CMV : A common virus in France

Two viruses are particularly severe on tobacco in France: cucumber mosaic virus (CMV) and potato virus Y (PVY). They can be found in almost all tobacco production regions, occurring alone or in combination with one another. Certain regions seem to be more affected by these viruses especially Southwest of France and the Alsace.
 
The symptoms observed on plants infected with CMV vary significantly, depending on the types of tobacco grown, and at times reminding those caused by TEV, TMV, AMV and TRSV:
- mosaic patterns, with or without darker than normal lamina areas along the veins (vein banding) and interveinal yellowing. Some strains causing bright yellow to white mosaic, like "aucuba", or yellow and cross-linked have been reported several times in the literature; we have noticed these symptoms only occasionally (figure 4b);
- various leaf abnormalities, of shape and appearance, such as the presence of blisters, a tendency to filiformis or curling (Abnormal leaf size and shape, Different viruses) (figure 5.)
- localised necrotic lesions (figure 6) consisting of small beige to brown traits (etch) arranged in rows or rings (figure 7), mosaics, chlorotic and necrotic lines which sometimes follow the veins of lamina and make the leaf look like an "oak leaf". This symptom is mostly located on the lower leaves and particularly occurs in dark tobacco
- widespread necrotic lesions cause the destruction of the terminal bud. They are accompanied  by numerous necroses on the leaves, veins, stem and pith. These symptoms, which may affect a large proportion of plants are found mainly on dark tobacco (figure 6a). They can be confused with symptoms  caused by TSWV (tomato spotted wilt virus). Similar symptoms have sometimes been associated with viral complexes : TMV + PVY-N  or TMV + CMV. Strains of necrotic CMV on tobacco have also been reported in the Philippines and Japan.
 
In general Burley tobaccos exhibit yellow ?mosaics? very well defined especially when the infections occurred early on young plants. Dark tobaccos are also frequently infected but they show milder symptoms than burley tobaccos. Virginia type tobaccos seem like the least affected by the virus.
 
Symptoms of cucumber mosaic virus can vary greatly. This is partly due to numerous strains found in the field . CMV strains are divided into two main groups I and II, the first one having 2 subgroups IA and IB. Their incidence on tobacco varies and depends on seasonal climatic fluctuations. 
 
CMV is transmitted by aphid vectors in a non-persistant manner. Virus infected plants are often scattered at random in the field in foci of different sizes. For further information of this virus, please, see the fact sheet on Cucumber mosaic virus (CMV).