Effective Management of Thousand Cankers Disease of Walnut through Disruption of Insect Vector Behavior

Progress report for SW20-913

Project Type: Research and Education
Funds awarded in 2020: $349,770.00
Projected End Date: 03/31/2022
Host Institution Award ID: G334-20-W7899
Grant Recipients: University of California; University of California, Division of Agricultural and Natural Resources; USDA - Forest Service, Pacific Southwest Research Station; USDA - Agricultural Research Service (ARS)
Region: Western
State: California
Principal Investigator:
Dr. Richard Bostock
University of California
Co-Investigators:
Dr. Daniel Kluepfel
USDA - ARS, Crops Pathology and Genetics Research Unit
Dr. Steven Seybold
USDA Forest Service
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Project Information

Abstract:

Thousand cankers disease (TCD) is a threat to English walnut (Juglans regia) and Paradox rootstock in California orchards, to California native walnut species, and to eastern black walnut in forests, plantations, and landscapes throughout the USA.  TCD is caused by a fungal infection following attack by the walnut twig beetle (WTB, Pityophthorus juglandis), which aggregates and transmits the pathogen, Geosmithia morbida.  Multiple infections girdle and kill branches and stems, with TCD often becoming lethal to the tree.  Current management options (only partially effective) are limited to general cultural practices (i.e., maintain tree vigor) and sanitation of infested materials.  Successful TCD management in orchards must include a more targeted strategy that includes the capacity to determine which host trees are likely to become infected and the ability to detect and deter the vector. Chemical ecology studies of the interaction between the WTB, pathogen, and plant host have identified an aggregation pheromone-based lure for WTB detection and repellent compounds to disrupt WTB host location, feeding behavior, and aggregation.  This project extends our studies on characterization of candidate attractive compounds to enhance the lure and incorporates field trials to further evaluate an optimized lure and a repellent mixture, initially in commercial orchards.  Common bacterial root, crown and stem diseases of walnut are often associated with TCD-affected trees, and orchard surveys will define the strength of this association. We will determine how these stresses may influence susceptibility of trees to beetle attack and pathogen colonization.  This information will inform our development of a risk rating system that draws in part from a clearer understanding of the association between predisposing stresses and TCD to guide grower deployment of WTB lures and repellents. Education and outreach to walnut producers, PCAs, and other stakeholders through face-to-face meetings, publications, and various online resources will be important activities and central to project success.  The Western SARE survey and evaluation tool will be used to assess grower awareness of TCD and its risk factors, as well as interest in adopting the WTB lure and repellent in orchard management programs.  This program will also engage our statewide network of UCCE orchard systems advisors who work with walnut growers by providing the latest research findings.  The principal outcome of this project will be enhanced competitiveness through increasing sustainability and resilience of walnut orchards as indicated by development and demonstration of an improved lure to monitor WTB populations and repellents to disrupt aggregation behavior. These fit well within an integrated pest management program, with potential for large-scale development and adoption by growers throughout California and elsewhere.

Project Objectives:

Note: The COVID pandemic forced closure of our laboratories during the first three months of the grant and has continued to constrain staffing, travel, and education and outreach efforts. Record-breaking wildfires in the region during 2020 prevented fieldwork and trapping studies during periods when WTB are normally most active (smoke inhibits them), delaying experiments until 2021. Supply chain issues in repellent availability and other materials also caused unanticipated delays. The death of Dr. Seybold and consequent loss of his expertise, research facilities, collegiality and inspiration created a huge void. COVID-related challenges also impacted personnel, resulting in staffing changes and revisions to the project scope. Changes in the following sections are incorporated, with further explanation in the Results and discussion section.

Four objectives underscore the overall goal of developing effective management tools for TCD.  Objective 1: Identify volatile organic compounds (VOCs) that attract WTB and larger bio-organic compounds that serve as feeding deterrents or stimulants.  This thrust extends ongoing work, with anticipated completion in year 1 of this project.  Within this objective, we also will continue to field test a repellent mixture for disrupting WTB aggregations and protecting trees from attack in commercial orchards (to be conducted years 1-2).  Our current lure, MBO, for monitoring WTB populations works well. Objective 2:  Assess influence of crown gall on WTB attraction, and determine if tumor-emergent WTB can carry and mediate transmission of crown gall bacteria (years 1-2).  Objective 3: Develop a tree risk-rating system based on association between TCD incidence and occurrence of crown gall diseases (years 1-2).  Objective 4: Conduct education and outreach with input from our producer cooperators and UCCE Orchard Systems farm advisors to the extent possible within restrictions imposed by pandemic, health department and university policy.  The research has largely been conducted in the Bostock lab (UCD), with support from members of the Kluepfel (USDA ARS) lab. Dr. Fichtner serves as Extension/Outreach Representative.  Researchers supported by the project include Megan Siefker (June 2020-present), Jason Simmons (April 2020-March 2021), and Dr. Corwin Parker (January-December, 2021).  

