Diseases is an alteration in one or more of the ordered sequentials series of physiological processes culminating in a loss of coordination of energy utilization in a plant as result of continuous irritation from the presence or absence of some factor or agent.
Virus is an infectious agent often causing disease, invisible with light microscope, small enough to pass through a bacterial filter lacking a metabolism of its own and depending on a living host cell for multiplication.
Virus produces visible or otherwise detectable abnormalities in plants which are recognized as symptoms. Mostly virus causes systematic infection i.e. the virus spreads through the host.
Transmission of viruses by vectors including arthropods is an important part of virus study. No description of virus is complete without information on the virus-vector relationship. Viruses are mostly transmitted by aphids, leafhoppers and white flies. Beetles and thrips as insect-vectors also transmit some viruses.
Disease of Fruit Crops
The major viral disease affecting fruit crops are apple mosaic, bunchy top disease of banana, mosaic of banana, decline disease of sweet orange, mosaic disease of papaya and leaf curl disease of papaya.
This disease can be transmitted by grafting. No insect – vector is known.
Symptoms
One of the common symptoms is yellow vein banding pattern. Young leaves show, small irregular creamy or yellow spots which are conspicuous against the dark green colour of normal leaf tissue. Also chlorotic bands develop along veins forming the vein banding pattern. No symptoms of this disease on fruits have been observed.
Control
It is caused by banana virus I and Musa virus I. The virus is generally transmitted by the insect-vector Pentalonia nigronervosa.
Symptoms
The very external symptom of this disease is that plants appear as irregular, nodular, dark-green to brown steaks along the secondary veins with chlorotic streaks on the underside of the lower portion of midrib. Badly affected plants are stunted. All leaves are reduced both in width and length, and stand erect and bunched together at the apex of the plant to form a rosette, a condition typical of the bunchy top disease.
Control
The virus causing banana mosaic is sap-transmissible from banana to cucumber and cucumber to cucumber, but not from banana to banana by sap inoculation. The insect-vector is Aphis gossypi.
Control
The virus is transmitted by the insect vector oriental citrus aphid (Taxoptera citricidus) and melon aphid (Aphis gossypii).
Symptoms
Infected plants show partial chlorosis. The plants show the symptoms of twig decline, leaf fall, heavy flowering or bearing of fruits which take colour even when immature, leaf rolling, sudden collapse slow decline.
Control
The disease is caused by Papaya Mosaic virus. This virus is spread by several species of aphids, like Myzus persicae, Aphis gossypii, Aphis malvae, Aphis medicaginis, Aphis craccivora, and Marcosiphum sonchi.
Symptoms
The initial symptoms vary according to the age of plant, condition of infection, season etc. Leaves show profuse mottling and puckering, chlorotic and malformed appearance, and often modification into tendril like structure. There is increase in number of lobes in leaf. The leaves are reduced in size. The old leaves get defoliated leaving a tuft of small ones at the top. The infected plants bear a fruit trees which remain abnormally small. Plants also show reduction in growth. It also affects photosynthetic activity of the leaf.
Control
The disease is caused by Tobacco virus 16 or Nicotiana virus 10. The virus is readily transmitted by grafting or juice inoculation. The most important insect-vector is the white fly (Bemisia tabaci).
Symptoms
The infected plant show a severe curling, crinkling and distortion of leaves, reduction of petioles, internodes and also main shoots. Leaves become dark-green, leathery and little. Plants fail to flower and sometimes bear few fruits.
Control
Disease of Vegetables Crops
The viral diseases vegetables are mosaic disease of brinjal, leaf curl of chilli, chilli mosaic, green mosaic disease of cucurbits, yellow mosaic in pumpkin and Okra and leaf curl in tomato.
The mosaic viruses are all sap-transmissible. Some of the insect-vector which transmit the disease are, Aphis gossypii, A malvae, and Myzus persicae.
