspray the lower canopy while delivering good/decent coverage to the treetops (as long as the ground speed is right.)
Gallons per acre
With the ground speed and nozzles selected, determine the GPA by checking the math you just did in the previous step. Park the sprayer on flat ground and completely fill the tank with clean water. With the nozzles just selected on the sprayer and using the sprayer and tractor settings for the right/appropriate ground speed, turn on the spray booms for a measured amount of time (one minute, two minutes, etc.) and then shut off the flow. Refill the sprayer with clean water using calibrated buckets or a hose with a flow meter to measure how much water was sprayed in the time the nozzles were “on.” Calculate GPM from the volume sprayed and the run time. Adjust GPM, as needed, using the system pressure or by changing nozzle sizes or parts (e.g., two- or four-hole swirl plates for disc/core nozzles) to deliver the recommended GPA.
Check coverage
Water-sensitive papers (WSP) are small cards with yellow coating on one side that turn blue where water (or fingerprints) touches the surface. To check spray coverage, place WSP at different heights in the trees in the orchard. This can be done several ways. If you have a pruning tower, use it to get up into one or more trees in the orchard and directly clip WSP to leaves or attach to nuts. Flag each WSP location so you can find it later. Another method is to attach WSP at different heights on a PVC pole and run the pole up through the middle of the tree canopy.
Once WSP are up in the canopy, spray clean water down the row where WSP are placed using the tractor settings and nozzle selection/location determined earlier. Take down WSP after and compare upper- and lower-canopy locations to see if coverage is generally uniform. You can measure coverage with a smartphone camera and apps, but a visual scan should be enough. Are the lower cards all blue? If so, the lower canopy is getting too much spray. One possible fix for this is to change out lower nozzles for a size smaller and repeat the test. If the upper cards are not getting much coverage, increase selective nozzle sizes that target the upper canopy and/or slow down the sprayer.
Spraying when relative humidity is low (<40%) can cut spray deposition in the upper canopy in half compared to spraying when relative humidity is higher (early morning). This can lead to poor pest control and/or development of pesticide resistance. Especially in warm summer months with low daytime humidity, night and early morning spraying is important to achieving good spray coverage.
Effective pest control with pesticide(s) is a key backstop in a good, cost-effective IPM program. Good spray coverage (and material selection/spray timing) ensures the backstop is solid.
Phytophthora can rot both crowns and roots of row crops (two healthy shallot plants on left) (all photos by S. Koike.)
Phytophthora is the genus name given to a group of fungus-like organisms that have tremendous impacts on plants. Recent research indicates Phytophthora is closely related to brown algae and diatoms. Of the many documented Phytophthoras, a few dozen species cause disease on vegetable, fruit, ornamental and forest plants grown worldwide.
Phytophthora has a notorious record for damaging crops. One of the earliest notable cases involved Phytophthora infestans, which caused epidemics of late blight on potato in Europe in the 1840s to 1850s. Devastating losses of potato crops resulted in famines, human suffering and death, and forced migrations. Another species, P. cinnamomi, caused the loss of hundreds of plant species in Australia. Such widespread decline threatens the plant and animal ecosystems in this region. And even closer to home, in coastal California and southern Oregon, P. ramorum (sudden oak death) has killed millions of tanoak and coast live oak trees, and imposed the destruction of millions of ornamental nursery plants due to state and federal regulatory measures. This article will focus on Phytophthora problems of annually grown row crops.
Types of Phytophthora Diseases
Phytophthora is a plant pathogen that resides in the soil. However, this soilborne pathogen can cause both belowground and aboveground diseases.
Root and crown rot
The majority of Phytophthora diseases involve tissues in contact with infested soil (Table 1). Roots are directly infected by Phytophthora in the soil; such roots become gray, brown or black in color. Roots later decay, with outer layers of the root sloughing off, leaving intact only the central wiry xylem core. While the stems and crowns of annual crops can be directly infected by Phytophthora present in the rhizosphere, it is common to have root infections progress up the root and into the crown. Diseased crown tissues likewise become discolored and decayed. More fibrous row crops like strawberry will also manifest discolored roots and crowns. However, these roots and crowns usually retain their structure and do not have the soft decay symptom.
All row crops having root and crown infections can appear delayed in development, stunted and deficient in nutrients due to non-functional roots. With time, these plants wilt, collapse and die. Fruit-bearing row crops can develop a gray to brown rot on fruit if such fruits are in contact with soil or puddled water. Fruit diseases can be seen on cucumber, melon, squash, pepper, tomato and strawberry. Postharvest decay can occur if fruits are infected in the field prior to harvesting.
