Tomato Spotted Wilt Virus took the fast train from Texas to the Southeast in the 1980s, becoming the No. 1 priority for farmers and researchers alike. By the new millennium, it had taken up residence in the upper reaches of the peanut belt.
Through a concerted, multi-disciplined effort at the University of Georgia, research and Extension came up with a TSWV Risk Index in the mid-1990s. The Index has served as a model for researchers and growers in the fight to minimize the virus. Research and Extension have also developed a TSWV Risk Index for the Virginia-Carolina region closely based on the Index developed in Georgia.
The mantra has been “you can't control TSWV, but you can only minimize its risk by certain practices.” That's about as good as it gets when dealing with the ever-changing nature of a virus. Last year was a mild one for the virus in both the Southeast and the Virginia-Carolina region. “Don't count on it (TSWV) not showing up again,” says David Jordan, North Carolina State University Extension peanut specialist.
University of Georgia Extension entomologist Steve Brown co-authored the Index with colleagues from the disciplines of plant pathology, entomology and agronomy to give farmers a way to deal with the risk.
When TSWV infects a host plant, it can cause a disease that severely weakens or kills that plant. Thrips spread the virus after feeding on infected plants.
The Index has had a positive impact on managing risks to TSWV. The idea is to combine various practices to lower the risk to TSWV.
Early on, planting date appeared to have a bearing on the amount of TSWV incidence in farmer fields. Thrips populations and peanut susceptibility to infection are at their highest in the early spring. Early-planted and late-planted peanuts tend to have higher levels of TSWV than peanuts planted in the middle of the planting season. Planting date is the second consideration when using preventative measures to reduce the risk of TSWV.
In Georgia, a planting date prior to May 1 has the highest risk of 30 points. A middle date of May 11-25 has 5 points, while a planting after June 5 has 20 points. In the V-C, planting prior to May 5 has the highest risk of 20 points. The mid-planting date of May 6-15 has 10 points. After May 15, the risk increases to 15 points.
While no variety is immune to TSWV, a few have demonstrated moderate levels of resistance. TSWV resistance has become a priority for peanut breeding programs for new releases. In the 2004 version of the Georgia Index, there are two categories each for susceptible, three categories for intermediate resistant varieties and three categories for moderately resistant varieties, ranging in point value from 50 points to 10 points.
In the V-C area, Perry, NC 9, NC 7 and NC 12C are highly susceptible. VA 98R and Wilson are in the mid-range of susceptibility while Gregory and NC-V 11 offer the best way to reduce risk to TSWV.
Plant populations affect TSWV incidence. While higher plant populations might not reduce the number of infected plants, it will increase the number of healthy plants that can fill in and compensate for infected plants Getting a rapid, uniform stand is important in reducing the risk to TSWV. The best scenario in Georgia is a plant population of more than 4 pants per linear foot of row. In the V-C, the best scenario is more than 5 plants per linear foot.
While insecticides have proven to be ineffective at suppressing primary infection of TSWV, phorate (Thimet 20G and Phorate 20G) have demonstrated consistent, low-level suppression of TSWV. Phorate may induce a defense response in the peanut plant that allows the plant to better resist infection or inhibits virus replication. Researchers have found the same to be true in the V-C region.
In Georgia, twin rows have shown a one- to two-point increase in grade and a 25 percent to 30-percent reduction in TSWV severity.
Research in both Georgia and the V-C region has shown that conservation-tillage reduces TSWV severity. North Carolina State University Extension has developed a Risk Index for farmers considering making the transition from conventional to conservation-tillage.
Researchers in Georgia have observed that the use of Classic can result in an increased expression of TSWV, even though it has not always resulted in significant yield losses.
No single factor can be used as a reliable TSWV control measure. Used in combination, the factors in the Index can reduce the risk.
As the researchers and Extension personnel in Georgia point out, “The University of Georgia Tomato Spotted Wilt Virus Risk Index for Peanuts was developed as a tool for evaluation of risk associated with individual peanut production systems. When high-risk situations are identified, growers should consider making modifications to their production plan to reduce their level of risk.”