The California Walnut Conference, the annual gathering for walnut growers and handlers, is returning with a new date and location for 2026. The event will take place February 19, 2026, at the Turlock Fairgrounds, marking a new chapter in its continued evolution and growth.
The annual California Walnut Conference is presented by West Coast Nut magazine in partnership with the California Walnut Commission (CWC). Together, these organizations have built a forum for information, networking, and solutions for walnut growers and handlers and industry suppliers.
“We know this event has become important to the walnut industry and allied community and hope this more centralized location and new date will help make the conference accessible to additional growers and handlers,” said Jason Scott, Publisher and CEO of West Coast Nut and JCS Marketing Inc.
This year’s conference will include a full day of seminars covering topics important to walnut growers and handlers, including research, production practices, trade developments, advocacy, and market dynamics. Continuing education will be offered eligible sessions.
“The California Walnut Conference is a cornerstone of our efforts to bring the industry together,” said Robert Verloop, Executive Director and CEO of the California Walnut Board and Commission. “We changed the location to Turlock in order to make the conference more accessible for all growers. It provides the opportunity for knowledge-sharing, collaboration, and dialogue that is essential to moving our industry forward into the future. With the challenges and opportunities ahead, this conference helps ensure we are aligned and working toward a strong and sustainable future for California walnuts.”
Based on overwhelming interest from last year, the Poster Board Research Sessions will be expanded in 2026. These sessions feature key findings from walnut industry-funded research in areas such as pest management, irrigation, fertility, varieties and rootstocks, and overall crop production. Researchers will be on hand to engage directly with attendees, answer questions, and provide insights into practical applications for growers.
The Walnut Industry Resource Center—first introduced in 2025—will return with even more nonprofit and government partners offering cost-share programs, educational materials, and grower-focused resources aimed at improving profitability and sustainability on the farm.
Don’t miss this opportunity to connect directly with handlers, processors, and the California Walnut Board and Commission staff to learn more about how the industry is working together to boost demand and increase prices for California walnuts.
Exhibitor and sponsorship registration opens this month. Interested businesses and organizations are encouraged to act quickly to reserve space. For sponsorship or exhibitor opportunities, contact sales@jcsmarketinginc.com.
Attendee registration will also open this month. Growers, handlers, and industry professionals can register at myaglife.com/events.
For more details or to register for this free event, visit myaglife.com/events or walnuts.org.
An integrated approach to spider mites is always recommended, including the use of beneficial insects, particularly fostering six spotted thrips populations, according to Todd Burkdoll, field market development specialist for Valent (photo courtesy UC Statewide IPM Program.)
As summer temperatures soar into triple digits across California’s Central Valley, the annual battle against spider mites has intensified. Growers with a history of infestations face a high risk once again, prompting early action.
“There’s a 95% chance that orchards that had spider mites last year have them again this year,” said Todd Burkdoll, field market development specialist for Valent. “Scouting for overwintering females, if you find adults out there, they’re laying eggs, and those eggs will hatch, and then it goes from one to two to four exponentially really fast. Get on them early, get those females under control.”
Burkdoll recommends an integrated pest management (IPM) approach that combines vigilance, targeted materials and biological control.
He emphasizes early detection. “When in doubt, scout,” attributing that advice to the late John Palumbo.
For chemical control, Burkdoll favors an insect growth regulator (IGR). “I like to go with an IGR [like] Zeal. It’s very specific. It’s basically a mite growth regulator and sterilizes the females, so the eggs that she lays are sterile. The eggs that are already laid, they turn black and die, and the nymphs don’t go from one to the next. So it’s a pretty effective tool.”
Despite its efficacy, proper application is critical. “The only Achilles heel is you have to get good coverage. You can’t spray and pray, go across the field three, four miles an hour and expect it to work, because to the degree you get coverage is to the degree you will get killed, period.”
He quoted the late Madera County Farm Advisor George Levitt: “If you don’t get good coverage, you’re not going to get control. Speed does not kill. Slow down, do it right the first time and you won’t have to go back and try to play catch‑up later on.”
Burkdoll expects moderate to heavy mite pressure in tree fruit, almonds and vineyards in 2025 and stressed the value of biological controls in IPM. “An integrated approach to spider mites is always recommended, and involved in that approach will be the use of beneficial insects, particularly fostering six spotted thrips populations.”
He cautioned against overuse of broad-spectrum insecticides like pyrethroids and organophosphates, saying they harm both pests and beneficial species. “They kill most arthropods, spiders, predatory mites, six spotted thrips… If you can back off on those or wait till the last minute or use something else that isn’t as broad-spectrum or a more targeted approach, that’s always an advantage in the long term.”
For dormant-season mite management, Burkdoll noted selective use of pyrethroids, especially when mixing up chemistries, to avoid resistance. “Going out there broad-spectrum with pyrethroids early season is putting you on a treadmill for other pests. You’re going to have to spray again and again. So using IPM, mixing up chemistries, using different modes of action, and they have to work; you have to have something that works.”
He recommends following UC guidelines for mite management rooted in decades of research. “Older chemistries that have been around, they’ve been thoroughly tested, gives you a pretty good ballpark of what you can use. Just mix up the modes of action. Don’t use the same group number.”
The 2025 Crop Consultant Conferencereturns September 24-25 at the Visalia Convention Center, bringing together California’s leading PCAs, CCAs, researchers and industry professionals for two days of learning, networking and innovation in the heart of Central Valley agriculture.
Hosted by Progressive Crop Consultant, MyAgLife and Western Region Certified Crop Advisers, the Crop Consultant Conference is California’s premier event for crop consultants committed to advancing sustainable, profitable farming. Attendees can look forward to a packed agenda that includes expert-led sessions, CEU opportunities, novel research and practical field-ready strategies designed to meet today’s challenges head-on.
Comprehensive, Flexible CEU Opportunities
One of the conference’s greatest strengths is its dual-format CEU education program. Attendees can earn continuing education units (CEUs) for CA DPR, CCA, FREP, NDA and AZDA through both in-person and online sessions, providing the flexibility busy ag professionals need.
Beginning April 1, 2025, a new expert-led online session launches on the first of each month, covering topics like tree nut economics, advanced irrigation management and more. In-person sessions during the conference will offer extensive CE credit opportunities, with online access to additional courses extended through Dec. 31, 2025.
“This flexibility allows PCAs and CCAs to balance their work schedules while advancing their knowledge and skills,” said Jason Scott, CEO of JCS Marketing, Inc. “And with registration priced at just $345, it’s an incredible value, translating to pennies per CE credit.”
Practical Knowledge and Cutting-Edge Innovation
This year’s program emphasizes practical, immediately actionable insights. Topics will include soil health, pest and disease management, regulatory updates, climate-smart farming practices and the latest advancements in ag technology.
“This strategic location in Visalia allows participants to connect directly with innovations shaping the future of crop consulting,” Scott said.
Attendees will have full access to the conference trade show, showcasing the newest products, services and technologies in agriculture. It’s a rare opportunity to see the tools that can enhance consulting practices and client results firsthand.
A Full Conference Experience
Beyond earning CEUs, participants will enjoy breakfast and lunch both days, a lively networking mixer and ample opportunities to build connections with industry leaders, decision-makers and peers.
“This event is more than just lectures; it’s a platform for collaboration, innovation and professional growth,” Scott said. “It’s where future-forward crop consulting happens.”
High Demand: Register Now
Given the conference’s comprehensive educational offerings, affordable pricing and outstanding networking opportunities, both registrations and sponsorships are expected to sell out quickly.
“This is your chance to be part of a transformative experience that will elevate your professional knowledge and expand your industry network,” Scott said.
Secure your spot today and join us in Visalia this September for the 2025 Crop Consultant Conference.
Almond shells are lightweight but bulky, making them difficult and costly to spread due to the high volume needed per acre. Given limited impacts on soil fertility and yield, this practice is most practical for alfalfa fields located close to almond shell sources (all photos courtesy S. Light.)
Almond shells were applied as a mulch to an established alfalfa field in Yolo County over a two-year period. The intent of the study was to see if alfalfa could serve as a sink for almond shell byproducts after processing without affecting stand productivity. Alfalfa is deep-rooted and fixes nitrogen, which may allow for application of high-carbon materials like almond shell mulches.
The first year, almond shells were applied in October 2021 to a three-year-old stand at 4 to 8 tons per acre. By spring 2022, the almond shells had mostly decomposed. Almond shells were applied to the same test plots in November 2022 at 12.5 tons per acre. Additional field treatments included gypsum at 2 tons per acre per year and an untreated control. In addition to yield, test plots were evaluated for stand vigor, percent cover (bare soil, alfalfa, weeds) and weed pressure. Soil fertility and soil health measurements were also collected during this trial, including aggregate stability, compaction, soil moisture and soil cracking.
