- Author: Santosh Bhandari
- Author: Elizabeth J Fichtner
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 52nd Tri County Walnut Day on February 1st, 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.
![4 ton orchard light interception 4 ton orchard light interception](/blogs/blogcore/blogfiles/104667.jpg)
- Author: Elizabeth J Fichtner
- Author: Mel Thayer
- Author: Robert Van Steenwyk
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.
![Figure 1. The covering of a group of walnut scales forms a daisy shape. Removing the cover reveals the yellow adult females Figure 1. The covering of a group of walnut scales forms a daisy shape. Removing the cover reveals the yellow adult females](/blogs/blogcore/blogfiles/103396.png)
![Table1 Table1](/blogs/blogcore/blogfiles/103400.png)
![Figure 2. Ten weeks after the delayed-dormant application of IGRs, frosted scale populations were observed on untreated (A) and IGR-treated (B) trees. Photo B is from an excised branch of a CentaurĀ® (34.5 oz/acre)-treated tree, illustrating the lack of maturation of frosted scale resulting from the delayed-dormant IGR treatment. Figure 2. Ten weeks after the delayed-dormant application of IGRs, frosted scale populations were observed on untreated (A) and IGR-treated (B) trees. Photo B is from an excised branch of a CentaurĀ® (34.5 oz/acre)-treated tree, illustrating the lack of maturation of frosted scale resulting from the delayed-dormant IGR treatment.](/blogs/blogcore/blogfiles/103397.png)
![Figure 3. Crawler emergence was recorded using double sided sticky tape traps that were changed approximately weekly. Crawler emergence is reported as a rate of the average number of crawlers from two subsample traps per cm2 length of twig per day. Treatment P?0.01 Figure 3. Crawler emergence was recorded using double sided sticky tape traps that were changed approximately weekly. Crawler emergence is reported as a rate of the average number of crawlers from two subsample traps per cm2 length of twig per day. Treatment P?0.01](/blogs/blogcore/blogfiles/103398.png)
![Figure 4. The average total number of crawlers emerged over the season was evaluated across treatments. P?0.01 Figure 4. The average total number of crawlers emerged over the season was evaluated across treatments. P?0.01](/blogs/blogcore/blogfiles/103399.png)
- Author: Kamyar Aram
Autumn is in the air, and while it is still high season for harvest for many crops, shorter days and cooler temperatures inevitably bring to mind that winter prep tasks are not far off. And while the winter is the off-season for most crops in our region, it is definitely the on-season for cover crops. This is because, in California where irrigation water is a premium resource, we typically rely on winter rains to water cover crops. While it is ideal to make plans in advance and get the seed order in ahead of the fall rush, there is still time to think about getting a cover crop in before winter storms settle in.
What are cover crops and what are their benefits?
Cover crops include a wide range of herbaceous plants grown with the primary goal of producing biomass for the soil. They offer many opportunities to address specific soil concerns, in addition to improving and maintaining soil health, in general.
Cover crops can:
- Improve soil physical & biological functions
- Improve water infiltration
- Contribute and retain soil nitrogen
- Allow machinery earlier access to wet fields
- Suppress weeds
- Support pollinators & other beneficial insects
Water infiltration is one of the main issues that motivate growers to plant cover crops in California, as roots open new channels and feed underground life. This is especially helpful in orchards and vineyards where soil compaction is especially difficult to remedy. Cover crops can also contribute significantly to nutrient management, both in the use of nitrogen-fixing legume species, or by just foraging left-over nitrogen at the end of the season and keeping it from leaching out of the soil profile. The organic matter that cover crops contribute also improves soil structure and how much water and nutrients it can hold.
Cover crop types and their applications
Cover crops are often broadly grouped by plant types that differ in their primary benefits. For example, grasses generally produce dense stands with extensive, fine, fibrous roots that improve water infiltration and are excellent at mining nitrogen. They are more amenable to driving on at earlier growth stages, and are usually easy to mow. Legumes can actually add nitrogen to the soil through their symbiosis with bacteria that sequester nitrogen from the atmosphere. Because of their high nitrogen content, their biomass generally decomposes more quickly than other cover crops, which can mean a quicker turnaround in a tight planting schedule. Plants in the mustard family produce deep, thick tap roots that open up the soil, and their flowers are favorites of pollinators and other beneficial insects, but if allowed to mature, their sturdy stems may take more work to chop and more time to decompose.
These different types can be used singly or in combinations. There are mixes that incorporate all three types to "hedge bets" against conditions that might disfavor one or another type, and to take advantage of the way that they can complement one another. Many seed companies have developed their own mixes with characteristics to serve specific needs and circumstances.
