Encyclopedia of Plant Viruses and Viroids

Virology, a branch of biological science was developed with the discovery of Tobacco mosaic virus in 1898. Viruses are known to infect all sorts of living organism and those known to be restricted within plant kingdom are referred as plant viruses. Although a few groups of plant viruses are known to infect insect vectors, so far none of the plant viruses is known to infect higher animals including human beings. The size of plant viruses ranges between 16 and 2000 nm and generally organized with nucleic acids and protein in either cubic or helical symmetry resulting in isometric or rod shaped architecture. During 1971, another class of infectious phytopathogen containing only nucleic acids and no protein coat, and causing diseases similar to plant viruses was discovered in potato and referred as viroid (Singh 2014).

Plant viruses are important constraint in world agriculture. Of all the phytopathogens, studies of plant viruses have received special attention as they are difficult to manage. Numerous plant viruses are presently known globally and almost all the crop species are affected by one or the other viruses. Plant viruses were discovered in late 1880s. Over the last three decades, application of molecular methods revolutionized our understanding of plant viruses. Viruses are of simple biological identity that undergo continuous changes in their genetic makeup and thereby exist in numerous variants. The continuous evolution in viruses poses a great challenge in their identification and classification. No single property of viruses is adequate to identify them.