In man's continued effort to enhance food production, new high yielding crop varieties and new techniques for crop and field management are being constantly evolved resulting in a parallel increase in disease occurrence which calls for their efficient management. Terms like IPM (Integrated Pest Management and IDM (Integrated Disease Management) have different meanings. Of the various management strategies available for disease management, the chemical strategies have so far dominated our thinking. Over emphasis on chemical control of plant diseases has caused serious imbalances in the agro-eco system.
Strangely, about 70% of the amount of chemicals sprayed on field crops does not stick to the plants. The enormous quantities of chemicals, poisonous to microbial life that fall on earth get mixed up well with soil.Some problems like non-target effects of chemicals as well as iatrogenic diseases are being experienced. Coupled with these, the increasing awareness of impending pollution; health hazards and development of resistance to control chemicals in microbial pathogens have intensi-fied the search for alternative strategies.
The shift to exploration of non-chemical strategies is likely to correct the imbalance in our approach and subsequently improve the understanding of IDM (Integrated Disease Management). In this article, we discuss some components of IDM which include not only chemical control but an array of other practices like cultural measures, quarantine measures, biological control, field sanitation and genetic engineering. All these could serve to blend strategies for efficient and economic management of diseases in crops.
Cultural practices are important non-chemical methods to manage the plant diseases. By adopting cultural practices the environment of the pathogen can be modified. Agricultural practices like crop rotation, field fallowing, application of organic amendments are known to reduce the incidence of root rot, wilt and root knot nematode etc. Disease control by cultural practices is mainly preventive and these practices reduce the inoculum density of soil-borne pathogens and are the only economical control. Complete success is made when the nature of pathogens at different environmental conditions is known. Different approaches of cultural control are discussed here.
Crop rotation is useful against those pathogens which do not persist for a long time in the soil in the absence of suitable host. In this method, host plants and non-host plants are rotated in a way that reduces the inoculum size in the soil. Successful - examples are Soybean-potato rotation against root rot of bean and wheat-oat rotation against take-all disease of wheat.
Disease free seeds: Seeds are known to carry pathogens both internally and externally.
The seed crop should be raised in an area where the inoculum build up is very slow as in case of seed potatoes which are grown when the temperature and moisture inhibit the perpetuation of insect vector for Potato Virus indexing is performed at the time of digging potatoes. Healthy plants are marked and sown in small Insect healthy plants are allowed to grow to obtain to seeds.
Also the seed treatment with various other control for many seed borne diseases. In plants can be removed so that the generation could be disease free. This to control viral diseases, loose smut ofwheat and whip smut of sugarcane. Selection of vegetative prop gules should be from a disease free field especially in case of banana suckers and sugarcane setts.
Field sanitation is another important aspect. Plant debris are important sources of harboring plant pathogen in its active or dormant stage. The field should therefore be free from the debris after harvest. Organic amendments increase the activity of saprophytic micro-organisms which may inhibit the pathogens. Cotton root rot, Black scurf of potato, Root knot of vegetables and Phytophthora rot of pepper are controlled effectively. Club root of cabbage which by acidic soils can be controlled by liming. Similarly, potato scab which is favoured by alkaline soils can be controlled by addition of sulfur. Also, addition of chitin and related substances to the soil @ 1 .2t/ha controls the root rot of bean caused by FusariumSolaniF.Sp. Pahseoli. In case of mechanically transmitted diseases like TMV in tobacco, pruning and cutting equipments should be cleaned before use.
Weeds serve as the secondary source of inoculum for the pathogens in addition to the primary host. So the field should be free from the weeds. Alternate hosts are known to play an important role in completing life cycles of many rust fungi. Eradication of alternate host (Berber's) for black rust of wheat resulted in complete elimination of the disease in USA. While in India volunteer wheat plants from Nilgiri hills play an important role for black rust epidemic.
Ratooning is a regular practice in sugarcane which results in development of grassy shoot disease. Flooding the field with water helps in control of Fusarium wilt of banana as under anaerobic condition the pathogen survives for a shorter period. Similarly, fallowing the field for one season may restrict the development of disease as in the case of chickpea wilt. Alterations in the date of sowing can help to prevent coincidence of susceptible stage(s) of crop with the conductive climate of the pathogen. To check ergot disease of pearl millet, sowing date is adjusted so that the flowering date does not coincide with the cool rainy season. Likewise, early-sown wheat crop escapes from rust disease and late-sown pea escapes from root rot and blight.
