Thrips-vectored viruses turn up in scattered tomato fields

01/08/2013 11:32:00 AM
Vicky Boyd

A virus first found in a Homestead tomato field three years ago continues to slowly spread, while a virus new to the United States turned up in a handful of south state tomato fields last summer.

Both groundnut ringspot virus and the newer tomato chlorotic spot virus are tospoviruses, a family of about two dozen viruses worldwide that also includes tomato spotted wilt virus, says Scott Adkins, a research plant pathologist with the U.S. Department of Agriculture Agricultural Research Service in Fort Pierce.

Both the newcomers remain isolated, having been found at relatively low levels in scattered fields, he told attendees of the recent International Pepper Conference in Naples.

Because the two viral diseases are so new to the state, researchers say they are still learning about their habits under Florida growing conditions and haven’t yet developed grower recommendations.

But they say they believe that the same control measures used for tomato spotted wilt virus also should help control the two newcomers.

Those measures include:

• starting with clean, virus-free transplants;

• planting varieties with the Sw5 resistance gene;

• using reflective mulch;

• using soft chemistry to preserve natural thrips enemies;

• destroying the crop as soon after harvest as possible; and

• avoiding planting a new crop when the old crop is still present in an area.

The thrips-tospoviruses relationship

Tospoviruses are spread by thrips, but only the immatures—or larvae—can actually pick up and become infected with the virus.

The virus remains virulent during pupation, and the winged adult that emerges is responsible for most virus transmission.

“If they don’t get it as a larvae, they can’t transmit it as an adults,” Adkins says.

The efficiency with which thrips can transmit disease varies among tospoviruses and among thrips species, Adkins says.

Groundnut ringspot virus

Field consultants with Jupiter-based Glades Crop Care Inc. noticed something unusual in a Homestead tomato field in 2009. Because the symptoms didn’t look like typical tomato spotted wilt, they sent them to the USDA for identification.

Eventually, USDA scientists identified the problem as groundnut ringspot virus, a tospovirus native to South America and also found in Africa, Adkins says. Until the Homestead discovery, it had not been found in the United States.

The virus has since spread and has been seen in scattered tomato fields south of Interstate 4, says Galen Frantz, senior crop management specialist with Glades Crop Care in Lehigh Acres. It also has been confirmed in peppers, eggplants and tomatillos.

A survey of 500 fields in eight other Southeastern tomato-producing states this summer did not detect GRSV, he says.

Florida remains the only state in which the virus is found.

Glades Crop Care plans to conduct trials this spring to study how the disease spreads within a tomato field, Frantz says.

In South America and Africa, GRSV infects not only Solanaceous plants—which include tomatoes, peppers and eggplants—but also legumes, such as peanuts.

A survey of Florida’s peanut acreage near Lake Okeechobee was conducted this summer, and none of 80 to 85 samples collected was positive for GRSV, Adkins says.

“We looked at other plants that showed similar symptoms and only found tomato spotted wilt virus, which was a surprise to us,” Adkins says. “And we only saw GRSV on solanaceous plants.”

Further genetic testing showed what scientists were calling GRSV in Florida actually was a hybrid that had genetic material from true groundnut ringspot virus as well as tomato chlorotic spot virus, or TCSV.

That might explain why the survey didn’t detect it in peanuts, Adkins says.

A new tomato virus

Until last spring, TCSV was known to occur in Argentina and Brazil but not in Florida, Jane Polston, a University of Florida plant pathologist in Gainesville, told attendees of the recent Florida Joint Tomato Conference.

But that has since changed.

Polston says she received 15 samples collected from Miami-Dade County on March 15 and 12 collected from Lee County April 12—all showing leaf necrosis.

Three different tests were used to pinpoint the culprit.

Immunostrips from Agdia can quickly confirm the presence of a tospovirus, but she says they won’t differentiate the species.

Polston then turned to PCR, a type of genetic fingerprinting, to try to narrow the organism’s identity.

To confirm her results, she sequenced the newcomer and compared the results to known viruses in the gene bank.

The verdict: TCSV.

It has since turned up in Hendry and Collier counties, Adkins says.

In South America, TCSV has been reported to have a host range that includes vegetables and ornamentals. Among them are peppers, tomatoes, tobacco, jimson weed and ornamental tobacco.

Both western flower thrips and flower thrips are known to spread TCSV in South America, she says.

“So I don’t think we’ll be losing this virus any time soon,” Polston says. “There’s a whole lot we don’t know about TCSV, especially in Florida.”

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