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The Crop Consultant Conference Returns as In-Person Event Over Two Days

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Progressive Crop Consultant Magazine’s popular two-day Crop Consultant Conference will return this year as a live conference and trade show, featuring seminars worth 10 hours of CCA and 8 hours of DPR continuing education credits, a live trade show, and the presentation of Western Region CCA Association’s popular CCA of the Year Award and honorariums and scholarships. The Crop Consultant Conference will be held on Sept. 16 and 17 at the Visalia Convention Center.

The Crop Consultant Conference has become a premier event held in the San Joaquin Valley each September for Pest Control Advisors and Certified Crop Advisers. Co-hosted by JCS Marketing, the publisher of Progressive Crop Consultant Magazine, and Western Region Certified Crop Advisers Association, the event brings industry experts and suppliers, researchers and crop consultants together for two days of education, networking and entertainment.

“We are excited to be back to doing our events in person, and expect another sell-out event for crop consultants in the Western United States,” said JCS Marketing Publisher and CEO Jason Scott. “Agriculture is a relationship-driven business and there is no substitute for live events.”

Topics for the two days of seminars include: Various seminars on managing pests and diseases in high-value specialty crops, tank mix safety and regulations, fertilizer management, soil health, new technology, new varieties and rootstocks and their impact on tree nut pest management.

The conference will conclude with two one-hour panels offering hard-to-get CCA credits and moderated by Western Region CCA related to nitrogen monitoring, use, application and management as well as the various regulatory requirements around irrigated nitrogen management.

Registration fees for the two-day event are $150, or less than $15 per CE unit. Pre-registration is required and can be done at progressivecrop.com/conference.

Click Here to Register Now

Identifying the Potential and Impacts of On-Farm Groundwater Recharge

Aquifers have become depleted from decades of overuse. Drilling deeper is an option for farmers, but prohibitively expensive for low-income residents in disadvantaged communities in the San Joaquin Valley.

A UC scientist believes managed aquifer recharge on agricultural lands close to populations with parched wells is a hopeful solution.

Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater. If relatively clean mountain runoff is used, the water filtering down to the aquifer will address another major groundwater concern: nitrogen and pesticide contamination.

“The recharge has the potential to clean up groundwater,” she said.

Five years ago, UCCE Specialist Toby O’Geen developed an interactive map (casoilresource.lawr.ucdavis.edu/sagbi/) that identifies 3.6 million acres of California farmland with the best potential for replenishing the aquifer based on soil type, land use, topography and other factors. Dahlke and her colleagues analyzed the map and identified nearly 3,000 locations where flooding suitable ag land will recharge water for 288 rural communities, half of which rely mainly on groundwater for drinking water. The research was published by Advancing Earth and Space Science in February 2021.

“If we have the choice to pick a location where recharge could happen, choose those upstream from these communities,” Dahlke said. “Recharge will create a groundwater mound which is like a bubble of water floating in the subsurface. It takes time to reach the groundwater table. That bubble floating higher above the groundwater table might just be enough to provide for a community’s water needs.”

 

Filling Reservoirs Under the Ground

Many climate models for California suggest long-term precipitation amounts will not change; however, the winter rainy season will be shorter and more intense.

“That puts us in a difficult spot,” Dahlke said. “Our reservoirs are built to buffer some rain storms, but are mainly built to store the slowly melting snowpack in the spring. In the coming years, all the water will come down earlier, snowmelt likely in March and April and more water in winter from rainfall events.”

She is working with water districts and farmers to consider a change in managing water in reservoirs.

“We want to think about drawing reservoirs empty and putting the water underground during the fall and early winter. Then you have a lot of room to handle the enormous amounts of runoff we expect when we have a warm atmospheric river rain event on snow in the spring,” she said. “However, farmers are hesitant. They like to see water behind the dams.”

Interest in groundwater banking has been lifted with the implementation of the 2014 Sustainable Groundwater Management Act (SGMA). The law requires governments and water agencies to stop overdraft and bring groundwater basins into balanced levels of pumping and recharge by 2040. Before SGMA, there were no statewide laws governing groundwater pumping, and groundwater was used widely to irrigate farms when surface supplies were cut due to drought.

“For some of the drought years, overdraft was estimated to be as high as nine million acre-feet a year,” Dahlke said.

Dahlke believes wintertime flooding for groundwater recharge can help water districts meet SGMA rules. “We have to do anything we can to store any surplus water that becomes available to save it for drier times, and our aquifers provide a huge storage for that,” she said.

Helen Dahlke, professor in integrated hydrologic sciences at UC Davis, has been evaluating scenarios for flooding agricultural land when excess water is available during the winter in order to recharge groundwater (photo by Joe Proudman, UC Davis.)

 

Farming Impacts

The Dahlke Lab is collaborating with UC ANR farm advisors and specialists as well as scientists at other UC campuses to learn about agronomic impacts of flooding a variety of agricultural crops, including almonds, alfalfa and grapes.

In the San Joaquin Valley, UCCE Irrigation Specialist Khaled Bali led an intermittent groundwater recharge trial on alfalfa. The researchers applied up to 16 inches per week with no significant impact on alfalfa yield.

“You could do groundwater recharge in winter and then turn the water off completely and still get a cutting or two of alfalfa before summer,” he said.

This past winter, Dahlke was prepared to flood 1,000 acres of land with water from the Consumnes River. Even though winter 2020-21 was another drought year, the research will go on. Her team was able to flood a 400-acre vineyard and, in collaboration with scientists from UC Santa Cruz, deploy sensors in the field to measure infiltration rates to better understand whether sediment in the flood water could clog pores in the soil. Her team also collaborates with Ate Visser of Lawrence Livermore National Laboratory in using isotope and noble gas data to determine the groundwater age and flow.

The Dahlke Lab’s groundwater banking project has planned more studies in groundwater basins across the state to close knowledge gaps on suitable locations, technical implementation and long-term operation. They also plan to address operational, economic and legal feasibility of groundwater banking on agricultural land.

Mitigating Tree Nut Stress and Disease Requires a Multi-Pronged Irrigation Approach

Nut growers are essentially paid in two ways: they can either produce higher yields or reduce the number of deductions from the processor. When nut trees aren’t receiving enough water, you’ll see an increased number of blanks, shriveled kernels and pinched kernels. It’s also common to see more disease and mite activity in the orchard. All of these issues can hurt final nut grade and yield, which ultimately affects grower payouts from the processor.

During this tight water year, it’s more important than ever to help growers implement a science-based approach to reduce the risk of disease and maximize orchard yields. In years of drought, salinity levels in the soil are going to rise, making it critical to keep orchards at proper field capacity so that trees do not suffer from physiological drought stress.

This is not as simple, however, as ensuring the right amount of water is applied to the orchard. Growers are going to be pressured to match water deliveries to crop stage and demands throughout the season, rather than honing in on evapotranspiration (ET) rates.

So the question for CCAs and PCAs is, “How do we help growers implement a strategic irrigation schedule?” My answer starts and ends with measurement. When you ask growers how much water they’re putting out, they often come back with a number of hours irrigated. But this number does not tell you how much water is being applied to an orchard; that can be drastically different based on application rates.

The first step in making the most of irrigation is working with your growers to understand how many inches of water per acre of soil the irrigation system puts out per hour. This piece of information is often overlooked but is essentially the backbone of a good irrigation schedule. Everything else should be calculated based on that application rate. When water is limited, it’s also helpful to know the exact depth of soil that needs to be wetted to avoid overirrigating.

An individualized irrigation plan and schedule takes into account a grower’s water quality and goals.

 

Measuring Orchard Demands

After the exact application rate is determined, growers can start designing an irrigation schedule that meets the needs of their orchards.

Irrigation scheduling maximizes the use of available water by applying the exact amount of water needed to replenish soil moisture. This practice offers growers numerous potential benefits:

  • Possibly reduces the grower’s cost of water through more efficiently timed irrigations.
  • Lowers fertilizer costs by limiting surface runoff.
  • Maximizes net returns by increasing crop yields and crop quality.
  • Aids in controlling root zone salinity accumulation through controlled leaching.
  • Decreases common disease pressures.

I once worked directly with a walnut grower who was able to increase edible yield by 4% just from having a better understanding of his irrigation output rates and adopting a more scientific irrigation approach. By measuring crop-water demand and optimizing water usage, the grower saw dramatic benefits in nut quality across the board, including both color and size advantages.

There are essentially three approaches to water management, and growers should use at least two of the following to design an irrigation schedule:

 

Soil-Based Approach

Soil-based methods are simply measuring how much water is being stored in the soil. If there is less water in the soil, and the water is held at a greater tension, it will be more difficult for the trees’ roots to take up that water, resulting in tree stress.

While irrigating based on the feel or appearance of the soil is a commonly used method, more precise measuring tools such as tensiometers, probes and electrical resistance blocks will provide more specific information that growers can utilize to produce healthier, more consistent nut crops.

Like the other approaches mentioned below, soil moisture readings can be used by themselves to schedule irrigations, but they are most beneficial when used in combination with other methods of irrigation scheduling.

Probes, for example, are very good at giving the user a snapshot of what the soil moisture looks like at varying depths. It will determine how deep in the soil profile water is infiltrating and ensure calculated water applications are not over- or under-irrigating trees. However, these probes do not tell you how often or how long to irrigate. Combining the data from the probes with ET measurements on a regular basis will provide enough information to make a strategic irrigation schedule.

 

Climate-Based Approach

This approach requires a deeper understanding and balancing of irrigation application rates and ET. This is the combination of transpiration, or water evaporation from plant leaves, and evaporation from the soil surface.

Many factors affect ET, including air temperature, humidity and wind speed; soil factors such as texture, structure and density; and plant factors such as plant type, root depth and canopy density, height and stage of growth.

Reference ET information is available from a variety of sources, but most well-known is the California Irrigation Management Information System (CIMIS) network of nearly 100 California weather stations that provide daily ET values.

Knowing how closely the amount of irrigation water plus rainfall matches estimates of real-time orchard ET can help make irrigation scheduling decisions, especially if this information is teamed with other measurement approaches.

 

Plant-Based Approach

Plants respond in different ways to keep their water supply and demand in balance, and most plant-based methods for irrigation management are based on the measurement of one or more of these responses.

The pressure chamber method for measuring the tension of water within the plant has been shown to be a reliable and commonly used measurement of stress in orchards.

One drawback of using a pressure chamber is that the tool doesn’t distinguish between types of stress. If you are utilizing a pressure chamber in a particularly weak or diseased portion of the orchard, it will not decipher between stress due to poor water supply, salinity and/or pests. This is why it’s important to measure crop-water demands with a climate- or soil-based approach in addition to the pressure chamber approach.

By using ET and a pressure chamber, we found that one almond grower was putting out the right amount of water in the orchard, but the watering interval was too wide. Once the water interval was shortened, trees experienced less stress and were able to produce a much better crop.

