A citrus inspector with the California Department of Food and Agriculture finds Asian Citrus Psyllid on the underside of a citrus tree. Image via CDFA.

published on April 26, 2021 - 12:56 PM
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As the world dealt with a pandemic, the citrus industry was fighting its own disease.

In the struggle against Huanglongbing (HLB), growers, researchers, advocates and testers alike had to adapt their defensive measures to meet social distancing guidelines. And as a promising new treatment arises that could effectively end the threat of a disease that devastated citrus industries in other parts of the country, both residential and commercial citrus growers have to maintain vigilance in order to keep a beloved fruit and a multi-million industry alive.

Following HLB’s first appearance in Florida in 2004, the state went from producing 80% of the nation’s citrus — not counting tangerines — to producing less than 42%, according to the United States Department of Agriculture.

The USDA calls HLB, also known as citrus greening disease, incurable, threatening the future of the industry.

Over the years, the Asian Citrus Psyllid — the insect that carries the bacteria that causes HLB — made its way across the nation, to Texas and to Southern California. The California Department of Food and Agriculture (CDFA) now maintains quarantine areas in Southern California stretching from Huntington Beach to Riverside County. In 2020, at least one insect carrying the disease has also been detected in San Diego County.

The psyllid has been detected as far north as Placer and Alameda counties, including all seven San Joaquin Valley counties.

Researchers aren’t so quick to assign the “incurable” status to HLB.

Coming out of the University of California Riverside’s Citrus Research Center is a treatment for HLB derived from a variety of finger limes in Australia.

Professor Hailing Jin at UC Riverside said she noticed that the finger lime showed resistance to the bacteria. She wondered if traits in the lime tree could be used in the fight against the disease.

They extracted a peptide in the tree that has shown significant promise in not only killing the bacteria that cause HLB, but also inducing immune responses in citrus trees as well as certain potato and tomato plants, inoculating plants and trees against future infections. So far, the treatment is being tested on five varieties of citrus, including sweet oranges, similar to navels, popular in the Central Valley.

After seeing very promising results, field trials were supposed to begin in early 2020. Most of the testing has to be done in Florida, where the disease is more prevalent. Because of Covid, the University of Florida had to close its campus, putting an end to the field trial, said Jin. They were able to start again in October.

“A lot of work slowed down because of the pandemic,” Jin said. The team at UC Riverside had been testing the peptide for five years. Working with trees means testing takes longer.

Field trials can take up to two years and is the last step before researchers feel confident enough to submit the treatment for approval.

Invaio Sciences has already licensed the peptide and begun their own field trials, said Jin.

Showing the effectiveness of the peptide treatment means farmers would no longer have to rely on antibiotics. Tests have shown the peptide kills bacteria faster and more effectively. And the way it kills bacteria makes it much harder for the bacteria to adapt to the treatment.

The treatment is still in the development stage and thus more expensive, but once companies can scale, the treatment should be more affordable, Jin said.

But before treatments are widespread, containing the disease is paramount.

Covid affected almost every level of research, said Melinda Klein, chief research scientist at the Citrus Research Board in Visalia.

Proposed experiments were postponed. Lab work in greenhouses was affected where social distancing couldn’t be maintained, Klein said. It did allow researchers time to review previous work and write papers and grants.

“It gave you a chance to catch up on the reading and computer work that goes along with research,” Klein said.

For pest detection though, work had to be continued. Testing is largely done by setting traps in homes that have citrus trees.

Normally, staff would go door-to-door to ask to go into backyards, said Victoria Hornbaker, director of the citrus pest and disease prevention division at the CDFA. Because of Covid and unrest in 2020, surveying was instead done by leaving notes on doors, letting homeowners opt into testing by scheduling an appointment when testers were in the area. This year, Hornbaker estimates they collected a third fewer samples, yielding 70,000 compared to a high of 110,000.

Despite the drop in testing, Hornbaker said it gave her confidence that they were able to find positives. There were two detections in new areas — one in Moreno Valley in Riverside County and another in San Diego County. To date, there still hasn’t been a detection of HLB in a commercial grove. In Riverside County, one psyllid with the bacteria was detected on a farm. The grower quickly isolated the tree and no tree was infected with HLB, said Hornbaker.

Unlike Texas and Florida, California has natural barriers to the disease. She thinks the dry weather has played a part in keeping the number of infected insects down. Geography also plays a role. The psyllid only travels two to three miles in a lifetime, and if they are happy with plenty of food, they tend to stay where they are. Difficult terrain such as the Grapevine and the Mojave Desert help keep the disease contained. Quarantines are meant to keep travelers from unwittingly giving the disease a ride.

Extra measures also keep infections low. Besides the insect, plants can also become infected by grafting. Nurseries have cooperated when it comes to guidelines for shipping plants, Hornbaker said.

Vigilance has played a key role in keeping HLB contained, said Hornbaker.

“We may see a cure for HLB in the not-too-distant future,” she said. “The important part for us in the interim is that we continue to keep people thinking about Asian Citrus Psyllid and Huanglongbing, protecting our citrus trees so we can allow that scientific breakthrough to catch up.”

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