Following identification of tomato spotted wilt virus (TSWV) in Alabama and Georgia in 1986, peanut production in the Southeast faced an unprecedented threat.
“TSWV was the worst virus in peanuts in the Western Hemisphere,” says University of Georgia Plant Pathologist Albert Culbreath.
Culbreath, speaking via Zoom technology to the 53rd Annual American Peanut Research and Education Society (APRES) Conference, held virtually for the second year in a row July 12-16, offered a timeline for the development, spread and management of the potentially devastating virus.
He says the infection timeline in U.S. peanuts began in South Texas. “It was identified in South Texas peanuts in 1971.”
He says Western flower thrips, a vector for the virus, showed up in Georgia in 1980. The virus was detected in Mississippi in 1983 and by 1985 South Texas peanut producers reported 50% losses to TSWV. “Some fields recorded 100% losses,” Culbreath says.
By 1990, Georgia producers were seeing field losses to the virus.
Recipe for infection
Culbreath says production regimes provided opportunities for the virus to spread. He calls production practices in the late 1980s and into the ‘90s a “recipe for severe epidemic.”
Spots on these peanut leaves are symptoms of Tomato Spotted Wilt Virus. (Photo by Albert Culbreath)
That recipe included:
- Near monoculture with 750,000 acres of mostly Florunner, a TSWV susceptible cultivar;
- Introduction of TSWV;
- Large host range for TSWV, including more than 700 species of plants;
- Western flower thrips were new to the area but tobacco thrips were endemic.
“A lot of factors came together. We had a rough road for the next few years,” Culbreath says. “In 1997, we saw 12% crop losses to TSWV.”
He says another factor was early planting. “Much of the crop went out in early April, some in late March.”
Lower field population is also a factor, he adds, and Georgia peanut farmers had cut back on plant populations. “In the absence of spotted wilt, that was possible,” Culbreath says. “Research had shown that reduced seeding rates, especially when they could be planted evenly, was a way to save on production costs without hurting yield. Spotted wilt flipped that.”
A bit of denial
He says denial also plays a role with any new disease threat. “Some tended to believe ‘it’s just another virus and it’s not likely to be a threat.’”
Culbreath says peanut mottle virus, identified in the Southeast in the 1970s, never materialized as a significant threat to peanuts. A similar situation occurred with the peanut stripe virus identified in Georgia in 1982.
TSWV was different.
“Thrips have a lot of hosts,” he says, offering ample opportunity for the virus to thrive. “Tobacco thrips cause unsightly injury to peanut plants, but typically does little real damage, and Western flower thrips do not reproduce on peanuts. As a virus vector, however, thrips are a clear and present danger.”
He says the tobacco thrips is a competent vector, a long-time pest on peanuts, tobacco, and cotton in Georgia. It reproduces well on peanuts and cotton and is active most of the year in Georgia.
The Western flower thrips, also a “competent vector,” was not known in Georgia until 1980. Identification may have been “coincidental” with detection of Western flower thrips and TSWV. “It is active any time blooms are present on a wide range of plants in Georgia.”
Peanut production in Georgia is a long process, from April to November. “We have little time when peanuts are not present,” Culbreath says. “When we add weed hosts, we have an even more complicated issue.”
Culbreath says the complex of factors favoring thrips and TSWV demanded a hard look at production practices. It took a lot of sweat, he says. “That’s how we came up with SWEAT—Spotted Wilt Eradication Action Team. We know TSWV is not going to be eradicated, but we had to dream big.
“We have many people, across all disciplines, tackling the problem. The effort includes pathology, entomology, agronomy, weed science, and breeding efforts.”
Southern Runner, released in 1986, showed some resistance to TSWV in Texas, Culbreath says. “It was never widely accepted.”
Georgia Green was accepted and became the dominant cultivar in the Southeast. “Georgia Green has moderate resistance,” Culbreath says.
He says Georgia Green responded to altered practices such as:
- Mid-May planting date,
- Increased plant population,
- At-plant insecticide (Phorate),
- Twin-row pattern,
- Reduced (strip) tillage.
“We use a risk assessment index, too,” Culbreath says. ‘It’s a grower tool that helps determine best actions in an integrated approach to managing TSWV.
“We saw a significant decrease in losses after we switched to Georgia Green and use of the Risk Assessment Index. One result was that by 2003 less than 2% of the crop is planted before May 1.”
Breeding efforts have been and will be critical, Culbreath says. “We knew we needed better varieties, so TSWV became a key in breeding efforts. Breeders see great potential to increase resistance from sources of both cultivated peanut and introgression from wild species.”
Culbreath says concerted efforts by a multi-discipline team made a difference in TSWV losses. “An integrated management system has been very successful in reducing or preventing TSWV losses. Inter-disciplinary — university, agency, state and company — inter-everything teamwork has been the key.” he says.
He cautions against complacency, against assuming, “The tomato spotted wilt problem has been solved. We can take our bows and rest on our laurels…because Bob Kemerait’s (UGA Extension pathologist) phone is still ringing.”
By the numbers
The numbers back up the caution.
“TSWV has not gone away. In 2019 and 2020, we saw an upswing in crop and dollar losses to spotted wilt. We will need to continue emphasizing an integrated approach to managing spotted wilt to buy time until cultivars with higher levels of resistance become available.”
A Navaho saying is appropriate, Culbreath says. “Coyote is always out there watching, and Coyote is always hungry.”
He adds, “We have to assume the same about TSWV.”