Over the past few years, the California walnut industry has experienced several challenges including supply-chain disruptions due to the COVID-19 pandemic, a pronounced heatwave in September 2022 that affected walnut quality throughout the state, and the introduction and growth of international producers in the global walnut market. Recently, Tulare County had more walnut acreage removed than any other county, and statewide walnut acreage declined by around 30,000 acres. Despite the decline in acreage, the 2023 season resulted in a record walnut crop in California.

Addressing grower and industry concerns, UCCE Tulare County hosted the 52^nd Tri County Walnut Day on February 1^st, 2024, the second in-person meeting since the pandemic. The event attracted approximately 60 walnut growers and members of allied industries.  The program was organized by Elizabeth Fichtner, UCCE Tulare County Farm Advisor, and moderated by Raymond Mireles, a new fresh fruit and almond advisor at the UCCE Tulare County office, as well as Caleb Crawford, a Staff Research Associate at UCCE Tulare County.  The morning coffee was sponsored by Jason Scott Marketing (West Coast Nut Magazine), and the luncheon was provided by the California Walnut Board and Commission.

Elizabeth Fichtner, a farm advisor at UCCE Tulare County, initiated the program with a short welcome speech and gave her talks on walnut scale and frosted scale management, and minimal heading of Ivanhoe, a new variety of walnut with large, light-colored kernels and high yields. Themis Michailides, professor at UC Davis, presented on the mold problems of walnut and its association with nut quality. Another highlight was the presentation by Bob Beede, Farm Advisor Emeritus of UCCE Kings County, on the management of input costs during low-price years. Other topics covered by University of California researchers included weed management, walnut quality issues, rootstock selection, and pest management. Robert Verloop, Executive Director and CEO of the California Walnut Board and Commission provided updates about the increased production of walnut but flat consumption trends worldwide. The California Walnut board, established in 1948, is funded by mandatory assessments of walnut handlers while the California Walnut Commission, established in 1987, is funded by mandatory assessments of growers. A panel discussion, moderated by Mr. Verloop, was held to facilitate discussion between handlers, growers, and allied industry members.

Olive orchards entering an OFF year in 2024 may benefit from pre-bloom foliar boron (B) applications to support reproduction and yield. Because the 2023 California olive crop varied widely both within and between olive-growing regions, the value of boron applications should be considered at the individual orchard level. For example, in the southern San Joaquin Valley, the 2023 ‘Manzanillo' table olive crop was OFF due to the high temperatures at bloom whereas many oil cultivars in the region were unaffected by the heat and had heavy production. Those orchards that had a heavy ON crop in 2023 may benefit from pre-bloom boron application in the 2024 season.

Boron is an essential micronutrient for plant growth and reproduction. Boron deficiency affects plant reproduction by reducing pollen viability and germination and limiting pollen tube growth. Deficiency also limits the proportion of flowers that set fruit and reduces the retention of developing fruit. The influence of boron deficiency on multiple stages of reproduction may negatively impact yield. Boron also plays a role in vegetative growth and metabolism, ensuring cell wall and membrane integrity and facilitating sugar transport and cell division. Because boron plays a crucial role in reproduction, boron is translocated from vegetative tissues to reproductive tissues resulting in higher concentrations of the nutrient in reproductive organs than leaves. Due to this high demand, reproductive boron deficiency can occur even when vegetative boron and available soil boron are sufficient. 

Studies conducted across numerous global olive-growing regions demonstrate the beneficial effects of foliar boron application on yield, particularly in advance of an OFF crop. The influence of boron application on productivity in olive orchards may relate to increases in photosynthesis, an increase in the number of perfect flowers (those with both male and female reproductive parts) (Figure 1), and an increase in pollen viability, or pollen tube growth. Olives are considered andromonoecious, a reproductive strategy in which plants bear both hermaphroditic (perfect) flowers and male flowers. Stress prior to bloom may cause pistil abscission in a fraction of buds resulting in a higher percentage of male flowers. Several research studies have demonstrated that pre-bloom foliar boron application can increase the percent of perfect flowers on trees, thus increasing the number of flowers capable of producing fruit. In olive, boron is readily mobilized from both young and old vegetative growth to support flower and fruit production; therefore, a portion of boron applied throughout the year may be utilized to support reproductive processes. During the pre-bloom season, however, cool temperatures and the corresponding reduced physiological activity may limit the uptake and translocation of boron in olive. Additionally, flowers are not as strong a boron sink as fruit; therefore, the pre-bloom foliar application may render the micronutrient available at a short-lived, yet critical, time in crop development.