Outcomes of Mulch Application on Crop and Soil Metrics The almond shell mulch did not reduce stand vigor as measured by the number of alfalfa plants per square foot. Alfalfa yields were likewise not significantly reduced (P>0.05), though they trended lower in almond shell plots for the first spring cutting (Fig. 1 and 3) and then evened out and were slightly higher than control plots in late summer for both years of this study (Fig. 2 and 4). Almond shells are high in C and low in N. Amendments with a high C:N ratio can tie up N as they break down. The slight reduction in spring yields might be due to initial spring tie-up of N for feeder roots from the almond shell application.
Figures 1-4. Alfalfa yields were not significantly different between the almond shell and control treatments (P>0.05), though trended slightly lower in the spring harvest followed by increased yields in midsummer where almond shells were applied to established alfalfa stands the previous fall in both 2022 and 2023. Gypsum likewise showed trends for higher alfalfa yields, indicating benefits to soil health at our study site with relatively high clay soils.
For soil health metrics, almond shell applications showed benefits of reduced soil cracking (Fig. 5) and soil compaction in the top three inches of soil (Fig. 6). Soil cracking is common in clay soils and can tear feeder roots apart in perennial crops like alfalfa. Soil compaction, a common problem in alfalfa due to equipment traffic during multiple harvests, can lead to yield and stand loss, poor water infiltration and reduced microbial activity. Our study shows almond shells provide an opportunity to mitigate surface soil compaction once the alfalfa has established. There were no changes to other soil health metrics like aggregate stability and bulk density after two years of this trial, nor did the almond shell mulch suppress weeds at rates applied.
Figure 5. One measure of soil health is the degree of cracking as soils dry. Applications of almond shell as a mulch to alfalfa growing on a relatively high clay soil significantly reduced levels of cracking.Figure 6. Soil compaction is measured using a penetrometer, which collects pressure (pounds per square inch (PSI)). Less pressure was needed to penetrate soil in the top 3 inches in plots that had almond shells applied as a mulch (not incorporated).
Apart from electrical conductivity (EC), which measures salinity levels in soil, soil measurements were not significantly different by treatment. Gypsum is a highly soluble salt, and EC was higher in the gypsum plots compared to the almond shell and control plots. Even though differences were not statistically significant, there were some interesting trends in soil measurements. Specifically, almond shells have about 30 pounds of potassium per ton (36 lb K₂O/ton), which can eventually leach into the root zone with rain or irrigation as almond shells decompose. In this project, there was more potassium in soils with almond shell mulch compared to gypsum or control plots. In addition, plots with almond shells had more total carbon and total organic matter. Soil samples were collected in the top foot of soil and shells were applied to the soil surface. It is likely the soil sampling depth affected our ability to measure differences in soil K and C. Other measurements like cation exchange capacity, magnesium, calcium and total N were not different by treatment.
Soil water measurements were collected in this trial. Infiltration measurements were taken for the first four inches of water applied. Infiltration measures the rate at which water moves into the soil. Infiltration was fastest in plots with almond shells for all 4 inches of water. However, the differences were only statistically significant for the 4th inch of water (Fig. 7). In a heavy rain event, rapidly moving water into the soil is advantageous for preventing runoff and retaining water in the fields. Saturated hydraulic conductivity measures the rate that water flows through saturated soil. Though not statistically significant, almond shell plots also had a faster saturated hydraulic conductivity (faster water flow rate) compared to other treatments.
Figure 7. Water infiltration was measured for the first 4 inches of water. This simulates how long it takes water to move into the soil during a heavy rain event. The almond shell plots had the fastest water infiltration (fewest minutes required per inch) for all measurements. The 4th inch of water is shown.
Volumetric water content was measured in the top 6 inches of soil. The total differences in soil water content at any point in the season were negligible for on-farm irrigation decisions. However, some interesting trends were observed. In rainy months, almond shell plots had higher water content after rain events, likely because of increased infiltration and hydraulic conductivity. However, in the summer months, almond shell plots had slightly less water in the top 6 inches of soil. These are the months when the yields trended slightly higher in the almond shell plots. Alfalfa is a crop that yields relative to water applied; this reduction in water content is likely due to the increased alfalfa yield in plots with almond shell application.
Cost Considerations and Logistical Factors of Shell Application Almond shells are both bulky and very lightweight, making them challenging to spread compared to other amendments. However, shells are a dry material, and transportation costs are not lost to water weight as with other soil amendments like compost. Good soil coverage requires a high volume of shells per acre, and multiple truckloads per field will be needed. This increases hauling and spreading costs. For the Sacramento Valley where this field trial was conducted, freight costs are $10 per ton within 50 miles and spreading costs are $15 per ton. These costs are high given the lack of measurable differences to soil fertility and yield. Thus, this practice is best suited for established alfalfa fields located near a source of almond shells to reduce freight costs.
Mulching is considered a soil conservation practice under both federal and state guidelines. However, mulch must be applied to a 2-inch depth and at a rate to achieve 70% soil coverage. At the highest application rate in our study (12.5 tons/acre), the depth of the mulch was under 1 inch.
Almond shell mulch rests on the soil surface of an alfalfa field. Researchers observed benefits such as reduced soil cracking and improved water infiltration without significant changes in soil fertility or productivity.
Key Takeaways In our study, almond shell mulch in an established alfalfa field showed benefits of reduced soil cracking, reduced soil compaction and increased water infiltration without negatively affecting overall stand health and yields at the rates of almond shells applied. Alfalfa has deep roots and fixes N, making it a resilient crop for diverting high-C almond shell waste byproducts from nearby orchards to alfalfa stands, improving organic matter recycling in the region. Since almond shells are not incorporated, any N tie-up would be slow and only in the soil surface. Incorporating almond shells to alfalfa stands prior to planting or applying shells to first-year stands is not recommended due to issues with tying up N with a high C:N product that could affect stand establishment and plant growth. This project was an initial evaluation and did not quantify the optimum application rate to alfalfa fields.
Funding was provided by the California Alfalfa and Forage Research Foundation. Thank you to our grower collaborator for supporting this work.
The original version of this article first appeared in the April/May 2025 issue of Hay & Forage Grower.
Figure 3. Recently released CB77 has a slightly larger, whiter seed compared to CB46. More lygus stings are visible in CB46 (photo courtesy B. Huynh.)
Black-eyed peas, also called cowpeas, are a bean speciesnative to Africa in the Vigna genus of legumes. Cowpeas were introduced to the United States as early as the 16th century by Spanish colonists and through the trans-Atlantic slave trade. “Blackeyes,” as they’re called locally, are grown by California growers on approximately 8,000 acres each year to produce a nutrient-rich food for consumers. Most production in California goes to the dry bean sector for canning and bagging. A small amount of the crop is produced for fresh consumption, similar to green beans or snap peas, and may be sold at farmers markets.
Blackeyes are an important crop in the San Joaquin and Sacramento valleys, where diverse crop rotations are common. A relatively drought-tolerant crop, blackeyes are usually flood or furrow irrigated. Growers rarely fertilize with nitrogen since blackeyes efficiently fix nitrogen from the air due to the plant’s symbiotic relationship with a root-inhabiting Rhizobia bacteria species. Additionally, blackeyes are moderately tolerant of salinity and can grow in conditions where yield declines would be expected for other summer annuals like corn and tomatoes. These characteristics of blackeyes can be an economic incentive to grow them in some years and will be important in California, particularly under hotter and drier conditions expected with climate change. Plant Breeding for the Future
As California shifts toward drier and more extreme weather, blackeyes, like most cultivated plants, will experience new pressures that growers will be first to manage. Heat stress and drought vulnerability, emerging and invasive insect pests, increased weed competition and the evolution of endemic and invasive diseases are some of these pressures. Plant breeding that considers these stresses will help the industry stay ahead of the curve.
UC has a long history of variety development for the California blackeye and garbanzo industries. For blackeyes, the current standard varieties are CB46 and CB50, which were released in 1990 and 2009, respectively. CB46 is high-yielding and has Fusarium wilt race 3 resistance, but it is susceptible to virulent and aggressive races of root-knot nematodes, Fusarium wilt race 4, aphids, lygus and late-season diseases known collectively as ‘early cut-out.’ Additionally, the market now prefers larger seed and whiter grain than what CB46 provides. CB50 is high-yielding, has larger seed size than CB46 and is resistant to Fusarium wilt races 3 and 4 and root-knot nematodes.
To support plant breeding efforts, new breeding lines and cultivars are trialed at research facilities and then on commercial farms to evaluate material across environmental conditions. New materials are evaluated against commercial standards for yield, quality and pest resistance. The cultivars and advanced lines that have been trialed across regions and years are described in Table 1. UCCE farm advisors have collaborated for more than 10 years with UC Riverside plant breeders to test improved lines and, over the last five years, have conducted 21 trials across seven locations in the Central Valley.
