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 [1, 2].  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.

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. However, we will continue to search for candidates emerging through our chemistry discovery pipeline that augment MBO potency (year 1) and then field-test any promising candidates in orchards (year 2).  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).  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).  Objective 4: Conduct a vigorous education and outreach program with input from our producer cooperators and UCCE Orchard Systems farm advisors.  This program will continue throughout the duration of the project.  The research plan distributes the work among the Bostock (UCD), Seybold (USDA FS), and Kluepfel (USDA ARS) laboratories, with input on planning and deployment from producer/advisor representative Moore.  Dr. Fichtner will serve as Extension/Outreach Representative. UCD researchers Simmons (Ph.D. degree expected September, 2020) and Audley (Ph.D. degree expected December, 2019) will continue as postdoctoral associates, emphasizing their respective expertise (Simmons – host and fungal chemistry, plant pathology; Audley – field-testing of lures and repellents, entomology). Additional detail is provided within the Materials and Methods, Educational/Outreach Activities, and Timeline sections.