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Virus takes toll on vegetable crops

Tomato spotted wilt virus (TSWV) has proven to be a costly and widespread problem for Southeastern vegetable producers, and researchers continue to search for ways to minimize the impact of the deadly disease.

TSWV now occurs consistently throughout the pepper and tomato production areas of North Carolina and Georgia, where it commonly infects 20 percent or more of the plants in individual fields, says George Kennedy, North Carolina State University entomologist.

The tobacco thrips is the principal vector responsible for the spread of TSWV into susceptible crops in North Carolina, although the Western flower thrips also is seen a vector in certain locales, says Kennedy.

Although TSWV is spread by adult thrips, it must be acquired by first-instar thrips feeding on infected host plants, he says. Wild plant species are important, he adds.

Kennedy is conducting research to help determine the role of weeds as sources for the spread of TSWV. In a field experiment conducted near Kinston, N.C., common chickweed plants infected with TSWV in October remained infected until they matured in late May and June of the following year.

Beginning in March and continuing into June, TSWV spread from the infected chickweed into surrounding plots of uninfected buttercup. The incidence of TSWV-infected buttercup plants was 0 percent in February, 1 percent in March, 14 percent in April and 42 percent in June.

Results of weed surveys conducted in North Carolina over two years indicated that the potential of each weed species to serve as a source for the spread of TSWV in spring could be classified readily as high, moderate or low, according to Kennedy.

Weed species in each category include the following: High — dandelion, sowthistle, smallflower buttercup, Rugel's plantain, mouseear chickweed and common chickweed; Moderate - prickly lettuce and purple cudweed; and Low — curly dock, oldfield toadflax, swine cress, wild radish, henbit, ribgrass, horseweed, white clover and cutleaf evening primrose.

“The relative importance of individual weed species at any given location will be determined by its abundance as well as its susceptibility to infection, and it's ability to support reproducing populations of vectors,” says Kennedy.

A number of other weed species included in the survey were not found to be infected with TSWV or did not support reproducing populations of vectors, he says.

Based on estimates from 2000, Georgia's tomato crop is planted on 6,206 acres with an estimated annual value of $83 million and estimated annual pesticide costs of approximately $2.5 million.

Tomatoes are affected by various insect pests, including armyworms, tomato fruit worms, tobacco hornworms, stink bugs, aphids, whiteflies, thrips and others. But the most severe constraint to production is thrips-vectored TSWV, says David Riley, University of Georgia entomologist.

“Recent work on thrips feeding suggests that tobacco thrips may be more important in early season transmission of the virus, which could lead to greater yield loss,” says Riley.

Thrips-vectored TSWV and thrips vectors can have a tremendous negative impact on tomato yields and can reduce quality through irregular ripening, he says.

The current recommended TSWV management program for tomatoes consists of early season insecticides, silver reflective plastic mulch and the use of resistant cultivars — BNH444 or BHN 555 — where the incidence of TSWV has been high.

“The perceived risk to tomato production still is high, but strategies are available to reduce the risk,” says Riley.

A thrips vector control program has been developed for tomatoes that offers short-term benefits to tomato growers in TSWV-affected production areas, he says. 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.

“Also, since transmission of the virus occurs through thrips feeding, the focus is on various tactics that prevent thrips feeding and kill thrips before they can feed,” says the entomologist.

Imidacloprid (Admire) is a systemic insecticide that has been shown to reduce thrips feeding enough to prevent visible feeding scars for days after the thrips are introduced to the treated leaf tissue.

In addition, lambda-cyhalothrin plus methamidophos (Warrior plus Monitor) and spinosad (Spintor) have been shown to be effective in killing thrips.

The current best chemical treatment, says Riley, is imidacloprid soil drench plus lambda-cyhalothrin plus methamidophos foliar treatments rotated weekly with spinosad treatments beginning as soon as the tomatoes were transplanted and continued weekly for at least four weeks.

Commercial validation of the insecticide treatment was positive for the fields where the treatments were followed exactly, but did not do well when substitutions were made, he adds, such as spinosad alone instead of the rotation combination.

“Insecticide resistance will be the major drawback of this strategy in the near future is excessive calendar treatments are continued commercially.

“For this reason, an early season treatment program that incorporates insecticide rotation currently is recommended with different treatments between early and late season to avoid consecutive selection pressure.”

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