Citrus research experts Tracy Kahn
and Richard Lee check propagated
citrus material in a greenhouse on the
UC Riverside campus.

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Pest threat puts citrus researchers on high alert

Issue Date: July 8, 2009

By Kate Campbell
Assistant Editor

When botanist Tracy Kahn heard last summer that the Asian citrus psyllid had been found in San Diego County, she said she felt sick, but not surprised. The pest has been showing up in citrus growing regions throughout the world, devastating commercial production.

She feared it was just a matter of time before it found its way to California. And, as curator of the nearly century-old Citrus Variety Collection at the University of California, Riverside, she has good reason for concern.

Kahn is in charge of one of the world's largest and most diverse citrus plantings—a 1,700-specimen, living museum of world citrus varieties. She's well aware that the priceless, outdoor collection is vulnerable to infestation by the psyllid and devastation by the tree-killing bacterium it spreads.

The presence of the pest in San Diego and Imperial counties triggered an immediate state and federal quarantine, and actions are under way now to eradicate the threat. Officials report traps in the six-square-mile quarantine area continue to catch psyllids.

Located about 100 miles away from the current pest infestations, researchers at UC Riverside aren't waiting for the worst. They've already secured greenhouses with double entries and small-mesh screening.

They've increased pest trapping in experimental plants and in shade, screen and green houses. They've also stepped up clonal material testing to ensure holdings are free of the huanglongbing bacterium that causes citrus greening disease and rapid tree death.

Although the university citrus facilities are outside the quarantine zone—for now—officials agree the threat is too close for comfort. They know already that thousands of acres of Florida citrus trees have been infected with the bacterium and hundreds of thousands of trees have died or been pulled out.

"I have a very specific job to do here," said Georgios Vidalakis, director of the Citrus Clonal Protection Program at UC Riverside. "That's to ensure citrus plant material introduced to California is safe, free of disease and pathogens, including citrus greening."


Georgios Vidalakis, director of the Citrus Clonal Protection
Program, follows up on the indexing of citrus plant material
entering California.

Vidalakis, a Greek-born plant pathologist, directs the state's 50-year-old clonal protection program. The cooperative effort includes participation by the California Citrus Research Board, the California Citrus Nursery Board and the California Department of Food and Agriculture.

State and federal laws require that any citrus material entering California pass through the Citrus Clonal Protection Program, where it's tested for a variety of plant diseases. Although a high percentage of new citrus imports arrive infected with diseases, citrus greening has not yet been found in those materials.

Vidalakis noted that as an early precaution, citrus plant material from infected regions has not been accepted by the program for nearly 40 years.

"The collection of plants under the Citrus Clonal Protection Program are invaluable," Vidalakis said. "They're the basis of California's citrus sector. Every citrus tree grown in this state, in the orchard or in the backyard, can be traced back to a modern tree in our collection, what we call foundation plant material."

He said the program supplies budwood "for every citrus orchard in California, nearly 30,000 buds a year. And, from each bud, a citrus nursery, which is our primary customer, can make up to 300 trees. In a year to 18 months, that could result in more than 9 million trees."

The state's citrus farmers produce nearly $2 billion worth of fruit annually, with oranges and lemons among California's top agricultural commodities. Citrus is also one of the state's leading agricultural exports. And there's an untold amount of backyard fruit produced in a state where citrus trees are commonly planted for household consumption and ornamental landscaping.

An economic analysis commissioned by California Citrus Mutual concluded that the Asian citrus psyllid "represents potentially one of the most devastating invasive species to ever threaten the California citrus industry."

Ultimately, if the pest were to become established in California, the economic impact is estimated at more than $350 million a year due to fruit and tree damage, increased pest treatments and added regulation. That number doesn't include costs if citrus greening were to become established in the state. In that case, costs would skyrocket.

Richard Lee, research leader for the UC Riverside-based National Clonal Germplasm Repository for Citrus and Dates, also calls the psyllid threat "very serious." The repository, part of the National Germplasm System, holds more than 400 disease-free citrus varieties.

"For all of us, the arrival of the Asian citrus psyllid has serious implications," he said. "If we found ourselves in a quarantine zone, or worse, with infected trees, that would be a nightmare."

The U.S. Department of Agriculture citrus repository contains the genetic material necessary to replicate and maintain healthy citrus stock throughout the United States.

The facility includes a laboratory, four greenhouses and a large screenhouse, recently retrofitted to ensure mesh tight enough to exclude the tiny psyllid. Every citrus plant within the repository has now been tested for the bacterium that causes citrus greening and, much to the relief of all citrus researchers at UC Riverside, it has not been found.

To further protect the state's and the nation's citrus reproduction capabilities, Lee said the entire holding has been replicated as a backup collection and will be stored in a secure location, still to be decided.

Ted Batkin, president of the California Citrus Research Board, which helps fund research at UC Riverside, said the UCR facilities are an important resource for the state's citrus sector and, along with several other university research programs, help in the development of new varieties and in solving pest and disease problems.

He added that the research board is setting up a new rapid-testing laboratory near the UCR campus to aid farmers in early disease detection to guard against the spread of citrus greening. He said proximity to the university's citrus experts and its facilities is an added benefit.

"What the industry and the public need to be concerned about now," Batkin said, "is that this valuable resource is vulnerable to pests and diseases, particularly to Asian citrus psyllid and the bacteria it carries.

"This situation is very serious, but I want to avoid the 'Chicken Little, the sky is falling' syndrome," he said. "If we aren't able to protect the citrus research plantings at UC Riverside, the California citrus industry will continue, but losing it will make things a whole lot harder."

Information on the Citrus Clonal Protection Program at UCR is available at www.ccpp.ucr.edu. To learn about the Asian citrus psyllid and citrus greening online, go to www.californiacitrusthreat.org.

(Kate Campbell is an assistant editor of Ag Alert. She may be contacted at kcampbell@cfbf.com.)

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.