Table 1. Descriptions of California blackeye cultivars and new breeding lines.
The variability in precipitation and average air temperature down the Central Valley can influence blackeye phenotypic traits. For example, in Five Points (southern San Joaquin Valley) from 2020 to 2025, the mean annual precipitation was 7.6 inches, whereas Davis (southern Sacramento Valley) had a mean annual precipitation of 17.1 inches. Similarly, average daily air temperature in June, July, August and September over the same five-year period in Five Points was 78 degrees F but was 73 degrees F in Davis. It is important to test experimental lines across regions to understand how they will perform in different environments.
California growers who serve on the California Dry Bean Advisory Board have identified high yield, seed quality and disease resistance as top priorities for plant breeding efforts. They have also emphasized the importance of regional acclimation. The following are some highlights from research funded by the California Dry Bean Advisory Board, USAID Feed the Future Innovation Lab for Legume Systems Research and California Crop Improvement Association, made possible through the generous support of numerous growers, bean harvesters and bean handlers.
Regional Trial Results
Yield results from 2020 to 2024 trials are summarized for the Sacramento and San Joaquin valleys (Table 2), where average yields ranged from 1,091 to 3,582 pounds per acre. The lowest yield occurred in 2020 and the highest in 2024, both in the Sacramento Valley. Interestingly, yields in the San Joaquin Valley were 27% lower than in the Sacramento Valley in 2024. While yields are usually higher in the San Joaquin Valley, the lower yields may have resulted from a prolonged heat wave and above-average nighttime temperatures, which caused significant crop losses.
Table 2. Annual blackeye bean cultivar and advanced breeding line yield (lb/ac) in the Sacramento and San Joaquin valleys.
Seed size is an important quality factor in blackeye production related to consumer preferences. Average seed size, reported as the weight of 100 seeds, for the last five years of trials is shown in Table 3. Seed size ranged from 20.0 grams per 100 seeds for CB77 (2020, Sacramento Valley) to 27.8 grams for CB50 (2024, Sacramento Valley). CB5 and CB50 consistently have the largest seed size across sites from year to year, while the experimental lines tend to have smaller seed size, similar to CB46 and CB77. Over the last five years, average seed size in the San Joaquin Valley was approximately 7% smaller than in the Sacramento Valley.
Table 3. Annual blackeye bean cultivar and advanced breeding line seed size (100 seed weight in grams) in the Sacramento and San Joaquin valleys.
An important insect pest of blackeyes is lygus, which kills fruits before they develop, resulting in direct yield loss. Lygus feeding, called “stings,” also damages and discolors seeds after pods develop (Fig. 1), which reduces yield and diminishes the quality of the beans. The results for lygus damage, shown as the percent of seed with lygus stings, are shown in Table 4. The average lygus damage ranged from 2% for CB77 (2020, Sacramento Valley) to 43% for CB5 (2021, Sacramento Valley). Average lygus damage in the Sacramento Valley over the last five years was 20%, while in the San Joaquin Valley it was 7%. Importantly, under the heavy lygus pressure in the Sacramento Valley, the five-year average lygus damage was lower for experimental line 07KN-74 and newly released CB77 compared to the commercial cultivars CB5 and CB46. This demonstrates a high yield potential and reduced need for insecticides to control lygus with the newer material. With little development of new insecticides for use in dry beans, the importance of insect-resistant varieties cannot be overstated.
Figure 1. Lygus susceptible (left) and lygus tolerant cowpea (right) (photo courtesy Rachael Long, UCCE.)Table 4. Annual blackeye bean cultivar and advanced breeding line lygus damage (% of seed damaged) in the Sacramento and San Joaquin valleys.
New material is also evaluated for disease resistance (data not shown). The evolution of Fusarium wilt provides an example of why plant breeding is critical to disease management. CB5 is an older blackeye variety that is susceptible to Fusarium wilt race 3. CB46 was released as a commercial variety with Fusarium wilt race 3 resistance. However, after years of production, CB46 started showing susceptibility to Fusarium wilt race 4. CB50 was then introduced as a new variety with resistance to both Fusarium wilt race 3 and race 4. During trialing, advanced breeding lines are grown alongside traditional cultivars (like CB5, CB46 and CB50) to quantify how new material compares to varieties already on the market. Lines are evaluated over multiple years to capture variability in pest pressure, weather and other yield-limiting conditions.
Research Outcomes
Recently developed varieties show resistance to disease and aphids, while new breeding lines show great promise for nematode or lygus resistance. Recently, CB77 was publicly released as an improved variety with similar yield and quality to CB46 but with resistance to cowpea aphid (Fig. 2). It also has a brighter white color than CB46 (Fig. 3). The California Crop Improvement Association currently holds foundation seed of CB77 and will distribute it to growers who successfully apply to grow certified seed for commercial production.
Figure 2. CB77 with resistance to cowpea aphid (left) and CB46, which is susceptible to cowpea aphid (right) (photos courtesy Rachael Long, UCCE.)
Lines N2 and 07KN-74 will be ready for public release within a year. Line N2 is a root-knot nematode-resistant line with high yields in the San Joaquin Valley. Root-knot nematodes damage roots, which diminishes water uptake and yield. As fumigants are phased out through regulatory processes, host resistance and alternative strategies for reducing root-knot nematode damage must be taken into consideration. Line 07KN-74 is a lygus-tolerant variety with moderate yield, similar to or slightly lower than the commercial standard CB46. This article summarizes the plant breeding and trialing efforts to improve blackeyes for the California industry. Yield, quality and pest resistance are important traits for plant breeding efforts, and this article summarizes years of data across multiple locations in the Central Valley. These evaluations are ongoing and provide growers with first-hand information on how new genetic material performs under commercial farming conditions. Growers who are interested in learning more or hosting on-farm trials should contact the authors, who would be glad to help make those arrangements.
Figure 1. A demonstration of one flux tower monitoring station and some of the instrumentation set up above the canopy and within the flux tower’s footprint.
In California, avocado (Persea americana Mill.) is primarily grown in southern and central parts of the state along the coast where 88% (USDA-NASS 2023) of the avocados are grown in the United States. These regions have semi-arid Mediterranean climates and currently face uncertain water supplies, mandatory reductions of water use and rising cost of water, and thus, efficient use of irrigation water is one of the highest conservation priorities. Moreover, due to increasing salinity in water sources and the fact that avocado trees are sensitive to salinity, effective irrigation is more critical to ensure optimal yield and high-quality avocado fruits. Many avocado growers have developed irrigation practices that enable good profitability; however, the continual increase in water costs and water restrictions due to drought and climate change has placed pressure on the industry to further enhance water use efficiency. Accurate information on crop water use along with irrigation best management practices are the immediate needs of the avocado industry under the current fluctuations in water availability, reliability and quality to sustain the profitability and sustainability of production. Hass is the predominant avocado variety in California, accounting for nearly 95% of the planted area (Hass Avocado Board 2020). This article summarizes some findings from our recent three-year study conducted on Hass avocado crop water use (actual evapotranspiration or crop water consumption) and crop coefficients.
Experimental Sites and Measurements The data used in this analysis are from the research conducted at Hass avocado orchards in four avocado sites in southern California, here referred to as site A (San Pasqual Valley, Escondido), site B (Via Vaquero, Temecula), site C (Orchard Hills, Irvine) and site D (West Saticoy, Ventura) (Table 1). The sites consisted of a wide range of climates, slopes and elevations, soil texture and conditions, tree spacings, soil types and conditions, and water sources, offering a good representation of the Hass avocado production systems in California.
Table 1. General information about experimental avocado sites.
A combination of eddy covariance and surface renewal equipment (flux tower, Fig. 1) was utilized to measure actual crop water consumption at each avocado site over a three-year period (2022-24). Several other sensors and equipment were used to monitor soil and plant water status, soil salinity and chloride, and high-resolution images were captured by unmanned aerial systems to evaluate canopy features.
Weather Variables Monthly average meteorological data over three years from 2022-24 were compared with the 10-year average (1995-2024) (Fig. 2). The data demonstrated all regions had a dry 2022 winter, a wet 2023 winter and a near normal (10-year average) 2024 winter. Overall, more variations were observed in the monthly maximum temperatures than the minimum values over the study seasons compared with the mean 10-year corresponding temperature data. A similar tendency was found across the experimental regions over the period. Except the fall, the entire 2022 season was warmer compared to the 10-year average. August 2022 had the highest mean daily maximum temperature for the recorded period of three years at 38.1 degrees C (100.6 degrees F) in the San Pasqual Valley and 34.8 degrees C (94.6 degrees F) in the Via Vaquero area. September 2022 had the highest mean daily maximum temperature over the three-year period at 30.9 degrees C (87.6 degrees F) and 28.3 degrees C (82.9 degrees F) in the Orchard Hills and the West Saticoy regions, respectively.
