On-going research

What's good for controlling tomato spotted wilt virus (TSWV) in peanuts isn't necessarily helpful in tobacco. This is one of several conclusions reached from on-going research dating back to 1986, says Paul Bertrand, University of Georgia Extension plant pathologist.

“We've made progress in the management of tomato spotted wilt virus in tobacco,” says Bertrand. “And, we've found that tobacco and peanuts appear to behave differently in a field infected with a tomato spotted wilt epidemic.”

Research has revealed, he says, that planting date probably isn't a useful option in controlling TSWV in tobacco. “We had seven trials, looking at tobacco planting dates from March 29 to April 26. In one Coffee County trial, the virus became progressively worse from the first planting date to the last. In another plot, about five miles from the first one, the virus was more severe in two of the middle planting dates.

“In Irwin County, about 35 miles from these first plots, there was substantially more virus in the first two planting dates. Forty or 50 miles from that plot, the virus was severe in the beginning, but dropped off in the later planting dates. Using all combinations of these results, we can't say for sure if you should avoid planting on a certain date,” says Bertrand.

Plant spacing — another factor in avoiding TSWV in peanuts — also doesn't appear to have an effect in tobacco, he says. “Last year, we looked at the effect of in-row planting spacing on the incidence of tomato spotted wilt. We all know that plants are becoming more crowded in the rows, partly to offset TSWV and partly for combining benefits.

“But we found last year that there's not much effect on the virus from crowded tobacco plants. It appears that tobacco may not behave like peanuts in regard to plant spacing or plant density,” notes Bertrand.

Other research has looked at suppressing TSWV through the use of Actigard insecticide. Actigard has been labeled for blue mold in tobacco that is 18 inches or larger, but it has not been labeled for use in greenhouses or plant beds, says the plant pathologist.

“This is just the opposite of what we need for suppressing tomato spotted wilt,” says Bertrand.

In 2000, Actigard trials were conducted on greenhouse plants and in plant beds, he adds. “The best suppression of TSWV came from an Actigard application of 3.25 grams per 7,000 plants, sprayed on the foliage in the plant house or plant bed followed by one to 1.8 ounces of Admire.”

Actigard, he says, requires five to seven days before it induces resistance mechanisms in the plant. For this reason, the Actigard treatments were made five to seven days prior to transplanting.

Admire was applied either at one ounce per 1,000 tray cells in the greenhouse with a drench or at 1.8 ounces per 1,000 combined with transplant water. The drench treatment was made one to three days prior to transplanting.

Actigard and Admire were not mixed in any of the trials, says Bertrand. “Actigard and Admire gave us 50 to 60 percent suppression of tomato spotted wilt. Following up with a field spray of either Actigard or another insecticide generally didn't appear to be needed.”

An Actigard program does involve some risk, he says. “Even if you use the suggested rates, you can assume that you'll have a one to two percent stand loss associated with the Actigard treatment. And that's one of the reasons it isn't labeled for use in plant beds or greenhouses.

“There's a justified fear in using too much of the material and causing significant damage.

“Rates higher than 3.25 grams per 7,000 plants are not safe. A rate of four grams per 7,000 plants caused significant stunting in trials conducted in the Carolinas. The maximum rate is on the borderline of being unsafe.”

Once plants treated with Actigard have been pulled, they should be planted within 24 hours or thrown away, says Bertrand. In North Carolina, an 85 percent stand loss was seen where more than 24 hours elapsed between pulling the plants and transplanting.

e-mail: [email protected]

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