ABSTRACT
In contrast to animals, plants are sessile organisms and are not significant transmitters of viruses except for some instances of pollen and seed transmission and movement of plants resulting from human intervention. Th us, the great majority of plant viruses are dependent for their spread (and therefore survival) upon effi cient transmission from plant to plant by specifi c vector(s). Plant-toplant spread ensures virus survival, oft en resulting in disease. It is therefore critical to understand this essential component of the virus life cycle and the life cycle of the vectors as well. Plant viruses cannot enter the host on their own. Th ey need a method of introduction that allows them to enter host tissues where they can multiply. One method, oft en used in experiments, but not of great importance in natural systems, is via mechanical damage but more important in nature is by a range of invertebrates and soil-borne fungal and plasmodiophorid species. Th ese species that act in this way are known as ‘vectors’. Th ey penetrate unwounded plant cells, usually when feeding on them or parasitizing them, and can acquire viruses from an infected plant and may subsequently transfer the virus (inoculate) to susceptible, healthy plants. Diff erent types of feeding behaviours are found in diff erent invertebrate vector species depending upon their mouthparts that may be adapted for chewing, sucking (lacerating-sucking, piercing-sucking) and sponging-lapping. Th e vector retains the virus in an infective form between acquisition and inoculation. In the fungal or plasmodiophorid vectors the virus is contained either on the surface or within the vector. Th e relationship between vector and virus is often very specific and the result of complex biological interactions. Th is specifi city operates both within and between vector groups. Th us, a virus transmitted by a member or members of one taxonomic group are not normally transmissible by a member or members of another taxonomic group. In most plant virus-vector interactions, the vector is just a carrier of the virus but, in a few cases, some vectors also support multiplication of the viruses and even transmit them to their progeny. Th e role of vectors in virus spread is of immense signifi cance. Understanding their biology and phenology is essential if strategies to control the virus diseases they transmit are to be developed.
