2. Introduction
It is a viral disease which affects all dicotyledonous plants
of which most important are tobacco and tomato.
China is the largest producer of tomato and tobacco
followed by India and USA.
Tobacco mosaic virus (TMV) the causal agent of mosaic
disease (MD), within the genus Tobamovirus under the
Virgaviridae family. Нis genus currently holds 37 viral
species. Infection of tobacco and other crops by TMV,
greatly diminishes crop yields, financial returns, and
threatens farmer.
3. Infects
Tobacco mosaic virus (TMV) infects more than 350 plant
species within nine plant families of nutritional, economic,
horticultural and ornamental importance.
weeds and members of the Solanaceae (e.g. tobacco, tomato,
potato, eggplant, pepper, and petunia), Brassicaceae (e.g.
cabbage, radish), Cucurbitaceae (e.g. cucumber, watermelon,
zuchini, pumpkin), and Legumineae (e.g. bean)
Ornamentals such as chrysanthemums (Chrysanthemum
indicum L.), and impatiens (Impatiens balsamina), are most
susceptible to TMV.
4. Transmission
The virus is rarely seed-borne and is not vector-
transmitted. In addition to transmission through
susceptible weeds and plant reservoirs.
TMV can also be transmitted mechanically through
use of contaminated cutting tools, handling, plant
cuttings, grafting, contact between plants or other
contaminated surfaces, soil and potting material.
5. TMV structure
TMV is made up of a piece of nucleic acid (ribonucleic acid;
RNA) and a surrounding protein coat. The complete virus is a
submicroscopic, rigid, rod-shaped particle.
The rod-shaped virus particles (virions) of TMV measure about
300 nm x 15 nm . A single TMV particle is composed of 2,130
copies of the coat protein (CP) that envelope the RNA molecule
of about 6,400 nucleotides
6. TMV replication
Once inside the plant, the virus releases its genetic code (RNA). The
virus particles disassemble in an organized manner to expose the
TMV RNA.
The virus RNA is positive-sense and serves directly as a messenger
RNA (mRNA) that is translated using host ribosomes. Translation of
the replicase-associated proteins (RP) 126- and 183-kDa, begins
within a few minutes of infection.
As soon as these proteins have been synthesized, the replicase
associates with the 3' end of the + sense TMV RNA for the production
of a negative sense, RNA. The - sense RNA is the template to produce
both full-length genomic + sense RNA as well as the + sense
subgenomic RNAs (sgRNAs)
7. These virus particles are very stable and at some point when the
cells are broken or the leaf dries up, they are released to infect new
plants. Alternatively, the + sense TMV RNA is wrapped in movement
protein and this complex can infect adjacent cells.
TMV uses its movement protein to spread from cell-to-cell through
plasmodesmata, which connect plant cells and eventually enters the
translocation system of the plant (xylem and phloem) and from
there it spreads to the entire plant.
11. Epidemiology
The TMV disease cycle and its epidemiology are intimately related
because the virus is completely dependent on the host for replication
and spread.
There is wide variation in disease incidence, depending on the time
of disease onset in the field and on cropping practices. For example, a
few plants could become infected early in the season, either from
TMV on the seed coat or by workers contaminating plants.
The disease could then spread rapidly throughout the field or
greenhouse by TMV-infected plants contacting healthy plants or by
equipment or workers. TMV can also survive or overwinter in
infected plant debris or perennial (weedy) hosts and perhaps in the
soil.
Agricultural practices, such as continuous cropping, have the
potential to be a particular problem, especially in greenhouse
facilities, where TMV inoculum may increase in more than one plant
species.
12. Managing
Purchase virus-free plants. Remove all weeds since these may harbor
TMV and all crop debris from benches and the greenhouse structure.
Set aside plants with the above symptoms and obtain a diagnosis.
Discard infected plants. Disinfest tools by placing them in disinfectant
for at least 10 min. Rinse thoroughly with tap water. Disinfest door
handles and other greenhouse structures that may have become
contaminated by wiping thoroughly with one of these materials.
Propagate plants via seed rather than vegetatively. Thoroughly wash
hands after handling tobacco products or TMV-infected plants. Do
not keep tobacco products in the pockets of clothing worn into the
greenhouse. Launder greenhouse work clothes regularly