Figure 2. Monthly mean daily maximum and minimum air temperatures over the study period compared to the 10-year average and monthly total rainfall over the study period compared to the 10-year average in regions 1-4 (A-D). Data from CIMIS stations of Escondido SPV# CIMIS 153, Temecula East III# CIMIS 237, Irvine# CIMIS 75 and Camarillo# CIMIS 152 were used for this analysis, representing sites A-D, respectively.
Salinity Effects Salinity within the soil profile varies over the season and between the seasons affected by rainfall, irrigation management, leaching practices, irrigation water quality, and soil types and conditions. The bulk electrical conductivity values measured by CropX sensor at site A (Fig. 3) demonstrated a decline from 388 µS/cm on June 23, 2022 to 95 µS/cm on Feb. 8, 2023 at this site. The salinity noticeably diminished after the wet winter 2023 in comparison with summer 2022 when a salt-affected condition was observed at some of the avocado sites. The avocado sites occasionally could experience salt accumulation more than the threshold while wet winter and appropriate leaching practices may have a significant impact on maintaining salt and chloride issues (Fig. 4). This may negatively influence the crop coefficient values under circumstances. The threshold ECe (electrical conductivity of the saturation extract, dS/m−1) for Hass avocado is reported to be 2.0 dS/m−1.
Figure 3. Half-hourly bulk electrical conductivity data from CropX sensor at 20 cm depth at site A. The data is reported for a 16-month period (June 9, 2022 through Oct. 8, 2023). A wet winter was observed in the 2023 season. The ECe value measured on two different dates (Sept. 17, 2022 and May 4, 2023) at the same depth was 1.69 dS/m−1 and 0.84 dS/m−1, respectively.Figure 4. Soil profile ECe observed values in different sampling points at site C from the survey conducted in September 2022 and April 2023 (after the wet winter 2023). The threshold ECe for ‘Hass’ avocado is reported to be 2.0 dS m-1.
Daily Crop Water Use While a similar crop water use pattern was found over the course of the measurement seasons in experimental sites, daily crop water consumption was generally greatest at site A. Variable daily crop water use was observed on each site over the season/s. For instance, it varied from 0.03 in d−1 to 0.18 in d−1 with an average of 0.11 in d−1 in the 2023 season at site A (Fig. 5a). Considering the tree spacings at this site, the crop water use ranged between 6.7 and 40.5 gallons per tree with an average crop water need of 24.6 gallons per tree in 2023. The values were, as expected from the weather data, lower in late fall and winter when conditions were cooler, and the days were shorter. Uniform daily crop water consumption over the summer months occurred more frequently than other months, specifically during the winter and part of the spring when the weather conditions were more unstable.
The observed daily actual crop water use and Spatial CIMIS ETo (reference ET) in each of the experimental sites were used to compute the daily actual Kc values at each site over the study seasons. The trends in daily Kc values were similar across experimental sites over the study period, with more Kc variability during late fall and winter months when compared with spring and summer months (Fig. 5b). During late-fall and winter, the weather is more unstable with more cloudy and rainy days and wet soils. More fluctuations in actual Kc values are expected under such circumstances. The daily Kc value varied from 0.61 to 1.10 with a mean of 0.75 at site A over the 992-day study period.
Figure 5. Daily actual crop water uses (a) and actual crop coefficient values (b) and 15-day average values at site A over a 992-day period (April 2022 to December 2024).
Seasonal Crop Water Use and Crop Coefficient Values Considerable differences were found in the seasonal crop water use measured across experimental sites and seasons (Fig. 6). The largest difference was 11.4 inches between site A and site D during 2024. However, the seasonal crop water use difference between avocado sites C and D was 2.1 and 2.4 inches in 2023 and 2024, respectively. Overall, greater crop water consumption was observed in each of the avocado sites in 2024.
The greatest seasonal crop water consumption was determined at an avocado site (site A) with the features of coarse sandy loam soil texture, 44% south facing slope, average elevation of 758 feet above mean sea level, plant density of 120 trees per acre, average canopy coverage of 88.7% and tree height of 23.2 feet (Fig. 6). In contrast, the least seasonal crop water use was observed at an avocado site (site D) affected by coastal climate with the features of loamy soil texture, 3% southwest facing slope, average elevation of 164 feet above mean sea level, plant density of 254 trees per acre, average canopy coverage of 75.9% and tree height of 12.5 feet.
Figure 6. Seasonal crop water use measured at the avocado sites in 2023 and 2024. The comparison demonstrates that the seasonal consumptive water use at avocado sites varied from 28.1 inches (affected by coastal climate) to 40.4 inches (an inland valley) over the two growing seasons of 2023 and 2024. Considering the tree spacings at the avocado sites, the seasonal crop water requirements may vary from about 3,000 gallons per tree (high density orchard affected by coastal climate) to about 9,000 gallons per tree (low density orchard under growing conditions of inland valley).
The results demonstrated there is considerable spatial and temporal variability in crop coefficient values of avocado orchards (Fig. 7). At site A, the average monthly Kc value varied between 0.70 in July-August and 0.85 in January. The south facing high slope along with the large canopy coverage are likely the most influential drivers in the environmental conditions of this avocado site, which tends to receive higher direct sunlight and light interception resulting in high crop coefficient values over the season. The monthly actual Kc value varied from 0.55 in July to 0.73 in January at site D, located at a low elevation. This specific site was more affected by the coastal fog influence than the others that could be a major reason for less crop water needs over the season.
Figure 7. Mean monthly actual crop coefficient (Kc) values at the experimental avocado sites. The observed daily actual evapotranspiration and Spatial CIMIS ETo on each site were used to compute the monthly mean Kc values over the study period. Standard deviation of the corresponding Kc values is shown on the bars.
The maximum difference was found between the monthly Kc values of site A and site D, ranging from 11.5 % greater in April to 27.0 % greater in July. Greater differences were observed during the June-September period when lower Kc values determined than in the other months of the year. A similar trend was found at site B with the lower difference values. Inversely, more differences (the values are relatively low) were obtained in the winter months at site C that could be caused by the green ground cover between tree rows during the winter at this site.
The results illustrated summer has the lowest crop coefficient values, increasing gradually from late September to a maximum in mid-winter, again gradually reducing during spring to a minimum in mid-summer. To be more precise, the findings revealed greater Kc act values of avocados during flower bud development and flowering through fruit set growth phases than the fruit development phase. Potential reasons for such a trend are:
• Avocado leaves have a thick waxy cuticle that may reduce water loss through leaf surface and stomata. Young leaves, flowers and young fruit do not have a fully developed cuticle and may lose more water. Researchers reported that during flowering, as some of the floral parts have stomata, the evaporative surface of the avocado tree canopy increases by up to 90%, leading to an increment of the total tree transpiration rate.
• Avocado requires high energy in fall and winter for oil accumulation in fruit and floral development, and the trees may transpire at a higher rate compared to grass (ETo) during these months for photosynthetic activity throughout the growing season.
A mean daily crop water use of 0.13 and 0.15 in d−1 was found for spring and summer (over the three study seasons), respectively, whilst the value for winter and fall was similar (0.08 in d−1) at avocado site A with maximum values. Considering the tree spacings at this avocado site, the average daily crop water requirements are estimated 29.2 and 33.7 gallons per tree in spring and summer and 17.7 gallons per tree in fall and winter. In a winter with normal or wet rainfall conditions, precipitation most likely provides sufficient water to compensate for avocado tree water needs. The study verifies this for 2023 and 2024 at all avocado sites.
Considerable spatial and temporal variability were found in crop water consumption and crop coefficient values of avocado orchards. Several factors impact the variability of these measures in avocados, including climate, slope and row orientation, elevation, height of trees, trees canopy coverage that provides a good indication of canopy size and the amount of light interception, irrigation management practices and salinity and/or soil differences. If avocado orchards are located in similar climatic regions, it appears slope and row orientation along with canopy coverage percentage are likely the most influential drivers on avocado crop water use. It needs to be noted that in the Northern Hemisphere, midday and daily total solar radiation is mostly greater on southern slopes than on northern slopes and the slope aspect influences incoming light intensity and as a result consumptive water use.
The seasonal crop water uses provided in this article are the seasonal water consumption measured for avocado orchards across avocado experimental sites. Excess irrigation can be considered beneficial water use for salinity and choloride management in avocado orchards. The amount of additional irrigation water to effectively drain salt from the crop root zone depends on the soil conditions, effective rainfall and quality of irrigation water. However, the total irrigation water that needs to be applied in an individual orchard over the season depends on seasonal crop water requirements, effective rainfall, water distribution uniformity and salt leaching requirements. Heat waves are another driver that may impact the total applied water in avocado orchards.
