PDF

Tomato Spotted Wilt of Peanut Identifying & Avoiding High Risk Situations

Steve Brown, Extension Entomologist; Jim Todd, Research Entomologist; Albert Culbreath Extension/Research Plant Pathologist; John Baldwin and John Beasley, Extension Agronomists; Bob Kemerait and Hannu Pappu, Plant Pathologists

Spotted Wilt Eradication Action Team Web Site

When tomato spotted wilt virus (TSWV) infects a host plant, it can cause a disease that severely weakens or kills that plant. This particular virus is capable of infecting an unusually large number of plant species, including several that are important crops in south Georgia. In recent years, peanut, tobacco, tomato and pepper crops have been seriously damaged by TSWV. The only known method of TSWV transmission is via certain species of thrips, which have previously acquired the virus by feeding on infected plants. The factors leading to the rapid spread of this disease in south Georgia are very complicated, and no single treatment or cultural practice has been found to be a consistently effective control measure. However, research continues to identify factors that influence the severity of TSWV in individual peanut fields.

Factors Affecting the Severity of TSWV on Peanut

Peanut Variety

No variety of peanut is immune to TSWV. However, a few varieties have consistently demonstrated moderate levels of resistence. In addition to resistence (reduced disease incidence), some varieties appear to have some degree of tolerance (reduced severity in infected plants) as well. Higher levels of resistence and tolerance are anticipated, since peanut breeding programs are now evaluating potential new varieties for response to TSWV.

Planting Date

Thrips populations and peanut susceptibility to infection are at their highest in the early spring. The timing of peanut emergence in relation to rapidly changing thrips populations can make a big difference in the incidence of TSWV for the remainder of the season. Optimum planting dates vary from year to year but, in general, early-planted and late-planted peanuts tend to have higher levels of TSWV than peanuts planted in the middle of the planting season. It is important for larger acreage peanut farmers to spread their harvest season. Some staggering of planting dates may be necessary but, to avoid spotted wilt pressure, it may be more effective to plant varieties with different time-to-maturity requirements as closely as possible within a low-risk time period. If peanuts must be planted during a high-risk period, try to minimize the risk associated with other index factors.

Plant Population

An association between skippy stands and higher levels of TSWV was noted soon after the disease began to impact peanut production in Georgia. More recently, research has confirmed the impact of plant population on TSWV incidence. Low and high plant populations may actually have the same number of infected plants, but the percentage of infected plants is greater in low plant populations. In other words, a higher plant population may not reduce the number of infected plants, but it will increase the number of healthy plants that can fill in and compensate for infected plants. In some cases, low plant populations may result in increased numbers of thrips per plant, thereby increasing the probability of infection. When plant populations are as low as two plants per foot, severe losses to TSWV have been observed even when other factors would indicate a low risk level. Getting a rapid, uniform stand with the desired plant population is a function of not only seeding rate but also seed quality, soil moisture, soil temperature and planting depth.

Insecticide Use

In general, the use of insecticides to control thrips vectors has been an ineffective means of suppressing TSWV. In theory, lowering overall thrips populations with insecticides should effectively reduce in-field spread of TSWV. However, insecticides have proven to be ineffective at suppressing primary infection, which accounts for most virus transmission in peanut fields. Despite the overall disappointing results with insecticides, one particular chemical -- phorate (Thimet 20G and Phorate 20G)-- has demonstrated consistent, low-level suppression of TSWV. The mechanism of phorate's TSWV suppression is not known, but the level of thrips control obtained with phorate is not greater than that obtained with other insecticides. Phorate may induce a defense response in the peanut plant that allows the plant to better resist infection or inhibits virus replication.

Row Pattern

Seven- to ten-inch twin row spacing using the same seeding rate per acre as single row spacing has become increasingly popular in Georgia. Research on irrigated peanuts has shown a strong tendency for significantly higher yields, a one to two point increase in grade, and reductions in spotted wilt severity that have averaged 25 percent to 30 percent. The reason for this reduction in spotted wilt is not fully understood.

Tillage

The tillage method a grower uses can make a big difference in peanut yields. There are many different methods to choose from, each with its own merits and disadvantages for a given situation. Strip tillage has been shown to have some strong advantages (including reduced soil erosion and reduced time and labor required for planting), but in some situations, yields have been disappointing. Unbiased tillage research is difficult to accomplish, but studies have consistently shown that peanuts grown in strip till systems have less thrips damage and slightly less spotted wilt. On-farm observations have confirmed these results, but more studies are needed to characterize the magnitude of the reduction. We do not suggest that growers should change their tillage method just to reduce spotted wilt, but we have included tillage in the risk index in an attempt to better identify total risks.

