Potential cure for ailing citrus trees is a dose of hope for industry
Attention home gardeners: Our beloved citrus trees may yet be saved from the incurable huanglongbing, a.k.a. HLB or citrus greening disease, thanks to natural immunities found in a rare and flavorful relative known as the Australian finger lime.
After five years of study, a team of UC Riverside researchers led by Hailing Jin, professor of plant genetics, identified which gene in the finger lime causes that immunity and extracted it to create an antibiotic that has killed the disease in young trees raised in laboratories and controlled environment greenhouses.
The treatment can be used as a spray or an injection, but don’t go rushing to your garden store just yet.
In a news release, UC Riverside announced it has entered into an “exclusive, worldwide license agreement” with Boston-based Invaio Sciences to produce and market the antibiotic, but Jin said production can’t begin until the antimicrobial peptide (a really small protein) that kills the bacteria has gotten government approval.
That natural resistance is what prompted Jin to study the finger lime, and related plants, to see if she could determine why it seemed immune to HLB infections. She used comparative analysis to look at the plant’s genes to see if she could identify what created the “internal immune response that protected it from infection ... and that allowed me to identify the responsive genes that contribute to these tolerances.”
Australian finger limes are also known as caviar limes because once cut, the small, pickle-size fruit spills into tiny caviarlike beads that provide an intense citrus taste.
Now that researchers have identified the peptide, Jin is hopeful the regulatory approvals will come quickly, since the protein is derived from a natural substance and appears safe to humans. Moreover, she said, the antibiotic is easy to manufacture and works in heat up to 130 degrees.
But Jin is adamant the treatment should not be called a cure, at least not until she’s completed her tests on mature trees in the field, growing outside of a controlled environment. Those tests have been delayed because of coronavirus shutdowns.
The Jin peptide is the most promising development in destroying the disease that has devastated Florida’s citrus industry. The disease ruins the citrus fruit, so it stays bitter and green, and over time will kill the tree.
The disease is spread by the Asian citrus psyllid, a tiny sap-sucking insect discovered in California around 2008.
And California’s $1.7-billion citrus industry is willing to do whatever it takes to destroy the disease-spreading psyllid, such as the controversial practice of spraying the trees with pyrethroid and neonicotinoid pesticides, which are toxic to bees.
In California, untended residential citrus trees are one of the biggest threats, according to UC Riverside entomologist Elizabeth GraftonCardwell. About 60% of California’s homes have at least one citrus tree, and the industry has been trying to keep people from planting more to contain the threat to commercial growers.
The University of California’s department of agriculture and natural resources has created an interactive map at igis.ucanr.edu that identifies hot zones of infection. In Southern California they’re mostly where Los Angeles, Orange, Riverside and San Bernardino counties meet. Enter your address to see how close your home is to those infection areas.
If you have an infected tree, Jin said the best approach is to remove it because her antibiotic won’t be available soon.
“If you leave a positive tree anywhere, it can serve as a reservoir for psyllid to spread the disease,” she said. “These psyllid can fly pretty far, miles away from an infected tree.”