Definition

A virus is a "set of one or more nucleic acid template molecules, normally encased in a protective coat or coats of protein or lipoprotein, which is able to organize its own replication only within suitable host cells.  Within such cells, virus production is a) dependent on the host's protein-synthesizing machinery, b) organized from pools of the required materials rather than by binary fission".

Etymology. 1392, "venomous substance," from L. virus "poison, sap of plants, slimy liquid," probably from PIE base *weis- "to melt away, to flow," used of foul or malodorous fluids (cf. Skt. visam "poison," visah "poisonous;" Avestan vish- "poison;" L. viscum "sticky substance, birdlime;" Gk. ios "poison," ixos "mistletoe, birdlime; O.C.S. višnja "cherry;" O.Ir. fi "poison;" Welsh gwy "fluid, water," gwyar "blood"). Main modern meaning "agent that causes infectious disease" first recorded 1728. The computer sense is from 1972. Adjective form viral was coined 1948.

Discussion

Viruses are submicroscopic.  They are not cells. Plant viruses come in many shapes and sizes.

The nucleic acid portion of viruses are RNA or DNA. A virus particle (nucleic acid + protein coat) is called a “virion”. Viruses are Biotrophs (need a living host). Viruses must spread from one plant to another in order to survive. Vectors help to transmit viruses from plant to plant. Vectors include insects (e.g., aphids, leafhoppers, beetles, mites, whiteflies, mealybugs), nematodes, some plants (e.g., dodder). Viruses must enter plant cells through wounds. Viruses move from cell to cell; and move through plants systemically. One virus may infect one or dozens of plant species. Each plant species is usually attacked by many different kinds of viruses.

Viruses do not divide, do not produce specialized reproductive structures; they induce host cells to produce more virus.

Viruses cause disease by:

            -utilizing cellular substances.

            -taking up space in cells

            -disrupting cellular components and processes.

            -disrupting metabolism, leading to development of abnormal structures.

The most common symptoms of viral diseases are:

            -reduced growth rate of the plant (stunting, dwarfing)

            -reduced yield

            -shortened life span of the plant

            -systemic symptoms: *mosaics, ringspots, lesions, chlorosis

Some plants show no symptoms when they are virus-infected.