I always encourage growers to utilize at least two of the three water management approaches because in the orchard, data or feedback you get from each approach doesn’t always line up perfectly. There’s always some subjectivity. Carefully evaluating the information provided by multiple sources can help determine the best irrigation schedule for your growers’ orchards this year.

 

Maximizing Irrigation Scheduling ROI

There is not a one-size-fits-all irrigation approach or schedule, and choosing which tools to use depends on water quality available to the grower and operation goals.

Combining a plant-based approach with a climate-based approach has been the most effective in my experience, but there are certainly benefits to monitoring soil moisture status in combination with either of the other two methods .

Companies like Wilbur-Ellis Agribusiness are beginning to offer irrigation scheduling services (Figure 1), that include weekly plant stress readings, soil moisture conditions and watering interval recommendations based on replacement ET.

Figure 1: Soil monitoring reports can help growers and crop advisors understand what is happening with soil moisture in real time.

These programs have been shown to reduce drastic swings in irrigation protocols, which in turn reduces pest and disease pressures on orchards. These programs also allow growers to focus on other aspects of irrigation and orchard management rather than worrying about interpreting results.

Another critical piece of irrigation management to keep in mind is system maintenance, which includes in-season flushing. Keeping a clean system with good distribution uniformity is essential when growers are trying to get the most out of orchard irrigation scheduling (Figure 2).

Figure 2: System maintenance is key to more efficient water delivery.

 

Key In-Season Flushing Protocols

  • Regularly flush laterals (monthly, bi-weekly if needed).
  • Flush complete system after fertilizer injections.
  • Flush from larger to smaller lines; mains and submains, then laterals.
  • When flushing lateral lines, ensure proper velocity and volume to purge contaminants.
  • Never open more than five to eight laterals at a time, as additional open lines will reduce the velocity of flow, which reduces the effectiveness of the flush (dependent upon the total number of laterals per block).
  • Always note pressures and flows at the initial system start-up. Changes in these parameters often indicate in-season issues.

Minimizing Drift from Orchard Spray Application by Spray Backstop System

The use of pesticides can be very effective in protecting trees from pests and diseases. However, many times this is also accompanied by negative impacts on humans and the environment. Off-target movement of chemical spray has always been a challenge for growers because it can contaminate the environment, reduce spray efficacy and impose liabilities.

California has stringent pesticide laws and regulations and orchard spray application is considered a high-risk operation. California law establishes a buffer between schools and any pesticide spraying location. Growers are required to notify the public when they spray pesticides. This includes schools, daycare facilities, and county agricultural commissions.

A prototype of the spray backstop system developed at the Digital Agriculture Lab at UC Davis.

Spray drift can be reduced by choosing the right type of nozzle, adjusting and calibrating sprayer settings, defining shelter zones and specifically modified practices in the downwind rows. Reducing the movement of spray droplets to sensitive areas might be accomplished by these methods, but they are in clear contrast with strategies that lead to a uniform on-target deposition. For instance, spraying with larger droplets can reduce the amount of drift, but it also decreases the effectiveness of the spray at higher parts of a tree. Likewise, reducing airflow results in reduced drift; however, it also diminishes the spray efficacy in zones farther from the sprayer, for example, on treetops. Drift can also be reduced when we use slower ground speed and higher liquid flow rate, but the impact is not significant.

Thermal view of spray cloud escaping the canopy from the top.

Spray Backstop

At the Digital Agriculture Lab at UC Davis, a sprayer attachment system called Spray Backstop was developed to minimize drift potential and possibly improve spray coverage on the treetops. The backstop system is a screen structure that can be raised above the trees using a foldable mast.

A test was conducted in a mature almond orchard to determine how much drifting could be reduced with the backstop system. A cotton ribbon loop was stretched around the trees to quantify the droplets scaping the tree canopy. The ribbon could capture all spray droplets that did not deposit on-target and were not blocked by the backstop system.

The orchard was sprayed with a mix of water and fluorescent dye. The ribbon was cut into sub-samples and analyzed with the fluorometry method. Comparing the ribbon samples from the test with and without the backstop showed that the Spray Backstop system could effectively block the spray droplets escaping the canopy from treetops or sides and reduce drift potential by 78%. Leaf samples were also collected from trees in both spray application conditions and analyzed by the fluorometry technique. Unlike the conventional drift control methods, using the spray backstop system does not change overall canopy deposition and could also help improve deposition on treetops.

A sprayer working in a young almond orchard in Northern California (all photos courtesy Digital Agriculture Lab.)

Adopting the spray backstop system into the orchard spray application practice will reduce the environmental degradation while protecting residential areas and schools from exposure to chemicals. On the other side, growers can adjust their sprayer for more air and finer droplets (that will improve spray coverage and efficacy in the upper canopy area) without being concerned about drifting because the backstop system can stop spray droplet movement above the trees. A uniformly applied treatment will significantly reduce the risk of crop failure due to pests and diseases.

The spray backstop system is simple and could be easily implemented without significant modifications to the grower’s spray rigs. This system can help growers to be compliant with pesticide regulations and maintain good environmental stewardship. You can find more information about the spray backstop project at the digital Agriculture lab website: digitalaglab.ucdavis.edu.

Spray backstop blocking the spray cloud from moving upwards.

Treating and Preventing Citrus Huanglongbing with a Stable Antimicrobial Peptide with Dual Functions

Citrus Huanglongbing (HLB, citrus greening disease) is caused by the vector-transmitted phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas). It is the most destructive disease which infects all commercial citrus varieties and threatens citrus industries worldwide (Bove 2006; Graham et al. 2020). Current management strategies include insecticide application to control the transmission vector Asian citrus psyllids (ACP) and antibiotics treatment to inhibit CLas (Barnett et al. 2019), but neither could control HLB effectively. Since the first report of HLB in Florida in 2005, citrus acreage and production in Florida decreased by 38% and 74%, respectively (Graham et al. 2020; Stokstad 2012). The disease has spread to citrus-producing states, including Texas and California. In severely affected areas, such as Florida, effective therapy is demanded because disease eradication is impractical. In recently impacted areas, such as California, preventing new infections is most urgent. Hence, innovative therapeutic and preventive strategies to combat HLB are urgently needed to ensure the survival of the citrus industry.

One of the most effective and eco-friendly strategies for disease management is to utilize plant innate immunity-related genes from disease-resistant or tolerant varieties for plant protection. Upon pathogen infection, plant defense response genes undergo expression reprogramming to trigger plant innate immunity. Plant endogenous small RNAs play a pivotal role in this regulatory process (Huang et al. 2019; Zhao et al. 2013b), including phytohormone- and chemical-induced systemic acquired resistance or defense priming, which can promote robust host immune responses upon subsequent pathogen challenges (Brigitte et al. 2017; Zheng Qing and Xinnian 2013).

 

Our Approaches

HLB tolerance has been observed in some hybrids (e.g., US-942 and Sydney hybrid 72 (Albrecht and Bowman 2011, 2012a)) or citrus relatives (e.g., Microcitrus australiasica, Eremocitrus glauca and Poncirus trifoliata) (Ramadugu et al. 2016). By comparative analysis of small RNA profiles and the target gene expression between HLB-sensitive cultivars and HLB-tolerant citrus hybrids and relatives (Albrecht and Bowman 2011, 2012a), we identified a list of candidate natural defense genes potentially responsible for HLB tolerance (Huang et al. 2020). One of the candidate regulators is a novel anti-microbial peptide (AMP), which we named “stable antimicrobial peptide” (SAMP). Here, we demonstrate that SAMP not only has antimicrobial activity but also has priming activity and can induce citrus systemic defense responses. This dual-functional SAMP can reduce CLas titer, suppress disease symptoms in HLB-positive trees and activate plant systemic defense responses against new infection.

 

Results

Through the comparative expression analysis of small RNAs and the target genes between HLB-sensitive cultivars and HLB-tolerant citrus US-942 (Poncirus trifoliata x Citrus reticulata) and microcitrus Sydney hybrid 72 (Microcitrus virgate from M. australis × M. australasica) (Huang et al. 2020), we identified a list of candidate plant immune regulators that are potentially contributable to HLB-tolerance. One candidate regulator is a 67-amino acid (aa) peptide, SAMP, that was predicted with antimicrobial activity (Park et al. 2007). SAMP has significantly higher expression levels in both HLB-tolerant hybrids US-942 and Syd 72 than the HLB-susceptible control trees. We further cloned SAMP genes from HLB-tolerant citrus relatives. We found that SAMP transcripts are closely related and have a significantly higher expression level in HLB-tolerant varieties. We further detected the 6.7kD SAMP in the phloem-rich tissue; bark peels of HLB-tolerant Ma and Pt but not in the susceptible Cs. These results support that the SAMPs are likely associated with the HLB-tolerance trait. According to our functional analysis of SAMP, we list the advantages of using SAMP to manage citrus HLB as the following:

 

SAMP has bactericidal activity and is heat stable.

We screened SAMPs from several citrus relatives using a C. Liberibacter solanacearum (CLso)/potato psyllid/Nicotiana benthamiana interaction system to mimic the natural transmission and infection circuit of the HLB complex. We found that the SAMP from Ma Australian finger lime (MaSAMP) had the strongest effect on suppressing CLso disease and inhibiting bacterial growth in plants. To directly determine the bactericidal activity of MaSAMP on Liberbacter spp, we developed a viability/cytotoxicity assay of Lcr, a close culturable relative of the CLas and CLso (Fagen et al. 2014; Leonard et al. 2012; Merfa et al. 2019). Using this assay, we found that MaSAMP can rapidly kill the bacterial cells within five hours, which is more efficient than the bactericidal antibiotic, Streptomycin. While the heat sensitivity of antibiotics is a major drawback for controlling CLas in citrus fields, we found that SAMPs are surprisingly heat stable. A prolonged exposure to extreme temperatures of 60 degrees C for 20 hours had minimal effect on MaSAMP, which still retained most of its bactericidal activity, whereas Streptomycin lost its antibacterial activity following the same temperature incubation. Thus, SAMP is a heat-stable, plant-derived antimicrobial peptide that can directly kill Lcr and suppress CLso in plants.

 

SAMP suppresses CLas in HLB-positive citrus trees.

To determine whether MaSAMP can also suppress CLas in citrus trees, we used the pneumatic trunk injection method to deliver the MaSAMP solution into the HLB-positive trees. We first tested with eight CLas-positive Citrus macrophylla with similar bacterial titer and disease symptoms for the treatment. At eight weeks following two doses injected, separated by two months of MaSAMP injection, the disease symptoms and the bacterial titer in all six treated trees were drastically reduced compared to the mock-treated (buffer only) plants and one tree with no CLas detected. We further tested with HLB-positive ‘Madam Vinous’ sweet oranges and Lisbon Lemon trees; those appeared to have declining symptoms and similar CLas titer. After MaSAMP treatments, the trees had developed symptomless new flushes, while mock trees exhibited symptomatic flushes (Fig. 1). The CLas titer was reduced in MaSAMP-treated trees, while it increased in the mock-treated trees. Taken together, these results demonstrate across three trials that SAMP injection can suppress CLas titer in three different HLB-susceptible citrus varieties and can cause trees in declining health to recover.

