Drought or tomato spotted wilt virus alone would be enough to significantly damage a tobacco crop. Together, they can be devastating, as Georgia producers are learning this year.
“Tobacco crop conditions are tough,” says J. Michael Moore, University of Georgia Extension tobacco specialist. “About 20 percent of Georgia's crop is infected with tomato spotted wilt virus, and we must assume that most of those plants will die. At this point, we're probably looking at a total yield loss of about 10 percent.”
Drought conditions also have taken a toll, he adds. “Tobacco growth has been very slow under these drought conditions. Georgia soils continue to be dry. Many of our growers are behind and the crop is late in topping out. Although 100 percent of our tobacco crop is irrigated, many ponds and streams have become dry,” says Moore.
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. There are, however, some promising preventative treatments.
“Tomato spotted wilt virus generally is less in fields were Admire was used, and fewer plants were affected when Admire was 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 have seen an advantage this 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 rootball 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.”
Other states in the Tobacco Belt have reported problems this year with tomato spotted wilt virus, but none as severe as Georgia, he says. “North Carolina and South Carolina haven't had widespread problems. The southern region of South Carolina has been hit hard, and there are hot spots in North Carolina, but nothing compared to what we've seen. Growers in Florida are reporting a 5 percent incidence of the disease.”
And, as if tomato spotted wilt virus wasn't enough, Georgia growers also are seeing losses due to rhizoctonia, says Moore.
“Losses to rhizoctonia have been worse this year than in recent memory, with a larger portion of the incidences occurring in plants grown in field beds. Soil moisture conditions were good to excellent at transplanting and provided an excellent environment for the growth of rhizoctonia.”
Budworm populations also are worse this year, he says.
“In addition to budworms arriving early, control has been made even more difficult by the drought, which contributes to the slow growth of tobacco and the cabbage-like appearance of the leaves closing tightly around the bud. Worms are protected in this tightly closed bud and have been difficult to kill.”