The USDA Agricultural Marketing Service and the California Avocado Commission jointly supported this research.
References
Hass Avocado Board and the CIRAD Market News Service. 2020. World avocado production prospects: California in transition.
U.S. Department of Agriculture National Agricultural Statistics Service., 2023. Quick Stats. Available online at: https://quickstats.nass.usda.gov/.
Figure 1. Limited microbial activity with reduced seed germination and vigor (left). Increased microbial activity (increased abundance and diversity) with increased seed germination and vigor (right).
Literally and figuratively, it is thelittle things that count when establishing crops. Yes, it is the microbes that we are talking about! When it comes to soil management, growers often have a plan to support their soil’s chemical and physical properties, but there is also a massive opportunity to see improvements in crop establishment by supporting soil biology. Bacteria, fungi and archaea live very close to plant roots or within the crop itself and have a symbiotic relationship to crops. The more active, abundant and diverse the microbiome, the better for crops. Around 80% to 90% of soil processes are impacted by the soil microbiome, which includes everything from soil health to quality and structure. In turn, that also means your soil microbiology can positively impact crop health, growth and overall performance throughout the growing season and after.
Some of the main benefits microbes provide an establishing crop include supporting germination, nutrient cycling, optimizing soil health, improved root development, pathogen defense and supporting stress tolerance.
Supports Seed Germination The spermosphere is a 2- to 12-mm area around your seed, and it’s the crop’s first interaction with the real world. This is where the symbiotic relationship between crops and microbes begins. Seeds naturally secrete enzymes and metabolites to provide microbes with a carbon source. In exchange, the beneficial microbes mineralize nutrients, reduce abiotic stressors and protect the seeds from pathogens.
Since this is such a brief yet vital stage, ensuring beneficial microbes are present and active at the time of planting can significantly impact early crop health by optimizing the habitat for germination, leading to more uniform stand and early root and shoot vigor (Fig. 1). There are interesting experiments that show sterilized seeds didn’t germinate as well compared to non-sterilized seeds because seeds’ germination enhances with the help of beneficial microbes. Also, native microbes help more than foreign microbes, according to the same research article.
Once the radical has emerged from the seed, the spermosphere no longer exists and has transferred into the rhizosphere, the next beneficial microbes crops encounter around the root zone.
Improves Root Development Microbes support an establishing crop with root development in two ways: First, some soil microbes produce phytohormones that promote plant growth and processes, and second, by improving soil structure and reducing compaction, soil microbes support root development.
Bacteria and fungi both assist with improving soil structure. Bacteria secrete extracellular polymeric substances that act as a glue and bind soil aggregates together. Fungi’s hyphae root network works as a net, holding soil in place, reducing erosion and acting as a nutrient superhighway. Once connected to the nutrient superhighway, the hyphae roots can determine which nutrients your crops need, seek out and find said nutrients and bring them back to the plant. Without these microbial networks, crops may struggle to access nutrients beyond their immediate root zone, limiting their ability to thrive, especially in harsh conditions.
When crops can grow healthier roots, they can also access more water and nutrients on their own, stimulating the plant’s overall growth.
Improves Nutrient Cycling Nutrient cycling has two key components: access and absorption. Once roots find nutrients, they also must be able to absorb them. Soil microbes can help an establishing crop both find and access nutrients. Recently dead and living plants release carbon as rhizodeposits in the soil. Beneficial microbes can use this carbon as food. In exchange, they provide the crop with other essential nutrients like NPK through atmospheric fixation or mineralization of organic matter (Fig. 2). The increase of organic matter decomposition provides an increase in NPK availability and absorption.
There are many microbes that aid in nutrient cycling. Two well-studied are rhizobium and mycorrhizal fungi. Rhizobium, for example, are nitrogen-fixing bacteria mainly associated with forming symbiotic relationships with plant roots. The bacteria form nodule structures on the root. There, colonies of bacteria convert atmospheric nitrogen into ammonia that can be used by the plant in exchange for carbohydrates. Plant arbuscular mycorrhizal fungi improve phosphorus availability by foraging with their hyphae root system.
Figure 2. Simplified plant and microbial nutrient cycling model
Optimizes Soil Moisture As previously mentioned, soil microbes can improve soil structure by binding soil aggregates together. Those microaggregates create pockets for roots to access water and air. Water plays a key role in enabling plants to access nutrients. For some nutrients, such as nitrates and sulfates, they depend on mass flow, or the movement of water, to carry nutrients to the plant. By optimizing water levels, nutrients become more mobile so they can move through well-structured soil to where they’re needed.
Drought-stressed plants uptake less nitrogen and restrict phosphorus uptake, stunting crop growth. Wet or poorly drained soil is susceptible to nutrient loss, which also negatively impacts crops and yields. A healthy microbiome leads to healthy soils that can both drain excess water and store water when needed.
Improves Stress Tolerance Young crops are especially susceptible to drought, cold, flooding and less-than-ideal salinity levels. To help crops manage stress, beneficial microbes secrete antioxidants, enzymes and osmoprotectants to reduce abiotic stress.
Beneficial microbes optimize water productivity. By improving water holding capacity, even during extremely dry conditions, crops have a better chance of establishing. Bacillus subtilis, for example, assists with stress tolerance by producing phytohormones, siderophores and enzymes to stimulate a crop’s natural defense mechanism against environmental stressors.
Impacts Positively to All Four Spheres A healthy and active soil microbiome doesn’t only support your crops at the beginning of the growing season. Yes, you want to create as hospitable of an environment as possible for seeds and newly germinating crops. By supporting microbes in the spermosphere and rhizosphere, you can improve growing conditions for your seeds and newly established crops. As crops continue to grow, some of the microbes in the soil will transfer to the phyllosphere. The phyllosphere is made up of the above-ground tissues of the plant where microbes live. An active phyllosphere supports your crops with managing abiotic and biotic stressors throughout the growing season. Come fall, the beneficial microbes in your soil help break down crop residue in the detritusphere, improving conditions and increasing available nutrients for the seeds you’ll plant in the spring.
Healthy microbes in any sphere positively impact the other three spheres (Fig. 3), which in turn positively impact your crops and soil each planting season.
Figure 3. All four spheres interact with each other and influence each other (courtesy PhycoTerra.)
Beneficial Microbes Are a Good Offense and Defense for an Establishing Crop Offensively, beneficial microbes help crops improve root development, nutrient cycling and optimize soil moisture. Defensively, beneficial microbes help crops manage biotic and abiotic stressors. The support of an active microbiome generally results in better growth and healthier plants overall, and higher yields, especially under less-than-ideal conditions.
Crops need an abundant, diverse and active microbiome to support them both offensively and defensively. Soil characteristics, organic matter levels, soil pH, soil depth, soil type, moisture content, soil structure, temperature, weather and agricultural practices can all impact the diversity and abundance of microbes in agronomic soil. Unfortunately, 75% of soil microbes on farms are dormant or too weak to support crops due to starvation.
The first step to leveraging beneficial microbes on your farm is to conduct a soil test to determine the biological health of your soil. Then, consider implementing farming practices that aid beneficial soil microbes so they can support your establishing crops and overall yields. These practices include crop rotation, no-till or minimal till, and providing the farm with optimal moisture, air, pH, food and nutrients for microbes. The great news here ismany practices that benefit soil physical and chemical health can also promote soil biological health, and vice versa. For example, a good soil structure can support better soil air and moisture levels, and thus soil microbial growth. In turn, this can further help crop establishment and growth. When it comes to establishing a crop, it really is the small things that count, the ones we can’t see but make a big impact on overall soil health and crop health. Hope you have a great year ahead with your crops and an abundant microbiome!
Resources
Why are your soil microbes dormant?: https://phycoterra.com/blog/why-are-your-soil-microbes-dormant/
Microbiome as a Key Player in Sustainable Agriculture and Human Health: https://www.frontiersin.org/journals/soil-science/articles/10.3389/fsoil.2022.821589/full
Management of Soil Microbes on Organic Farms: https://eorganic.org/node/34646#
Benefits to Plant Health and Productivity from Enhancing Plant Microbial Symbionts: https://pmc.ncbi.nlm.nih.gov/articles/PMC8072474/#
Study improves understanding of how bacteria benefit plant growth:
https://news.ucr.edu/articles/2023/07/24/study-improves-understanding-how-
bacteria-benefit-plant-growth#
Microbial controls on seed germination: https://www.sciencedirect.com/science/article/pii/S0038071724002657
Microbial co-operation in the rhizosphere: https://academic.oup.com/jxb/article/56/417/1761/484466
Early results suggest integrating natural habitat into pest management strategies could be a cost-effective way to enhance biological control (photo courtesy H. Cohen.)