Figure 1: New growth leaves form buffer- (set as mock, left) or SAMP-treated (right) HLB-positive ‘Madam Vinous’ sweet oranges trees.

 

SAMP treatment safeguards healthy citrus trees from CLas infection.

Protecting healthy trees from CLas infection is critical for managing HLB. The establishment of defense priming in plants can promote faster and/or stronger host immune responses upon pathogen challenges (Brigitte et al. 2017; Zheng Qing and Xinnian 2013). To determine whether MaSAMP has priming activity, we applied it by foliar spray to citrus plants. We found that MaSAMP applications triggered prolonged induction of defense response genes. Thus, SAMP can potentially “vaccinate” uninfected citrus trees and induce defense responses to combat against HLB. To test the protection ability of SAMP on citrus trees, we applied the MaSAMP solution or buffer as mock treatment by foliar spray onto young, healthy ‘Madam Vinous’ sweet orange trees. Five days after treatment, the trees were exposed to ACP carrying CLas under the “no choice feeding” condition for 21 days. We treated trees with MaSAMP solution by foliar spray every two months subsequently. The result indicates that MaSAMP-treated trees have a lower infection rate.

 

SAMP disrupts the outer membrane and causes cell lysis of the bacterial cell.

To understand the mechanism of MaSAMP bactericidal activity, morphological changes of Lcr post-MaSAMP treatment were observed using transmission electron microscopy. Application of 10 μM MaSAMP to Lcr caused cytosol leakage and the release of small extracellular vesicles after 30 minutes of incubation. The Lcr cells were lysed within two hours of incubation. We isolated the membrane fraction from the MaSAMP treated Lcr and detected the enrichment of MaSAMP in the outer membrane fraction compared with the inner membrane fraction. Thus, MaSAMP likely disrupts mainly the outer membrane of Lcr and breaks the bacterial cells, which leads to cell lysis.

 

SAMP has Low toxicity.

Because SAMP is internalized by citrus, it is important to test its phytotoxicity. We injected different concentrations of MaSAMP solution directly into citrus leaves and found that MaSAMP has little phytotoxicity. Furthermore, we found that MaSAMP can be detected in fruit tissue of both Australian finger lime and trifoliate orange by Western blot analysis and is very sensitive to human endopeptidase Pepsin, a major gastric enzyme produced by stomach chief cells. Thus, MaSAMP in Australian finger lime has already been consumed by humans for hundreds of years and can be easily digested (Figure 2). These results suggest a low possibility of toxicity of SAMP on citrus and humans, although additional safety assessment tests are necessary for regulatory approval.

Figure 2: Fruits of Australian finger lime contain MaSAMP, which has already been consumed by humans for hundreds of years and is easily digested.

 

Conclusion

Current methods for HLB management include insecticidal control of the vector (Stansly et al. 2014), antibacterial treatments (Blaustein et al. 2018; Gottwald 2010; Hu et al. 2018; Zhang et al. 2014) and nutrient supplements (Rouse 2013; Zhao et al. 2013a). The overuse of insecticides and antibiotics is known to pose threats to human and animal health and select for resistance in the target insect population (Tiwari et al. 2011). Further, current bactericidal or bacteriostatic treatments mostly involve sprays of antibiotics, such as streptomycin and oxytetracycline, which are likely to select for antibiotic-resistant bacteria strains and disrupt the citrus microbiome and ecosystem and may further affect the effectiveness of these antibiotics for medical antibacterial treatment in humans and animals.

On the contrary, SAMPs have a distinct mode of action and tend to interact with the bacterial cell membrane through nonspecific mechanisms, making the emergence of resistant bacteria less likely (Jochumsen et al. 2016; Rodriguez-Rojas et al. 2014). Moreover, SAMP kills bacteria faster than antibiotics, reducing bacterial generations and further lowering the possibility of evolved resistance (Fantner et al. 2010). Most importantly, the heat stability of SAMP can provide a prolonged and durable effect in the field compared to heat-sensitive antibiotics. SAMP not only kills bacteria cells but can also prime plant immune responses to prevent/reduce infection. In our greenhouse trials, SAMP has been shown to treat HLB-positive trees and inhibit the emergence of new HLB-infection in healthy trees. Field trials, which can take several years, are currently being initiated in Florida to confirm the efficacy of SAMP in controlling HLB. Field trials also include testing multiple peptide application methods for citrus growers to prevent and treat HLB.

Contact Hailing Jin at hailingj@ucr.edu for more information.

 

References:

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Rouse, B.B. (2013). Rehabilitation of HLB Infected Citrus Trees using Severe Pruning and Nutritional Sprays. Proc Fla State Hort Soc 126, 51-54.

Stansly, P.A., Arevalo, H.A., Qureshi, J.A., Jones, M.M., Hendricks, K., Roberts, P.D., and Roka, F.M. (2014). Vector control and foliar nutrition to maintain economic sustainability of bearing citrus in Florida groves affected by huanglongbing. Pest Manag Sci 70, 415-426.

Stokstad, E. (2012). Agriculture. Dread citrus disease turns up in California, Texas. Science 336, 283-284.
Tiwari, S., Mann, R.S., Rogers, M.E., and Stelinski, L.L. (2011). Insecticide resistance in field populations of Asian citrus psyllid in Florida. Pest management science 67, 1258-1268.

Zhang, M., Guo, Y., Powell, C.A., Doud, M.S., Yang, C., and Duan, Y. (2014). Effective antibiotics against ‘Candidatus Liberibacter asiaticus’ in HLB-affected citrus plants identified via the graft-based evaluation. PloS one 9, e111032.

Zhao, H., Sun, R., Albrecht, U., Padmanabhan, C., Wang, A., Coffey, M.D., Girke, T., Wang, Z., Close, T.J., Roose, M., et al. (2013a). Small RNA profiling reveals phosphorus deficiency as a contributing factor in symptom expression for citrus huanglongbing disease. Mol Plant 6, 301-310.

Zhao, H., Sun, R., Albrecht, U., Padmanabhan, C., Wang, A., Coffey, M.D., Girke, T., Wang, Z., Close, T.J., Roose, M., et al. (2013b). Small RNA Profiling Reveals Phosphorus Deficiency as a Contributing Factor in Symptom Expression for Citrus Huanglongbing Disease. Molecular Plant 6, 301-310.

Zheng Qing, F., and Xinnian, D. (2013). Systemic Acquired Resistance: Turning Local Infection into Global Defense. Annual review of plant biology 64, 839-863.

Cover Cropping to Achieve Management Goals

Cover crops can provide many benefits to growers, like improving water infiltration and reducing nutrient loss. However, growers in California’s southern San Joaquin Valley worry that the lack of consistent winter rainfall and high cost of water in the area make cover cropping impractical (Mitchell et al. 2017).

In this article, we’ll discuss how well different cover crop mixes suppressed weeds, provided resources for beneficial insects and improved water infiltration. We’ll also delve into water requirements and effects on soil nitrate.

Irrigated (left) and non-irrigated (right) soil builder mix in Shafter on March 11, 2021.

Location of Trials & Cover Crop Species Selection

These trials took place at UC-managed research farms in Shafter (Kern County) and Parlier (Fresno County). The Parlier research farm is the Kearney Agricultural Research and Extension Center and will be referred to as “Kearney” for the remainder of this article.

Four common cover crop mixes were planted at Kearney and five mixes were planted at Shafter (Table 1). Three of the mixes were similar in both locations. We worked with Kamprath Seeds to select cover crop species based on mixes that had done well in a demonstration trial in Shafter the previous year. Two beginner-friendly simple mixes were also evaluated.

Based on the results of our trial, we have outlined some suggestions for trying out cover crops on your farm. Our suggestions depend on your goals for planting cover crops and your concerns about fitting them into your existing cropping system.

 

Goal: Weed Suppression

Based on the trial results, if your goal is weed suppression, you may consider the following.

Grasses appeared to be most effective at suppressing weeds, especially Merced rye which grew vigorously in both irrigated and non-irrigated plots at both locations. Brassicas also contributed to weed suppression, while legumes were least competitive with weeds, especially at Kearney.

If your goal is weed suppression, consider higher percentages of grasses and brassicas in your seed mix.

Irrigated (left) and non-irrigated (right) barley & common vetch in Shafter on March 11, 2021.

Higher seeding rates led to greater weed suppression. If weed suppression is your goal, consider seeding at rates higher than recommended.

If it is not realistic to purchase enough seed for higher seeding rates, pre-irrigating before the first rain to allow weeds to germinate, followed by cultivation prior to planting, can help knock back weeds that may compete with cover crops at germination.

If pre-irrigation is not possible, consider waiting for weed seeds to germinate after the first decent rain, then cultivating your field before planting your cover crop.

However, try to not wait too long, as colder temperatures may inhibit germination. This may have been another reason why cover crop establishment was not as vigorous at Kearney; we planted on December 18 when the high temperature was 53 degrees F and the low was 40 degrees F. Compare this to the planting date of November 5 in Shafter where the high was 20 degrees F warmer (high of 73 degrees F, low of 52 degrees F).

Irrigated (left) and non-irrigated (right) rye & peas in Shafter on March 11, 2021

With legume species, in particular, less initial germination correlated with a low percentage of legume species in the final stand. Grasses, however, germinated well in both locations and dominated the final stand.

Therefore, if you cannot plant until later in the winter, consider seeding grass seeds in higher proportions, especially if weed suppression is your goal.

Irrigating directly after seeding may help cover crops establish more vigorously and compete with weeds. If you are not able to irrigate your cover crop, planting right before a rain event can help your cover crops establish a better stand.

Do not assume that there is enough moisture in the soil to help your cover crops properly germinate if you are planting after a rain event.

Irrigated (left) and non-irrigated (right) soil health mix in Shafter on March 11, 2021.

 

Goal: Provide Resources for Beneficial Insects

The legumes in both locations did not perform well, so they did not offer much sustenance for pollinators.

The brassicas in Shafter started blooming at the beginning of February, while the radishes at Kearney started blooming in mid-March. This provided forage and diversity for the pollinating bees from the surrounding almond orchards. Butterflies were also observed on these flowers.

Ladybugs were seen on the grasses in both locations, which can serve to keep aphid populations low on surrounding or subsequent crops.

 

Goal: Improve Water Infiltration

At Kearney, water infiltration was measured using a mini disk infiltrometer before planting cover crops and right before termination of cover crops.

Infiltration rates were higher when cover crops were in the ground compared to the bare soil prior to planting cover crops (Figure 1).

