UC program aids in citrus disease fight


Issue Date: April 4, 2018
By Christine Souza
Greg Greer, manager of the Rubidoux Citrus Quarantine Facilities for the University of California, Riverside, checks on citrus samples stored in an incubator. The facility works with the on-campus Citrus Clonal Protection Program to assure citrus budwood remains free of pathogens.
Photo/Christine Souza
Nichole Ginnan, a graduate student at the University of California, Riverside, removes leaves from citrus budwood and prepares a sample for DNA extraction.
Photo/Christine Souza

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 high-quality 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 U.S., 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.

Riverside County Agricultural Commissioner Ruben Arroyo said HLB has been confined so far to residential citrus trees in Los Angeles, Orange and Riverside counties. Because of psyllid finds in a number of citrus-growing counties, state and federal quarantines have been established to help prevent the spread of the insect and to try to keep the disease out of the state's commercial groves.

"We're doing everything that we can possibly do to prevent the spread of the psyllid and the disease," Arroyo said. "If we continue to try to prevent the movement, hopefully by that time, the research will have caught up and identified a rootstock that is disease-tolerant but still gives a good-tasting piece of fruit."

Riverside County citrus grower John Gless has property inside a psyllid quarantine area. Being in a quarantine zone, he said, involves an added treatment prior to harvest and requires added measures, such as packing fruit inside the quarantine zone.

"We're going to have to learn to live with (the psyllid), but we are putting up a good fight as an industry," Gless said.

For his farm, Gless said he plans to plant new acres of citrus and "keep thinking positively," adding, "If we can get some resistant rootstocks, that's going to be the key to the future."

(Christine Souza is an assistant editor of Ag Alert. She may be contacted at csouza@cfbf.com.)

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