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Methods of plant disease control
- 1. Lakhimpur Telahi Kamalabaria College Department of Botany HomeAssignment Methods of Plant Disease Control Submitted to_ Mr. Bharat Hazarika Assistant Professor Botany Department, L.T.K. College,Azad
- 2. Submitted by_ Introduction: Plant disease may be defined simply as a malfunctioningprocessin the plantas a whole or any partof it caused by any of the several factors. The study of plant disease is called Phytopathology or Plant pathology. Accordingto G. N. Agrios (1978), plant pathology is the study of- a. The livingentites and the environmentalconditionsthat cause diseases in plants. b. The mechanismsby which these factors producediseasein plants. c. The interaction between the disease-causing agents and the diseased plants. d. The methodsof preventingdisease, alleviating the damage it causes, or controlling a disease either before or after it developsin a plant. Plant diseases are caused by physical, chemical or biological factors of the environment. Diseasecaused by biotic agents such as viruses, bacteria, fungi, nematodesetc. Disease caused by physical or chemical factors such as air pollution, water, frost, nutrition, etc. many plantdiseases result from multiple factors, some of which are biotic (internal) and other abiotic (external); the role of each factor is determined by the environment. The purposeof plantpathology is developingcontrolfor all plantdisease. The object is also to save the yield which is destroyed by plantdisease and to make it available to the cultivators and to the hungry and ill-clothed peopleof the world. Principles of Plant Disease Control: Sri Biswajit Das L.T.K. College, Botany Dept. B.Sc 2nd Semester Roll No.- 49
- 3. Study of plant pathology to acquire information on system, caused and mechanisms of developmentof plantdisease in no doubt interesting and scientifically justified; the solution of control problem is the ultimate purpose of the plantpathology. Dependingupon thetypesof pathogen, the host and either interaction, control methods vary from onedisease to another. Therefore control measuresfor the disease of a particular plant or plant population areto be planned according to the natureof the pathogen, its modeof perennation and method of spread. The variouscontrolmeasuresmay be divided as-(a) Regulatory, (b) Cultural, (c) Biological, (d) Physical and (e) Chemical dependingon the natureof the agents employed to controlthe disease. A. Regulatory methods: These methodsare employed in order to prevent the importand spread of pathogensinto the country or individual states. Regulatory controlmeasure is implemented by meansof quarantinesand inspectionsof plants in the field or warehouse and often by eradication of somehost plants. Quarantines and inspections-There are lost evidencesto indicate that many of the plant pathogens have been introduced unknowingly from onecountry to the other where such pathogens did not exist before. Plant quarantinesare steps taken by a country or state through legislation to preventthe entry of foreign plant pathogens presentin diseased plantingmaterials into the country. In order to keep out foreign plant pathogens and to protect the countries farms, gardens, forests, etc., the quarantinemeasuresprohibit or restrict entry into country or state from foreign countries. Several voluntary inspection system of plantmaterials for plantingare also introduced in different countriesor states, for the purposeof controllingthe entry or disease producingpathogens. B. Cultural methods: Under this, the ‘control is achieved through the activities of man and through the genetic or culturalmanipulation of plants, but without the use of any biological, physical or chemical agents.’ Some of the objects of these methods are to eliminate the
- 4. pathogen from the plantor form the area in which the plantsgrow, while other objects are to increase the resistance of the host to the pathogen or to create conditionsunfavorableto the pathogen and also to obtain pathogen-freepropagativematerials from infected plants. Host eradication-When a pathogen has been introduced in a new area inspite of quarantineor quarantineshave notbeen established in time to preventsuch introduction or to preventthe introduction of new pathogens from diseased host plants, all the host plantsshould be removed and burned-thismeasurealso results in elimination of the pathogen that host plants carry and in prevention of greater losser from the spread of the pathogen to moreplants, host eradication to preventthe spread of many diseases is