In this article we will discuss about:- 1. Forms of Virus 2. Structure of Virus 3. Symptoms of Viral Diseases 4. Control Measures.
Forms of Virus:
The forms of virus are diverse. When observed under an electron microscope, they appear spherical or rod-like or cubical or filliform or polyhedral or coiled or twisted like a screw or bopyramidal or hexagonal or prism-like etc.
On the basis of forms, the viruses may be grouped into three classes:
(1) Cubical or box-like, e.g. Vaccinia virus.
(2) Helical, when twisted like a screw or coiled e.g., tobacco mosaic vims, etc.,
(3) Mixed symmetries or Combined symmetries, e.g. bacterial virus, herpes simplex virus, etc.
Structure of Virus:
The virus is non-cellular and particulate. The present conception is that the body of virus is composed, of a central core filled up with nucleic acid with a thick protein envelope. This protein envelop is called the capsid. There are numerous long protein particles, known as capsomeres inside it.
Viruses either contain DNA or RNA but both do not occur in the same capsid. RNA of tobacco mosaic virus is embedded in the capsomeres. When the bacterial virus lives as a parasite inside the body of bacteria, it is known as bacteriophage or phage.
At the time of reproduction, a virus attaches itself to a bacterial cell or to the tissue of a host and an aperture appears on the host cell through which the nucleic acid only enters into the host and the protein coat is left on the outside as a “ghost”.
The nucleic acid then replicates inside the host cell and produces new viral protein and thus a new virus is formed. In this way numerous viruses are formed. Thus the viruses multiply to produce exact replicas of themselves in the tissue of hosts. The host cell ultimately dies and releases the numerous virus particles.
DNA viruses are bacterial virus, pox virus, herpes simplex virus, etc. and the RNA viruses are tobacco mosaic virus, spherical and rod-like plant virus, influenza virus, etc.
Symptoms of Viral Diseases:
Viruses produce various symptoms or visible effects on their hosts. The symptoms may be primary or systemic.
(a) Primary Symptoms:
The primary is initial symptoms are local reactions at the actual spots of inoculation.
They are usually of two kinds:
i. local lesions
ii. clearing of veins.
The local lesions appear as dots or rings of various types. They are the result of death of cells in these areas. Clearing or chlorosis of the tissues and around the veins is known as vein clearing.
(b) Systemic Symptoms:
The prominent systemic symptoms are as follows:
1. Mosaic:
In this case, usually the leaves of host’s show mosaic pattern or patches of yellow and green colours. These patches are due to chlorosis and may be circular or irregular.
2. Chlorosis:
It is the uniform yellowing or formation of different shades of green without any pattern. This is due to dis-balance of normal development of chlorophyll.
3. Necrosis:
It implies the death or disintegration of the host cells. This is one of the symptoms of various virus diseases. Usually it appears as small spots on the leaves or streaks on the stems. Occasionally it extends over large areas causing the death of host plants.
4. Rings:
These appear as light green or yellowish rings on the surface of leaves which are the results of chlorosis and necrosis. These rings are usually concentric to a central spot.
5. Distortion:
It is common symptoms of virus diseases. In this case, dwarfing and distortion of plant members or the entire plant takes place. Sometimes characteristic symptoms such as malformations and overgrowths are also found in some virus diseases.
Control Measures of Virus:
The following measures are adapted to check the spread of virus diseases:
1. Eradication of diseased plants.
2. Destruction of weeds which harbour viruses by burning.
3. Isolation of suspicious diseased plants.
4. Sanitation in case of highly infected plants is essential.
5. Removal of refuse of infected leaves of crop plants to a distant place far away from the healthy plants.
6. Heat treatment to kill or at least inactive viruses which cannot tolerate high temperature.
7. Inoculation of young plants with mild strains of virus for making them resistant to virulent strains.
8. Washing of hands with germicides before and after working in the field.
9. Sterilization of agricultural implements.
10. Preparation of virus-free seed beds.
11. Spraying of insecticides to kill insect vectors.
12. Growing resistant varieties of plants.
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