This article throws light upon the top four methods of controlling plant diseases. The methods are: 1. Cultural Methods 2. Physical Methods 3. Chemical Methods 4. Plant Quarantine.
Controlling Plant Disease: Method # 1.
(a) Selection of Geographical area which on the basis of the favourable temperature and humidity requirement for a particular crop but unflavored to the fungi and bacteria.
(b) Selection of field:
Many soil borne diseases are controlled by proper selection of the field. It is quite possible that a particular field soil contains a pathogen species. In that case the particular crop is not sown in that field for several years. The causal organism of Red rot of sugarcane Colletotrichum falcatum survives in the soil for several months. Water drainage is also taken care of while selecting the field.
(c) Choice of the time of sowing:
The susceptible stage of plant growth and the favourable environment for pathogen should not match at the same time.
(d) Disease escaping varieties:
Certain varieties of crop due to their growth characteristics are able to escape from disease. This disease escaping characteristics of the crop is not genetic rather it is due to growth habits and time of maturation. Early maturing variety of pea is capable of escaping powdery mildew and rust.
(e) Selection of seed:
To avoid seed borne diseases, healthy and disease free seeds are essential.
(f) Crop rotation:
Crop rotation is essential for controlling soil borne diseases and pathogens.
Removal and Destruction of Diseased Plant Organs, eradication of alternate and collateral hosts and sanitation of Fields.
(h) Modification of cultural Practices:
Cultural practices such as – distance between the plants, time and frequency of irrigation, transplantation time and method, mixed cropping, amount and property of fertilizer and compost etc. can be changed to reduce losses caused by the disease.
(i) Eradication of Insect Vectors:
Insects serve as vectors for many diseases. Eradication of such insect vectors is essential for the control of pathogens.
Examples of some diseases and their insect vectors are as follows:
Agar : A gelatin-like substance obtained from sea weed (red algae Gracilaria, Gelidium etc.) and used to prepare culture media on which microorganisms are grown for study.
Alternate host : One of the two kinds of plants on which a parasitic fungus (e.g. black rust of wheat caused by Puccinia graminis trifid) must develop to complete life-cycle.
Anthracnose : A leaf spot or fruit spot type of disease caused by fungi that produce their sexual spores in an acervulus.
Antibiosis : The phenomenon in which a substance produced by one microorganism is harmful to another organism.
Antibody : A protein produced by specific stimulation when a foreign antigen enters into the blood of an organism. Antibodies get attached with the antigens and make them ineffective or harmless.
Antigen : A substance (usually a protein, lipid or carbohydrate) which after entering into a body activates the production of antibody.
Bacteriophage : A virus which infects specific bacteria and kills them.
Bacteriophage : A chemical or physical agent that prevents’, multiplication of bacteria without killing them.
Blight : A non-restricted tissue disintegrating symptom characterized by general and rapid killing of leaves, flowers & stem.
Blotch : A disease characterized by large and irregular spot or lesions on leaves, shoots and stems.
Canker : A necrotic or sunken lesion Oil a stem, branch or. fruit of a plant (e.g. citrus canker caused by Xanthomonas citri.
Carries : A plant or an organism which carries an infections agent but does not show symptoms of disease produced by the agent.
Chlorosis : Yellowing of green tissue due to chlorophyll destruction.
Damping off : Destruction of seedling near the soil surface, resulting in the falling of seedling on the ground.
Die Back : Progressive death of shoots and roots generally starting at the tip.
Disinfectant : An agent that kills or inactivates pathogens in the environment or on the surface of the plant, prior to infection.
Downey Mildew : A plant disease in which the mycelium & spores of the fungus appear as a Downey growth on the host surface.
Endemic Disease : A disease which regularly occurs on a particular area of earth or country.
Epidemic Disease : A wide spread & severe outbreak of a disease.
Etiolation : Yellowing of the plant due to deficiency of light.
Exclusion : Control of plant disease by excluding the pathogen or infected plant material from disease free areas.
Exudate : Liquid discharge from plant tissue.
Facultative parasite: An organism that is usually saprophyte under certain conditions may become parasite.
Facultative saprophyte : An organism that is usually parasite but may also live as a saprophyte.
Gall : A swelling produced on a plant as a result of infection by certain pathogens.
Gummosis : Production of gum by plant tissue.
Heterotrophy : An organism depending on an outside source for organic nutrients.
Hyperplasia : Excessive development due to increase in the number of cells.
Hypertrophy : Excessive growth due to increase in size of cells.
Immunity : The state of being exempted from infection by a given pathogen.
Infection : Establishment of the pathogen in the host.
Infections disease : A disease caused by a pathogen which can spread from a diseased to a healthy plant.
Latent virus : A virus that does not induce symptoms in its’ host.
