Spread by insects

TSWV: Challenge for tomato growers Tomato spotted wilt virus (TSWV) is one pathogen in a group of viruses referred to as Tospoviruses, which are becoming increasingly important as pathogens on a worldwide basis and have been problematic in North Carolina in recent years.

An understanding of the complexity of this viral group only began to emerge in the last 10 years. These particular viruses are capable of infecting an unusually large number of plant species, including several that are important cultivated crops.

Diseases attributed to TSWV were first reported in Australia about 1915, and until 1990 TSWV was considered to be unique. However, now there are at least 12 distinct viruses, which have been taxonomically separated in the Tospovirus genius; once all considered TSWV.

When TSWV infects a host plant, it can cause a disease that severely weakens or kills that plant. In recent years, peanut, tobacco, tomato and pepper crops have been seriously damaged by TSWV in North Carolina.

TSWV and other Tospoviruses are spread by small insects called thrips, which have previously acquired the virus by feeding on infected plants. Nine species of thrips have been reported as vectors of Tospoviruses and specificity between thrips and tospovirus species has been shown.

In western North Carolina, the primary vector species in tomatoes are considered to be both tobacco and western flower thrips, with tobacco thrips being most common. Tospoviruses replicate in their thrips vectors, thus the insects not only spread the virus, but also serve as a viral host.

Thrips cannot transmit Tospoviruses unless they acquire the virus during their immature stages. In fact, it is believed that only the first instar of immature feeding thrips can acquire TSWV from infected plants.

When larvae feed on infected plants, ingested virus crosses the mid-gut barrier and enters the salivary glands. The disease is then carried to other plants as the insect matures into migrating adults. Transmission occurs when the virus moves into the plant with saliva during feeding.

The TSWV/thrips complex is considered persistent, i.e., once the insect acquires the virus; it is potentially able to transmit the virus for its entire life.

While the insect remains infective for life, there is no evidence of direct passage from one generation of thrips to the next, i.e., the virus is not transmitted from adult to egg.

Even though TSWV is considered to be a persistent virus, it behaves in a somewhat non-persistent nature in crop plantings.

The life cycle of thrips in North Carolina is three to four weeks and is being constantly interrupted during tomato pest management and cultural operations. The tomato spotted wilt virus itself contributes to mortality within the thrips population, especially tobacco thrips.

It is also known that higher temperatures increase the rate of vector mortality due to the virus. This may explain in part why there is a higher incidence of TSWV in earlier tomato plantings than in later plantings here in North Carolina.

The dynamic nature of TSWV in North Carolina's diverse agricultural environment is complex and not well understood. However, in the past, TSWV has been observed to be more severe in some geographical areas than in others.

Sometimes, even small geographic areas within a county may have a tendency to have more or less TSWV than surrounding areas. For example, TSWV has generally been less of a problem in eastern Henderson County an apple production area where western flower thrips predominate, than in western Henderson County a tomato, greenhouse, corn and forage crop production area.

Incidence of TSWV has generally increased in the entire state, and the predictability of geographic variations in the future is uncertain.

Early spring problems Thrips populations and susceptibility to infection are at their highest in the early spring. Experience has shown that timing of crop planting in relation to rapidly changing thrips populations can make a significant impact in the incidence of TSWV for the remainder of the season.

The vast majority of TSWV problems are due to primary infections. Primary infections to North Carolina tomato fields are likely initiated in two different ways. The TSWV has a very wide host range including many perennial and winter-annual weed species.

Over-wintering thrips (all life stages may be found over-wintering in North Carolina) acquire the pathogen from these hosts and vector it into adjacent tomato plantings. In the way of observation in the mountain region of North Carolina, thrips populations and the resulting incidence of TSWV tend to be more severe following mild winters due to vector survival.

Thrips carrying the TSWV may also be blown in from other less temperate production areas to our south on strong weather fronts, in effect, long distance inoculation.

Optimum planting dates vary from year to year but, in general, early-plantings tend to have higher levels of TSWV than crops planted in the middle of the planting season due to population dynamics of the vector.

In 1995, I witnessed a 20 acre, early season tomato planting which was devastated with more than 90 percent plant mortality due to TSWV.

A second mid-season, five acre planting less than 30 feet from the devastated field, which was still in place, but separated in time by three weeks sustained losses of less than three percent.

No single treatment or cultural practice has been found to be a consistently effective control measure. Management of the TSWV is not possible once the plant is infected.

Therefore, many growers attempt to focus on management of the insect vector. In general, the use of insecticides that have good efficacy against the thrips vectors like methamidophos (Monitor) and methomyl (Lannate) have been an ineffective means of suppressing TSWV.

Currently many tomato growers in North Carolina may be spending too much money attempting to control thrips in an effort to prevent TSWV, with very limited success.

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 tomato fields.

As described above, primary infection is due to populations of thrips that migrate from adjacent, non-treated areas or are blown in from long distances (a "thrips cloud") quickly infecting plants.

Once a vectoring thrips has established a feeding relationship with the tomato plant for as little as 15 minutes (somewhat dependent upon plant age), the plant is infected. Killing the thrips in an effort to contain TSWV at that point accomplishes little.

If anything, a pre-transplant application of the thrips suppressive insecticide imidacloprid (Admire 2F), which is a lot cheaper than other chemical controls, is of benefit because it is taken up by the insect vector during the feeding process (i.e., less dependent upon timing of application). But again, secondary transfer of TSWV is of little significance.

Control warranted There are times when controlling flower thrips is warranted. Besides the TSWV issue, thrips can cause damage when they feed in shoot tips of young plants, and within flowers causing blemishes on fruit resulting from small oviposition punctures.

The blemishes on fruit are often not severe enough to cull fruit, but the fruit may be reduced to a lower quality grade.

BHN Research of Bonita Springs, Fla., has recently released a couple of tomato varieties, BHN 444 and BHN 555, which have consistently demonstrated high levels of resistance to TSWV. However, fruit quality may not be comparable to varieties currently grown for fresh market in North Carolina.

Additional information Growers can learn more about tomato spotted wilt virus in the Tomato Session at this year's Southeast Vegetable and Fruit Expo held in conjunction with AgTech 2000. The joint conference will take place Dec. 11-13, 2000, at the Sheraton Greensboro Hotel at Four Seasons in Greensboro, N.C.

In addition to the many vegetable and fruit topics, AgTech will add topics concerning new technologies in all types of agriculture, focusing on cotton, soybeans, tobacco, peanuts, corn and wheat. In addition to the educational sessions, an extensive trade show will be held throughout the three-day event.

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