The following points highlight the eight chief methods used for the transmission of plant viruses. The methods are: 1. Seed Transmission of Virus 2. Transmission by Vegetative Propagation 3. Transmission by Mechanical Means 4. Transmission by Cuscuta 5. Soil Transmission 6. Insect Transmission 7. Transmission by Fungi 8. Some Soil Inhabiting Viruses have Nematode Vectors.
Method # 1. Seed Transmission of Virus:
Transmission through the seeds of the host plant was earlier considered to play a minor part in the spread of virus diseases. Recently Bennett (1969) listed 53 viruses which are transmitted by seeds of about 124 plant species.
The seeds are important in the spread of a few viruses of legumes, wild cucumber, tomatoes, and curly top virus of beet sugar. In the latter case the seeds carry a high percentage of the virus. The virus, however, does not enter the embryo. It is carried in a portion of the seed of the diseased plants.
Method # 2. Transmission by Vegetative Propagation:
It is one of the chief methods of transmission of virus diseases especially of Potato, Rose, Sugarcane, Raspberry, Strawberry, Turnips, Bulb plants, fruit trees and many ornamentals.
The vegetative parts, the infected plants such as the tubers, bulbs, roots, offshoots, buds and scions which are used for propagation, will contain the virus present in the parent. The new plants raised by the above-mentioned vegetative methods are nearly always infected.
Method # 3. Transmission by Mechanical Means:
Many mosaic viruses are transmitted mechanically from diseased plants to healthy ones by the following methods:
(i) By contact of infected and healthy leaves brought about by wind.
(ii) By rubbing the juice of the diseased plants over the surface of the leaves of healthy plants.
(iii) By grafting infected buds on to healthy plants.
(iv) Agricultural implements also play quite an important part. The knife used for cutting the seed pieces and the pruning shears will spread the disease.
(v) Some viruses spread below ground by contact between the roots of diseased and healthy plants.
(vi) Handling plants at planting time and in cultural operation will also help in the spread of viruses such as Sugar beet. Curly top virus and Cucumber mosaic virus.
Method # 4. Transmission by Cuscuta:
In many cases Dodder (Cuscuta) serves as a transmitting agent and an effective bridge between the infected host and the healthy plants by establishing intimate biological contact through its haustoria.
Method # 5. Soil Transmission:
Quite a number of viruses are transmitted through the soil. Common examples of soil borne viruses are Potato mosaic virus, Oat mosaic, Wheat mosaic, etc. In all these cases the disease is contracted from the soil.
Method # 6. Insect Transmission:
Some plant and animal viruses are spread and complete particles introduced into host cells by arthropod vectors and even by dog-bite as in rabies. Among the arthropods most important agents of spread of virus diseases are the insects.
The insect which carries the disease is called a vector. The insect vectors which play a major role in the dissemination of plant viruses are the Aphids, Leafhoppers, Flee beetles, Scale insects, thirps and White flies.
Most of the insect vectors are sucking insects. Aphids transmit more plant viruses than any other insects. Leafhoppers come next in the list. About three hundred plant virus diseases are known to have insert vectors.
The insert obtains virus through its mouth parts at the time of feeding on the diseased plant. It is then inoculated in the healthy plant by means of the mouth part. Inoculation in many cases must be in a certain tissue or upon young leaves.
The virus may remain active in the body of the vector for many days. Instances are however, known when infectivity is soon lost. There are also cases where a vector cannot infect a healthy plant immediately after it has fed on a diseased plant.
There is delay in the development of infective power within the vector. This period of development of infectivity for the virus within the vector is called the incubation period. The duration of the incubation period varies with different viruses from a few hours to days
There also appears to be some relationship between the plant viruses and the insect vectors which transmit them. The precise nature of this relationship is still unknown. The virus disease of sugar beet known as curly leaf or curly top is spread by the leaf-hopper Circulifer tenellus.
Other sucking insects which feed on sugar beet are unable to transmit this virus. On the other hand peach aphid is the vector of Sugar beet mosaic virus. The leafhopper does not transmit this virus. Thirps transmit the spotted-wilt virus. All vectors of yellow group of viruses are leafhoppers and of mosaic group are aphids.
Method # 7. Transmission by Fungi:
The first proof of the fungus as a vector of plant viruses was found by Gorgon in 1958. Fie found that the diseased lettuce was invariably infected by a soil chytrid, Olpidium. Later he discovered that the fungus acts as a reservoir and vector of the big vein virus.
The virus acquired by the fungus remains in the oospore. The latter germinates and produces the zoospores which function as infective agents and penetrate lettuce roots. Similarly tobacco necrosis virus has been reported by Teakle (1960) to enter roots of its host by the zoospores of O. brassicae.
Method # 8. Some Soil Inhabiting Viruses have Nematode Vectors:
Animal viruses may gain access to the higher animals through the mouth and nose from dust or contaminated food. Besides infection from outside, virus may also be transmitted from cell to cell but the internal transmission need not be in the form of virus particles.