The plants such as Cuscuta and Cassytha have

agents which transports a microorganism from one host plant to another plant
are called vectors. The spread or dispersal of inoculums from its source to
healthy host plant is called dissemination. Fungi, Bacteria, nematode and virus
are main pathogens which are transmitted by vectors.. Some plants and fungi act
as vectors for different pathogens. For example, the big-vein disease of
lettuce was thought to be caused by a member of the fungal division Chytridiomycota, namely Olpidium brassicae. Eventually the
disease was thought to be viral. But after that it expressed that the virus was
transmitted by the zoospores of the fungus and also survived in the resting
spores. Since then, many other fungi in the Chytridiomycota have been shown to
vector plant viruses. Many plant pests that seriously effect important crops
depend on other plants, often weeds, to harbour or vector them; the distinction
is not always clear. In the case of Puccinia
graminis for example, Berberis
and related genera act as alternate hosts in a cycle of infection of grain.
More directly, when they twine from one plant to another,
parasitic plants such as Cuscuta and Cassytha have been shown to convey
phytoplasmal and viral diseases between plants. Insect vectors of viruses are
the most important, composite and widely dispersed plant disease agents. Aphids,
leafhoppers, whiteflies, thrips, mealy bugs, beetles and mites are main insect
vectors transmitting plant pathogens. Insects were the first known vectors of
plant virus-related diseases, i.e., leaf hoppers (Deltocephalis dorsalis) vector of rice stunt virus disease. Vectors
classification in different groups give some idea about the mechanism of virus
spread by vector. Insect vectors have wide host range. To develop proper
techniques to control vector-borne diseases virus–vector affiliation
understanding is vital. Vectors of viral diseases can be managed by cultural
control, biological control, and chemical control. Vector transmission of a
pathogen can be categorized depending on its ability or effectiveness. Efficacy
is based on the fact that how effectively vector can transmit the pathogen when
it gets a chance of spreading pathogen. It depends upon the capability of
vector to transfer the pathogen in a specific time. Insect vectors also spread  some bacterial pathogens of plants. Some bacterial
parasites rely on insect vectors for movement and disease spread. Most of the
fungal pathogens are not dependent on mobile vectors like insects or mites.
They mostly spread to plants mostly by wind, rain, or soil, though, some fungi
are specific for spread by insect vectors.