The worst-ever incidence of tomato spotted wilt virus in North Carolina has flue-cured tobacco growers east of I-95 worried, says a North Carolina State University Extension plant pathologist.
At least two counties, Bertie and Duplin, are looking at 20 percent yield reduction from the virus.
“The virus is really bad in a strip where we have the highest concentration of tobacco in the state — east of I-95,” says Tom Melton, North Carolina State Extension plant pathologist, who notes the problem is not widespread across the state.
South Carolina producers are facing a TSWV “epidemic as far up as Kingstree and Turbeville,” says DeWitt Gooden, Clemson University Extension tobacco specialist. “It's been a problem in the southern part of the state for several years. This is the first year of the widespread incidence in our tobacco growing area,” he added.
“TSWV is very bad in this area,” reports Bryant Spivey, Duplin County, N.C., Extension agent. “In the worst-case scenario some fields have incidence up to 60 percent. All fields have at least some virus.”
Spivey estimates 15 percent to 20 percent total tobacco stand loss in Duplin County. The virus has also hit Pitt, Bertie, Jones, Pender and Onslow counties hard.
Melton says TSWV has been an increasing problem over the past several years in North Carolina. “Ten years ago, we would find the virus in a field or two at 25-percent to 30-percent incidence,” Melton says. “In the last four or five years, we've had TSWV in levels consistently higher than 10 percent.”
There is no cure for the virus. Once a plant is infected, it eventually dies. Thrips spread the virus.
“At this point in the season, farmers can do nothing — only prepare for next season,” Melton says. Admire in the greenhouse is the only choice growers now have. Research with Admire in combination with Actigard in the greenhouse has shown a reduction in TSWV incidence. Also, experimental treatments of Actigard as a foliar spray in the greenhouse were more effective than those applied after transplanting.
In his research and visits to farmer fields, Melton has noticed trends about fields with the most incidence of TSWV.
Burning down fields adjacent or close to tobacco land may worsen the problems. “Farmers need to burndown their fields with herbicides that are near tobacco land about a month or so ahead of transplanting to allow the thrips to move out of the field,” Melton says. “Burning down a field within a week or two of transplanting may allow the thrips to move back into the (tobacco) field.”
While there's no cure for the virus, scientists are continuing the search for answers. In North Carolina, for example, Melton is studying the biology of the thrips, the interaction between weeds and thrips and the virus to gain a better understanding of the problem.
Work also continues on developing plant resistance to the virus, “but I'm not real encouraged” with the resistance avenue, Melton says. “Resistance to TSWV, with the exception of peanuts, has not held up in other crops.”
In Georgia this season, some 20 percent of the crop is infected with the virus.
“In terms of dollars and pounds, we may lose more than Georgia to TSWV this year simply because we grow more tobacco,” Melton says.
Tobacco sucker control season begins
Even with delayed growth from drought and tomato spotted wilt virus, chemical sucker control programs should be under way across most of the Georgia-Florida tobacco belt.
Suggested programs, says University of Georgia Extension tobacco specialist J. Michael Moore, include starting early (when 30 to 40 percent of the plants are considered to have reached the early button stage) with at least a 3 percent solution of a fatty alcohol. Additional applications of fatty alcohol should be applied five to seven days later with increasing concentrations of 4 and then 5 percent, he says.
“After this point, the first harvest should occur, before maleic hydrazide (MH) is applied,” says Moore. “Five to seven days after the last application of fatty alcohol, a three-way mix of 5 percent fatty alcohol, 2.25 pounds of active MH — available in a variety of solutions and dry concentrations — and 2 quarts of Prime-Plus may be applied as the fatty alcohol solutions, to run down the stalk.”
This program, he adds, should result in excellent sucker control and should minimize MH residues in the cured leaf.
“Growers are urged to limit the use of MH and to supplement MH with applications of the fatty alcohols and Prime-Plus in order to reduce the residues of MH in tobacco offered for sale. Don't chance having tobacco rejected because of excessive MH residues,” notes Moore.