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Research aims to counteract citrus disease

Issue Date: February 24, 2016
By Kate Campbell
To protect California citrus groves from the plant disease HLB, researchers focus on fighting the insect that carries the disease and on early detection of HLB itself.

With an emphasis on early detection, California citrus farmers are racing along with researchers to find a cure for the deadly bacterial plant disease huanglongbing, also called HLB or citrus greening.

They are working to control the Asian citrus psyllid, an invasive pest that can spread the disease, while maintaining the health of their trees. They also help fund research while tracking the latest scientific findings. As more residential citrus trees test positive for HLB, the pace of research is picking up.

The U.S. Department of Agriculture announced in early February it had awarded $20.1 million nationally in grants for university research and cooperative extension projects to help fight the disease.

Beth Grafton-Cardwell, director of the University of California Lindcove Research and Extension Center in Exeter and a leading expert on both the Asian citrus psyllid and HLB, said California research emphasizes psyllid control and techniques for early detection of the disease.

Most of the disease research is taking place in Florida, where HLB is widespread. USDA said the disease has affected more than 75 percent of Florida citrus crops and threatens production across the United States.

HLB has also been found in Georgia, Louisiana, South Carolina and Texas, as well as on several residential trees in Southern California. A total of 15 U.S. states or territories are under full or partial quarantine due to the presence of the psyllids, which signals the eventual arrival of HLB.

Grafton-Cardwell said researchers in California are working on advanced biochemical PCR (polymerase chain reaction) techniques. The approach uses molecular genetics to target minute quantities of DNA or RNA to indicate presence of HLB in a tree, but has its limitations.

"One of the problems with this approach is that if an infected psyllid first lands on a tree leaf and injects the bacteria, it creates a localized infection, just a spot," she explained. "It could take years for the bacteria to move throughout the tree."

She said the goal is to find the bacterium in trees much closer to the time of initial infection. Early detection would allow farmers to take timely action and pull out infected trees or even those suspected of infection, she said, noting regulators will likely wait for positive test results before ordering pullouts. Researchers are concerned that may be too late to stop the infection's spread.

There are several early-detection research projects going on now in California, she said.

Researchers are working on developing a mobile VOC (volatile organic compound) sniffer device that could be used in citrus groves to pick up traces of HLB off-gassing from infected trees. Devices being tested are sensitive to VOCs below parts-per-billion levels and are relatively compact, portable and low-cost, Grafton-Cardwell said.

"That system is about a year or two away, which is much closer than the estimated 10 years to 15 years it will take to get disease-resistant rootstock," she said. "The (VOC) system is working now, but we need to figure out how early is early in terms of detection."

Another early-detection approach involves testing citrus leaves to detect metabolites produced by the bacteria or the tree. In Florida, dogs are being trained to detect the presence of the bacterium. California researchers are also testing a twist-tie system that would indicate HLB presence in trees, and scientists are developing field DNA test kits.

"If we could move up detection techniques even by six months it would help," Grafton-Cardwell said.

High-volume laboratories for testing psyllids, citrus leaves and plant tissue for HLB play an important role in the disease-detection process, she said. These labs have the staff and equipment to handle more material than other kinds of testing facilities.

In Florida, the situation got out of control because symptoms were not apparent, according to Greg McCollum, a USDA research plant physiologist. He told researchers attending a summit on the pest and disease last fall that once the HLB pathogen was detected in a Florida citrus grove, "we found the pathogen everywhere."

In California, with more than 270,000 acres of commercial citrus groves at stake and extensive ornamental citrus plantings, he said the state is ahead of the curve with its early-detection research—and that he hopes it can stay that way.

"Currently, the standard three-pronged approach—controlling psyllids, removing infected trees and only planting clean nursery stock—is still the most important management strategy," McCollum said.

The Visalia-based Citrus Research Board is contributing funds to a number of early-detection research projects, board President Gary Schultz said. The board is concentrating some of its funding on field testing of early-detection techniques by growers.

Schultz said board subcommittees want to determine what early-detection capability might mean in terms of farm economics. Grower costs might include new equipment, testing services, tree pullout and disposal expenses, soil treatment, replanting and production losses.

"Reliable data on the potential economic impact of fighting the disease in the coming decade is currently being evaluated, " Schultz said.

"The experience in Florida has been helpful in terms of timing issues in the pest and disease progression," he said. "We've asked Florida and Texas growers what they would have done differently. There has been excellent collaboration between growers in the citrus states about how to handle this problem on the ground."

The psyllid and HLB are a major priority of the main citrus-producing states and it's a priority of the USDA and leading universities, Schultz said.

"Where we'll be in the next three to five years is hard to say, but we wake up every day and work on it," he said.

(Kate Campbell is an assistant editor of Ag Alert. She may be reached at

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

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