Research and Extension experts are developing a risk index for tomato spotted wilt (TSWV) for the Virginia-Carolina region, after a marked increase in the virus this season. Like their fellow associates in Georgia, however, they realize there's no cure for the virus, only measures that may keep producers on the positive side of risk.
Knowing the history of the virus and its effect on peanuts, North Carolina State and Virginia Tech experts will be developing an index similar to the one used by producers in Georgia to lessen the risk to the virus. They hope to have it in the hands of producers by next season.
TSWV first devastated peanuts in Texas in the early 1980s. By the late 1980s, the virus had made its way to Georgia. By the early- to mid-1990s, the virus had become the Number One limiting factor for production in Georgia peanuts.
In the Virginia-Carolina, the incidence of the virus has been at a manageable, but increasing level over the last 15 years.
When TSWV infects a host plant, it can cause a disease that severely weakens or kills that plant. The virus is spread by thrips. But getting rid of the thrips doesn't mean eliminating the virus. The virus over-winters in a number of weeds and then is spread by thrips.
“This disease is a moving target,” says Jack Bailey, North Carolina State University Extension plant pathologist. “We've watched it march across the U.S.” Georgia had less TSWV incidence for most of the season, but experienced an increase in the virus close to harvest.
The increase in TSWV incidence in peanuts this season has mirrored an increase in tobacco. “We've seen a significant increase in tomato spotted wilt virus in the last year and we need to get a better handle on it and stay on the least-risk side,” says Rick Brandenburg, North Carolina State Extension entomologist.
The V-C researchers are looking at a model similar to the one developed in Georgia, but adapted to the Upper Southeast region for growing peanuts. The index will include factors such as resistant varieties, planting dates, twin vs. single rows, seeding rates and tillage practices.
“There's nothing you can do to eliminate TSWV,” Brandenburg stresses. But certain practices lessen the risk to the virus.
Variety is chief among the factors that can lessen the incidence of TSWV.
In tests, Perry, a recent release in North Carolina, held up under low TSWV pressure, says Brandenburg. The jury is still out on how it will hold up under heavy TSWV pressure.
Gregory and NC-11 look good under low and high pressure, but NC-9 is highly susceptible to TSWV. “We've got to look at the interactions of these practices along with the varieties.”
Breeding lines in Tom Islieb's program at North Carolina State look promising in regard to resistance to TSWV.
Early in their fight against TSWV, researchers in Georgia were able to lock into a source of resistance in a commercial peanut variety and develop new varieties with TSWV resistance. Georgia Green's TSWV resistance is the major reason it has become the dominant variety in the Southeast.
After variety selection, planting date and planting population are important in lessening the risk to TSWV.
Researchers have documented a trend toward greater incidence of TSWV when plant population goes down.
For that reason, experts are recommending two to four plants per foot of row. “The producer needs to make sure he has the thickest stand growing vigorously in the spring,” Bailey says. Getting a stand of less than two plants per foot of row increases the risk of TSWV. Experts recommend planting when conditions are right for rapid, uniform emergence.
When to plant also has an effect on TSWV incidence. In Georgia, for example, peanuts planted early and late tend to suffer more from TSWV, while peanuts planted in mid-May appear to have less risk to the disease.
Because the planting window is narrower in the V-C than in Georgia, “we don't have a good feel for planting dates that will reduce TSWV incidence,” Brandenburg says.
Twin-row patterns have a positive effect on lessening TSWV incidence. “There's less risk with twin rows than with single rows,” Brandenburg says.
At-plant application of Thimet has been shown to decrease risk to TSWV. “Insecticide gave a slight decrease in thrips, but there's a limit to what an insecticide will do with TSWV,” Brandenburg says.
V-C researchers point out that even with a risk index, there's a limit to the amount of control.
An index, according to those who developed one in Georgia, combines what is known about individual risk factors into a comprehensive, but simple evaluation of TSWV risk for a given field. A low risk doesn't imply that a field is immune from any TSWV losses.