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California Industry on High Alert

The Tomato Magazine
August 2007

By Lisa Lieberman

T he discovery of the deadly tomato yellow leaf curl virus (TYLCV) earlier this spring in California has the tomato industry on high alert.

“TYLCV is recognized as the worst, most pathogenic tomato virus there is,” says Robert Gilbertson, a plant pathologist at the University of California, Davis. Diseased plants were found in a Brawley
greenhouse, near the California/Mexican border. Experts believe that the virus could have been brought in via tomato transplants from Mexico or Texas, or that it could have been carried in by whitefl ies. Prior to the 1990s, the TYLCV was only known to occur in the Middle East and Africa, Gilberston says. Later, the virus was introduced into the Dominican Republic and the Caribbean where it then spread to Florida, Georgia and Louisiana. In 2005, it was discovered in Northern Mexico for the first time.

“Wherever the virus has spread, it’s caused signifi cant losses to tomato production,” Gilbertson points out. The virus causes tomato plants to become stunted and grow abnormally upright. The
leaves turn bright yellow and the fl owers fall off the plants, usually before fruit sets. The virus is not transmitted in the seeds or by touch; it’s only transmitted through the white fly, Bemisia tabaci, in particular.

“The whitefly is a good vector,” Gilbertson notes. “That’s how it spread so efficiently from the Caribbean to the southern United States.” As soon as the virus was discovered in Brawley, California Department of Agriculture offi cials quarantined the greenhouse where it was found and destroyed all infected plants.

“We surveyed the area around the infected greenhouse and found nine residential locations that had diseased plants. All were destroyed. We haven’t seen any signs of the disease spreading beyond that area, so we think that’s an encouraging sign,” reports Steve Lyle, spokesman for the CDFA. Bemisia tabaci is the most common type of whitefly in Imperial County, but doesn’t do nearly as well in northern California where temperatures are much cooler, Gilbertson explains. The vector is limited to the southern areas of the state, although it can push itself as far north as Kern County and into southern Fresno County.

Greenhouse growers, however, need to be on the lookout for the virus, especially if they have received transplants from Imperial County, Gilbertson warns. Whiteflies carrying the virus can be extremely dangerous. Just a few infected fl ies can infect 100 percent of the plants in a fi eld within a short period of time.

One of the big differences between TYLC and other diseases is it spreads much faster. When whitefl ies feed off an infected plant, they pick up the virus in just a few minutes and spread it to the next healthy plant. One white fly can infect up to 100 plants.

“This is such a serious disease that we’re monitoring the fi elds with the same level of seriousness as if we were monitoring for the Med fly,” Gilbertson explains. “We don’t want this virus to get established in California. The fact that we identified it so quickly was critical, and we’re cautiously optimistic that we’ve limited exposure. But the virus has shown it has a sneaky way of getting established wherever it is introduced.”

When the TYLCV was first introduced into the Dominican Republic in the early 1990s, it temporarily wiped out the entire tomato industry.

“They had hundreds of acres of processing tomatoes--a $10 million industry,” Gilbertson exclaims. The Dominican Republic government was able to help put the industry back on track by imposing mandatory tomato-free periods during the summers.

“During June, July and August, growers aren’t allowed to do any plowing or plant any tomatoes,” he adds. “The insect only lives for one month, and the virus isn’t transmitted through the eggs, so having this three-month free period, basically cleans the virus out of the whole system.”

The problem, though, is that the virus can also infect weeds in the fi elds, which can serve as reservoir hosts and don’t always show symptoms. “By the end of the growing season, the virus comes back. But, the delay is enough so that early maturing hybrids and resistant hybrids can achieve good yields,” Gilbertson says.

In the Dominican Republic, June, July and August aren’t critical growing months for the local canneries, so the three-month tomato free periods have worked out relatively well, the researcher notes. In
Florida, Georgia and Louisiana, where TYLC has also established a foothold, threemonth, tomato-free periods also are being considered.

“These states haven’t enforced the free periods because it would take a significant effort to get everyone on board. Right now, they’re relying on chemical controls for the whiteflies. They’re also working on developing varieties resistant to the virus,” Gilbertson reports.

Admire, a neonicotinoid, is the commonly used chemical for controlling whiteflies, the researcher says. So far, it’s been a very good material. It’s not highly toxic to animals, and while the white flies have shown some signs of becoming resistant to it, they don’t seem to be increasing their resistance very quickly. In Florida, offi cials are also encouraging growers to destroy the vegetation in their fields once they’re done with the growing season. This helps with sanitation. Some growers in the southern area of the state are also beginning to minimize production during certain times of the growing season to
help reduce the spread of the virus.

In California tomato growing regions such as Fresno, there’s a natural tomato-free period from November to early February where there are no tomatoes in the fields, Gilbertson says. “With diseases like spotted wilt virus, you have a wide host range for the disease, but with TYLCV, the virus doesn’t generally do well in too many other plants. So, while it’s true that it can survive at low levels, the bottom line is that with the right measures, it’s still possible to produce healthy tomato crops with TYLCV,” Gilbertson promises.

© 2007 Columbia Publishing

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