As if Florida vegetable growers don’t already have a full plate of plant diseases, a new virus is slowly spreading through South Florida, causing sporadic infections in tomato, pepper, eggplant and tomatillo along the way.
Researchers, such as Scott Adkins, an Agricultural Research Service research plant pathologist, say growers and consultants shouldn’t panic over the discovery of groundnut ringspot virus (GRSV), which is closely related to tomato spotted wilt virus (TSWV).
Instead, he and others recommend adding GRSV to the list of pests you or your consultant already scout for.
If you do find something suspicious, the researchers encourage reporting it to your county agent or vegetable Extension specialist so they can confirm identification and map its spread.
And since both GRSV and TSWV are spread by thrips, the researchers say they believe the same management measures growers follow for TSWV also should help control the newcomer.
More questions raised
Charlie Mellinger, a plant pathologist and principal in Jupiter-based Glades Crop Care, had occasionally seen symptoms similar to TSWV on tomatoes in the Homestead area for several years.
Although TSWV is common in North Florida’s tomato production area, it’s uncommon in the southern part of the state, Mellinger says. Most times when he sent in a sample to Adkins’ lab for identification, it came back negative for TSWV.
In 2009, a post-doctoral researcher from Australia, Craig Webster, was working in Adkins’ lab at the U.S. Horticultural Research Lab in Ft. Pierce.
He ran a general test for tospoviruses, the genus of viruses that includes GRSV, TSWV and tomato chlorotic spot virus. When Webster and Adkins analyzed the results, they discovered GRSV.
“We’re trying to resolve plants that we had identified as TSWV in South Florida—we think now they were GRSV,” Mellinger says. “We’re not really sure what our past incidence of TSWV was in South Florida since we find plants that test positive with both [viruses].”
Since the initial confirmation, researchers have more questions than answers about the newly identified disease, Adkins says.
To help address some of those unknowns, USDA’s National Institute of Food and Agriculture has provided one-time seed money to address issues associated with emerging and new plant and animal pests and diseases, including GRSV.
Representatives from ARS, universities, the private sector and the grower community will work cooperatively in the effort, Adkins says.
Surveys of tomato production areas will begin this fall in South Florida and move up the Atlantic coast next year with the tomato season, Adkins says.
A similar survey will be conducted in Southeast peanut production areas, including the several hundred acres around Lake Okeechobee grown mostly for the boiled peanut market.
From South Africa to South America to Florida
When GRSV was confirmed in the Homestead area in 2009, it marked the first time the disease had been found in the United States.
It is native to South Africa, where it causes stunting of the peanut plant and small, distorted leaves with severe yellowing and concentric ringspots. GRSV also has been identified in tomatoes in Brazil and Argentina during the 1990s.
One of the challenges is very little has been written about GRSV in literature, so researchers have very little to go by, Adkins says.
Since the initial discovery in Miami-Dade County, GRSV has been found in tomato fields in Collier, Hendry, Lee, Manatee, Martin and Palm Beach counties.
In addition, it has been found in pepper, eggplants and tomatillo in peninsular Florida from Manatee County south.
Based on grower and scouting reports in 2010, sporadic infections were seen throughout south Florida’s tomato production area at low levels, typically 2 percent to 3 percent, Adkins says.
How wide a host range the virus has is one area in which the researchers hope to gain more knowledge.
Tomatoes, peppers, eggplants and tomatillo are members of the Solanaceous family whereas peanuts are members of the legume family.
Host plants can be infected during any growth stage.
During the 2010 season and again this year, infected plants were also found in greenhouses, Adkins says.
When tomato plants are infected in the seedling or plug stage, they typically die before reaching the field he says. Frequently, GRSV symptoms are very similar to those of tomato spotted wilt virus, Adkins says.
If tomato plants are infected at an early stage, symptoms include inward rolling leaves, leaf bronzing and dark brown leaf flecking.
As the infection progresses, brown streaks form on the main stem and leaf petioles, and wilting or death occurs in the top portion of the plant. Fruit may be deformed, show uneven ripening and may have raised bumps or rings.
But if you see necrosis on tomato stems, there’s a good chance it’s GRSV, Adkins says. In other crops, such as peppers and eggplants, the symptoms are visually nearly identical to TSWV.
The only way to positively confirm which disease is present, or whether a sample contains both, is through laboratory molecular testing, Adkins says.
“At this point, I’d say it’s based on where you’re located,” he says. “If you’re on the peninsula and you see strong TSWV symptoms, I’d suspect GRSV. In South Carolina, people wouldn’t give GRSV a second thought” because TSWV is common.
Thrips spread the virus
Western flower thrips are a confirmed vector of GRSV, Adkins says.
The larvae, or immature thrips, pick up the virus as they feed on infected plants. They remain infectious when they reach adulthood and transmit the virus as they feed on healthy plants.
This is the same infection mechanism that occurs with Western flower thrips and tomato spotted wilt virus.
One of the goals of the multi-state research project is to conduct tests to determine whether other thrips, such as common blossom or yellow flower thrips (Frankliniella schultzei), melon thrips (Thrips palmi) or chilli thrips (Scirtothrips dorsalis), also can vector GRSV, he says.
Mellinger says there’s strong circumstantial evidence that points to F. schultzei also being a vector of GRSV.
In the Homestead area, he found tomato fields with higher GRSV incidence adjacent to watermelon fields with high populations of F. schultzei.
GRSV management strategies
Although little is known about GRSV, Adkins says he surmises that the same management techniques growers use for TSWV would probably help control GRSV.
Those include starting with clean transplants, applying a systemic insecticide at planting, using metallized mulch on planting beds, using selective insecticides to preserve beneficial insects and using TSWV-resistant tomato varieties with the Sw5 gene.
The Sw5 gene also appears to impart resistance to GRSV, Adkins says, citing research results.
For more information on GRSV, download the University Florida EDIS PP282 at http:// edis.ifas.ufl.edu/pp282.