2022 Orchard Cover Crop Trial
Cover cropping can be as much art as science, if not more so, which means that experience and experiment are essential. To this end, in the fall of 2022, I teamed up with Tom Johnson of Kamprath Seed and a local walnut grower to test out some different cover crops with the hope to improve water infiltration in the orchard. Our trial included a winter triticale, a hybrid grass known for abundant root production without too much aboveground growth, a mustard “pollinator blend” that included a mix of mustard and radish types, and Kamprath Seed's walnut mix, which includes a full complement of grasses, mustards, and legumes. The grower added a forage, spring triticale, which has more of an upright growth, adding a second triticale type for comparison. It turned out to be a lucky year to have cover crops in the orchard, as it was an exceptionally wet winter, putting many fields feet underwater, especially where infiltration was poor. Overall, all of the cover crop types established rather well in most of the planted rows, though there was a great deal of variation throughout the orchard, showing how conditions can affect the crop. Nevertheless, the walnut mix (with legumes, mustard, and grasses) consistently produced the highest total biomass, even in places where the mustard mix and straight triticale came up somewhat thin (see chart below). This highlights the advantages of diversity, but as Tom pointed out during our spring field day for the trial, results may vary considerably from year to year, and each type or blend will have advantages and disadvantages. You can learn more about the trial and the qualities of different cover crops in a video version of the field day on my YouTube channel.
Getting started with cover crops
If you are inspired, I think it is with good reason. Cover crops are tools with great potential and they really engage farming know-how and problem-solving muscles. As in all new things, it's always best to start small and experiment. There are many resources available for learning the basics or refining your existing knowledge about how to effectively employ and benefit from cover crops. In 2021, UCCE and the Resource Conservation District in Contra Costa County together developed Cover Cropping Opportunities in Specialty Crops, a series of recorded webinars and virtual site visits that explore the world of cover cropping from many different angles. These videos offer a wealth of information, instruction, and demonstrations from experienced practitioners and experts.
In addition, below are listed some other excellent UC resources on cover cropping:
Cover Cropping Opportunities in Specialty Crops video series by Contra Costa County Resource Conservation District and UC Cooperative Extension
Grasses, Mustards, & Legumes: An Orchard Cover Crop Field Day video featuring Tom Johnson of Kamprath Seed by UCCE Specialty Crops in the East Bay/Mt Diablo Region
Covercrops for California Agriculture UC ANR Publication Number 21471
Cover Cropping for Vegetable Production UC ANR Publication Number 3517
Cover Cropping in Vineyards: A Grower's Handbook UC ANR Publication Number 3338
Cover Crop Management in Annual Farming Systems Blog Post by UCCE Agronomy Advisor, Sarah Light
Cover Crop Best Management Practices published by the Almond Board of California in collaboration with UC Davis and UC ANR.
Cover Crops for Walnut Orchards UC ANR Publication Number 21627e
Cover Crop Selection tool by UC ANR SAREP
/span>/span>- 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.
![Figure 1 Figure 1](/blogs/blogcore/blogfiles/90407.jpg)
![Figure 2 Figure 2](/blogs/blogcore/blogfiles/90411.jpg)
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- Author: Cheryl Reynolds
‘Tis the season for baking lots of tasty treats. Breads, cookies, cakes, and candy are just a few that come to mind. What makes many of these treats so tasty is the addition of almonds or walnuts to the list of ingredients.
In California, we are lucky to be at the center of almond and walnut production. According to the California Department of Food and Agriculture's (CDFA's) latest Agricultural Statistics Review, more than 99% of the almonds and walnuts produced in the United States are grown in California.
Almond and walnut growers work tirelessly to supply enough nuts to not only satisfy domestic demand, but also for export. Worldwide, almonds rank as the largest specialty crop export. California is the top almond producer in the world, accounting for about 80% of all almonds grown. For walnuts, California ranks as the second largest producer in the world. To keep up with this demand, almond and walnut growers must be constantly aware of pests, diseases, and abiotic problems that can affect the tree and growing nuts.
The University of California Statewide Integrated Pest Management Program (UC IPM) has recently published revised Pest Management Guidelines for almonds and walnuts, helping growers prevent and manage pest problems with the most up-to-date information.
Revisions in the Almond Pest Management Guidelines include:
- A new section on bacterial spot, a new disease of almond in California found in the Sacramento and northern San Joaquin valleys
- A renamed section on fruit russeting, revised from the old powdery mildew section
- Significant revisions made to the management section of navel orangeworm, one of the major pests attacking California almonds
- Improvements on how to do dormant spur sampling section with easier-to-understand information on monitoring and thresholds
Revisions in the Walnut Pest Management Guidelines include:
- Updated information on the association between walnut twig beetle and thousand cankers disease
- New sections for Botryosphaeria and Phomopsis cankers, branch wilt, and paradox canker
- Significant changes to the walnut husk fly management section
Both the almond and walnut revised Pest Management Guidelines also include updated information on fungicide efficacy, weed management, and vertebrate management.
Authored by University of California specialists and advisors, the Pest Management Guidelines are UC's official guidelines for monitoring and managing pests in California crops. For more information on pest management in these or other crops, visit the UC IPM website.