Symptoms
Field infected brinjal shows a pronounced mosaic mottling of younger leaves, which are visible clearly against transmitted light. Leaves of diseased plants are reduced in size and occasionally malformed. Affected plants, are dark green and dwarfed and they do not bear fruits as much as healthy ones. The flowers show colour-break in petals.
Control
Occasionally this disease is caused by Vicotiana virus 10. The virus is transmitted by the insect vector Bemisia tabaci.
Symptoms
Infected plants show serve reduction in leaf size, curling of leaves and extreme dwarfing of plants. The leaves turn paler and become pubescent.
Control
Chilli mosaic disease is a commonly occurring ailment of the Chilli. The virus is transmissible by contaminated hands, implements and contact. The insect-vector responsible for transmission is Aphis gossypii, A. euouymi and Myzus persicae.
Symptoms
The symptoms are characterized by distortion of leaves with puckering, blistering and pronounced mottling. The leaves are often reduced in size and become filiform. Flowers and fruits may not form in
Cases of early infection, or may remain small and distorted.
Control
The disease is easily transmitted in juice by the mechanical methods of inoculation, and through the see of melon. No insect – vector is yet been determined. Cucumber, bottle-gourd, snake-gourd, pumpkin, squash and water melon suffer from green mosaic disease.
Symptoms
The diseased plants develop a well-marked light or dark-green mottle, together with blistering and distortion of the leaves and stunting of plant. The fruits especially of cucumber and bottle gourd, are often mottled and malformed.
Control
Yellow mosaic disease is widespread in cucurbit growing areas especially pumpkin and squashes. The disease causing virus is transmitted by insect-vector Bemisia tabaci (white fly).
Symptoms
The young developing leaves of infected pumpkins plants show a faint vein yellowing. In subsequent leaves the intensity of vein yellowing increases gradually till the characteristic symptoms of vein-yellowing and yellow patches develop in leaves.
Control
Yellow vein mosaic disease of okra is a serious problem. When the infection is early the entire crop is destroyed. The virus responsible for this disease is Hibiscus Virus I. Virus is transmitted by the insect-vector Bemisia tabaci.
Symptoms
The main symptoms of yellow vein mosaic are vein clearing and veinal chlorosis of leaves. The yellow network of veins is very conspicuous and the veins and veinlets are thickened. In severe cases the chlorosis may extend to the interveinal areas and may result in complete yellowing of the leaf. Fruits are dwared, malformed and yellowish green in colour.
Control
The leaf-curl disease is a serious menace to the tomato crop. The virus is transmitted by insect-vector Bemisia tabaci.
Symptoms
The plants stunted leaves and internodes are much reduced in size, with the result that leaves are curled and crowded together. The leaflets are reduced in size and their margins are curled inwards or outwards. The infected plants are pale in colour and have a tendency to produce more stunted lateral branches, resulting in a bushy growth. This disease induces partial or complete sterility.
Control
VIII. Disease of Oilseed Crops
Some of the important viral diseases of oilseed crops are mentioned here.
This disease is very much prevalent in groundnut growing areas. This disease is caused by the virus transmitted through Aphis craccivora aphids.
Symptoms
In the beginning the diseased plants show faint mottling and vein necrosis of the young leaflets. Subsequent leaves may be completely chlorotic or may show a mosaic pattern of dark green veins leading to light green leaflets. Later on leaves become reduced in size and the petiole and rachis are shortened the axis of the plant ceases to grow, thus making the plant resettled. Diseased plant look stunted and is of bushy appearance.
Control
It is one of the important disease of sesamum. This disease is caused by Nicotiana virus 10. This virus is transmitted by an aphid.
Symptoms
Downward curling of the infected leaves is the most conspicuous symptoms of the disease. Reduction in size, bittleness and dark colour are the most common features of infected leaves. In severely infected plant capsule formation does not take place.
Control
IX. Conclusion
Now a days the viral diseases of plants are becoming a meance because they cannot be cured by an chemical treatment, the measures suggested for their control are only empirical. They are, more or less, prophylactic and meant for prevention of infection, eradication of the source of inoculum, or protection against the disease-carrying vectors.