PROSSER, Wash. – In the not too distant future, grape growers may be able to detect virus-infected grapevines in the field using a portable sensor that evaluates changes in the light reflecting properties of leaves. The sensor will speed detection and save money now spent in random testing to find infected vines.
Grapes are susceptible to numerous diseases. One of the most baffling is grapevine leafroll, a complex malady associated with about nine viruses. It can delay ripening, reduce yields by 50 percent or more and harm fruit quality. Once infected, vines cannot be treated. The disease can be eradicated only by pulling and replacing the vines, an expensive proposition.
In the first study to report the possibility of using leaf spectral data for virus disease diagnosis in a perennial woody species, scientists at the Washington State University Prosser Irrigated Agriculture Research and Extension Center found differences in the light reflectance data from leaves of infected and healthy leaves of two red grape varieties.
Eileen Perry and Francis Pierce, who are, respectively, assistant director and director of WSU’s Center for Precision Agricultural Systems, analyzed data collected with a spectral radiometer from leaves of two red grape cultivars showing symptoms of grapevine leafroll 3, one of nine viruses associated with grapevine leafroll disease in Washington. The reflectance properties were compared with leaves from non-infected plants.
Naidu Rayapati, a grape virologist, compared the analysis with results generated by RT-PCR, a molecular diagnostic tool, to learn whether any spectral features could be correlated with the disease in living plants. “PCR exponentially amplifies genomic segments of the virus into millions of copies so we can visually diagnose the presence of the virus,” Rayapati said. “It helps you find a needle in a haystack.
“We have found in two grape varieties–Merlot and Cabernet Sauvignon–that there are certain differences in the wavelength of the spectrum that are unique to a disease. We would like to see if there are any differences in white grape varieties that don’t show any changes in pigmentation,” he said.
The scientists have since begun analyzing light reflectance data for Chardonnay white grapes. “The Chardonnay is not as symptomatic visually,” Perry said.
In late summer, the leaves of red grape varieties turn red with the principal veins in the leaves remaining green. Symptoms are less pronounced in white grapes, evidenced by a slight yellowing and cupping of the leaves in some varieties. Concord grapes may show no symptoms at all.
Short of laboratory testing, there is no way to know for certain if a suspect vine is infected. Two laboratory methods are used currently, and both involve destructive sampling.
“You have to go out to the vineyard, collect samples to be tested, bring the samples to the lab where it is ground and processed for detection,” said Rayapati.
Costs for testing can add up as well. The most reliable test–polymerase chain reaction–enables researchers to identify the DNA of low concentrations of viruses. It costs in the neighborhood of $40 per PCR assay, per virus, per sample.
“If you have one sample and you want to check for six different leafroll viruses, that would cost $240,” Rayapati said.
The end product of the research could be a handheld instrument that would enable growers to screen plants in the field. “Ideally, you wouldn’t have to go through and grab leaves randomly off vines or select vines randomly,” said Perry. “You would have a better idea of which plants were infected or likely to be infected.”
“Even if we can achieve 80 percent success with this kind of technology,” Rayapati said, “it will help us tremendously in terms of cutting down the cost and speeding the process of testing.”
The study was published online Jan. 19 as an Article in Press in the peer-reviewed journal “Computers and Electronics in Agriculture.” It can be reviewed at http://tinyurl.com/grapesensor.
Perry will begin new duties as a research scientist with the Victoria Department of Primary Industries in Australia in early March. She will continue to work with Rayapati and Pierce on this project
The work is being funded by WSU wine grape funds.