Figure 1: Infiltration rates in both fields at Kearney before the cover crops were planted (blue) and while the cover crops were fully established (orange).

There were no significant differences in infiltration across seed mixes. Therefore, based on our results, there is not one particular mix we would recommend to improve water infiltration.

Having roots in the ground improves the ability of water to enter the soil surface. Roots create channels for water to enter so that more water can be stored in the soil.

With lower infiltration rates, water is more likely to run off the surface. This is why avoiding bare soil is important if your goal is to increase the amount of water that can be stored in your soil.

 

Concern: Will the Cover Crops Tie Up Soil Nitrogen?

In Shafter, we took soil samples zero to six inches deep on April 21 in the irrigated plots five weeks after termination. The fallow samples had an average soil nitrate level of 45.8 mg/kg, which was similar to the soil nitrate level that we measured before planting the cover crops.

Two of the mixes had soil nitrate levels that were very close to that of the fallow area: the brassica pollinator mix and the soil builder mix. Those mixes were mostly or entirely brassicas, so they decomposed quickly enough that the nitrate they had taken up had returned to the soil. The other three mixes (soil health, rye and peas, and barley and vetch) led to significantly lower soil nitrate levels.

Grasses have higher C:N ratios, which meant that soil microbes took longer to decompose their residue and needed to mine soil nitrogen to do so.

 

Concern: Can I Really Grow Cover Crops Without Irrigation?

Shafter
All of the non-irrigated plots contributed some amount of biomass. This means that even if you cannot irrigate your cover crops, you can still reap some soil health benefits.

For four of the mixes, the irrigated plots produced at least twice as much biomass as the non-irrigated plots. In contrast, the non-irrigated brassica plots produced almost as much biomass as the irrigated brassica plots (Figure 2).

Figure 2: Aboveground dry biomass was collected in Shafter on March 17, 2021.

Kearney
The differences in biomass between the irrigated and non-irrigated plots were not significant, except for the irrigated Merced rye and peas and the irrigated barley and vetch. These two irrigated mixes yielded nearly twice as much biomass as their non-irrigated counterparts (Figure 3). This biomass was mostly attributable to the grasses in both mixes.

Figure 3: Aboveground dry biomass was collected in Kearney on March 22, 2021.

The non-irrigated side produced an impressive amount of biomass given that it only received 3.71 inches of water.

The mixes that performed best without irrigation were the Merced rye and peas and the soil builder mix. The daikon radish performed well in the soil builder mix without supplemental irrigation, and the Merced rye performed well in the rye and peas mix without supplemental irrigation. Therefore, Merced rye and radish would be good options if you are unable to irrigate your cover crops.

 

Conclusion

Based on the results of our trial, cover crops can still produce biomass without irrigation in the San Joaquin Valley, even during a drier winter. While they may not produce huge amounts of biomass without irrigation, having roots in the ground is still beneficial for suppressing weeds, providing beneficial insect habitat, increasing water infiltration and feeding your soil biology.

Cover crops turn sunlight into carbon that feeds your soil microbes through their root system. Therefore, by simply maintaining living ground cover, you are feeding your soil. In addition, when you return cover crop residues to your soil, microbes will decompose them and slowly release available nutrients that can be used by your cash crop.

If you are growing cover crops in a limited water environment, consider planting Merced rye and brassica species, such as daikon radish or mustards. These are also good options if you plant your cover crops later in the winter.

Irrigated (left) and non-irrigated (right) brassica pollinator mix in Shafter on March 11, 2021.

Triticale also grows well with limited water and decomposes more quickly than Merced rye, which is something to consider if you are disking in your cover crop and planting an annual crop shortly after.

In both Shafter and Kearney, the legume species did not perform well because of the high soil nitrate levels and low water conditions. If your field is in a similar situation, it might be better to save money and not buy legume seeds.

You should also think about the cash crop that will follow the cover crop. For example, you do not want to plant a brassica cash crop after a brassica cover crop. This might increase pest pressure for your brassica cash crop.

Interested in trying out cover crops? The USDA and the California Department of Food and Agriculture have programs to help you pay for it. For more information about the USDA resources, reach out to your local USDA Natural Resource Conservation Service office. For more information about the CDFA Healthy Soils Program or getting started with cover crops, reach out to Shulamit Shroder at sashroder@ucanr.edu or Jessie Kanter at jakanter@ucanr.edu.

 

Resources

Mitchell, J. P., Shrestha, A., Mathesius, K., Scow, K. M., Southard, R. J., Haney, R. L., Schmidt, R., Munk, D.S. & Horwath, W. R. (2017). Cover cropping and no-tillage improve soil health in an arid irrigated cropping system in California’s San Joaquin Valley, USA. Soil and Tillage Research, 165, 325-335.

Vineyard Water Management During Drought Years

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This past winter was a dry year with a total of <8.5 inches of precipitation (November 2020 to March 2021) and that was significantly lower than the historical average of 11 inches for the San Joaquin Valley (SJV). Winter drought and freeze temperatures last November and December (minimal temperature was as low as 20 degrees F) caused some severe damages on grapevines, and the delayed spring growth (DSG) was mostly reported by growers. DSG includes erratic bud break, stunted shoot development, dead shoot tips and excessive berry abscission with suckers pushing vigorously at the base of the vine. Some DSG symptoms were very similar to the symptoms of drought or grapevine water stress. Generally, most DSG vines recover after an irrigation event starts; however, the impact of DSG on yield might persist (poor fruit set and small bunch size), and growers likely suffer from the economic loss.

In the face of upcoming drought challenges, growers need to focus on three main areas to improve irrigation efficiency and maximize yield and quality: soil, grapevine canopy and weather condition.

 

Soil

Most grape roots are in the top three feet of soil, and this is the main area where our irrigation works. Due to the lack of winter precipitation, dormant or early season irrigation is necessary to restore the soil moisture, which supports grapevine budbreak as well as early season shoot elongation. Given that the severe drought condition persists in most of SJV, growers need to assess the soil dryness and moisture content at individual sites, since different soil types can hold moisture quite differently (Table 1). Sandier soil holds water poorly in comparison to clay soil. Conversely, water percolates more rapidly in sandy soil than clay soil. Therefore, with the same amount of precipitation or irrigation, clay soil can hold more than sandy soil and grapevine/cover crop can deplete the soil moisture much faster at sandier sites. Soil type can be accessed through UC Davis SoilWeb (casoilresource.lawr.ucdavis.edu/gmap/), or can be analyzed through commercial laboratories by collecting representative soil samples at a site. Cover crop and middle row vegetation should be mowed earlier under drought conditions to preserve soil moisture.

Table 1: Representative values for available water content, rooting depth and allowable depletions for different soil types. Table is elaborated in Raisin Production Manual (L.P. Christensen 2000).

After understanding the different soil types and their water-holding characteristics, growers need to make the decision for first irrigation based on the soil moisture content, and that can be quite important during the current drought condition. Numerous tools are available for growers to use to measure soil moisture content, the most commonly used method being ‘feel and appearance’, where growers use a shovel or auger to dig out soil samples at different depths and feel the moisture by squeezing the soil in their hand.

Currently, growers have access to many inexpensive soil moisture sensors which can help to measure soil moisture in real time. There are mainly two types of soil moisture sensors: one measures soil water tension (e.g., tensiometer or WaterMark™) and the other measures soil volumetric water content (e.g., Neutron probe and capacitance sensor). Soil-water tension tells you how hard it is for the grapevine roots to pull water from the soil particles, and the reading is typically negative with units of centibar. The more negative the reading, the harder it is for the grapevine roots to absorb water. Growers can set up the pre-determined value of soil-water tension (e.g., -30 centibar) at a certain soil depth {e.g., two feet). Once the soil-water tension reaches the value, the irrigation should start. The pre-determined soil-water tension is usually between -30 and -40 centibar in SJV, varying across different soil types. However, soil-water tension does not tell you how much water has been depleted or the available water content. Soil volumetric water content, on the other hand, offers the percentage of water volume versus the total soil volume and tells you the amount of water stored in the soil and how much you need to irrigate to maintain the desired water content. Growers should irrigate the vineyard when 30% to 50% of allowable water is depleted throughout the root zone (Table 1). The soil volumetric water content is also well correlated to plant water stress measured by midday leaf water potential, and growers can potentially use soil volumetric water content to assess grapevine water stress (Figure 1).

Overall, growers can use both soil-water tension and soil volumetric water content mentioned above to monitor the soil moisture either indirectly or directly and schedule irrigation.

Figure 1: Soil volumetric water content is correlated with midday leaf water potential. Data were collected from various irrigation treatments: 0.2 ETc, 0.6 ETc, 1.0 ETc and 1.4 ETc. Figure is elaborated in Williams and Trout 2005.

 

Grapevine Canopy

Grapevine canopy growth (e.g., budbreak and shoot elongation) depends on the availability of water and nutrients in the soil. Lack of soil moisture during the dormant season increases the risk of freeze damage and hinders the start of budbreak and early season shoot elongation, causing DSG. DSG usually occurs when the grapevine suffers water stress at the beginning of the growing season. If the drought condition persists, shoot elongation might be hampered, and the shoot tip might die off due to lack of water. Under severe drought stress, inflorescences will die off and cause significant yield loss.

To prevent early season vine water stress, soil moisture is the key measurement to decide when to irrigate as was previously mentioned. However, canopy appearance and visual assessment can help to confirm the success of an irrigation program. First, upward-growing shoot tips and tendrils are the most obvious signs of a healthy canopy. Second, a pressure chamber to measure midday leaf water potential might offer a powerful tool to validate the irrigation program (Table 2).

Table 2: Midday leaf water potential readings and related grapevine water stress.

Grapevine water demand is directly related to the amount of sunlight captured by the canopy, and a larger canopy needs more water than a smaller canopy due to receiving more light. Therefore, as the shoot grows and the canopy expands, grapevines generally use more water. Most grapevine irrigation recommendations are based on the canopy size and climatic condition. Canopy management (e.g., leafing, shoot tucking/thinning and hedging) has the potential to influence the canopy size and amount of light received, and ultimately water use.

Soil water tension sensor (left) and soil volumetric water content sensor (right)

Grapevines at different growth stages have different sensitivity and tolerance to water stress. From budbreak to bloom, the grapevine is very sensitive to water stress, and even a mild stress will hinder growth and cause irreversible yield loss. Generally, water stored in the soil profile after winter precipitation is enough to support vine growth. However, in years of drought such as this year, soil moisture may be inadequate to support growth, and irrigation is needed to replenish soil moisture. From bloom to fruit set, the grapevine is also sensitive to water stress, and severe stress causes poor set and yield loss. Fruit set to veraison is a good time to apply some stress if growers are looking to reduce berry size (e.g., smaller berry) and improve berry quality (e.g., color). However, the benefit of improved berry quality might come with the sacrifice of yield. Typically, in SJV, mild stress is recommended at this period to balance quality and yield. From veraison to harvest may be the best time to apply some stress to advance berry ripening and reduce disease pressure (e.g., bunch rot). However, growers need to avoid severe stress which results in excessive defoliation, since a healthy canopy is required for photosynthesis. Postharvest, it is generally recommended to replenish the soil profile, since the photosynthetic active canopy still produces the carbon to refill the reserve of trunk and roots, and the reserve will be used to support the vine growth of the following season. Postharvest irrigation in abundant quantities can also help to leach the salts and alleviate the concern of salinity.

