Different crop plants are attacked by different
diseases when growing in the fields and the diseases are significantly
reduced the yield of the crops. Below are the some information about
crops — wheat, rice and cotton are attacked by different diseases
during their period.
Agriculture is the backbone of our national economy.
Our farmers are doing their best to increase crop productivity to meet
the demands of food, clothing, shelter forever increasing the burgeoning
population of Pakistan, but the greatest threat to crop production is
the loss to crop plants by pest and diseases. The insect pests have
become quite apparent whereas, the damages to crop plants by fungi,
bacteria, actinomycetes, viruses, nematodes and non-pathogenic factors
do not manifest so clearly to alarm the farmers. Plant diseases have
played a major role in causing losses of food production in the world
over of the various plant diseases. Seed-borne diseases play an
important role in causing heavy loss of wheat yield. According to a
rough estimate, the loss from rust and smut diseases of wheat was about
130,000 metric tons every year.
Bread wheat is the staple food crop in Pakistan and
is cultivated on some 8.3 million hectares of which, nearly 75% is
irrigated. It is a cool season crop and is subject to attack by numerous
diseases. It is cultivated under a wide range of ecological and climatic
condition at altitude from sea level to about 150 meters above sea level
and rainfall from 10 cm to 100 cm per annum. The wheat crop is planted
generally in October/November and harvested by the end of March in the
south and the end of May in the north. It provides favourable conditions
for the development of a number of diseases. The losses in the decrease
of wheat yield is tremendous due to attack of several diseases in the
crop. The stunted plants, growth, leaf, stem spots galls, shriveled or
discoloured grain. Most of wheat diseases are caused by fungus
parasites, but a few are caused by bacteria. The wheat diseases may
cause unbearable reduction in grain yield. Numerous insect pests also
attack wheat at various stages of plant development. It has been
reported that wheat crop in Pakistan suffered from four major seed-borne
diseases viz; loose smut (Ustilago tritci), complete bunt or
stinking or hill bunt (Tilletia, foetida), Karnal or partial bunt
(Neovossia indica) and flag smut (Urocystis tritici) of
the seed borne diseases of wheat loose smut of wheat caused by Ustilago
triticii (Pers.) Rostr is one of the major diseases which has been
prevalent in most of the wheat growing countries of the world. The
appearance of loose smut, complete bunt or hill bunt on wheat crop is
one present in all parts of Pakistan.
LEAF RUST: Leaf
rusts are the most serious diseases of wheat. The rust that attack wheat
are leaf rust, stem rust and stripe rust. The leaf rust disease, caused
by a fungus Puccinia recondita Rob. Ex. Desm fsp. tritici
is found all over the world in yield and also reduce grain quality to
considerable extent. Leaf rust generally occurs, when moderate climatic
temperature prevail for a long time and it causes considerable losses.
Some wheat varieties are susceptible to this disease. The leaf rust
fungus enters the wheat plant through the stomata. It appears on the
green plant as orange to orange-brown pustles. These pustles are more on
the upper leaf surface than on the lower. Several leaf rust resistance
varieties have been developed in the country.
Stem rust caused by fungus (Puccinia graminis triticii) probably
is the most serious of wheat diseases and may cause heavy yield
reduction. Stem rust occurs all over the country. It causes severe
losses under favourable condition. It is found wherever, wheat is grown.
Stem rust in wheat is characerized by elongated reddish-brown pustles
that occur on the upper stem, leaves, sheaths, floral bracts. These
contain brick-red uredospores. Both stem and grain turn brown, come dry
brittle. Stem rust is composed of many parasite strains (races). There
are about 275 known races in the world. There is a great need to develop
stem rust resistance wheat varieties.
Stripe rust of wheat (Puccinia glumarum) is called yellow rust
shows a wide-spread phenomenon on wheat crop, which may damage, when the
crop is at milk stage or earlier. Infection in early growth stages
reduces root and top growth and grain yield. It occurs mostly in
northern foot hill and to a lower extent in the central plains.