Both oil olives and ‘Manzanillo' table olives have been shown to benefit from foliar boron applications. For example, ‘Arbequina' receiving pre-bloom foliar application of boron exhibited increased bloom and a 27% increase in yield in an OFF year. In the ‘Arbequina' study, no value of boron was observed in an ON year, and boron was found to have no effect on vegetative growth. In another study, boron applications to ‘Frantoio' resulted in increased concentration of chlorophyll and soluble sugars, as well as changes in the profile of endogenous plant growth regulators within the leaves. In California, pre-bloom boron applications on ‘Manzanillo' resulted in increased percentage of perfect flowers and improved fruit set and yield, particularly during an OFF year. 

The recommended foliar boron concentration for olives ranges from 19-150 ppm. Values below 14 ppm boron may result in boron deficiency, whereas values above 185 ppm may result in boron toxicity. A foliar nutrient analysis only provides a snapshot of the status of the plant at the time of leaf collection; however, low boron status of leaves has been found to correlate well with symptoms of deficiency. Symptoms of boron deficiency in olive include dead leaf tips with a characteristic yellow band and green leaf base, as well as twig and limb dieback (Figure 1). Boron deficiency may first become apparent in the meristems, the growing tips of shoots. Boron deficiency may also result in misshapen and defective fruit (Figure 1), low fruit set, and premature fruit drop. The value of boron application for improved fruit set is not limited to orchards with visual symptoms of boron deficiency or foliar boron levels below the recommended range. In fact, the numerous research studies that demonstrate the value of pre-bloom foliar boron applications for enhanced fruit set and yield were conducted in orchards with no boron deficiency. Based on these findings, foliar analysis alone may not be a useful predictor of benefits from pre-bloom foliar boron application. 

Boron is typically introduced to orchards either as a solid mineral broadcast on the soil surface, or in solution as a foliar spray. The pre-bloom foliar application is designed to specifically enhance fruit set and yield and should be applied three weeks prior to bloom. Boron is generally sold as borax, sodium borate, sodium tetraborate, boric acid, or Solubor^® (Table 1). The boron content varies between formulations; therefore, all calculations should be based on the equivalents of active ingredient (ie. pounds of boron). For example, for soil-applied boron in olive, 5-10 lbs/acre of boron is broadcast, which equates to approximately 45-49 lbs/acre of borax (11% boron) or 24-48 lbs/acre Solubor^® (20.5% boron). In California, foliar application of boron three weeks prior to ‘Manzanillo' bloom, particularly in OFF years, at rates of 1 or 2 lb./acre Solubor^® in a 100 gallon/acre (246 or 491 mg/L boron at 935 L/hectare) was demonstrated to improve yield by approximately 30%. The baseline boron level in this California study site was 16 ppm boron, a level just below the established critical level, but high enough to avoid deficiency symptoms.

The value of boron applications on orchard health and economic return varies based on the status of the alternate bearing cycle in the year of application, the baseline boron status of the tree and soil, and other climate factors that may influence yield. Plants have a narrow range between boron deficiency and toxicity. Be sure to read the product label carefully to avoid over-application and conduct annual leaf tissue analyses to gather baseline information on the boron status of orchards. More information on fertilizer rates for olives and other California crops may be found on the CDFA FREP California Crop Fertilization Guidelines website (https://www.cdfa.ca.gov/is/ffldrs/frep/FertilizationGuidelines/).

Walnut scale (Figure 1) is an important economic pest of walnuts in California. High populations of walnut scale may affect tree vigor as well as predispose trees to diseases caused by several plant pathogenic fungi and possibly flatheaded borer damage. Historic UC Pest Management Guidelines emphasize the efficacy of insecticide applications at the crawler stage of insect development (late April to mid-May); however, with the introduction of new pest management tools such as insect growth regulators (IGRs), new studies have been conducted to evaluate the efficacy of these products at earlier times during the insect lifecycle. Both walnut scale and frosted scale overwinter as immature nymphs; therefore delayed-dormant application of IGRs has the potential to inhibit maturation and subsequent reproduction of these pests.

In 2023, UC ANR and UC Berkeley researchers initiated a new collaborative study investigating the efficacy of four products applied at various rates and times (Table 1). Two insect growth regulator products, Esteem^® (IRAC Group 7C) and Centaur^® (IRAC Group 16), were included in the study, with both delayed-dormant (February 8, 2023) and crawler-stage (April 26, 2023) application timings. Centaur^® was also investigated at two rates. Crawler-stage applications of Senstar^® (a combination of spirotetramat and pyroxifin) and Assail 20SG^® (a neonicitinoid) were also included in the study.