Figure 2: Crop coefficient (Kc) is correlated with canopy size measured by leaf area. Figure is elaborated in Williams and Trout 2005.

 

Weather Conditions

As was previously mentioned, grapevine water use depends on two main factors: canopy size and weather. Most growers know crop evapotranspiration (ET) and can calculate grapevine water demand over a given period through the calculation: grapevine evapotranspiration (ETc) = reference evapotranspiration (ETo) × crop coefficient (Kc). Growers can simply use ETc to irrigate the grapevines weekly by using gallons/vine/week or hours/vine/week after factoring in the application rate. Kc is related to canopy size (Figure 2, see page 38) and ETo is related to weather condition (Figure 3). ETo is strongly correlated with sunlight and the ambient temperature. A sunny and cloudless day will drive more grapevine water use than a cloudy and foggy day or a smokey day as occurred last year due to the wildfires. Similarly, a forecasted heat wave will cause severe water stress if there is lack of irrigation. Water stress coupled with temperatures >100 degrees F disrupts berry growth and sugar accumulation and causes yield loss and maturation delay.

Figure 3: Sunlight is strongly correlated with ETo and ambient temperature. Data points are extracted from last 10 years’ average during months of August, September and October at CIMIS station #56 in Los Banos.

Currently, there are different ways growers can adjust irrigation based on weather condition: 1) National Weather Service (digital.weather.gov/) provides the weather forecast as well as forecasted ETo. Growers can adjust the irrigation amount based on forecasted weather and ET. 2) UCCE is launching weekly crop ET reports (ucanr.edu/sites/viticulture-fresno/Irrigation_Scheduling/), so growers do not need to calculate weekly grapevine ET or gallons/vine/week themselves. Irrigation can be simply followed on the ET reports.

Finally, growers need to put economic consideration into water management. Water might be better used for younger blocks than a vineyard which is near the end of its lifespan, and it also makes more economic sense to use water for the cultivar which has a better price when the water is scarce. The take-home message on vineyard water management is:

  • Check soil moisture at all levels.
  • Assess canopy, vine water status and weather condition.
  • Spend water when it is needed the most.
  • Salinity becomes an important factor in determining water management.
  • Mow cover crop or middle row vegetation early to preserve soil moisture.
Upward growing shoot tip and tendril (left), before pressure bomb, bagging leaf before cutting the petiole (top right), during pressure bomb, using magnified glass to observe popping water from the petiole (bottom right).

References

Williams, L. and Trout, T. 2005. Relationships among Vine- and Soil-Based Measures of Water Status in a Thompson Seedless Vineyard in Response to High-Frequency Drip Irrigation. Am J Enol Vitic. 56: 357-366.
L. P. Christensen. 2000. Raisin Production Manual. University of California Agriculture and Natural Resources Publication 3393.

Can Artificial Intelligence Enhance the Profit and Environmental Sustainability of Agriculture?

The global population is projected by the United Nations to reach 9.7 billion people by 2050. To satisfy the needs for food and fiber for that many people, agricultural production should increase by >70%. Globally and here in the U.S., studies suggest that improved management of soil resources, irrigation water and nutrients through digital agriculture (See Sidebar Digital Agriculture on page 9.) tools can be the key to increasing agricultural production (Fig. 1). According to USDA, sustaining the economic viability of agriculture is only one of the goals that must be achieved to fully satisfy the long-term food and fiber needs of the American people. Other key goals are to reduce the environmental footprint of agriculture and to improve the quality of life for farm families and communities.

Figure 1: Economic feasibility analysis showing the relative value that agricultural technologies are expected to add to the current global crop production to achieve the United Nation’s 2050 food production goals. Data extracted from Goldman Sachs, July 2016, “Precision Farming: Cheating Malthus with Digital Agriculture”

 

Improving Production Systems

Agriculture in the Southwestern U.S. is vital to the prosperity of rural communities and the national economy, both for internal consumption and for international export. Many crops are grown in the Southwestern U.S., including field crops and many specialty fruits, vegetables and nuts. In particular, around $12 billion/year in income is generated by agriculture in California’s Salinas River Valley and in the Colorado River Basin, including regions in Southern California that use Colorado River water for irrigation. In these regions, agriculture employs more than 500,000 workers.

Sustaining and improving the agricultural productivity in the region in the long term is, however, under threat as growers face major challenges: climate change, disease and pest outbreaks, increasing salinity and diminished and/or degraded soil and water resources, to mention a few. The region has experienced prolonged and major droughts since 2000, with the possibility of streamflow reductions by more than 50% by 2100. Future requirements to preserve groundwater may remove 500,000 acres from production in California alone. Minimizing salt and nutrient loading is increasingly mandated for effective water reuse. Uncertainty in the water supply in these regions is particularly troublesome, as agriculture there consumes 39% of the U.S. total irrigation water (almost 32 million acre-feet per year.) Additionally, climate change threatens to increase insect, pathogen and weed pressures and geographic distribution, while public interest and increasing on-farm costs push toward reducing pesticide use and organic farming.

Our current understanding of agricultural production systems indicates that Crop Yield is a complex function of Genetics × Environment × Management × Space × Time interactions. Understanding why yield deviates from optimal over space and time in different landscapes is key to adjusting management cost-effectively. When looking at the multi-year productivity of farmland, a rule of thumb might say that yield varies over time in response to climate as much as it changes across different spatial scales (within a field and across multiple fields) due to the variability in soil and landscape features.

Failure to adapt management to the dynamic spatial and temporal variability of crop growth often results in crop loss or over-application of agronomic inputs, which can lead to economic loss and environmental degradation. Real-time crop growth models that can leverage information from very high spatial and temporal resolution satellite imagery and ground networks of sensors, such as weather stations, are great candidates for guiding site-specific tailored agronomic management. Moreover, when combined with artificial intelligence, crop growth models can help improve farm management while considering the tradeoffs between different sustainability aspects at different spatial scales. For example, how to increase field scale profitability while reducing regional-scale environmental impacts.

 

Current Research

With the overarching goal of increasing agricultural profitability by reducing and optimizing inputs to increase yield and curb losses from abiotic and biotic stressors in the Southwestern U.S., UC Riverside recently started a five-year project on the use of artificial intelligence and big data from high-resolution imagery and ground sensor networks to improve the management of irrigation, fertilization and soil salinity as well as to enable early detection of weeds and pests. The project is led by Elia Scudiero, a professional researcher in UC Riverside’s Department of Environmental Sciences, and includes several co-investigators at UC Riverside and UC ANR, USDA-ARS, University of Arizona, Duke University, Kansas State University and University of Georgia. To accomplish its goal, the project relies on many collaborations with ag tech industry partners, including Planet Labs, Inc. Through the collaboration with Planet Labs, the project investigators will use daily high-resolution (around 12 feet) satellite imagery to monitor crop growth and soil properties.

Some of the artificial intelligence applications that the project will develop, for a variety of crops, include crop inventorying, soil mapping, estimations of crop water use and requirement, estimation of plant and soil nutrient status, analyzing the feasibility and cost-effectiveness of variable-rate fertigation across selected regions in the Southwestern U.S. and detecting weeds and pathogens. In the remainder of this article, we will provide a general overview and some preliminary results from some selected applications: Water use and water requirement estimations, mapping soil salinity with remote sensing and detecting biotic stressors.

Figure 2: Left panel shows the map of evapotranspiration on November 10, 2019 for a selected region near Yuma, Ariz. using Landsat 8 imagery. This map has a red circle that is shown in the panel on the right illustrating evapotranspiration (ET) for the selected point for the season. Vegetation data from Landsat are combined with high-resolution meteorological data to estimate ET on an hourly time basis. The red bars illustrate rain events and the black bar illustrates the amount and timing of irrigation that would be needed to avoid unacceptable soil moisture depletion.

 

Water Use and Water Requirement Estimations

A key element of this project will be to provide reliable estimates of crop water use and irrigation forecasting at a very high resolution daily. Current remote-sensing-based evapotranspiration models often suffer from infrequent satellite overpasses, which are generally available weekly or every two weeks at the 30- to 100-foot spatial resolution. Such sporadic information is a limitation, especially for vegetable crops, which can have very fast-growing cycles. To overcome this and other limitations, the project is integrating daily meteorological information (from state and federal networks) and high-resolution satellite data from Planet Labs with BAITSSS, an evapotranspiration (ET) computer model developed by Dr. Ramesh Dhungel (a Research Scientist in the project) and colleagues. The current version of BAITSSS leverages information from the Landsat 8 satellite platform (NASA). An example application of the model is shown in Figure 1, where BAITSSS is combined with soil moisture modeling to forecast when irrigation is needed to supply crop water demand.

The project’s lead on water use and requirement estimations, Dr. Ray Anderson (USDA-ARS U.S. Salinity Laboratory, Riverside, Calif.), says that “along with irrigation management, we believe the daily imagery from Planet Labs will allow growers and irrigation managers to see evapotranspiration anomalies within a field. These anomalies could indicate irrigation issues or decreases in plant health due to other abiotic stressors such as salinity or nutrient deficiency. Identification of these anomalies will ensure more efficient field scouting and earlier identification of issues before permanent yield loss occurs.”

 

Mapping Soil Properties

Accurate knowledge of spatial variability of soil properties, such as texture, hydraulic properties and salinity, is important to best understand the reasons of crop yield spatial variability. Scudiero’s Digital Agronomy Lab at UC Riverside is developing novel tools based on machine learning to automate near-ground sensing of soil properties and remote sensing of soil salinity in irrigated farmland. Soil salinity maps are very useful to inform field-scale irrigation practices (e.g., to calculate the amount of irrigation water needed to avoid harmful accumulation of salts in the soil profile.)

In this project, Scudiero and his team will be using field-scale soil maps of soil salinity collected in the past (since the early 1980s) and throughout the next five years by UC Riverside, the USDA-ARS U.S. Salinity Laboratory, the University of Arizona and other collaborators to generate soil salinity maps for the entire Southwestern U.S. Figure 3 shows the soil salinity map produced for the western San Joaquin Valley by Scudiero and colleagues (see the additional resources section.) In particular, Scudiero’s team will use the ground information to calibrate the Planet Labs time-series imagery to predict soil salinity in the root zone (e.g., the top four feet of the soil profile.) Satellite imagery alone is generally not sufficient to predict soil salinity. Other stressors (water stress, nutrient deficiency) have similar imagery properties to salinity. However, in short periods (two to five years), salinity remains fairly stable throughout the soil profile, contrary to other more transient stressors. Because of these differences in temporal variability between stressors, multi-year time series can be used to detect and map crop health reduction due to soil salinity.

