Viral diseases of economic crops       
By M. Mithal Jiskani, Assistant Professor (Plant Pathology) Sindh Agriculture University, Tandojam.

The viruses are parasitic in nature and cause the most infectious group of diseases, of all forms of livings (including human being, animals and plants). The viruses are very small to that of all others and can not be seen with necked eye, nor with the help of commonly used microscopes, hence are regarded as sub microscopic, nucleo protein particles, multiply inside living cells. Viruses cause diseases by upsetting the metabolism of the cells, but not by consuming cells or killing them with toxins.

The total number of viruses known to date is well over a thousand, and new viruses are added to this almost every month. More than half of all known viruses attack and cause diseases of plants. One virus may infect one or dozens of same or different plant species, and one plant may be attacked by one or many different viruses.

The importance of plant viruses in relation to crop production, can be realized from the fact that among the various factors responsible for low yields, viral diseases are prominent and cause losses in world's crop production amounting to many million rupees, which comes next only to losses caused by insect pests. 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. On nation wide bases, the record showed that amongst the major viral diseases of economic crops, recorded in Pakistan (Table), 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, tomatoes and pulse crops are considered very serious, during different cropping seasons, at different locations of Pakistan; and in certain cases, fields have been found to show as much as 100 percent damage.

However, it is quite difficult to present accurate estimates of the losses due to viral diseases. It is becoming obvious that: now, it is upon the crop protectionists including plant pathologists and entomologists as well, to design and formulate ways or mean to combat all enemies of the crops, so that the growers (farmers) may try to minimize the losses caused by plant viruses to the crops.

IDENTIFICATION OF VIRAL DISEASES

Various external and or internal changes, reactions or alterations (signs) on or in the plants, due to any abnormality as a result of the pathogen (disease causing agent) is termed as symptoms. Actually, abnormal appearance on or 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. It must be remembered that a virus not only causes just one type of symptoms. Mostly viral infection results in more than one type of symptoms. There may be a series of symptoms as the disease persists within the plant. For example, stunted growth and dwarfing, may be associated with necrotic symptoms and in extreme cases, the necrosis may spread to the whole plant to cause plant death.

TABLE: MAJOR VIRAL DISEASES OF ECONOMIC CROPS.
 

MOST COMMON EXTERNAL SYMPTOMS: 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 symptom development on the leaves. 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, tumors or galls, bunchy top, witches broom, rosette, enation and necrosis.

MOSAIC: Mosaics characterized by intermingled patches of normal and light green, yellow or white areas of the leaves or fruits, or are whitish areas intermingled with areas of the normal color of flowers or fruits. The mosaic depends on the intensity or particular pattern of discoloration. The mosaic type symptoms may be described as mottling, streak, ring pattern, line pattern, vein clearing, vein banding, chlorotic spotting, etc. 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.

VEIN CLEARING: Veins become clear due to destruction of chlorophyll in the vein tissues.

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 discolored without any spotting patterns and become yellow, although some vein clearing may be present. Viruses causing the true yellows diseases show a tendency to produce virescent flowers, to break the dormancy of axillary buds and induce cessation of flowering, to be leafhopper transmitted, and to be relatively sensitive to heat treatment.

RING SPOTS: Ring spots, characterized by the appearance of chlorotic or necrotic (usually circular) 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.

DWARFING AND STUNTING: The plant size is reduced due to shorter internodes, smaller leaves, fruits and various other plant parts.

TUMOR (GALLS): Unusual swelling or development or transformation produced as a result of viral infection.

BUNCHY TOP: Leaves or branches become bunched at the top of plants.

WITCHES BROOM: Appearance of broom like growth, due to pathogen.

ROSETTE: Short, bunchy habit of plant growth.

ENATION: Malformation or tumor or leaf like out growths on the leaves and roots referred to as enation.

NECROSIS: Death of cells or tissues.

LESS COMMON SYMPTOMS: A large number of other less common virus symptoms are also described. These symptoms may be accompanied by other symptoms on other parts of the same plant and include: leaf roll (e.g., Potato leaf roll), leaf and stem distortion (e.g., bean common mosaic virus BCMV), rubbery wood (e.g., apple rubber wood), pitting of stem (e.g., apple stem pitting), pitting of fruit (for example, pear stony pit) and flattening and distortion of stem (for example, apple flat limb).

COTTON

Cotton leaf curl is recorded as most destructive diseases, while, sometimes stenosis (stunt or small leaf) also become important.

