This story was originally posted by modern farmer and is republished with permission.
It begins as imperceptible, dormant for two or even four years. It’s undetectable. But little by little, the signs are coming out. Individual branches of a tree show signs of nutrient deficiency or possibly excess water. Branches will begin to yellow and weaken, becoming shrunken. Then the fruit will turn, become small and refuse to ripen, and sometimes fall early. The fruit is safe for human consumption, but it tastes like battery acid.
And once the tree reaches that point, there is no going back. THE the tree will die within a few years, no matter what surgery you attempt. That’s why when growers see trees infected with Huanglongbing, known as HLB or citrus greening, they immediately seek to remove the tree. There is no other option.
“We destroy all the trees that are infected. We monitor and weed out those where we can. We use biological controls with the loss. We use all the tools in the bag,” says Jared Plumlee, senior vice president of agriculture at ranch stalls, in Orange Cove, California. Plumblee oversees approximately 7,000 acres in the central San Joaquin Valley, growing navel oranges, valencias, tangerines, and even lemons and grapefruits. There’s no sign of HLB in the Booth Orchards yet, and Plumlee aims to keep it that way. The ranch also has its own on-site packing plant, where they only pack their own product. This is both to foster trust with consumers, so they know every piece of fruit in a Booth box is from this farm, and to keep potentially infected fruit out.
HLB is an insect-borne disease Asian citrus psyllid, which infects trees with a slow-growing bacterium as it feeds on their shoots. It usually spreads when the insect crosses borders in fruit or tree cuttings, but global warming is speeding things up. The transmission of citrus greening is temperature dependent, both to ensure the survival of the psyllid and to make the host trees most vulnerable. The temperatures between 60 and 90 degrees Fahrenheit allow disease to develop. Research has shown that areas that remain within this range for at least half of the year have the most cases of HLB.
As global temperatures rise, citrus greening infestations can — and will — move further north. Tracking the spread of HLB is, in some ways, tracking global warming.
Growers can often unknowingly graft an infected tree branch onto their otherwise healthy stock. This is how citrus groves in Texas, and most especially Florida, fell victim to the disease. HLB was first discovered in Florida in 2005, where it quickly tore through the state’s orange and grapefruit groves, infecting nearly 90% of citrus trees. Almost 20 years later, last season’s orange production is only 16 percent yield in 2003. And overall citrus production continues to decline, every year for the past five years. This year’s orange yield is expected to be 25% less than last year’s final production.
Across the country, California producers have been paying close attention to what their Florida colleagues have been through, and they have no desire to follow the same path. “We’re seeing these numbers (from Florida), and it’s very, very scary,” Plumlee says. Since California’s annual temperatures are squarely within the HLB’s optimal transmission range, growers are being as proactive as possible, even pushing through state legislation that allows citrus growers to essentially step in and investing money in research and eradication programs. “We’ve done pretty well so far; it still hasn’t been found in commercial orchards,” says Plumlee.
California is home to approximately 300,000 acres of citrus production across the state. Infected trees have been found in California, Los Angeles and Orange counties and along the coast near San Diego. But, so far, the bacteria has stuck to residential trees or other easily removed ones.
“Last year, nursery stock was sent from South Carolina from a nursery that had citrus canker,” says Victoria Hornbaker, director of the Citrus Pest and Disease Prevention Division at the California Department of Food and Agriculture. “But our team was able to react incredibly quickly to get to those places, collect that nursery stock, destroy it, and do a one-mile survey of all the citrus trees around those places to make sure we didn’t see no symptoms of citrus canker in the environment, so this is a good example of how quickly we can mobilize and respond to potential citrus issues.”
The citrus industry is not just big in California, but across the country. Florida may be known for its oranges, but it’s mostly oranges grown for juice. California has always been the birthplace of fresh citrus, producing 85% of the country’s table fruit. “If we lose California citrus, we also lose our domestic and international market,” says Hornbaker. This means importing more citrus from outside the country, raising prices and losing a huge amount of revenue all around.
This is why California growers place such importance on being proactive and why there is an incredible amount of research on citrus greening from California institutions. University of California, Riverside is working on a treatment that effectively kills bacteria, although it is still being tested in the industry. Growers are experimenting with higher density plantings, putting more trees in the ground per acre, to get a higher yield in less time. Although, as Plumlee explains, this also has a significant downside. “The longevity of a plantation like this might not be 50 years; it might only be 25. Because once the trees fill up, you kind of hit that plateau on what your production level may be. So, against HLB, you will return the field faster.
Breeders are also working to find new varieties that are less susceptible to HLB. THE Sugar Belle Tangerine, the hybrid of a clementine and a Minneola, showed promising resistance. The new variety was born from University of Florida research and was released to growers across the state in 2009. More than a decade later, Sugar Belle is among the most widely grown varieties in the state. .
Curiously, there seems to be a correlation between citrus fruit size and resistance to bacteria, although it’s unclear if size is a determining factor or just a coincidence. But Neil McRoberts, a professor of plant pathology at the University of California, Davis, says grapefruits and large oranges are less resistant to disease, with smaller tangerines showing more disease resistance. “Because our citrus fruits come from so few different progenitor lines, they have no natural resistance to the bacteria. So none of our favorite citrus types and varieties have much resistance in general,” says McRoberts.
There might be a cure there. But that won’t be on the horizon in five or even 10 years. less citrus trees that can stand and continue to produce a crop,” says McRoberts. “There are promises there, but it’s slow work.”
In the meantime, Plumlee and other producers have no choice but to keep going and keep growing. “You can’t just give up and quit. You keep doing science and testing and trying to solve this puzzle. But, in the short term, there’s not much you can do that we haven’t already done. Growers like Plumlee have a culture that is vital to the state and the nation and a disease they are fighting to keep at bay. For now, they just have to hold on — for a decade or two — until more effective methods become available.
“That’s the scary part,” Plumlee says. “If we had something today, straight out of the lab, that works, we still have 10 or 15 years to prove that it actually works. And then another 10, probably before everything is implemented in the industry. So we know time is not our friend.