Protection Methods

- During cultivation
 
There is no curative method to effectively control  tomato spotted wilt virus (TSWV) during cultivation. Generally, an infected plant will remain so all its lifecycle.

If outbreaks occur in nurseries and are detected early, the few plants showing symptoms of TSWV should be quickly eliminated and in no case transplanted later. Infected plants do not always show symptoms and therefore it may be impossible to eliminate all diseased plants from the nursery. In United States, it has been  observed that in case of a low infection occurring in the nursery the  impact on the development of the epidemic in the field is very little. This may not be the case in all situations.
 
Insecticide treatments are needed to control thrips populations on tobacco. A number of products can be used both at the soil level and as foliar application. Unfortunately, these treatments are often ineffective in controlling outbreaks of this virus, especially in open field. Indeed, the thrips vectors often come from outside the field and transmit the virus during their feeding, even before the insecticide has time to act. Populations of Frankliniella occidentalis resistant to several insecticides (dimethoate, acephate, oxamyl, fenpropathrin) have been  reported, they do not contribute to improve the efficiency of chemical control with regard to these insects. This resistance  should make us understand why it is important to alternate insecticides with different modes of action. Control of Thrips tabaci seems easier than that of other thrips. To be effective, treatment should be performed early, ensuring a good spray penetration, in order to reach the places where the thrips seek refuge.
  
- Next crop
 
In countries where the infections  occur very early, they shoud be prevented or minimised as much as possible. To do this, one can eliminate the weeds and nymphs found on the soil. Disinfection of the latter may be considered. Protection of the young plants shoud be assured by using agrotextiles (unwoven fabrics mesh ...). The mechanical barrier thus created delay contamination. Weeds around the nursery area should be removed in order to eliminate virus sources or vectors. This is particularly important in the case of this virus. Weeding should also be carried out carefully in the field.
 
One should also avoid setting a nursery or a tobacco crop in the vicinity of a crop susceptible to this virus (figure 1) and its vectors. As it was shown for aphids, covering the soil with an aluminised plastic film reduced the number of thrips present in tobacco fields, and the impact of this virus. It will also be desirable to intervene in the crop rotation crops  that are not attractive for vectors.
 
Currently, farmers no not have varieties resistant to TSWV. No source of resistance is known in Nicotiana tabacum. On the contrary, many species of Nicotiana are sensitive; therefore opportunities to get inter-specific hybrids are low. Past or present breeding programs have tried or are trying to develop resistance or tolerance reported in Nicotiana alata and in genotype Polalta (getting its resistance to TSWV from the latter species) and in Nicotiana africana. The nature of this  resistance is not yet clearly determined.
 
Advances in molecular biology have opened up exciting opportunities for virus control. Several new strategies have been developed to control virus development in plants. They resulted in transformed plants with genes coding, for viral capsid protein, a satellite RNA, or viral replicase. If in the case of TSWV such transformed genotypes were obtained, none of them has been used extensively in field.

U.S. situation

While no current management practices will completely control the effects of TSWV on tobacco crops, some tools that can help moderate the disease have emerged in the last few years. Thrips are able to transmit TSWV very quickly, and most of these virus-carrying thrips come from outside the tobacco field. Over-the-top insecticides do not kill the thrips quickly enough to stop the spread of the virus. This type of spraying has not been successful in reducing disease incidence. However, some disease suppression has been noted on imidacloprid-treated plants in Georgia and North Carolina. Therefore, applying imidacloprid in the greenhouse to control aphids and other insect pests may help suppress TSWV. The application of acibenzolar-S-methyl, a growth regulator, alone or in combination with an insecticide, to seedlings in the greenhouse shows promise for being an effective and economical management tactic. Most economically important TSWV infections apparently occur within the first week or two after transplanting; many may occur during the first few days. Thus, protection should be in place before transplanting. Application of any chemicals after the virus has infected the plant will be of little, if any, benefit.

(Mina Mila - North Carolina State University)