Figure 3: 2013 remote sensing soil salinity (electrical conductivity of a saturated paste extract, ECe) for the zero to four feet soil profile in the western San Joaquin Valley. The map was generated using Landsat imagery with a resolution of about 900×900 ft. Current research is investigating the use of Planet imagery with a resolution of 12×12 ft to generate soil salinity maps across all irrigated farmland in the Southwestern U.S.

 

Biotic Stress Detection

Within this project, UC Riverside scientists are using smartphone pictures, drone imagery and Planet Labs imagery (20 inch and 12 feet resolution) to develop artificial intelligence classifiers for early biotic stress detection. Figure 4 shows an example of a survey carried out by project collaborators Sonia Rios (UC ANR) and Robert Krueger (USDA-ARS) at UC Riverside’s Coachella Valley Agricultural Research Station in Thermal, Calif. Preliminary analyses show that 20×20-inch resolution imagery from the Planet SkySat satellites can be used to identify the presence of weeds against bare soil ground coverage in citrus and date palm. Single-date imagery cannot be used to successfully distinguish between different weed species. The project investigators hope that repeated satellite imagery, together with drone imagery, will be successful at identifying the emergence and extent of weeds and other biotic stress at the field and farm scales. To develop such tools, project investigators are carrying out controlled experiments at the UC Riverside research farms with controlled weed and pathogen pressures on a variety of vegetable crops.

Figure 4: Planet SkySat imagery from UC Riverside’s Coachella Valley Agricultural Research Station. In panel A, the high-resolution (~20×20-inch resolution) is shown in natural colors and using the Normalized Difference Vegetation Index (NDVI). The different crops present at the site are highlighted in panel A. Panels B, C and D show pictures taken as a ground-truth for the machine-learning weed classifier under development in the project. The project is also collecting imagery from unmanned aerial vehicles (UAV) at selected test sites to detect biotic stress (weeds and pathogens), panel E (photos courtesy R. Krueger (B, C and D) and E. Scudiero (E).)

 

Training to Growers and Consultants

From conversations with stakeholders in California and other states in the Southwestern U.S., we learned that most growers are committed to maintaining the quality and profitability of soil, water and other natural resources in the long term. Additionally, many growers recognize the potential of integrating digital agriculture technologies in their daily decision-making. Nevertheless, investing in new technology always requires considerable commitment. Therefore, growers would like to have more information on the cost-effectiveness of state-of-the-art technologies and on the suitability of technology to their local agricultural system. To address these and other questions, the project team is establishing a multi-state cooperative extension network to develop training programs for growers and consultants on the topics of precision agriculture, digital agriculture and the use of soil and plant sensors in agriculture. These training activities, scheduled to start in 2021, will include contributions from university personnel and industry members.

This article provided an overview and some preliminary results for the University of California Riverside, or UCR, -led project on “Artificial Intelligence for Sustainable Water, Nutrient, Salinity, and Pest Management in the Western US”. The project is funded by U.S. Department of Agriculture’s National Institute of Food and Agriculture (Grant Number: 2020-69012-31914). Through September 2025, the project will investigate the use of daily high-resolution satellite imagery and data science to identify inefficiencies in agronomic management practices and to support improved irrigation, fertilization and pest control in the irrigated farmland across the Colorado River Basin and Central and Southern California.

Additional information on the project and the content of this article can be requested from Elia Scudiero (elia.scudiero@ucr.edu). Information about the project’s cooperative extension events can be requested from ai4sa@ucr.edu. Further information about research on remote sensing of soil salinity and evapotranspiration can be found in the additional resources section.

 

Additional Resources:
Dhungel, R., R.G. Allen, R. Trezza, and C.W. Robison. 2016. Evapotranspiration between satellite overpasses: methodology and case study in agricultural dominant semi-arid areas: Time integration of evapotranspiration. Meteorol. Appl. 23(4): 714–730. doi: 10.1002/met.1596.
Scudiero, E., Corwin, D.L., Anderson, R.G., Yemoto, K., Clary, W., Wang, Z.L., Skaggs, T.H., 2017. Remote sensing is a viable tool for mapping soil salinity in agricultural lands. California Agriculture 71, 231-238. doi: 10.3733/ca.2017a0009

Developing a Nitrogen Fertilizer Plan for Olive Orchards

Nitrogen management plans (NMP) for California olive orchards are essential for the Irrigated Lands Regulatory Program and can increase net return. A good NMP has the potential to increase yield, improve oil quality and mitigate biotic and abiotic stresses while reducing nitrogen losses from the orchard.

Olives differ from other orchard crops in California in that they are both evergreen and alternate bearing. Individual leaves may persist on the tree for two to three years. Leaf abscission is somewhat seasonal, with most leaf drop occurring in late spring.

Rapid shoot expansion occurs on non-bearing branches during the hottest part of the summer (July/August) on ‘Manzanillo’ olives in California. The fruit on bearing branches limits current-season vegetative growth. Olives bear fruit on the prior year’s growth, and the alternate bearing cycle is characterized by extensive vegetative growth in one year followed by reproductive growth the following year (Figure 1). With bloom occurring in late April to mid-May, fruit set can be estimated in early July, allowing for consideration of crop load while interpreting foliar nutritional analysis in late July-early August.

Figure 1. Illustrative model of the alternate bearing cycle of olive (courtesy Carol Lovatt, UC Riverside.)

 

Critical Nitrogen Values

Foliar nitrogen content in July/August should range from approximately 1.3% to 1.7% to maintain adequate plant health. The symptoms of nitrogen deficiency manifest when foliar nitrogen content drops to 1.1% N. As leaves become increasingly nitrogen deficient, foliar chlorosis progresses from yellow-green to yellow. Leaf abscission is common at nitrogen levels below 0.9%. Nitrogen deficiency in olive is associated with a reduced number of flowers per inflorescence, low fruit set and reduced yield.

Excess nitrogen (>1.7%) adversely affects oil quality. Oil with low polyphenol concentration is associated with orchards exhibiting excess nitrogen fertility. Since polyphenols are the main antioxidant in olive oil, reduced polyphenol levels are associated with reduced oxidative stability.

Nitrogen content may impact orchard susceptibility to biotic and abiotic stresses. For example, while excess nitrogen content has been associated with increased tolerance to frost prior to dormancy, it is associated with sensitivity to low temperatures in spring (post-dormancy). High nitrogen content has also been associated with increased susceptibility to peacock spot, a foliar fungal disease on olive.

 

Foliar Sampling for Nitrogen Analysis

By convention, foliar nutrient analysis is conducted in late July to early August in California. Fully-expanded leaves are collected from the middle to basal region of the current year’s growth at a height of about five to eight feet from the ground. To capture a general estimate of the nitrogen status of the orchard, samples should be taken from 15 to 30 trees, with approximately five to eight leaf samples collected per tree. Leaves for analysis should only be collected from non-bearing branches. Growers may find it beneficial to make note of the ‘ON’ and ‘OFF’ status in the historical records of each block. The orchard bearing status, combined with anticipated yield and foliar analysis, will guide decisions for nitrogen applications the following year.

 

Nitrogen Distribution in the Tree

Over 75% of the aboveground nitrogen in the olive tree is incorporated in the vegetative biomass (Figure 2). The twigs, secondary branches, main branches and trunk account for approximately 33% of aboveground nitrogen. 23% of the aboveground nitrogen is harbored in the fruit, with the majority in the pulp (19%). Fruit is only an important nitrogen sink during the initial phase of growth. As fruit size increases, the N concentration decreases due to dilution.

Figure 2. Approximate distribution of nitrogen in the aboveground portion of olive trees (source: Rodrigues et al. 2012.)

 

Estimation of Nitrogen Removed from the Orchard

The easiest component of orchard nitrogen loss to estimate is the N in the harvested fruit. A ton of harvested olives removes approximately 6-8 lbs N from the orchard. The quantity of nitrogen in the fruit varies slightly between olive varieties (Table 1). Growers can use the Fruit Removal Nutrient Calculator for Olive on the CSU Chico website to gain estimates of N removal by the three oil varieties (Arbequina, Arbosana and Koroneiki) and the Manzanillo table olive. This tool was developed by Dr. Richard Rosecrance, a professor at CSU Chico, and Bill Krueger UCCE farm advisor. To access the Fruit Removal Nutrient Calculator for Olive, visit rrosecrance.yourweb.csuchico.edu/Model/OliveCalculator/OliveCalculator.html.

Pruning may generate a second component of nitrogen loss from orchards. The best practice to mitigate nitrogen loss from pruning is to reincorporate the pruned material into the orchard floor by flail mowing. The nitrogen in this organic material will gradually become available to the trees through mineralization.

In mature orchards, the wood removed by annually pruning is approximately equal to the annual vegetative growth. Consequently, the input and removal of nitrogen in vegetative growth is cyclic and almost equal in mature orchards. In young orchards, nitrogen inputs are utilized to support vegetative growth and little N is removed from the orchard in prunings or crop. During this time, nitrogen must be supplied to meet the demand to support vegetative growth. It is estimated that approximately 2.5 lbs N is required to produce 1000 lbs fresh weight of tree growth.

 

Nitrogen Use Efficiency

Not all the nitrogen supplied to the orchard from fertilizer and other inputs (i.e., organic matter, irrigation water) is utilized for tree growth and crop production. A fraction of nitrogen is lost from the orchard ecosystem through processes such as runoff, leaching and denitrification. Efficiency varies among orchards, with some orchard systems exhibiting higher nitrogen utilization rates than others. The efficiency generally varies from 60% to 90%. Higher values denote more efficient use of nitrogen inputs. To estimate the amount of nitrogen to supply an orchard, the demand is divided by the estimated efficiency. For example, if nitrogen demand is 50 lbs. per acre and efficiency is estimated at 0.8, then 62.5 lbs N per acre should be applied.

Nitrogen management plans are site-specific and designed to meet orchard and crop demand while reducing environmental losses. Nitrogen utilization is never 100% efficient. Nitrogen use efficiency can be maximized by minimizing losses from irrigation and fertilization practices while utilizing foliar analysis and knowledge of alternate bearing status to fine-tune applications.

 

References

Fernández-Escobar, et al. 2011. Scientia Horticulturae 127:452–454.
Hartman, H.T. 1958. Cal Ag. Pgs 6-10.
Rodrigues, M.A. et al. 2012. Scientia Horticulturae 142:205-211.

