September 28, 2013

 Researchers Get Closer to Early Detection
Of HLB-Infected Citrus Trees

By Patrick Cavanaugh, Editor

Using Volatile Organic Compound sniffers with a suitcase-size gas chromatograph, and magnetic resonance spectroscopy as well as DNA sequencing of known huanglongbing (HLB)-infected citrus trees, scientists are getting closer to a very early diagnosis that could help with early detection and possible cures of the disease.

It’s all happening while quarantines expand in the Central San Joaquin Valley after the HLB vector, the Asian Citrus Psyllid (ACP), has been found on sticky traps or live in trees.  None of the collected ACPs have tested positive for HLB. Still, the only known tree that has been positively infected with HLB was a lone tree in a neighborhood in Hacienda Heights, near Los Angeles, in March 2012.

“We are just verifying how accurate these early detection techniques are, and it is being done by scientists at UC Davis and UC Riverside,” noted Beth Grafton Cardwell, director of Lindcove Research and Education Center, and research entomologist at UC Riverside.

The Citrus Research Board focusing on a pre-systematic detection platform is funding all the work. The tested plant tissue is from the infamous Hacienda Heights tree, the only known tree in California ever to be infected with HLB. That tree was eventually destroyed, but not USDA researchers took many samples.

Scientists are collecting material from trees and testing with different antibodies and small RNA to determine if these early detection systems really do work,” Mary Lou Pole, Vice President Science and Technology Citrus Research Board, based in Visalia. “We have found some samples to be suspect-positive, but the only way to confirm this is through the USDA’s PCR test, and of course it comes up negative. And of course that’s the whole point of early diagnostics, and that’s to pick it up before the PCR test.”

Some homeowners have actually taken out about five trees, in total, after they were suspect with the early diagnostics due to early Volatile Organic Compound (VOC) detection.

“The PCR test can only be positive if it tests tissue where the bacteria is, which could be hit or miss, so we are hoping to find a way to early-detect an infected tree with other methods,” said Pole.

“Some other diagnostics include host plant response proteins, which are secreted by bacteria that move systemically throughout the tree,” Pole explained. “So, instead of looking for the bacteria, we are looking for a signature of molecules associated with the bacteria that is moving through the plant.”

“We are getting close to finding a good way to detect early, and information should be coming in near future,” said Carolyn Slupsky, UC Davis Food Science Technology Associate Professor and Nutritionist. “We want to know the metabolic pathways, the changes in the plant, before the plant starts to yellow,” she said.

“Plants are always producing metabolites, and an HLB infected tree will give off a certain pattern of metabolites that can be detected by chemical analysis using magnetic resonance spectroscopy,” Slupsky said.

“We are also working on understanding when the pathogen infects the tree. Ultimately, we want to find a way to kill the bacterial pathogen that causes HLB,” Slupsky said.

Diagnostic tools are being developed that should help in the early detection.

Cristina Davis, professor in the UC Davis Department of Mechanical and Aerospace Engineering, and Abhaya Dandekar, professor in the UC Davis Department of Plant Sciences, collected samples of VOCs emitted from HLB-infected trees in Florida every month for a year in order to “train” the mobile sensor to recognize the “smell” of HLB.

“The idea is to extract a group of compounds that create the signature for the presence of HLB,” Dandekar said. A software program develops an algorithm that lets the machine know it is detecting HLB. Davis is working with Applied Nanotech, Inc., in Texas to commercialize this artificial nose.

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