Researchers find way to treat, prevent killer citrus disease

UC Riverside discovers Australian finger lime peptide can kill, prevent the spread of fatal citrus tree disease huanglongbing

RIVERSIDE, CALIF. – New research affirms simple proteins found in an Australian plant can destroy the deadliest citrus tree disease and help prevent infection.

Huanglongbing (HLB), more commonly referred to as citrus greening, gets its name from the discoloration it causes while also making citrus fruits bitter and worthless. It has wiped out citrus orchards across the globe, most notably in Florida and Texas, causing billions in annual production losses. All commercially important citrus varieties are susceptible to it, and there is no effective tool to treat HLB-positive trees, or to prevent new infections.

However, new UC Riverside research shows that a naturally occurring peptide found in HLB-tolerant citrus relatives, such as Australian finger lime, can not only kill the bacteria that causes the disease, it can also activate the plant’s own immune system to inhibit new HLB infection. Few treatments can do both.

Research demonstrating the effectiveness of the peptide in greenhouse experiments has just been published in the Proceedings of the National Academy of Sciences.

The disease is caused by a bacterium called CLas that is transmitted to trees by a flying insect known as the Asian citrus psyllid (ACP). One of the most effective ways to treat it may be through the use of this antimicrobial peptide found in Australian finger lime, a fruit that is a close relative of citrus plants. Peptides are similar to proteins but are smaller and have fewer amino acids, the building blocks of proteins, and usually combine to form complex proteins.

“The peptide’s corkscrew-like helix structure can quickly puncture the bacterium, causing it to leak fluid and die within half an hour, much faster than antibiotics,” explained Hailing Jin, the UCR geneticist who led the research.

When the research team injected the peptide into plants already sick with HLB, the plants survived and grew healthy new shoots. Infected plants that went untreated became sicker and some eventually died.

“The treated trees had very low bacteria counts, and one had no detectable bacteria anymore,” Jin said. “This shows the peptide can rescue infected plants, which is important as so many trees are already positive.”

The team also tested applying the peptide by spraying it. For this experiment, researchers took healthy sweet orange trees and infected them with HLB-positive citrus psyllids.

After spraying at regular intervals, only three of 10 treated trees tested positive for the disease, and none of them died. By comparison, nine of 10 untreated trees became positive, and four of them died.

In addition to its efficacy against the bacterium, the stable anti-microbial peptide, or SAMP, offers a number of benefits over current control methods. For one, as the name implies, it remains stable and active even when used in 130-degree heat, unlike most antibiotic sprays that are heat sensitive—an important attribute for citrus orchards in hot climates like Florida and parts of California.

In addition, the peptide is much safer for the environment than other synthetic treatments. “Because it’s in the finger lime fruit, people have eaten this peptide for hundreds of years,” Jin said.

Researchers also identified that one half of the peptide’s helix structure is responsible for most of its antimicrobial activity. Since it is only necessary to synthesize half the peptide, this is likely to reduce the cost of large-scale manufacturing.

The SAMP technology has already been licensed by Invaio Sciences, whose proprietary injection technology will further enhance the treatment.

Following the successful greenhouse experiments, the researchers have started field tests of the peptides in Florida. They are also studying whether the peptide can inhibit diseases caused by the same family of bacteria that affect other crops, such as potato and tomato.

“The potential for this discovery to solve such devastating problems with our food supply is extremely exciting,” Jin said.

Pest prevention

The disease has been out of the news of late because there have been no cases of HLB infected trees and few finds of psyllids in the state’s primary citrus growing region between Porterville and Woodlake in Tulare County. Since a peak of 200 disease-spreading pests in 2016, Tulare County has seen the number of psyllids drop to just three in 2020, and two of those were caught while being processed at a packinghouse.

Christopher Greer, assistant ag commissioner/sealer for Tulare County, said the dwindling numbers are cause for celebration but not a victory lap.

“If we become like Florida or Texas, if HLB wiped out the citrus industry here, that would financially impair California and devastate the ag industry here,” Greer said. Tulare County alone represents $1 billion in gross citrus production and is home to twice as many citrus packing plants as any other county in the state.

Greer said most of the decline is thanks to a regulatory program began by the California Department of Food and Agriculture (CDFA) in 2017. The program implemented procedures statewide to remove all stems and leaves from fruit before leaving the field because the pest feeds on the foliage and not the fruit itself. CDFA also ordered trucks to tarp all of the fruit shipments and required packing houses to wash and wax all fruit to remove any remaining pests. Locally, the county funded a citrus abatement program in 2018 to either remove abandoned orchards or reinstitute pest management programs in neglected orchards.

The biggest threat to citrus remains residential citrus trees because they are not monitored by the Ag Commissioner’s office. Greer said there are still large numbers of psyllids found in Southern California and all have been found in backyards or in orchards next to residential neighborhoods with citrus trees.

“It is much harder in residential settings than commercial settings,” Greer said. “But having a treatment would be huge win for the industry.”

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