Invasive pests can wreak havoc on orchards, and keeping them in check often means frequent pesticide applications. With increasing pesticide regulations and the rising costs of chemical applications, growers of all crop types need reliable alternatives for controlling key pests. Our research explores one such strategy: leveraging natural habitat to enhance pest control. By studying how natural vegetation, including both large protected areas and smaller on-farm hedgerows, impacts pest control, we’re asking how strategically incorporating native plants into an operation may reduce pest pressure and potentially cut down on pesticide use.
Small-scale plantings like hedgerows and floral strips have frequently been found to improve biological pest control and attract pollinators but there are challenges to implementing these approaches. First, some studies have shown small-scale plantings only deliver better pest control when they are within 100 to 1,000 meters of larger patches of natural vegetation (Chaplin-Kramer et al. 2011; Heath and Long 2019) and may thus be less effective in simple habitat-poor landscapes. Other studies also show substantial variation in the effects of natural vegetation on pests, natural enemies and crop yields (Karp et al. 2018). Sometimes adding natural vegetation works great but other times it seems to have no impact. This variation makes it difficult to develop broadly applicable management guidelines.
These inconsistent effects of noncrop vegetation on pest levels and crop yields might be attributed to the following explanations: 1. Researchers failing to monitor the entire biological control community and thus overlooking key interactions. 2. A poor understanding of how on-farm practices interact with landscape-scale processes. 3. Measuring pest levels and biological control in ways that are not relevant to the economic decisions made by growers (Karp et al. 2018, Chaplin-Kramer et al. 2019). For growers to determine whether natural vegetation is likely to deliver useful biological control and reduce expenditures on pesticides, we need studies that comprehensively survey the entire community of animals providing biological control, integrate experiments at the farm and landscape scale and measure biological control in a way that fits within the framework growers already use to make pest management decisions.
Figure 1. Bird species richness (a) and abundance (b) are significantly higher in natural habitats compared to orchards and significantly higher on orchard margins compared to interiors (Source: Cormier et al. in prep.)
Ventura County Study Targets Orchard Pest Control We have recently begun a study in lemon and avocado orchards in Ventura County, where we are assessing whether natural vegetation can suppress arthropod pest outbreaks below economic thresholds. We are compiling data on pest levels in orchards using both our own standardized surveys and data collected by PCAs. We are also comprehensively surveying the wildlife community that may contribute to pest control, including bats, wild mammals, birds and insect natural enemies. Critically, our study design allows us to assess how farm-level practices interact with landscape-scale variables. Half our study orchards are close to a large riparian corridor with extensive natural vegetation and half are more than 1 kilometer away. Additionally, half our orchard survey sites have bare margins and half have vegetated margins. This study design enables us to assess how both small on-farm plantings and large patches of natural vegetation interact to influence pest levels and wildlife abundance and diversity. We are also evaluating which specific plant species and plant traits in hedgerow plantings attract helpful insect predators, insectivorous birds, parasitoids and pollinators.
This study is in the early stages, but several key preliminary results have emerged. First, birds can be pest control allies. In orchards close to large patches of natural habitat, we found significantly more insect-eating birds like yellow-rumped warblers and bushtits. These species are known to consume pest insects like aphids, scale and ants. We found no evidence of bird damage to crop trees or fruit, but these effects will vary depending on crop type because avocado and lemon are not species typically targeted by birds. The diversity and abundance of beneficial bird species was significantly higher on orchard margins than interiors (Fig. 1). Additionally, bird diversity and abundance were significantly higher in orchards that had small patches of natural vegetation on their margins even if they were far from the large riparian zone. If you or your grower have natural vegetation nearby, you may already have a hidden pest control workforce, and growers might be able to attract more of these workers by planting the right vegetation on margins. We are currently analyzing exactly what vegetation characteristics attract beneficial insectivorous birds with the aim of providing precise guidance on how to put birds to work on farms.
We also find that hedgerows and floral strips attract the right bugs. Large patches of native vegetation and noncrop habitat around orchards bring in more beneficial insects, including wild bee pollinators and predatory parasitoid wasps. The closer orchards were to the large riparian zone, the stronger this effect. Our orchards close to the river had both fewer pests and more parasitoids (Fig. 2). We are curious if growers who have hedgerows experience increases in pest populations and pesticide costs. This might tell us if intentional hedgerow plantings harbor pests and will be a key future analysis.
A major concern for growers is whether adding native plants could inadvertently attract more pests. The good news? So far, we haven’t seen an increase in pest populations in orchards with hedgerows or floral strips. In fact, we see evidence for greater abundance of pests in orchards that are farther from the riparian zone (Fig. 2). We also anecdotally heard some growers needed fewer pesticide applications in blocks closer to natural vegetation.
Figure 2. Farms closer to riparian habitat have fewer insect pests than sites further away. Closer sites also have more insect natural enemies.
Actionable Strategies for Growers and Consultants Our study suggests some practical tips for incorporating native vegetation into farms. First, start small. Even a 10-by-10-meter hedgerow can make a difference over time when it comes to diversifying habitat for beneficial species. Choose native species that attract beneficial insects like buckwheat and salvias. Adding species with woody structure like laurel sumac, coyote bush and willows can attract birds that need cover and places to perch and nest. Second, think year-round by selecting plants that provide bloom year-round so that you can provide pollen and nectar for insects outside of your crop’s bloom period. Check out this UC site for more information on planting a hedgerow: ucanr.edu/sites/default/files/2018-12/295420.pdf. Finally, monitor and adapt. Keep an eye on pest levels and beneficial insect activity. Work with your PCA to track changes and adjust as needed.
Our study is ongoing, but early results suggest integrating natural habitat into pest management strategies could be a cost-effective way to enhance biological control. Healthier landscapes and support for beneficial insects go hand in hand.
Interested in learning more about our study? Reach out to us! We are a group of researchers from Cal Poly, Pomona, UCCE, CSU Long Beach and UC Santa Barbara with backgrounds in entomology, ecology, animal behavior and restoration. Any questions can be directed to Liz Scordato, Hamutahl Cohen and Erin Questad atescordato@cpp.edu, hcohen@ucanr.edu and ejquestad@cpp.edu, respectively.
References Chaplin‐Kramer, R., O’Rourke, M. E., Blitzer, E. J., & Kremen, C. (2011). A meta‐analysis of crop pest and natural enemy response to landscape complexity. Ecology Letters, 14(9), 922-932.
Heath, S. K., & Long, R. F. (2019). Multiscale habitat mediates pest reduction by birds in an intensive agricultural region. Ecosphere, 10(10), e02884.
Karp, D. S., Chaplin-Kramer, R., Meehan, T. D., Martin, E. A., DeClerck, F., Grab, H., … & Wickens, J. B. (2018). Crop pests and predators exhibit inconsistent responses to surrounding landscape composition. Proceedings of the National Academy of Sciences, 115(33), E7863-E7870.
Interactions in the rhizosphere. Plants influence their rhizosphere microbiome through exudation of compounds that stimulate (green arrows) or inhibit (red blocked arrows). Most microbes affect neither the plant nor the pathogen because they occupy different ecological niches (commensal microbes) but may affect every other organism to somewhat through a complex network of interactions.
“Soil health” and “healthy soils” have become popular topics in recent years as evidenced by the increased number of government programs and commercial products aimed at improving soil health. The desirable properties of healthy soils are efficiency and efficacy of nutrient cycling, capacity to hold and release plant-available water, an environment conducive to root growth, supportive of beneficial soil organisms and improved resilience of the vine to stress from pests, diseases, drought and/or heat.
Characteristics of a healthy soil are those that promote healthy plant growth:
• A living matrix of plant residues, plant roots, animal residue and microorganisms.
• Porous, with a range of pore sizes that allow a balance between water and air in the soil and space for a complex network of microorganisms (bacteria, fungi, etc.), microarthropods and roots to establish. • Chemically balanced to allow for nutrient cycling and conducive to the environmental needs of different types of soil organisms in the soil food web and vine roots. • High in organic matter, which adds nutrients and microbes to soil; those microbes support essential ecological functions of soil, including recycling of nutrients.
Different vineyards and different soil types support different soil ecosystems. What would be considered healthy for sandy soils may not be the same as what is considered healthy for clay soils. Assessing whether the functioning of the soil ecosystem is optimal for any given crop/soil combination is difficult as comparisons between combinations are not necessarily valid.
Roles of Microorganisms in Soil Health Healthy functioning of soil is promoted by complex networks of microorganisms and their grazers, such as beneficial microarthropods. The microbiome of a soil is composed of a host of organisms, including but not limited to bacteria, fungi, protists, nematodes, earthworms and microarthropods. Within these groups, some species can be beneficial, others pathogenic. This can be true even within a genus. For example, the bacteria Pseudomonas fluorescens is beneficial, while Pseudomonassyringae is a pathogen.
