The plant cells are protected externally by the cell wall. Hence, plant viruses have a considerable obstacle to overcome when trying to establish themselves in the host plant cell.
Some plant viruses are transmitted through contaminated seeds, pollen or tubers. Tobacco Mosaic Virus (TMV) and a few others may be carried by wind or animals and may enter when the leaves are mechanically damaged.
Soil nematodes transmit viruses such as the Tobacco ringspot virus while feeding on roots.
Tobacco necrosis virus is transmitted by parasitic fungi.
Insects serves as the most efficient agents of transmission particularly leafhoppers and aphids which as sucking insects.
They pick up the viruses on their mouth parts while feeding on an infected plant and directly transmit the viruses to the next plant they visit.
Some viruses maybe stored in the gut of the insects.
The wound tumor virus multiplies in the leafhopper tissues before reaching the salivary glands and being transferred to the plant.