Viral diseases of
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.
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
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.
Yellow dwarf, Stunt
Maize & Sorghum
Mosaic, Grassy shoot, Chlorotic streaks
Leaf curl, Stenosis
Mosaic, Leaf roll
Leaf curl, Mosaic
Okra or Bhindi
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
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
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
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
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
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
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
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.
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
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
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
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
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
WHEAT AND BARLEY
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
TRANSMISSION: Dodder and 11 species of aphids and 6
species of hoppers.
PERPETUATION: Through cereal and grass hosts as well
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.
MAIZE AND SORGHUM
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
PATHOGEN: Bean yellow mosaic (BYM) and Common bean
DISTRIBUTION: Generally wherever field beansare
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
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
PATHOGEN: Groundnut rosette virus (GRV).
DISTRIBUTION: Africa, Java and Pakistan.
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.
TOMATO AND TOBACCO
MOSAIC has equal importance as very important viral
disease of tomato and tobacco crops.
PATHOGEN: Tobacco mosaic virus (TMV)
DISTRIBUTION: World wide.
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.
CONTROL: 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.
CONTROL: 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
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.
LADY'S FINGER (BHINDI OR OKRA)
YELLOW VEIN MOSAIC is considered very important
PATHOGEN: Hibiscus or Bhindi yellow vein mosaic
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
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).
DISTRIBUTION: World wide.
HOST RANGE:Wide range of hosts than any other virus.
SERIOUSNESS: Cucurbit, peppers, spinach, tomatoes,
beets, beans, banana, crucifers, lilies, zinnia and many
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
HOST RANGE: All kinds of citrus plants or trees.
SERIOUSNESS: Orange, lemon, grapefruit, limes etc.
TRANSMISSION: Rootstocks, buds, grafts, dodder and
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
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
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