Researchers gear up for battle against disease that’s devastating U.S. citrus crop

02/07/2014 5:00 AM

02/07/2014 4:06 PM

In a secure Biosafety 3 containment facility at the University of California, Davis, researchers are required to de-robe, pull on scrubs and pass through negative pressure doors – something like an airlock – before they can begin their work. Leaving requires a shower and more airlocks.

“Everything is completely contained,” said MaryLou Polek, whose organization helps fund some of the research done at the facility. “They can’t even take a notebook out. They have to email their results out.”

Biological weapons? Well, in a way.

The building is called the UC Davis Research Containment Facility. And in this state-of-the-art operation, researchers work with exotic pests and pathogens that threaten U.S. agriculture and natural resources – things such as the glassy-winged sharpshooter and the brown marmorated stink bug.

They’re also studying what may be the biggest threat these days to the U.S. citrus industry: the Asian citrus psyllid.

An insect that’s no bigger than the head of a pin, the Asian citrus psyllid is responsible for spreading a disease called citrus greening that is wreaking havoc on citrus growers.

Growers in Florida have been grappling with it for years, and government officials recently stepped up their efforts to combat it. And growers in California – another huge producer of citrus crops – are on edge, as the insect and the disease have appeared in the state but not yet caused widespread problems. In labs on both ends of the country, researchers are employing different strategies to detect the disease, stop its spread, and hopefully cure already-sick trees.

“Florida is concerned about therapeutics – their industry is already so highly affected,” said Polek, vice president of science and technology for the Citrus Research Board, based in Visalia, Calif. “If they don’t find a way to cure their plants, they’re sunk. In California, we’re trying to not get to that point.”

Citrus greening is also known as Huanglongbing, or sometimes yellow dragon disease, and it first appeared in Asia during the late 1800s. It has decimated citrus crops in Asia, Africa, the Arabian Peninsula and Brazil.

The insect that spreads the disease – that pinhead-sized Asian citrus psyllid – was first seen in Florida in 1998. The disease itself appeared in Florida in 2005 and rapidly spread to all 32 citrus-growing counties.

The disease migrates when infected plants are moved and come into contact with the carrier insect, which can transmit it to other trees.

The bacterial disease is not a threat to humans or animals, but takes hold in a tree and eventually causes it to produce green, bitter-tasting fruit, ruining crops meant for sale to consumers or juice-makers.

Krysta Harden, deputy secretary of the U.S. Department of Agriculture, recently toured citrus groves near Tampa.

“A healthy orange makes you want a glass of juice,” she said. “And then to see this hard, small green fruit is devastating.”

Prakash K. Hebbar, national coordinator for the USDA’s citrus health response program, said that while it took five or six years for the damage to really be seen, it’s now showing up in reduced yields and lower quality of fruit. Much of the recent reduction in citrus production is because of the disease.

According to USDA statistics, the number of acres bearing oranges in Florida dropped from 588,000 to 429,000 between 2003 and 2013 – a reduction of 27 percent. In California, the drop was from 198,000 to 175,000, or 12 percent.

Rep. Vern Buchanan, a Republican from Sarasota, Fla., said growers in his state are suffering from their smallest orange crop harvest in 24 years. He pushed for citrus-greening funds in both the omnibus budget bill passed by Congress in January and the farm bill passed by Congress this week. The president is expected to sign the farm bill Friday.

The farm bill contains $125 million in mandatory funding for citrus-greening research and $125 million in discretionary funding for the disease.

Buchanan, from his walks through orange groves in recent years, described fruit “that just looks like it’s dying.”

“It’s turning colors, doesn’t have its natural state, is usually one-third the size of normal,” he said. “Some trees look like they’re going to be dead in a year or two.”

Unlike a cold snap that affects one year’s crop, with citrus greening the tree itself dies, he said.

“It’s really been a lot more aggressive in the last couple years,” he said. “Other states are at risk, but we have the most citrus and the most to lose.”

Even so, those other states are on high alert.

In addition to Florida, the disease has been found in Georgia, Louisiana, South Carolina, Texas and California.

When an infected tree is found in those states, officials swoop in and eliminate it. In Southern California, one infected tree was found in a resident’s personal garden; officials negotiated with the homeowner, removed the tree and established a quarantine to keep nursery stock from moving out of the area.

Although that tree was in Southern California, Polek from the Citrus Research Board said growers in the San Joaquin Valley in the central part of the state are on the lookout for the insect.

“So we know it’s up here, somewhere,” she said.

Around citrus groves, yellow sticky traps are positioned every quarter-mile or so to capture the insect.

Among her group’s research strategies are to find better ways to detect the disease in trees. While that one tree in Southern California was determined to have the disease using one diagnostic method, different diagnostic tests found another seven trees showing signs of the disease. Officials talked with those seven homeowners and replaced the trees with non-citrus plants, Polek said.

“We’re trying to protect our commercial groves,” she said.

Carolyn Slupsky, an associate professor in both the Department of Nutrition and the Department of Food Science and Technology at the University of California, Davis, said one of the first steps is to understand exactly how the pathogen affects the plant – a difficult prospect, since it will only grow in a citrus tree, not in a test tube, as do many other pathogens.

Researchers want to detect the disease before it becomes visible, since it can take two or more years following infection for outward symptoms to be evident.

In a 2012 article in the Journal of Proteome Research, Slupsky and her colleagues found the juice produced by infected-but-outwardly healthy trees differed from juice produced by completely healthy trees; the finding could help researchers design methods for early detection.

Researchers are also exploring whether gasses from a tree could lead to early detection.

“Most of Florida has been affected by the disease – I’m not sure how many completely clean groves there are,” Slupsky said. “Now it’s a matter of time when it will show up elsewhere.”

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