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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.

Viral diseases of economic cropsThe 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.


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.







Yellow dwarf, Stunt

Maize & Sorghum

Streaks, Stripe


Mosaic, Grassy shoot, Chlorotic streaks


Leaf curl, Stenosis






Leaf curl








Mosaic, Leaf roll


Leaf curl, Mosaic


Leaf curl




Mosaic, Yellows

Okra or Bhindi

Yellow vein


Bunchy top


Leaf curl, Mosaic




Mosaic, Leaf curl

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


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).


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.


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


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.


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.

SYMPTOMS: Typically, mass of stunted, crowded shoots arise from diseased seed pieces or ratoon stools. Some shoots maybe devoid of chlorophyll and stool may die. Stalks may have long, delicate, scaly buds that develop chlorotic stools. White to yellow, well defined stripes occur on thin and chlorotic leaves. Numerous tillers may also occur at base of the stool.

CONTROL: Plant disease free cuttings.


This disease is most prevalent in low lying, poorly drained areas and potassium deficient soils.

PATHOGEN: Sugarcane chlorotic streak (SCSV).

DISTRIBUTION: Pakistan, U.S.A., Australia and Java.

HOST RANGE: Several grasses.

TRANSMISSION: Stalk cuttings, leaf hopper and running water.

SYMPTOMS: One to many yellowish to whitish streaks with wavy, irregular margins occur on both sides of leaves, leaf midribs and sheaths. Streaks are short at first but later on may extend the entire length of leaf blade. Erect leaves and wilt in young plants. Young shoots may stunt or die.

CONTROL: Plant resistant varieties. Hot water treatment at 52OC. Use disease free material. Rouging of infected crop plants and weeds. Improve soil drainage. Proper application of potassium fertilizers as per requirement.


YELLOW DWARF is considered most important viral disease of wheat and barley, but in Pakistan, is not recorded as destructive.

PATHOGEN: Barley yellow dwarf virus (BYDV).

DISTRIBUTION: Africa, Asia, Australia, Europe, New Zealand, North and South America.

HOST RANGE: Wide variety of gramineous plants.

SERIOUSNESS: Barley, oats, wheat, rye, lawn weeds and gasses.

TRANSMISSION: Dodder and 11 species of aphids and 6 species of hoppers.

PERPETUATION: Through cereal and grass hosts as well as aphids.

SYMPTOMS: Yellowing and shortening of leaves, stunting of plants, reduced tillering but severely infected barley plants may show excessive tillering. Sometimes the head fails to emerge. Inflorescences are smaller and number and weight of kernels is reduced. Many of the flowers are also sterile. The root system is drastically reduced in weight but shows no symptoms.

CONTROL: Cultivate resistant varieties. In some cases, early or late sowing and high seed rate may also be helpful. Control of vectors.


STREAK DISEASE has been reported as viral disease but has not importance with reference to damaging the crop.

DISTRIBUTION: Asia and Africa.

HOST RANGE: Maize and sorghum.

TRANSMISSION: Through leafhoppers (not seed or sap).

SYMPTOMS: Initially, circular, colorless spots occur on lowest exposed portions of young leaves. Spots are scattered at first but later on become closer. Narrow, broken, chlorotic stripes occur along veins. The stripes may coalesce to form wider stripes.

CONTROL: Sow resistant varieties.


MOSAIC is considered as major threat to almost all BEAN CROPS.

PATHOGEN: Bean yellow mosaic (BYM) and Common bean mosaic (CBM).

DISTRIBUTION: Generally wherever field beansare grown.

TRANSMISSION: Yellow mosaic is transmitted through aphids only but common mosaic is seedborne, may also transmit through aphids, pollens and by mechanical mean.

PERPETUATION: Wild sweet clover helps the virus to survive.

SYMPTOMS: Differences between symptoms may vary greatly between plants. However, in both diseases, the general symptoms include dwarfing, excessive branching or bunches, leaf cupping and typical symptoms of mosaic.

CONTROL: Elimination of wild sweet clover is recommended for yellow mosaic. Disease free seed is suggested for common mosaic. Resistant varieties or rouging of infected plants are best way against both diseases.


ROSETTE is alone important viral disease of groundnut.

PATHOGEN: Groundnut rosette virus (GRV).

DISTRIBUTION: Africa, Java and Pakistan.

TRANSMISSION: Mechanically.

SYMPTOMS: An overall stunting of plants with typical rosette or clumped appearance is common symptom. Affected plants have flattened growth at the top portions associated with leafy growth and malformed buds. Young leaflets become faint in colour followed by chlorosis. Chlorotic and mottled leaf, and blossom and pod formation is reduced. Early infection causes small, sessile flowers that do not open.

