A virus is a set of one or more nucleic acid template molecules, normally encased on a protective coat or coats of protein or lipoprotein that is able to organize its own replication only within suitable host cells. Translation of the genome (to produce proteins) or transcription and replication (to produce more nucleic acid) takes place within the host cell and uses some of the host's biochemical "machinery". Viruses do not capture or store free energy and are not functionally active outside their host. They are therefore parasites (and usually pathogens) but are not usually regarded as genuine microorganisms (Hull, 2002). Most viruses are restricted to a particular type of host. Some infect bacteria, and are known as bacteriophages, whereas others are known that infect algae, protozoa, fungi (mycoviruses), invertebrates, vertebrates or vascular plants. However, some viruses that are transmitted between vertebrate or plant hosts by feeding insects (vectors) can replicate within both their host and their vector. Plant viruses are viruses that affect plants. Like all other viruses, plant viruses are obligate intracellular parasites that do not have the molecular machinery to replicate without a host. Plant viruses are pathogenic to higher plants (www.dpvweb.net). This web site is mostly concerned with information related to plant viruses in India.
The discovery of plant viruses causing disease is often accredited to Martinus Beijerinck who determined, in 1898, that plant sap obtained from tobacco leaves with the "mosaic disease" remained infectious when passed through a porcelain filter. This was in contrast to bacteria microorganisms, which were retained by the filter. Beijerinck referred to the infectious filtrate as a "contagium vivum fluidu", thus the coinage of the modern term "virus”. After the initial discovery of the ‘viral concept’ there was need to classify any other known viral diseases based on the mode of transmission even though microscopic observation proved fruitless. In 1939 Holmes published a classification list of 129 plant viruses. This was expanded and in 1999 there were 977 officially recognized, and some provisional, plant virus species (http://paulstownallotments.com/Diseases/Diseases.html). In India virus like infections were reported by many plant pathologists in 19th century, but recognized as distinct one early during 1900s in various crops like cardamom, bhendi/okra, sugarcane, tomato, legumes and others. Mollison in 1990 reported katte or mosaic disease of cardamom in North Canara (Uttara Kannada) district of present Karnataka state as a severe damaging disease in the crop. Kulkarni in 1924 recorded early reports of mosaic of sugarcane, tobacco mosaic on tobacco in Maharashtra, Karnataka, Gujarat and Deccan plateau of Karnataka, Andhra Pradesh and Tamilnadu with up to 50% infection in some fields. India now has a record of more than 300 plant viral diseases in various crop plants and weeds (Nayadu, 2008).More recently virus research has been focused on understanding the genetics and molecular biology of plant virus genomes, with a particular interest in determining how the virus can replicate, move and infect plants. Understanding the virus genetics and protein functions has been used to explore the potential for commercial use by biotechnology companies. In particular, viral-derived sequences have been used to provide an understanding of novel forms of resistance. The recent boom in technology allowing humans to manipulate plant viruses may provide new strategies for production of value-added proteins in plants.