The presence of a virus

   
In France, viruses in nurseries are rather rare. This situation is partially due to:
- the almost complete  disappearance of tobacco mosaic virus (TMV), a contact transmitted virus. Before the widespread cultivation of resistant varieties TMV could easily persist in nurseries from one year to another or could be re-introduced and spread during plant manipulation;
- very low frequency of viruliferous aphids in late winter and spring that transmit two major viruses of tobacco in France, the cucumber mosaic virus (CMV, figure 1) and potato virus Y (PVY);
- the fact that nurseries are sheltered provides a mechanical barrier against aphids.

Figure 1	Figure 2

 
In countries where climatic conditions are milder during the planting season, the situation is very different and viral infections may occur. Given the high density of seedlings, a few viruliferous aphids are sufficient to provide many infections (see additional information on virus transmission in the Viruses section).
 
It is always important to detect as early as possible the first virus-infected plants, like the plants showing mosaic patterns (figure. 2), and eliminate them quickly. For more details on viruses, see the sections of the following topics: Abnormal leaf shape and Leaf mosaics.