Walnut safety and canker disease

Walnuts, an $825 million crop in California, have been free of food safety scares to this point and need to be kept that way. And the walnut industry needs to do its part to keep its reputation strong by guarding against the spread of a new disease that is taking its toll on black walnut trees.

Separate speakers made those points during the 41st Tri-County Walnut Day in Visalia, Calif., which drew more than 200 to listen to talks on irrigation management, stress factors, insect pests and new walnut varieties and rootstocks.

Dennis Balint, CEO of the California Walnut Commission, said growers and packers should expect increased oversight by the federal government on food safety issues and traceability.

“We can expect intense scrutiny,” he said. “There have been surprise [Food and Drug Administration] inspections up and down California.”

Because of adverse impacts from any food safety scare, Balint said, the industry has “no choice” but to maintain good management practices and ensure food safety “from farm to family.”

Balint said one threat to the industry’s reputation could be posed by any grower who is selling uninspected walnuts directly “to export agents.” He said such actions defy the commission’s marketing order and its definition of a “handler” and could bring fines of $1,100 per day.

“This poses a potential problem of infestation and a threat to food safety,” he said, recounting a shipment to Istanbul of unpackaged product. “There are liability issues.” Moreover, the person selling 500 pounds of inshell walnuts or 200 pounds of shelled walnuts outside marketing rules may be skirting commission assessments.

Balint said California, which produces all the nation’s English walnuts, has had back-to-back banner years with close to 435,000 tons in 2008 and a record 436,000 tons in 2009.

The new disease that is posing a problem mostly for black walnut trees throughout the Western states is called the “thousand cankers disease.” Elizabeth Fichtner, a University of California Cooperative Extension farm advisor on orchard crops in Tulare County, described the symptoms growers should look for and warned them to take care with rootstocks.

Many of the state’s English walnut varieties are on Northern California black walnut rootstock. It’s not known how susceptible those rootstocks are to the disease, but Fichtner warned the industry to keep an eye on wood from rootstocks that might end up in areas vulnerable to infection.

“Don’t ship infected rootstocks,” she said, showing a map that shows a large area of the native range of back walnuts, mostly in the Eastern United States. She pointed out there is an established black walnut industry that includes sales to retail, commercial baking and making of candy and ice cream.

California counties where there have been confirmed finds of thousand cankers disease include Alameda, Los Angeles, San Benito, San Joaquin, San Luis Obispo Solano, Sutter, Tulare and Yolo.

It was first found in Yolo County in June 2008. There are now only a few infected English walnut trees, but that number is expected to rise.

Fichtner said one sign of the disease is small holes in the cankers, from which the walnut twig beetle, which carries fungal spores that cause the problem, can emerge. She said growers should take special care when checking for the disease and be aware that symptoms look similar to shallow bark canker, which is not a significant threat in orchards.

“This beetle is only attacking weak English walnut trees, so maintaining orchard health is the best defense,” Fichtner said.

That notion – maintaining orchard health – was woven through the Visalia program as speakers looked at other pests and management practices.

Marshall Johnson, an entomology specialist with the UC Kearney Agricultural Center in Parlier, talked of pests that included the Pacific flat-headed borer, walnut husk fly, codling moth and the 10-line June beetle.

While the first two pests do direct damage to the nut, Johnson said, “the other two can take out whole orchards.”

Johnson said biological controls are not effective in combating the codling moth, but there are insecticides that can be used along with pheromone traps for monitoring. Effective products include Altacor and acetamiprid.

He said the walnut husk fly also can be controlled with products that include Assail and Provado.

The Pacific flat-headed borer is a beetle that attacks weakened or stressed trees, Johnson said, adding that is one more reason to maintain orchard health. The beetles pupate under the tree bark. One effective control, he said, is painting trees with water-based latex paint. It is also best to prune out and destroy affected branches.

Johnson said he is conducting a study funded by the California Tree Fruit Agreement in Reedley, which represents growers of peaches, plums and nectarines, on control for the borer.

The 10-line June beetle thrives in sandy soils, Johnson said. It attacks the roots of trees, and one sign of its presence is emergence holes in the soil at the base of a tree that are the diameter of a pencil.

Because the grub from the pest is as much as 5 feet underground, it’s difficult to get material to the beetle to destroy it. Research is under way on efforts to inject materials to control the pest.

David Goldhamer, UC soil and water specialist at Kearney, talked of hedgerow yields, saying they were higher in the first six years because water use was more efficient because surface evaporation was lower due to larger canopies.

Goldhamer warned that stressing walnut trees cuts yields, but he added that trees can recover full productivity within two years. His research simulated drought conditions and showed that in the year following deprivation yields were cut as much as 80 percent. “There were fewer nuts and less vegetative growth, but the walnut trees have the ability to come back.”

One way of cutting surface evaporation is to put drip irrigation under the soil, however, Goldhamer said that’s a challenge because of root intrusion into emitters and the amount of water saved may amount to only 10 percent.

He said a pressure chamber can be used to gauge water loss, but it cannot adequately characterize a field. “They can gauge a few trees, but they are expensive.”

Canopy temperature is another gauge. Goldhamer said thermal satellite imagery does not do that well, and the best bet is flyovers by unmanned drones, something that is not permitted in California.

Goldhamer’s conclusion: “There is not a good time to stress walnuts. If it must be done, post-harvest is likely best.”

Chuck Leslie, staff resource associate with the UC Davis walnut breeding program, cited three newer, recently released varieties: the Joseph Sexton (“it may be good for hedgerows but it’s not the most beautiful”), the Harold Forde (“it harvests close to Chandler and has good color”) and the Felix Gillet (“a large vigorous tree with an open canopy and large kernels”).

Just released in late January was the Ivanhoe, which Leslie said is “an uncle of the Chandler with good color.” But it’s susceptible to blight, he said. Just released, it may be in short supply.

Bob Beede, UC farm advisor for Kings County, talked of use of Etephon for hull removal and his research into its use with the Serr variety walnut. In short, he said, “It works better on other varieties.”

He also talked of use of ReTain for loss of pistillate flowers in walnuts, particularly in the Serr variety. Beede said it is important to use the product at 20 percent to 30 percent bloom, to provide good coverage and to do so “when there are a lot of flowers on the ground.”

“But it is not a cure-all for poor water management, low fertility, nematodes, excess shading and sustained stress.”

TAGS: Tree Nuts
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