HLB fight focuses on protecting commercial citrus


Issue Date: August 9, 2017
By Kevin Hecteman
Rich Colwell, a mandarin grower in Penryn, says he is worried about the arrival of the Asian citrus psyllid in Placer County last year. The insect, which can spread the deadly bacterium huanglongbing, was carried into the county by a homeowner who brought citrus trees from Orange County.
Photos/Kevin Hecteman
Rich Colwell, a mandarin grower in Penryn, says he is worried about the arrival of the Asian citrus psyllid in Placer County last year. The insect, which can spread the deadly bacterium huanglongbing, was carried into the county by a homeowner who brought citrus trees from Orange County.
Photo/Kevin Hecteman

A discovery in a Riverside backyard has added more urgency to California's work to fight a fatal citrus disease.

The backyard grapefruit tree had HLB, also known as huanglongbing or citrus greening. HLB, if left unchecked, could ravage California's citrus production, as it already has done in Florida. HLB has not yet reached California's commercial citrus orchards—and the focus of many a researcher is how to keep it that way.

"There are literally hundreds of scientists working on every imaginable method of changing the situation," said Beth Grafton-Cardwell, an entomologist and director of the University of California Lindcove Research and Extension Center in Exeter. "What's foremost in the minds of the California researchers is coming up with what we call an early-detection technique."

The present means of detecting HLB in a citrus tree involves a test called polymerase chain reaction. Though the PCR test can confirm the presence or absence of HLB, the tree has to have been infected for some time before the test will pick it up.

"That can take months to years before the bacterium distributes itself through the whole tree, and meanwhile, the psyllid is spreading it and spreading it," Grafton-Cardwell said.

Carried by an insect called the Asian citrus psyllid, HLB is the fastest-spreading disease she's ever seen, she said, because of the psyllid's efficiency in reproduction.

Early-detection methods involve picking up signs of distress from the tree, using high-tech and low-tech detection methods.

"It might be that the tree, when it becomes infected, gives off a different smell or volatile profile than a healthy tree, and that can be detected by gas chromatograph or by dogs," Grafton-Cardwell said. "The microbes that are on the surface of the leaves shift when the tree becomes unhealthy, and that can be swabbed and measured. Or it might be that the bacterium is producing proteins that circulate through the tree, and those are different than a healthy tree. Or the tree might have an immune response where it produces certain metabolites or proteins that, when it's sick, is part of its immune response."

Work is under way at UC Davis, she added.

"What we're hoping is that they will catch the disease really early in the infection, within weeks or maybe a month or two, instead of nine months to two years," Grafton-Cardwell said. "If we can figure out which trees are infected and get rid of them, then we're basically slowing or halting the spread of the disease."

In addition, beneficial insects—tiny wasps from Pakistan called Tamarixia radiata—are being deployed. The wasps lay their eggs in psyllid nymphs and will spread naturally, Grafton-Cardwell said. But they're expensive—75 cents per wasp—and trillions of them are needed to have any sort of an impact on a pest that's in Southern California to stay.

None of this will eradicate HLB, but it will buy time for researchers who are searching for a cure. Avenues of research include resistant rootstock or transforming the psyllid so it will no longer spread the disease, Grafton-Cardwell said.

Although HLB has not yet been found north of Los Angeles, the psyllid has been detected as far north as Placer County, northeast of Sacramento, where it was found last summer.

"That's a really dangerous situation in Placer County," said Alyssa Houtby, director of government affairs for California Citrus Mutual. "What happened was: A homeowner moved from Orange County to Placer County, and they brought with them their potted citrus trees that had ACP."

Orange County is under quarantine for the psyllid, meaning the trees should not have left the area.

This worries Rich Colwell, owner of Colwell Thundering Herd Ranch in Penryn and president of the Placer County Mountain Mandarin Growers' Association. Though disappointed by the psyllid's arrival, Colwell said Placer County farmers "were not unprepared, nor were we that surprised."

He and his fellow mandarin farmers have been preparing for a potential psyllid invasion for years, he said, working with the county agricultural commissioner, UC Cooperative Extension and the state to educate themselves. A Placer County psyllid quarantine covers 200 square miles but does not include any commercial mandarin groves.

"That would severely restrict our ability to make a living until the restriction is lifted," Colwell said. "We're very interested in what preventive measures could be taken."

The wasp program is one of the options Colwell is interested in, noting that one wasp release has taken place in the county.

The psyllid first showed up in California in 2008, near the Mexican border, Houtby said; HLB was first detected in 2012 in Hacienda Heights, in Los Angeles County.

For the first time this year, the state budget set aside $10 million specifically for psyllid and HLB research and outreach. The money will go toward biological controls, such as raising and releasing beneficial insects; increased trapping to monitor then psyllid's spread; public outreach and education; and costs associated with surveying and quarantine efforts.

"The industry spends about a million dollars a year on a variety of communications tactics that include billboards, public service announcements, advertisements, media outreach," as well as mailers and outreach to local elected officials, Houtby said. Social media has been a big part of the effort, Houtby said, with videos, competitions, pledge drives and messages on Twitter and Facebook.

"Our primary message to homeowners is to not move citrus, to keep your homegrown citrus at home, and second to cooperate with the Department of Food and Ag," she said.

(Kevin Hecteman is an assistant editor of Ag Alert. He may be contacted at khecteman@cfbf.com.)

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.