- (Public Value) UCANR: Promoting economic prosperity in California
- Author: Elizabeth J Fichtner
- Author: Carol Lovatt
The alternate bearing (AB) nature of olive is one of the top physiologically-driven challenges faced by olive growers. AB refers to the tree's habit of producing a heavy crop in one year followed by a light crop the next year. The heavy crop is referred to as the “ON crop,” which is characterized by large yields with small size fruit that may mature late and have reduced commercial value due to size. Conversely, the “OFF crop” has characteristically low yields with large sized fruit that may not be cost effective to harvest. AB adversely affects the consistency of the fruit supply, thus having a negative economic impact on every step within the production chain from farm to consumer. Because mitigation of AB can best be achieved by management of crop load, UC researchers have conducted recent studies evaluating the efficacy of a new chemical flower thinning strategy using naphthaleneactic acid (NAA) (Figure 1) applied at full bloom to only on side of the tree annually or biennially to reduce the severity of AB and maintain higher annual yields of commercially valuable size fruit.
There are currently four known mechanisms by which the ON crop reduces flowering and fruit number to contribute to AB of ‘Manzanillo' table olive orchards. The first mechanism of AB is the suppression of summer vegetative shoot growth by the current crop (Figure 1). Flowers and fruit on olive are borne on one year-old shoots; consequently, the vegetative growth in the current year provides the nodes at which inflorescences form in the subsequent spring. Research studies conducted in Tulare County California have demonstrated that the fruit's suppression of vegetative growth is irreversible after pit hardening. As a result, crop management strategies designed to reduce the current season's crop must be implemented by June to promote summer vegetative growth and increase return bloom the following year. The second mechanism of AB is inhibition of floral development (typically initiated in July) by the current season's crop. The third mechanism of AB is the inhibition of spring bud break. As a result of bud break inhibition, even floral buds that have formed may not open at bloom. Last, the current season's crop causes the abscission of floral buds. Since the OFF crop has an effect opposite to that of the ON crop, once AB is initiated in an olive tree, cycles of ON and OFF floral intensity and cropping are perpetuated by the opposing effects of high and low crop loads on these mechanisms.
Historically, olive growers have used NAA, a plant growth regulator, as a fruit thinning agent to reduce the current season's fruit load. Upon application, NAA is absorbed by leaves and developing fruit and is translocated to the peduncle where it incites an abscission layer at the point of attachment to the stem. As an olive fruit thinning agent, NAA is typically applied 12-18 days after full bloom, i.e., during fruit set. Treatments are made with an NAA ammonium salt product, such as Liqui-Stik Concentrate (Loveland Products) applied as a dilute spray (300-500 gallons per acre). Chemical thinning with NAA can be risky; too early an application may result in overthinning, whereas too late an application may not thin sufficiently. Additionally, hot temperatures (> 100°F) within one week of application may enhance the efficacy of NAA resulting in excess thinning. Due to the greater risk of spring heat waves in the south, chemical thinning has been more commonly utilized by table olive growers in the Sacramento Valley than in the southern San Joaquin Valley.
With support from the California Olive Committee, researchers have been evaluating the use of NAA at full bloom (rather than 12-18 days after bloom) as a crop management tool to reduce the severity of AB, which is measured as alternate bearing index (ABI) on a scale from 0 (no AB) to 1 (total AB, crop one year, no crop the other year). These studies tested a full bloom NAA application to one side of the tree with the goal of eliminating crop on one side of the canopy (Figure 2) while maintaining crop on the opposite side. Annual and biennial applications of NAA to just one side of the tree were compared. To implement the annual NAA application strategy in a commercial orchard, growers would apply NAA to one side of the tree at full bloom in year 1, then on the other side of the tree in year 2. In the biennial strategy for NAA application, there would be a year of rest (no treatment) between the application to one side of the tree in year 1 and the other side in year 3.
After 4 years of research, NAA application either annually or biennially at full bloom did not affect cumulative total yield. However, full bloom NAA applications to one side of the tree significantly reduced the severity of AB from near total AB (ABI = 0.94) for the untreated ON/OFF control trees by 20% when applied annually and 38% when applied biennially. The results indicate that annual total yields were more uniform from year to year, especially for trees treated biennially, which improves the economics of all steps in the production chain from farm to consumer.
