Tomato Spotted Wilt Virus: TSWV in Vegetable Crops: Tospoviruses

Tospoviruses In Solanaceae and Other Crops in The Coastal Plain of Georgia

Management of TSWV

Thrips in Solanaceous Crops
David Riley, Robert McPherson, Lenny Wells, Steve Brown

The rationale for a thrips-TSWV risk management program is based on two main assumptions that: (1) certain pre-season field measurements can be consistently associated with high incidence of tomato spotted wilt during the following season and (2) certain control tactics can consistently reduce the impact of TSWV in the target crop. The pre-season field measurement currently are based on TSWV host weed or crops and their relative importance in producing viruliferous (virus-carrying) thrips. In the tomato crop, several years of data collected in Tifton, Georgia, indicate 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. Thrips-TSWV control tactics should be targeted for early in the crop growing season to have the greatest impact on yield. Virus transmission that occurs later in the season may not be as important in terms of yield, but in tomato can be very important in terms of TSWV- irregular ripened fruit. 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 after harvest. Data from 2001 suggested that the tomato plants with slight to moderate symptoms during early to mid-season result in the greatest amount of TSWV - irregular ripened fruit in the harvest, not plants with symptoms appearing late-season. Thus, the critical control period for reducing TSWV-related yield loss is early to mid-season in tomatoes. Lateseason control of thrips for thrips-related yield quality loss, such as dimpling of the fruit, is likely to be much less important than the TSWV damage.

Management of the thrips vector can be difficult because immature thrips can acquire TSWV from infected weed host plants surrounding vegetable fields (Groves et al. 2001b). The year-to-year differences in timing and intensity of TSWV are related to the thrips and virus in our weed populations. In early spring, prior measurements indicated that TSWV incidence in winter weeds in January-February >2-8 percent is associated with a high incidence year and <2 percent, a low incidence year for TSWV. This information will be used to establish an assessment of TSWV risk for vegetable crops, which will determine in part the level of within-season vector control tactics that should be used. Because within-season TSWV and vector management procedures (e.g., reflective mulches, intensive chemical control of thrips and resistant cultivars) are costly and management intensive, the availability of an assessment of TSWV risk before the season will enable growers to limit their use to those situations in which they are truly needed.

Because the virus replicates in the vector as it matures, viruliferous adults can quickly spread the virus when they move into the field before thrips can be controlled (Ullman et al., 1997). However, some success in control tactics for thrips vectors has been documented (Riley, 2001; Riley and Pappu, 2000). Some of the primary control tactics available in tomato include the use of reflective plastic mulch (Greenough et al., 1990), host plant resistance to the virus (Kumar et al., 1993 and 1995), and insecticides combined with other tactics (Brown and Brown, 1992; Riley and Pappu, 2000). In tomato and pepper, simultaneous evaluation of multiple control tactics for thrips and tomato spotted wilt management in tomato have been done (tomato - Riley and Pappu, 2000, 2004; pepper - Reitz et al., 2003). Improvements in commercially available materials have recently occurred, specifically the release of TSWV-resistant tomato cultivars "BHN- 444, BHN-555, and BHN-640" (BHN Research Inc., Bonita Springs, Fla., USA), the TSWV-tolerant pepper "Heritage" (Harris Moran-Harris Seeds, Rochester, NY), and the metallic reflective plastic mulch "RepelGro" (ReflecTec Foils Inc., Lake Zurich, Ill., USA), that have improved management options in tomato and pepper in recent years.

In tobacco, conventional insecticides labeled for beetle and aphid control generally provide effective short-term suppression of thrips infestations. Although these materials reduce thrips populations for several days after the foliar application is made, the seasonal incidence of TSWV that is vectored by thrips is not suppressed. Repeated applications of certain insecticides, once or twice a weeks for up to 6 weeks, can reduce the incidence of TSWV symptomatic plants. However, repeated foliar applications in the early tobacco growing season do not always suppress TSWV. Also, this practice of repeated early-season insecticide applications is not recommended to tobacco growers, because it can lead to insecticide resistance and not be very economical. A beneficial insecticide treatment for TSWV suppression is a greenhouse tray drench application or a transplant water applica-tion of the neonicotinyl insecticides imidacloprid (Admire) or thiamethoxan (Platinum). Growers are advised to treat their crop with one of these materials prior to transplanting, because this thrips management practice consistently reduces the seasonal incidence of TSWV. In tomato, a combination of imidacloprid (Admire) soil drench, plus weekly foliar applications of methamidophos (Monitor – SLN label) and lambda-cyhalothrin (Warrior) in the first 4 weeks after transplant significantly decreased TSWV and increased yields. Since tomato is a very high value crop, this expense can be justified. In pepper, imidacloprid (Admire) at planting can help to reduce the incidence of TSWV in the spring crop.

