Research and Extension people can't help but be concerned about tomato spotted wilt virus. They're getting questions from producers about how to manage the virus that affected more than 40 percent of the tobacco in Georgia, while increasing its incidence farther north.
The topic dominated discussion among a group of tobacco specialists at an informal gathering in Asheville, N.C.
Sitting across the table from each other, research and Extension experts from different states talked about the problems facing the tobacco industry at the bi-annual Valent Tobacco Seminar. The crop protection manufacturer brings together specialists from flue-cured and burley areas in off years of the Tobacco Workers Conference.
Talk of a tobacco buyout as well as budget constraints at land grant universities in the South also played a major role in the discussion.
The specialists say the seminar gives them an opportunity to interact with each other and catch up on research and industry activities.
While snow and ice forced specialists in Kentucky and Maryland to cancel, specialists from Georgia, South Carolina, Tennessee and Florida had plenty to discuss.
TSWV received the most attention.
In Georgia, plant pathologist Natalia Martinez, along with colleagues in Florida, South Carolina and Virginia, is the first to document mixed infections of TSWV and Impatiens Necrotic Spot Virus (INSV) in tobacco. INSV had previously been detected in peanuts in Georgia and Texas.
INSV and TSWV are closely related viruses and are both vectored by thrips. INSV has been observed in North Carolina and Kentucky in tobacco as well. It is also found in peanuts.
A study in 2002 found that out of eight sites in the Southeast, one location in Georgia had 30 percent of the tobacco showing mixed infections of TSWV and INSV; 22 percent in one Virginia site and 18 percent in another location in South Carolina.
Using the standard ELISA test to detect viruses, as well as an Immunostrip developed by Agdia, which is similar to a pregnancy test — two lines positive, one line negative — Martinez also found that in most fields severely affected by spotted wilt, TSWV is present in 30 percent of plants that weren't expressing symptoms. “There is more infection than we really can see,” she said.
Because TSWV hits plants when they are young and green, secondary infections from other disease-causing organisms can take over and kill the plant before the diagnosis is made. Alex Csinos, University of Georgia plant pathologist, says the new problems may be a result of new varieties.
Researchers are also looking at weed species for possible clues to the TSWV puzzle. Bob McPherson, a UGA Extension entomologist, and colleagues looked at 13 weed species over a three-year period. The study found thrips in weed species ranging from Carolina geranium to Virginia pepperweed from broomsedge to wild radish.
While Frankliniella fusca or tobacco thrips, Frankliniella occidentalis or western flower thrips are the most common vectors of the virus, the researchers documented two new species of thrips in tobacco and weeds.
On the crop protection front, Actigard received a third-party registration label for TSWV in Florida, Georgia, South Carolina, Virginia and North Carolina in 2003. UGA Extension Entomologist David Jones says it provides a weapon against TSWV. Admire also helps with cutworms and other insects.
“Producers can reduce TSWV by 50 percent to 60 percent by using Actigard and Admire together,” says Alex Csinos, University of Georgia Extension plant pathologist. “Admire works on thrips as a repellent; Actigard activates the plant's defenses. Actigard can be applied in the field or in the greenhouse. Tests have shown that the greenhouse application is the most important application.
Csinos points out that younger plants tend to be more susceptible to TSWV infection than older plants. With Actigard, expect to see some stunting for three to five days after applications. “You need to have the plants at the height you want them before you spray Actigard,” Csinos says.
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