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