Foliar blights
Some Phytophthora species can produce airborne or splash-dispersed spores that can infect leaves, stems and fruit that are not touching the ground (Table 1, see page 14). Initial symptoms include small, brown or gray lesions. Such lesions rapidly expand to affect large areas of the foliage, causing it to collapse. Fruits can also be infected by these aerial spores, resulting in fruit rot. Collectively, such diseased foliage and fruit are called blights. As previously mentioned, one of the best known foliar Phytophthora diseases is late blight of potato and tomato caused by P. infestans. Phytophthora capsici causes both root and crown rot diseases as well as foliar blights on cucurbits and other vegetables. While not formally called a “blight,” P. ramorum causes aboveground diseases on foliage, twigs, branches and trunks of many woody trees and shrubs.
Fruit in contact with soil or water infested with Phytophthora can develop discolored rots.
Biology and Disease Development
Phytophthora species are labeled with the common name “water molds.” This is an appropriate name because these organisms are closely connected to water. If sufficient soil moisture is present, Phytophthora will grow mycelium like fungi. If host roots and favorable soil water conditions are present, Phytophthora will produce asexual reproductive structures called sporangia. Sporangia are flask- or oval-shaped structures within which are made zoospores. Zoospores released from these sporangia will swim in the soil water in the direction corresponding to increasing gradients of root exudates, land on roots and initiate infections. Sporangia and zoospores are short-lived structures; if a host root is not found or if soil conditions become too dry, these structures shrivel up and die.
In addition, Phytophthora forms a second type of structure that is spherical, with a thick resilient cell wall, that is called an oospore. Oospores are sexual structures that allow Phytophthora to recombine genetically and form diverse genotypes and strains. With their thick walls, oospores enable the pathogen to survive periods when the soil is dry and host plants are absent. Oospores are the likely means by which these pathogens are spread when contaminated soil is moved from field to field.
The flask-shaped sporangia of Phytophthora produce zoospores; when sporangia break open, zoospores swim out and infect host roots. 2022-07-14T23:45:47Z
Detection and Diagnostics
Confirmation that Phytophthora is causing a disease requires laboratory testing. Traditional culturing methods, in which pieces of diseased plant tissue are placed into Petri dishes containing selective agar media, are still very useful. More advanced and rapid detection tools include serological methods (such as lateral flow devices or ELISA) in which specifically designed antibodies detect the antigens of Phytophthora and molecular methods (such as qPCR or RPA) in which molecular markers target the Phytophthora DNA. In seeking confirmation of Phytophthora diseases, make sure the diagnostic lab has experience with Phytophthora and uses the appropriate tests.
Diagnostic precision is needed because several soilborne pathogens including Phytophthora cause similar symptoms on row crops. Plant pathogenic species of Pythium and Phytophthora in particular cause very similar root rots, crown infections, foliage yellowing, leaf wilting, poor overall growth and death of the plant (Table 2). Even systemic vascular wilt pathogens such as Fusarium and Verticillium can cause aboveground symptoms that resemble Phytophthora root and crown diseases (Table 2). With so much economic capital committed to the growing of high-value row crops, guessing which pathogen is responsible for losses is too risky. Knowing which pathogen is causing plant loss will help the grower optimize disease management strategies.
Managing Phytophthora Managing soilborne Phytophthora diseases uses strategies like those deployed against other soilborne pathogens. However, unlike many other soilborne pathogens, post-plant chemical control is a viable option against Phytophthora.
Diagnosis
Have qualified professionals confirm that Phytophthora is the issue; in some cases, it is useful to also know which species of Phytophthora is involved.
Site selection
Choose fields that do not have a history of Phytophthora problems and that have well-draining soils. Soils higher in clay content have been associated with increased risk of Phytophthora.
Crop rotation
If Phytophthora is a concern, avoid back-to-back plantings of the same susceptible crop. Rotate with crops that are not known to be susceptible to the Phytophthora present at that location. Selection of non-susceptible crops will depend on the identification of the Phytophthora species (Table 1).
Irrigation management
Because Phytophthora is dependent on wet soil conditions, carefully schedule irrigations to prevent over watered, saturated soils. Low-flow irrigation systems such as drip irrigation for strawberry and microsprinklers for tree crops, can help discourage Phytophthora outbreaks. When possible, route excess or ponding water away from the production area with the use of ditches, raised beds or slopes.
Sanitation
Sanitation refers to measures used to prevent the introduction or spread of the pathogen in the growing location. Because Phytophthora resides in soil, avoid moving mud-encrusted farm implements from infested areas to clean fields. Avoid using transplants, cuttings and other vegetatively propagated materials that show disease symptoms and are infected with Phytophthora.
