Eleven years after a disease that kills citrus trees was discovered in Southern California, the number of infections is rising — but experts and researchers are still fighting it.
In Southern California, the disease has hit Orange and Los Angeles counties the hardest, but infections have also been found in Riverside, San Bernardino and San Diego counties.
The higher number of cases in Orange and Los Angeles counties may result, in part, from the presence of ports, where insects may arrive with shipments, said David Morgan, environmental program manager for the California Department of Food and Agriculture. Other factors include the higher density of people and backyard citrus trees and the climate.
Portions of all five counties are under quarantine and the boundaries are expanded as new infections are found outside those areas. Citrus plants can’t be moved off properties inside quarantine areas, though washed fruit without stems and leaves can be shared in small amounts within the quarantine area.
Roughly half of Orange County and smaller regions of the other four are under quarantine, a California Department of Agriculture map shows.
Huanglongbing, or citrus greening disease, is caused by a bacterium transmitted between citrus trees by a tiny insect called the Asian citrus psyllid. Symptoms of the disease include yellow-mottled leaves, stunted growth and fruit production, and deformed, bitter fruit. Most infected trees die within a few years.
The disease was discovered in the U.S. in Florida in 2005, according to the U.S. Department of Agriculture. Since then, it has reduced the state’s citrus production by 75%.
Asian citrus psyllids were found in California in 2008. Four years later, the disease was found on a tree in Hacienda Heights in Los Angeles County.
The disease’s spread in California has not risen to levels seen in Florida, and so far there have been no infections in commercial groves, according to the California Department of Agriculture’s Citrus Pest and Disease Prevention Division — only in residential citrus trees.
The infection numbers are increasing, though.
As of Monday, May 8, there were 5,007 confirmed cases of the disease statewide, department data shows.
In 2022, 1,342 infections were confirmed across California — hundreds more than in any previous year — and 825 infections have been confirmed in approximately the first third of 2023.
“Fortunately, it’s not spreading as fast as we thought it might when it first appeared, but it’s still spreading, which is a negative,” Los Angeles County Agricultural Commissioner spokesperson Ken Pellman said.
“We were very lucky that we’d seen what had happened over in Florida, and we were able to start a really good control strategy in California before it got away form us,” said Morgan, who is based in Riverside and runs three biological control facilities, in Riverside, at Cal Poly Pomona and in Arvin.
Florida had additional factors helping spread the disease, including hurricanes that “blow insects everywhere” and the presence of a landscaping plant that carries the disease, Morgan said.
Victoria Hornbaker, director of the department’s Citrus Pest and Disease Prevention Division, also credits the community with helping slow the disease.
“If we weren’t getting the cooperation that we are from residents — we really do get overwhelming cooperation from residents — we wouldn’t be in such a good spot as we are today,” Hornbaker said.
Greening is worst in Orange County
Of the 5,007 Huanglongbing cases in the state, about 69% of them, 3,457, have been in Orange County.
Jose Arriaga, Orange County agricultural commissioner/sealer of weights and measures, said in an email that the office assists the California Department of Food and Agriculture with maintaining regulations inside quarantine boundaries, which currently include about 470 square miles of the county.
Arriaga wrote that the office’s efforts include taking part in monthly outreach events, providing translation services for the public and industry, and offering guidance to nurseries and growers on the regulations.
Los Angeles County has had 1,106 infected citrus plants.
Pellman said that, for three years after the first LA County infection was discovered in 2012, “things were blissfully quiet — and then it started popping up everywhere.”
The disease has maintained a “steady march” since then, Pellman said.
Of the remaining infections, San Bernardino County has had 246; Riverside County 168; and San Diego County has logged 30.
At the state’s Mount Rubidoux Field Station in Riverside, Morgan and other researchers work to control the number of Asian citrus psyllids in California.
They do that by raising and releasing tamarixia radiata, a type of wasp that attacks Asian citrus psyllids.
“What we really have here is a wasp factory,” Morgan said.
People don’t notice the tiny insects, over 28 million of which have now been released, Morgan said.
“They’re about the size of a period on the end of a sentence,” he said.
The wasps, he said, lay their eggs in or on the Asian citrus psyllids. When the eggs develop, they eat the psyllid.
The process was initially developed by UC Riverside, which first released the wasps in 2011 — before the first confirmed case of the disease in California. Later, the project was transferred to state officials.
“We do the research, we do the groundwork, we prove the concept works, and then we do what’s called a technology transfer,” said Georgios Vidalakis, a UCR professor and UC extension specialist in plant pathology who also directs the Citrus Clonal Protection Program.
In order to raise the wasps, they must also raise Asian citrus psyllids, and plants to host them.
The field station grows curry leaf plants rather than citrus trees, which Morgan said are related to citrus but grow more quickly and don’t carry the disease. The plant is used in South Asian cuisine.
The psyllids are placed on a curry leaf plant in a cage, and left to mature and lay eggs, which hatch into nymphs. Wasps are then added to the enclosure and lay eggs on the nymphs. The new wasps are then collected, and strategically released in urban areas to seek out the Asian citrus psyllid.
“We’re reaching a point where the number of diseased plants is getting difficult to handle,” Morgan said.
Scientists battling the disease
At UCR, research into citrus greening disease continues.
Scientists are working on all three elements involved: the bacterium, the insect and the citrus tree, Vidalakis said.
“If we manage to disrupt any of those three elements, then what we call the ‘disease triangle’ doesn’t come together,” Vidalakis said.
A collaboration between UCR and UC Davis is using computer modeling to simulate the disease’s bacterium, which Vidalakis said cannot be cultured in a lab.
“We can train computers to think they’re the bacteria,” he said, and researchers can then see how the bacteria reacts to different conditions.
In another case, scientists are crossing citrus species in hopes of producing hybrids more tolerant to disease, which Vidalakis said appears to be working, though the fruit produced by the current hybrids is “more of a lemon type.”
Several years ago, Hailing Jin, a UCR professor, discovered an antimicrobial peptide — a molecule involved in the plant’s immune response — in Australian finger limes that kills the Huanglongbing bacterium.
Jin said that the peptide can both make trees resistant to infections, like a vaccine, as well as control the disease in trees that are already positive for the disease.
The peptide is also more resistant to California’s heat than antibiotic treatments currently in use, which Jin said can lose their activity within hours due to high temperatures.
“It would be nice to use an eco-friendly method or natural product,” Jin said.
“People have been eating the peptide from the finger lime fruit for years.”
The peptide is currently being developed for commercial use by Invaio Sciences, which Jin said UCR agreed to give exclusive license to.
Residents’ cooperation is helping
As research continues, experts ask that everyone continue doing their part to prevent the disease from spreading.
“We ask this with all our heart, and all the scientific information we have,” Vidalakis said.
He asks that people avoid moving citrus plants around the state, and don’t bring plants into California from another state, which is prohibited for citrus plants.
Residents with citrus plants may share the fruit in small quantities, once it’s washed and free of any other plant material, inside quarantine areas.
Vidalakis suggested that anyone looking for a specific kind of citrus tree first check UCR’s Citrus Variety Collection, where it may be available.
Anyone who suspects a tree is infected with the disease can contact the California Department of Food and Agriculture Pest Hotline: 800-491-1899.
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