also carried out in fields, nurseries, greenhouse, etc. through elimination of the ready source of inoculumswithin the crop. Croprotation-It is obviousthat continuouscroppingin a particular field providesthe opportunity for perpetuation and intensification of pathogenic organisms. Soil pathogensattacking plantsof one species can be often eliminated from the soil by plantingcrops, belonging to other species not attacked by the pathogen, for 3 or 4 years. Sanitation-This practice includesall measuresaimed at eliminating or reducingthe amountof inoculum presentin a plant, field or warehouseand at preventingthe spread of the pathogen to other healthy plans and plantproducts. Improvement ofgrowing conditions of plants-It has been observed that the vigour of the plant often help to increase the resistance against pathogen attack. Creating unfavourable conditions to the pathogen- Proper aeration of stored plant productsdriesup their surfacerapidly and inhibits the germination and infection by bacterial and fungal pathogens presenton them. Tissue culture- Controlof the vascular diseases of some ornamentalplantpropagatingby cuttings may be donethrough tissue cultureof the meristem tips, because those pathogens are
- 5. unableto reach the apical meristems until very late stages of the disease free cuttings for creating new healthy plants. C. Biological methods: Plantdisease may be controlled biologically by selecting and breedingplants for resistance to particular pathogens or by usingsome other micro-organismswhich are either antagonistic to the pathogens itself. Breeding and use of resistant varieties-The best methodsof controlling plantdisease would be by meansof resistant varieties. If plantvarieties resistant to disease could be found or developed, then all crop-lossesresulting from disease could be avoided. A few plantsamong the diseased remain virtually unaffected by the pathogen those survivor plantsprobably remain healthy because of their resistance characters; if such plantsare propagated asexually and continueto be resistant to pathogen for several years, they may become the stoke plants for the developmentof resistant varieties. Cross protectionandinterference- The term cross protection is used specifically for the protection of a plantby a mild strain of a virusfrom infection by a strain of the same viruswhich causes much moresevere symptoms. Hyperparasitism-Thisis the control of pathogenic micro- organismswith other micro-organismsor viruses which parasitise or antagonize the pathogens- bacteriophyages, mycoparasites; nematophagousfungi, etc. are the best example of Hyperparasitism. Control throughtrap crops and antagonistic plants- Some plantswhich are not susceptible to few sedentary plant-parasitic nematodesproduceexudateswhich stimulate the hatching of eggs of those nematodes- the larvaeremain within those plants but are unableto develop into adultsand lay eggs and they die finally; such typeof plantsare called trap crops. D. Physical methods: Various typesof radiation and temperaturearethe physical agents that are used in controlling plantdisease. Control by heat treatment-Followingtypesof heat treatment (high temperature) are is use:
- 6. a. Soil sterilization by heat- This practice is usually donein seed beds, cold farms, greenhouse, etc. by the heat carried in live steam or hot water. b. Hot water treatment of propagative-Hot water treatment of seeds, bulbs, nursery, stocks, etc. is usually doneto kill plant pathogens which may be presenton the insideand outsideof those organs. c. Elimination of virusesfrom plantsby heat- Heat treatment in controlling virus diseaseis oneof the most successfuland widely used therapeutic methods. d. Hot air treatment of storage plantorgans- This treatment of storage organsoften removesthe excess moisturefrom their surfacesand hastens healing of woundsthereby preventing their infection by some weak pathogens. Control by refrigeration-This is the widely used measureof controlling post-harvest disease of fleshy plantproduct. Temperaturesat or slightly above the freezingpoint do not kill the pathogen presenton or in the tissues but inhibit or greatly retard the growth and activities of such pathogens hence the spread of existing infection and the initiation of new infection are prevented. E. Chemical methods: The use of chemical compoundstoxin to pathogens is the most common meansin controllingplant diseases in the field, in the greenhouseand also storage. Mostof the chemical are used to control diseases of foliage and other aerial plantparts; some chemical are used to disinfect and protect seeds, tubers, bulbs, etc. from infection; some are used to disinfect soli, others to disinfect warehouses, to protect fruitsand vegetables from infection, to treat wounds, etc. Insecticides are used to controlinsect vector of some pathogens. Methods of plant disease control with chemical- a. Foliage spraysand dusts-Chemical are usually applied as spraysor dustson the foliage of plants to control fungaland bacterial disease. Most of the fungicidesand bactericides are protectant in their action- hence they mustbe sprayed or dusted on the plantsurface beforethe arrival or the
