Thrips and Spotted Wilt Management in Tomato

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Figure 1: Greater yield loss in tomato with earlier virus (TSWV) transmission at Tifton, GA in field plots during Spring 2000 (C. Chaisuekul, D. Riley, and H. Pappu)

Tomato is currently one of the most important vegetable crops in Georgia, second only to the Vidalia onion crop in terms of value, and is worth approximately $80 million annually. Currently, the greatest threat to tomato in Georgia is thrips-vectored Tomato spotted wilt virus (TSWV).

In Georgia, TSWV and thrips vectors have had a tremendous negative impact on the yield of various crops including peanut, tobacco, tomato, and pepper, causing an estimated $100 million in losses annually across all of these crops. In tomato, TSWV can often reduce marketable yields by 50% besides greatly increasing the incidence of irregular-ripened tomatoes. The thrips species primarily responsible for transmitting this virus in tomatoes are western flower thrips (Frankliniella occidentalis), tobacco thrips (Frankliniella fusca), and others (for example, Frankliniella bispinosa and onion thrips, Thrips tabaci).

Based on recent studies by Drs. David Riley and Hanu Pappu at the University of Georgia - Coastal Plain Experiment Station at Tifton, a thrips-TSWV management program has been developed for tomatoes that offers both short-term and long term benefits to tomato growers in TSWV-affected production areas in Georgia and the southeastern USA. The rationale for this program is based on the discovery that, in the tomato crop, early-season virus transmission has a much greater impact on yield than if the virus is transmitted to the plant later in the growing season (see Figure 1).

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Photo by Alton N. Sparks, Jr., University of Georgia, Bugwood.org
Thrips tabaci
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Thrips tabaci
2511060
Photo by David Riley, University of Georgia, Bugwood.org
Irregular ripening of tomato caused by TSWV.
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Irregular ripening of tomato caused by TSWV.

Also, since transmission of the virus occurs through thrips feeding, Dr. Riley's research program has focused on various tactics that prevent thrips feeding, kill thrips before they can feed, and/or reduce the attractiveness of the crop so that less thrips occur on the plant.

These approaches have been targeted toward the critical thrips-control period, early in the tomato growing season. Virus transmission that occurs later in the season may not be as important in terms of yield, but there is still the problem of irregular ripened fruit that can occur (see photo of tomato fruit above).

Irregular ripening caused by TSWV can show up after commercial fruit have been treated with ethylene for ripening and can result in reduced tomato quality. The treatments evaluated in 1998-2000 resulted in less numbers of irregular-ripened fruit in the best treatment, imidacloprid soil drench plus lambda-cyhalothrin plus methamidophos foliar treatments beginning as soon as the tomatoes were transplanted. In addition, host plant resistance in the tomato cultivar BHN444, and silver reflective mulch greatly improved yields (see Table below).

Results of 1999 tomato test at Tifton, GA in terms of main plot (BHN444 resistance and silver reflective mulch) and subplot (different length periods of insecticide control of thrips) effects on number of thrips, % TSWV, and $ yield per acre, respectively.
BHN444-silver mulch 47 b 28% b $7,233 a
Sunny Hyb.-silver mulch 49 b 57% a $4,721 ab
Sunny Hyb.-black mulch 64 a 67% a $3,602 b
Admire + four weeks of foliar sprays 53 b 28% c $6,685 a
Admire + two weeks of foliar sprays 57 b 56% ab $6,102 a
Admire + eight weeks of foliar sprays 13 c 44% ab $5,781 a
Admire + one week of foliar sprays 70 a 58% ab $3,777 b
Untreated check 73 a 67% a $3,580 b


The results from several field trials like the 1999 test reported here all indicated that TSWV could be reduced and yields enhanced with the right combination of specific treatments. The role of each treatment is still being quantified. For example, foliar sprays without the imidacloprid treatment in the 1998 test didn't do as well as foliar plus the imidacloprid treatment in terms of improving yields. Further investigation revealed that imidacloprid was needed to reduce thrips feeding (see Figure 2) which aids in reducing virus transmission when combined with other treatments.

Figure 2: Average feeding scars per 6 thrips compared to applied Admire rate (µl per plant) and Admire leaf residue (ppm) on tomato leaves. (C. Chaisuekul and D. Riley, 2000)

The results of several studies demonstrated that thrips and TSWV could be reduced considerably with host plant resistance, reflective mulch, and insecticide treatments. The use of reflective mulch just on the shoulder of the bed aided in early season soil warming in the middle of the bed while still providing a reflective surface to repel thrips (see photo).

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Photo by Alton N. Sparks, Jr., University of Georgia, Bugwood.org
Staked tomato plants on silver-painted plastic mulch.
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Staked tomato plants on silver-painted plastic mulch.

From the research results and input from the State Vegetable Extension Specialist, David Adams, a program was commercially validated in fields in 2000 to begin refinement of this Early TSWV Management Program. Grower input helped to select the best treatments. For example growers have observed that excessive applications of lambda-cyhalothrin plus methamidophos tends to "harden" the tomato plant and this has been noted to lower yields, so excessive insecticide is not only a problem late in the season because of residues, it can also be economically unwise. Currently, this program is being validated in commercial tomato fields and has resulted in 12% and 46% yield increases in two fields in Tift County, Georgia. This joint research and extension project will continue into 2001 with further refinements as the project matures.

In summary, management of thrips as vectors of TSWV in tomato should target a reduction in thrips feeding where the transmission of the virus occurs. This includes tactics that will either directly prevent thrips feeding, kill thrips before they feed, and/or reduce the attractiveness of the crop to prevent thrips from occurring on the plant. These tactics should focus on the early season in the tomato growth cycle in order to have the greatest impact on yields. To affect the severity of the disease on tomatoes, the best option is the use of host plant resistance where it is available.

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Photo by David B. Langston, University of Georgia, Bugwood.org
Leaves appear bronzed, caused by tomato spotted wilt virus.
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Leaves appear bronzed, caused by tomato spotted wilt virus.

Future areas of research that Dr. Riley is exploring are how TSWV symptoms (see leaf symptoms in photo above) occur in tomatoes relative to thrips and mechanical inoculation, the relationship of TSWV inoculation with irregular ripening of tomatoes, factors that affect thrips feeding, and integration of host plant resistance with other management tactics. Like the TSWV Risk Index currently in place for peanuts in Georgia, similar programs need to be developed based on sound IPM research and effective extension implementation for tomatoes and other vegetable crops affected by this deadly plant virus.

For more information contact:
David Riley
Department of Entomology
University of Georgia
P.O. Box 748
Tifton, GA 31793
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