Soil microbes play an important role in nutrient cycling in the soil. Decomposers break down organic matter, making it available as an energy and nutrient source for other organisms. Macronutrients such as potassium and phosphorus, which are often immobile in soil, are made available to the vine by some soil microbes.
Soil microorganisms improve soil structure. Bacteria play an important role in aggregate structure and stability. They produce sugars that hold the mineral parts of the soil together. Fungal hyphae weave soils together as do plant roots. Collectively, soil minerals, roots, bacteria and fungi comprise soil aggregates.
Some microbes are biological control agents that antagonize or compete with deleterious microorganisms. For example, predatory nematodes are beneficial. As fungi and bacteria, respectively, Trichoderma spp. and Bacillus subtilis are other examples of well-known biocontrol agents.
Plant growth-promoting bacteria produce chemicals that stimulate vine growth, and amoeba protists stimulate lateral root formation by producing a plant hormone mimic. A vine might react to these compounds like a plant hormone. Other types of bacteria convert nutrients into forms more available to the vine.
Arbuscular mycorrhizal fungi (AMF) live in the soil and on vine roots in a symbiotic relationship with the plant. The plant delivers photosynthates to the fungi for energy, and the fungi provide additional water and nutrients such as phosphorus and nitrogen to the plant. AMF have structures called hyphae that extend great distances through the soil. Hyphae are essentially long tubes that can transport water to vines from areas beyond the root zone. This helps the vine cope with drought. Hyphae also play a role in soil structure.
Soil rich in organic matter supports a diverse microbial ecosystem that helps improve structure, nutrient cycling and plant resilience. Healthy vineyard soils often contain visible root systems and fungal hyphae interwoven through soil aggregates (photo courtesy Katie Bruce, Niner Wine Estates.)
Soil Microbial Consortia Soil microbial species do not function in isolation. The survival and success of any one type of soil microorganism is dependent on the presence and activity of many other collaborating microbes. One type of organism provides the resources another type of organism needs or changes the environment such as to favor a different type of organism. Collaborations of multiple species of bacteria and fungi are referred to as a soil microbial consortium. Applying compost to the field can be a method for delivering or manipulating these synergistic soil microbial consortia.
Like other food webs found in nature, soil food webs are composed of multiple trophic levels or positions in the food chain. Communities of organisms perform important ecological functions, such as contributing to plant productivity, decomposing dead and decaying matter, and returning energy and nutrients for use by plants. Numbers decrease as you move from bottom to top, but the biomass per individual increases from bottom to top. Soil food chains may be more complex than aboveground food chains, as they tend to exhibit a greater incidence of omnivory that are capable of foraging on multiple trophic groups.
The three basic pathways that energy is moved between and within trophic levels are roots, bacteria and fungi. Pathogenic fungi, bacteria and nematodes and their consumers comprise the root pathway. The bacterial pathway is made up of bacteria that feed on dead plant material (saprophytic), those that cause diseases in plants (pathogenic), plus the organisms that feed on them, such as protists and bacterial-feeding nematodes.
Fungi found in the fungal pathway include species that are saprophytic, pathogenic and/or mycorrhizal. This pathway also includes consumers of these fungi. Some mesofauna organisms occupy other trophic levels as secondary, tertiary and quaternary predators. Such organisms include protists, nematodes, mites, fly larvae, centipedes, spiders and beetles. The conversion and movement of energy and nutrients around the soil ecosystem is what allows the functions of decomposition, mineralization and soil aggregate formation to occur.
Soils with collaborative suites of microbial species are likely to be more resilient than single species, which are more vulnerable to disease or stress. Species within these communities turn “on” and “off” according to different environmental signals, such that when one classification of soil organisms declines, another one can fill that same role or function. An analogy is an orchestra that features different instruments at different times in a performance. Unfortunately, naming the species composing different soil consortia and their ecological functions in soil health is still in its infancy.
Monitoring Soil Microbiome and Soil Health Most methods for identifying and quantifying soil microbes are indirect. The methods include measures based on soil aggregation, biomass (estimated by a phospholipid fatty acid profile or counting cells under the microscope), biological activity such as production of extracellular enzymes, and identification by matching DNA fingerprints found in a soil sample to the known genomes of species of bacteria, fungi, protists or nematodes.
Aggregate stability can be a good measure of soil health because it reflects both physical structure and biology. The bulk density of soil is not a direct measure of soil aggregates but is related. A qualitative way of judging aggregate stability is to take a small sample of soil and drip water on it. If the soil crumbles and falls apart, that is an indication of poor aggregation. If the sample absorbs the water, that is a sign the soil has good structure and ability to hold water. Even if all the species of microorganisms in a soil are unknown, measuring aggregates comprised of bacteria and fungi is useful for monitoring changes through time.
Knowing the functional activity of fungi and bacteria provides a general description of the soil ecosystem and soil health. Functional activity can be measured as enzymes metabolizing specific substrates in soils containing cellulose, amino acids or phosphorus, for example.
Monitoring these and other variables can inform decisions about ground cover, cultivation and fertilization toward the goals of reducing compaction, improving soil aggregate stability, increased water infiltration and disease suppression. The limitation of this type of description is that it does not identify or differentiate what genera or species of these organisms are present. The diversity and complexity of the soil microbiome is crucial to the healthy function of the soil.
Techniques like aggregate stability tests and microbial enzyme analysis help monitor soil health and guide management practices (photo courtesy Katie Bruce, Niner Wine Estates.)
Biological Indicators Soil ecology is the study of the complex interactions between the environment and myriad soil biota. No single measure can capture all the variables that contribute to soil health, but choosing measurements that complement each other can help. Interpreting simple measurements of broad groups like fungi or bacteria is difficult because it does not distinguish pathogens from beneficials.
The biomass of bacteria and fungi can be estimated. Phospholipid fatty acid profiles or cell counts are two methods for estimating microbial biomass. Use of viability stains can distinguish active from dormant organisms. Measuring the ratios between fungi and bacteria can be useful as well because it reflects disturbance. A well-functioning vineyard soil will have a higher ratio of fungi to bacteria, which is promoted by reducing or eliminating tillage to keep vegetation with living roots in the system and avoiding the disruption of the physical characteristics of the microbial habitat.
Measuring respiration in the soil provides a picture of how much life there is in the soil, but it is hard to interpret because it combines respiration of roots, microorganisms and their consumers. Although these measures provide rough estimates of biomass, they do not reflect “who” is there.
Soil organic matter is composed of both living and decaying material. The active or living portion of total soil organic matter can be quantified using a technique based on changes in the color of a potassium permanganate solution mixed with soil. Measurements using this method correlate positively with soil biological activity and are sensitive to management practices.
Current research is being performed to identify sentinel species of microorganisms. If there are genetic markers for these organisms, then identifying specific soil microorganisms is possible. For example, DNA can be extracted from soil. Strands of DNA are replicated using polymerase chain reaction techniques. Those strands are compared to the known genomes of different organisms. The longer the strand of DNA replicated determines how specific identification can be. As the genomes of more soil microbes are mapped, identifying the composition of the microbial community in the soil will become more accurate and useful. This research is still in its infancy.
Encouraging and Conserving Soil Microbial Ecosystem Diversity of plants in the vineyard increases the diversity of the soil microbial community. This can be achieved with cover crops and grazing. Planting a blend of multiple species of grasses and legumes accomplishes this. Soil covered with vegetation is typically healthier than bare ground.
Applying compost is an excellent way of introducing more carbon into the soil. Compost can potentially inoculate soil with beneficial microbes, provide nitrogen in organic forms and increase soil organic matter overall. The carbon and nitrogen provided by compost feeds both vines and soil microorganisms.
Reducing tillage as much as possible is advisable. Excessive tillage disrupts the soil food web. The mechanical action of tilling severs earthworms and breaks up soil aggregates, which are habitat for beneficial soil bacteria. Hyphae of AMF are torn. Soil organic matter is lost to the atmosphere from tillage, reducing the food source and habitat of many soil microbes. Microorganisms are redistributed in space, separating them from their habitats and food sources such as predators from prey, decomposers from material that needs decomposing, and beneficial relationships between microbes and roots. Organisms surviving a tillage event will need to repopulate and recreate communities within the soil.
Conserving and encouraging the microbial community of the soil is crucial to improving and maintaining soil health. Differences between soil types and the necessities of vineyard management make comparisons difficult. Developing a soil health management program for any vineyard takes time, dedication and the willingness to experiment. Appreciating the role of the soil microbial ecosystems will contribute to the success of a grower’s efforts in improving and maintaining a healthy soil.
