Viruses are transferred from infected plants to healthy ones by a number of agencies described below, and this is known as transmission of viruses.
1. By grafting:
A large number of plants are vegetatively propagated. Grafting is the most common method used in propagating fruit and ornamental plants.
When an infected plant is used for grafting with a healthy plant, the virus is transmitted through the cell solutions flowing from the infected part into the healthy parts.
2. By seeds:
Seed transmission of viruses is not common. Mosaic viruses of cucurbits and legumes are transmitted by seeds. But in the crops which are vegetatively propagated by the use of setts, tubers, rhizomes, bulbs, and corms, etc., viruses are transmitted by these organs.
3. By contact:
Viruses can be transmitted merely by contact or slight rubbing of the infected and healthy plant organs. Such transmission is quite easy in a thickly populated field where one plant is always in close contact with the other and even the slightest movement of the wind can help in rubbing the organs of one with the other. Viruses usually gain entry through the injuries caused on the plant surfaces.
4. By air and water:
Unlike fungi and bacteria viruses are not easily transmitted by air and water. The tobacco necrosis virus has been observed to be transmitted by both air and water.
5. By soil:
Tobacco mosaic virus lies in the soil with the plant debris after the harvest of the crop, and infects the new crop, when sown in the same field.
6. By tools and agricultural operations:
At the time of agricultural operations such as topping, pruning, weeding, rogueing or irrigation, the agricultural tools and the hands of the worker may get contaminated by the juice of an infected plant if it gets injured. These tools and the hands of the workers transfer the viruses to the healthy plants, the contact with which cannot be avoided ordinarily.
7. By smokers:
Tobacco mosaic virus remains infective in dry tobacco leaves for as long as 31 years. This virus is very resistant to high temperatures. It can spread by the fingers of the smokers, by the smoke and the un-burnt pieces of cigarettes, cigars and biris. Tobacco chewers’ sputum can also transmit virus if it is there in the tobacco leaves.
8. By store house:
Viruses, such as tobacco mosaic virus is transmitted to the new stock of tobacco, when it is stored in the same store house, where the infected leaves were stored.
9. By insects:
In nature, viruses are ordinarily transmitted by aphids, jassids and white flies which are the sucking insects. Most of the crop diseases viruses are transmitted by insects, and for this simple reason some virologists believe that there would have been no virus diseases if there were no insects. One virus can be transmitted by several species of insects and vice versa.
Insect vectors taking part in transmission, first suck juice from the infected plant. Now if it visits a healthy plant, the virus may be transferred and the new plant becomes infected. But in some cases a vector fails to infect a healthy plant soon after it has fed upon a diseased plant but it can infect the plant after some time.
This period taken by the virus in developing infectivity within the vector is known as incubation period. Incubation period is different for different viruses and varies from hours to days. In such cases the insect vectors act as alternate hosts, in which the viruses undergo some changes before infecting the real or say, the primary host.
All the insect vectors cannot transmit different viruses indiscriminately. Curly top virus of beet-root is transmitted only by a leaf hopper Eutettix tenellus, and cannot be transmitted by any other sucking insects found on beet-root leaves. Mosaic virus of beet-root is transmitted only by ‘peach aphid’. Stunt virus of paddy is transmitted only by a leaf hopper Nephotettix apicalis. From the plants, simultaneously infected with a mixture of tobacco mosaic and cucumber mosaic viruses, only tobacco mosaic virus is transmitted by needle prick, whereas cucumber mosaic virus is transmitted by aphids alone.
The above-mentioned examples show that there exists some close relationship between the virus and its insect vector. Though this relationship is not properly understood in some cases, it is not very difficult to comment upon it. All the species of insect vectors differ to a lesser or greater extent in their habit, morphology, anatomy and physiology.
An insect which feeds on the pollen grains will not suck the sap, and one feeding on the leaves may not feed on the stems, or other organs. Viruses are sometimes present only in the cells of deeper tissues and as such they can be sucked up only by the insects having long proboscis. Surface feeders cannot transmit such viruses. Similarly the vectors feeding on the deeper tissues do not feed on the surface tissues.
In case, where there is a definite incubation period, the virus passes through the entire alimentary canal and coelome of the vector, then enters into the salivery glands and can go out with the saliva through the proboscis of the insect when it punctures into the host tissue. In passing through this long way it has to come in contact with several intestinal secretions. Some viruses multiply and grow while passing through the body of the vectors and others pass out as such without undergoing any change.
Such viruses are favoured by the physiological activities of the vectors and pass out of it, retaining their infectivity. Physiological activities of a particular vector cannot suit all the viruses and therefore this differential adaptation of viruses to the vector is also related with the transmission of a particular virus and the reason for this is also related to their physiological behaviour.