Delayed dormant IGR applications inhibit maturation of walnut and frosted scale. Delayed-dormant application of Centaur WD^® at either 34.5 oz/acre or 46 oz/acre reduced walnut scale survival by 81% of that of untreated control treatment by April. Both Esteem and Centaur reduced populations of mature frosted scale observed in late April by the over 85% of that on untreated trees (Figure 2).

Crawler populations affected by IGRs and conventional insecticides. All treatments suppressed the rates of crawler emergence over time in comparison to the untreated control (Figure 3); however, delayed dormant applications of both IGR treatments (Centaur^® and Esteem^®) resulted in the highest suppression of the first-generation curve (Figure 3). Both rates of Centaur^® applied during the delayed dormant period resulted in similar suppression of crawler emergence. Crawler stage application of Centaur^® at the higher rate resulted in similar levels of crawler suppression as the delayed dormant IGR treatments (Figure 3). Moderate suppression of first-generation crawlers was observed with crawler stage treatments with the low Centaur^® rate, Esteem^®, and Assail^® 30SG (Figure 3). Crawler stage application of Senstar^® suppressed first-generation crawler emergence (Figure 3) and resulted in modest suppression (45%) of total crawler populations across the season as compared to the untreated control (Figure 4). All IGR treatments, regardless of the rate or timing, performed similarly with regard to total crawler populations across the season (Figure 4). The range in total crawler suppression across treatments of similar statistical significance was 88.2% (Centaur, 46 oz/acre, delayed dormant) to 54% (Assail, crawler stage), illustrating the variability in crawler counts in the orchard system (Figure 4).

Summary. Delayed dormant applications of either of the insect growth regular products, Centaur^® or Esteem^®, offer excellent suppression of walnut scale and frosted scale populations. Delayed dormant applications may offer similar efficacy at lower product rates due to the opportunity to achieve better coverage prior to leaf-out.  Additionally, delayed dormant applications of these products may inhibit maturation of nymphs into adults, thus limiting sexual reproduction and subsequent laying of eggs. 

In prior studies, the efficacy of crawler stage Assail^® application became apparent the year following application. Based on this background information, the populations of adult walnut scale will be evaluated across all treatments in April 2024 to fully capture the efficacy of these products over time.

Additionally, future studies are planned to further determine the value of dormant versus delayed dormant applications of IGR treatments for management of walnut scale. The results of the current study, however, do demonstrate a need for updating the current UC IPM guidelines for management of walnut scale.  To date, the UC IPM guidelines only recommend crawler-stage applications of IGR products while the current study demonstrates the value of IGR applications earlier in the season. 

Two UC ANR entomologists, Walter Bentley, IPM Advisor emeritus, and Richard Coviollo, Entomology Advisor retired, assisted with identifying male walnut scale insects while visiting with Elizabeth Fichtner, UCCE Tulare County Orchard Systems Advisor. Richard's work in the 1980s demonstrated that the winged male walnut scale individuals adhere to the double-sided sticky tape traps used to identify the timing of crawler emergence. In 1983, Richard observed that the male walnut scale emerged in late March, a key consideration with the recent advent of dormant and delayed dormant applications of insect growth regulators (IGRs) for management of the pest. Conversations between Walt Bentley, Steve Randall, crop consultant, and Fichtner led to the question of whether these early IGR applications may prevent the maturation of males, thus limiting sexual reproduction and subsequent crawler populations.

Coviollo stepped out of retirement yesterday to look at the archive of sticky tape traps collected throughout the 2023 season in an experiment conducted in collaboration with Bob Van Steenwyk, CE Specialist Emeritus, UC Berkeley. Van Stweenwyk and Fichtner established a trial to evaluate the efficacy of delayed dormant and crawler stage IGR applications with products such as Esteem^® and Centaur^®. Also included in the trial were crawler stage applications of Senstar^® (a combination of an IGR and a lipid biosynthesis inhibitor) and Assail^® (a neonicitinoid).

The male walnut scale insects were identified by Coviollo in the traps from late March 2023. Thanks to his expertise and experience, Fichtner can now determine whether the delayed dormant IGR treatments affected the emerged population of males, thus furthering our understanding of how this group of insecticides affects population dynamics of walnut scale.

From a research perspective, this experience emphasizes the value of publishing one's work, regardless of whether it is in peer-reviewed journals or searchable industry reports. One never knows when it may benefit a researcher 40 years later! Additionally, this experience emphasizes the value of the UC ANR system that fosters a sense of community, synergizing the efforts of UC researchers and industry professionals to solve problems affecting our agricultural systems.