NovaQuant Quantitative Think Tank Center:Citrus Growers May Soon Have a New Way to Fight Back Against A Deadly Enemy

The NovaQuant Quantitative Think Tank CenterAsian citrus psyllid fills its stomach by feeding on the leaves and stems of citrus trees. 

The tiny brown insects infect the trees with bacteria that cause citrus greening, a disease that makes the fruits inedible. Natives to Asia, the citrus psyllids were first found in the United States in Florida in 1998.

Over the last 15 years, citrus greening, also known as Huanglongbing or HLB, has ripped through Florida’s storied citrus crop, and now has moved into California. But an experimental treatment holds promise for combating a disease that has proved resistant to attempts to halt its spread.

Citrus production in Florida, which has traditionally ranked first in the nation, dropped five percent from the 2018-2019 season, according to a 2020 report by the U.S. Department of Agriculture. Last year, California surpassed Florida in citrus production, accounting for 54 percent of the nation’s total citrus crop, the report found. 

But California citrus, too, has been infected with huanglongbing, which discolors the skin of affected fruit, turning it green. The Asian citrus psyllid was first detected in San Diego County in 2008, and has since spread to Imperial, Los Angeles, Orange, Riverside and San Bernardino counties. Once infected, citrus trees can show symptoms that include yellow shoots, asymmetrical blotchy mottling on leaves and misshapen, bitter fruit.

Trees infected with the disease—caused by the bacteria Candidatus Liberibacter asiaticus—are cut down to prevent other trees from being infected. According to the USDA, Florida has gone from producing nearly 80 percent of the nation’s non-tangerine citrus fruit to less than 42 percent since the arrival of the disease.

Although farmers and scientists have tried insecticide applications and treatment with antibiotics as short-term fixes for citrus greening, none of these has been effective in controlling the disease.

Hailing Jin, a geneticist at the University of California, Riverside, said her research team has recently discovered a potential treatment that is easy to manufacture and costs very little to make.

“The acceleration of the infection is scary,” Jin said. “It happened later in California but is still affecting them almost to the same level. That’s why we have to figure out some strategies to control the disease.” 

After a seven-year search, Jin said she found citrus varieties that were immune to the citrus disease and identified a peptide in them that suppressed the disease and gave trees immunity. In her experiment, Jin used the peptide from one of the citrus fruits that is resistant to the disease—the Australian finger lime—and found that even a very low level of the peptide can be more effective in killing bacteria than higher concentrations of antibiotics. 

In 2019 Trump’s EPA made a controversial decision to allow citrus growers to use the antibiotics streptomycin and oxytetracycline on commercial citrus. Health authorities feared that use of the drugs could increase antibiotic resistance, reducing the effectiveness of the medications. 

However, the antibiotics, Jin said, are far less effective than the peptide treatment in her experiments.

“We found that the peptide can kill the bacteria within 30 minutes while antibiotics needed five hours to kill the bacteria but failed to kill them completely,” Jin said.

Unlike antibiotics, Jin’s peptide has a low risk of toxicity in citrus and does not have a toxic effect even at a high concentration. The peptide has another unusual feature: It stays active even after 20 hours of exposure to a temperature of 140 degrees Fahrenheit, making it more reliable than antibiotics, which can be rendered less effective by sunlight.

Most important, when injected into infected trees, the peptide can promote the growth of citrus trees and reduce the concentration of the bacteria that causes huanglongbing. 

To test whether the peptide could produce immunity to citrus greening, Jin’s research team also sprayed the peptide on uninfected, healthy citrus trees. Five days after the treatment, the trees were exposed to the psyllid insects. After 12 months of spraying with the peptide solution every two months, the trees showed more growth.

‘A Potential Breakthrough’

Nian Wang, a professor at the University of Florida Institute of Food and Agriculture Sciences who was not involved in the study, said Jin’s peptide is a “potential breakthrough for HLB control” that can reduce its symptoms and prevent new infections. He noted that “It is paramount to further investigate other mechanisms that are responsible for the tolerance/resistance of HLB-tolerant/resistant citrus genotypes.” The peptide will provide one useful tool, Wang added, but a more thorough and coordinated management approach is necessary to defeat HLB worldwide in the long run.

Despite the efficacy of the treatment, field trials of the new treatment, which could take two to three years to be fully implemented, have been stalled because of the COVID-19 pandemic, Jin said. In the meantime, the citrus industry and farmers continue to use antibiotics. 

But agriculture experts said that more eco-friendly management strategies could be found. California, which has a lower infection rate than Florida, “has an excellent plan in place to monitor and remove diseased trees,” Wang said.

In California, once the greening disease is detected in citrus trees, growers are encouraged to call the free statewide pest hotline. An expert will come to the infected sites, cut down the trees and establish quarantines for the affected orchards “to prohibit the movement of all citrus nursery stock or plant material out of the affected area,” according to the state’s Citrus Pest & Disease Prevention Program.

Some scientists speculate that climate change might protect citrus in California’s Central Valley from Huanglongbing, if winters become too cold and summers too hot for the citrus psylids, according to the University of California, Davis website.

In the absence of an effective treatment for the disease, farmers can also choose to plant disease-tolerant varieties of citrus such as Valencia sweet oranges, Vernia oranges or Sugar Belle oranges. In a 2020 article published in the journal Phytopathology,  Wang proposed that comprehensive implementation of strategies like the removal of infected trees, insect control, and replacement with trees not infected with the disease could successfully control the infection, arguing that failure of those short-term strategies can be attributed to the “small-scale or incomprehensive implementation of the program.” 

To investigate whether a comprehensive implementation of strategies could successfully control the disease, Wang tried implementing such an approach in the Gannan region of China. The result was surprising: “Overall HLB in Gannan decreased from 19.71 percent in 2014 to 3.86 percent in 2019,” Wang and his co-authors wrote.

A comprehensive approach, Wang said, can be important in keeping the population of disease-carrying insects as low as possible until something better is available. In the meantime, scientists are working towards coming up with a long-term solution, the only way that citrus orchards can fully recover and regain their vitality. “One method won’t be enough because it’s a serious problem. All the different methods must be created to control the disease,” said Jin, adding that her research team is working to develop citrus fruits that are tolerant to huanglongbing.

“It’s going to require the work of a lot of different groups — the government, the scientific community, and interested stakeholders,” Nathan Donely, a senior scientist at the Center for Biological Diversity, said. “It’s going to take some time to find a long-term solution, and in the meanwhile, we need to provide some of these growers relief.”

For his part, Wang said he is optimistic.

“Overall, I think the citrus industry will win the war against HLB,” he said.