Timeline:

Field testing of the lure and repellent in our producer cooperator orchards is central to the goals of the project (Objective 1). Drs. Seybold and Audley, with assistance of Dr. Bostock, will oversee this work conducted over years 1 and 2 during the indicated periods. Completion of trials and associated data analysis will be a major milestone. Within this objective, we will complete ongoing analyses of VOCs and extractives from walnut bark and fungal tissue during Apr-Nov of year 1, under the direction of Drs. Bostock, Seybold and Simmons. Although not critical for successfully testing the field repellent, the results will be informative for Objective 2, and may identify compounds to enhance the MBO lure, which would be an important milestone. Objective 2, the impact of bacterial crown and stem diseases on bark chemistry that influence WTB behavior, will be directed by Drs. Bostock, Kluepfel and Simmons, with input from Dr. Seybold (years 1&2). Identifying a chemical “signature” of trees predisposed to attack by WTB would be an important milestone. We will also determine if tumor-emergent WTB carry crown gall bacteria, an important milestone and scientific discovery if such is the case (directed by Kluepfel, Bostock and Simmons). Objective 3 (led by Dr. Seybold) concerns developing an orchard risk-rating system informed by orchard surveys during summer and winter in both project years. Validating association between other stress factors and TCD will be an important milestone. Objective 4 is the Education/Outreach component of the project, under principal direction by Dr. Fichtner, with support by Drs. Bostock, Seybold and Kluepfel. This program will be ongoing throughout the two-year project, and is described elsewhere in the proposal. A Gantt Chart timeline, with additional detail and interim and end-of-project milestone indicators, and logic model are attached.

Gantt Chart - Bostock

LogicModel-Bostock WSARE

Cooperators

Click linked name(s) to expand/collapse or show everyone's info
  • Al Albertson - Producer
  • Megan Siefker (Researcher)
  • Dr. Corwin Parker
  • Brent Barton - Producer
  • Hal Crain - Producer (Educator)
  • Dr. Elizabeth Fichtner
  • Stan Lester - Producer
  • Jerry Moore - Producer (Researcher)
  • Jason Simmons

Research

Hypothesis:

The central hypothesis of this research is that cost-effective deployment of the walnut twig beetle (WTB) repellent for mitigating thousand cankers disease (TCD) in commercial walnut orchards in California will require knowledge of the repellent's range of efficacy from a point source, understanding seasonal patterns in WTB flight behavior, and identification of trees that are most at risk of attack by WTB.

Materials and methods:

Objective 1 focuses on characterization of semiochemicals that attract WTB and field-testing of lure and repellent mixtures in commercial orchards. We are building upon work with the WTB aggregation pheromone, MBO, now commercialized and used widely for detecting invasive populations of WTB in North America and Europe (Seybold et al. 2013;  Seybold et al. 2015). However, this lure is only effective within a 10-15 meter radius of host trees. We will attempt to improve the lure by adding host- and/or fungal-produced components to elicit optimal attraction. VOCs will be collected with solid-phase microextraction (SPME) fibers or Twister® sorbent stir bars placed in proximity to the source tissue and then separated and identified by gas chromatography-mass spectrometry (GC-MS) (McCartney et al. 2018).  Source tissues will include bark of Juglans species that vary in attraction to WTB, cultures of G. morbida, and G. morbida-infected English walnut bark. VOC sampling in-field will be carried out in which PDMS-based SPME fibers are suspended in stainless steel tea strainers by branches of Juglans major and Juglans californica for 24 hours. VOCs will be thermally desorbed from SPME and Twister sorbents for analyses by GC-MS.  VOCs will also be extracted directly and identified from source tissues on a larger scale by using conventional biochemical methods and liquid chromatography (LC)-MS. 

