Georgia tobacco growers have a much-needed new tool for battling tomato spotted wilt virus in 2003, with Actigard being granted a third-party special local needs label. Tobacco plants treated with Actigard prior to transplanting have shown increased resistance to tomato spotted wilt virus.
Actigard is not a pesticide, explains Alex Csinos, University of Georgia plant pathologist. “It's a material that activates a plant's natural defense mechanism to combat tomato spotted wilt virus. It's difficult to predict the amount by which the disease will be decreased because disease occurrence is dependent on several factors, including level of virus pressure in any given field,” he says.
Before tobacco transplant growers can apply Actigard to plants, they must obtain a special local needs label from the Flue Cured Tobacco Stabilization Corporation. Applications for the label are available at local county Extension offices or may be downloaded from the University of Georgia Tobacco Website at www.georgiatobacco.com.
“Producers should try and get their label in hand by Feb. 15 so they have it in plenty of time to spray the tobacco plants five to seven days before transplanting them to the field,” says J. Michael Moore, University of Georgia Extension agronomist. Growers should check with their local Extension agents or University of Georgia tobacco specialists for application rates, he adds.
“It is very important that producers follow the recommended application rates,” says Moore. “Over time, applying Actigard can cause severe plant damage. Growers are solely liable for the use of Actigard for this purpose — not Flue Cured Stabilization, the Extension Service or Syngenta, the maker of the material.”
Tobacco specialists continue to recommend that producers spray transplants with Admire three days before transplanting, but stress that growers should not mix the two chemicals and spray at one time.
Individual field incidence rates of tomato spotted wilt virus are reported to be as high as 85 percent of the plants in a field, he says. Many fields, says Moore, had 50-percent incidence levels in less than six weeks after transplanting.
“Yield losses generally are expected to be approximately one-half of the symptom incidence level,” he says. “As this rate, the 2002 season may be as bad as the 1999 season when tomato spotted wilt incidence was estimated at 35 percent of stand and yield loss was 18 percent. Hundreds of tobacco acres in Georgia were replanted this year due to damage from tomato spotted wilt virus.”
Once a plant is infected with tomato spotted wilt virus, there's not much a grower can do to save it, notes Moore. Research, however, has revealed some promising preventative treatments.
“Tomato spotted wilt virus generally is less in fields where Admire is used, and fewer plants are affected when Admire is applied as a drench to plants while they still were in the greenhouse. Transplant water treatments were less effective than tray drench treatments.
“Experimental treatments using Actigard as a foliar spray in the greenhouse were more effective than those treatments applied after transplanting. The incidence of tomato spotted wilt virus was reduced by Actigard foliar treatments. Actigard in sequence with Admire in the greenhouse appears to have given increased control over either individually,” says the agronomist.
Growers also saw an advantage this past year in plants grown in a greenhouse as opposed to those grown in field beds, he says. “The source of the transplant doesn't have as much effect on severity of the virus as which fields you are planting into. Greenhouse plants don't wilt down in the field as severely as those grown in field beds. Greenhouse plants have a root-ball that can be taken to the field.”
University of Georgia researchers continue to look for reasons why the incidence of tomato spotted wilt virus is worse in some years than others, says Moore. One possible correlation might be the amount of rainfall in March. It appears, he says, that levels of the virus are less severe when more rainfall is received in March.
“This possibly might affect the survival and maturity of thrips as they emerge as juveniles.”
Georgia's tobacco production in 2002 decreased 8.56 million pounds from 2001, to 55.7 million pounds. This represents a 13 percent reduction from 2001's 64.2 million pounds. Tomato spotted wilt virus and other diseases took a toll on this past year's crop. Yields averaged 2,100 pounds in 2002, compared with 2,460 pounds per acre in 2001.
Individual field incidence rates of tomato spotted wilt virus were reported to be as high as 85 percent of the plants in a field in 2002, says Moore. Many fields, he adds, had 50-percent incidence levels in less than six weeks after transplanting.
“Yield losses generally are expected to be approximately one-half of the symptom incidence level,” he says. “Hundreds of tobacco acres in Georgia were replanted last year due to damage from tomato spotted wilt virus.”
Actigard 50 WG contains the active ingredient acibenzolar-S-methyl and would be applied at a maximum of 0.5 ounce per 25,000 to 50,000 plants in plant houses, float houses and outside seed beds, and a rate of 0.5 ounce per acre for post-transplant field application. Actigard has been evaluated since 1997 for management of tomato spotted wilt virus in Georgia.