s this summer fades into fall, Dr. George Kennedy will be one step closer in his quest to provide tobacco, tomato, pepper and peanut producers with a tool for fighting the potentially devastating tomato spotted wilt virus. He’s betting that if farmers mow the summer weeds around their fields closely they will be able to break the cycle that allows the virus to persist throughout the year as the tiny, winged insects known as thrips carry it from weeds to row crops and back.
While Kennedy and Dr. Clyde Sorenson, both entomologists, conduct their experiments at the Border Belt Research Station and on farms where the disease has proven problematic in recent years, Dr. James Moyer will be fighting tomato spotted wilt virus on another front — his laboratory on N.C. State University’s campus.
Moyer, a plant pathologist, is studying the virus’ genetics and evolutionary biology, learning more about what makes the virus and its relatives so adept at overcoming resistance in both the floral and field crops that they attack.
For the past 15 years, the researchers have been in the vanguard of the College of Agriculture and Life Sciences’ attack against one of the world’s top ten economically important viruses. TSWV causes millions of dollars in estimated losses annually, infecting some 800 plant species in 70 countries.
In their attempt to help growers in North Carolina and beyond wage an effective counterattack, College faculty members have embarked on an intelligence mission to learn more about the virus and what makes it such a formidable foe. They’ve also trained award-winning graduate students who have, in turn, become leading experts on the disease, and they’ve provided growers with an arsenal of knowledge and tools to minimize losses.
Dr. Margo Daub, head of the College’s Botany Department, says that the impact of this work has been monumental, particularly for the floral crops industry.
“The virus used to be a huge problem in the industry, and now they can manage it,” she says. “Jim Moyer played a major role in understanding the basic biology of the virus and helping growers apply this understanding. He really saved the floral industry.”
Moyer’s involvement with TSWV began in the mid-1980s, when he was recruited by Dr. Ron Jones, plant pathologist, and Dr. Jim Baker, entomologist, both with the College’s Cooperative Extension Service. Until that time, the virus had been, as Moyer puts it, “not much more than an intellectual curiosity.”
Discovered in 1915 in Australia and thought to be the lone plant-infecting virus in the Bunyaviridae family and the sole member of the Tospovirus genus, TSWV had kept a relatively low profile, occurring only in isolated pockets around the world.
But in the 1980s it erupted with a vengeance throughout the Southeastern United States. Tomato, peanut and tobacco crops were hurt, and some bedding plant greenhouses were completely wiped out, with individual growers losing hundreds of thousands of dollars, Moyer says.
Growers were stymied by several problems: For one, they found the disease difficult to diagnose. The stunting, stem-browning, necrotic lesions, plant death and other symptoms that it causes are also characteristic of other problems that are managed differently.
“In tomatoes, it looks like chemical injury — round, necrotic lesions, like someone dropped acid on them,” Moyer explained. “In gloxinia, it looks like the fungus Phytophthora, with rot around the crown. In other plants, it looks like bacterial canker.”
What’s more, TSWV’s symptoms not only vary greatly from crop to crop, they also differ depending on the season and the infected plant’s age.
Perhaps more important, N.C. State’s Plant Disease and Insect Clinic and other diagnostic labs found that the tests that should have helped them pinpoint the problem — and thus the best means of control — were unreliable.
Within short order, Moyer and his graduate students figured out why many of the tests for suspected tomato spotted wilt virus were coming up negative: In those cases, TSWV simply wasn’t present. But another virus was. They called the virus that they discovered impatiens necrotic spot virus, or INSV.
“The dogma up until that point had been that TSWV was in a monotypic group, but since the discovery of INSV at least 12 more distinct viruses have been discovered in the Tospovirus genus,” Moyer explains. “INSV is similar to TSWV, but it is serologically distinct — in other words, it requires a different test.”
N.C. State researchers developed a diagnostic test that continues to be the industry standard. Meanwhile, extension specialists found ways to help growers manage the viruses, which aren’t susceptible to chemicals and which quickly overcome resistant varieties — even those that are genetically engineered.
In the floral crops industry, propagators no longer turn to weak, leftover plants as the starting point for next year’s crop. Instead, they propagate from tissue culture. And growers screen their greenhouses to keep out thrips.
While effective, such control measures are costly, so the American Floral Endowment, the North Carolina Tobacco Foundation, the U.S. Department of Agriculture’s Southern Regional Integrated Pest Management Program, and numerous private firms continue to support N.C. State’s ongoing Tospovirus research financially.
That research included Moyer’s and Daub’s successful efforts to develop a genetic engineering technique that led to resistant chrysanthemums, New Guinea impatiens and tobacco. While those experiments have important implications for medical and plant breeding research, the resistant varieties haven’t been commercially produced because of intellectual property issues.
Other N.C. State researchers are also looking at new techniques for instilling TSWV resistance in tobacco and other crops, while Moyer’s lab is working to gain a better understanding of TSWV’s ability to rapidly overcome such resistance.
Thus far, they’ve identified the part of the tomato spotted wilt virus genome that contains the gene responsible for overcoming resistance, and they are homing in on the specific gene.
Meanwhile, Kennedy, a William Neal Reynolds professor of entomology, is focusing on the ways the virus and its vector — thrips — interact, how the virus persists in nature throughout the year, and what determines its spread from weeds to field crops.
Students have also conducted important research on the Tospoviruses. Indeed, three graduate students studying TSWV have won the Department of Plant Pathology’s top academic honor, the annual Nusbaum Scholar Award. In 1991, the winner was Marcus Law, who helped discover INSV; in 1997, it was Wenping Qiu, who found that genes in host plants and insects can cause a TSWV population to reassort genome segments into an isolate that can overcome resistance; and, in 1998, Sonia Herrero won for her investigation into whether transgenic plants can be a viable option for disease control.
While the war on TSWV is still being fought, the inroads made by students, researchers and Extension specialists have been substantial.
“In the College of Agriculture
and Life Sciences, we talk a lot about the value of collaborations that
span the basic-to-applied continuum, as well as the interconnectedness
of our teaching, research and extension efforts,” Moyer says. “You can
really see this when you look at the history of our work with tomato
spotted wilt virus. It’s been a truly problem-driven effort that has
had a real impact for our growers.”