Avocado Invasive Insect Pests

Globally, invasive insect pests cause substantial damage to agricultural crops and natural environments. In the U.S. alone, crop and forest production losses from invasive insects and pathogens have been estimated at almost $40 billion per year (Pimentel et al. 2005). On average, a new non-native invertebrate is introduced into California every 40 days. With respect to insects and mites, about one-third of these become pests (Dowell et al. 2016), and economic losses in California are estimated at more than $3 billion per year (Metcalf 1995).

Rapid growth of U.S. demand for fresh avocados has increased the fruit’s prominence in retail sales and diets. California is the largest producer of avocados grown in the U.S. The value of U.S. avocado production measured at approximately $392 million in 2017, (NASS, 2018). There are more than 3,000 avocado growers in the state farming on approximately 50,000 acres of land, with Ventura County leading the state in most acres planted and harvested in recent years (California Avocado Commission [CAC], 2020).

California avocados are particularly special because less than 1% of the state is suitable for growing them. A healthy, single ‘Hass’ avocado tree can produce up to 200 pounds of fresh fruit each year, which is ~500 pieces. In 2018 alone, California produced around 350 million pounds. Avocados are considered a specialty crop and are not cheap to grow because of water and land costs, and fruit therefore usually demands a high premium price.

Traditionally, insecticide use in California avocado orchards has been minimal due to relatively few pest species, which, for the most part, have been under adequate levels of biological control (Hoddle 2005). However, due to the increase of international trade, illegal importation and the smuggling of foliage, branches with leaves, whole plants and budwood, invasive insect and mite pests have begun to threaten the economic viability of avocado production in California.

Beetle gallery formation in an avocado branch (photo by A. Eskalen.)

Polyphagous and Kuroshio Shot Hole Borers

Polyphagous Shot Hole Bore (PSHB) is an invasive ambrosia beetle from Southeast Asia that was first detected in Los Angeles County in 2003 (Gomez et al. 2018, Rabaglia et al. 2006). PSHB is a pest of great concern in Southern California as it can attack over 300 tree species and reproduce on a subset of these that are capable of supporting beetle reproduction and growing the fungi that cause fusarium dieback.

PSHB has a strong symbiotic relationship with several fungi, including Fusarium euwallaceae and Fusarium kuroshium, the causal agent of Fusarium Dieback (FD) disease (Eskalen et al. 2013). The list of reproductive hosts includes several native oaks, maples, sycamores, willows and avocado. The fungus disrupts the vascular transport of water and nutrients on their host tree that eventually causes branch dieback. The most common symptoms of the disease include sugar or gum exudates, dieback, wilt and ultimately host tree mortality. Reports of dieback symptoms in El Cajon, San Diego County led to the discovery of another species, the Kuroshio shot hole borer (KSHB) in 2013.

KSHB is also present in commercial avocado orchards in San Diego County. KSHB was originally limited to the San Diego region but has since spread to Orange, Los Angeles, Ventura and Santa Barbara counties. Presumably, PSHB and KSHB were introduced accidentally into Southern California via wooden products and/or shipping material (e.g., pallets and dunnage) from Southeast Asia. KSHB is morphologically indistinguishable from PSHB, but species can be separated using molecular techniques.

The fungus gets into the woody tissue of the tree when a female PSHB excavates a gallery and inoculates the gallery walls with fungal spores that it carries in special structures called mycangia (Beaver, 1989). The female PSHB and her offspring feed exclusively on the fungal spores. PSHB adults engage in sibling mating within the natal gallery. Offspring sex ratios are female biased to maximize reproductive output such that one male can inseminate many of his female siblings. Once fully developed and mated, female PSHB exit their natal gallery to start a gallery of their own and repeat the cycle. Unlike their sisters, male PSHB do not disperse since they are not capable of flying and do not possess mycangia, which are specialized structures for fungus storage.

Pesticide efficacy may be limited, and this is attributed to the fact that PSHB feeds on fungi and not directly on wood tissue. This reduces exposure to active ingredients when feeding (Eatough Jones et al. 2017). Cultural management practices include: 1) removal of infected branches to reduce local beetle numbers and fungal spread in the infested area; 2) chipping (< 1 inch) and solarizing infested wood; and 3) avoiding movement of infested materials to new areas (if transportation of materials is necessary, it must be covered(UC IPM 2017).

These management strategies can perhaps abate the severity of local infestations. Also, limiting the spread of infected/infested trees include minimizing firewood transportation. Sterilizing pruning tools with household bleach or another cleaning solution can reduce the spread of the FD pathogen. The PSHB was found in Israel in 2009 in commercial avocado orchards where it damages trees. To date, there has been no avocado tree fatality due to the PSHB or FD in California. For more information regarding PSHB, visit ucanr.edu/sites/pshb/.

Severe Laurel wilt vascular staining on avocado (photo by A. Eskalen.)

Redbay Ambrosia Beetle and Laurel Wilt Complex

An invasive ambrosia beetle, the redbay ambrosia beetle (RAB; Xyleborus glabratus) is a serious pest currently spreading through the Florida avocado industry and has been responsible for significant yield reductions since its discovery in 2005. The beetle was first detected in the U.S. in Port Wentworth, Ga. in 2002 and was probably introduced via infested wooden packaging material (Crane 2011).

This beetle has been slowly spreading across the southeastern U.S. and is currently found as far west as east Texas. Therefore, California growers need to be aware that this pest-disease complex may spread to California avocados. What makes this complex dangerous is that the vector, RAB, has a symbiotic relationship with the fungal pathogen Raffaelea lauricola that causes Laurel Wilt Disease (LWD). Native to Southeast Asia, RAB has similarities to our current ambrosia pest, PSHB. However, the fungus associated with LWD is unlike the disease here in California. Trees become infected when female beetles attack host trees and introduce the pathogen into the xylem while boring their galleries. The infection restricts the flow of water in the tree, induces a black discoloration in the outer sapwood and causes the leaves to wilt. Tree mortality is so rapid that leaves do not fall from dying branches.

Symptoms of RAB beetle and LWD infestations include (Carrillo et al. 2017):

  • Wilting of leaves and young stems. You can usually see dead leaves hanging on branches.
    Color change in leaves from light green to dark purplish green or greenish brown.
  • Stem and limb dieback.
  • Trunk and major limbs that show dried sap which has a white appearance and is a crystalline, powder-like material.
  • Dark streaks in the sapwood. Normally sapwood should be white to yellowish white in coloration with no dark staining or streaking. To check for this symptom simply remove a section of the bark to check for discoloration, which, if present, may indicate fungal infection.
  • Small, dark holes in the sapwood and what looks like loose wood dust and frass indicate wood boring beetles are present.
RAB entry holes in avocado (photo by S. Rios.)

LWD affects redbay (Persea borbonia) and other tree species of the Laurel family (Lauraceae), including avocado. R. lauricola is introduced into host trees when adult RAB colonizes a tree. Adult RAB are very small (~1/16-inch-long), dark brown to black in color and spend most of their life within the tree. Larvae are white, legless grubs with an amber colored head capsule and are found within galleries throughout infected trees. Female beetles can produce flightless male offspring without mating, but females may mate with their male offspring or brothers to produce males and females. Females greatly outnumber males in populations. In the Southeast U.S., RAB’s lifecycle from egg to adult appears to take 50 to 60 days, and there appear to be multiple overlapping generations per year (Hanula et al. 2008). Female beetles emerging from galleries may reinfest the same tree or disperse in search of new hosts. Host trees can remain standing for years and may continue to serve as host material for beetles for several months after initial colonization. Flight activity peaks in the late afternoon and early evening.

It is known that ambrosia beetles are notoriously difficult to control with insecticides because they are protected from residues by living inside the tree most of their life versus being outside the tree. RAB can fly short distances, but LW fungus spreads more quickly through the movement of insect-infested plant material, such as firewood.

Additionally, the pathogen also spreads to other ambrosia beetle vectors. This happens when beetles feed on diseased trees and become contaminated with spores of R. lauricola. Spread can also occur through root grafting between trees. Sanitation is the most effective way to manage this problem. Scouting for wilted branches and quickly removing them has been key to successful early intervention and eradication.

It has been suggested to remove symptomatic trees immediately upon their identification. However, once the appearance of frass and streaks start to show in the wood, this is a sign that the tree has already been infected and has been for some time. As soon as a grower sees the wilt in the branches, it’s time to move quickly. Verticillium wilt and Phytophthora root rot can be mistaken for LW, so avocado growers should check for these diseases before removing trees.

The Florida avocado industry has implemented a universal detection and suppression program with the goal of preventing or limiting the incidence of LW in commercial groves and depressing ambrosia beetle populations (Carrillo et. al 2017). Surveying for the symptoms of LW is a key component to limiting the spread of the disease. Growers and their workers should survey groves weekly or more often if an infestation is detected in an adjacent grove. Early detection, removal and destruction of LW affected trees is the most important practice for controlling LW.

Contact insecticides are ineffective because the pathogen vectors (i.e., the ambrosia beetles) are primarily inside the tree. The first goal is to avoid infestation with the beetles by maintaining a healthy tree as stressed trees are more attractive to colonizing beetles. Beetles prefer trees in orchards that have dense canopies with overlapping leaves and branches.

Chipping infected trees is effective in reducing the spread of the disease. However, chips must be as small as possible (one square inch or smaller) and dried quickly so that the wood is not conducive to fungal growth. A potential drawback to this method is that the aroma of the chipped wood can attract other wood boring insects. Research in Florida is currently being conducted to determine if different commercial formulations of insecticides can be effective in controlling the beetle. For more information regarding LW and RAB, please visit sfyl.ifas.ufl.edu/miami-dade/agriculture/laurel-wilt—a-disease-impacting-avocados/.

Laurel wilt branch die back on avocado (photo by S. Rios.)

Avocado Lacebug

The avocado lacebug (ALB), Pseudacysta perseae (Hemiptera: Tingidae), was first detected in California on backyard avocado trees in Chula Vista and National City, San Diego County in 2004. This pest was first described in Florida in 1908 and has since been reported from Mexico, Guatemala, Puerto Rico, Jamaica, the Dominican Republic and parts of northeastern South America. The native range of ALB is uncertain. Molecular studies suggest that the western areas of Mexico may be the evolutionary center of origin for this pest and it was spread unintentionally from this area, probably on avocado trees into eastern Mexico, Florida and the Caribbean (Rugman-Jones et al. 2012).

After a lull of approximately 13 years, reports started coming of ALB damage to avocados outside of San Diego. In October 2017, well established, reproducing populations of lace bugs were confirmed in commercial Hass avocado groves in Oceanside and De Luz in San Diego County and in Temecula in Riverside County (CAC, 2017). Around this time, infestations were reported from backyard avocados in Culver City in Los Angeles County. Genetic analysis suggested that these new, more damaging populations of ALB were different to those originally detected in San Diego in 2004, and it’s likely that a second introduction of ALB into California has occurred, possibly from Florida (Rugman-Jones and Stouthamer unpublished molecular data).