Complete bunt is also a serious
disease in the certain wheat growing area of the country. Species of
this pathogen occur in Pakistan, but T. foetida is prominent.
This disease is not very common in the country. In some areas of
Pakistan, this disease frequently occurs.
The main smuts that attack wheat crop are but or stinking smut, loose
smut and flag smut.
LOOSE SMUT: Loose
smut caused by the fungus Ustilago nuda may occur wherever wheat
is grown, but it is most abundant under humid, sub-humid conditions. The
disease usually occurs between January to April. The 25°C temperature
or slightly above and Relative Humidity (RH) of 80% generally favours
the attack of disease. The disease appears at the flowering stage of the
wheat crop. Colour is black and full of smut, which consists of fungual
spores. The spores are blown away, by wind leaving behind only naked
rachis. This disease generally occurs during Janurary to February.
Flag Smut is generally found in traces in the foot hill areas of
Pakistan. The smut diseases of wheat can be controlled by treating the
seeds with fungicides viz. Baytan, Benlate and Ridomil at 0.1%.
COMMON BUNT OR STINKING SMUT:
Bunt or stinking smut (Tilletia caries and T: foetida) is found
on wheat all over the world, but more prevalent in the drier regions of
the world. It is an economic threat to wheat, especially for winter
wheat. Bunt causes reduction in yield as well as an improper quality of
grain. Common bunt shortens the plants to as much as half the height of
normat plants. Smutted wheat heads usually are radish-green or
grayish-green in colour before they mature. Losses from bunt are
considerably decreased by growing resistant wheat varieties.
ROOT ROT: It
is a common disease in wheat crop.
POWDERY MILDEW: Powdery
mildew (Erysiphe graminis tritici) is found in many wheat growing
countries and may cause considerable damage under moist cool conditions.
Heavy attacks of powdery mildew reduce the size of wheat kernels and
yield of grain, particularly when infection occurs between the tillering
and jointing stays of the wheat plant. Powdery mildew is most
conspicuous on the leaf blades, but is also may develop on the sheaths
and on the floral bracts. Diseased parts of the plants become white or
gray which often covers the entire blade. Sulphur dusts or fungicides
are used for the control of this disease. Mildew is not a serious
problem, but develops occasionally in root wet springs in certain areas.
In addition to the above disease, ear cockle, bacterial ear rot, is
occasionally encountered in the central plains.
INSECT PESTS: Wheat
may be attacked by numerous insect pests. Severe losses occur somewhere
every year, in wheat growing areas of the world. The principal world.
The principal insects that attack the wheat plant are one the Hession
fly, wheat joint-worm, wheat-stem saw fly, wheat straw-worm, green bags
and other aphids, several species of grass hoppers, Mormon crickets,
wire worms, chinch bugs, wheat stem maggot, cut worms, stink bugs, bill
bugs and army worms. Several other insects seriously damage wheat in the
storages or in godowns.
Some of the insect pests of wheat are: Termites: (Odontolermes
obesus); Wheat stem fly: (Atherigone Naqvii); Army worm: (Mythimna
separate); Aphids: (Schilzophis spp); Wheat crop is attached
by many insects, and they caused economic damage to crop.
APHIDS (MACROSIPHUM MISCANTHI):
The aphids generally suck the sap from the wheat plant. It is bred
rapidly during cold season and remain active during February and March,
at the time of ripening of the crop. The most identified aphids are:
green aphids, full black aphids, black thrips.
WEEDS OF WHEAT:
Weeds of wheat are: Avena fatua,
Phalais minor, Anagallis avensis, Chenopodium album, Chenopodium murale,
Melilotus alba, Convolvulus arvensis, Vicia hirsuta, Carthamus
oxyacantha, Asphodelus tenuifolius, Rumex dentatus and Cyperus
rotundus. The weed can be controlled manually, mechanically,
culturally and chemically. However, the most easy and less time
consuming is chemical weed control.
mosaics and streak mosaic are important virus diseases of wheat.
