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Biological Chemistry

Detecting Citrus Blight

Agriculture: Portable spectrometer identifies signature volatile organic compounds before it’s too late

by Sarah Webb, special to C&EN
February 28, 2014 | APPEARED IN VOLUME 92, ISSUE 9

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Credit: Hilda Gomez/USDA
Citrus greening disease damages fruit by causing it to ripen backward, leaving green at the base.
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Credit: Hilda Gomez/USDA
Citrus greening disease damages fruit by causing it to ripen backward, leaving green at the base.

Citrus greening disease, a bacterial tree infection, has devastated citrus groves throughout the U.S. and the world. Citrus growers have few treatment options or methods for early diagnosis.

Now, researchers report a portable spectrometry technique to detect signature volatile organic compounds of the disease before visible clues develop and when it’s still possible to keep the infection from spreading (Anal. Chem. 2014, DOI: 10.1021/ac403469y).

Citrus greening disease, also called Huanglongbing (HLB) disease, is caused by Candidatus Liberibacter species, which can live in trees for years, allowing them to silently infect neighboring trees. By the time one tree’s leaves begin to yellow and its fruit is damaged, it’s too late to save nearby trees. If growers could detect the disease early, they could remove infected trees before they cause irreparable damage.

Mechanical engineer Cristina E. Davis of the University of California, Davis, has been developing a portable chemical-sensing system, based on gas chromatography/differential mobility spectrometry, that sniffs out VOCs produced by organisms. She and her colleagues placed the spectrometer next to the leaves of citrus trees in a greenhouse and saw dramatic differences in the combination of VOCs released from infected trees as compared with healthy ones. They also measured the VOCs released from naturally infected trees in an outdoor grove in Florida over a period of 16 months.

Using these data, the researchers 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.

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 conducting additional studies to pin down how early the device can reliably detect HLB infection.

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