Introduction

Plant viruses are viruses that effect on plants. Plant viruses are obligate intracellular parasites. Plant viruses are pathogenic to higher plants. Viruses also cause many important plant diseases and are responsible for huge losses in crop production and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease but often there is leaf yellowing either of the whole leaf or in a pattern of stripes or blotches, leaf distortion e. g. curling and other growth distortions e. g. stunting of the whole plant, abnormalities in flower or fruit formation. Plant viruses are classified into groups based on nature of genome six major groups. Plants viruses are transmitted into an other plants with the help of cell wall and viruses cannot penetrate them. Most plant viruses are therefore transmitted by a vector organism that feeds on the plant or in some diseases are introduced through wounds made, for example, during cultural operations e. g. pruning. A small number of viruses can be transmitted through pollen to the seed. Plant viruses are important pathogens.

The study of plant viruses has made important contributions to the understanding of viruses in general—for example, the recognition of viruses as pathogens, the structure of virus particles, and the infectious nature of RNA. Viruses can be control with the help of different methods. With the help of chemical and bioloigical methods. We control the plant viruses use a viruses free plant material.

Classification of plant viruses

Classification of plant viruses divided into groups. Plant viruses are classified into single stand and double stands which may bi positive and negative stands. Plant viruses have plylum, class, order, famly, ganus and species. Examples of plant virusesTobacco mosaic viruses,Tomato spottes wilt virus, Tomato yellow leaf curl virus, cucumber mosaic virus,potato virus,cauliflower mosaic virus,African cassava mosaic virus, plum pox virus,brome mosaic virus and potato viruse.

The genus takes its name from the discovery of tomato spotted wilt virus (TSWV) in Australia in 1919. It remained the only member of the family until the early 1990s when genetic characterisation of viruses discovered in plants became more common. There are now at least twenty viral species in the family with more being recorded and described on a relatively regular basis. Together, these viruses have been documented infecting over eight hundred different plant species from 82 different families. Cucumber mosaic virus is a plant pathogenic virus in the family Bromoviridae. It is the type member of the plant virus genus, Cucumovirus. This virus has a worldwide distribution and a very wide host range.

In fact it has the reputation of having the widest host range of any known plant virus. It can be transmitted from plant to plant both mechanically by sap and by aphids in a stylet-borne fashion. It can also be transmitted in seeds and by the parasitic weeds.

African cassava mosaic virus and South African cassava mosaic virus are distinct species of circular single-stranded DNA viruses that are whitefly-transmitted and primarily infect cassava plants. These have thus far only been reported from Africa; related species of viruses Indian cassava mosaic virus, are found in India and neighbouring islands Sri Lankan cassava mosaic virus, though cassava is cultivated in Latin America as well as South East Asia. Nine species of cassava-infecting geminiviruses have been identified between Africa and India based on genomic sequencing and phylogeneticanalysis. This number will probably grow due to a high rate of natural transformation associated with CMV.

Plum virus also known as sharka, is the most devastating viral disease of stone fruit from the genus Prunus. The disease is caused by theplum pox virus and the different strains may infect a variety of stone fruit species including peaches, apricots, plums, nectarine,almonds, and sweet and tart cherries. Wild and ornamental species of Prunus may also become infected by some strains of the virus. Brome mosaic virus is a small (28 nm, 86S), positive-stranded, icosahedral RNA plant virus belonging to the genus Bromovirus, family Bromoviridae, in the alphavirus-like superfamily. BMV commonly infects Bromus inermis and other grasses, can be found almost anywhere wheat is grown, and thrives in areas with heavy foot or machinery traffic. It is also one of the few grass viruses that infects dicotyledonous plants, such as soybean; however, it primarily infects monocotyledonous plants, such as barley and others in the family Gramineae. Need and significance of plant virual taxanomyViruses are very small pathogenic particles made up of nucleoprotein.

The study of plant viruses is so important because they cause diseases to the economically important crops. They cause a great loss to the quality and quantity of the crops. Plant viruses show various types of symptoms such as colour breaking, chlorosis, mottling, vein clearing, vein bending, leaf curl, decrease in size, distorted growth, etc. The plant viruses are very simple and are very host specific. Yellow mosaic symptoms on lettuce caused by Lettuce mosaic virus. Yellow vein-banding symptoms on grapevine caused by Grapevine fanleaf virus. Fruit distortion on eggplant fruit caused by Tomato bushy stunt virus. A healthy fruit is shown on the left. Bark scaling caused by Citrus psorosis virus. Sometimes the virus is restricted to certain parts of the plant e. g. the vascular system discrete spots on the leaf but in others it spreads throughout the plant causing a systemic infection. Infection does not always result in visible symptoms.

Review of literature

Virus disease of plants was known long before the discovery of bacteria.

1. Breaking of flower colour of tulips (as early as 1576).
2. Transmission of leaf variegations from the scion to the stock of woody plants as early as 17oo.
3. Tobacco mosaic was identified by Swietch in Holland in 1857.
4. Adolph Mayer (1886) was the first to point out that tobacco mosaic is readily transmissible and infectious. Believed that bacteria are the cause of the disease.
5. Iwanowski (1892) confirmed some of the results Mayer. He demonstrated that the power to infect was lost if the sap was previously heated. He reported that infectiousness was retained even when sap was passed through bacteria proof filters.
6. Bejierink (1896) demonstrated that tobacco mosaic infectious agent could diffuse through an anger membrane and concluded that tobacco mosaic was caused by a non-corpuscular ‘ Contagium vivum fluidum’ which he called as Virus.
7. Stanley (1935) crystallized tobacco mosaic virus with ammonium sulphate and concluded that the virus was an autocatalytic protein that could multiply within the living cells. He considered that virus was a globulin containing no Phosphorus. For his discovery he was awarded a Nobel Prize.

Conclusions

1. Tmv are first viruse which discover first. Tmv cause a disease tombaco mosaic.
2. Plant viruses are diverse group of plant pathogens. They infect and cause disease in many crop plants. Because these pathogens depend on the normal cellular machinery of their plant host for reproduction.
3. It is difficult to eliminate them without damaging the host plant. The most management strategies for diseases caused by plant viruses are directed at preventing infection of plants.