The word ‘virus’ means poison. Viruses are the most infectious group of diseases and are parasitic in nature. It is possible to describe them in terms of properties of chemical molecules.
They consist of only one kind of nucleic acid enclosed within a protective of protein or lipoprotein called “capsid or sell”. In most plants, viruses, only one kind of protein is found but some of the large viruses, may have several different proteins each have different function. Complete virus particle is called “virion”.
The importance of plant viruses in relation to crop production, can be realized from the fact that, viral diseases are prominent and cause losses in world’s crop production amounting to many million rupees. Plant virus diseases may damage leaves, stems roots, fruits, seed or flowers and may cause economic losses by reduction in yield and quality of plant products. The severity of individual virus diseases may vary with the locality, the crop variety, and from one season to the next. Some virus diseases have destroyed entire planting of certain crops in some areas, for example, cotton leaf curl virus, banana bunchy top virus, viral diseases of chilies and tomatoes as well as of pulse crops are considered very serious.
The record showed that amongst the major viral diseases of economic crops, cotton leaf curf curl virus found to be most important since last decade. The disease was first time recorded in 1967 at Multan. In Sindh, it was first reported in 1996 at Ubauro, district Ghotki. It is reached up to New Saedabad, district Hyderabad during 1999-2000. The CLCV was considered a minor disease until 1987, but in 1991-92, infested 35,000/- acres and 294,000/- acres during 1992-93 causing a huge production and monetary loss to the nation.
It is quite difficult to present accurate estimates of the losses due to viral diseases, because it varies from year to year and from one region to the other. The aim to elaborate all above actual and potential hazards, damages and losses were determine the importance of viral diseases.
Symptoms: Actually the symptoms are observable effects of a virus on growth. development and metabolism of an infected plant. Abnormal appearance in the plant is usually the first indication of a virus infection in nature. Severe disease symptoms may occur only when virus has infected the plant systematically. In the early days of plant virology, symptoms were the main means by which a virus disease was diagnosed. Plants infected with viruses may develop various type of symptoms on all their parts.
The most common kind of symptoms produced is reduced growth rate of the plant, resulting in virus degree of dwarfing or stunting of the entire plant. Almost all viral diseases seem to cause some degree of reduction in total yield, and the length of lives of virus-infected plants is usually shortened.
The most obvious symptoms of virus-infected plants are usually those appearing on the foliage, but some viruses may cause striking symptoms on the stem, fruit, and roots, with or without symptoms’ development on the levels. All symptoms can be grouped as follows.
Infection type: In almost all virus diseases of plants occurring in the field, the virus is present throughout the plant and the symptoms produced are called systemic symptoms.
Local Symptoms: In many plants the virus cause the formation of small, usually necrotic lesions only at the points of entry and the symptoms are called local symptoms.
Masked symptoms: Plants, that usually develops upon infection with a certain viruses, may remain temporarily symptoms less under certain environmental conditions and such symptoms are called masked symptoms.
Chronic symptoms: Plants may show acute or severe symptoms soon after infection. They may lead to death of the lost. If the host survives the initial shock phase, the symptoms tend to become milder in the subsequently developing part of the plants, leading to partial or even total recovery.
Slow or quick decline: The symptoms may progressively increase in severity and may result in gradual or quick decline of the plant.
Common symptoms: The most common types of plant symptoms produced by virus infections are mosaic, mottle, vein-clearing, vein-banding, yellows, ring-spots, chlorosis, dwarfing and stunting, tumours or galls, bunchy top, witches broom, rosette, enaction and necrosis.
Mosaic: Mosaics characterized by intermingled patches of normal and light-green, yellow or white areas of the leaves, or are whitish areas intermingled with areas of the normal colour of flowers or fruits. The viruses causing most mosaic diseases are mechanically transmitted and usually have aphid vectors in nature, are generally resistant to brief heat treatments and do not stop flowering or effect the dormancy of buds.
Mottle: An irregular pattern of indistinct light and dark areas called mottle.
Vein-clearing: Destruction of chlorophyll in the vein tissues termed as vein-clearing. Vein-Banding: Bands of green tissue along the vein, while the tissues between vein become chlorotic.
Yellows: When chlorophyll disappears completely due to chlorosis, yellowing, bronzing, or reddening, the foliage of the host becomes uniformly discoloured without any spotting patterns and become yellow, although some vein-clearing may be present.
Ring spots: Ring spots, characterized by the appearance of chlorotic or necrotic ring spots on the leaves and sometimes also on the fruit and stem. Most ring spot causing viruses are not transmitted by either aphids or leafhoppers, but some of them are transmitted by nematodes.
Chlorosis: Yellowing of green tissues due to chlorophyll destruction are considered as chlorosis.
Dwarfing, and stunting: The plant size is reduced due to shorter internodes, smaller leaves and fruits and reduction of size of various other plant parts.