In addition, both annual and biennial applications of NAA to one side of the tree at full bloom had positive effects on the yield of commercially valuable size (CVS) fruit (medium plus large) compared to the untreated ON/OFF control trees. Biennial NAA application at full bloom reduced the ABI for CVS fruit 43% compared to the untreated control trees, whereas annual NAA application at full bloom only reduced the ABI for CVS by 12.5%. Importantly, application of NAA at full bloom to one side of the tree biennially resulted in 40% greater cumulative yields of medium plus large fruit than untreated control trees, with annual treatment increasing yield of medium plus large fruit only 20%. The increased and more uniform yields of CVS fruit resulting from biennial NAA application at full bloom to one side of the provide growers with greater, more reliable annual income. Moreover, biennial application of NAA (once every three years) is half the cost of annual NAA application. Since NAA is applied to only one side of the tree at full bloom, for which total removal of the crop is desired, the risk of over thinning with NAA is eliminated in this strategy compared to the standard practice of applying NAA to the whole tree during fruit set. Whereas a grower must decide to treat once every three years based on floral intensity and without knowledge of the year's fruit set, the fact that only one side of the tree is treated lessens the effect of a subsequent poor set. The NAA standard practice provides the grower a window of 12-18 days after full bloom to evaluate fruit set when deciding to treat. If the grower sprays NAA according to the standard practice and set is subsequently negatively impacted (potentially by even NAA itself interacting with high temperatures), the whole tree will be affected.
The results of this research demonstrate the potential value of NAA applied at full bloom to shift the crop load to one side of the tree and then the other side biennially. This technique essentially creates a bearing and non-bearing side to each tree, allowing for unsuppressed vegetative growth on the treated side and documents the need for a rest period with no NAA application until year 3 to allow the tree to fully recover. Biennial application of NAA at full bloom to one side of ‘Manzanillo' olive trees successfully reduced the severity of AB and increased yields of commercially valuable size olive fruit better than annual application of NAA or the untreated ON/OFF control trees. The use of NAA at bloom to mitigate AB warrants further investigation. Researchers are also actively investigating the use of pruning 28 days after full bloom to only on side of the tree annually or biennially as a means of mitigating AB to achieve consistent yields of commercially valuable size fruit.
- Author: Elizabeth J Fichtner
- Author: Charlotte Burks
- Author: Mel Thayer
- Author: Robert Van Steenwyk
Walnut scale (Figure 1A) 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. Historic UC Pest Management Guidelines emphasize the efficacy of insecticide applications at the delayed dormant (March) stage of plant development or at the early crawler stage (late April-early May) (Figure 1B) of insect development. Both walnut scale and frosted scale overwinter as immature nymphs; therefore dormant application of insect growth regulators (IGRs) has the potential to inhibit maturation and subsequent reproduction of these pests. New research is needed to evaluate the efficacy of dormant IGR treatments for management of walnut and frosted scale.
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 dormant (February 8, 2023) and early crawler-stage (April 26, 2023) application timings. Centaur® was also investigated at two rates. Early crawler-stage applications of Senstar (a combination of spirotetramat and pyroxifin) and Assail 20SG (a neonicitinoid) were also included in the study.
The study will address the impact of dormant IGR applications on populations of viable adult frosted scale and walnut scale in late April, just prior to anticipated walnut scale crawler emergence. Additionally, the influence of all treatments on walnut scale crawler populations will be evaluated. The results from the study will be shared on the UCCE San Joaquin Valley Trees and Vines website (https://www.sjvtandv.com/) and in the UCCE Tulare County's “In a Nutshell newsletter”.
- Author: Elizabeth J Fichtner
As California's nut industry has grown, so too have the opportunities for UC ANR farm advisors and specialists to present the latest innovative research topics to the state's nut growers, crop consultants and farm managers. UCCE farm advisors utilize large industry-sponsored trade show venues and magazines to extend unbiased, third-party generated data to our primary clientele groups who produce California's almonds, pistachios, walnuts, and pecans. The partnerships between our applied research community and agricultural marketing companies such as Malcolm Media and Jason Scott Marketing, publishers of Pacific Nut Producer (PNP) and West Coast Nut (WCN), respectively, magnifies the reach of UCCE's extension efforts beyond county lines, thus fostering regional and statewide impacts of otherwise local programs.
Annual trade shows such as the California Tree Nut Conference, the Tree and Vine Expo, and the California Walnut Conference attract hundreds of growers to centralized locations in the Central Valley. The educational content at these conferences, including presentations and oversight of panel discussions, is contributed by UC ANR academics with wide-ranging research expertise. The value of this partnership is underscored by the unbiased role of UC scientists in the research arena, particularly at venues designed primarily as marketing events. These trade show events generally engage over 1000 growers per event, allowing UC-generated information to reach greater numbers of contacts than that of our local meetings or newsletter publications. The added benefit is that the costs of the venue and associated luncheon are covered by the corporate sponsors presenting their products in the trade areas.