Measuring TSWV Risk

Many factors combine to influence the risk of losses to TSWV in a peanut crop. Some factors are more important than others, but no single factor can be used as a reliable TSWV control measure. Research data and on-farm observations, however, indicate that, when combinations of several factors are considered, an individual field's risk of losses due to TSWV can be estimated. There is no way to predict with total accuracy how much TSWV will occur in a given situation or how the disease will affect yield; but by identifying high risk situations, growers can avoid those cultural practices conducive to major yield losses. The University of Georgia Tomato Spotted Wilt Risk Index for Peanuts was developed as a tool for evaluation of risk associated with individual peanut production situations. When high risk situations are identified, growers should consider making modifications to their production plan (i.e., variety, planting date, seeding rate, etc.) to reduce their level of risk. Using preventive measures to reduce risk of TSWV losses is the only way to control the disease. After the crop is planted, there are no known control measures.

The index combines what is known about individual risk factors into a comprehensive but simple estimate of TSWV risk for a given field. It assigns a relative importance to each factor, so an overall level of risk can be estimated. The first version of the index was developed in 1996 and was based on available research data. Small plot studies and on-farm observations have been used to evaluate index performance each year since release of the first version. In research plots where multiple TSWV management practices were used, as little as 5 percent of the total row feet were severely affected by TSWV, compared to more than 60 percent in high-risk situations. Yield differences were more than 2,000 pounds per acre in some cases. Results of these and other validation studies have been used to make modifications in the 2000 version of the index. Future changes are expected as we learn more about TSWV.

Keep in mind that the risk levels assigned by this index are relative. In other words, if this index predicts a low level of risk, we would expect that field to be less likely to suffer major losses due to TSWV than a field that is rated with a higher level of risk. A low index value does not imply that a field is immune from TSWV losses. Losses due to TSWV vary from year to year. In a year where incidence is high statewide, even fields with a low risk level may experience significant losses.

The University of Georgia Tomato Spotted Wilt Risk Index for Peanuts

For each of the following factors that can influence the incidence of tomato spotted wilt, identify which option best describes the situation for an individual peanut field. An option must be selected for each risk factor. Add the index numbers associated with each choice to obtain an overall risk index value. Compare that number to the risk scale provided and identify the projected level of risk.

Examples

If you want to do all you can to minimize spotted wilt:

Following strip tillage (5), C99R peanuts (15) are planted in twin rows (5) on May 10 (5) using Thimet in the furrow (5) with a final stand of 4.5 plants per foot of row (5).

Risk Index = 40

You can try a high-point variety and still avoid high risk:

Following strip tillage (5), Flavorunner 458 peanuts (50) are planted in twin rows (5) on May 8 (5) using Thimet in the furrow (5) with a final stand of 4.5 plants per foot of row (5).

Risk Index = 75

A low-point variety alone is not enough:

Following conventional tillage (15), C99R peanuts (15) are planted in single rows (15) on April 5 (25) using Temik in the furrow (15) with a final stand of 1.5 plants per foot of row (25).

Risk Index = 110

Thimet or Phorate alone is not enough:

Following conventional tillage (15), SunOleic 97R peanuts (50) are planted in single rows (15) on April 15 (25) using Thimet in the furrow (5) with a final stand of 1.5 plants per foot of row (25).

Risk Index = 135

If you prefer not to use Thimet or Phorate:

Following strip tillage (5), Virugard peanuts (20) are planted in twin rows (5) on May 8 (5) using Temik in the furrow (15) with a final stand of 4.5 plants per foot of row (5).

Risk Index = 55

If you must plant early:

Following strip tillage (5), Georgia Green peanuts (20) are planted in twin rows (5) on April 10 (25) using phorate in the furrow (5) with a final stand of 3 plants per foot of row (10).

Risk Index = 70

If you're not equipped for strip tillage or twin rows:

Following conventional tillage (15), Georgia Green peanuts (20) are planted in single rows (15) on May 10 (5) using Thimet in the furrow (5) with a final stand of 4.2 plants per foot of row (5).

Risk Index = 65

If you grow peanuts like they did in the 1970s:

Following conventional tillage (15), Florunner peanuts (50) are planted in single rows (15) on April 8 (25) using no at-plant insecticide treatment (15) with a final stand of 1.5 plants per foot of row (25).

Risk Index = 145

Trade names are used only for information. The Cooperative Extension Service, the University of Georgia College of Agricultural & Environmental Sciences, does not guarantee or warrant published standards on any product mentioned; neither does the use of a trade or brand name imply approval of any product to the exclusion of others that may also be suitable.

Bulletin 1165/Revised February, 2001

The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or disability.

An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.

Gale A. Buchanan, Dean and Director

Back to Georgia Extension Service Publications