In what is being called the most severe outbreak of tomato spotted wilt virus in the history of the U.S. Tobacco Belt, Georgia growers are eyeing unprecedented stand and yield losses.
“Tomato spotted wilt virus has been terrible,” says Martie Boyd, a producer in south Georgia's Berrien County.
“We're seeing up to 60 percent damage in some of our fields. I've even seen some fields that are up to 85 percent lost, and that's not worth harvesting. It's the worst I've ever seen.”
And tobacco isn't the only victim of the virus, says Boyd. “I also grow vegetables, and I can't hardly grow bell peppers anymore. I believe growing bell peppers on bare ground is a thing of the past. The virus doesn't affect my squash, but we've got it on tobacco, peppers, tomatoes and peanuts,” he says.
“Tomato spotted wilt virus is the big story this year, and we're going to have to learn to live with it,” says grower Steve McMillan, also of Berrien County. “If we don't learn to deal with it, we won't stay in the tobacco business. We're looking at 50 to 70 percent infection in some fields, and stand losses reaching as much as 30 percent. We don't have enough damage for insurance to do us any good, but we won't make our pounds this year. That's a bad place to be.”
Steve's brother, Tim, says the tobacco production season began on a positive note. “We probably had a 99 percent stand at the start of the season. We didn't see hardly any tomato spotted wilt virus in the first two weeks.
“In the third or fourth week, we saw it at levels of about 10 to 15 percent. Every week after that, we could see more of the virus in our fields. We probably have about 60-percent infection across our entire crop. There's no way we'll ever make our pounds this year,” he says.
Prior to this year, the worst outbreak of tomato spotted wilt virus on Georgia tobacco was in 1999, says J. Michael Moore, University of Georgia Extension tobacco specialist.
“In 1999, we lost an estimated 18 percent of our yield with a stand loss of 35 percent, averaged across the entire state,” says Moore. “We'll probably surpass that this year. We're already at 15 percent yield loss and 30 percent stand loss, and the season is far from over.”
Once a plant is infected with tomato spotted wilt virus, there's not much a grower can do to save it, says Moore. However, there are some promising preventative treatments.
The virus, he says, generally has been less in fields where 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, he reports. Actigard in sequence with Admire in the greenhouse gave increased control over either treatment individually, he adds.
University of Georgia research also is looking at the origin of the thrips that vector tomato spotted wilt virus, says Bob McPherson, research entomologist. “Where are these thrips coming from? Over the past three years, we've found confirmed vectors on cutleaf evening primrose, wild radish, carrots, common chickweed, flowering dogwood, narrowleaf vetch, broomsedge, and soybean and tobacco foliage and blooms.
“We've found a lot of vectors in a lot of plants that are found in and around the tobacco farmscape. We begin the season with a lot vectors coming from many different places. It'll be a tough problem to solve, but we're making progress,” he says.
The answer to solving the tomato spotted wilt virus problem may lie in resistant tobacco varieties, says Mike Stephenson, research coordinator with the University of Georgia Coastal Plain Experiment Station.
“Four experimental lines have looked pretty good,” says Stephenson. “They are showing 10 to 20 percent damage without Admire or Actigard. Additional lines with thrips resistance also look promising, averaging about 20 percent damage.
“Breeding thrips resistance is an interesting approach. We don't have a firm grasp on how many sub-species of the virus exist, and that makes it difficult for breeding purposes. The virus changes faster than we can breed a variety. Thrips resistance along with Admire and Actigard could produce a means of defeating the disease.”
Researchers also are examining weather data to determine if there are correlations between the incidence of the virus and certain weather patterns.
“We've taken data from 1989 through 2000 and looked for relationships between a variety of environmental factors and tomato spotted wilt,” says plant pathologist Lenny Wells. “During these 11 years, we found that whenever we received 8 inches of rainfall or more during the month of March, tomato spotted wilt virus was lower compared to those years when we received fewer than 8 inches of rain in March.
“Based on those findings, we've been able to predict the severity of the disease for the past two years. In 2001, we received about 9.5 inches of rain in March, and the virus incidence was less than 10 percent. This year, we had about 4 inches of rain in March, and the severity is much worse.”
All of this, he adds, is relative to location. “From one field to the next, you may have a higher disease incidence during the same year. But if you look at the same location over several years, the incidence of the virus will be lower in those years in which we received less rainfall in March. The trend holds steady, but it's relative to location,” says Wells.
The theory from this research, he says, is that heavy rainfall during March could be causing a harmful effect to the immature thrips populations, decreasing the number of potential vectors.