TSWV index developed for Virginia-Carolina region

For peanut farmers, tomato spotted wilt virus (TSWV) is an unwelcome visitor. While you may not be able to stop the visit, you can minimize its impact by using certain practices before you plant.

Peanut farmers in the Virginia-Carolina region will have a tool to minimize the impact of tomato spotted wilt virus (TSWV) this season. Extension specialists and researchers in Virginia and North Carolina have developed a TSWV Advisory Index similar to the Risk Index developed in the mid-1990s at the University of Georgia.

The Advisory Index for Managing TSWV in North Carolina and Virginia Peanuts focuses on variety, planting date, plant population, in-furrow insecticide/nematicide and tillage. TSWV has become a major problem for peanut farmers in Virginia and North Carolina.

The Index assigns high-, medium- and low-risk point values to the practices. The key is to consider the strength and weaknesses of each cultural practice in developing a TSWV-management strategy that fits your farm. Practices that add up to 60 or less represent a low risk to TSWV; 65-85 is a moderate risk while 90 or more represents a high risk to TSWV. The authors expect to publish the Index the middle of March.

“It's important to remember that there's no cure for TSWV,” points out David Jordan, North Carolina State Extension peanut specialist. “But there are cultural practices that can help the growers minimize the risk to this devastating disease.”

TSWV has become an increasingly major pest in peanuts and other crops in North Carolina and Virginia over the past several years. In 2002, incidence hit an all-time high in both states. The virus infects a wide range of plants and is spread by thrips. Like all viruses, it's difficult to forecast changes and severity from year to year. The virus has marched across the peanut belt in the span of about two decades, making stops in Texas first, then the Southeast and now the V-C region.

Poor, inconsistent stands increase the risk of TSWV, making plant populations one of the most important elements in reducing incidence of the virus.

To illustrate how the Index works in the field, Jordan and the authors offer high risk, low risk options as well as a compromise between the two extremes.

For example, a low-risk option would have the farmer planting Gregory (which is 20 points on the Index) after May 5, but before May 15 (10 points on the Index) in strip-tillage (5 points on the Index) at a plant population of five plants per linear row of foot (5 points on the Index) and using Thimet 20G (Phorate 20G) in-furrow (5 points on the Index). This scenario shows a low risk of 45 points.

A high-risk scenario includes using none of the management options designed to minimize TSWV. For example, planting Perry before May 5 in conventional-tillage at a plant population of two plants per linear row foot, and using no in-furrow insecticide yields a score of 115 points, represents the highest risk to TSWV incidence.

An in-the-field scenario might be to compromise between the two extremes, Jordan says.

For example, on finer-textured soils with a history of Sclerotinia blight and Cylindracladium black rot (CBR), plant Perry between May 6 and May 15 in conventional-tillage at a plant population of five plants per linear foot of row, and using Phorate 20-G in-furrow. This scenario scores a 70 on the Index, which reduces the risk to a moderate level.

On coarse-textured soils with a history of Sclerotinia blight in the extreme northern range of North Carolina or up into Virginia production, moderate risk would involve planting VA 98R prior to May 5 in strip-tillage at five plants per linear foot of row and using Phorate 20G in-furrow.

Practices such as planting date, variety selection, seeding rate and insecticide can be used without additional equipment purchase. Twin rows or reduced-tillage systems may require additional equipment.

Among the five components of the Advisory Index, plant population is probably the most important, closely followed by variety selection and planting date, Jordan says. Tillage and insecticide choice aren't as important factors in reducing risk to TSWV as plant populations.

For example, a plant population of five or more per linear foot of row has a low-risk rating of 5; three to four plants per linear foot of row has a medium-risk rating of 15; two or less plants per linear foot of row has a high-risk rating of 25.

Virginia-type varieties Gregory, NC-V 11 and runners Georgia Green and C99R have the most resistance to TSWV. VA 98R and Wilson have moderate resistance. Susceptible varieties include Perry, NC 9, NC 7 and NC 12C.

A mid-range planting date of May 6-May 15 reduces risk to TSWV. Planting before May 5 or after May 15 increases risk of TSWV incidence.

Research in the lower Southeast as well as in the V-C region has found that Phorate 20G (Thimet 20G) does reduce TSWV incidence. It should not be the over-riding consideration for selection, however.

Strip-tillage into a killed cover crop reduces TSWV incidence over conventional-tillage.

Index authors are Christie Hurt, a North Carolina State graduate student, Rick Brandenburg, Extension entomologist; Jordan; Barbara Shew, Extension plant pathologist; Tom Isleib, peanut breeder; and Mike Linker, IPM coordinator at North Carolina State; Ames Herbert, Virginia Tech Extension entomologist; Pat Phipps, Virginia Tech Extension plant pathologist; Charles Swann, Extension peanut specialist; and Walt Mozingo, Virginia Tech coordinator of the PVQE program.

For more information, contact your local Extension office.

e-mail: [email protected]

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