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CHARACTERISTICS OF VIRUSES

Viruses are another very common and very important cause of plant diseases. While bacteria are small, viruses are even smaller. Bacteria can be seen with a light microscope, but viruses can typically only be seen using an electron microscope.

Viruses are also much simpler structurally than bacteria. Bacteria have true cells. Viruses do not. Most viruses are composed of a piece of genetic material with a protein coat. Viral genetic material can be either deoxyribonucleic acid or DNA (the same kind of genetic material found in human cells), or ribonucleic acid or RNA. Most known plant viruses contain RNA.

Because viruses are so simple, they do not have the "machinery" necessary to reproduce. Thus after infecting a cell, they "hijack" the cell and force it to produce viral particles.

Viruses can be transmitted (moved from plant to plant) in several ways. Many are mechanically transmitted. These viruses can be moved about by a grower handling an infected plant, then handling a healthy plant, or simply by two plants rubbing together. Other viruses are transmitted by insects (these insects are called VECTORS). These viruses are picked up by the insects as they feed and then are moved from plant to plant as the insects move from plant to plant. Some insect-transmitted viruses actually reproduce in the insect vector as well as in the plant host.