- 7. establishment of the pathogens there. Sporeof pathogens requiremoisture before their germination- hence spraysor dustsseem to be most effective when those are applied before, duringor after every rain, especially duringthe period or possibility of infection. b. Treatment of seedsand other propagativestocks- Seeds, tubers, bulbs, roots, etc. are generally treated with various chemicals to preventtheir decry after plantingby controlling pathogens carried on them or presentin the soil. Chemicals may be applied on seeds and other propagativematerials like tubers, bulbs, corms, roots, etc. as dustsor as thick water suspensionsmixed with those materials, or they can be soaked in water solution of the chemical and then dried. c. Soil treatment- Soil, in which trees, ornamentals, vegetables, etc. are planted in often treated i.e. fumigated with volatile chemicals to control fungi, bacteria and nematodes. This treatment is doneseveral daysor weeksbefore planting. d. Treatment of tree wounds-Accidentalcuts and woundsmode on the bark of branches and trunksduringpruningor wounds resulting duringthe removalof infections by fungiand bacteria mustbe protected from dryingand also from becoming gateway of entry of new pathogens. e. Controlof postharvest diseases- Many fruitsand vegetables are subjected to fungalattack in storage after their harvest. A number of fungicidechemical are used for control of such post harvest diseases. f. Disinfection of warehouses-To avoid infection of stored productsby pathogens, the storage rooms are cleaned thoroughly and the debris left over there from previousyears is removed and burned. g. Controlof insect vector- This is very importantespecially when the pathogen is introduced or disseminated by insect vector. Application of insecticides to controlviruses, and fungaland bacterial spores carried by insects is very successfulmethod.
- 8. Types of chemicalsused to control plant diseases- Various chemical have been used now a daysfor crop protection as fumigants, soil treatments, dusts, sprays, pests, paints and systemic. a. Copper compounds-Bordeauxmixtureisthe most commonly and widely used copper fungicideall over the world. This fungicidecontrols many bacterial and fungalleaf spots, blights, anthracnoses, cankers, downy mildews, etc. b. Sulphur compounds-Numerousinorganicand organic sulphur compoundshavebeen used as good fungicideto control a variety of diseases. The organicsulphur compounds comprisethe most modern fungicides. They includethiram, ferbam, ziram, nabam, zineb, etc. there are all derivativesof dithiocarbamic acid. c. Mercury compounds – Inorganicmercury compounds HgCl2 known ascorrosive sublimate or dichlorideof mercury and Hg2Cl2 known ascalomel are used for soaking the seeds, rhizomesand corms of many vegetables and flowersto control some bacterial and fungaldiseases. d. Quinones – Chloranil and dichlone, these two quinone compoundsareusually used commercially as fungicides. e. Benzenecompounds – Beingtoxin to micro-organisms, benzenecompoundsareused commercially as fungicides. Daconil2787, abroad spectrum fungicideis effective against leaf spots, blights, fruitspots and rots, certain powdery mildews, etc. f. Heterocyclic compounds – It includessomeheterogeneous group of fungicides, of which the best one is captan. Captan is sold as captan, orthocide, etc. it’s a very good and effective fungicidefor control of leaf spots, blights, fruitrots, etc. g. Other organic fungicides – There are some chemically diverse compoundsactingas good fungicidesfor some diseases. Polyram is a foliar and seed protectant fungicide, it also controls rusts, downy mildews, leaf spots, blights of various plants.
- 9. h. Antibiotics – Many antibiotics such as streptomycin, tetracyclines, cycloheximide, griseofulvin, etc. are the most importantto control plant diseases. These are the productsof one micro-organism and toxin to other micro-organism. These antibiotics act on the pathogen or on the host directly or they act after undergoingtransformation within the host.