Different viruses

Many viruses disturb the development of young tobacco leaves. These pests, in addition to causing multiple defects in colouring (mosaics, yellowings), induce changes in lamina shape a symptom that can be quite spectacular. The nature and intensity of these alterations may fluctuate depending on the viruses and strains.
 
For example, in the case of cucumber mosaic virus (CMV) outbreaks, the newly formed leaves are frequently more or less blistered (figure 1), curved down or severely distorted. This is the case also for alfalfa mosaic virus (AMV) (figure 2). For CMV, the leaves are often smaller, sometimes with serrated edges.
 
Among the viruses occurring in France, only cucumber mosaic virus (CMV) and tobacco mosaic virus (TMV) can induce quite frequently filiform-type symptoms (thread- or filament-shaped) which can be more or less marked:
- when they are moderate, the leaves are simply narrower than usual (figure 3);
- they can affect only a portion of the lamina, such as its tip (figure 4);
- in extreme cases, the leaves are reduced to midrib only (figure 5).
 
In severe tobacco mosaic virus (TMV) outbreaks infected leaves may be particularly blistered (figure 6). Tomato spotted wilt virus (TSWV), in addition to necrotic spots on leaves sometimes causes plant stunting and apex curving (figure 7). Some necrotic strains of potato virus Y (PVY) frequently cause embossing and more or less pronounced lamina curling (figure 8).
 
These examples, as well as those mentioned in other fact sheets, make one  realise the potentialities of certain viruses to induce leaf deformation. In order to determine these viruses more precisely, please, do refer also to the section of Abnormal leaf colour, to the fact sheet on the symptoms of Leaf mosaic and to those of tomato spotted wilt virus (TSWV), cucumber mosaic virus (CMV), tobacco mosaic virus (TMV), and potato virus Y (PVY).
  

• Other viruses that cause severe deformation of tobacco leaves :

- tobacco leaf curl virus (TLCV), viruses belonging to the family Geminiviridae, and to Begomovirus genus. This virus causes stunting of plants and curled, twisted leaves with enations on the underside of the leaf along the veins. The virus is transmitted by the whitefly, Bemisia tabaci which is its main vector.
 
- beet curly top virus (BCTV), belongs to the family of Geminiviridae and to Curtovirusgenus. This virus causes curly and embossed leaves, with irregular and prominent veins. The plants are sometimes stunted. The leaves are rough from time to time and eventually turn yellow. Several species of leafhoppers (Circulifer spp., Agallia albidulla) transmit this virus in a persistent mode.
 
- tobacco rattle virus (TRV) belongs to the genus tobravirus. Symptoms caused by this virus are curled up, spoon-shaped leaves, with spots, necrotic rings and patterns, sometimes quite similar to those of TRSV (tobacco ring spot virus). Leaf tissues have a brittle texture and a dry sound when touched. Systemic necrotic lesions are visible on the stem and leaves. Several species of nematodes belonging to the genera Trichodorus spp. and Paratrichodorus spp. transmit this virus. These nematodes remain infectious for several months or even years. There is a low transmission rate through seeds in some weeds.

- tobacco stunt virus (TStV), is classified as Varicosa virus. There are various symptoms,  such as: curling up of young leaves, chlorosis and necrosis of the veins, necrotic spots on older leaves, mottling, filiformis, shorter internodes and stunted plants. Its natural vector is Olpidium brassicae.
 
For these viruses, please see the fact sheet of Other parasitic tobacco viruses.