Virus particles are extremely small and can be seen only with an electron microscope. Most plant viruses are either rod-shaped or isometric (polyhedral). TMV, potato virus Y (PVY), and cucumber mosaic virus (CMV) are examples of a short rigid rod-shaped, a long flexuous rod-shaped, and an isometric virus, respectively. Viruses consist of an inner core of nucleic acid (either ribonucleic acid [RNA] or deoxyribonucleic acid [DNA]) surrounded by an outer sheath or coat of protein (referred to as the capsid). The capsid is further enclosed by a membrane in most human and animal viruses that helps the virus pass through the cell membrane in these types of cells. Since the cell membrane in plants is surrounded by a rigid cell wall, plant viruses require a wound for their initial entrance into a plant cell. Wounds in plants can occur naturally, such as in the branching of lateral roots. They may also be the result of agronomic or horticultural practices, or other mechanical means; fungal, nematode, or parasitic plant infections; or by insects. In some cases, the organism creating the wound can also be carrying and can pass or transmit the virus. Organisms that transmit pathogens are called vectors.

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