As an Extension specialist, Georgia's Steve L. Brown likes to walk into a field whenever there's a problem and offer a solution. But that's not always possible.
“About 10 years ago, we were walking into peanut fields and scratching our heads,” says the plant pathologist. “We didn't recognize the problem and we didn't know what to do about it. The problem became worse, and we did more head scratching. Then, we got busy and conducted a lot of research. Now, this research finally is paying off.”
The “problem” Brown speaks of is tomato spotted wilt virus (TSWV) and University of Georgia researchers have made significant progress against the disease during the past decade.
In reviewing some of the “basics” concerning TSWV, Brown says it is now known that thrips vector the virus. “We know the disease is caused by a virus, and that the virus is transmitted by thrips — these tiny insects we've had with us all along. But we didn't have the virus until about 15 years ago,” he says.
Thrips still are the only known means of transmitting the virus, he notes. “That's important to remember. There's a lot of talk about seed transmission, and about the virus living in the soil, but there's never been evidence to support any of this.
“There's no evidence of seed transmission, even though we can find the virus in the seed coat. But the virus never makes it into the embryo and into the resulting seedling that comes from the seed,” says Brown.
Eight species of thrips worldwide are known to transmit viruses like tomato spotted wilt, he says. Unfortunately, he adds, Georgia has two of these species and possibly a third.
“Tobacco thrips are by far our most common species, and it is a vector of tomato spotted wilt virus. We also have some Western flower thrips. In a given year, 10 to 15 percent of our thrips population may be Western flower thrips. Florida thrips also may be contributing, although they are rare in Georgia, and usually are found only in the extreme southern part of the state. Tobacco thrips is our main vector,” says Brown.
In attempting to understand TSWV, it's important to remember that not all thrips carry the virus, he says. “We can have extremely high thrips pressure, and we may or may not have a high incidence of tomato spotted wilt. It depends on how many of those thrips are carrying the virus. And they get the virus by feeding on another plant containing the virus.”
Primary infection occurs, explains Brown, when thrips carrying the virus move into a peanut field or surrounding areas. They're picking up the virus in another location, flying into the peanut field and inoculating the plants.
This primary infection occurs throughout the season, not just during spring, he adds.
“We also know that peanut susceptibility may vary with age. We know a young plant may be more susceptible to the virus than an older plant. And there's recent evidence that transmission may be more efficient in lower temperatures. Even though infection can occur throughout the year, the time during which that plant first comes out of the ground seems to be important.”
Secondary infection, continues Brown, is infection that occurs within a field, from one plant to the next. “We know that we get some secondary infection, especially where the incidence of the virus is very high. But generally, that isn't very significant compared to primary infection. We think most of the virus is coming in from outside sources.”
Foliar insecticides that are used to control thrips can offer temporary relief, he says. “We can spray, in a very intensive program, and actually see some differences. But we've never been able to show that those visual differences hold up over time. By season's end, sprayed and un-sprayed plots look the same, and we've never seen a yield response from foliar insecticides.”
Plants showing symptoms of tomato spotted wilt in a field might represent only about half of the infected plants, he says. For some reason, plants may or may not show symptoms of the virus, or there might be a delay in the symptoms. Researchers don't fully understand what happens when the virus gets into the plant, says Brown.
No peanut varieties are immune to TSWV, he says, but there is much variation among the susceptibility levels of varieties. “Our most resistant varieties have a common heritage. These include Georgia Green, Southern Runner, MDR-98 and C99-R. Other varieties, such as ViruGard, were bred from very susceptible parents.
“We don't understand the mechanics of resistance, but we're fortunate to have germplasm that carries resistance to tomato spotted wilt. A lot of research is being done to breed plants for resistance, and some excellent varieties will become available in the next few years.”
Tospoviruses are the group of viruses, says Brown, that include TSWV. “When tomato spotted wilt was first discovered, it was so different from other viruses that they made an entirely different category for it. Since then, they've found other viruses that are similar to tomato spotted wilt. There are at least 10 serologically different viruses in this group.”
Viruses in this group that affect peanuts include bud necrosis, which is a very serious problem for peanut producers in India. Although the virus hasn't been found in the United States, the thrips that vector the virus — Thrips palmi — have been introduced into Florida.
“We know the vector is there, but we've never picked up that species of thrips in Georgia. The potential is there — that species could move up and bring us a new and different type of tospovirus.”
Another tospovirus — Impatiens necrotic spot virus — has been found in Georgia and is very similar to TSWV, says Brown. Researchers don't know how this virus would react to some of the control measures used for TSWV, he adds. The vector for this virus is Western flower thrips.
Peanut yield losses to TSWV can amount to about 20 to 40 pounds per acre, he says. This is $4 to $10 per acre for each percent of row feet affected in a field.
“Whenever we have a field that's 50 percent infected — 50 percent of the row feet are showing symptoms such as severe yellowing and stunting — research indicates that we'll lose 1,000 to 2,000 pounds of yield per acre. In the last couple of years, we haven't had that many 50-percent infected fields in Georgia. Hopefully, those kinds of losses won't be too common.”
Early season TSWV infection is much worse than late season, notes Brown. “If we get the stunting early in the season, those plants won't recover. Late-season yellowing on a full-grown plant won't hurt us nearly as bad as early season infection.”
The levels of tomato spotted wilt virus seen in Georgia from 1995 to 1997 were a real threat to the survival of the peanut industry in the state, he says. Losses to the disease were significant during that time period, he adds.
“We've seen relief during the past three years, but the virus hasn't gone away — it's still a very serious threat. We've seen a decline in the virus because of the changes growers have made in their production practices. These include those factors in the risk index, including a shift in varieties and planting dates.
“But the pressure is still there. If you don't believe it, take one of our older varieties, like Florunner, and plant it in April at about 80 pounds of seed per acre. You'll see heavy pressure from tomato spotted wilt.”
Looking at the effect of TSWV on worldwide peanut production, Brown says the virus has diminished as a yield-limiting factor in Texas while the incidence of the disease has increased greatly in the Virginia-Carolina region in recent years. Last year, the V-C region reported some fields with a 40 percent incidence of TSWV.
TSWV is rare in Argentina, he says, but it is present and possibly could be increasing.
Research in recent years has focused on why the incidence of TSWV varies from field to field, he says. “We can have a high incidence of the virus on one side of the road and not so much on the other side. We want to know why.
“We now know that severity is affected by many factors, including variety, planting date, plant population, at-plant insecticide, row patterns and tillage. Adjacent crops still is a question mark. We think it's a factor but we don't know enough to explain it. And there could be other factors influencing the incidence of tomato spotted wilt.”
The University of Georgia's Tomato Spotted Wilt Risk Index is a tool for managing the problem, says Brown. “It's a way to assess the risks associated with a particular set of production practices. Research has shown a $6 to $11 per acre increase in net return for each point reduction on the index. If you can bring down that number, it'll mean money in your pocket.”