Tumor or galls: Unusual swelling or development or transformation produced as a result of viral infection.
Bunchy top: Increasing of leaves or branches at the top of plants.
Witches’ broom: Appearance of broom like growth for example, lilac witches’ broom.
Rosette: Short, bunchy habit of plant growth for example, Peach rosette.
Enation: Malformation or leaf like or sometimes tumours like out growths on the leaves and roots. Leaf like outgrowth referred to as enation for example, pea enation mosaic or cotton leaf curl virus.
Necrosis: Death of cells or tissues.
Less common symptoms: A large number of other less common virus symptom is also described. These symptoms may be accompanied by other symptoms on other parts of the same plant and include: leaf roll, leaf and stem distortion rubbery wood pitting of stem flattening and distortion of stem.
Microscopic symptoms: In addition to the microscopically visible symptoms, several histological and cytological abnormalities also appear in virus infected plants.
Hyperplasia: Excessive development due to increase in number of cells.
Hypertrophy: Excessive growth due to increase in size of cells.
Hypoplasia: Under-development of tissue or plant due to smaller or fewer cell division.
Hypotrophy: Under-development of tissue or plant due to reduced cell enlargement.
Atrophy: Complete absence of multiplication and enlargement of cells. Control: Several methods for control of plant viruses are being practised, which may be divided in to three main group viz.: (1) resistance or immunity to virus(2) prophylactic measures and (3) direct protection.
1. Resistance: Healthy or vigour or host plant confers no resistance or immunity to virus disease. Plant breeding for hereditary resistance to virus is of great importance, and many plant varieties resistant to certain virus diseases can be produced. Immunisation of plant with an avirulent strain of a virus so that the plant will be protected from infection by the virulent strains of the same virus is also experimentally possible, but has not been employed in plant virus control in the field because of the danger to other plants, danger from double infections, and danger from mutations.
Prophylactic measures: Prophylactic measures imply protection of the host from exposure to the virus before infection. These measures include: (a) exclusion, (b) eradication, (c) sanitation and (d) control of vectors.
Exclusion: The best way to control a virus disease is to keep it out of an area through quarantine, inspection, and certification.
Eradication: Eradication of source of infection from the field may help to control the diseases in some cases. Eradication can be done by several ways, for example, eradication of alternate hosts, eradication of wild or other over-wintering hosts including weeds, eradication of alternate hosts, eradication of wild or other over-wintering hosts including weeds, eradication of virus from diseased plants or plants (propagative parts of the host such as seeds, tubers, and bulbs).
Sanitation: This refers to use of healthy seed obtained from healthy crop or seed must be treated by suitable methods, rouging or removal of diseased plants as well as extra suckers and mother plants from a crop, treatment of nursery beds and or plants, improved cultural practices such as planting season, proper manuring and irrigation application, mixed cropping, crop rotation till age operations.
Control of vectors: Plants may also be protected against certain viruses free seed and vegetative parts, viz. tubers, bud-wood etc., viz tubers, bud-wood, etc., is the single most important measure for avoiding virus diseases of many crops, especially those lacking insect vectors. Several types of inspection and certification programmes are also effective to produce virus free seeds, tubers and nursery stock used for propagation. The insect vectors, mites, fungi, nematodes and dodder transmit plant viruses. All these living carriers could be controlled by many mechanical and chemical methods.
Direct protection: Direct protection includes heat treatment and viricides, as follows: Heat treatment: Once inside a plant some viruses can be inactivated by heat. Seeds are usually dipped in hot water (35oC - 54oC) for a few minutes or hours, while the actively growing plants are usually kept in greenhouses or growth chambers at 34o-40oC for several days, weeks, or months, after which the virus in most of them is inactivated and the plants are completely healthy.
Viricides: No chemical substances are available yet for controlling viral diseases of plants. Several substances, however, such as cytovirin, 2-thiouracil, 8-Azoguanine 2,4-dichlorophenoxyacetic acid, zinc sulphate, malachite green, and quinhydrone are used as foliar sprays, as solutions for the watering or immersion of plants, have been reported to suppress symptoms and diminish the virus concentration; but treated plants revert to the original condition after treatment has cased. None of these substances has yet had any commercial application for controlling virus diseases in the field. On the other hand, foliar application of certain growth regulating substances, such as gibberllic acid, has been effective stimulating growth of the virus suppressed auxiliary buds in yellows type diseases and resulting in increased fruit production. Similarly, sprays with gibberellic acid can overcome the stunting induced by certain viruses, for example, corn stunt virus and severe etch virus on their respective hosts corn and tobacco. Certain yellows type diseases (aster yellows and mulberry dwarf) can be controlled by immersing the roots of infected plants in, and / or spraying the plants with the antibiotics Aureomycin and Archromycin of the tetracycline group.
