Potential cure for citrus disease heartens farmers’ hopes

Cure for the citrus greening disease HLB could be within sight, the time line to deliver it to farmers is still uncertain

By Ching Lee
California Farm Bureau Federation

CALIFORNIA – After years of research, a potential cure for the deadly plant disease citrus greening may be on the horizon, though it will take some time to get a product to farmers.

The University of California, Riverside, announced last week its scientists have found a new treatment that shows promise for controlling citrus greening, also known as huanglongbing or HLB, which has severely damaged many trees in Florida and other citrus-growing regions.

Though the disease has so far not been found in California commercial citrus groves, it represents a threat that could bring significant harm to the Golden State’s $3 billion citrus business, if not for ongoing efforts to control infestations of the Asian citrus psyllid, the insect that spreads the disease.

The new treatment involves an antimicrobial peptide found in Australian finger limes, which can naturally tolerate the HLB bacteria, said UC Riverside geneticist Hailing Jin, who discovered the compound after a five-year search. Unlike the antibiotic sprays Florida growers have been using to fight the disease, the peptide can remain stable even when used in 130-degree heat, she noted. It is also easy to manufacture and safe for humans, she added.

The peptide, which effectively kills the bacterium that causes HLB, can be applied by injection or leaf spray—and only needs to be reapplied several times a year, making it cost-effective for growers, according to UC Riverside. It can also be developed into a vaccine-like spray that can induce young, healthy trees’ innate immunity to the bacterium.

In her experiments, Jin noted how leaves of infected trees appeared healthy again a few months after treatment, with the bacteria “drastically reduced.”

That the university has entered into a licensing agreement with Massachusetts-based Invaio Sciences to develop the product is important, said Victoria Hornbaker, director of the Citrus Pest and Disease Prevention Program at the California Department of Food and Agriculture.

“When you’ve got the interest of a commercial partner, that shows you that you’ve got something there,” she said.

Though Hornbaker called news of the new treatment “super exciting,” she and others familiar with the research cautioned that a product won’t be available overnight.

“What we’ve heard from our partners at the Citrus Research Board and others is we’ve still got a ways to go before this proves to be commercially viable,” said Casey Creamer, president and CEO of California Citrus Mutual, adding that more work needs to be done in the next year or two by researchers here and in Florida to determine how well the product works in commercial groves.

“With any innovative technology, we want to make sure a solution is good for the environment, safe for those who work in and around the groves, and is accessible to growers,” said Rayne Thompson, vice president of government relations and public policy for the Sunkist Growers citrus cooperative.

UC Riverside spokeswoman Jules Bernstein said Jin has already published a patent and is in the process of submitting her work for peer review.

As someone who’s seen up close the destruction HLB has done to Florida citrus orchards, Tulare County grower Rod Burkett, who also works as a pest control advisor, said he’s “tickled to death” about prospects for a new treatment for citrus greening. The disease causes infected trees to produce small, misshapen and bitter fruit; most infected trees eventually die within a few years. Because there is no cure, the one defense farmers have is to control the psyllid, with one of those methods being the use of pesticides, which Burkett said could become less effective over time.

“The problem is all the pesticides that are registered for the (psyllid) are already being used for other insects,” he said.

Since the Asian citrus psyllid was first detected in a San Diego County backyard citrus tree in 2008, the pest has spread to 29 of the state’s 58 counties, reaching as far north as Placer County. Sacramento County became the newest region to be added to the state’s quarantine zone, after the insect was found last year in a Sacramento neighborhood.

Discovery of the state’s first HLB-infected tree occurred in 2012, at a home in the Los Angeles suburb of Hacienda Heights. HLB has since been found in four counties—Los Angeles, Orange, Riverside and San Bernardino—all in residential properties.

Combating the pest and the disease has taken a “multipronged approach” that includes millions of dollars spent on research, monitoring, establishing quarantines and procedures to control spread of the psyllid, and public education and communication, Creamer said.

“I don’t believe there’s any stone that people aren’t turning over and looking at,” he said. “You’d like it to be faster and be less costly, but when you step back and look at the program and what we’ve done in California, I would say it’s been a huge success—but we still have a long way to go.”

Hornbaker agreed, citing the lack of confirmed HLB cases in a commercial grove 12 years after the first psyllid infestation as a testament to the work being done by farmers, CDFA staff and partners, plus homeowners whose cooperation and support have been key to psyllid and HLB detection and removal of infected trees.

Detection is done mostly by trained staff who set traps for the pest and do visual surveys to look for both the psyllid and the disease. Most trapping occurs in areas where the psyllid is not expected to be found, so that when it is detected, the state can react quickly and knock down the population to achieve local eradication, Hornbaker said. The state has also been looking to sniffer dogs, which are being trained to aid in detection of both the psyllid and the disease.

Though pesticide treatment has been a key method for reducing psyllid populations, the state has also relied on biological controls such as a parasitic wasp, Tamarixia radiata, which is being raised in two facilities in Southern California. CDFA deployed more than 4 million parasitoids last year, Hornbaker said, focusing on new areas where the psyllid is found, along transportation corridors and around HLB detection areas.

Though the new UC Riverside treatment could turn out to be the silver bullet farmers have been waiting for, she said, the goal of the state program has always been to expand the toolbox to provide more options for fighting the psyllid and the disease.

“We’re doing what we’re doing right now to give science a chance to come up with a cure,” Hornbaker said.

Ching Lee is an assistant editor of Ag Alert. She may be contacted at [email protected].

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