CONTROL: Sow resistant varieties. Diseased plants should be uprooted and destroyed.


LEAF CURL is recorded as problematic disease.

TRANSMISSION: White fly, Bemissia spp. is the main vector in nature, and graft transmission is reported successful for producing the disease.

SYMPTOMS: Diseased leaves are markedly reduced in size, become slightly thick and brittle and dark green in colour. Affected plants are stunted and bear scanty capsules having poor seed setting. Early infection may result in severe reduction in yield.

CONTROL: Diseased plants and weeds must be collected and destroyed. The vector white fly may be controlled.


MOSAIC has equal importance as very important viral disease of tomato and tobacco crops.

PATHOGEN: Tobacco mosaic virus (TMV)


HOST RANGE:The virus may affect more than 150 genera of primarily herbaceous, dicotyledonous plants including many vegetates, flowers and weeds.

SERIOUSNESS: Tobacco and tomato show symptoms, but symptomless on grape and apple.

TRANSMISSION: Contaminated hands, tools or equipments. Nursery plants, Sap, Grafting and Dodder, Seed transmission in apple, pear and grape. Occasionally by contaminated jaws and feet of insects.

PERPETUATION: Through plant debris, seed, cigarettes and snuff. The viruses retain infectious in dried mosaic infected leaves when heated at 120OC for 30 minutes, at dilution of 1:1000000. Remain infectious for more than 50 years in TMV infected leaves kept dry in laboratory.

SYMPTOMS: Consist various degrees of: Chlorosis, curling, mottling, dwarfing, distortion and blistering of leaves, Dwarfing of entire plant, Dwarfing, distortion and discoloration of flowers. In some plants, development of necrotic areas on the leaf. TOMATO: Mottling of older leaves, mottling with or without malformation of leaflets. Long, pointed and sometimes shoestring like leaflets. Reduced fruit set.

Sanitation, Resistant varieties, 2 years crop rotation, removal of infected plants, weeds and alternate hosts. Avoid chewing and smoking tobacco during handling crop or washing hands with soap. Spray milk on plants and dipping the hands in milk during handling the crop.


LEAF CURL is considered as secondarily important viral disease of the tomato crop that may also cause considerable loss in quality and quantity of produce.

PATHOGEN: Tobacco virus 16 or Nicotiana virus 10

TRANSMISSION: White fly, Bemissia tabacci.

SYMPTOMS: Dwarfing, twisting and curling of leaves, mottle vein clearing, excessive branching, stunting of plants and partial or complete sterility.

Planting resistant varieties.


LEAF CURL is considered most important viral disease of chili after wilt caused by fungi.

PATHOGEN: Nicotiana virus 10

TRANSMISSION: Through graft or white fly, Bemissia tabacci.

SYMPTOMS: Curling of leaves, accompanied by thickening and swelling of veins. Clusters of leaves with reduced size. The whole plant assumes bushy appearance with stunted growth. Fewer flowers and fruit, but if are formed, are much reduced in size and are curled.

CONTROL: Planting resistant varieties.


YELLOW VEIN MOSAIC is considered very important disease.

PATHOGEN: Hibiscus or Bhindi yellow vein mosaic virus.

TRANSMISSION: White fly, Bemissia tabacci.

SYMPTOMS: Vein clearing, vein chlorosis, yellow veins enclosing green patches of the leaf. Veins are thickened on lower surface of the leaf. Fruits are develop malformed and reduced in size, mostly are yellow, small, tough and fibrous.

CONTROL: Planting resistant varieties. Eradication or rouging of infected plants and weeds.


MOSAIC is recorded as major problem in almost all members of cucurbit.

PATHOGEN: Cucumber mosaic virus (CMV).


HOST RANGE:Wide range of hosts than any other virus.

SERIOUSNESS: Cucurbit, peppers, spinach, tomatoes, beets, beans, banana, crucifers, lilies, zinnia and many weeds.

TRANSMISSION: Sap and several insects. Mostly 6 spp. of aphids, including Myzus persicae and Aphis gossypii. Not through seed in cucumber but is in some hosts.

PERPETUATION: Through weeds, flowers and crop plants.

SYMPTOMS: Cotyledons turn yellow and wilt. Young leaves become mottled, distorted, wrinkled and their edges begin to curl downward forming rosette like clump. The plants appear dwarfed, leaves become half to their normal size. Few runners, flowers and fruits. Older leaves develop chlorotic and necrotic areas that cover entire leaf and killed leaves hang down or fall off. Fruit shows pale green or white areas intermingled with dark green, raised areas. Often form rough, wartlike projections, cause distortion, are somewhat misshapen but have smooth gray white colour with some irregular green areas. Often have bitter taste and upon picking become soft and soggy.

