It is the protection coat of the virus or capsid which is highly adapted to this mode of transmission. Each type of virus is very specifically associated to a vector. The most active vectors for phytoviruses are the representatives of the homopters (for example, the greenfly).
Viruses transmitted by insects can be classified in two broad categories :
- The non-circulating viruses which are brought by mouth pieces during transmission. They consist in non persistent and semi persistent viruses.
- Circulating viruses which accomplish a complete cycle in the vector before settling in the salivary glands and being introduced in a new plant by the salivary channel. They are of the persistent type.
Modes of transmission of viruses by a vector are defined according to :
- The time necessary for the insect to acquire the virus from an infected plant;
- The inoculation period which corresponds to the necessary time to transmit the virus to a sound plant;
- The latent period which defines the period between the moment the vector has acquired the virus and the moment it can transmit it to another plant ;.
- The retention period which corresponds to the time during which a vector having acquired a virus is still able to pass it on to another plant.
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