walnut blight Luke Milliron
Walnut blight disease can cause significant economic loss.

New weapon in fight against walnut blight

Walnut blight can have a serious economic impact for growers if it is not controlled.

Walnut growers have a new tool to help manage blight disease in their orchards — Kasumin 2L, manufactured by Arysta LifeScience, is the trade name for kasugamycin, and is available as part of a strategy to control the disease.

The new bactericide was discussed at a recent University of California Cooperative Extension breakfast meeting at Yuba City. “It’s great to have another chemistry in the rotational loop for blight management in walnuts,” says Emily Symmes, UCCE integrated pest management advisor.

Walnut blight can have a serious economic impact for growers if it is not controlled. The pathogen overwinters in dormant buds, and at bud break in the spring, the bacteria can infect buds, flowers, and developing nuts. If inoculum levels are high and there is a warm, wet spring, the disease can take off.

It is most prevalent in the Sacramento Valley, Symmes says. “It may be in the mid- to southern Central Valley, but it depends on spring conditions.” Development of walnut blight involves what she calls the “disease triangle. There are three components to the disease triangle that must all be present for it to occur. They include inoculum, susceptible plant tissue, and environmental conditions.”

Rainfall causes the pathogen to spread, she notes. “As things dry, we’re in better shape.”

In terms of controlling the disease, “Management of walnut blight depends almost exclusively on the use of protective chemical treatments.” 

USE IN ROTATION

Over the years, growers have used copper and mancozeb to control blight. “Prior to the registration of kasugamycin this year, we really relied on just those two materials.” Kasumin should be used in rotation with the other crop protection chemicals, Symmes says. “It’s not really a standalone. We now have the three materials — mancozeb, copper, and Kasumin — and you can create rotations as you go through the season.”

She points out that Kasumin is supposed to be tank-mixed. “And there are also quite a few label restrictions in California for Kasumin. It’s going to be a maximum of two applications per year for now. The registrant is working to potentially get that up to four applications per year. It’s good news for the industry in terms of blight management.”

Other restrictions include a pre-harvest interval of 100 days. Proper application timing and coverage are important for efficacy of control, Symmes notes. Additionally, “A critical note is that we must manage resistance development in the bacterial population by rotating chemistries as much as possible—it really wasn’t an option prior to the third material being registered,” she added. “It’s taken over a decade to get Kasumin registered because it’s considered an antibiotic. So manage your resistance so we can keep these tools as long as we possibly can.”

UNIQUE MODE OF ACTION

According to Arysta LifeScience, the active ingredient in Kasumin, kasugamycin, features a unique mode of action with no known cross-resistance to other bactericides. “It disrupts the function of the ribosomes, which shuts down the energy of the bacteria,” Layne Wade, Arysta LifeScience’s technical services manager, says.

He notes that Kasumin can already be applied up to four times per year in other states. “The California Department of Pesticide Regulation is reviewing the latest soil dissipation studies, and is expected to approve four applications in the future.”
Walnut blight became an issue for farmers many years ago, Wade says. “It has been a problem since the 1800s. A lot of walnuts started in the Anaheim area of Southern California, and this bacteria became a problem right from the beginning.”

Walnut blight can cause significant economic losses, and “can be very devastating if left uncontrolled,” says Luke Milliron, UCCE orchard systems advisor in Butte, Glenn and Tehama counties. “There have been UC studies showing that it can wipe out half the crop.” Early infections result in nut drop in June and July. As to susceptibility, he notes that “early-leafing varieties are going to be at a much higher risk.” he said.

Milliron credits Plant Pathologist Jim Adaskaveg at UC Riverside for his research on controlling walnut blight, noting that Adaskaveg found the copper-mancozeb, kasugamycin-mancozeb, and kasugamycin-copper rotation to be effective.

‘NOT A SILVER BULLET’

Kasumin offers hope to growers who include the newly-registered bactericide in their chemical rotations, Milliron says. “Folks are excited about Kasumin — but it’s not silver bullet. The greatest benefit of Kasumin is that it will hopefully preserve the efficacy of mancozeb for years.”

Jim Pengray, pest control advisor at Colusa County Farm Supply, says “we have varieties that are very vulnerable to blight. Vina and Ashley are susceptible varieties, and Tulare is moderately vulnerable. Regardless of which variety it is, walnut blight rears its ugly head each year.”

Pengray has a client with an orchard in Solano County that was planted to Vina in 1986, and walnut blight has been severe. Different combinations of materials have been tested over the years, including copper, mancozeb, Serenade, and Regalia. He intends to try Kasumin on 400 acres of the Vina trees.

“We’re excited about Kasumin,” he says. “We hope it will be a big player against walnut blight.”

As for the future of Kasumin, Pengray says, “The jury is out on this product.”

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