Bot is a knotty problem in California walnut orchards

Bot is a knotty problem in California walnut orchards

California walnut growers are dealing with the fungal disease Botryosphaeria (Bot) dieback which is attacking some orchards. Bot has been around for years, but has slowly and persistently developed in walnuts, says Themis Michailides, University of California Cooperative Extension plant pathologist.    

Growers packed the room at a recent meeting in Yolo County, Calif. to learn how to control Botryosphaeria (‘Bot’) dieback, a yield-reducing fungal disease attacking some of the state’s walnut orchards.

The meeting held in Woodland was organized by the University of California Cooperative Extension (UCCE).

“Bot has been around for years, but has slowly and persistently developed in walnuts,” said Themis Michailides, University of California Cooperative Extension plant pathologist based at the Kearney Agricultural Research and Extension Center in Parlier.

“When growers realized they were losing yields, it was a point for the industry to do some research and try to control it.”

Bot in commercial walnut orchards is found from Glenn County in the north to Ventura County in the south, he says.

The pathogen also infects pistachios, almonds, and pecans, plus 35-plus native trees and shrubs, including sequoias and blackberries.

Michailides points out that the disease was not introduced by agricultural crops since it has been present in the native species.

“These fungi were in California before the crops were introduced,” he said.

Spores accumulate

The disease spores build up and spread throughout orchards, infecting the trees.

“They accumulate and accumulate,” Michailides said. “This is the condition we have in California right now.”

Bot is waterborne, airborne, wind-borne, and transmitted by insects.

As the disease develops, cankers with distinct margins form on the tree branches and spurs. The cankers are loaded with spores of Bot and-or Phomopsis.

“Phomopsis is a different disease from Bot but is very similar, so we lump them together when talking about symptoms and management,” said Katherine Pope, UCCE orchard systems farm advisor in Yolo, Solano, and Sacramento counties, who also spoke at the Bot meeting.

The branches, leaves, and fruit of infected walnut trees become blighted, in which the plant tissue dries up and dies. The fruit dries up, and when it drops off the peduncle is still attached to the husk. Shell staining and kernel decay occur.

Cankers in the spurs kill the buds. It resembles spur dieback caused by frost damage. Vegetative and flowering buds killed during the previous fall or winter do not emerge in the spring.

Tree damage caused by scale insects, pruning wounds, and other types of injuries can precipitate Bot infection. In addition, damage from walnut blight disease (Xanthomonas campestris pv. Juglandis) creates an entry point for Bot.

Other diseases, including walnut blight, display similar symptoms in walnut trees, Michailides says, which complicates disease identification.

First seen in piscahios

Michailides said Bot was first noticed in California pistachios in 1985. The disease also attacks young almond trees.

“In walnuts and pistachios, it will not kill the tree,” he said. “In almonds, it will kill the tree.”

 “It will take several years before we have good management of this disease, It’s not an easy disease to control,” said Michailides.

Pope estimated that a quarter of the walnut growers in Yolo County are having trouble with Bot.

“It seems to have hit the older Hartleys (variety) particularly hard,” she said.

In terms of recommendations, Pope said, “We are recommending both cultural controls and fungicide. Pruning out dead and dying wood decreases the inoculum load in the orchard that could infect neighboring tissue. Prune what can be seen now, and go in after harvest when leaves are off and diseased branches are easier to spot.”

Pope says pruning’s should ideally be removed from the orchard and burned; otherwise, they can spread the airborne stage of the disease when they get wet.

UC researchers are conducting studies on whether chipping wood in the orchard could be an effective alternative to burning.

“Control scale in the orchard,” Pope said. “Scale injuries provide an entry point for the disease. Spray to protect vulnerable green tissue early in the season.”

UC research is currently conducted on efficacy of different sprays and spray timing.

Pope says a number of sprays effective at controlling Bot in pistachio are also labeled for use in walnut, including Pristine (replaced by Merivon), Luna Experience, Luna Sensation, Fontelis, Quilt Xcel, Abound, Quadris Top, Bumper, Quash, and Inspire Super.”

Grower testimonials

Walnut growers at the meeting shared on how Bot is impacting their walnut groves.

“It’s a big concern,” said Stan Lester of Lester Farms in Yolo County. “It’s a huge challenge to try to control this disease.”

“In the Winters area, we see very sick orchards. I think it’s from a combination of things - the drought, a cold spell in mid-December, and the Botryosphaeria which has been building up.”

He added, “I think we’ve seen it in older orchards, predominantly Hartleys, where these trees are 25-40 years old and have other things going on with them like deep bark canker.”

“In walnuts, we’re not seeing it in the newer and younger varieties,” Lester explained. “I’ve also observed that the Paradox rootstock, because of its hybrid vigor, is stronger and more resilient to all this stuff going on.

Dan Hrdy, owner of Citrona Farms in Winters, said, “It’s scary. It sounds like it can occur from all natural processes, besides agricultural sources, and the disease is everywhere.”

Walnut grower Rory Dorrough of Fairfield added, “This year it (Bot) became an economic problem for us. Initially, I thought it was freeze damage, but this is reoccurring. I’m looking for control measures.”

Walnut grower Dave Scheuring of the Gold Oak Ranch in Yolo County said, “I think there is some risk of hysteria and we really need to develop and utilize the dormant bud assay for determining the disease before we go into wholesale spraying programs that may not be needed.”

Gale McGranahan, UC walnut breeder emerita, said, “What I don’t know is if there are differences in susceptibility among the varieties, or if we could breed for resistance to Bot. It appears to be a serious problem.”

Mike Martin of Martin Orchards in Winters says UC researchers could use more funding assistance from the walnut industry.

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In terms of future research plans, Michailides noted there needs to be an emphasis on latent infections of green fruit, postharvest disease development, and disease management.

He concluded, “We still have a lot of questions and I hope we will have more answers.”

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