While Georgia peanut producers were encouraged this past year by the lowest incidence of tomato spotted wilt virus (TSWV) since the early 1990s, another, less well-known virus showed a considerable increase in many fields.
Impatiens necrotic spot virus (INSV) was first detected in peanut fields in Georgia and Texas in 1998, explains Lenny Wells, University of Georgia research plant pathologist. “We surveyed fields in six counties in 2001, and we found INSV in every field in every county. The incidence of the virus was up considerably from the previous two years,” says Wells.
INSV, he says, is an important disease of ornamental plants and often has a major effect on the greenhouse industry. INSV is closely related to tomato spotted wilt virus, he adds, and the viruses are believed to exhibit similar symptoms on peanuts, including concentric ringspots, “oak leaf” patterns of chlorosis, bronzing of leaves, stunting, wilting and death.
“Currently, only Western flower thrips is confirmed as a vector of INSV,” says Wells. “However, there is evidence that tobacco thrips also may transmit the virus.”
During 1998, INSV was detected in Georgia peanut fields in Tift and Mitchell counties, in the southwest region of the state, he says. “As a result of those findings, we conducted a survey of Georgia peanut fields for INSV during the 1999 and 2000 growing seasons.
“During August of 1999 and June, July and August of 2000, we sampled peanut plants from 15 counties. INSV was not detected in 1999. However, during 2000, INSV was detected in seven fields — three from Coffee County, two from Colquitt County and one each from Randolph and Sumter counties. Overall, INSV was detected in 2 percent of all plants sampled. Most of the infected plants were in Coffee County, where 36 percent of the plants tested positive for the virus,” says Wells.
This past year, surveys for INSV were conducted in six Georgia counties — Berrien, Crisp, Coffee, Randolph, Sumter and Tift, he says, and the virus was found in every field in every county. Forty-six percent of the infected plants were symptomatic and 36 percent were non-symptomatic, he says.
“Each of the plants showing symptoms of the virus also were infected with tomato spotted wilt virus. Of the non-symptomatic plants, 23 percent also had tomato spotted wilt. Most symptoms we're seeing were produced by tomato spotted wilt virus. Tomato spotted wilt may make plants more susceptible to INSV, but we're not sure of that at this point.”
Since the incidence of INSV increased in 2001, researchers may expand their surveys to more counties in 2002, says Wells. “We want growers to be aware that INSV is out there, but we don't want them to be alarmed. INSV, though present, does not appear to pose a major threat to peanuts in Georgia at this time. Nevertheless, continued monitoring of the virus is important.”
University of Georgia researchers also continue to look at the biology and behavior of the insects transmitting TSWV to help develop better ways of managing that virus, notes Wells.
“Many important aspects in the understanding and development of new tactics to combat TSWV come from studying the insects that transmit the virus. Two species of thrips — tobacco thrips and Western flower thrips — are considered the main vectors of TSWV in Peanuts.”
By collecting tobacco thrips (the most abundant vector of TSWV in south Georgia) from covered and uncovered potted peanut plants in weedy and weed-free fields during early spring, researchers have learned that it is unlikely that thrips are emerging from the pupal stage in the soil in sufficient numbers to have an impact on TSWV incidence, says Wells.
“Thrips colonize crops by emerging from cool-season weeds from which they migrate to crop hosts. Due to the relatively mild climate in south Georgia, tobacco thrips are believed to survive the winter — thrips can survive temperatures of less than 20 degrees F.) and probably reproduce year-round on a variety of weed hosts and on cover crops such as wheat.”
In addition, brachypterous thrips — non-flying thrips found at higher percentages during cooler months — probably do not play an important role in TSWV transmission during the growing season, since their numbers are reduced as the warmer planting season approaches, and very few are found to be capable of transmission whenever crops are in the field, he says.
“They can, however, harbor the virus over the cool seasons of the year, perpetuating the virus cycle. Cycles have been observed in the proportion of virus-transmitting thrips collected from field populations of tobacco thrips during the early season.
“The timing of these cycles may vary slightly from one year to the next and probably are influenced by thrips population buildup, which appears to be related to environmental conditions in the early spring.”
Research, says Wells, also supports the theory that thrips have a difficult time locating crop hosts in reduced-tillage systems, since very few thrips were collected from uncovered peanuts in fields where weeds were abundant.
“The opposite effect was observed in weed-free fields where higher numbers of thrips were collected from uncovered plants,” he says.