COTTON LEAF CURL VIRUS (CLCV)

This disease is also called leaf crinkle. A virus causing leaf curl of cotton was first recorded in Nigeria (1912), Sudan (1924), Tanzania (1926), Philippine (1959). In Pakistan, this disease was first time recorded in 1967 at Multan (Punjab) on some cotton plants. It was considered a miner disease until 1987, but in 1991 92, it becomes severe and since 1992 93 causing a huge production and monetary loss to the nation. In Sindh, this disease was first reported during 1996 at Ubauro, district Ghotki, and is reached up to New Saedabad, district Hyderabad, during 1999 2000. It is quite difficult to present accurate estimates of the losses due to cotton leaf curl disease, because the losses vary from year to year and from one area to the other. Sometimes the cotton fields have been found to show as much as 100 percent damage.

PATHOGEN: The disease causing virus belongs to Gemini group, sometimes refer as Gossypium virus 1.

DISTRIBUTION: Africa, Nigeria, Sudan, Tanzania, Philippine and Pakistan.

HOST RANGE: More than 30 different crop, weed and ornamental plants are reported as hosts.

SERIOUSNESS: Cotton, lady's finger, tomato, chili, cucurbit (especially water melon), beans, sunflower, sesame, soybean, cow peas, egg plant (brinjal), sun kukra, china rose, thorn apple (dhatura), mint (podina), holly hock (gul e khera), zinnia, AK (Calotropis), shesham (talhi) and citrus species.

TRANSMISSION: The disease transmitted by feeding of the white fly, Bemissia tabacci within 6.5 hours. A single female, carrying virus, can infect many plants. White fly is known to survive on as many as 53 host plant species, and is responsible for transmitting 23 crop diseases in region. At global level, white fly infests 600 different plant species. The virus is not transmitted by sap, seed or soil.

PERPETUATION: The disease causing virus survives in several different plant hosts, from which it may spread.

SYMPTOMS: Upward and downward curling of leaves accompanied by small and main vein thickening (SVT & MVT) on leaves, pronounced on underside. If a diseased leaf is viewed from beneath against the light, thickened vein found darker green and opaque than normal. In extreme but not in frequent cases, formation of cup shaped or leaf laminar (veins) outgrowth called "enation" appears on the back or underside of the leaf. The newly produced leaves are small, excessively crinkled and curled at the edge. Primary stem often tends to grow taller than normal. The internodes being elongated and irregularly curved but sometimes the whole plant is stunted. The flowers checked in growth and become abortive. Bolls remained small in size and failed to open. All parts of badly hit plants are very brittle and ready broken.

CONTROL: Cultivation of resistant varieties is only safe measure. Crop rotation with non host crop. Proper use of irrigation and fertilizers. Potassium fertilizer improves the disease resistant power in plants. Vector, white fly must be controlled. All alternate hosts (including weeds) must be eradicated before, during and after cotton crop. Deep plowing with short duration in fallow lands help to control weed hosts. The disease (CLCV) is not seed transmitted but use of healthy seed, acid delinting and chemical seed treatment is recommended as preventive measure. Use of proper cotton production technology is economical and most effective for management of all diseases (including this).

STENOSIS (STUNT OR SMALL LEAF)

DISTRIBUTION: Pakistan and India.

SYMPTOMS: Leaves develop in clusters, are malformed and of different shapes and sizes. Enations are produced on lower surface of veins. Flowers may remain small with balls never forming. Infected plants can easily pulled out of the ground, having a large number of adventitious roots.

CONTROL: No known control.

SUGARCANE

Mosaic, grassy shoot and chlorotic streaks are considered major viral diseases of sugarcane.

MOSAIC

PATHOGEN: Sugarcane mosaic virus (SMV).

DISTRIBUTION: Wherever sugarcane is grown.

HOST RANGE: The disease causing virus has a wide range and infects a large number of grasses.

TRANSMISSION: Aphids, Mechanical, and is seedborne in corn.

PERPETUATION: Grasses and infected sugarcane crop.

SYMPTOMS: Newly leaves are unrolled from spindle. Irregular oval or oblong, pale green blotches of various sizes occur on leaves, with various widths. Stunted shoots, twisted and distorted leaves in some cultivars. Mottling of stem, causing death of tissue and cankered areas in other cultivars may also occur.

CONTROL: Plant resistant varieties. Rogue out infected plants.

GRASSY SHOOT

PATHOGEN: Sugarcane grassy shoot virus (SGSV)

DISTRIBUTION: Pakistan, India, Taiwan and Thailand.

HOST RANGE: Sugarcane and sorghum.

TRANSMISSION: Infected sugarcane seed pieces, mechanically by cutters or cutting knives and aphids.

PERPETUATION: Infected sugarcane crop.
 

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