Soil-borne wheat mosaic sometimes causes extensive damage to susceptible
STREAK MOSAIC: Wheat
streak mosaic is a destructive virus disease. The first symptom is
against green smottling of the leaves, followed by appearance of light-coloured
streets along the veins and stunting of the plants. Infected plants have
yellowish-green to markedly yellow mettled and striged leaves. Infected
plants produce either poorly filled heads with shriveled kerned or the
head at all.
This disease is most prevalent on
wheat in temperate humid to sub-humid areas. This disease reduces the
stand, yield, all quality of wheat, especially when the temperature and
relative humidity are high. The fungus most commonly associated with
fusarial blight head on wheat is Gibberella zeae. Partial control
measures for this disease include crop rotation, suitable soil
preparation, early seedling and seed treatment.
CROWN, FOOD AND ROOT ROTS: The
diseases include Polythium root rot, Helminthosporium crown and root
rot, snow mold, Rhizoctonia blight and foot or culm rot caused by Cercosporella
and Leptosphaeria. Underground parts of the wheat are affected.
After wheat, rice is the major summer monsoon Kharif
food crop of Pakistan, after cotton, being the second largest foreign
exchange earning, it is an immensely important commodity for the
national economy. Rice is cultivated over an area of 2.422 million
hectares giving production of 4.671 million tons of rice with an average
yield of 1930 kg/ha (1999-2000). During the growth, the crop plant
normally faces many constraints, and among them plant diseases have
played a significant role reducing the crop productivity. Most of the
important rice diseases are seed borne, which affect the germination of
seed, produce seedling blight, brown leaf spot, and blast disease. The
infected seed becomes the main source of spreading them under field
conditions. Nearly, 75 different diseases of rice caused by fungi,
bacteria, nematodes and viruses have so far been described from
different parts of the world. Of these, at least, 15 rice diseases have
been found to occur in Pakistan. The most important and widely prevalent
diseases in rice growing areas in Pakistan and their remedial measures
are given below: These diseases as well as others occur in many
countries, where rice is grown.
STEM ROT (SCLEROTIUM ORYZAE CAT.):
Stem rot is caused by the fungus (Sclerotium
oryzae). This disease is known to emerge in the rice growing areas
in both nurseries and fields. This disease causes severe damage to rice
product. The diseases appear at early as well as at late stage of plant
growth. At the beginning, a small black lesion develops on the outer
leaf sheath near the water level due to infections produced by sclerotia.
The infected parts of the plants get rotted and also show dark black at
the base of the plant. The ears become sterile at affected shoots
finally dry up. The disease appears generally in July to August.
Stagnant water is mainly responsible for the appearance of this disease.
The infected fields may be treated with fungicides, such as Benlate,
Vitavax, Derosal, etc. Bordeaux mixture spray is also suitable. Water
management, destruction of the infected rice stubble and proper use of N
fertilizer may reduce the attack of this disease. Removal of infected
plant in the best treatment.
KERNEL SMUT (TILLETIA
BARCLAYANA BREF. SACE): It seems to appear on all the
IRRI rice growing areas of the country. But, infected areas vary from
location to location. The disease appears in the form of minute black
streaks at the time of crop maturity. Grain may be affected wholly or
partially. The disease is caused by sporidia, which are produced in
abundance by the germinating spores, which remain alive in the soil and
germinate during the flowering period of the next crop season.
WHITE TIP (APHELENCHOIDES BESSEYI):
The white tip nematodes disease is
most commonly observed in the rice growing fields. Infected plants
generally develop chlorotic or white spot or the leaves. Plants become
stunted, lack vigour and produce very small panicles. Affected panicles
show high sterily distorted kernels. The soils is also a important
source of infection. Chemical treatment of soil with Temik, Furadom
Diazinon @ 1-2 kg/acre will help to read the incidence of disease.
