By Vicky Boyd
Although all of the tomato plants infected with tomato yellow leaf curl virus in a Southern California greenhouse were destroyed, plant disease experts say it will be at least another year before they know whether they eradicated the disease.
Part of the concern stems from the virus’ ability to infect other closely related plants, such as malva and nightshade, without showing symptoms, says Bob Gilbertson, a plant pathology professor at the University of California, Davis.
“Our big concern is these symptomless reservoir hosts will continue to maintain the virus,” he told a press conference at the American Phytopathological Society’s annual meeting in San Diego, July 30. “That’s how the virus has been able to persist in Florida. We hope it doesn’t happen in Southern California.”
Considered by some to be the worst tomato virus worldwide, tomato yellow leaf curl virus is endemic to Florida and was confirmed in Texas and Sonora, Mexico, in 2006. It also was confirmed in Arizona in February.
Leaves of infected plants turn yellow and cup upward. If the plants are infected early enough, growth will be stunted and flowers will abort, preventing any fruit set.
It is spread by silverleaf and sweetpotato whiteflies.
Tomato yellow leaf curl virus was discovered for the first time in California in Brawley high school greenhouse in March. Students had contacted Eric Natwick, a UC Cooperative Extension farm adviser, after they noticed their plants were turning yellow and the leaves cupped upward.
UC Davis researchers confirmed the problem was tomato yellow leaf curl virus.
The plants were destroyed, and inspectors from the Imperial County agriculture commissioner’s office surveyed the half dozen commercial tomato fields in the county as well as tomato plants in home gardens.
None of the plants in commercial fields showed symptoms, although plants in several home gardens were infected, Gilbertson says. They were destroyed.
In addition, inspectors surveyed weeds around the greenhouse and found several that showed symptoms. Those, too, were destroyed.
“We are cautiously optimistic that we have contained the outbreak, but this remains to be seen,” Gilbertson says.
Imperial County inspectors will reinspect areas around the greenhouse, residential properties and commercial tomato fields during the 2008 growing season.
In addition, inspectors this season will survey commercial tomato fields in Kern, Fresno and Merced counties. Whiteflies have been found in those counties, but colder weather helps keep populations lower than in the Imperial Valley, he says.
The source of the infestation remains a mystery, although Gilbertson has some theories.
The plants were started from seeds, and tomato yellow leaf curl virus is not a seed-transmitted disease. So it had to come in on infected whiteflies blown in from Mexico, he says.
“These whiteflies are very delicate and can’t fly on their own for more than 7 to 10 miles,” Gilbertson says. “But they can get caught up in wind currents and be blown for several hundred miles. We believewe’re not 100 percent surethese whiteflies were blown into Southern California.”
He says he believes populations of infected whiteflies built up in the Brawley greenhouse, then moved out to residential properties, where they infected plants.
Genetic testing shows the strain of the disease found in the greenhouse is nearly identical to that found in northern Mexico, Gilbertson says.
For more information and to download a flier on the virus, visit the UC IPM Web site at http://www.ipm.ucdavis.edu/PMG/r783103311.html.