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Tomato Spotted Wilt Virus

Tomato Spotted Wilt Virus of Vegetable Crops

Photo:Thrips-vectored Tomato spotted wilt virus (TSWV) is one of the more severe problems affecting tomato and pepper production in the southeastern USA. Losses can range from a few percent to entire fields being abandoned. Since these two crops in the Southeast represent over $440 million in farm-gate value, the economic impact can be severe. TSWV has severely challenged traditional pest management practices in tomato and pepper because it is severe, unpredictable, and lacks highly effective control options. Most of the new information on TSWV management in pepper and tomato is now summarized at this linked USDA RAMP funded website.

Photo to your left illustrates speckling of tomato leaves caused by tomato spotted wilt virus, and the photo below is a tomato plant on the left that is severely stunted by the virus which is typical of an early infection.

TSWV wilted tomato plant

TSWV is spread solely by tiny insects known as thrips. In the southeastern US, the principal vectors are the tobacco thrips (Frankliniella fusca) and the western flower thrips (F. occidentalis), shown under magnification in the photos below.  Tobacco thrips vectors dispersing from weed hosts are able to transmit TSWV to susceptible crops quickly, often before they can be killed by insecticides applied to the crop.  Early season spread of TSWV from non-crop weed hosts into susceptible crops has the greatest impact on yield.

Subsequent spread within fields (or secondary spread) by large populations of western flower thrips developing on the crop can at times be problematic in tomato and pepper. Western flower thrips are extremely difficult to control because they are highly tolerant to insecticides commonly used on tomato and pepper, and they prefer to reside deep within the blossoms where they are difficult to reach with insecticides. Additionally, western flower thrips infestations are exacerbated by applications of certain insecticides that eliminate natural enemies of thrips.

tobacco and western flower thrips

In addition to their role as vectors of TSWV, both thrips species are important pests in their own right due to the damage that results from their feeding.  High populations of tobacco thrips on young plants early in the season can stunt plants and delay crop maturity, especially in pepper, whereas feeding on the developing fruit by western flower thrips causes blemishes on fruit which reduces marketability.

Effective management of TSWV in tomato and pepper requires the use of multiple tactics because single control practices often are not sufficient or the most economical. The use of TSWV-resistant tomato and pepper cultivars (also see table below) is our most important tactic for the management of this pest. Resistant cultivars do not appear to have much effect on thrips numbers, but can have strong levels of resistance to the virus. The main challenge to this tactic is the potential for resistance breaking strains of the virus. Thus, resistant cultivars need to be re-evaluated annually at multiple locations. Other important tactics include reflective mulch, systemic acquired resistance, insecticidal sprays, careful weed management, and combinations of these control practices or tactics have had significant results on reducing TSWV.  Each of the available management tactics and tools has a different set of advantages and limitations which you should consider before using.

Pepper resistant to TSWV

Tomato and pepper cultivars resistant to TSWV are critical to managing this problem. The current commercial resistance is based on a single gene; Sw5 gene in tomato and tsw gene in pepper. These resistant pepper lines and resistant tomato lines are available, and to some extent are grown commercially in the southeastern USA (See table above). However, resistance-breaking strains of TSWV that overcome resistance based on the Sw5 gene in tomato and the tsw gene in pepper have occurred in many areas where resistant cultivars have been grown. Thus, pepper and tomato lines have to be yearly re-evaluated in the field.

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