Our laboratories have established methods to field-test improved lures and repellents for disrupting WTB aggregations and to protect trees (Audley et al. 2020b;  Homicz et al. 2021).  Flight responses are measured by trap catches to the standard MBO lure. We have field-tested known WTB repellents in various combinations (Audley et al. 2020a;  Audley et al. 2020c), which are evaluated for their potential to reduce flight response to MBO-baited traps (positive control) and then later as a potential behavioral disruptant to reduce colonization and damage to baited trees (Seybold et al. 2013). Based on trapping results, commercial availability, and cost of the semiochemicals tested, we conclude that a two-compound repellent consisting of R-(+)-limonene and trans-conophthorin at defined release rates may constitute an effective tool for protecting trees from colonization by WTB (Audley et al. 2020a;  Blood et al. 2018). We are evaluating this repellent mix at the USDA’s Juglans National Clonal Germplasm Repository in Winters, CA, and the Plant Pathology Armstrong Research Farm at UC Davis, using the current MBO lure as attractant. 

Note: The following procedure as originally proposed was simplified to accommodate limited staffing and a restricted experimental window. The final report will document the new procedure.  To test the repellent under orchard conditions, trees assigned to the positive control treatment group will receive a single 15 ml plastic bottle of the MBO lure (Chen and Seybold 2014). Trees assigned to the repellent treatment group will receive the MBO lure plus the repellent. Negative control trees will not receive any semiochemicals. All semiochemical release devices will be placed on the trunk at 2 m off the ground on the north-facing side of each tree. To assess WTB landing rate, two clear acetate sticky sheets will be pinned to corks glued to the surface of each tree. One sheet will be randomly assigned to a cardinal direction and placed on a branch at 2 m off the ground. The second sheet will be placed on the opposing side of the tree on a branch at 4 m off the ground. Secondarily, a single 3 m tall pole will be placed 5 m away from each tree, randomly assigned to either the east or west side of the tree. A single sticky trap sheet will be attached to this pole, and an MBO bubble cap lure (1.2 mg/d release rate) (Seybold et al. 2013) will be pinned beneath each sheet. Effective radius of the repellent will be determined by placing additional sticky sheet landing traps on trunks of each neighboring tree within the row and each tree immediately across from the treated trees in both adjoining rows. WTB landing rates on these neighboring trees will be compared among the three treatment groups.

Wherever possible, a completely randomized design will be used, but if WTB population density is not uniform in an orchard (determined through funnel trap monitoring), a blocked design will be used for assigning trees to treatments. Trials will be conducted during late spring-early summer (April – June) and late summer-early fall (August – October), when WTB are active in flight (Chen and Seybold 2014).

Objective 2 determines if infection by the bacterial pathogen, Agrobacterium tumefaciens (crown gall) increases host apparency and susceptibility to WTB attack.  Our field observations indicate attack by WTB and severity of TCD is often higher in orchards with a history of crown gall (Seybold et al. 2016).  Crown galls and/or the associated bacterial pathogen appear to have the capacity to provide a substrate for enhanced development of new WTB adults.  We will sample a mature orchard with a history of crown gall disease. The potential association of WTB with A. tumefaciens will be examined by processing tumor-emergent WTBs for the presence of A. tumefaciens with both an A. tumefaciens selective isolation medium and species-specific PCR (Yakabe et al. 2012).  These data will facilitate development of informed management decisions to sustain long-term orchard health, and contribute to the basis for a tree risk-rating system.

Objective 3. Previous surveys of 'Chandler' walnut trees have revealed a strong interaction between the number of WTB entrance/emergence holes (= holes) through the outer bark on Paradox rootstock and main stems and incidence and severity of crown gall disease of the rootstock (Seybold et al. 2016). These holes are a sign of attempted and completed reproduction by WTB. Thus, there is risk-rating potential for this disease to serve as an indicator for predisposition of trees to attack by WTB and a subsequent long-term decline of tree health.

Due to travel restrictions and staffing limitations, as well as a reassessment of useful parameters to include in a risk rating scheme, we revised our orchard survey plan. In 2021 we surveyed a local commercial English walnut orchard (grower J.H Meek & Sons, Inc., Woodland, CA) with a history of crown gall and TCD. We assessed incidence/severity of crown gall disease (measured as stem gall circumference), TCD symptoms as expressed as staining of branches, scion trunk, and rootstock, and number of WTB entry/exit holes. 

Research results and discussion:

The following describes progress during the first year of the project, including efforts into the first quarter of year two, referencing where appropriate the changes in the original project plan due to the challenges mentioned in the revised objectives section.

