- Author: Cameron Zuber
- Author: Elizabeth J Fichtner
The removal of nuts remaining on almond trees from the prior year's crop is an important winter sanitation practice for the management of navel orangeworm (NOW), Amyelois transitella. Residual nuts are called ‘mummies' and the process of removing the mummies is referred to as a ‘mummy shake' because they are mechanically shaken from trees. This practice is conducted during the dormant and delayed dormant season, a time when orchard access may be thwarted by the winter rains.
Most growers strive to have the mummy shake complete by mid-January when buds are dormant and less likely to abscise from the vibration caused by a mechanical shaker. As the flower buds progress toward bloom, they become more sensitive to the shaker vibration and more likely to abscise. Studies conducted in the 1980s (Sibbett et al.) established that the shaking of mummies by January 31 (approximately 8 days prior to bloom) at a Kern County site did not adversely affect yield; however, the authors cautioned growers of the risk of delaying mummy shakes further, particularly on early blooming varieties and in locations in the southern San Joaquin Valley1. Because bud development and bloom date advance with increasing latitude, the potential risk of early and mid-February mummy shakes was investigated by W. Asai (Pomology Consulting, Turlock, CA) in the northern San Joaquin Valley. This work, conducted at a more northern latitude, suggested shakes conducted in early February may not compromise yield2.
The lack of yield detriment attributed to a mummy shake-mediated bud loss may seem counterintuitive; however, simple concepts of tree physiology may help explain this phenomenon. Consider that only approximately 30% of the flowers on a tree set a crop. A given tree does not have the carbohydrate stores needed to set every flower. As a result, the loss of a subset of flower buds may have little effect on overall yield. Naturally, the risk of crop loss increases the closer the shake approaches bloom, and both research groups suggested that mummy shakes be complete prior to the pink bud stage of development.
Although rainy years make it difficult for growers to access orchards and complete orchard sanitation tasks, the heightened soil moisture adversely affects NOW survival in comparison to dry winters. Mummy nuts on the ground support enhanced NOW survival on a dry orchard floor than on moist soil with winter vegetation in the row middles. The next step in managing overwintering populations of NOW is destruction of mummies by flailing or mowing. Flailing and mowing should be completed by March 1, prior to the emergence of NOW. The emergence profile of NOW varies by location, but the first flight generally starts in late March.
Growers who have not completed their winter sanitation practices by the end of January should walk their orchards to assess bud development in consideration of a delayed mummy shake. Winter sanitation can reduce now damage by up to 80%, so an early February shake may be worth the effort if orchard access is possible and bud development has not advanced into pink tip. For more information on NOW management, visit www.ipm.ucdavis.edu.
- Author: Elizabeth J Fichtner
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/).
The dairy and nut industries are dominant contributors to Tulare County's agricultural economy, with milk consistently ranking as the highest valued ag product, followed close behind by pistachio and almond in the top 10. The dairy industry provides economic value to a biproduct of the almond industry-- almond hulls. Almond hulls are incorporated in the herd diet, thus reducing the quantity of forage required for feed in a region sustaining a multi-year drought. This practice increases the water use efficiency of both the dairy and orchard systems.
In October 2022, the UCCE Tulare County Orchard Systems Program, led by Dr. Elizabeth Fichtner, provided an opportunity for cross-disciplinary dialogue between the animal science and plant science communities. Dairy researchers from across the United States visited Tulare County as part of a USDA-funded research team focusing on the economic and environmental sustainability of dairy enterprises. The team was hosted by Dr. Noelia Silva del Rio, UC ANR Dairy Herd Health Specialist, at the UC Davis Veterinary Medicine Research and Extension Center in Tulare, CA. The group had the opportunity to visit a commercial walnut orchard during harvest operations, using the field site as a platform for discussion of cross-disciplinary topics including nutrient management, food safety, composting protocols, and water use efficiency. Additionally, dairy researchers observed the extensive damage on walnut trees caused by Agrobacterium tumefaciens, an organism familiar to all scientists for its role in revolutionizing molecular biology, but infamous to walnut growers and plant pathologists as a bacterial plant pathogen causing crown gall.
- Author: Caleb Crawford
- Author: Elizabeth J Fichtner
Scheduling irrigation events based on crop use allows growers to meet the water demands of crop production wile reducing both the risks of over- and underwatering. Together with improvement of irrigation system efficiency, the integration of the crop water use concept to irrigation scheduling may additionally aid growers in adhering to policies designed to conserve water.
UCCE has partnered with the Department of Water Resources (DMR) to generate weekly Etc reports for almond, pistachio, citrus, raison grapes, wine grapes, walnuts, and stone fruit. The Etc report is an estimate of the weekly soil moisture loss for each crop based on weather data gathered from CIMIS (California Irrigation Management Information System) stations. For the southern San Joaquin Valley, the report is based on data from the following six CIMIS stations: Merced, Parlier, Lemon Cove, Panoche, Five Points, and Stratford. For each crop, local UCCE farm advisors have monitored leaf out in the spring to make estimates of when to start the current irrigation season. Because leaf out may vary extensively by variety, the leaf out date estimates are selected at the approximate midpoint for a given crop. To use the Crop Water Use Report, select the crop of interest as well as the closest CIMIS station to the block in question to determine the approximate water use for the past week as well as the accumulated water use in the current season. Additionally, the report allows growers to estimate water use in irrigation systems with varying efficiencies, and to estimate the number of gallons applied per tree or vine over the prior week. Last, the Water Use Report provides an estimate of the next week's ETc based on an average from previous years.
The Crop Water Use Report is located on the UCCE Tulare County website (https://cetulare.ucanr.edu/) under the agriculture section in the Nut, Prune, and Olive Programs. It is also emailed out weekly from the UCCE Fresno County office. If you would like to be added to that electronic mailing list, please send an email to Mae Culumber (firstname.lastname@example.org).
The Etc reports provide valuable estimates of crop water use but are not a substitute for a grower's direct measurement of water stress using tools such as the pressure chamber. The water use report, however, does provide a “cheat sheet” for growers to use as a reference for their irrigation practices. Growers should consider local weather conditions, the exact leaf out dates of varieties under their management, the efficiencies of their irrigation systems, soil type, and historic water use and crop production information to customize irrigation practices at each site.
- Author: Elizabeth J Fichtner
- Author: Dani Lightle
- Author: Themis Michailides
- Author: Emily Symmes
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.