Seed treatment is another important measure and this may be of two types. a) physical and b) Chemical.
Physical treatment (Heat therapy) is quite effective in eradication of internal seed borne inoculum. Heat therapy can be conveniently grouped into two broad categories.
Hot water treatment : The method demonstrated by Jensen against loose smut of wheat caused by Ustilagosegetumtritici.
INTEGRATED DISEASE MANAGEMENT IN CROPS
Soaking wheat seeds in water at 20-30°C for 4-6 hours, then dipping in water at 49° for 2 minutes followed by drying before planting kills the dormant mycelium resulting in complete eradication. This method is effective against many seed borne bacterial diseases such as brown spot of rice (Xanthomonasoryzae) and cotton blight (Xanthomonasaxanopodispv. Mal-vaceanim). An increase in temperature or soaking duration possibly reduces the germination. Solar heat therapy : is another method, advised as an alternate to hot water treatment for loose smut of wheat by Luthra and Sattar. Infected wheat seeds are soaked in ordinary water for 4 hrs. (8 AM to 12 PM) and then excess water drained out. Seeds are then spread on tarpaulin and exposed to sunlight from 12 PM to 4 PM. The limitation is that this method is effective only in hot sunny days and this method is not commonly adopted by farmers owing to the fact that incomplete drying of water soaked seeds results in poor germination, which is undesirable.
Hot air treatment:is effectively utilized to manage the seed borne viral diseases. Tomato mosaic virus of tomato can be controlled by heating the dry seeds for 3 days at 70°C) Red rot of sugarcane and grassy stunt are also controlled by treating the setts at 40° - 45oC)by moist hot air for a period of 30 minutes.
The use of chemicals for seed dressing is an age old practice. Organomercurial were effectively used for controlling many seed borne diseases such as wheat bunt, Helminthosperium leaf spot of maize, Rice brown spot, loose smut of oats etc. Owing to their hazardous nature, they are now banned for use in agriculture. Alternatively, fungicides Chloranil, thiram, captan, maneb 'etc. are quite popular among farmers which control numerous diseases such as damping-off of beans, chillies, pea, seedling rot of groundnut. seed rots and blights of maize. Systemic fungicides such as carboxin (smuts), oxycarboxin (rusts), metalaxyl (downy mildew of pearl millet) have added further Possibilities of effective control of pathogens located deep inside the seeds. Antibiotics in mixtures are very effective isolation and also reduce the possibilities of development of antibiotic resistant mutants. The table below presents and idea of the Variety of chemicals employed in disease control.
Group Common name Commercial name Diseases
1. Sulphur :
Elemental Sulphur (15-20Kg/ha) Powdery Mildews
WettableSulphur (2-4Kg/ha) Powdery Mijdew disease
Ziram Ziride, Cuman L Leaf spot, Ergot of pearl
millet and sorghu
Thiram Thiride (0.2%) Seed Treatment
Ferbam Fermete Apple scab, banana leaf spot
Zineb DithaneZ78 cotton necrotic leaf spot
(0.2%) Groundnut leaf spot,
Maneb IndofilM-45 (0.2%) Downy mildew, Rust, spot and blight
Nabam Dithane A40 (0.2%) leaf blight and leaf spot diseases 2. Copper fungicides :
Copper sulphate Downy mildew,
apple scab, leaf spot, leaf blight
Bordeaux mixture (1 %)
Bordeaux paste (10%)
Copper oxychloride Fytlam (0.25%) PM, white rust of crucifiers, wilt pathogens
3. Mercuric fungicides :
4. Heterocyclic Nitrogen
Captan Orthocide (2g/kg) Seed treatment
Folpet Phaltan (0.1-0.15%) Rose black spot
Captofol Difolatan (0.2%) Mango anthracnose,
late blight of potato
5. Systemic fungicides :
a) Benzimidazole groups
Benomyl Benlate (0.1%) PowderyMildew
Carbendazim Bavistin (0.1%) Soil borne pathogens, sheath rot and blight
Carboxin Vitavax (2g/kg) Smut
Oxycarboxin Plantvax(0.1%-0.2%) Rusts
Metalaxyl Ridomil,Apron(4glKg) Downy Mildew
Triademefon Bayleton (0.1%) Powdery mildew.
