Portable Spectrometer Sniffs Out Citrus Blight

Citrus greening disease has devastated citrus groves throughout the U.S. and other parts of the world, with few treatment options or ways to spot infection early. To help curb the disease’s spread, researchers report a new way to detect the disease before visible clues develop. They use a portable spectrometry method to screen for volatile organic compounds released by infected trees (Anal. Chem. 2014, DOI: 10.1021/ac403469y).

Citrus greening disease, also called Huanglongbing (HLB) disease, is caused by a bacterium, Candidatus liberibacter, that can live in trees for years, allowing it to silently infect neighboring trees. By the time one tree’s leaves begin to yellow and produce damaged fruit, it’s too late to save nearby trees. “It’s almost a perfect storm of a disease,” says Cristina E. Davis of the University of California, Davis. So as scientists search for a cure, they’d also like to have a way to detect the disease early, so that growers can remove infected trees before they cause irreparable damage.

To detect citrus greening, growers have relied on polymerase chain reaction methods to look for bacterial RNA in samples from trees. But the method is expensive and time consuming, and can give false negatives because the pathogenic bacteria don’t distribute evenly within a tree’s tissues, Davis says.

Davis, a mechanical engineer, has been developing a portable chemical-sensing system based on gas chromatography/differential mobility spectrometry that sniffs volatile organic compounds (VOCs) produced by organisms. The gaseous chemicals waft into the device, and it identifies them by their movement within an electric field.

Plants constantly release VOCs into the air, and changes in the mix of compounds can indicate changes in their health. The problem of citrus greening was an important one in plants, she says, so Davis collaborated with citrus researchers to see if her team’s method could help.

First, they placed the spectrometer next to the leaves of citrus trees in a greenhouse in Lake Alfred, Fla., and saw dramatic differences in the combination of VOCs released from infected trees compared to healthy ones. Then, over 16 months, they studied the VOCs released from naturally infected citrus trees in an outdoor grove in Florida, which allowed the researchers to monitor changes in a more realistic setting, including seasonal variations.

Using the data from these experiments, Davis and her colleagues constructed a statistical model of the patterns of VOCs coming off healthy and diseased trees. They then used this model to accurately predict the infection status of all the trees tested in another orchard, where citrus greening disease had begun to take hold. With additional studies, they identified some of the most significant biomarker compounds released by trees at different stages of infection.

The work is impressive and provides a technique that’s nearly ready for practical application in the field, says Nigel B. Perry, a senior scientist with Plant & Food Research, a government-funded research institute in New Zealand.

The instrument is still a prototype and will require further development for routine use by growers, Davis says. She and her colleagues are now working on additional studies to make faster, cheaper measurements, and to pin down exactly how early the device can reliably detect HLB infection. They also think the method could prove useful for monitoring other agricultural diseases.