In a sunlit backyard somewhere in northeast Los Angeles, a homegrown citrus tree blossoms. A stranger stands a few meters back in contemplation. She scans the tree’s billowing foliage. She moves in, grabs a fruit, and turns it over in her hands, looking for the telltale green flush at the navel.
It’s the 20th tree she’s seen today, and none have caused her worry. But she keeps searching, house to house, backyard to backyard. Bountiful clementine trees, whimsical potted finger limes, and mangy Meyer lemons: All are potential threats.
The visitor is a horticultural technician, one of many dispatched by California’s Citrus Pest and Disease Prevention Division, and she’s hunting for trees infected with citrus greening disease, a contagious bacterial disease pulsing through Los Angeles County. The bacterium that causes citrus greening disease won’t harm humans. But if left unchecked, it could wipe out the entire country’s supply of citrus fruits. And it all starts with one sick tree.
Ten years ago in Florida, citrus greening disease (also called Huanglongbing or yellow dragon disease) tore through commercial groves and slashed commercial orange yield by 70 percent. It gained its foothold by spreading through the backyards of citrus hobbyists, tree by domesticated tree. A similar story played out decades ago in China, where the disease was first named; in Brazil in 2004; and in Texas in 2012. The nimble bug continues to infiltrate groves in these far-flung locations. For growers, it’s a constant fight.
Now, in Southern California, about 2,300 infected citrus trees have been discovered and destroyed. “The clock is ticking as to when infected trees in commercial groves will be found,” says California grower John C. Gless of Gless Ranch Inc. With modern science, though, Californians have a chance to write a different story.
The bacterium that causes citrus greening disease is a stealthy killer. Lurking inside even the smallest cutting from your aunt’s kumquat tree or the most ordinary potted pomelo could be Candidatus Liberibacter asiaticus, or CLas, a bacterium that hinders fruit development, renders juice unpalatably sour, and eventually kills the tree—the course of citrus greening disease. The disease is asymptomatic for months, while the tree remains contagious. “You can’t see it, you don’t know it’s there, but HLB is a death sentence for citrus,” says Gless.
Before that death sentence is executed, though, CLas spreads from an infected tree to others through the air. It hitches a ride with Diaphorina citri, a flying insect about the size of a grain of rice, mottled brown with black dots on the tips of its antennae. Diaphorina citri sucks in bacteria while feeding on the phloem of new growth citrus leaves and then moves on to other citrus trees, depositing a small amount of CLas as it goes along. In Florida, hurricane winds blow bacteria-laden insects into commercial groves where they quickly infect thousands of other trees. California’s seasonal Santa Ana winds have potential to do the same.
The CPDPD and their door-to-door technicians focus on slowing the spread of the disease, hemming its boundaries. “We were able to learn from the things that occurred in Florida and Texas so we can be more proactive at preventing Huanglongbing,” says Victoria Hornbaker, director of CPDPD. From her office in Sacramento, Hornbaker keeps a running tally of infected trees alongside a map of their locations. To predict where the disease might spread, CPDPD uses a data algorithm developed by Tim Gottwald and colleagues at the USDA Agricultural Research Service labs in Fort Pierce, Florida. The model inputs geographically tagged data such as temperature distribution, locations of likely disease introduction like ports, and places devoid of citrus trees like office buildings and public parks. It outputs a blotchy map that the CPDPD uses to decide which neighborhoods to poke around.
In April, the map output northeast L.A., a neighborhood with a previous outbreak. Hornbaker’s techs rushed out to homes to inspect dozens of trees each day, looking for outward signs of citrus greening disease like blotchy leaves and green-tinted mature fruit. If they suspect a tree is sick, they pluck a few leaves, bag them, and send them to their Sacramento lab for genetic analysis. They also look for Diaphorina citri insects, and if they find one, they attempt to catch it in a bottle of alcohol and send it to the lab. If the Sacramento lab finds DNA from the citrus greening disease bacterium inside any of the samples, the CPDPD issues a “citrus quarantine.”