CONTROL: Resistant varieties. Use of disease free seedlings. Elimination of alternate hosts (including weeds). Control of insect vectors.


TRISTEZA DISEASE OF CITRUS is an important viral disease of about all citrus plants or trees.

PATHOGEN: Citrus Tristeza virus (CTV).

DISTRIBUTION: South America, South Africa, Asia and Australia.

HOST RANGE: All kinds of citrus plants or trees.

SERIOUSNESS: Orange, lemon, grapefruit, limes etc.

TRANSMISSION: Rootstocks, buds, grafts, dodder and aphid.

PERPETUATION: Infected hosts, sometimes 100% trees of an orchard carry the virus.

SYMPTOMS: Symptoms vary on the citrus species. More or less sudden wilting and drying of the leaves followed by death of tree or dieback of twigs and partial recovery may occur. Leaves become small, more or less yellow, stand upright. Death and decay of feeder rootlets and extends to primary root.

CONTROL: Use of disease free rootstocks, buds and grafts. Removal of infected trees.


BANANA BUNCHY TOP is considering a very severe disease, causing a huge production and monetary loss to banana growers in all over Sindh. The disease was first recorded from Fiji in 1879, in India during 1940. Now this disease is prevalent also in Pakistan. The symptoms of banana bunchy top disease were first time recorded in December 1988 at coastal areas of district Thatta, Sindh, but the disease was confirmed during July 1991.

PATHOGEN: The bunchy top of banana is caused by banana bunchy top virus (BBTV) Magee.

DISTRIBUTION: Banana bunchy top disease is widely distributed among banana growing countries viz. Australia, Fiji, Egypt, Sirilanka, Bunion, Island, Pakistan and India.

HOST RANGE: No alternate host has been reported.

TRANSMISSION: The disease causing virus is being transmitted by an insect victor, black aphid, Pantalonia nigronervosa Coq. The aphids generally live round the base of pseudostem. The virus is not transmissible by mechanical mean, but is systematic, and found on all parts of the infected plants. Suckers are considered a major source for spread of the disease.

PERPETUATION: The disease perpetuates through infected plants and virus migrates through infected plant parts, suckers and vector.

SYMPTOMS: The symptoms may appear at any stage of banana plant growth. The leaves become bunched together at the apex of plant and a rosette is formed. All the symptoms produced could be categorized in following distinct groups.

Stunted and erect leaves: Emerging leaves are progressively shorter, narrower and more erect than normal.

Stunted plants: Infected plants do not usually grow taller than two to three feet, hence are markedly stunted.

Dot dash lines on leaves: The irregular, nodular, dark green dot dash lines or streaks appear running down the leaf stalk between the leaf blade and pseudostem.

Colour, thickness and size of leaves: Diseased leaves become pale yellow, brittle and much reduced in size.

Small, deformed bunch and bell: Only on plants, that are infected late, bunches and flower bells are stunted and deformed. Often the bunch stem will curve outward.

Dot dash lines on bell: Dark green dot dash lines occur on the bottom of the flower bell. Sometimes the flower bracts bend upward.

CONTROL: No resistant varieties have been evolved, hence following preventive measures must be adopted. Regular visit of banana may help to sort out diseased suckers and plants for eradication. Quarantine regulation must be implemented, so that the diseased suckers and other plant parts should not be transported to disease free areas. Certified, disease free suckers should be planted. Extra new suckers must be removed. Row to row and plant to plant space may be maintained. Insect vectors that is aphid be controlled. Weeding may be confirmed during interculturing with short interval. Proper use of irrigation and fertilizer could also be helpful. Modern production technology is safe way to save the crop.


LEAF CURL is important viral disease of papaya.

DISTRIBUTION: Where papaya is grown.

TRANSMISSION: White fly, Bemissia tabacci.

SYMPTOMS: Almost all the leaves of the plant are reduced in size and show malformation and sever curling, crinkling and distortion. The margins of the leaves are curved or rolled downward and or inward. Vein clearing and thickening also take place. The leaves become brittle and growth of the plant is arrested. Sometimes, plants become partially or completely sterile depending on growth stage and severity of the disease. If the fruits are formed, these are disfigured and mummified.

CONTROL: Planting resistant varieties. Eradication of diseased plants. 

Mithal Jiskani;

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