Pest and disease management in walnut orchards is predicted to look different in the future.

A roadmap for sustainable pest management was developed by members of the Sustainable Pest Management Workgroup and Urban Subgroup in collaboration with the California Department of Pesticide Regulation (DPR), CDFA and CEPA. The roadmap defines sustainable pest management as a process of continual improvement that integrates an array of practices and products aimed at creating healthy, resilient ecosystems, farms, communities, homes and gardens. By 2050, the plan is intended to promote human health and safety, ecosystem resilience, agricultural sustainability and economic vitality.

When it comes to walnut blight control, national and international regulatory challenges are increasing with the use of the copper product mancozeb and antibiotics. UC Riverside Plant Pathologist Jim Adaskaveg told growers and crop consultants at the 2025 California Walnut Conference that increased dependency on biological products or the identification of new modes of action is inevitable.

“Current treatments are on a hit list by regulators,” Adaskaveg warned.

Blight management will remain a top priority in walnut production in a changing climate. Adaskaveg said that overuse of copper mancozeb has led to complete resistance in other plants. With an uncertain regulatory future for mancozeb in the EU and other countries, including the U.S., the EPA has proposed the cancellation of dimethyldithiocarbamates.

The goal for the walnut industry is to keep mancozeb registered for as long as possible so the pathogen remains sensitive in mixtures with other copper products (copper and kasugamycin). One solution is the use of dodine and kasugamycin in mixture rotations as effective alternatives with different modes of action that have high performance in favorable environments. Adaskaveg said another goal is to identify other synergistic compounds with different modes of action that can be used in mixture rotations.

He added mixture rotations and the development of resistant cultivars will be important tools for overcoming blight.

The development of a new decision support tool to improve navel orangeworm (NOW) model accuracy is another avenue for the future. Houston Wilson, assistant UCCE entomology specialist and conference speaker, said the next step in NOW management will include integration with automated traps, regional data interpretation, high-resolution weather data, tree phenology models and a grower coordination component.

Wilson highlighted the need for regional coordination for long-term management, noting NOW moves between orchards and the timing and extent of NOW movement remain unclear. Management in a single orchard can be undermined by colonization from nearby orchards. New tools, Wilson added, could provide better regional monitoring, risk prediction and coordinated adoption of best practices.

Regulatory challenges and other relevant issues walnut growers will face in the future were addressed by panelists Taylor Roschen, governmental advocate with the law firm Kahn, Soares and Conway; UCCE IPM advisor Jhalendra Rijal; and Ethan Mathews, director of government affairs for Wilbur-Ellis.

“Stay engaged, stay involved and speak up,” Roschen advised walnut growers about legislative proposals that affect their use of crop protection materials.

Roschen, who is also a member of the Sustainable Pest Management Workgroup, said there were some wins for California agriculture in 2024. An agricultural exemption is being sought for AB 2552, which would ban first-generation anticoagulant rodenticides. AB 1963 will require DPR to complete reevaluation by Jan. 1, 2029. A signing message noted that no more one-off pesticide bans or reevaluations will be allowed. A governor’s proposal for a mill increase to 33.9 was negotiated to lower the increase to 30 mills over four years.

Anticipated issues in the California legislature going forward include invasive species, debate regarding the role of the Office of Environmental Health Hazard Assessment in exposure limits, the precautionary principle (whether exposure limits are to be hazard-based or risk-based) and one-off pesticide bans.

Regulatory actions include pesticide use notifications with public notifications 24 hours in advance of restricted-use products and 48 hours prior to fumigant use. Roschen said a fumigant alternatives study could be released in the next few months and may lead to more regulatory actions.

Insect Pests Expected to Become More Problematic
Rijal said warmer temperatures, higher levels of carbon dioxide in the atmosphere and unpredictable precipitation are all going to affect insect populations in the future. Their range is expected to expand, and more generations may be produced. Trees stressed by drought conditions may also invite more pest damage.

“We will need to think in the long term for pest management,” Rijal said.

NOW is already producing more generations, particularly in southern growing regions. A new pest, the carpophilus beetle, has been found in walnuts in Stanislaus County. It overwinters in nuts on the ground. Rijal said this pest was detected in an older variety, Hartley, not in Chandler. Walnut growers will also need new cultural practices to manage codling moth. It is critical to hit the second generation with an insecticide application, Rijal said, but new ways to manage this pest are needed.

Stressed crops will attract more insect pests, including flatheaded borer, which has placed more pressure on young walnut trees over the past seven to eight years. Rijal said this pest kills young trees and orchard damage is increasing. Insecticide sprays are part of the solution, he noted, but they need to be integrated with other practices.

Awareness of these emerging pest issues is important, Rijal said. The public, policymakers, growers and researchers all need to work to develop new management strategies.

More MRLs Expected
Mathews said non-tariff barriers for walnuts, impeding market access, are certain.

“There will be more MRLs in the future, and there is no sign of them abating,” he said.

“There is often no scientific justification for these; they are protectionist,” he added, with the EU leading the way in new barriers.

To combat the harm imposed on the export market for walnuts due to MRL issues, Mathews said there is a need for a risk-based system.

He emphasized that agriculture is already using conservation measures in crop production and expressed hope that the incoming administration in Washington will understand agriculture’s needs.

Mathews said he is expecting some healthy debate on pest management issues between USDA and EPA.