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

View the project final report

Commodities

• Nuts: walnuts

Practices

• Education and Training: on-farm/ranch research
• Pest Management: disease vectors, integrated pest management

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 are only partially effective and 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 extended our studies to further evaluate an optimized lure and a repellent mixture, including a trial in a commercial orchard.  Common bacterial root, crown and stem diseases of walnut are often associated with TCD-affected trees. In orchard surveys, we determined the strength of the association between crown gall disease and severity of beetle attack and TCD incidence. This information clarifies anecdotal observations and provides clearer understanding of the association between crown gall as a potential predisposing stress and TCD that may guide grower deployment of WTB lures and repellents. The unforeseen challenges described elsewhere in our report severely impacted project execution throughout the performance period. Education and outreach to walnut producers, PCAs, and other stakeholders was limited to a webinar, publications, and available online resources. Nonetheless, our statewide network of UCCE orchard systems advisors who work with walnut growers remain engaged and will continue to provide latest research findings and recommendations on TCD to clientele.  The principal outcome of this project will be enhanced competitiveness through increasing sustainability and resilience of walnut orchards as indicated by demonstration of an effective 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.

Note: The COVID pandemic forced closure of our laboratories during the first three months of the grant and continued to constrain staffing, travel, and education and outreach efforts throughout. 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 field experiments until 2021. Supply chain issues in repellent availability and other materials also caused unanticipated delays. The death of Dr. Seybold prior to the start of the project and consequent loss of his expertise, research facilities, collegiality and energy created an insurmountable void throughout the project. 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, modified from the original plan, were pursued that underscore the overall goal of developing effective management tools for TCD. Objective 1: Field test a repellent mix for disrupting WTB aggregations and protecting trees from attack in experimental orchards and a commercial orchard. Objective 2: Determine if tumor-emergent WTB carry crown gall bacteria. Objective 3: Develop a tree risk-rating system based on association between TCD incidence and occurrence of crown gall disease. 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 the pandemic and within health department and university policy.  The research was largely conducted in the Bostock lab (UCD), with support from members of the Kluepfel lab (USDA ARS). Dr. Fichtner served as Extension/Outreach Representative. Although not formally listed as a cooperator, Dr. Kari Arnold, UCCE Area Orchard and Vineyard Systems Farm Advisor, Stanislaus County, contributed to outreach and education efforts.  Researchers supported by the grant include Megan Siefker (Lab Assistant, June 2020-September 2022), Jason Simmons (graduate student; April 2020-March 2021), and Dr. Corwin Parker (post-doctoral associate; January-December, 2021).