What researchers don't know about tomato spotted wilt virus is a lot, says Alex Csinos, University of Georgia research pathologist, but they continue to learn more each year.
“We think the disease is complicated by many factors,” said Csinos during this year's Georgia Tobacco Tour. “Just about anything you can think of has something to do with tomato spotted wilt virus in some fashion.”
Researchers believe, he says, that the severity of the virus is affected by weather conditions. “Weather conditions affect not only the development of virus symptoms on the plant, but they also affect the development of thrips, the insects that transmit the disease to tobacco plants,” says Csinos.
Other factors that affect the development of the virus include weeds and host crops planted around a field, he says. In addition, the location of a field can be a factor, he adds.
“This is an extremely complex system, and you can't pick out just one thing and say if we do this one thing we'll be free and clear of the virus. That's not going to happen. It's going to require a very intensive management program, and that's where we're headed with this research. But there are many things we still don't know about tomato spotted wilt virus,” says Csinos.
There have been indications from growers, researchers and Extension specialists that factors such as plant source, host weeds, planting date and weather conditions all have an effect on the virus, says Csinos.
“I think they're all right. Our problem is pinning it all down. It's like a needle in a haystack. We know there are a million things going on out here on the farm, and every farmer does things a little bit different. That may make a difference in whether one farmer has more tomato spotted wilt than another.
“We don't yet know the exact criteria that affects how much of the virus you'll have on your farm. But we do know that when farmers do certain things, such as pushing back their planting date, they have less of the virus. Our goal is to come up with a full-fledged management program that we can plug in. You may not be able to use all of the recommendations, but you might can plug in three or four things that'll decrease your risk of getting the virus,” he says.
The Bowen Farm located at the Coastal Plain Experiment Station in Tifton, Ga., always has been a “hot” spot for tomato spotted wilt virus, says Csinos. In past years, when statewide levels of the virus were at 4 to 6 percent, the Bowen Farm would get 40 to 50 percent, he adds.
“We use this location as our tomato spotted wilt virus research areas mainly because we can put out a test and be sure to get the disease every year. We have another banner year in 2004 with a 50 to 60 percent average on the farm.”
Researchers on focusing on several areas, says Csinos, including the effect of the age of tobacco transplants on the amount of tomato spotted wilt virus.
“About two or three years ago, we had a researcher who found that whenever he inoculated young plants, he had more virus than if he inoculated older plants. We have tried to further this finding by conducting tests during the past three years.”
The first and second years of the tests showed predicted results, he says. The older plants tended to be tougher, and they had less of the virus than the younger plants when transplanted in the field.
“This is what we expected because the younger plants were more tender than the older plants. Now, in the third year, it's difficult to tell, but it looks as though the younger and older plants are running about even. This will be a harsh year, so maybe we can't rely on just plant age. It may have to be tempered with other management tools.”
The test this year showed that a K-326 young transplant that was six to seven weeks old at transplanting had about 69 percent symptomatic plants. NC-71 transplants of the same age were running at about 48 percent infection.
“There's a little difference going on there with the two varieties, and that has been consistent over the years. It has not been statistically different every year, but there always is a trend. An older transplant of K-326 was running at about 67 percent while an older transplant of NC-71 was running at 57 percent.”
Adding Actigard and Admire dropped the infection rate to 30 to 35 percent, he says. “That isn't news, but we were hoping we could combine those applications with plant age. However, it looks as though the older and younger plants are running about even this year. It'll be difficult deciding next year if we need to do this again. We have two years saying there is an effect and one year saying there isn't one.”
Researchers also continue to look at the timing of Admire and Actigard applications on tobacco, says Csinos.
“Most everyone is very familiar with the fact that Actigard and Admire applied in the greenhouse and the floathouse will give you a reduction in tomato spotted wilt of about 50 percent over the non-treated plants. We're looking at whether or not there's a strategic time during that post-transplant period when we can apply Actigard and get a boost in the amount of disease control.”
This is the third year of the test, he explains, and Actigard was applied at 0, 14, 28 and 42 days post-transplant, at one half ounce.
“Right now, the data is saying that anything at 28 days shows a pretty good drop in the amount of the disease. For example, in the control — with no application in the greenhouse or field — we're running at 55 percent.
“If we make one field spray of Actigard and Admire, it's down to 15 percent. If we use Actigard and Admire in the greenhouse, with no field treatment, we're running at 33 percent. So, there seems to be some effect there, and the 28-day application always looks to be the best. Last year, 28 days was the best treatment.”
Researchers also believe that the amount of thrips occurring in a location has an effect on the amount of tomato spotted wilt virus, says Csinos, since thrips transmit the virus to the plants.
“We're looking at thrips to see if there's a peak of thrips numbers that we can use as a trigger for making applications of Actigard and Admire. We're using sticky cards, and we pick those up every 48 hours and count the thrips, so we have a pretty good idea.
“We had a sticky card in the field beginning two weeks before transplanting. We know how many thrips were there before we started, and we know what happens to the thrips population all the way through the growing season. We want to see if we can use the number of thrips as a trigger for making these applications.”
Two weeks prior to transplanting, thrips numbers were running at about five per card during a 48-hour period, he says. Thrips numbers reached their peak in mid-May. Last year, numbers went up during the first of May and came back down, he says.
“Last year, we had more thrips earlier. This year, they peaked about two weeks later than last year. However, the 28-day application last year coincided almost identically with the peak. So there could be something there.”
Researchers also have seen that these peaks are related to other factors, says Csinos, like thrips moving from dried rye grass. Warmer temperatures also make thrips more active, causing them to reproduce in greater numbers.
“We did not identify the thrips species — we just counted total thrips coming into the field. We estimated that 11 million thrips would be in a 1-acre field per day. So it's obvious we're not going to eliminate thrips. We're going to have to manage the virus through some other method. We're basically using thrips as an indicator of what we think is occurring in the field.”
Previous research has shown that planting dates have some effect on the amount of tomato spotted wilt virus in a field, says Csinos. “The problem we've seen is that there's a difference every year, and we don't know what the difference is going to be. We don't know if we'll see a higher incidence of the virus early or late. We can't predict what that difference will be at planting.”
Many factors can affect tomato spotted wilt virus, he says, and what is happening at one location may not be happening at another. “What occurs consistently on one farm may not occur at a farm 10 or 20 miles down the road. That's why it's so important to identify only the modifiers of the disease, and then use only those that might affect your farm or crop.”
At the Bowen Farm in Tifton, three planting dates were used this year, explains Csinos. March 23 was the earliest date, with the second planting date two weeks later, and the third planting date two weeks following the second.
“We had three planting dates, two weeks apart, and we looked at the differences in treated and untreated. Untreated, early had by far the greatest amount of the virus. There's a nice trend in terms of virus reduction whenever the planting date was delayed. Planting date has been a very key factor in reducing the amount of tomato spotted wilt, even in the absence of treatments.
“There are so many modifying factors out there. Every farmer will have to look at each individual farm and decide how the criteria can apply to his operation. Under certain conditions, we can use some of these known factors and do a pretty good job of reducing the amount of the disease.”