The currently labeled insecticides/nematicides that are applied in tobacco fields in PPI applications are not effective in reducing TSWV infection. In fact, some of these materials, when no other tray drench, transplant water or foliar applications are made, actually have higher incidence of TSWV symptomatic plants than the plots where no PPI insecticides/ nematicides are used. Many of the insecticides applied for budworm and hornworm control are effective in reducing thrips populations; however, these applications are made later in the season (usually 6-8 weeks after transplanting) and have no impact in suppressing TSWV infection.

In tomato studies at Tifton, three tactics, i.e., early season insecticides, the TSWV-resistant tomato and reflective mulch, reduced the incidence of TSWV and improved tomato yields when the level of TSWV exceeded 17 percent. Averaged over three years, there was considerable economic incentive for using all three tactics in a preventative manner. Obviously, when TSWV incidence is low, e.g., 2 percent as in 2000, none of the tactics show a yield response. Unfortunately, a pre-season prediction of TSWV severity is not currently available, although some overwintering host plant surveys show promise in this area (Groves et al., 2001b). Based on these studies, control tactics should be implemented in regions where TSWV is a problem in order to mitigate long term risks to production. We recommend a cautious approach when using insecticide because of the threat of insecticide resistance in thrips (Kontsedalov et al., 1998). This is an additional reason to focus treatments on early season tomato and not season long. The reflective metallic mulch is an effective tactic that appears to be cost effective as long as possible harvest delays from soil cooling are not a concern. Also, the commercial acceptability of the fruit of the BHN444 globe type is not as high as standard deep globe, blocky type tomato hybrids and may not be acceptable to some growers if the TSWV risk to production does not warrant sacrificing fruit marketability. If a risk index can be developed for TSWV in tomato such as has been done in peanut (Brown et al., 1999), then the selection of tactics can be based on the severity of predicted TSWV incidence, with high risk involving all available tactics and moderate risk using some selection of one or two tactics. This would help to preserve the efficacy of the existing tactics for thrips vector control and mitigation of the negative economic impact of TSWV on tomato.

Several treatments (control tactics) evaluated in 1998-2002 resulted in less numbers of irregular-ripened fruit and high tomato yield relative to an untreated standard. Several of these tactics have been commercially validated as well, but in high-TSWV risk situations, multiple tactics with additive effects will be required to economically reduce the impact of the virus. Table 3 lists the current effect treatments for TSWV/thrips management in tomato, but it should be noted that often multiple tactics are needed together to achieve commercially acceptable control levels. The best insecticide treatment, imidacloprid soil drench plus lambda-cyhalothrin plus methamidophos foliar treatments beginning as soon as the tomatoes were transplanted, needs to be targeted at early season if it is to be as effective. Late-season insecticide treatments are completely ineffective for reducing thrips-vectored TSWV. The only purpose of controlling thrips late season would be to reduce direct thrips feeding damage to the fruit. Likewise, the metallic silver plastic mulch is most effective early season when the reflective surface is greatest. Certainly, host plant resistance in the tomato cultivar BHN444, and the metallic silver reflective mulch have greatly improved yields where used in areas of high TSWV incidence.

It is important to remember that, since transmission of the virus occurs through thrips feeding, a program that prevents thrips feeding, kills thrips before they can feed, and/or reduces the attractiveness of the crop so that less thrips occur on the plant, will be the most effective. Feeding consists of slight to deep probing and it is currently being investigated which type of feeding activity results in the most virus transmission. This is extremely important because the anti-feeding effect of imidacloprid seems to work well in pepper and tomato, but not so in peanut. It is also important to note that even though a product like imidacloprid can reduce thrips feeding initially, this alone may not be enough to reduce virus transmission since behavioral effects like this can be overcome with starvation. So it is important to not only repel thrips feeding, but also remove the thrips from the host plant as quickly as possible before repellence is overcome. This can be< done either through mortality of the thrips (e.g., lambda-cyhalothrin plus methamidophos foliar treatments) or overall repellence off of the crop through the use of metallic silver mulch.

The research on control tactics is being updated each year so that the amount of risk reduction associated with each tactic can change somewhat from year to year. If a new tactic is discovered, then the relative ranking of tactics can also change, so it will be important to use the most current risk-management program available.

Table 3. Thrips-vectored TSWV control tactics and/or treatments that have been found effective in experimental trials.

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