  • Publication of studies relevant to project. Four papers were published in peer-reviewed journals, appearing during the first year (2020) of the project. The studies reported in these papers, supported mostly from prior funding, are central to the continuing studies on the repellent and the main theme for this project. A fifth paper, a new isothermal PCR assay we developed for in situ detection of Geosmithia morbida in TCD-afflicted walnut trees, was submitted to Plant Disease in late 2020. The paper is being revised to add additional experimental results and will be resubmitted soon. The following papers were published:

Audley JP, Bostock RM, Seybold SJ. 2020. Trap assays of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Curculionidae: Scolytinae), reveal an effective semiochemical repellent combination. J. Chem. Ecol. 46:1047-1058.

Audley JP, Homicz CS, Bostock RM, Seybold SJ. 2020. A study of landing behaviour by the walnut twig beetle, Pityophthorus juglandis, among host and nonhost hardwood trees in a northern California riparian forest. Agric. For. Entomol. 22:338-348.

Audley JP, Dallara PL, Nelson LJ, Hamud SM, Bostock RM, Seybold SJ. 2020. Trapping failure leads to discovery of potent semiochemical repellent for the walnut twig beetle. J. Econ. Entomol. 113:2772-2784.

Homicz CS, Audley JP, Chen Y, Bostock RM, Tauber CA, Seybold SJ. 2021. Walnut twig beetle landing rates differ between host and nonhost hardwood trees under the influence of aggregation pheromone in a northern California riparian forest. Agric. For. Entomol. 23:111-120.

Accepted with revision: 

Simmons JD, Yaghmour MA, Seybold SJ, Bostock RM.  Development of a recombinase polymerase amplification assay with qualitative end-point detection for Geosmithia morbida, the causal agent of thousand cankers disease in walnut. Plant Dis. (in revision)

In addition, Jason Simmons completed the M.S. degree in Plant Pathology on 31 March 2021 with thesis of the same title as the above manuscript.

  • Trapping of WTB. We processed a backlog of trap catches from walnut twig beetle (WTB) flight monitoring projects that took place in walnut orchards at Wolfskill Experimental Orchards in Winters, CA and the Armstrong Plant Pathology Research Field in Davis, CA between 2013 and 2019. These samples were collected from 4-unit Lindgren funnel traps baited with the male aggregation pheromone of the WTB. Four traps from each site were emptied weekly and saved in labeled ziplock bags. Trap catches were sorted under a dissecting scope and counts of male and female WTB as well as other associated insects were recorded. While many of these samples were processed immediately after collection, a large number of them were placed in cold storage and recovered in early 2020 after the passing of Dr. Seybold. Processing these samples was important because data from these long-running experiments identify peak flight trends, enabling us to maximize WTB trapping numbers at experimental sites for repellent trials. Figure 1 captures annual trends at the two sites for the 2015 to 2019 trapping seasons, the most complete trapping datasets.

It is apparent from the graphs that there is variation between years, between sites, and among traps within sites, but there are general trends. There is a peak flight period beginning late May/early June that often extends into July. There can also be another period of flight activity later in the summer, beginning in August and extending through late October. In some years, there was another upsurge in WTB activity in early spring (March-April) at the Wolfskill site. We are examining archived weather data at these two sites to determine if there is a relationship between WTB flight activity and temperature and possibly other factors. 

Figure 1. Left panel, WTB trapping numbers at the Armstrong Research Farm. Right panel, WTB trapping numbers at the Wolfskill Experimental Orchards. Years are indicated.

 

  • Objective 1. Walnut bark chemistry and repellent field tests. COVID restrictions and regional wildfires in 2020 (WTB flights are inhibited by smoke) prevented field assessments of repellent efficacy until 2021. Two sites, one at the Wolfskill Experimental Orchards in Winters, CA, and another at the Armstrong Plant Pathology Research Farm, were sampled weekly for 7 weeks, from 9/1/2021 until 10/19/2021. Although a more detailed analysis of the results will be presented in our final report in 2022, at this time we can say that the repellent cocktail of R-(+)-limonene and trans-conophthorin has an effective distance of about 2 meters from the point source, but not beyond that. This is consistent with our view that in order to be cost-effective, any deployment of the repellent in orchards will likely involve targeting single-trees identified as high risk (see Objective 3) during periods of peak WTB flights.

