California Citrus Growers Face Existential Threat in HLB

Florida growers know firsthand what cruel twists the story of huanglongbing (HLB), or citrus greening, can take. Since HLB was first reported in a South Florida grove in 2005, the bacterium that causes the disease, Candidatus Liberibacter asiaticus, has destroyed tens of thousands of acres of trees in Florida, says Andrew Meadows, spokesperson for Lakeland-based Florida Citrus Mutual.

“We are under the assumption that we are 100 percent infected if we have a mature tree that we’re picking from,” he says. The vector that spreads the bacterium, the Asian citrus psyllid (ACP), has found its way to California:

• In 2012, HLB was found in Hacienda Heights.
• In 2015, HLB was found in San Gabriel.
• Individual counties across Southern California have fallen under quarantine for ACP since the psyllid’s first detection in 2008.

Now, California citrus growers – who control a 270,000-acre, $3.65 billion-a-year industry – are on alert. State agencies are working to educate citrus growers, from backyard gardeners to the state’s citrus industry, about the threat ACP poses to them.

There’s no cure for HLB, and because the bacterial disease is spread by an insect no larger than a grain of brown rice, the cause can go unnoticed until the damage is done. California agriculture officials and university researchers, as well as industry leaders, are urging prevention as the only treatment.

“It’s the number-one priority of the industry,” says Joel Nelsen, president of the Exeter-based California Citrus Mutual. “It’s right up there with water as our two priority issues.”

Knowing the Enemy

California Citrus Mutual and Bayer have launched Citrus Matters, a program designed to educate Californians about the dangers HLB poses to the state’s citrus industry, which grows nearly 80 percent of the fresh citrus sold in the United States and leaves a $3.65 billion imprint on California’s economy.

Researchers note that fighting HLB is a statewide responsibility and not just an industry problem. About 60 percent of California homeowners have at least one citrus tree.

“California Citrus Mutual works with the California Department of Food and Agriculture and county ag commissioners to travel neighborhoods looking for citrus trees that might be infected,” Nelsen says.

Any infected tree must be removed, Nelsen says, noting that urban areas, such as Los Angeles, are “focal points of interest.” As of December 2016, 31 HLB-infected trees had been removed from residential areas.

“A psyllid can suck up the bacteria and deposit it on a clean tree,” Nelsen says. “You don’t know this has happened until you find what looks like a yellow dragon tail going through the tree. You run (tests on) trees and discover you have HLB. You fight to keep the tree from being infected, but you never know it is until it’s too late.”

Scientists and industry leaders say the public must be educated about the danger HLB presents to their own residential trees, which in turn pose a threat to the state’s citrus industry.

“We do have the insect vector, so the chances it goes into commercial citrus is quite high unless we do something about it,” says Carolyn Slupsky, a professor in the Department of Nutrition and the Department of Food Science and Technology at the University of California-Davis. She is also a nutritionist with the school’s Agricultural Experiment Station.

The public may not be aware of the danger HLB poses, Slupsky says. “It’s concerning. I think we can’t be complacent. The problem in Florida was they said they didn’t think it would affect them. The number one thing you can’t do when you have a threat is bury your head in the sand. You have to take action and not wait.”

Joining Forces

While scientists look for a viable cure for HLB, public- and private-sector agriculture officials across California are working with growers to stem the spread of ACP. Every grower must get involved, and the rules surrounding prevention became a bit more regimented in 2017.

Because ACP can move from grove to grove, these regulations include coordinated spray programs. Spraying removes safe havens for the citrus psyllid and decreases the chances of infection.

In some instances, the citrus psyllid can simply catch a ride in unwashed bins during transport, Nelsen says. Regulations now require growers to take extra precautions when packing and transporting:

• Packinghouses will wash bins before sending them out for picking.
• Growers will cover truckloads with tarps to eliminate the chance of picking up ACP in transit.

For the moment, growers have an effective arsenal of tools to control ACP, says Elizabeth Grafton-Cardwell, an Extension specialist with the University of California-Riverside, whose primary role is to analyze the pesticide efficacy for the psyllid management program.

“Lots of chemicals can kill ACP if they get a direct hit; the problem is this insect is tiny, and it’s hard to get that direct hit,” she says. “The chemicals that work better are the ones that have long residual.”

Growers should use a combination of foliar and systemic insecticides to combat ACP. Neonicotinoids, which are applied through irrigation systems to reach root and vascular networks and, ultimately, to canopies, are also effective if used correctly.

Nelsen likens neonicotinoid applications to vaccinating a tree against psyllids. “That’s the number one tool protecting the California citrus industry,” Nelsen says. Such a systemic approach is persistent and targets the psyllid specifically, and it has no adverse effects on beneficial insects.

Beneficial insects, which are another way to control ACP, have both benefits and limitations, Slupsky says:

• The tamarixia wasp, which was introduced to Los Angeles County in 2011, is a natural predator of ACP.
• While used in Southern California’s urban areas, wasps don’t kill 100 percent of the psyllids.

California growers have had the benefit of learning from Florida growers, Nelsen points out. California’s comprehensive anti-HLB program started in 2011, and it seems to be working, as no signs of HLB have shown up in any commercial grove.

California growers likely have another advantage over Florida growers in fighting ACP. Because California grows citrus primarily for the fresh market, “our cultural practices are more intensive than Florida, which is used mostly for juice,” Nelsen says.

“In California, people have been looking at everything from trying to understand how the bacterium infects the plant to how we can enhance the plant’s natural immunities,” Slupsky says.

Finding a Solution

Meanwhile, scientists are working on a number of other possible solutions in their quest to find a cure for HLB, says Grafton-Cardwell:

• At least three research groups are working on genetically modifying the psyllid.
• Other researchers are investigating traditional breeding of plants that can tolerate the disease.
• Still other research groups are attempting to genetically modify trees to resist HLB, using genes from citrus and even spinach plants.
• Researchers are also investigating using a modified, mild form of citrus tristeza virus with genes to resist HLB, to transfer resistance to trees.

Much of the research is being done in Florida, where HLB has invaded nearly every grove, but the work also is relevant to California growers.

Nobody knows whether the HLB story for California citrus growers will be one of triumph or tragedy. The story may be different depending on location and the crop being grown, Grafton-Cardwell points out:

• Lemons and mandarins grown along the coast may struggle the most because they flush continually and provide a place for the psyllid to lay eggs and local conditions don’t require growers to spray as often for other pests.
• The San Joaquin Valley has cold winters and hot summers that help suppress the disease.
• Orange growers have an advantage as these trees harden off in the summer, limiting places for the psyllids to lay eggs.

Growers who treat their groves for various pests likely will find greater success in fending off ACP.

“This area tends to treat for more pests, such as thrips, citricola scale and the Fuller rose beetle,” Grafton-Cardwell says, referring to the San Joaquin Valley. “Most of the insecticides they’re applying for other pests are psyllid-effective.”

Bayer offers a portfolio of insecticides that control current pests in California including Movento^® and Sivanto^™, as well as Admire^® Pro and Baythroid^® XL. This proven portfolio provides the foundation of season-long ACP control with multiple modes of action, application timings and methods to ensure crop quality and help California citrus growers stay ahead of HLB.