Preparing for a New Citrus Pathogen

WASHINGTON, Dec. 28--A nasty bacterial disease of citrus can be quickly detected by a sensitive genetic test, if the pathogen enters this country from abroad, Agricultural Research Service administrator Floyd P. Horn announced today.

Some 88 million orange trees in Sao Paulo, Brazil, are infected by a Xylella bacterium that has reduced yields in Brazilian experimental orchards from 37 to 100 percent, depending on the orange variety. The disease is citrus variegated chlorosis, CVC for short.

"If this pathogen sneaks into the United States, federal and state regulators will be ready," said Horn. "Thanks to the work of our researchers, they'll know how to handle it."

Florida and other citrus-growing states are home to leafhoppers and other insects that could transmit CVC from tree to tree. But the pathogen also hides in dozens of woody plants and trees. The danger is that it could conceivably enter the U.S. in another plant species undetected until it appears in citrus, according to researchers with ARS, the chief research arm of the U.S. Department of Agriculture.

ARS plant pathologist John Hartung said the CVC pathogen is only one strain of the bacterium Xylella fastidiosa. Telling the strains apart is one of the chief obstacles to rapid, accurate detection of CVC.

Hartung led development of the genetic test at ARS' Fruit Laboratory in Beltsville, Md. The test distinguishes the citrus pathogen from other strains of X. fastidiosa, including one that infects Brazilian coffee plants.

"The coffee pathogen is so similar to the citrus pathogen that a sensitive DNA test like PCR is the only way to distinguish between the two," said Hartung. PCR, short for polymerase chain reaction, enables researchers to distinguish one pathogen from another by comparing unique segments of DNA. The method is similar to those used by law enforcement agencies.

"The citrus industry needs a good diagnostic procedure to understand how CVC spreads and to develop effective control procedures," Hartung noted. Two or three other PCR tests have been developed since the ARS test, he added.

Florida state scientists who regulate imported plants and certify bud wood say PCR testing for CVC is in their future plans. The Brazilian citrus industry is adapting PCR testing to certify that bud wood used to propagate trees is free of the CVC pathogen.

Hartung is collaborating with Brazilian plant pathologists to learn how to deal with the pathogen while it's confined to Brazil and Argentina. The state of Sao Paulo has twice as many citrus trees as Florida. Brazilian exports fill the U.S. demand for orange juice that Florida can't meet.

Experiments are underway in the quarantine greenhouses at Beltsville to see if the citrus strain infects coffee and vice versa. Preliminary findings in Brazil suggest that they don't.

ARS plant epidemiologist Tim Gottwald and colleagues at the U.S. Horticultural Research Laboratory in Orlando, Fla., work in ARS' exotic plant disease program--a watchdog for diseases that threaten U.S. agriculture. It's not the legally imported bud wood that worries Gottwald as much the plant material that travelers from abroad carry in their luggage and plant in groves or yards, he said.

"The first line of defense is exclusion," said Gottwald. "But given enough time, all pathogens eventually move around among the citrus growing areas. We're trying to have the research and control method in place so that if a crisis occurs, we'll be prepared."

Scientific contacts: John S. Hartung, ARS Fruit Laboratory, Beltsville Agricultural Research Center, Beltsville, Md., phone (301) 504-6374, fax (301) 504-5062, jhartung@asrr.arsusda.gov; and Timothy R. Gottwald, ARS U.S. Horticultural Research Laboratory, Orlando, Fla., phone (407) 897-7347, fax (407) 897-7309, gott@magicnet.net.