Projects Underway for Better Huanglongbing Detection in Infected Trees
By Patrick Cavanaugh, Farm News Director
The big problem with Huanglongbing Disease (HLB), also known as citrus greening, is that an infected tree, despite having no visual symptoms, could quietly be a massive reservoir of HLB. The main vector, the Asian Citrus Psyllid (ACP), could easily pick up HLB from that tree and spread the disease throughout the orchard.
Currently, the main push to contain the possible spread of HLB is by controlling ACP. Yellow sticky traps are positioned near every commercial orchard in California, as well as near non-commercial trees in neighborhoods. Finding a psyllid on a trap triggers mandatory spray programs to help eliminate all psyllids in the area.
“Yes, one of the most critical questions in managing in HLB is the trees can be infected for a long time before you can visually see the ACP there,” said Neil McRoberts, an associate professor of plant pathology, University of California, Davis.
“There is a group of scientists, funded by the Citrus Research Board of California (CRB), working on different early detection methods. Those are methods where you would be able to tell the tree was infected before you could see the tree is infected,” said McRoberts.
The CRB funded field trials in Texas over the last few years that have narrowed the field of competitors to two or three techniques. The CRB will continue to fund the two leading contenders in that race to determine the leading early detection technology.
In the first technique, researchers analyzed all the microbes that live on a non-infected leaf surface and studied how that profile of microbes changes when the tree gets infected. Detecting a change in microbe profile could indicate the tree is not as healthy as it should be.
“Researchers take the microbes that live on an infected tree, extract the DNA from those microbes and run the DNA through a sequencer. The sequencer identifies which types of microbes are there,” he said.
McRoberts said sampling for microbes is easy. Researchers use an industrial-sized swab on the leaf surface. “You literally clean the leaf surface with the swab until it’s squeaky clean, put that swab in a bottle and send it to a lab. When it gets to the lab, they extract the DNA out of it and the rest of the process happens from there.
In selecting which trees should undergo microbe swabbing, McRoberts noted that the ACPs tend to attack the groves from the outer edges, inwards. “At different times of the day, the light will be on different edges of the grove depending on where the sun is and how warm it is. You can target your sampling towards the places where you’re more likely to find it, but still, finding those initial little infections is tough.”
The second method is a technique called metabolic profiling. A newly infected tree starts to produce different proteins and other chemicals in response.
Still another research strategy is analyzing things that stay in the tissue. “There’s a change in the profile of metabolites in the tree. If you run those metabolites through a mass spectrometer, the mass spectrometer will spit out a profile. You can tell the difference between a healthy profile and an infected profile,” McRoberts said.
“This is how the dogs come into the picture. Everybody knows that citrus has an odor. When the trees are infected, the profile of the chemicals and the composition of that odor change. We can’t smell it, but a very sensitive electronic detector device can sometimes pick it up. Remarkably, dogs can pick it up. We think that’s what happening with the canine detectors; they’re picking up some change in the smell of the trees.”
McRoberts said that dogs are amazingly accurate in detecting trees with HLB disease. “The best that we can tell from the trials involving dogs, the false positive rate is less than 1 in 1,000. I’m very confident with the detector dogs,” McRoberts said.
Featured photo: Adult Asian citrus psyllid (Photo by J. Lewis). Courtesy of Citrus Research Board