Viruses are minute parasitic, ultramicroscopic disease producing organism which were discovered by J. Wanowrki in 1892. He thought it small bacterium causing tobacco Mosaic. Hunger 1905 and Freiberg Stanley Para crystalline form. The virus is an obligatory parasitic pathogen viruses cannot be placed in either plant or animal kingdom. Science which deals with viruses is called virology. They may be plant viruses, Mammalian viruses, insect viruses and Bacterial viruses or Bacteriophage.
Some virologists told that viruses are non living objects. They are crystalline particles of definite shape and size which can reinfect new host. Some give view that viruses are living objects. Viruses possess character of growth and reproduction. Viruses do not survive in non living tissues hence are parasitic bacteria. Viruses are primitive form of living mater and thus regard viruses as intermediate between non living organisms.
Properties of Viruses:
(1) They are transmissible from unhealthy to healthy plants.
(2) They are infilterable through fitter paper.
(3) They can multiply only within the living host cell.
(4) They possess the capacity of variations and adaptations.
(5) They are highly resistant to acid, alkalies and salts.
(6) They are resistant to high temperatures.
(7) There is no direct effect of sunlight on viruses.
Transmission of Viruses:
Plant viruses can be transmitted from one host to the other in following ways:
(a) By seeds: The mosaic viruses of cucurbits and legumes are transmitted by seeds. Seeds carry a high percentage of viruses. The virus does not enter the embryo. It is carried in a portion of the seeds of diseased plants.
(b) By Vegetative propagation: This is one of the chief methods of transmission of virus disease especially of potato, Raspberry, Strawberry, Zulips, Bulb plants, fruit trees and many ornamentals. The diseased vegetative parts such as tubers, bulbs, roots, buds, cuttings, grafts which are used for propagation will contain the virus present in the parent plant.
(c) By mechanical means: Hand, tools, agriculture implements carry with them several viruses which are thus transmitted from unhealthy plant to the healthy ones.
(d) By soil: Tobacco mosaic virus in the soil with plant debris after the harvest of the crop and infects new crop when sown in same field.
(e) By insects: Insects are natural agents of spread of virus diseases. The insect which carries the diseases is called a vector. They are apluds, Grasshoppers scale insects with flies. Insect obtains viruses through its mouth parts. It is then passed into healthy plant by mouth parts. The virus may remain active in the body of
Vector for many days and this period is known as incubation period.
Viral diseases are:
Crop plants are the common victims of virus infection. Most important disease caused by viruses in plants in mosaic disease on tobacco, sugar cane, cucumber, tomato and peach. Mosaic diseases are not fatal. Mosaic disease causes mottling or variegation of leaves. There is enormous loss in the crop in quality and quantity of the yield. The diseases are mosaic and streak of sugar cane, leaf curl of tobacco, mosaic of cucurbita, ground nut rosette, and leaf roll of potato.
Symptoms of Virus diseases:
(1) Chlorosis: Due to the presence of viruses chlorophyll of the green organs disappears at places, leaving yellowish spots, this is called chlorosis. Presence of yellow stops at places in green tissue appears like a mosaic pattern and therefore the disease with such symptoms are known as mosaic diseases.
(2) Yellows: Complete leaf of host plant becomes, yellow, there being no mottling.
(3) Vein cleaning and vein bounding:
Disappearance of chlorophyll along the vein of the leaves is known as vein clearing and when chlorophyll surrounding the virus disappears the symptom is known as vein bonding.
(4) Necrosis: This causes groups of cells in diseased plants to collapse and die. Necrosis occurs both in stem and leaves. Necrotic sports are also known as Lesions. In some virus diseases there are over growths such as galls or enations. Other common symptoms are stunting and dwarfing of plants, crinking pucking, and reduction in the size of leaves and sterility and curling of leaves.
Control of virus diseases:
(1) By destruction of infected plants and isolation of suspectible plants.
(2) By elimination of insects use of insecticidal dusts and strays reduce the changes of infect transmission.
(3) By selecting the seed free from virus infection.
(4) By developing the virus resistant varieties.
(5) By doing the heat treatment of infected plants.
(6) By fumigation of insecticides, sprays, phenol etc.
.jpg)