BACTERIAL LEAF BLIGHT (XANTOMONAS
COMPESTRIS PV. ORYZAE): The disease appears both in
nurseries and field after transplantation of the rice crop. The
characteristic symptoms of the disease develop yellow to white water
soaked stipes at the margins of the infected leaves. The grain at
maturity also becomes infected and finally become grey or
yellowish-white. Resistant varieties and disease free healthy seed
should be used for disease control. Suitable chemical such as ceresan,
streptomycin, pencillin and coliocidin also may help in the control of
the disease. Avoidance of deep water in the nursery/field also helps to
reduce the incidence of the disease.
BLAST (PYRICULARIA ORYZA CAV.):
Blast is one of the most important
destructive diseases of rice crop. It is caused by fungus (Piriculara
oryzae). It generally occurs throughout the major rice growing areas
in Pakistan. However, its prevalence and intensity may region to region,
because of the climatic conditions and pattern of varieties under
cultivation. The fungus can infect leaves nodes, glumes and panicles of
rice plant at any growth stage. The spot of whitish, greyish or bluish
dots appear on the leaves. Heavily infected leaves soon dry up plants.
August and September are the suitable period for appearance of the
disease. Higher doses of NP also help in the spread of the disease. Seed
of rice should be fully treated with Benlate, Bayton, Topsin-M before
raising nursery. Before transplanting the nursery, the field should be
sprayed with suitable insecticides.
NARROW BROWN LEAF SPOT (HELMINTHOSPORIUM ORYZAE.):
This disease is caused by fungus (Cercospora
oryzae). Other diseases are leaf smut, black sheath rot, sheath
blight, white leaf, etc. The disease occurs in all the rice growing
areas of Pakistan. Its intensity occurs depending upon the weather
condition of the rice growing areas. The furgers attacks the plant
leaves at any growth stage, causing seed rot, seedling blight, brown to
dark brown spots of varying size appearing on the leaves. The
temperature for such fungus varies from 24°C to 36°C and presence of
dew on the leaf surface helps in germination of diseases. Seed treatment
with fungicides Benlate, Baytan, Bayleten, Topsin-M, etc. helps in the
removal of seed borne inoculum.
Other insects and pests, which attack the rice plants
are: Rice bug (Leptocorisa acuta), Stalk or Stem borers (Chilo
plajulellus of Chilo simpl.ex), Leaf beetles (Lema oryzae),
Stored grain insects (Rhizopertha dominica), Rice water weevil (Lissorhoptrus
simplex), Armyworn (Laphygna frujiperda), Rice armyworn (Spodoptera
mauritia), Rice leaf miner (Agromyza oryzae).
It has been reported that substantial losses have
been occurred to the attacks of diseases, insects and pests on the rice
crop. So, it is necessary to control and eradicate such diseases of rice
crop as far as possible.
There are many diseases and pests, which attack
cotton crop through the country. Among them the most series is leaf curl
virus disease of cotton, which was first observed near cotton growing
area of Multan in Pakistan during the year 1967 and has been observed
since then. In the beginning, the disease did not attract serious
attention, because it occurred casually up to minor economic importance.
However, this disease has gained economic importance for the last 4 to 5
years. lt is reported that in 1987, the incidence was up to 80% in
certain fields and in the year 1991-92 according to survey conducted by
Extension Wing of Agriculture Department, Multan Region, the cotton area
affected by this disease was about 35000 acres out of which about 7000
acres were severally affected during 1992-93, it infested about 300,000
acres causing a huge production and monetary loss to the nation. This
disease was mainly observed in many districts of southern Punjab.
The presence of innoculum of CLCV with low intensity
has been observed in July 2000 at 202 spots infecting 2078 acres in the
districts of Sahiwal, Okara, R.M. Khan, D.G. Khan and Rajanpur.