Plant-associated arbuscular mycorrhizal fungi (AMF) participate in soil carbon storage, improve soil aggregation and promote plant health and crop yield. Like other perennial crops, citrus trees create associations with AMF (Wu and Srivastava 2017; Xi et al. 2022) which have been shown to improve crop nutrition (Wu and Zou 2009), enhance tolerance against abiotic stressors like drought (Wu et al. 2019) and induce better root development (Wu et al. 2012). Due to multiple benefits of AMF to soil and plant health, AMF has gained much attention, leading to a rapidly expanding market in mycorrhizal biostimulants focused on improving crop yield and root development of horticultural crops (Igiehon and Babalola 2017; Chen et al. 2018).
In agricultural systems, abundance of AMF can be negatively impacted by intensive cultivation, leading to a decrease of AMF spores and infective mycelium; thus, native AMF are often promoted by cover cropping and reducing soil disturbance (Bowles et al. 2016b). AMF inoculation can be successful in soils with limited native AMF, poor soil health and low productivity (Verbruggen et al. 2012; Rog et al. 2025). Our study aimed to investigate the effects of inoculated and uninoculated triticale cover crops on soil health and carbon storage in the alley and tree rows of a commercial lemon orchard in the Californian Central Coast region.
Experimental Design and Soil Analysis
The study was conducted at a commercial citrus orchard located in San Luis Obispo County between fall 2019 and spring 2023. The testing site was a 6-acre block planted with Citrus limon (L.) Burm. f.‘Lisbon Lemon’. The experimental design was a randomized complete block design with three blocks and three treatments in each block. Treatments included a control (bare fallow with no herbicide application), a cereal cover crop (Triticale (Secale x Triticum L.) drill-seeded at 110 pounds per acre and a cereal cover crop inoculated with AMF (110 pounds per acre inoculated with commercial AMF inoculum Rhizophagus intraradices, 300 propagules per gram at 10 pounds per acre). Cover crops were seeded in alley rows every growing season from 2020 to 2022. The timing of cover crop seeding and AMF inoculation was selected in accordance with winter rain events. In winter 2022-23, early rain promoted germination of cover crop seed from previous years and no AMF inoculation was applied. Cover crops were completely rain-fed, with no supplemental irrigation during dry months, and were mowed in June each year.
In the first three years, soil samples were collected between trees from the tree rows and from the center of the alley row at 0 to 6 inches and 6 to 18 inches depth. In year four, soil sampling was adjusted to better understand the link between the position on the orchard floor, microbial community structure and soil carbon dynamics as affected by the cover crops and AMF inoculation. Composite soil samples were taken from each plot at four functional locations defined as follows: between two trees on a berm (Location 1), the transition section where the berm ends but no cover crop is grown (Location 2), the cover crop edge (weeds in control plots) (Location 3) and the center of the alley row (Location 4; Fig. 1). Fresh soil subsamples were sent to Ward Laboratories for soil microbial community structure analysis using phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA). Soil samples were sieved, air dried and analyzed for total soil C (%), permanganate oxidizable carbon (POXC) and mineralizable carbon (Min C). Min C, also referred to as soil respiration, is an indicator of microbial activity, while POXC, also referred to as active carbon, can be considered a microbial food source. Both Min C and POXC are commonly used as indicators of soil health (Moebius-Clune et al. 2016; Norris et al. 2020).
Figure 1. Modified sampling scheme across the orchard floor. Location 1 was between two trees on a berm, Location 2 was in the transition section where the berm ends but no cover crop is grown, Location 3 was in the cover crop edge (weeds in control plots) and Location 4 was in the center of the alley row.
The Overlooked Role of Weeds and Native AMF. After two years of treatment implementation, there was no effect of cover crops or inoculation with AMF on the total soil C, POXC and Min C in the tree or alley row (data is not shown). Likewise, there was no treatment effect on total soil C, POXC and Min C at any of the four locations across the orchard floor after three years of treatment implementation (Table 1). Our control treatment had weeds (predominantly Malva and some filarees during soil sampling) which added C sources to soil, likely causing the lack of a significant difference between cover crop treatments and control.
Our PLFA and NLFA microbial biomass data showed no significant persistent inoculation impact on the microbial community compared to the non-inoculated cover crop and the weedy control treatment plots after three years of treatment implementation. We did not observe any differences in microbial biomass between the cereal cover crop and the weedy control treatment, indicating weeds supported the microbial community similarly as the cereal cover crop (Fig. 2).
Figure 2. Mean abundance of AMF biomass NLFA (ng/g soil) and total bacteria biomass PLFA (ng/g soil) for the control, a cereal cover crop (CC) and a cereal cover crop inoculated with mycorrhizae (CC M) treatments in four functional locations: 1) on top of the berm next to the tree, 2) in the fallow area next to the berm, 3) on the fringe of the cover crop area and 4) in the middle of the cover crop area) in the 0- to 6-inch-depth increment. Error bars represent standard error (n=3). Different uppercase letters indicate significant differences between locations.
The lack of effect of the inoculated cover crop compared to the non-inoculated cover crop and control treatment on soil C metrics and microbial community structure in our study may be attributed to native AMF species forming associations with plants growing in the alley row and leading to similar results as those observed in the inoculated plots (Wilson et al. 2009; Bowles et al. 2016a,b; Agnihotri et al. 2021; Lin et al. 2023). These findings suggest native AMF can be successfully promoted in the alley rows by weeds or a cover crop and may provide a more effective strategy than inoculation in perennial citrus orchards. Therefore, AMF inoculations may not add value in soil with existing plant cover and low soil disturbance. Cover Crop Effects Extend Beyond Area Directly Underneath Plant Cover In contrast to treatment comparisons, soil C indicators showed greater values in the alley rows compared to the tree row (Table 1). In the topsoil, the cereal cover crop, weedy control and repeated additions of tree prunings in the alley row supported more microbial biomass, including AMF (Fig. 2), and had higher C storage potential compared to the tree row that had less plant matter input in the top soil and potentially experienced C loss due to pulses of C mineralization during dry-wet cycles associated with frequent irrigation (Lundquist et al. 1999; Denef et al. 2001; Lopez-Sangil et al. 2018). The location effects in this orchard trial suggest different managent of trees and alley rows leads to soil heterogeneity across the orchard floor.
Table 1. Average total soil carbon (%) concentrations, average permanganate oxidizable carbon (POXC, mg C kg soil-1), average mineralizable carbon concentrations (Min C, mg C kg soil-1 day-1) and the respective standard errors of the mean (n = 3) for the control, cereal cover crop (CC) and cereal cover crop inoculated with mycorrhizae (CC-M) treatments in four functional locations: 1) on top of the berm next to the tree, 2) in the fallow area next to the berm, 3) on the fringe of the cover crop area and 4) in the middle of the cover crop area) in the 0- to 6-inch-depth increment. Different uppercase letters indicate significant differences between locations within the same carbon measurement.
The management in the alley rows more positively impacted soil C cycling indicators and soil health with some carry-over from the cover cropped alley rows into adjacent areas. More specifically, total C and POXC concentrations declined from the center of the alley rows to the area next to the citrus trees (Table 1). In addition, total C and POXC were greater in location 2, which represents the plant-free area adjacent to the cover crop and impacted by wheel traffic, compared to location 1 immediately next to the citrus tree. An orchard floor gradient assessment from the center of the cover cropped row toward the tree row with bare soil showed a potential carryover effect from the cover crop row into the fallow area between the trees and the edge of the cover crop. Citrus trees may potentially receive more nutrients outside of the berm by extending lateral roots into alley rows and by making associations with AMF that have extensive hyphal networks. Therefore, lemon trees may benefit from soil improvements from cover crops grown in the alley row.
Mycorrhizae Affect Carbon Cycling through Recruitment of Bacteria We used correlation analysis to evaluate the relationships among microbial groups and C cycling indicators (Table 2). In our dataset, AMF and saprophytic fungal biomass were positively correlated with total bacterial biomass (Table 2), which supports previous findings on the cooccurrence of AMF or saprotrophic fungi with bacteria in soil niches (Yuan et al. 2021; Zhang et al. 2022). We also observed no correlation between NLFA biomass of AMF and saprotrophic fungi. These findings suggest both AMF and saprotrophic fungi are able to recruit bacteria but the two groups of fungi occupy different soil niches. Correlation assessment of linking AMF NLFA biomass with soil C indicators showed no direct significant impact of AMF on soil C storage. In contrast to AMF, bacterial biomass was significantly correlated with all soil C indicators supporting studies where bacterial community was a major contributor to soil organic C accumulation (Zhang et al. 2020; Guo et al. 2021; Hu et al. 2023). As such, our findings suggest AMF may have an indirect influence on soil C dynamics by promoting bacterial biomass. Therefore, management practices that promote AMF, such as cover crops and reduced soil disturbance, help build soil organic matter and store carbon.
Table 2. Pearson’s correlations between AMF biomass NLFA, saprophytic fungi NLFA, total bacteria biomass PLFA, Min C (mg C kg soil-1 day-1), POXC (mg C kg soil-1) and total soil carbon (%).
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