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Tomato Spotted Wilt Virus — Growing Worry for California’s Central Valley

By Lisa Lieberman

The Tomato Magazine
April 2006

Tomato spotted wilt virus (TSWV) hasn’t generally been a problem in California, but recently the virus has started showing up in more fields in the Central Valley, especially in the Fresno/Huron growing regions.

“When I first saw it, it was in a small, concentrated area,” says Michele LeStrange, farm adviser for the University of California (UC) in Tulare and Kings counties. “This last year, in 2005, the amount of symptoms was much more dramatic, and several hundred acres of tomatoes were disked just prior to harvest.”
Symptoms of TSWV include stunted plants and leaves turning brown. More important is that the virus can also cause off-color rings around the tomatoes which make them unappealing to consumers at the supermarket or unusable for processing, especially if the end use is as peelers.

Not a New Virus
“It’s not a new virus, but it’s a hot topic,” LeStrange adds. “Not many viruses are vectored by thrips, but this is one of them. That’s one of the things that make this virus unique.”

With TSWV, only the first and second instar larvae of the thrips can feed on the virus, acquire it and, as adults, transmit it to plant hosts.

“If they haven’t picked up the virus when they’re young, the thrips can’t spread it as adults,” LeStrange points out.

Since ultimately it’s the combination of the thrips vector and the actual TSWV itself that causes the infections in plants, it’s important to note than any inoculum sources must be suitable hosts for both the thrips vector and the TSWV, LeStrange says.

When adult thrips with the virus do infect large numbers of plants in a field, the fruit can be so badly scarred that it either has to be disked or sold to processors for lower prices. Plants infected with the disease often show symptoms of yellow to necrotic blotches on the leaves or dark russet spots. Some plants die back completely. These foliar symptoms, however, resemble several diseases.

“The real symptoms for this disease show up in the fruit,” LeStrange explains. “The diseased fruit have distinctive rings on them, so if you were looking at a pale green tomato with darker green rings or a red fruit with yellow or lime green rings, that’s very noticeable.”

In more severe infestations, the disease will start showing up in small green fruit at an early stage.

Impact Had Been Minor
Although TSWV has probably been around the area for a long time, growers generally only had problems with a few plants being affected here and there. So, the question LeStrange and fellow researchers are trying to answer is what and where is the inoculum source that is infecting the thrips?

Researchers are in the process of surveying hundreds of plants to determine which plants are hosts to the virus during the off-season. So far, they’ve only been able to find the virus in crops and not in any weeds or ornamentals, which is a good sign, LeStrange says.

“There’s probably a crop or two that really makes a good host for the virus and thrips. We just don’t know for sure what they are yet,” LeStrange admits. “When we can put a finger on them, we can encourage growers who are neighbors not to plant susceptible crops right next to each other.”

Researchers say that it’s important to keep an eye on the TSWV since the virus has a history of appearing sporadically in an area for a few years, then suddenly surging to epidemic proportions. Wherever TSWV incidence has increased enough to cause economic losses, it seems to remain a chronic problem. This pattern has been repeated in India, Australia, Hawaii and some areas of the southern United States. In the South, such as North Carolina and Georgia, TSWV has been a problem in tomatoes, peppers, peanuts and tobacco for several years, LeStrange says.

“When you have all those crops growing in an area simultaneously, the virus becomes a bigger problem,” she notes. “There needs to be a break in the cycle.”

Also Impacting Peppers, Lettuce
In California, although TSWV mostly impacts tomatoes, it also affects peppers and possibly lettuce, LeStrange says. It’s also possible that the virus comes from other plants such as foothill weeds, winter-time crops or ornamentals. In Merced, researchers have already found the virus on at least one ornamental.

Scott Stoddard, a UC farm adviser in Merced County, says that a disease very similar to TSWV or the virus itself became a problem in peppers in the early ‘90s, wiping out much of the industry in the area. In tomatoes, if the virus infects the plants early on, it will kill transplants within four to six weeks. If the transplants are already well underway, some fruit on the plants will become badly disfigured, he points out.

“If there’s already fruit on the plant when the plant gets the disease, then that fruit will mature out fine,” he says. “All the tomatoes that set after that will be pretty disfigured, or if you have plants that have been out in the field for six weeks and are starting to bloom, sometimes the fruit never really sets.”

With a late season infection, growers can still go in and harvest the fields and get a partial crop. Even though the plants don’t look healthy, there still may be some harvestable healthy fruit, he adds.

Seed companies have developed varieties with resistance to TSWV, but these are used mainly in Florida and Mexico. Commercial varieties used in central California do not have any resistance to this disease, Stoddard says. There has been some work done to come up with resistant varieties, although none of those are commercially available yet.

At the moment, there’s no really effective way to treat the disease, aside from trying to prevent it from happening in the first place by reducing thrips as much as possible in the fields, Stoddard says. Some research from other areas has shown that insecticides can reduce the severity of the disease, if timed properly. This would hopefully ease thrips damage as well as TSWV.

“We don’t have any management recommendations for control in tomatoes because this has always been a rare, nuisance-type disease,” Stoddard notes, explaining that the problem seems to have gotten a lot worse over the past couple of years, so more people are becoming aware of it.

© 2006 Columbia Publishing

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