ALB has been recorded feeding on avocado, red bay and camphor, which are all in the Lauraceae family.

Avocado lace bug is a true bug with sucking mouth parts. Lace bugs use these needle-like mouthparts to feed on the undersides of leaves. Through feeding, leaf cells and pierced cell contents are extracted and ingested, preventing photosynthesis.

Adult avocado lace bugs are small, winged insects, about 2 mm in length (slightly longer than 1/16 in), with black bodies, yellow legs and antennae, and are visible to the naked eye (UC IPM 2021). ALB live in colonies on the lower surfaces of mature leaves, often adults, eggs and nymphs are found together. Eggs are laid in an irregular pattern, sometimes in loose rows, attached to the lower leaf surface and are covered with irregular globules of a black, sticky, tar-like substance excreted by adults. To the naked eye, eggs will appear like grains of black pepper or dirt.

Eggs hatch into wingless nymphs that are capable of walking. Nymphs go through a gradual metamorphosis, shedding their exoskeleton several times as they grow in size, finally developing wings and becoming flying adults. Nymphs are dark red-brown to black and covered with spines. They feed for approximately two to three weeks before maturing into adult males and females which mate, and females then lay eggs, starting the cycle over. In California, ALB populations tend to peak over early summer, around June, then decline in fall, sometime around September (Humeres et al. 2009).

ALB restrict their feeding to the undersides of leaves. Feeding initially causes small white or yellow spots on the surface of the leaves as individual cells dry out. These necrotic areas look like tip-burn caused by excessive salts, but in this case, the necrotic areas are islands of dead tissue in the interior of the leaf surrounded by living tissue. It is suspected that feeding damage can provide entrance for pathogenic fungi, in particular Colletotrichum spp., which are leaf anthracnose fungi. As lace bug colonies grow, brown necrotic areas develop where there has been heavy feeding damage. Heavy feeding can cause striking leaf discoloration and early leaf drop (Hoddle 2004; Hoddle et al. 2005). Avocado lace bug nymphs and adults do not feed on fruit. However, heavy feeding damage to leaves will likely have a detrimental effect on yield, which may result from the loss of photosynthetic capacity in damaged leaves.

Relatively little is known about biology and ecology of ALB in California. In Florida, the most important biological control agents are two egg parasitoids including Oligosita sp. (a Trichogrammatid wasp) and an unidentified mymarid wasp. Green lace wing larvae and other generalist predators are also thought to be important natural enemies. A predatory thrips, Franklinothrips vespiformis, is reported to be the most important natural enemy of the avocado lace bug in the Dominican Republic and is similarly abundant on ALB-infested avocados in Escuintla Guatemala.

Insecticide treatments for other sucking pests currently registered for use on avocado in California will likely provide control of avocado lace bugs (Hoddle et al. 2005). In a trial reported in 1998, J. E. Peña, University of Florida (UF), showed that citrus oil, M-Pede (insecticidal soap) and Mycotrol (a Beauveria fungal species) all controlled lace bug, but it was not indicated how long the effect lasted. Researchers at UF have shown that citrus oil and M-Pede provided short-term lace bug control. Results of small tree trials or weathered residue tests have indicated that carbaryl, imidicloprid, cyfluthrin, carbaryl, fenpropathrin and malathion provide excellent control of avocado lace bug nymphs. Spinosad, abamectin and mineral oil are much less effective at providing control (Humeres et al. 2009b; Byrne et al. 2010). For more information on lace bugs, please visit biocontrol.ucr.edu/avocado-lace-bug.

The presence of frass indicates the presence of RAB (photo by A. Eskalen.)

Proactive Management and Early Detection

The concept of early detection and rapid response is fundamental to effective invasive species management. Developing collaborative relationships with avocado research colleagues in different countries can provide insight into new potential pest problems that could eventually find their way to California (Hoddle et al. 2009). Developing this idea further can result in the development of proactive biological control and IPM programs in advance of the anticipated arrival of new pest species (Hoddle et al. 2018).

Ideally, finding a new pest species in the early stages of the invasion and quickly containing and treating the new infestation may reduce costly long-term management, especially if eradication is possible. Rapid and decisive containment actions are an important consideration as controlling invasive pests, especially fruit feeders, in California avocado orchards will always be a challenge and expensive should they establish widely.

Avocado lace bug adult and eggs (photo by M. Hoddle.)

 

References

Byrne, F. J., Almanzor, J., Tellez, I., Eskalen, A., Grosman, D. M., & Morse, J. G. 2020. Evaluation of trunk-injected emamectin benzoate as a potential management strategy for Kuroshio shot hole borer in avocado trees. Crop Protection, 132, 105136.

Byrne, F. J., Humeres, E. C., Urena, A. A., Hoddle, M. S., and Morse, J. G. 2010. Field evaluation of systemic imidacloprid for the management of avocado thrips and avocado lace bug in California avocado groves. Pest management science, 66(10), 1129-1136.

Carrillo, D., J. Crane, R. Ploetz, E. Evans, and J. Wasielewski. 2017. Brief Update to Laurel Wilt Recommendations – Ambrosia Beetles. Univ. of Florida, IFAS Extension Notice.

Crane, J. (2011) Laurel Wilt and the Redbay Ambrosia Beetle threaten Florida’s avocado and native trees in the Laurel Family. Univ. of Florida, IFAS Extension Homeowner informational brochure.

Coleman, T. W., Poloni, A. L., Chen, Y., Thu, P. Q., Li, Q., Sun, J., and Seybold, S. J. 2019. Hardwood injury and mortality associated with two shot hole borers, Euwallacea spp., in the invaded region of southern California, USA, and the native region of Southeast Asia. Annals of Forest Science, 76(3), 1-18.

Dowell, R. V., R. J. Gill, D. R. Jeske, and M. S. Hoddle. 2016. Exotic macro-invertebrate invaders in California from 1700 to 2015: An analysis of records. Proceedings of the California Academy of Sciences Series 4 63(3): 63-157.

Hanula, J.L, Mayfield, A.E. III, Fraedrich, S.W., and Rabaglia, R.J. 2008. Biology and host associations of the redbay ambrosia beetle, (Coleoptera: Curculionidae: Scolytinae), exotic vector of laurel wilt killing redbay trees in the southeastern United States. Journal of Economic Entomology 101: pp. 1276-1286.

Hoddle, M. S. 2004. Invasions of leaf feeding arthropods: why are so many new pests attacking California-grown avocados? California Avocado Society Yearbook, 87, 65-81.

Hoddle, Mark S., J. G. Morse, Richard Stouthamer, Eduardo Humeres, Gilsang Jeong, William Roltsch, Gary S. Bender. 2005. “Avocado lace bug in California.” California Avocado Society Yearbook 88: 67-79.

Hoddle, M. S., Arpaia, M. L., and Hofshi, R. 2009. Mitigating invasion threats to the California avocado industry through collaboration. Calif. Avo. Soc. Yrbk., 92, 43-64.

Hoddle, M. S., K. Mace, and J. Steggall. 2018. Proactive biocontrol: A cost effective management option for invasive pests. California Agriculture 72(3): 1-3.

Hughes, M.A., Smith, J.A., Ploetz, R.C., Kendra, P.E., Mayfield, A.B., Hanula, J., Hulcr, J., Stelinski, L.L., Cameron, S., Riggins, J.J. and Carrillo, D. 2015. Recovery plan for laurel wilt on redbay and other forest species caused by Raffaelea lauricola and disseminated by Xyleborus glabratus. Plant Health Progress, 16(4), pp.174-210.

Humeres, E. C., Morse, J. G., Stouthamer, R., Roltsch, W., & Hoddle, M. S. 2009a. Detection surveys and population monitoring for Pseudacysta perseae on avocados in southern California. Florida Entomologist 92(2): 382-385.

Humeres, E. C., Morse, J. G., Stouthamer, R., Roltsch, W., & Hoddle, M. S. 2009b. Evaluation of natural enemies and insecticides for control of Pseudacysta perseae (Hemiptera: Tingidae) on avocados in Southern California. Florida Entomologist, 92(1), 35-42.

Lu, M., Hulcr, J., and Sun, J. 2016. The role of symbiotic microbes in insect invasions. Annual Review of Ecology, Evolution, and Systematics, 47, 487-505.

Mayfield, A.E. III, Barnard, E.L., Smith, J.A., Bernick, S.C. , Eickwort, J.M., and T.J. Dreaden. 2008. Effect of propiconazole on laurel wilt disease development in redbay trees and on the pathogen in vitro. Arboriculture & Urban Forestry 35: 317-324

Mayorquin, J. S., Carrillo, J. D., Twizeyimana, M., Peacock, B. B., Sugino, K. Y., Na, F., and Eskalen, A. 2018. Chemical management of invasive shot hole borer and Fusarium dieback in California sycamore (Platanus racemosa) in southern California. Plant disease, 102(7), 1307-1315.

Mead, F. W., and Peña, J. E. 2016. Avocado lace bug, Pseudacysta perseae (Heidemann)(Insecta: Hemiptera: Tingidae). Entomol. Circ, (346).

Metcalf, R. L. 1995. Invasion of California by exotic insects. California Agriculture 49(1): 2.

Pimentel D, Zuniga R, and Morrison D. 2005.Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecol Econ 52(3):273–288.

Ploetz, R. C., Pérez‐Martínez, J. M., Smith, J. A., Hughes, M., Dreaden, T. J., Inch, S. A., and Fu, Y. 2012. Responses of avocado to laurel wilt, caused by Raffaelea lauricola. Plant Pathology, 61(4), 801-808.

Rios, S., B. Faber, P. Mauk, A. Eskalen, and M.L. Arpaia. 2018. Redbay Ambrosia Beetle Poses Potential Threat to California’s Avocado Industry. California Association of Pest Control Advisors. February 2018. 22(1):36-38, Pp 36-38.

Rivera, M. J., Martini, X., Conover, D., Mafra-Neto, A., Carrillo, D., & Stelinski, L. L. 2020. Evaluation of semiochemical based push-pull strategy for population suppression of ambrosia beetle vectors of laurel wilt disease in avocado. Scientific reports, 10(1), 1-12.

Rugman-Jones, P. F., M. S. Hoddle, P. A. Phillips, G. Jeong, & R. Stouthamer. 2012. Strong genetic structure among populations of the invasive avocado pest Pseudacysta perseae (Heidemann) (Hemiptera: Tingidae) reveals the source of introduced populations. Biological Invasions 14: 1079-1100.

University of California Integrated Pest Management Guidelines for Avocado <https://www2.ipm.ucanr.edu/agriculture/avocado/> Accessed 20 March 2021.

Umeda, C., Eskalen, A., and Paine, T. D. 2016. Polyphagous shot hole borer and Fusarium dieback in California. In Insects and diseases of Mediterranean forest systems (pp. 757-767). Springer, Cham.

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