Fungicides
For some crops, applying fungicides to the crop may provide some protection. Because Phytophthora is not a true fungus, fungicides with modes of action effective against oomycete diseases are necessary. The repeated use of products having the same mode of action can result in Phytophthora populations that are insensitive (=resistant) to those products; therefore, fungicide applications should include products having different modes of action.
Resistant or tolerant cultivars
There appear to be relatively few row crop cultivars that are genetically resistant to Phytophthora; on the other hand, some cultivars (e.g., the strawberry cultivar Radiance) are known for their increased susceptibility to Phytophthora diseases.
The IPM components for managing soilborne Phytophthora diseases are also relevant for foliar Phytophthora problems and include proper diagnosis, site selection, crop rotation and genetic resistance. For foliar Phytophthora, the mode of irrigation can be extremely critical; the use of overhead sprinkler irrigation can exacerbate Phytophthora blights on cucurbits, tomato, potato and pepper, for example. Sanitation is a key factor for the prevention of epidemics of late blight since P. infestans can over-season on infected plant material. For example, diseased potato tubers, left in old fields or lying in cull piles near production areas, are a source of airborne spores that can infect new plantings. Managing foliar Phytophthora diseases relies more heavily on protectant fungicide sprays than control programs targeting soilborne Phytophthoras. For this reason, careful field scouting plays a critical role for early detection of symptoms and deployment of fungicide tools.
Figure 1. CRS female (overturned) showing crawlers. Crawlers move around looking for feeding spot, settle and spend life on the same spot (photo courtesy UC Statewide IPM Project.)
California red scale (CRS) is an armored scale insect that affects all citrus varieties. It attacks all aerial parts of the tree including leaves, fruits, twigs and branches by sucking on plant tissue with its long filamentous stylet. Heavy infestations cause leaf yellowing and drop, dieback of twigs and occasional death of the infested tree. Heavily infested fruits with patches of California red scale may be downgraded in the packinghouse. For managing CRS, an integrated approach that combines mating disruption, insecticides and biological control using Aphytis melinus is used. Growers and PCAs have long relied on using pheromone cards for monitoring males and use of degree days to predict future events for making treatment decisions. Insecticide applications give the best results when the population is at the most susceptible stage (immatures) and is uniform.
In a normal year, monitoring for CRS begins on March 1. PCAs put out pheromone cards and call a biofix when first males are caught on the trap (biofix is the first event in CRS seasonal cycle.) In 2023, we got many calls from PCAs that their traps were empty in March. Our observation at the Lindcove REC center was the same. First males were found on trap cards on the week of April 10 and common consensus for biofix day was April 11. This situation is true for major citrus growing counties in the San Joaquin Valley.
Figure 1. CRS female (overturned) showing crawlers. Crawlers move around looking for feeding spot, settle and spend life on the same spot (photo courtesy UC Statewide IPM Project.)
How Do Lower Temps Affect CRS and its Management?
Like all other insects, development of CRS is temperature dependent. Season starts with surviving overwintering females. As the temperature increases and heat units accumulate, gravid female produces crawlers (Figure 1). Crawlers only remain mobile until they find a suitable location to begin feeding. Once they start feeding, they do not move and go through development being attached to the feeding spot. Crawlers go through active feeding stage (instars) and a dormant period (molting). Females molt twice and males molt four times and emerge as fliers. Males are the only other moving stage (Figure 2). Males find and mate with third instar females. Afterwards, gravid female starts producing crawlers, hence completing the life cycle. In the San Joaquin Valley, there are four complete generations of CRS. In years with warm winters/hot summers, partial fifth generation (immatures/males) have also been reported.
Figure 2. CRS male adult (top) and CRS males on trap card (bottom) (photos courtesy UC Statewide IPM Project.)
CRS does not develop below 53 degrees F, and it is the lower developmental threshold (LDT) temperature based on which degree days for CRS development are calculated. Using CIMIS station data for four counties (Kern, Tulare, Fresno and Madera), I calculated the cumulative degree days above the LDT for CRS since 2020. Figure 2 shows degree days in 2023 trails below those for years 2020-22. Effects of low average daily temperature were first noticed when biofix was delayed by about four weeks in all four counties. Figure 2 (see page 8) shows number of males/trap at Lindcove REC station, where biofix and first-generation flight peak were about four weeks later than in 2022. Second-generation flight started on the week of June 19, which is also about four weeks delayed. This means CRS is developing, but at a slower rate than it had been in earlier years (Figures 2 and 3). It takes 550-degree days after the male flight for crawler emergence. Expect second-generation crawler emergence, third-generation male flights and consequent generations to be delayed. Relatively cooler spring and early summer temperatures mean less heat units/day and delayed development, thereby affecting male flight and crawler emergence, which will in-turn affect spray timing for CRS control in the 2023 season. Visit lrec.ucanr.edu/Citrus_IPM/Degree_Days/ for degree day updates in Kern, Tulare, Fresno and Madera counties.
