Photo: Courtesy of JP Krausz
Tobacco mosaic virus (TMV) was the first virus discovered.
In 1889, Martinus Beijerinck, found that ‘tobacco mosaic disease’ was caused by a pathogen able to reproduce and multiply in the host cells of the plant. He called it ‘virus’ (from the Latin virus, meaning poison) to differentiate this form of disease from those caused by bacteria.
Tobacco yield losses
due to TMV are currently estimated at only 1%, because resistant tobacco varieties are routinely grown. However, TMV affects other crops, and losses of up to 20% have been reported in tomatoes.
TMV can be a major problem because, unlike most other viruses, it does not die when the host plant dies and can withstand high temperatures. Thus, the virus can survive on implements, trellis wires, stakes, greenhouse benches, containers and contaminated clothing for many months.
It can also survive in crop debris on the soil surface and infect a new crop planted on contaminated land.
Tobacco products, particularly those containing air-cured tobacco, may carry TMV too.
The virus cannot be transmitted in the smoke of burning tobacco, but smokers, especially those who roll their own cigarettes, could possibly carry the virus on their hands and transmit it to healthy plants.
Sap-feeding insects such as aphids cannot transmit TMV. However, chewing insects such as grasshoppers and caterpillars do occasionally transmit the virus. They are not considered important vectors, however.
Transmission
Tobacco mosaic virus is usually spread from plant to plant via ‘mechanical’ wounds caused by contaminated hands, clothing or tools such as pruning shears and hoes. This is because TMV occurs in very high concentrations in most plant cells. When plants are handled, the tiny leaf hairs and some outer cells are inevitably damaged and leak sap onto hands, tools and clothing.
Seeds from infected plants can also carry the virus on their seed coats. The earlier the age at which the mother plant is infected, the more likely it is that the virus will contaminate the seed coat during seed harvesting. When the seed germinates, the virus may enter the seedling through small cuts caused by transplanting and handling, or during the germination/emergence process.
Once inside the plant, the virus releases its genetic code (RNA). The plant mistakes this for its own RNA, and starts to produce viral proteins.
The virus then spreads to neighbouring cells through microscopic channels in the cell walls (plasmodesmata), and eventually enters the translocation system of the plant (xylem and phloem). From here, it spreads to the entire plant.
Signs and symptoms
Symptoms first appear about 10 days after infection. The plants do not usually die, but growth can be seriously stunted. In the case of tomatoes, certain TMV strains can cause deformed fruit, non-uniform fruit colour and delay ripening.
Specific symptoms depend on the host plant, age of the infected plant, environmental conditions, the virus strain and the genetic background of the host plant.
However, common signs include mosaic-like patches (mottling) on the leaves, curling of leaves and the yellowing of plant tissues.
Managing the virus
No chemicals can cure a plant infected with a virus, and TMV is no exception. As mentioned before, however, resistant plant varieties are available.
You will need to consider adaptability, potential yield and resistance to other important diseases when selecting varieties.
Ultimately, effective TMV management involves using virus-free seedlings or plants and implementing strict hygiene procedures:
- Use new potting mix and new or thoroughly cleaned seedling trays when growing seedlings;
- If infected plants are discovered, either remove and destroy the plants and restrict access to the area, or always work in the affected area last and decontaminate yourself and your equipment afterwards;
- Remove all crop debris from the land, seedling production beds and benches in greenhouses;
- Place tools in a disinfectant solution for at least 10 minutes and rinse thoroughly with tap water;
- Disinfect door handles and other greenhouse structures that may have become contaminated by wiping thoroughly with recommended disinfectants;
- Thoroughly wash your hands with recommended disinfectants, such as carbolic soap, or a mixture of non-fat milk powder at 20% weight/vol, 10% bleach, and 70% ethanol, after handling tobacco products or TMV-infected plants. Make sure that the solutions are fresh, and replace regularly (it is recommended that the bleach solution be replaced every four hours).
If you are a seedling producer, ensure that greenhouses are within a clean zone and control the movement of people, plants, vehicles and materials into the greenhouse areas.
Treat each greenhouse as a separate unit, with protective clothing, tools, gloves and bins in each. These items should not be moved between units.
The ARC’s Industrial Crops unit (Rustenburg) has a virus diagnostic laboratory and conducts diagnostic services for nurseries and farmers.
Email Phillip Mphuti at [email protected].
