TOBACCO MOSAIC VIRUS(TMV) ALL
DETAILS OR DESCRIBE
Tobacco mosaic virus (TMV) is a positive-sense single stranded RNA virus, genus tobamovirus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns, such as "mosaic"-like mottling and discoloration on the leaves (hence the name).
Structure
Tobacco
mosaic virus has a rod-like appearance. Its capsid is
made from 2130 molecules of coat protein (see image to the left) and one
molecule of genomic single strand RNA, 6400 bases long. The coat protein
self-assembles into the rod-like helical structure (16.3 proteins per helix
turn) around the RNA, which forms a hairpin loop structure (see the electron
micrograph above). The protein monomer consists of 158 amino acids which are
assembled into four main alpha-helices, which are joined by a prominent loop
proximal to the axis of the virion. Virions are ~300 nm in length and ~18 nm in diameter. Negatively stained electron microphotographs show a
distinct inner channel of ~4 nm. The RNA is located at a radius of ~6 nm and is
protected from the action of cellular enzymes by the coat protein.
Symptoms
Symptoms
vary with the species of plant infected and the environmental conditions. In
some cases environmental conditions bring out symptoms while other conditions
mask or hide symptoms. Symptoms associated with TMV infections:
stunting
mosaic pattern of light and dark green
(or yellow and green) on the leaves
malformation of leaves or growing points
yellow streaking of leaves (especially
monocots)
yellow spotting on leaves
distinct yellowing only of veins
Some of
the above symptoms can also be caused by high temperature, insect feeding,
growth regulators, herbicides, mineral deficiencies, and mineral excesses. TMV
diseases cannot be diagnosed on the basis of symptoms alone.
Pathogen Biology
Hosts for
TMV include tobacco, tomato, and other solanaceous
plants. Currently, yield losses for tobacco due to TMV are estimated at only 1%
because resistant varieties are routinely grown. In contrast, losses of up to
20% have been reported for tomato. In addition, poor fruit quality may reduce
the value of the crop on the commercial fresh market.
Transmission from plant to plant
TMV is
very easily transmitted when an infected leaf rubs against a leaf of a healthy
plant, by contaminated tools, and occasionally by workers whose hands become
contaminated with TMV after smoking cigarettes. A wounded plant cell provides a
site of entry for TMV. The virus can
also contaminate seed coats, and the plants germinating from these seeds can
become infected. TMV is extraordinarily stable. Purified TMV has been reported
to be infectious after 50 years storage in the laboratory at 4°C/40°F.
Life-Cycle (Replication) of Tobacco Mosaic
Virus (TMV):
Plant
viruses like TMV penetrate and enter the host cells in toto
and their replication completes within such infected host cells. Inside the
host cell, the protein coat dissociates and viral nucleic acid becomes free in
the cell cytoplasm.
Treatment
There are
no cures for viral diseases such as mosaic once a plant is infected. As a
result, every effort should be made to prevent the disease from entering your
garden.
Fungicides
will NOT treat this viral disease.
Plant
resistant varieties when available or purchase transplants from a reputable source.
Do NOT
save seed from infected crops.
Spot
treat with least-toxic, natural pest control products, such as Safer Soap, Bon-Neem and diatomaceous earth, to reduce the number of
disease carrying insects.
Harvest-Guard
row cover will help keep insect pests off vulnerable crops/ transplants and
should be installed until bloom.
Remove
all perennial weeds, using least-toxic herbicides, within 100 yards of your
garden plot.
The virus
can be spread through human activity, tools and equipment. Frequently wash your
hands and disinfect garden tools, stakes, ties, pots, greenhouse benches, etc.
(one part bleach to 4 parts water) to reduce the risk of contamination.
Avoid
working in the garden during damp conditions (viruses are easily spread when
plants are wet).
Avoid
using tobacco around susceptible plants. Cigarettes and other tobacco products
may be infected and can spread the virus.
Remove
and destroy all infected plants (see Fall Garden Cleanup). Do NOT compost.