AGRICULTURE
DISEASES OF DIFFERENT CROPS

Agriculture is the backbone of our national economy

By S.M. ALAM
June 23  - 29, 2003
 

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.

WHEAT DISEASES

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: 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: 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: 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.

PARTIAL BUNT: This disease is not very common in the country. In some areas of Pakistan, this disease frequently occurs.

WHEAT SMUT: 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 25C 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: 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: Soil-borne mosaics and streak mosaic are important virus diseases of wheat. Soil-borne wheat mosaic sometimes causes extensive damage to susceptible wheat varieties.

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.

SCAB: 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.

RICE DISEASES

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 24C to 36C 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.

COTTON DISEASES

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.

DISEASE TRANSMISSION: 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 conclusions.

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 disease inoculum.

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.