By Dan Rahn
Tomato spotted wilt swept through Georgia faster than any invasion since Sherman's.
Scientists had no cure for the disease, and the more they understood about it, the more they knew they didn't know.
From its emergence in Georgia in 1985 until last year, spotted wilt was growing out of control, claiming at its peak $40 million worth of the state's peanut crop and millions more of tobacco, peppers and tomatoes.
Then, after a 13-year struggle, researchers and farmers reached a turning point. For the first time, tomato spotted wilt wasn't worse than the year before. In the state's huge peanut crop, the virus's damage was only about one-fourth its 1997 peak.
The turnabout came after researchers introduced a counterattack from half a dozen directions, giving farmers an index of options that improve a crop's chances - although, in many cases, scientists still aren't sure quite why they work.
"We have some hypotheses," said Albert Culbreath, a UGA plant pathologist on the front lines of the battle. "But we have no conclusive explanation as to why any of these things work."
As you learn more about the disease and the tiny insects that spread it, you begin to understand what researchers and farmers are up against.
When it appeared out of nowhere in 1985, tomato spotted wilt was the only known member of the Tospovirus genus in the Bunyaviridae family of viruses. By far the most costly and cosmopolitan of what is now a dozen tospoviruses, it infects hundreds of plants worldwide. In Georgia, that includes two major farm crops (peanuts and tobacco) and two others (tomatoes and peppers), along with dozens of landscape plants and an untold number of weeds.
"It's a much more complex system than most plant viruses, which are seed-borne or mechanically transmitted," Culbreath said. "Tomato spotted wilt is vectored by thrips. And it replicates in both the plant and the thrips."
Vectored by thrips
Of the eight vectors, he said, the two most common in Georgia are tobacco thrips (Frankliniella fusca) and western flower thrips (Frankliniella occidentalis).
Western flower thrips arrived in Georgia in 1983, just ahead of the virus. "Some speculate that they brought in spotted wilt," Todd said. "They're active year-round on any plant that's got a flower."
Not so with tobacco thrips. "In the winter, we have a hard time finding them," he said. Adult tobacco thrips aren't even like each other: Some can fly, but others have nonfunctioning wings. The latter can have flying offspring, though, and the percentage of each varies through the year.
Western flower thrips don't have that split population. And no one knows why tobacco thrips do. "We think it's a safety mechanism for the species," Todd said. "A remnant is mandated to be left behind. The rest can fly off. That could be important. These (nonflying) tobacco thrips can infect volunteer peanuts and other host plants, leaving a reservoir of the virus in the field."
The year's first tobacco thrips generation appears each March on "volunteer" peanuts, plants that emerge from a field harvested the previous year. "Some years we have five or six generations," Todd said. "Their life cycles are 30 to 35 days from egg to egg."
Not all thrips can spread spotted wilt. "The larva acquires the virus in feeding," said Hanu Pappu, a UGA plant pathologist who works closely with Culbreath and Todd, focusing on more basic research. "The adult can ingest it but has a midgut membrane through which the virus cannot pass. When the larva takes it in, the virus survives the molting process and accumulates in the salivary gland of the adult thrips. The thrips can then transmit the virus for life as it injects saliva into plants in feeding."
A complex virus
And tomato spotted wilt is by itself a complex virus. Tospoviruses' structure - an enveloped, spherical package of proteins - makes it "more like viruses that infect people and animals than most other plant viruses," said John Sherwood, head of the UGA plant pathology department.
Spotted wilt causes stunting and chlorosis in plants. "As with all viruses, it alters the plant's metabolism and physiology to favor the virus," he said. "Viruses don't carry along the tools to do these things. They steal them from their plant hosts."
A plant virologist at Oklahoma State before coming to UGA in 1997, Sherwood has worked on tomato spotted wilt and its thrips relationship since the 1980s. "We know now that, like human immunodeficiency virus, tomato spotted wilt virus has many strains, some more aggressive than others," he said. "But there's much we still don't know. We don't know whether the new tospoviruses we've found, for example, are new viruses or have been there all along."
Pappu found that the virus isolated from Georgia sources was a more homogenous group when compared with spotted wilt from other parts of the world. "That tells us geography has some influence on the variation," he said. "It gives us some idea of the virus's evolution."
Its evolution may be a key to its complexity.
"The virus may have acquired the ability to infect plants during its evolution," Pappu said. "Spotted wilt has some effect on thrips but is not lethal, as it can be to some plants. That may be because it didn't originate as a plant virus but as a virus of thrips. Also, most plant viruses carry a gene that enables them to move through cells in the plant. Spotted wilt and other tospoviruses have that gene, but the other Bunyaviridae viruses don't."
SWEAT fights back
The collaboration in Georgia and nearby states is "as close to a pure team approach as anything I know," Culbreath said. "It's the only way it could work."
In Georgia and Florida, scientists work in a group called "S.W.E.A.T." (Spotted Wilt Eradication Action Team). But the whole world is working on this one.
There's a meeting of virologists, entomologists and plant pathologists in Italy in 2001, Todd said. There's also a tospovirus conference every five years and an international thrips conference. "A lot of people in a lot of countries are working on this."
For farmers, most of the critical successes so far have come from Culbreath, Todd and others like them. "We work mainly on management," Todd said. "We're the clod-kickers, the ones who make it work for farmers."
More than a decade into the fray, the "clod-kickers" have learned enough to make some reliable predictions. In short, they've found six things that affect the risk of spotted wilt: variety, planting date, plant population, insecticides, row pattern and tillage.