The farm advisor community similarly capitalizes on the high subscriber numbers of serial publications to extend written extension materials to our clientele base through the state. PNP, established in 1995, reaches nearly 10,000 subscribers and has relied on the UC ANR farm advisor community for relevant, science-based editorial content. For example, Brent Holtz, PhD, UC ANR has authored the monthly Almond Tasks editorial for 15 years. The promulgation of information is heightened by both the circulation volume and the availability of an online archive of articles.
The rich editorial provided by trade magazines includes articles written by UC academics and associated collaborators, as well as articles composed by journalists based upon the work or presentations conducted by UC academics. Subscribers should always take note of article authorship in trade magazines. Articles written by journalists are not proofread or edited by the UC research community; consequently, small errors may be unintentionally propagated in the trade literature. In the past decade, the demand for editorial on nut crops has increased so dramatically that the research community cannot meet the demand. Consequently, trade magazines have had to rely on technical articles composed by freelance journalists. Researchers appreciate the work of the ag- journalism community and do their best to provide information, quotes, and figures for such articles. Regardless, there should be a distinction between articles written by the research and extension community and those authored by our peers within the journalist community.
The written and spoken content presented for industry functions may vary considerably from the content farm advisors print or post in UC forums, such as UC newsletters and grower meetings. Trade shows are funded by marketing; therefore, research that demonstrates a lack of product efficacy (or perhaps even a product detriment to a crop or commodity) will not be welcome at trade shows. As a result, certain topics may be excluded from the agendas at trade events. Similarly, higher emphasis may be placed on agenda items qualifying for continuing education credits than on horticultural practices or techniques. Rest assured, UC ANR academics continue to provide third party, unbiased extension materials by maintaining UC-sponsored meetings for growers and publishing our newsletter editorials both in print and online. UC extension resources include our website, UCCE San Joaquin Valley Trees and Vines (https://www.sjvtandv.com/about-the-authors), individual farm advisor websites and blogs, and as well as the UCCE Growing the Valley podcast (https://www.growingthevalleypodcast.com/).
- Author: Elizabeth J Fichtner
- Author: Dani Lightle
- Author: Themis Michailides
- Author: Emily Symmes
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Walnut scale (Quadraspidiotus juglansregiae) is a common pest of walnut throughout California's Central Valley. Populations of the pest in commercial walnut orchards (Figure 1 A and B) appear to have increased over the past decade, inciting a recent surge of research on the biology and management of walnut scale. University of California researchers have recently monitored crawler emergence patterns in multiple walnut growing regions, thus improving the phenological models utilized in timing insecticide applications. Other studies have focused on evaluation of insecticide chemistries for management of the pest, including softer insecticide chemistries and insect growth regulators (IGRs). Additionally, walnut scale has been found to interact with canker-causing, plant pathogenic fungi in the canopy, thus exacerbating diseases that may further compromise orchard productivity (Figure 1 C, D and E).
Despite its name, walnut scale is not unique to walnut. It is native to North America and has a wide host range including woody shrubs, deciduous plants, and conifers. In California, walnut scale populations sampled from commercial orchards all represent a single species; however, the population composition of walnut scale across other hosts and regions has not been assessed at a genetic level. In California walnut orchards, walnut scale has historically been considered a minor pest. In the past decade, the insect has transitioned from an orchard inhabitant to a pest as the effective insecticides for walnut scale control have been cancelled.
The causes of walnut scale's recent emergence as a pest are not fully understood. It is speculated that populations of walnut scale are higher than historic levels. The interaction of scale with canker fungi, such as Botryosphaeria spp., may also be a new relationship for walnut scale. These pathogenic fungi have recently emerged as economic pathogens of walnut despite having been previously endemic in California. Generally, disturbances in natural or agricultural ecosystems may induce changes in the roles of ecosystem inhabitants, such as insects, pathogens, weeds, and other microbes. For example, prior to the introduction of DDT and other similar chemical insecticides for the management of codling moth, scale insects were not considered economic pests of walnut orchards in California. Utilization of these insecticides disrupted natural enemies (Figure 1F) of several scale species, resulting in a disruption in the orchard ecosystem and necessitating management of scale populations.
Walnut Scale and Botryosphaeriaceae. Walnut scale predisposes trees to infection by Botryosphaeriaceae fungi both directly and indirectly. A direct association between scale and fungal infection is observed when the scale body is lifted from the shoot to reveal a developing canker (Figure 1D). In this direct interaction between the scale and the pathogen, the feeding activity of the insect likely created an infection court (mode of entry) for the pathogen to infect and colonize the branch (Figure 1E). Infestation of twigs by walnut scale also predisposes new growth to infection and disease development. In this indirect interaction between the insect and the pathogen, the canker develops on plant tissue that remains uncolonized by the insect (Figure 1C). Research studies with three different pathogens in the Botryosphaeriaceae family suggest that disease levels are 60-70% higher on branches colonized by scale than on uncolonized branches.
