ABSTRACT
The natural spread of a vector-borne virus requires three basic components, namely, the virus itself, the host and the vector. The relationships between host and virus, host and vector and between virus and vector are subject to the influence of environmental conditions as well as human interference whereby the ecosystem is changed to improve agriculture. The major host factors which can influence plant susceptibility to virus infection include inherent genetic traits and plant age at the time of infection. The role of infected plants to serve virus sources depends on virus concentration and leaves of the same plant may vary in this regard. The natural spread of vector-borne plant diseases depends on vector dispersal and host selection; these are therefore crucial factors in disease epidemiology. The type of virus-vector relationship gives some idea about the possible distances of spread of a disease and also helps in determining the appropriate control measures.
