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Tri-County Walnut Day, an annual event hosted by Tulare County UCCE, was held on a foggy February day but still attracted local walnut growers and crop advisors who braved the drive to hear timely presentations on walnut production.
Well known for tackling topics important to walnut growers, Walnut Day was organized by Leonel Jiminez, UCCE county director, and UCCE farm advisor Elizabeth Fichtner.
CEO of California Walnut Board and Commission Robert Verloop led the list of speakers with an update on the walnut industry. The high points included record December kernel shipments and rebounding sales in key markets Turkey, Middle East, Germany and Netherlands. Year to date versus 2024, inshell was up 45% and kernel up 12%.
Looking at the current California walnut picture, there are more than 3,700 growers farming 380,000 acres of walnuts. In 2025, more than 800,000 tons of walnuts were produced, approximately 200,000 tons more than the 2024 crop. Verloop noted that the California walnut orchard age profile presents a solid foundation going forward with not one single age range dominating.
There are 67 walnut handlers with 80% of the volume handled by 17 handlers. 90% of the volume is handled by 27 handlers. Verloop said he would like to see a consolidation there with handlers focusing on growing the market, not just pushing supply into the market.
Nematodes
Plant parasitic nematodes in orchard soils stunt growth of young trees and contribute to lower yields over the life of an orchard. Andreas Westphal, UCCE Riverside plant pathologist and nematologist, noted the challenges in maintaining tree health when root lesion nematodes are present in the soil.
Using resistant or tolerant walnut rootstocks can be part of the answer to improving crop efficiency. Westphal noted the difference between resistance trait, which deals with reproduction in the nematode population, and tolerance, which is the reaction of the plant to nematode. Nematode resistant and tolerant rootstocks as well as rootstocks with multiple resistances are available to growers.
Soil treatments are another avenue to fight nematode damage. Work is being done to identify preplant soil treatments including chemical nonfumigant materials and anaerobic soil disinfestation. Westphal noted the high costs per acre with ASD as the reason for alternative solutions. Soil treatments without chlorpicrin are possible and becoming cost effective. Registration for one promising biofumigant, Dominus, is being pursued.
Post plant applications are also helpful to maintain yields.
Cover Crops
Kamyar Aram, UCCE advisor, outlined some of the advantages of cover crop seed mixtures used in walnut orchards. Regardless of what type of cover crop grown, Aram said it is important it be planted where the tree roots grow to achieve full benefits. Wider bands of cover crops provide more soil benefits.
In walnuts, cover crops offer significant opportunities to enhance soil health, manage nitrogen, suppress weeds and improve water infiltration during winter. Cover crop options include brassicas for soil compaction, legumes for nitrogen fixation and grasses for carbon-rich biomass.
Winter active mixtures of grass and legumes are terminated prior to spring and can often be grown without supplemental irrigation.
Aram advised growers to research cover crop mixtures, start with a smaller planting and experiment with different types of cover crops. Keeping records of planting and termination dates is also recommended.
The website westerncovercrops.org is a good resource.
āThe amount of water applied is not the only factor that determines
tree stress.ā Ken Shackel
Maximize Harvest
Another useful tool for walnut growers to consider is ethephon. Clarissa Reyes, UCCE farm advisor, has been conducting research on use of ethephon in Solano and Chandler orchards to determine timing.
The plant growth regulator ethephon (Ethrel) is used to speed up hull split and separation for an earlier harvest, removing nuts with a single shake. It is typically applied when nuts have reached packing tissue brown.
Proper timing is crucial to maximize the benefits of improved kernel quality without causing premature leaf drop, which can impact the next year’s crop.
Reyesā initial experiments with Solano and Chandler show the earliest times of harvest at certain ethephon concentrations in that year.
In Solano, 80% of walnuts reached hull split stage 4 after 13 days with the 900 ppm treatment on Sept. 15 and after 19 days with the 300 ppm treatment.
In Chandler, 80% of walnuts reached hull split stage 4 after 14 days with 300 and 900 ppm treatment, with Reyes noting there was rain after the application. Trees not treated reached hull split stage 4 after 18 days.
Spring Irrigation
A USDA-NRCS funded study is testing a delayed irrigation approach.
āTrees may hold some surprises now that we are watching,ā UC plant scientist Ken Shackel said. This commercial scale implementation of plant-based irrigation is being done to determine just how much water walnut trees need to produce a crop.
The five year plan began in 2025 and is evaluating the delayed irrigation effect. Delay treatment plots do not begin irrigation until the SWP of the trees drop below the threshold of 2 bars below baseline. Yield and nut quality data showed in the delay plots, yield average was 2.11 tons per acre compared to 2.30 in the control plots. Percent edible was higher, but percent of extra light kernels was lower.
Shackelās conclusions at the end of the first year of the trial noted that many sites are showing large differences in watering regimes compared to ET. The amount of water applied is not the only factor that determines tree stress. Depending on the location and soil, he said that substantially delaying the first irrigation may be safe, but there are also examples of not starting soon enough.
Flatheaded Borer
The Pacific flatheaded borer is not an especially new pest in California walnuts, but tree damage caused by these wood-boring insects has become obvious in the past few years. Samaneh Sakaki, UCCE associate specialist in Stanislaus County, said young trees are particularly vulnerable to this wood-boring insect. In 2023, she said numerous branch samples taken from an orchard were infested.
The most common species in California walnuts is Chrysobothris mali. Females lay eggs in bark crevices and wounds. The larvae chew into bark and feed and tunnel in cambium. Mature larvae overwinter under the bark and pupate in the spring. Adults mostly emerge from May through July.
Symptoms of tree damage by PFB include brown sap in mature trees, visible wounds, feeding channels, dead or flagged branches and exit holes in branches.
Practical IPM options for PFB infestations include preventing sunburn on newly planted first year trees. Protecting trunks by painting with 50-50 white latex paint and water. Keeping mature trees vigorous and pruning out all infested wood and removing from the orchard.
Sakaki said a trial found that the triangular trap is most effective with yellow and red colors capturing the most PFB during the season. A combination of red and yellow triangular traps with a lure works best.
Sakaki said an economic survey is planned to assess cost of damage, tree loss and control measures. Data will be provided for extension support, policy planning and research funding.
Cecilia Parsons | Associate Editor
Cecilia Parsons has lived in the Central Valley community of Ducor since 1976, covering agriculture for numerous agricultural publications over the years. She has found and nurtured many wonderful and helpful contacts in the ag community, including the UCCE advisors, allowing for news coverage that focuses on the basics of food production.
She is always on the search for new ag topics that can help growers and processors in the San Joaquin Valley improve their bottom line.
In her free time, Cecilia rides her horse, Holly in ranch versatility shows and raises registered Shetland sheep which she exhibits at county and state fairs during the summer.