Triademenol Bayton (0.1%) Powdery mildew, root rot
Bittertenol Baycor (0.1%) Apple scab and leaf spots
Trycyclazole Beam (0.1%) Rice blast
Trycyclazole Calixin (0.1%) Powdery mildews leaf spot
f) OP compound
IBP Kitazin(0.1%) Blast
Hinoson Edifenphos(0.1%) Blast
Dinocap Karathane(0.1-0.2%) Powdery mildew
Group Common name commercial name Diseases
Aureofungin Aureofungin sol Tanjore wilt of coconut Leaf spot
Cycloheximide Actidione 0.001% and stem rust of wheat and barly
Griseofulvin Actispray powdery mildew Powdery mildew of
Streptomycin sulphate Phytomycin Citrus canker, black armof cotton
Streptocycline Agrimycin 100 Apple fire blight BLB of rice,
Black arm of cotton.
Blasticidine ____ Rice blast
These chemicals definitely, yield effecitve results upon integration with other practices.
Quarantine plays an important role in excluding the pathogen from a place where it is not already established. Numerous diseases which were introduced in India from other countries are listed in the following table.
Year Pathogen Country
1879 Coffee rust (Hemileiavastatrix) Ceylon
1883 Potato late blight (phytophthorainfestans) Europe
1910 Grape downy mildew (Plasmoparaviticola) Europe
1912 Maize downy mildew (Scleropthoraphilippinensis) Java
1938 Rubber powdery mildew (Oidiumheveae) Malaya
1940 Banana bunchy top (Fusariumoysporum tsp. cubense Ceylon
1953 Potato wart (Synchytriumendobioticum) Netherland
In India DIP (Destructive Insect and Pest) Act was enacted in 1914 to prevent the entry of pathogens from other countries or from other states. Quarantine stations are established in major airports, seaports and land frontiers. All the plants and plant products should be accompanied with the phytosanitary certificate issued by competent authorities to pass through the quarantine. All these products will be checked and fumigated, if necessary both at entry and exit. Quarantine maybe of embargo or domestic. Embargo bans the import of some plant materials from a particular country where the disease is more prevalent. e.g., Sugarcane cuttings from Australia, Cocoa from Africa and Srilanka, Rubber from South America and West Indies, Sunflower seeds from Peru and Argentina are totally banned. Domestic quarantine prevents the entry of pathogen from one state to another state.Movement of potato seed from west bangal and Nilgiri of Tamil Nadu Caradamon from Anamalai hills of Kerla and Cardamom from Anamalai hills of Kerala and banana suckers from Gujarat and Maharashtra are restricted to control the spread of disease toother states.
Biological control is the reduction of the amount of inoculum or disease producing activity of a pathogen accomplished by or through one or more organisms other than man. Antagonists are biological agents with the potential to interfere with the life process of plant pathogens. Antagonists are the equivalent of "natural enemies" used in Entomology. Antagonism involves antibiosis, competition, parasitism and predation.
Mycoparasites used in biological control are fungal parasites of fungi. A necrotrophicmycoparasite kills its host, sometimes without infecting it and then uses the nutrients released from the deal hyphae. A microorganism may produce a siderophore that is antibiotic in the sense that the compound produced inhibits the growth of another microorganism nearby. Lysis may result from antibiosis, from competition, from necrotrophic parasitism or from enzymatic digestion of the host cell walls by a predator. In Hype parasitism one organism parasitizes another microorganism resulting in Iysis and death of that organism. e.g.Cinccinobolus on powdery mildew, Eudarlucaaustralis on rust and Verticilliumhemileia on Hemileiavastatrix. A few mechanisms and their interactions have been listed below the table.
Mechanism Pathogen Antaginist
Antibiotic Competition/antibiosis Agrobacterium tumifaciensAvirulentAgrobacteri um spp.
Phythium, Gaurnannomyces Peniophoragigantea
Graminis var. tnticiRhizoctoniaPseudomonas spp.,
Phytophthora Trichoderma spp.
