Plant virus disease occurs because of spread of viruses in nature. It is triple interaction between the plant, the virus and a means of virus spread. It also means that for their own survival the viruses must be transmitted from one host to another during every season and from one field to field or region to region. Transmission is classified into two broad categories, ‘vertical transmission’ wherein the virus is passed through the plant seed/propagule to the next generation or through the vector egg to its progeny and ‘Horizontal transmission’ in which the virus spreads to neighboring plants or across regions by movement through other agents like insects, fungi, mite and nematodes and in which carry the virus only for a definite period. (Nayadu, 2008).
These techniques are cheap and easy methods of plant propagation but provide the ideal opportunity for viruses to spread to new plants. Economically important viruses can spread systemically through most vegetative parts of the plant. A plant once systemically infected with virus remains infected for its life long. Thus any vegetative parts of a plant taken for propagation such as tubers, bulb, corms, runners and cuttings will normally be infected (Hull, 2002). Viruses can be spread by direct transfer of sap by contact of a wounded plant with a healthy one. Such contact may occur during agricultural practices, as by damage caused by tools or hands, or naturally, as by an animal feeding on the plant.
Plant viruses need to be transmitted by a vector and forms the largest and most significant vector group and particularly includes, aphids, whiteflies, hoppers, mealybugs, thrips, beetles, Plasmodiophorids, mites and nematodes (Hull, 2002).
About one seventh of known plant viruses are transmitted through the seed of at least one of their infected host plants. Seed transmission provides a very effective means of introducing virus into plant crop at an early stage, giving randomized foci of primary infection throughout the planting. Thus, when some other method of transmission can operate to spread the virus within growing crop, seed transmission may be persist in the seed for a long period’s so that commercial distribution of seed-borne virus over long distance may occur. In some case viruses are transmitted from plant to plant via pollen grain. The extent to which infected pollen is a significant factor in the spread of viruses in the field has not been thoroughly assessed. It may well be more important economically with cross pollinated woody perennials than with annual crops. With certain viruses, infected pollen may cause only the resulting seed become infected when a healthy plant is pollinated (Hull, 2002).
Dodder (Cuscuta spp) is a parasitic vine on higher plants. There are many different species with different host ranges, some of which are extensive. Dodder would transmit virus from plant to plant. Transmission by dodder is in some of respects similar to grafting. However, graft compatibility is limited to quite closely related plants usually within genus. Dodder on the other hand can be used to transmit a virus between distantly related plants. One of the main experimental uses of dodder transmission has been to transfer viruses from hosts where they are difficult to study to useful experimental plants (Hull, 2002).