Last year was one many Georgia tomato growers won’t quickly forget, when whiteflies swarmed over their fields and left a destructive virus to ravage their crop.
Having discovered firsthand the consequences of being unprepared for tomato yellow leaf curl virus, growers are turning to University of Georgia scientists to find solutions.
Researchers at the University of Georgia are working to find out more about the insect that transmits the virus, as well as some solutions that will give growers a marketable crop.
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After Georgia growers began finding tomato yellow leaf curl virus in their field several years ago, many have begun spraying every week to reduce the whitefly populations and prevent virus transmission.
Unfortunately, says Rajagopalbabu Srinivasan, a research entomologist with the University of Georgia, spraying often reduces the whiteflies’ natural enemies and increases the populations of the pest.
“Whiteflies have natural enemies, but the sprays are wiping them out,” he says.
“The whiteflies survive the sprays better than their natural enemies. Some whitefly populations are also developing resistance to commonly used insecticides such as imidacloprid.”
Part of the solution, says Srinivasan, is to plant varieties that are resistant to the virus. Growers have had access to several resistant tomato varieties since the early 21st century, but most growers prefer not to grow them.
“Resistant varieties are grown on less than one-third of the tomato acreage in Florida,” he says. “They don’t produce the beautiful tomatoes of the susceptible varieties, and the shelf life isn’t as long.”
But a crop of tomatoes from a resistant variety is better than no crop from a susceptible variety, Srinivasan says.
Problem spreads rapidly
Every year, whitefly peaks coincide with early tomato development, when the plants are most vulnerable to infection. Once a few susceptible plants are infected, tomato yellow leaf curl virus can fill an entire field with stunted plants that produce no tomatoes.
Even though growers spray insecticides, it takes one whitefly carrying the virus only a few minutes of feeding to infect a tomato plant.
Some growers have already planted resistant varieties, but often grow them alongside susceptible varieties. Srinivasan wanted to know if that was a wise practice, or if resistant varieties may in fact harbor the virus and increase the chances that whiteflies would pick up the virus and transmit it to the susceptible plants.
So in 2010, he used funding from a Southern IPM Center IPM Enhancement grant to study how whiteflies interact with both resistant and susceptible varieties, if resistant varieties could harbor the virus, and if whiteflies would transmit the virus from the resistant to the susceptible varieties.
Not only did the resistant varieties become infected, but whiteflies that fed on infected resistant varieties could acquire the virus and transmit it to a nearby susceptible variety.
Srinivasan concluded that growing the two varieties contiguous to each other may actually increase the chance of a yellow leaf curl virus epidemic.
In addition, because infected resistant cultivars show no symptoms, growers would not realize the field was infected until susceptible cultivars were infected. No variety has been developed that is resistant to whitefly feeding.
The initial findings from the Enhancement grant landed him and his Georgia colleagues a larger multi-year USDA Agriculture and Food Research Initiative (AFRI) grant, adding cooperators from South Carolina and Florida. With the AFRI grant, he hopes to find growers optimal solutions that will allow them to have marketable yields of tomatoes that will be desirable to consumers.
To find those answers, he and his colleagues are studying whiteflies more intensely, to understand what attracts them to plants and to discover other plants, such as weeds, that may be sources of whitefly populations and/or the virus.
They also plan to test several methods of whitefly control, including cultural practices like reflective mulch and periods of fallowness.
The researchers will also compare resistant and susceptible varieties for fruit quality, ripening and shelf life. Some of the newly-developed resistant varieties seem to be more promising in terms of marketable quality, so Srinivasan hopes growers may be more willing to grow them without adding susceptible varieties to the field.
Their analyses will ultimately result in a decision support website that will allow growers to plug in several variables to find out what varieties they should plant.
“For instance, they could choose an insecticide and a mulch and include their history of leaf curl virus, and see what options they have,” Srinivasan says. “The site would tell them how much risk they’ll have with the options they choose and how much they would gain or lose by planting one variety over another.”
In a given year, a grower could save thousands of dollars per acre by using a combination of cultural controls and resistant cultivars with significant reductions in pesticide use. Fewer pesticides would mean more whitefly natural enemies, leading to fewer whiteflies and a reduced possibility of infection.
For growers who have already lost a crop to yellow leaf curl virus, the website won’t come a moment too soon.
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