By Len Wilcox
Early detection of citrus trees infected with huanglongbing (HLB) has become the target of researchers around the world, and scientists from the University of California (UC) now believe they have the answer. Early detection is vital for identifying HLB-positive trees before physical symptoms of the disease appear and it spreads throughout a grove.
SMELLING THE SICKNESS
UC Davis scientists, led by Cristina Davis, have developed a technique to detect HLB-diseased citrus that involves identifying the scent of volatile organic compounds (VOCs). Through the services of a commercial laboratory developed by the scientists, growers are now able to identify HLB-diseased trees quickly and cost effectively, according to Davis.
The process utilizes early detection of VOCs released by diseased plants.
“All living things release VOCs,” Davis explained. “These organic compounds are chemicals that join together to make up specific, signature odors, such as the smell of orange blossoms or the pungent odor of garlic. The VOCs that create odors that are detectable by humans are at high levels. The scent of infected trees is well below human detection.”
Davis, a professor who is the head of the UC Davis Bioinstrumentation and BioMEMS Laboratory, started work to identify HLB VOCs in 2008. Testing proved the concept, which scientists evaluated over four growing seasons.
“We are ready to make this available to growers,” Davis said. “We can have an answer within days of receiving the sample.”
In the laboratory, cutting-edge research was used in the design and implementation of analytical sensors. The sensors can rapidly, accurately and specifically detect extremely small concentrations of chemical and biological materials. Such technologies can make an impact on everything from public health to national defense.
One of the first areas to benefit from this research is the citrus industry, with the lab’s work on early detection of HLB.
“We tested lemons, Valencia and grapefruit in Texas, Florida and California, in the field and the greenhouses,” said Davis. “The signal changed over time, but we could still lock in on it. After numerous field trials, we proved we could identify HLB 90 percent of the time.”
According to Davis, the process for growers is simple. They can contact the new lab (XTB Laboratories, phone 530-754-9004) and request a test kit. The kit is a collection device that the grower places in a tree. Collection is automatic, and the grower ships the collector back to the lab for testing.
IMPROVING ON EARLY DETECTION TECHNIQUES
HLB, also known as citrus greening disease, has become one of the greatest challenges for citrus growers around the world. It is a devastating disease for which no consistent cure has been found, and it spreads quickly through an infected area.
Thus, early detection and quarantine of infected trees are vital moves to prevent the disease from invading HLB-free, citrus-producing regions. But, until now, early detection has been extremely difficult. At present, visual inspection is the most common method to diagnose citrus greening. However, once the disease is observed, it is too late to save the tree, and perhaps an entire grove.
Also, visual inspection is problematic. It is open to subjective interpretation, and diagnostic errors can be higher than 30 percent. Diagnosis may be made more difficult by other biotic or abiotic plant-health problems.
Currently, early disease detection relies primarily on field scouting for disease symptoms, such as yellow shoots, leaf blotchy mottle, lopsided fruit with green color remaining on the end, and high levels of starch in the leaves. The symptoms can initially be confused with diseases like citrus blight and certain nutrient deficiencies.
Unlike VOC testing, leaf testing will not detect the pathogen in the tree for three to nine months after infection, and the symptoms don’t show up in the tree for a year or more after infection. Yellow shoots, leaf blotchy mottle and lopsided fruits with color inversion and aborted seeds are all characteristic of HLB, but they do not always occur together in the same tree. They can be distorted or masked by symptoms of other diseases or, in some cases, induced by conditions unrelated to HLB. Thus, early scientific testing is needed to follow up on visual observations to determine if a tree is infected.
While VOC testing is 90 percent certain, Davis suggests growers should confirm the presence of HLB before destroying the tree. Follow-up testing should be done by established methods, such as electron microscopy, serology, DNA probes, enzymatic assay, enzyme-linked immunosorbent assays, conventional polymerase chain reaction (PCR) and quantitative PCR, which are all used for the diagnosis and confirmation of HLB.
Len Wilcox is a freelance writer in Sanger, California.
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