Although several hundred bark samples, including samples from both G. morbida-inoculated and non-inoculated trees, were obtained from Juglans major, Juglans californica, and ‘Paradox’ hybrid rootstock (Juglans regia X Juglans hindsii), and extracted according to the protocols described in the materials and methods, only a small number of these were analyzed and partially interpreted. The loss from the project of the graduate student researcher responsible for this objective due to health issues in March 2021, as well as closure of co-PI Seybold’s analytical lab facility following his death, has precluded further progress on this aspect. Thus, we were unable to continue the portion of objective 1 concerning walnut bark chemistry and the search for tree host and fungal semiochemicals that influence WTB behavior.

  • Objective 2: Assess influence of walnut bacterial diseases – crown gall and shallow bark canker – on bark chemistry and WTB attraction, and determine if tumor-emergent WTB can mediate transmission of crown gall bacteria (years 1-2).  We are unable to pursue the bark chemistry/WTB attraction studies for the reasons indicated above. However, we are actively investigating tumor-emergent WTB for the presence of crown gall bacteria with support from the Kluepfel laboratory.  Initial results are promising, with evidence that WTB can carry Agrobacterium. Work is underway to verify isolated bacteria are tumorigenic Agrobacterium tumefaciens.  This work will continue during year 2 of the project.
  • Objective 3: Develop an orchard risk-rating system based on association between TCD incidence and occurrence of root, crown and stem diseases and other site-related stresses (years 1-2). We are focusing on the association between crown gall and TCD because anecdotal observations over the years in various orchards suggest that trees severely affected by crown gall are often affected by TCD. Data from three commercial orchards are being analyzed, but the data from one of these orchards (Meek’s orchard) are most robust and compelling. This is an older commercial orchard with a very high incidence of crown gall and significant incidence of TCD. However, the orchard continues to be commercially viable and the grower has kept it in production. Data were collected from over 200 trees selected at random during spring and summer of 2021. For each tree, we determined diameter at breast high (DBH), and estimated crown gall severity based on the percent of the circumference of the tree base galled, bark staining as a TCD symptom, and WTB strikes. The TCD/WTB severity assessments were parsed for rootstock, scion (main trunk), and visible branch symptoms. ANOVAs on crown gall vs. TCD/WTB severity were performed and the crown gall percent vs. TCD/WTB severity data are summarized in Figures 2 A-E.

There are highly significant associations between crown gall severity and scion and branch stains (TCD symptoms) as well as between crown gall severity and WTB strikes in scions and rootstocks. The only comparison that did not have a high significance was rootstock TCD stains, which was generally very low across the board. Factorial ANOVAs using crown gall percent*tree DBH indicated that DBH was not a factor.

Figure 2. Box plots of crown gall severity (percent of base) vs. TCD symptoms (stains) or WTB strikes in a commercial orchard. A) crown gall vs. rootstock stains; B) crown gall vs. scion stains; C) crown gall vs. rootstock WTB strikes; D) crown gall vs. scion WTB strikes; E) crown gall vs. branch stains. See text for further explanation.

 

Key research findings summary

  • There is a peak flight period beginning late May/early June that often extends into July. There can also be another period of flight activity later in the summer, beginning in August and extending through late October.
  • The repellent mixture has an effective distance of about 2 m from a point source, making single-tree deployment the most likely format if it is to be used successfully in a commercial orchard.
  • Initial results indicate that trees with crown gall disease are at high risk of WTB attack and TCD.
  • The WTB can emerge from crown galls and may carry crown gall bacteria. It is unknown whether crown gall-contaminated WTB can transmit the bacteria to cause crown gall.

References

Anonymous. 2019. R Core Team,  R: A language and environment for statistical computing. https://R-project.org/. Vienna, Austria. .

Audley, J. P., Bostock, R. M., and Seybold, S. J. 2020a. Trap assays of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Curculionidae: Scolytinae), reveal an effective semiochemical repellent combination. J. Chem. Ecol. 46:1047-1058.

Audley, J. P., Homicz, C. S., Bostock, R. M., and Seybold, S. J. 2020b. A study of landing behaviour by the walnut twig beetle, Pityophthorus juglandis, among host and nonhost hardwood trees in a northern California riparian forest. Agric. For. Entomol. 22:338-348.

Audley, J. P., Dallara, P. L., Nelson, L. J., Hamud, S. M., Bostock, R. M., and Seybold, S. J. 2020c. Trapping failure leads to discovery of potent semiochemical repellent for the walnut twig beetle. J. Econ. Entomol. 113:2772-2784.

