The dreaded disease tomato spotted wilt virus (TSWV) and its effect on tomato production is the focus of a multi-state research effort that includes a predictive model developed at North Carolina State University.

“They’re looking at being able to predict the severity or the potential of TSWV — that’s a big part of this overall project,” says Stormy Sparks, University of Georgia Extension entomologist. “If we can learn to predict if it’s going to be a mild or severe year, we have a better idea of what inputs to put into a crop.”

The research also is looking at resistant tomato varieties, says Sparks. “We’ve got about 10 varieties we’re looking at in Camilla at the Stripling Irrigation Research Park and others at Tifton and Ridgeville. In vegetables and tomatoes, resistant varieties are by far to best way to manage tomato spotted wilt virus. The problem is that we’re looking at about 30 varieties of tomatoes in Tifton, and almost all of them have one source of genetics,” he says.

Resistant varieties definitely are the preferred method for managing the virus, he adds. “The problem is that when we lose that source of resistance, we lose all 30 varieties at once. History tells us that when it changes, it changes dramatically. We’ll lose it all at once, and then we’re back to trying to manage it with other techniques,” says Sparks.

One of the methods for controlling the virus is the use of plastic mulch, he says. The standard for the industry in spring production is black plastic. It warms the soil, says Sparks, and promotes good plant growth. However, it has no effect on TSWV.

The standard for managing TSWV, he says, is reflective plastic mulch. “It is a very shiny, basically aluminum-coated plastic mulch. It reflects UV light, and it repels thrips and makes it hard for them to find the plant. It has an obvious effect on reducing TSWV, but it also has an obvious effect on reducing plant growth because we’re not heating the soil,” says Sparks.

A combination of the two types is the heat-strip plastic mulch. It has a black strip down the middle that heats the soil where the transplants are planted. “The silver shoulders hopefully will give you enough light to repel thrips and reduce TSWV.”

Heavy infestation

Last year, at the Camilla location, 40 to 50 percent of the plants in the test were infected with TSWV, and there was a significant reduction in the virus with both the silver much and the heat stripe.

“This year, only 3 percent to 4 percent of the plants were infected with TSWV. Last year, we looked at the economics of reducing the virus. This year, we’re looking more at the cost of these production tools when we don’t have TSWV. The silver mulch costs more, it reduces plant growth and probably yield, so we’ll be looking at that cost in the absence of TSWV,” he says.

Another point of the research is to predict TSWV, says Sparks. “If we can get a prediction model that tells us when it’s going to be a bad year, we’ll have a better idea of when to use some of these tools. If we know it’s going to be a mild year for the virus, maybe we can back off some.”

In the Camilla trials this past spring, half of the plants were treated with Admire the day after transplanting. There’s a consistent reduction in TSWV when the plants are treated with Admire versus the untreated plants, says Sparks. “We saw that last year, but there’s probably little cost benefit this year due to the low levels of the virus, so we’ll be looking at the economics of that treatment. We’ve known for years that Admire reduces TSWV. But the question that has been asked multiple times and no one apparently can answer is do the other soil-applied insecticides reduce the virus? We put out a large replicated trial this spring in Tifton to take a look at that. We had 1 percent to 2 percent TSWV in that trial, so we still don’t know the answer to that question.”

Another trial involves treating half of a plot with Actigard, a foliar-applied chemical that activates a plant’s defense system. This particularly test has been inconsistent, says Sparks. “Sometimes it shows impacts on TSWV and sometimes it doesn’t. It you put it on too heavily or too frequently, it will stunt plants. We’ll be looking at the cost effectiveness of these applications. This year, we’re getting an economic evaluation and the costs of a lot of these technologies.”

As far as the predictive model for TSWV, researchers are looking weekly at virus incidence; collecting thrips samples from plants, blooms and sticky traps; and also putting out petunia traps and changing them weekly.

Whenever a thrips with TSWV feeds on petunias, it leaves a very localized lesion, explains Sparks. “So by changing plants weekly, we can get an idea of how much TSWV is moving in the environment. Part of this overall project is to try and estimate the potential for TSWV in a given year.

“With vegetables and tomatoes, the predictive model may not be as potentially valuable as in some other crops. Decisions such as which varieties to grow and which plastic to lay are made three and four months prior to planting. You can’t really change that if your prediction is four to six weeks out. But if the model will work here, it will help us greatly in the tools that are available to us.”

Turning to varieties, Sparks says there is another source of genetics that was developed in Florida, and researchers are working with two or three varieties of tomatoes with a different genetic base for resistance.

“They had problems with it last year. One variety looked good and one did not. Hopefully, we’ll have an additional source of genetics in the future.”

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