Scientists lend growers a hand

Photo courtesy of the ARS.
California produces 99 percent of the commercial U.S. walnut supply and two-thirds of world supply.

Nutritionists are just scratching the surface of the many health benefits of diets rich in fruits, vegetables and nuts; the world’s oldest known tree food, the walnut, may be the biggest superstar of all. It was the first whole food to gain U.S. Food and Drug Administration approval of its health claim, and the nut’s omega-3 fatty acid content may help reduce the incidence of heart disease, cancer, stroke, obesity, clinical depression and other ailments.

Walnut history

Walnuts date back to 7000 B.C. and were considered a food of the gods in ancient Rome, thus the name Juglans regia for the god Jupiter. English walnuts, which comprise virtually all U.S. fresh consumption, are Persian nuts. The inaccurate reference to England has been traced to shipping on British vessels. The black walnut is native to this country, but its fruit isn’t commercially viable due to a hard shell.

Production

Virtually all walnuts for U.S. use are grown in California’s San Joaquin region. The state harvests 334,400 short tons annually on 215,100 acres. More than 30 English walnut varieties, all hybrids of Juglans regia, are grown, with Chandler, Hartley, Seer, Vina and Payne being the most commonly produced varieties.

Trees demand fertile, deep, well-drained, nonstratified, loamy soils. Roots can extend 10 feet. They come into nut production five to seven years after planting and may bear walnuts for up to 100 years. They are mechanically harvested in the fall.

Agricultural Research Service (ARS) and University of California-Davis scientists are involved in studying the problems associated with walnut production in order to breed stronger varieties. Much of that work occurs at the ARS’ National Clonal Germplasm Repository for Fruit and Nut Crops in Davis, Calif.

Genetic mapping at the repository

The repository’s research walnut orchard of 1,600 trees is the world’s largest and most genetically diverse walnut orchard. The center manages a smaller group of container trees in a greenhouse. Their unique collection enables scientists to uncover and help preserve the most valuable traits of the varieties.

Mallikarjuna K. Aradhya, walnut collection manager and geneticist, has sampled the trees for genetic material. He isolated unique DNA duplications called simple-sequence repeats to help identify varieties. The identifying markers enable not only repository staff to correctly identify trees, but also helps to prevent errors by growers and nurseries.

Last year, Aradhya launched a four-year study to refine the genetic markers, which will aid breeders in early selection of the best seedlings. Specifically, they will be able to zero in on those trees bearing highly desirable traits, such as kernel plumpness and optimal leafing time. Early selection will benefit growers due to the length of time required for the production of marketable walnuts.

Battling aflatoxin

Part of the walnut inspection process includes monitoring aflatoxin levels to ensure food safety. Aspergillus flavus and A. parasiticus are two fungi that infect walnuts and other crops. ARS scientists in the Western Regional Research Center’s Plant Mycotoxin Research Unit at Albany, Calif., found that the walnut variety Tulare is highly resistant to these fungi.

Photo by Peggy Greb, Courtesy of the ARS.
Chemist Russell Molyneux prepares walnut pellicle samples for analysis of gallic acid content.

Chemist Russell J. Molyneux discovered that a natural compound in walnut tannin, gallic acid, is released when attacked by Aspergillus and prevents it from producing aflatoxin. Tulare walnuts were shown to have far greater levels of gallic acid than other varieties. The trait can be bred into other walnut varieties, and the discovery may help scientists develop new ways to battle aflatoxin production in other crops.

Putting an end to deep bark canker

ARS scientists in Beltsville, Md., are developing ways to pinpoint deep bark canker in walnut trees. The problem is caused by the bacterium Brenneria rubrifaciens, which produces a red pigment. The disease affects the English walnut, causing vertical cankers that ooze bacterium-filled sap.

The cankers aren’t evident in young trees, but B. rubrifaciens is thought to be present long before there is visual evidence. To help control the disease in both nursery and orchard settings, scientists have developed a genetic detection method.

Crop improvement

Repository scientists often collaborate with University of California-Davis researchers. The 2008 improvement program applied research findings to breeding with 38 selections and 10,000 seedlings at or near the evaluation stage. Breeding for early harvest is a key interest. The program also works for high-yield, disease-resistant varieties. Breeders are developing a cultivar with blackline disease resistance and improving rootstock for the control of Phytophthora, nematodes and crown gall.

Photo by Scott Bauer, courtesy of the ARS.
Technician Kathy Reynolds, Entomologist Doug Light (center) and Field Development Manager Scott Lingren inspect a trap baited with pear ester for use in codling moth monitoring in a walnut orchard.

Walnut breeding began in 1948, with 10 varieties released in 1968. Virtually all the walnut varieties sold by California nurseries are UC-Davis cultivars. Chandler, patented in 1979, accounts for 90 percent of nursery sales. The current focus of the breeding program is on early harvest with high yields, large light-colored kernels and low blight scores. Sexton, Gillet and Forde, patented in 2006, all harvest before Chandler. A number of additional promising earlier-harvesting selections are now being propagated in nurseries and grower trials for establishing large-scale field trials next year and possible patenting and commercial release as new cultivars.

Production improvement and greater disease and pest resistance

In the area of production and postharvest, scientists have developed methods to clonally propagate superior walnut rootstocks using in vitro micropropagation, hardwood cutting and root grafting techniques. Two promising selections: RX1 (moderate resistance to Phytophthora citricola and P. cinnamomi) and VX211 (high vigor and nematode tolerance) have been patented and several nurseries are licensed to produce and sell them. Orchard trials with nursery grafted trees or liners of VX211, RX1 and other promising clonal rootstocks are being established in conjunction with farm advisers, nurseries and growers.

This year’s research includes identifying potential new sources of disease resistance in the repository collection, which can then be incorporated into rootstock breeding and selection. A new source of crown gall resistance has been discovered, which will be useful in developing resistant rootstocks. Other projects include screening for Phytophthora resistance and nematode resistance and tolerance. Scientists believe they have pinpointed the first known source of true root lesion nematode resistance among Juglans species.

UC-Davis is gaining an understanding of the role of canopy position and light exposure on quality-related problems. Shading- related pellicle darkening and kernel shrivel are associated with low light levels. However, there appears to be a link to water use as well that requires further investigation.

UC-Davis investigators also are successfully controlling codling moth. Aerosol pheromone puffers are demonstrating good results in trials over a wide area. Pheromone-mating disruption delivery via meso emitters also shows promise for smaller orchards.

More information is available online at www.walnutresearch.ucdavis.edu and www.ars.usda.gov/main/main.htm.

Based in Greensboro, N.C., the author writes articles about horticulture, landscaping, agriculture and travel. She has been a contributor to Moose River Media publications for three years.