Blood, B. L., Klingeman, W. E., Paschen, M. A., Hadziabdic, E., Couture, J. J., and Ginzel, M. D. 2018. Behavioral responses of Pityophthorus juglandis (Coleoptera: Curculionidae: Scolytinae) to volatiles of black walnut and Geosmithia morbida (Ascomycota: Hypocreales: Bionectriaceae), the causal agent of thousand cankers disease. Environ. Entomol. 47:412-421.

Chen, Y. G., and Seybold, S. J. 2014. Crepuscular flight activity of an invasive insect governed by interacting abiotic factors. PLoS One 9:15.

Homicz, C. S., Audley, J. P., Chen, Y., Bostock, R. M., Tauber, C. A., and Seybold, S. J. 2021. Walnut twig beetle landing rates differ between host and nonhost hardwood trees under the influence of aggregation pheromone in a northern California riparian forest. Agric. For. Entomol. 23:111-120.

McCartney, M. M., Roubtsova, T. V., Yamaguchi, M. S., Kasuga, T., Ebeler, S. E., Davis, C. E., and Bostock, R. M. 2018. Effects of Phytophthora ramorum on volatile organic compound emissions of Rhododendron using gas chromatography–mass spectrometry. Analytical and Bioanalytical Chemistry 410:1475-1487.

Seybold, S. J., Dallara, P. L., Hishinuma, S. M., and Flint, M. L. 2013. Detecting and identifying the walnut twig beetle: Monitoring guidelines for the invasive vector of thousand cankers disease of walnut. in: University of California Agriculture and Natural Resources, Statewide Integrated Pest Management Program, Oakland, California.

Seybold, S. J., Dallara, P. L., Nelson, L. J., Graves, A. D., Hishinuma, S. M., and Gries, R. 2015. Methods of monitoring and controlling the walnut twig beetle, Pityophthorus juglandis. United States Patent Publication Number 2013/0014428A1. United States Patent and Trademark Office. United States.

Seybold, S. J., Fichtner, E. J., Lampinen, B. D., Leslie, C. A., Hasey, J. K., Y., C., and Bostock, R. M. 2016. Impact of walnut twig beetle on English walnut health, productivity, and management: A synthesis of biotic and abiotic methods of assessment. .

Yakabe, L. E., Maccree, M. M., Sudarshana, P., McClean, A. E., Parker, S. R., Wechter, W. P., Presting, G., Marutani-Hert, M., and Kluepfel, D. A. 2012. Novel PCR primers for detection of genetically diverse virulent Agrobacterium tumefaciens biovar 1 strains. Journal of General Plant Pathology 78:121-126.

Participation Summary
3 Farmers participating in research

Education

Educational approach:

Due to COVID restrictions and other staffing issues, we were unable to conduct educational program for growers.

Educational & Outreach Activities

4 Journal articles
1 Other educational activities: Lead instructor (Bostock), Remote teaching of PLP120 Introductory Plant Pathology in spring quarter, 2020 to 33 UC Davis students (TCD and other tree diseases are discussed)

Participation Summary:

Education/outreach description:

A set of PowerPoint modules with voice over containing updated information on thousand cankers disease tailored for English walnut growers is in preparation and will be completed by the end of the project.

Learning Outcomes

Key areas taught:
  • Teaching not conducted

Project Outcomes

1 New working collaboration
Project outcomes:

The principal outcomes of the project thus far are in research with the following key findings:

  • There is a peak flight period beginning late May/early June that often extends into July. There can also be another period of flight activity later in the summer, beginning in August and extending through late October.
  • The repellent mixture has an effective distance of about 2 m from a point source, making single-tree deployment the most likely format if it is to be used successfully in a commercial orchard.
  • Initial results indicate that trees with crown gall disease are at high risk of WTB attack and TCD.
  • The WTB can emerge from crown galls and may carry crown gall bacteria. It is unknown whether crown gall-contaminated WTB can transmit the bacteria to cause crown gall.

These findings will inform how the repellent could be deployed in commercial English walnut orchards to reduce risk of TCD and thereby extend longevity and sustainability of orchards.

Unfortunately, the ongoing COVID pandemic and its unprecedented impacts on personnel and field, education and outreach activities, together with the loss of Dr. Seybold, created insurmountable challenges to the project as originally conceived. We are requesting a no-cost extension to complete some activities in order to partially address gaps in project performance, but the final outcome will be incomplete and disappointing with respect to what we originally planned.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.