UC program aids in citrus disease fight

At war with the Asian citrus psyllid since it was found in San Diego County in 2008, California citrus growers and packers have had unprecedented success in slowing the spread of the tree-killing bacteria the psyllid can carry.

People in the citrus business say part of that success relates to the testing and distribution of clean citrus plant material through the University of California, Riverside.

The Citrus Clonal Protection Program at UC Riverside tests clonal material to ensure that citrus varieties introduced into California remain free of pathogens.

Joel Nelsen, president of California Citrus Mutual, said he believes work such as that done by the program has helped defend California citrus from the tree-killing bacterial disease huanglongbing or HLB, also called citrus greening.

Although it was first detected in residential citrus trees in Southern California in 2012, HLB has not yet been found in the state’s commercial citrus groves.

In Florida, Nelsen said, spread of the Asian citrus psyllid and HLB led to the loss of 70 percent of commercial citrus production.

“Florida is the barometer,” he said. “If you contrast that with what we’re doing, our commercial industry is still alive and vibrant 10 years after the discovery of the psyllid.”

He described the Citrus Clonal Protection Program as “a vetting program to determine if certain rootstocks and/or certain varieties can produce successfully in California without carrying this disease.”

Program director and plant pathologist Georgios Vidalakis said he recognizes that a decade of fighting the psyllid and HLB is wearing on citrus growers.

“I tell them, ‘Please hang on, because in California, you are already making history,’” he said. “We are the only place that we’ve got the disease and we’ve got the insect for the last 10 years, and we don’t have an HLB epidemic in our commercial citrus.”

Vidalakis said diagnostic research at the program’s Citrus Diagnostic & Research Laboratory tests about 20,000 samples of citrus material annually.

“The motto of the National Clean Plant Network is, ‘start clean, stay clean.’ If you don’t start clean, the financial investment to produce highquality fruit for the consumer ends up at great risk,” Vidalakis said.

He said diagnostic technology has “increased dramatically” the number of samples tested, leading to the near disappearance of a specific type of pathogen called a viroid.

The program was established in 1956 as the Citrus Variety Improvement Program and is a cooperative program among the university, California Department of Food and Agriculture, U.S. Department of Agriculture and California’s $3.3 billion citrus sector.

Before citrus budwood can enter the United States, it must go through the USDA National Germplasm Inspection Station in Beltsville, Md. After inspection, material is forwarded to UC Riverside under a special permit, where it is tested for pathogens.

“The general category of pathogens that we are looking for is graft-transmissible pathogens of citrus,” Vidalakis said. “Without coming from a program like this, 95 percent of the time, the material will have one or more pathogens in it.”

If researchers find citrus budwood contains pathogens, the material moves through therapy and diagnostics at the nearby Rubidoux Quarantine Facility in Riverside, off campus.

Once the budwood receives a “clean bill of health” from that facility, Vidalakis said, the program’s registered budwood source trees move to the UC Lindcove Research and Extension Center in Exeter. There, 400 clean citrus varieties are available to California nurseries that are licensed to propagate trees, as well as to growers, researchers and citrus enthusiasts.

Eight years ago, the state’s citrus business initiated legislation to create assessments in order to invest in addressing the psyllid and HLB. Nelsen said the sector started with a $15 million program, which increased to more than $25 million — including federal support — because of the expansion of the psyllid.

“We now have a budget of $40 million to do a number of things: educate the homeowner, trap, treat where appropriate and find HLB, which means a lot of lab analysis and sampling,” Nelsen said. “This program has just gotten larger as we continue to fight and suppress the population of Asian citrus psyllid.”

To aid in the fight, the California Citrus Research Foundation and UC Riverside plan to open a new, $8 million Biosafety Laboratory Level 3 facility in Riverside this spring, for testing of infectious materials — needed to conduct research on HLB bacteria. It would be the second BSL3 lab in the state, along with one at UC Davis.