Tomato Spotted Wilt Virus

Emerging Menace

Tomato growers in California, who produce about 90% of the nation’s processing tomatoes, saw widespread symptoms of tomato spotted wilt virus (TSWV) during the 2008 production season. And while TSWV was severe in many fields, says Tom Turini of University of California Cooperative Extension, both identification of symptoms and damage could be difficult. “Curly top virus and adverse weather conditions have also plagued tomato production this year,” says the Fresno County vegetable crops farm advisor. “And in some fields, this has made diagnosis difficult.”

That’s problematic for growers because getting an accurate diagnosis is the first step in determining if they have a problem with TSWV, which only started causing serious problems in 2004, says Turini. TSWV symptoms on tomato include a bronzing or wilting of the leaves, which may be associated with brown necrotic spots and dead leaves or shoots. The symptoms that appear on the fruit, however, are the dead giveaway: Concentric rings that give the tomatoes an alien appearance.

Growers who suspect they have a problem can, of course, contact their local farm advisor. But now growers can diagnose TSWV on their own, says Turini. A company called Agdia manufactures a product called Immunostrips for Tomato Spotted Wilt Virus. Its form is much like a home pregnancy test, says Turini. It uses the same antibodies as in the ELISA test, which many growers are familiar with, but is much quicker.

“You put a leaf in a bag, grind it up, stick the strip in, and you’ll see results in minutes — one line if it’s negative, two lines if it’s positive,” he says. 

Get After It

If the test for TSWV is positive, Turini said growers have few options to reduce the risk of serious loss due to the virus but to protect their crop by embarking on an aggressive insecticide program. TSWV is vectored by thrips, and while nine species of thrips are capable of transmitting the virus, by far the most important vector in California is Western flower thrips, Frankliniella occidentalis. These thrips are a formidable foe for several reasons, said Turini, and while insecticide applications can be a component of a TSWV control program, there are serious limitations.

First, the females lay 200 to 400 eggs. Second, they have a rapid generation time, going from egg to adult in just eight days in ideal conditions, which means they can more easily develop resistance to insecticides, making a good rotation program vital. Finally, they’re simply difficult to kill. “Both the larval and adult forms end up in places, such as down deep in the flowers, where they don’t get direct contact with insecticides,” he says.

The good news is that there are insecticides available that Turini’s trials — which are being funded by the California Tomato Research Institute — have shown to be effective. The three insecticides that have separated themselves from the pack are: methomyl (Lannate, DuPont Crop Protection), dimethoate (various manufacturers), and a product just introduced this year, spinetoram (Radiant, Dow AgroSciences). While all these products are effective, Turini strongly emphasizes that they must be rotated because of concerns the thrips will develop resistance.

Searching For Answers

Asked why TSWV seems to be getting worse each year, Turini sighs audibly. “If we can find a reservoir — where the virus is during the winter months when there are not that many hosts — we might be able to answer that question,” he says. “This winter we will be taking a close look at where the most severe outbreaks have been found.”

In the meantime, he advises growers to avoid planting in areas where hosts with the virus are present at planting. There’s a long list of crop hosts besides tomatoes, with peppers and radicchio among the most obvious. Among the reported weed hosts are sowthistle, prickly lettuce, malva, bindweed, common sunflower, black nightshade, hairy fleabane, jimsonweed, lambsquarters, purslane, pigweed, Russian thistle, and tree tobacco.

Turini says he realizes it’s tough for growers to avoid all those hosts, particularly if they don’t have a lot of ground to choose from, but there is one last way to minimize the risk of your tomato crop contracting TSWV. “If you know you’re going into a high-risk situation,” he says, “consider a resistant variety. There are some good ones now.”