Walnut Scale Identification. Walnut scale is an armored scale with a scalloped waxy coating (Figure 2). The adult female scale is revealed by lifting the waxy coating. The body of the adult female walnut scale can be differentiated from that of the San Jose Scale (a non-native armored scale) by the ridges on the body. Conversely, the adult female San Jose Scale has a smooth body and a pronounced nipple on the coating (Figure 2).
Walnut Scale Lifecycle. Walnut scale nymphs overwinter in orchards and metamorphose to the adult stage in spring. The females remain non-motile but the adult male is winged and able to fly. After mating, the females lay eggs and the eggs hatch in two to three days and then the crawlers emerge. Crawler emergence is in late April to mid-May in California, depending on the climate and location. For example, initial crawler emergence has been observed as early as mid-late April in Contra Costa andTulare County orchards, and as late as
early May in Tehama County orchards (Table 1).
Female crawlers move around, allowing the scale to colonize the current season's shoots. Once a female crawler finds a place to settle and begin feeding, she secretes the scale cover. Male crawlers migrate to the margins of the female cover and settle. Initially the scale cover is white, but it changes to gray or brown in about a week. The female then goes through two instar stages and the male goes through four instars before maturing to the adult and a second generation is initiated in the same season. Second generation crawlers emerge in late July to early August with crawler populations peaking in late August to early September. Second generation crawler activity is generally complete by the end of October; however, at some sites, crawlers may be active into mid-November. These second-generation crawlers will settle and molt before winter.
Detection of Crawler Emergence. To target insecticide applications to the crawler stage, double-sided sticky tape can be wrapped around walnut scale-infested branches in mid-April (Figure 3A). Emerging crawlers stick to the tape as they navigate to find feeding sites. Tapes should be changed weekly and checked under a magnifying glass to observe crawlers (Figure 3B).
Chemical Control. Walnut scale can be managed with either broad-spectrum insecticides or IGRs. IGRs are a preferable tool for use in an integrated pest management program because they are less disruptive to natural enemies than contact insecticides. IGRs work by disrupting the molting process; therefore, their activity is not realized immediately but rather over time as the insect develops. When applied at the delayed dormant stage (ie. March), IGRs would have the opportunity to inhibit the maturation of the overwintering scale to the adult stage. When applied in late spring (ie. late April-early May), IGRs may impact egg hatch as well as the development of the first-generation nymphs.
IGRs including buprofezin and pyriproxyfen are effective for management of walnut scale. In UC trials, pyriproxyfen was only tested as a delayed dormant application, whereas buprofezin was tested at both at the delayed dormant and the crawler stage timings. In season, the delayed dormant buprofezin application was superior to the crawler application for reducing crawler populations; however, applications of the product at either timing yielded similar suppression of scale populations in the subsequent season. Only one IGR application per year should be adequate for management of the pest. Effective monitoring and decision-support can help determine the need to treat on a yearly basis. For more information, on monitoring visit the Sac Valley Orchards website (https://www.sacvalleyorchards.com/walnuts/insects-mites-walnuts/walnut-dormant-monitoring-and-treatment-decisions/).
Products other than IGRs also exhibit efficacy in walnut scale management. In UC trials, acetamiprid, spirotetramat, and bifenthrin/imidacloprid, when applied at the crawler stage, were effective for scale management. The full impact of both spirotetramat and bifenthrin/imidacloprid were not fully realized until the following season.
Summary. Walnut scale had become an insidious pest in walnut orchards. Orchards with high disease pressure caused by fungi in the Botryosphaeriaceae family will benefit from both management of walnut scale and pathogens. If populations of the scale are high, or appear to have increased recently, it may be time to apply an IGR for scale management. IGRs may offer scale suppression over multiple years and often the efficacy of products may not manifest until successive seasons due to the mode of action.
For more information on walnut scale and other pests and diseases of walnut, visit the UC IPM website (ipm.ucdavis.edu). Research reports from studies conducted with support from the California Walnut Board are posted online (walnutresearch.ucdavis.edu). Mention of any particular chemistries or trade names does not constitute a recommendation and are for informational purposes only. Always consult with your licensed crop consultant and adhere to the pesticide label and local and state regulations. Additionally, check with certifier to determine which products are organically acceptable.