Similarly, out of 1010 spots 74 spots of whiteflag, 166 spots of iassid
and 18 spots thrips have also been observed. White fly has also been
recording in July 2000 in Multan, Vehari, Layyah, Pakpatan and
Bahawalpur. Cotton is hit by a virus scare of the leaf and design
practically every year the disease is not limited to any specific crops.
Many others are often its like Pakistan, the incidence of disease is
common in India, Bangladesh, Sri Lanka, Nepal, Bhutan.
Several infected plants may show spirally twisted
leaf petioles, fruiting branches and, to a lesser extent, the main stem,
which tends to grow tall with elongated inter-nodes. However, in some
cotton varieties, the inter-nodal distance is reduced and the affected
plants become stunted in early infection with adverse effect on
fruiting. There is reduction in boll number and boll weight resulting in
loss of cotton yield. This disease has been recorded in other countries
as well. The symptoms of this disease appeared as curling of leaf
margins, either upward or downwards and a peculiar crinkled appearance
produced by veins. Chlorotic spots may also appear.
It has reported that CLCV was
successfully transmitted by white fly (Bemisia tabaci) from
naturally infected bean (Phaseolus vulgaris) and cheeseweed (Malva
parviflora) to cotton. Plants were collected, where more than 40% of
the bean plants displayed symptoms and 100% of the plants were infected
with white fly. Besides cotton, white fly is known to survive on as many
as 53 host plants and is responsible for transmitting 23 crop diseases
in our region. Research regarding field surveys and laboratory studies
on the disease and white fly has been conducted by the scientists in a
research centre during 1991-92 and 1992-93.
The extensive field survey was conducted from Rahim
Yar Khan to Sahiwal to record the incidence of CLCV, during the months
of July and August-1992. The report indicated that population of white
fly on cotton was present at almost all locations with few exceptions.
In many cases, the attack was for above the economic threshold level.
Their field surveys further indicated that incidence of disease varied
from district to district and variety to variety. The vector white fly
was present in all the field surveys irrespective of its population
density (0-34 nymphs/leaf). The data showed that S-12, CIM-70 and MNH-93
were the most susceptible varieties at all places, whereas other
varieties like NIAB-78, CIM-240 and CIM-109 at some places had minimum
and at other places maximum infestation, leading to uncertain
Research workers have been conducting laboratory
studies successfully, in transmitting this disease by using white fly as
vector from cotton to cotton and cotton to cowpea, French beans, okra,
tobacco and soybean. They provided an evidence that white fly sucks the
cotton leaf curl virus from a diseased plant and on migration to a
healthy plant transmits the virus to it. Once injected, the virus spread
in the entire plant. The experimental data do not support the
apprehension that disease can spread through cotton seed. The eggs of
white fly generally do not contain virus. However, larvae and pupa can
acquire the virus, while feeding on a diseased plant.
They further confirmed white fly as a vector by
conducting field cage studies. The crop exposed to white fly was
compared with crop in the cages. They reported that the caged crop was
healthy and free from CLCV disease. In these plants, the flowers were
healthy and boll tormation was of optimum size. Whereas, the crop which
was infested by white fly had disease symptoms stunted growth, poor
flowering and small sized boll formation. It is suggest that the
following measures may be taken to control the disease:
Remedial measures to control of vector (white fly)
through specific insecticides.
MANAGEMENT OF VECTOR ON ALTERNATE HOSTS:
Cowpeas, okra and cotton are three
such crops, on which disease transmission takes place in the shortest
possible time. Therefore before sowing of cotton, okra, cowpea and
cotton sprouts should be taken care off for white fly population or
EVOLUTION OF RESISTANT/TOLERANT VARIETIES:
Resistant/tolerant varieties appear
to be the only dependable measure for control of leaf curl virus.
Therefore, sufficient availability of seed of relatively tolerant
varieties should be made available to the growers in the main virus
affected areas. The sowing of highly susceptible varieties such as: S-12
and CIM-70 may be withdrawn.