Leaf mottling : A disease caused due to Zn deficiency (specially in Citrus spp.) in which new leaves develop inter-veinal chlorosis, get reduced in size, plant becomes bushy and the branches show die-back symptoms.
Lesion : A localized area of discoloured, diseased tissue.
Mildew : A plant disease caused by a fungus in which the mycelium and spores are seen as a whitish growth on the host surface.
Mosaic : Symptom of certain viral diseases of plants characterized by intermingled patches of normal and light green or yellowish colour.
Mycorrhiza : Symbiotic relationship between roots of higher plants and fungal mycelia which is essential for the growth of these plants.
Necrosis : The death of cells or of tissues.
Obligate parasite : A parasite that in nature can grow and multiply only on living organisms.
Parthenogenesis : Formation of embryo without fertilization.
Pathogen : An disease causing agent in plant.
Plasmogamy : Fusion of cytoplasm’s of two cells.
Polymorphism : Having various forms in a life cycle. The rust fungus is allomorphic as it produces five different types of spores in its life-cycle.
Pustule : Small blister like elevation of epidermis.
Quarantine : Control of export and import of plant to prevent spread of diseases or pests.
Race : A genetically distinct mating group within a species; also a group of pathogens with distinct pathological or physiological characteristics.
Resistance : The ability of an organism to overcome, completely or partially the effect of a pathogen.
Rickettsia like Organisms : RLOs a prokaryotic microorganism having a cell wall and obligate intra-cellular parasite.
Ring spot : A circular chlorotic area with a green centre; symptom of many viral diseases.
Rot : The softening, discolouration and disintegration of a succulent plant tissue as a result of fungal or bacterial infection.
Russetting : Brownish roughened areas on fruit skin produced as a result of excessive cork formation.
Rust : A disease of grasses and other plants giving a rusty appearance to the plant and caused by uredinales (rust fungi).
Saprophyte : A organism which lives on dead and decaying organic matter.
Scab : A rough, crust like diseased area on the surface of a plant organ. A disease in which such areas are formed
Scorch :Burning of leaf margins as a result of infection or unfavorable environmental conditions.
Smut : disease caused by Ustilaginaceae, characterized by masses of dark, powdery spores.
Spot : Disease symptom in which certain restricted tissue
Susceptibility : The inability of a plant to resist the effect of a pathogen.
Susceptible : A plant or species which is incapable of resisting the effect of a pathogen.
Toxin : A compound produced by microorganisms and being toxic to a plant or animal.
Vector : An insect able to transmit a pathogen.
Vein banding : Bands of green tissue along the veins while the tissue between the veins become chlorotic.
Viroid : A naked nucleic acid which resembles virus but is devoid of protein coat.
Wilt : Loss of rigidity and dropping of plant parts wholly or partially.
Yellows : Yellowing and stunting of host plant.
Controlling Plant Disease: Method # 2.
(a) The hot water treatment method of Jensen was developed in 1887 which was used to control loose smut disease of wheat, barley and Oats. Until the development of systemic fungicide hot water treatment was the only method to control loose smut. Hot water treatment is also effective in the control of nematodes.
(b) Solar energy treatment to control loose smut was first developed by Lutlzra. In this method seeds are first rinsed or soaked in water for 4-5 hrs. before drying them in scorching sun.
(c) Hot air treatment for the control of virus in propagating stocks was first developed by Kunkal in Peach yellow.
Controlling Plant Disease: Method # 3.
(a) Seed treatment with fungicide before transplanting.
i. Soil treating chemicals:
It is used for controlling such soil borne diseases which attack on seeds or seedlings. The examples of such chemicals are – Formaldehyde, Captan, Thiram, Zineb, Organo-mercurials, PCNB, Ethylene dibromide, vapam etc.
ii. For Externally seed borne diseases, chemicals such as formalin, copper carbonate, captan, organo-mercurials (Agrosan GN and Ceresan) are used for seed treatment.
iii. For Internally seed borne diseases (i.e. loose smut), hot water treatment and solar treatment are used.
iv. Systemic Organic Compounds are effective chemicals for controlling both externally and internally seed borne diseases eg. Oxanthin derivatives (Plantvax and Vitavax), Benlate, Bavistin, Demosan.
v. For controlling air borne diseases, foliar application of chemicals is more effective.
vi. The common copper fungicides are: Perenox, Perelan, Blitox, Cuprokyt, Cuprosanand Fytolan. Its use is comparatively better than that of Bordeaux mixture.
(b) Seed dressing with organomercurials and systemic fungicides.
Controlling Plant Disease: Method # 4.
Plant quarantine can be defined as a legal restriction on the movement of agricultural commodities for the purpose of exclusion, prevention or delay m the establishment of plant pests and diseases in areas where they are not known to occur.