The S.W.E.A.T. recommendations go to farmers in a neat package that was the brainchild of UGA Extension Service entomologist Steve Brown. The "UGA Tomato Spotted Wilt Risk Index for Peanuts" assigns numbers to the recommendations and adds a correction factor based on previous losses to spotted wilt.
The index gives farmers a choice of ways to lower the risk of the virus. The only certainty is that they can't just choose one. "No one factor is sufficient on its own," Todd said. "Variety, planting date, whatever - each has to be used in conjunction with other factors to be effective."
The best is Georgia Green, a 1995 UGA release that consistently has 50 percent less of the spotted wilt found in susceptible varieties. Not since Florunner's 1969 debut has a new variety so quickly dominated. In Georgia, Florida and much of Alabama, growers planted 80 percent Georgia Green in 1998.
Three other varieties - FL-MDR-98, Virugard and Southern Runner - also have moderate resistance. Planting one of the four is "the single most important thing a grower can do," Culbreath said.
But no one knows just why the new varieties are resistant. That's a little worrisome, scientists say, since spotted wilt mutates faster than most known plant viruses.
"We're in a manageable situation right now," said Bill Branch, the UGA peanut breeder who developed Georgia Green. "There are different levels of resistance: immunity (no disease), highly resistant, resistant, susceptible, very susceptible and dead. We're at 50-percent resistance now. If we get the resistance too high, this virus will mutate. Then we'll be stuck with something we can't manage."
The best new varieties all get their resistance from the same source. Dan Gorbet, a University of Florida breeder who developed FL-MDR-98, soon will release a line with two resistance sources. Genetic diversity, though, isn't something you can always count on. "Georgia Runner has the most diverse genetic background out there," Branch said. "But it's susceptible."
Branch said any new variety will have to have spotted-wilt resistance, but that alone isn't enough. "If it has high resistance and high yields but doesn't taste good," he said, "it won't be acceptable."
Peanut varieties take 10 to 20 years to develop. The best now were selected just as spotted wilt was arriving. When breeders started screening, the resistance was already there. "If that hadn't happened, we'd be in a heap of trouble in the Southeast right now," Gorbet said.
Over the years, a pattern emerged: The worst spotted wilt is in the earliest- and _latest-planted peanuts. The effect varies slightly from year to year, but it's still predictable. If everything else is the same, peanuts planted in early May have less spotted wilt. "We think that's related to thrips emergence and early-season population dynamics," Culbreath said.
A biotechnology technique called TAS ELISA (Triple Antibody Sandwich, Enzyme-Linked ImmunoSorbent Assay) may enable some fine-tuning of the recommendations. "With ELISA, we can detect one of the proteins the virus makes as it replicates inside the insect," Pappu said. That pinpoints the thrips that can transmit the virus.
In a three-year ELISA study, Pappu found that from zero to 8 percent of the thrips in a field can transmit the virus. The tiny figures are small comfort. "You can have 2 million to 3 million thrips per acre," Todd said. "A very low percentage will do the job just fine."
Planting in early May avoids a major April peak in virus-transmitting thrips Pappu found in his study. "The seasonal dynamics of the thrips vectors may allow us to further correlate with the extent of disease," he said.
Plant population, spacing
"It's a dilution effect," said John Baldwin, a UGA crop and soil scientist. "We want to end up with four plants per foot. We've got to have a good stand in 14 days. We tell growers to wait and plant high-quality seed under optimum conditions."
Baldwin's research generated the data behind the index recommendation on twin-row spacing, which puts seeds in twin rows 7 to 10 inches apart. He found its benefit while studying whether twin rows would enable peanuts to cover row middles quicker. He found the effect he had sought and more: Twin-row peanuts always had less spotted wilt.
"The plant population is the same," he said. "But in every case, over three years in nine locations, we had less spotted wilt with twin-row patterns. All current varieties responded with less spotted wilt in twin rows."
Higher plant populations and twin-row spacing take advantage of peanuts' tendency to compensate. Twin rows just do it better. "If it weren't for this virus," Culbreath said, "we could have one plant per foot and have a field of solid peanuts. But that would be disastrous with spotted wilt."
Tillage and insecticide
"Thrips are color-sensitive, and they apparently use sight to hone in on their host," Culbreath said. "We think the crop residue in strip tillage interferes with their ability to do this. But we don't know that for sure."
They don't know why it helps to use the insecticide phorate at planting, either. In intensive studies, Culbreath and Todd found insecticides can control thrips, but in most cases have little effect on the virus. But phorate provided consistent, low-level suppression of spotted wilt, even though it was no better at thrips control than other chemicals.
"Something is going on in that plant that causes the virus to be less successful at colonizing the plant," Todd said. "We suspect the phorate may trigger an acquired systemic resistance, like an immune response."
Studies this year will address that. "But learning conclusively exactly what's happening will require more intensive chemical analysis," Culbreath said. "We've been sidetracked, just putting out fires. But it's fascinating in terms of what's going on."
Actually, no one knows for certain what's going on in any of the indexed findings. Researchers know what works. They just don't know why. However, the team at least has learned some of the right questions to ask. And they've acquired many of the tools they need to answer them.
"There's a lot to learn about tospovirus symptoms," Todd said. "For instance, there are different strains of the virus and a huge array of symptoms. Are there strains tobacco thrips can vector and other strains western flower thrips can vector? There's a lifetime of work here for a pile of people."
Access http://sacs.cpes.peachnet.edu/spotwilt/ for more information.
Dan Rahn, a former English teacher and newspaper reporter, is the extension news editor for the UGA College of Agriculture and Environmental Sciences. He has bachelor's and master's degrees in English education from Georgia Southern University.