During quarantine, the movement of citrus trees, fruits, and cuttings is prohibited—Hornbaker’s technicians spread the word by flyering houses and holding meetings. “We are boots on the ground, doing our job to give science a chance to find a solution,” says Hornbaker.
Fortunately, researchers in academia and industry are searching for a more permanent fix. A few years ago, Hailing Jin, a plant molecular geneticist at University of California, Riverside, isolated a peptide from the citrus greening–tolerant finger lime tree that confers tolerance in other citrus trees. The peptide can be injected directly into the phloem of healthy trees. The tree will then mount a defense and create an immunological memory, similar to the action of a human vaccine.
The peptide can also act like an antibiotic. Injected into sick trees, it will infiltrate the cell membrane of the CLas bacteria and burst it. “Normally, a peptide either has antibacterial activity or they can prime the host to be more immune against a pathogen. Ours is unique that it has both activities,” says Jin. She has since partnered with Massachusetts-based company Invaio Sciences to bring a product to market in the next few years. “We need to find the best formulation, the best application, and the best delivery method, so we can control Huanglongbing,” says Jin.
Another company, Southern Gardens Citrus—a longtime player in the Florida citrus industry—has developed a different peptide and a viral vehicle for its delivery. Taken from the playbook for human vaccines, the company is using Citrus tristeza virus (like some COVID-19 vaccines use an adenovirus) as a vector to administer a spinach plant–derived peptide called defensin, an antimicrobial. Jin says that preliminary results look promising.
Other researchers are looking for ways to attack the bug rather than the bacteria. Koppert Biological Systems in São Paulo, Brazil, has engineered an insecticidal spray that contains the fungus Cordyceps fumosorosea, which disrupts feeding behavior of Diaphorina citri. Others are betting on RNAi, small bits of engineered genetic material that, if injected into a citrus tree, would be sucked in by the insect as it feeds, head toward a target gene, silence the crucial gene’s expression, and kill the insect.
Attacking the bug or bacteria will slow the spread of the disease, as will destroying infected trees. But the endgame might be to engineer trees that are not susceptible to citrus greening disease in the first place. “The ultimate goal is resistant rootstock,” says Gless. The conventional way is to identify resistant citrus varieties and selectively breed them over decades. One Florida grower discovered a naturally resistant variety of mandarins in his groves a few years ago. But it’s unclear whether growers can find enough resistant varieties to fill grocery stores.
Breeders can speed up the process through genetic modification, but GM fruit is not popular with consumers, especially in urban California. One workaround is to use the gene editing tool CRISPR-Cas9 to delete a chunk of a citrus tree’s genome that’s responsible for causing disease symptoms in the tree. If no new genetic sequence is introduced, the tree doesn’t qualify as a genetically modified organism. Nian Wang and his team at the University of Florida are busy perfecting this technique for citrus trees. They have already created modified sweet orange trees that they will test against citrus greening disease over the next few years.
Of course, none of these techniques has yet reached the average grower like Gless. “We don’t have a cure, so for now we’re being a proactive as we can,” he says. Gless Ranch sprays insecticide on trees, to keep out the insects that spread citrus greening disease, but of course, insects can eventually develop resistance to traditional foliar sprays. In the Coachella Valley, a growers’ consortium has banded together to coordinate their insect spraying activities and figure out what to do next. It’s a lot of work. “It’s meeting after meeting, and we’re putting a lot of time and effort into this battle,” says Gless.
Meanwhile, Victoria Hornbaker’s team at the CPDPD remains vigilant. At the Redlands farmers market on the outskirts of Los Angeles, the CPDPD erected a booth, a bright-orange canopy emblazoned with their website address. Underneath, a poster titled “What to look for” displays pictures: a patchy-colored leaf from an infected tree, a collection of Diaphorina citri nymphs, and Diaphorina citri adults perched on a doomed leaf. “Protect your community’s citrus!” the sign urges. Hopefully, local residents, hobbyists, and farmers are listening. And hopefully a solution will come sooner rather than later.