Erwiniaamylovora Erwinia, herbicola
PenecilliumAspergillus Pseudomonas fluorescens
Competition for Attachment Sites
Agrobacterium tumifaciensAvirulent Agrobacterium spp.
Hypevirulence Endothiaparasitica Mycovirus
Parasitism Pseudomonas syringe
pv. glycinea Bdellovibriobcateriovorus
Coniothirium min itans
Pythium spp. Pythiumoligandrum
Fusariumoxysporurn non pathogenic
f.sp. batatasF, oxysporum
Biocontrol agent Trade name Target pathogen
Agrobacterium radiobacter No gall Agrobacterium tumifaciens
Pseudomonas fluorescens Dagger G Pythiumultimum
Gliocladiumvirens Gliogard Rhizoctoniasolani
Trichodermaharzianum Trichodex Uncinulanecator
T. harzianurrvT.polysporum BINAB-T Heteribasidionannosum
Bacillus subtilis Quantum400 Fusarium
While in India too, the commercial bio formulations are available for soil application (Trichodermijn, Antagoncombi), Seed treatment (Antagon TV), foliar spray (Trichodex) and root dip (galltrol). The formulations are commercially produced by several firms like Green Tech Agro Products, Coimbatore, Tamil Nadu which account for coverage of .20000 ha approximately.
Research on formulation is carried out at IARI, BHU, PAU, DU, TNAU, Annamalai University and many other institutes. Major obstacles from the implementation of bicontrolprogrammes for plant diseases into IDM are the lack of understanding of the ecology of the pathosystems, mass production and delivery of control agents, selection and enhancement of control agents coupled with the specificity of control agents to only one disease on one crop in a small geographic area. More research is needed in this area. Integrated Disease Management System in Crop.
In some soil borne diseases, use of resistant varieties is the only solution.Resistant variety itself frequently falls to new races and therefore breeding for resistance is a continuous programme. In several of the presently grown resistant varieties the resistance has been brought from wild spp. Resistant varieties have been a great success with rust and smut dieases of cereals, wilt of cotton etc.
A few resistant varieties
Rice blast CO4, CO 25, TKMI
Rice BLB TKM6, IR42
Rice RTV IR26
Wheat Black rust LermaRojo, SafedLerma, Sonalika
Wheat Karnal bunt Kalyansona, PV18
Pearl millet downy mildew HB-1
Red gram wilt NP 15, NP38
Cotton wilt Pratap, Verum, Jayadhar
Late blight of potato Kufrikishan, Kufrisindhuri
A desired foreign gene is introduced into a plant for a particular trait and this yields a transgenic plant. Biotechnology plays a key role here in the management of crop diseases. Many transgenics have been developed, mostly against viral pathogens. Some of the examples of transgenic are listed here.
Crop Pathogen Gene Involved
Tobacco Alfalfa mosaic virus Coat protein expression
Tobacco rattle virus Coat protein expression
Tobacco Mosaic virus Coat Protein expression
Cucumber mosaic virus Coat Protein expression
Tomato aspermy virus Satellite RNA expression
Pseudomonas syringaepvtabaci Tab toxin resistant gene
Tomato Toamto spotted wilt virus Nucleio capsid gene
Rice Helminthosporiumoryzae HS Toxin gene
Potato Leaf roll virus Expression antisense RNAs
Bean Rhizoctniasolani Chitinase gene
If a plant is systemically inoculated with a mild strain of virus, then the same plant develops resistance against a highly virulent strain of the same virus. This is successfully exploited in tomato against TMV, potato against Potato spindle tuber viroid and in citrus against citrus tristeza virus.
All the above components of IDM when integrated and used in harmony control diseases in crops very effectively and economically. In the recent days, with more vigorous research in ecology, economics and Pests & Disease control, is now all the more important to understand nature's methods of regulating population. IDM cannot be devised in the absence of assured knowledge of epidemiology. Thus IDM approaches a multidisciplinary solution to disease control and employs sophisticated methods of experimentation, analysis and synthesis implying that the success of IDM should not be assessed on a short-term basis.
Alok Kumar Singh, Reshu Chaudhary and R.S. Sengar
Department of Agricultural Biotechnology
Sardar Vallabhbhai Patel University of Agriculture & Technology, Meerut- 250110