In the midst of their worst losses ever to tomato spotted wilt virus, Georgia tobacco growers need some good news about this killer disease. And University of Georgia scientists are certain they have it. Help, they say, is on the way.
Preliminary results from a second year of studies are confirming what the first year's research revealed. Treating tobacco plants early with a combination of two chemicals will dramatically reduce infections of spotted wilt. In fact, it can almost eliminate it.
"We're quite confident of what we have with this treatment," said Alex Csinos, a plant pathologist with the UGA College of Agricultural and Environmental Sciences.
One of the products, Admire, is an insecticide already labeled to control flea beetles and aphids in tobacco, Csinos said. The other, Actigard, is an exciting addition that isn't yet labeled for tobacco.
Plant Defense Activator
"We believe it will be labeled for use on tobacco next year," Csinos said. "It's not a pesticide. It's a plant defense activator. It doesn't kill anything, but acts much the way a booster shot for flu works with people."
Most plants have natural defense mechanisms to ward off diseases. Actigard boosts those natural defenses. "It gives the plant the opportunity to defend itself," Csinos said.
Because it's not a pesticide, it's an extremely safe product, too. Labeled in Europe for use on wheat, grapes and some vegetables, "Actigard has no effect on humans," Csinos said.
Actigard alone greatly reduced spotted wilt in the UGA research. But the one-two punch of the two products together was deadly.
Admire, Actigard: Deadly Duo
The UGA studies were conducted at four locations each year. Each trial compared plants left untreated with those treated with Admire alone, Actigard alone and a combination of the two.
The scientists gave the treated tobacco seedlings a "tray drench" treatment in the greenhouse first. They added three weekly sprays after the seedlings were transplanted to the fields.
"At one location, 30 percent of the untreated tobacco plants were infected with spotted wilt," said Hanu Pappu, a UGA molecular virologist who has worked with Csinos on the two years of research. "With the Admire treatment, the rate was 12 percent. It was 5 percent with Actigard and only 1 percent with the combination."
'Possibility of True Control'
The second-year figures, Pappu said, are based on plants with symptoms of spotted wilt. His exhaustive studies of plant samples will eventually show the precise percentage of infected plants, with or without symptoms.
The early results are enough, though, to confirm what the scientists found in the first year's results. In every case, he said, the combination was better than either product by itself.
"This gives us the possibility of true control for the first time," said Paul Bertrand, a UGA Extension Service plant pathologist. "All we've had to this point is some moderate level of suppression of the disease."
1999 Disease Damage Heavy
Suppression certainly hasn't been enough this year. Bertrand figures the virus has killed 35 percent to 45 percent of the state's tobacco plants.
Tobacco growers are allotted a certain number of pounds of "quota" leaf they can grow. They usually plant 10 percent to 20 percent more than that to be certain they can make their quota. But the buffer hasn't been big enough this year.
"More tobacco farmers are filing for crop insurance than in the past 10 years," Bertrand said. "The crop isn't in yet, but I suspect that 50 percent to 60 percent of the growers won't make their pounds."
Drought and other diseases have hurt the crop, too. "But spotted wilt is responsible for 80 percent of the shortage," Bertrand said.
Could Save Millions of Dollars
In 1997 -- the worst year yet -- spotted wilt cost Georgia growers $12.7 million, or about 8 percent of the $158 million crop. This year, the losses could be much higher.
Had growers been able to use the Admire-Actigard treatment this year, though, losses could have been much lower. "If these studies held true, we'd be looking at a 5-percent to 10-percent stand loss and no loss of pounds," Bertrand said.
(Dan Rahn is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.)