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Investment in walnut research paying dividends

Research to improve California walnut production goes back a long time, but continuing efforts have had significant impacts on crop production and profitability.

The month of January marked a milestone for the California walnut industry, as University of California researchers, farm advisors, and specialists met with the Production Research Committee of the California Walnut Board for the 50th anniversary Walnut Research Conference.

The annual meeting, organized by the UC Walnut Work Group, includes reports on current research projects funded by the CWB, and proposals for new and continuing research aimed at improving California walnut production.

Members of the committee evaluate the proposals, and with recommendations from UC, allocate funding. About $2 million in funding is available each year for research projects in genetic improvement, orchard management, pest management, and plant pathology.

Funding comes from assessments of walnut handlers. The assessment for 2017-18 and going forward is $0.0400 per pound of kernels of assessable walnuts.

Joe Grant, research director with CWB, says research to improve California walnut production goes back to the 1940s and ‘50s, but continuing efforts at the annual conference have had significant impacts on crop production and profitability. Collaboration between UC researchers and growers to solve challenges in walnut production have produced results that continue to benefit the industry.


Both Grant and former CWB research director Dave Ramos agree that UC research and development of improved walnut varieties have had a significant impact on California walnut production. Field trials and comparisons of walnut varieties play an important role in bringing new varieties to growers.

Ramos says the UC walnut breeding program has basically revolutionized the industry. Prior to the release of new varieties in the 1960s, growers were dealing with walnut varieties that were limited in production areas and had challenges with diseases and pests.

The release in 1968 of the Chandler variety was a huge benefit to growers, he says. Proof of the value of Chandler is in the total acreage now in production and its contribution of 50 percent of the annual walnut tonnage.

Improvements in rootstocks also have elevated production potential for walnut growers, Grant says. Historically, the industry was based on imported seedling trees, but development of Paradox rootstock by the renowned horticulturalist Luther Burbank added years to tree health and production. This hybrid of a native walnut and English walnut was resistant to nematodes and could withstand high soil moisture conditions.


UC research projects in walnuts were also some of the first to use biocontrol practices. The walnut aphid was an early pest, Grant says, with infestations leaving trees covered in black sooty mold. UC Riverside researchers were successful in finding and bringing in a parasitoid from Iran to control the walnut aphid. Unlike earlier biocontrol efforts, the parasitoid became established in the Central Valley and increasing populations brought the aphid population under control.

Understanding diseases and how they affect walnut production has also been a large part of the UC research over the last 50 years. Grant says walnut black line disease was a significant problem for growers, who saw their 15-year-old trees meet early deaths. UC Davis Plant Pathologist John Mircetich determined that the disease was caused by a graft-transmitted virus.

Studies in orchard management have also led to changes in tree spacing, as well as canopy management to improve walnut yields.

Recent research approved by the Production Research Committee includes genetic improvements. Gene analysis is helping researchers better understand which genes control which parts of production, including nut maturity, pellicle color, and tree structure.

New research is looking at genes responsible for resistance to soil pathogens, as well as identifying disease resistance genes for walnut diseases, including crown gall, Phytopthora, and root diseases — all with the goal of saving growers the costs of fumigants, fungicides, and pesticides.

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