Figure 3. Number of males collected in pheromone trap at LREC. Note first fliers started four weeks later and peaked four weeks later. Second-generation fliers stared on week of June 19 in 2023. Expect about three-week delay in crawler in second-generation crawler emergence.
Management Monitoring
Monitoring for California red scale and applying treatments to target the most susceptible life stage/generation is key to managing CRS. Goal is to maintain CRS populations at low levels to minimize fruit contamination at harvest. UC IPM guidelines has a list of updated recommendations.
Mating disruption
Mating disruption such as Checkmate CRS prevents or delays mating of males with females. Unmated females do not produce crawlers and stay as third instar females which is the preferred stage of parasitism by Aphytis melinus. Application of mating disruption (180/acre) prior to the onset of first or second generation (March or May) have shown to provide best results (Grafton-Cardwell et al. 2021).
Biological control
Parasitic wasps Apytis melinus and Comperiella bifasciata are important natural enemies that help manage CRS. However, these parasitoids can be susceptible to insecticides used for other pests, so their effectiveness depends on careful monitoring and use of selective insecticides (UCIPM Guidelines 2022).
Figure 4. Cumulative degree days above the lower developmental threshold, 53 degrees F, for CRS in Madera, Fresno, Tulare and Kern counties. Note that 2023 (black line) trails below other years.
Insecticides
Several insecticides have proven efficacy against CRS (UCIPM Guidelines 2022, Grafton Cardwell 2016). However, a number of populations have developed resistance to organophosphates and carbamates. Field observations show resistance to pyriproxyfen may have developed (UCIPM Guidelines 2022). Where resistance to carbamates or pyriproxyfen is suspected, use of alternate chemicals such as buprofezin, spirotetramat, mating disruption and release of Aphytis may provide better results (UCIPM Guidelines 2022).
California Red Scale Trial, 2022
A field trial to evaluate multiple insecticide treatments on California red scale was conducted at the Lindcove Research and Extension Center in 2022 (Gautam and Dhungana 2023). Treatments were randomly assigned to single-tree plots that were organized into blocks based on pretreatment counts of CRS/twig on 25 July, 2022. Treatments were applied on July 28 in 750 gallons of water, except for Movento which was applied in 250 GPA, and Centaur which was applied at 1,000 GPA. Post-treatment evaluation was done by rating twigs on September 23 and twigs and fruit on October 12 for the presence of live CRS. We also rated fruit for infestation by CRS, 0=no scale, 1=1-10 scale/s, 2= >10 scales/fruit. The insecticides applied were Movento at 10 oz, Sivanto at 14 oz, Centaur at 46 oz, Senstar at 20 oz and Esteem at 16 oz.
The insecticide that provided the best control in terms of reducing the percentage of fruit infested with >10 scales was Movento (Figure 5). Treatments, namely Centaur, Senstar, Sivanto and Movento, significantly reduced the total CRS/fruit compared to control (Gautam and Dhungana 2022).
Figure 5. Mean live CRS per fruit on October 12 counts following insecticide treatment. All treatments were applied with 1% oil.
Treatments should be applied to provide thorough coverage according to the size of the trees, except for Movento which is recommended at 250 GPA. See UC IPM guidelines for CRS for more application details and recommendations.
References
Gautam SG, SK Dhungana. 2023. California red scale insecticide trial, 2022. https://doi.org/10.1093/amt/tsad067
Grafton-Cardwell, EE, JT Leonard, MP Daugherty, DH Headrick. 2021. Mating Disruption of the California Red Scale, Aonidiella aurantii (Hemiptera: Diaspididae) in Central California Citrus. 114: 2421-2429
Grafton-Cardwell, EE, SJ Scott, and JE Reger, 2016. California red scale insecticide efficacy trial, 2016. https://doi.org/10.1093/amt/tsx044
UCIPM guidelines 2022. Citrus Pest Management Guidelines: Selectivity of Insecticides and Miticides.
https://ipm.ucanr.edu/agriculture/citrus/selectivity-of-insecticides-and-miticides/
UCIPM guidelines 2022. Citrus Pest Management Guidelines: California red scale and yellow scale. https://ipm.ucanr.edu/agriculture/citrus/california-red-scale-and-yellow-scale/
