The word virus is derived from the Latin language, which means morbid poison. Various serious diseases such as Influenza, Polio, and Smallpox are caused by virus. Viruses are obligate parasites. They are studied under Electron microscope. Among the plant viruses, Tobacco Mosaic Virus (TMV) has been studied in great detail.
Brief history of virus:
According to scientists of olden times, the viruses are poisonous which are carried by air and cause diseases. In 1770 the diseases known as ‘leaf roll of potato’ was studied for the first time. In 1785, viral disease was also noticed in Germany.
Pasteur (1867), studied viral disease in insects and was detected in Jawa and certain control measures were also recommended. Tomato mosaic disease was studied in England in 1887 but its detailed investigations were carried out in America. Iwanowski in 1898 reported that disease-causing entities of Tobacco Mosaic can pass through Bacteria-proof filters. Beijerinck (1898) called the infectious fluid of T.M.V. as Contagium vivum fluidum and subsequently it was called as Virus. Virus multiplies only in the living cells. Loffer and Forsch (1891) detected viral diseases of foot and mouth.
Size and shape of virus:
Recent investigations using electron microscope and X-ray photography have revealed their detailed structure. The viruses are 20-80 mm (m. or 1 micron = 1/1000 mm or millimicron 1/1000m = 1/1000 x 1/1000 mm = 1/10,00,000 mm in width and 100-50 mm in length. In shape, the virus may be like rods as in TMV (Tobacco mosaic virus), like spheres as in polio virus or may be like tadpoles as in Phage virus the size of TMV is 300 x 15m. The smallest virus is almost equal to the size of a molecule of oxyhaemoglobin, i.e., 3-5 mm.
Structure of virus:
Angiosperm, bacterial and insect viruses have been found to be composed by nucleoproteins. The .infectious particle of a virus or the virus consists of a core of nucleic acid, partly or wholly surrounded by a sheath of protein (capsid). DMA is the core in most virus-infecting animals or bacteria and RNA is the core in some viruses infecting plants. The protein sheath appears to be made up of several subunits packed together around the nucleic acid core. There is striking resemblance in chemical composition between virus particles and the chromosomes of plants and animals. Both carry genes, the unit of hereditary characters.
Transmission of Virus:
Virus disease may spread by the following means:
1. By mechanical means to running tool, grafting tools and by man himself.
2. By vegetative propagation of the diseased plant parts.
3. By seed and pollen grains, e.g., bean mosaic.
4. By soil, e.g., wheat mosaic.
5. By insect vectors widely distributed in nature.
6. By nematodes, e.g., colocasia mosaic.
7. By weeds, e.g., cuscuta.
8. By fungi, e.g., tobacco necrosis.
9. By mites and other similar insects.
10. By parasitic phanerogams found growing in abundance in nature.
Control measures :
1. Removing the source of infection of the viruses.
2. Burning and burrowing deep the infected parts of the plant.
3. By cultivating resistant varieties of the crops.
4. By the use of chemical Protestants well tested before their use.
5. By killing insect vectors.
6. Rotation of susceptible hosts.
7. Biological control.