One of several looming uncertainties for Georgia tobacco producers is how severe the incidence of tomato spotted wilt virus (TSWV) will be in 2005. In recent years, the virus has damaged as much as half of the state's crop, and most growers routinely prepare for infestation levels of at least 20 to 30 percent.
It's difficult if not impossible to predict the level of TSWV incidence this year, but there are some indicators that might provide clues to growers, says J. Michael Moore, University of Georgia Extension tobacco specialist.
“We can look at what has occurred in the past in terms of rainfall,” says Moore. “We had a very wet late March and early April. If that caused young thrips to drown or die because of disease, then that would be a good thing.”
In Georgia, tobacco thrips and Western flower thrips are the main vectors of TSWV in crops. Tobacco thrips are true foliage feeders and are the major carrier of TSWV in tobacco. Tobacco thrips are also the dominant species in the winter.
Researchers agree that March rainfall can be a good indicator of TSWV incidence in tobacco. Rainfall from this past March indicates that growers may have a lower TSWV incidence than in 2002 and 2004.
Heavy rainfall in March has a detrimental effect on thrips populations because immature thrips are washed off plants with heavy rainfall and don't survive as well. Winter weed data may be a weaker indicator of potential TSWV compared to rainfall, but researchers believe a combination of both may become the best indicator of potential TSWV infection.
Winter weeds from this past year show that pressure from the virus may be greater than in 2003.
“We know from weed surveys conducted in the various counties that the last round of weeds showed a very high level of TSWV,” says Moore. “There already is a good presence of the virus in the field. I'd say there's a pretty good chance that we're going to have a significant amount of TSWV.”
Winter weeds collected in March from locations in Berrien, Coffee, Grady and Tift counties had an average of 4.65 percent TSWV infection in 2002 (a dry spring with a high incidence of TSWV in crops), 0.3 percent in 2003 (a wet spring with a low incidence of TSWV in crops), and 4.1 percent in 2004 (another dry spring with a high incidence of TSWV in crops).
This year's winter weeds had an average of 4.18 percent TSWV infection, but there was a lower thrips population this past March compared to dry weather during March of 2002 and 2004.
Another indication of lower thrips populations corresponding to less TSWV incidence in tobacco may be the number of flower thrips in blueberry flower clusters in March, says Moore. In the high TSWV years of 2002 and 2004, there was an average of 25 to 30 flower thrips per blueberry flower cluster (a blueberry flower cluster averages five to seven flowers). In 2003, there was a low incidence of flower thrips in blueberries. This year, most blueberry flower clusters are averaging one to two thrips, with a few going as high as eight to nine.
Growers who are waiting for a TSWV-resistant tobacco variety will be disappointed to learn that a release probably is still years away due to the market's lack of acceptance of genetically modified organisms. A genetically modified variety tested in Tifton, Ga., during the past four years has shown 99 percent resistance to TSWV.
And, as if TSWV wasn't enough to worry about, there might be another virus concern on the horizon, says Moore. “I've been to Florida recently to look at a plant bed that was found to have cucumber mosaic virus. Some of those plants have been put in the field, and the virus is present and showing symptoms in a number of plants across the field. I'm very concerned that with the virus in north Florida, it's very close to the Georgia crop, and we'll get more of it than we want sooner or later.”
For this year, Moore and other researchers came up with a TSWV management plan for tobacco. Growers who are following the plan, he says, will have significantly lower virus levels than those who don't follow it. The plan is as follows:
TSWV Management Plan for Tobacco
Transplant after April 7. **
For best results with Admire, use greenhouse plants.
a. Admire at 1.4 to 1.8 ounces per 1,000 plants as per Growers Guide (pg. 112).
i. Apply and rinse off foliage three to seven days before transplanting.
ii. T-rail plants: avoid excessive irrigation after application.
1. Water only to maintain plants. Trays should not drip.
Bare root plants:
a. Admire at 2.4 to 2.8 ounces per 1,000 plants in transplant water as per Growers Guide (pg. 113).
All plants (greenhouse and bare root plants):
a. Actigard at 1 ounce per 100,000 plants as per Growers Guide (pg. 113).
i. Apply Actigard only to plants large enough and old enough to be transplanted.
ii. Apply Actigard five to seven days before expected transplant date.
1. Actigard-treated plants are useable for 14 days after treatment.
2. Do not re-treat plants with Actigard.
3. Do not hold Actigard-treated plants overnight after removal from beds or greenhouses.
Pre-water beds during periods of drought/wind before transplanting. A day or two delay in planting is better than planting Actigard-treated plants in dry beds. Actigard-treated plants do not tolerate stress.
Use transplant water.
**For any site in any year, there is a two-thirds chance of less tomato spotted wilt virus by delaying planting until after April 7.