Published on 01/20/05

Weeds help predict severity of killer crop virus

By Brad Haire
University of Georgia

Tomato spotted wilt virus hurts many crops in Georgia. But its severity varies from year to year. University of Georgia scientists are developing an alert to help vegetable farmers know how bad it will be each year before they plant.

"If a grower knew how bad TSWV might be before transplanting in the spring, he could make some management decisions that could save money or better protect the crop," said David Riley, an entomologist with the UGA College of Agricultural and Environmental Sciences.

Since appearing in Georgia in the late-1980s, TSWV has cost Georgia farmers hundreds of millions of dollars in damage to crops like peanuts, tobacco, peppers and tomatoes.

The carrier

TSWV is carried by tiny insects called thrips. They get the virus when they feed on infected plants. When they leave those plants, they can carry the virus to healthy ones. Millions of thrips per acre can visit a field in a year.

Prevention is the only "cure" for the virus. Once a plant gets it, it will grow poorly or die.

Thrips' populations drastically decline in winter. But a few, Riley said, retreat to the weeds that surround fields. They survive there until spring and then return, possibly with the virus, to freshly planted vegetables.

In the weeds

Starting last summer, Riley and other CAES scientists began an extensive survey of weeds and thrips numbers in Brooks, Colquitt, Decatur and Tift counties. A large portion of Georgia's vegetable crop is grown in these counties. Each month, weed samples are taken from two field sites in each county.

They want to know if weeds near the fields have TSWV and if thrips are feeding and reproducing on these weeds. They will focus on the data taken in February and March.

High numbers of samples with TSWV and thrips at this time will indicate a high risk for the spread of the virus. If this happens, an alert will be issued.

With this information, farmers can decide how best to protect their crops in the spring. "This can be another tool and service we can provide that farmers can use to know what to expect," he said.

Farmers can help. They can fill gallon-size plastic bags with weeds, particularly chickweed, cudweed, sow thistle, swine cress and Carolina geranium, from around fields planted with peppers or tomatoes. They can submit samples to their county UGA Extension Service agent or contact Stan Diffie at (229) 386- 3374.

Warm winter weather tends to increase thrips populations, Riley said. He's noticed a correlation, too, between the tree pollen amounts and thrips populations.

Thrips like to eat, among other things, pine tree pollen. "The increase and decrease of pollen seem to mirror the increase and decrease of thrips populations each year," Riley said.


To protect crops, vegetable farmers spend extra money on TSWV- resistant crop varieties and insecticidal sprays to control thrips, he said.

Many Georgia vegetables are grown in fields of raised beds wrapped in plastic film, mostly black. This helps farmers better control the crop environment.

Some farmers have started using more expensive reflective metallic film. Many believe it disorients thrips and keeps them from landing on crops' leaves.

Forecast tools exist for other crops. One developed by North Carolina State University predicts each year the possible severity of blue mold, a fungus that attacks tobacco. And research by CAES scientists has shown that Doppler radar can help peanut farmers know when to effectively apply fungicides.

Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.