In this article we will discuss about the measures and principles used for controlling plant diseases.
Measures to Control Plant Diseases:
Till the appearance of disease symptoms the pathogen has taken command over the plant. In order to avoid such a situation it is essential to protect plants from disease i.e., protective and preventive measures should be undertaken. Therefore it is said prevention is better than cure.
The control measures for plant diseases have been classified into two categories:
1. Prophylaxis.
2. Immunization.
1. Prophylaxis:
Prophylaxis includes methods which protect the plant from the attack and infection by the pathogen or from the environmental factors which favour disease development.
These methods have further been divided into three sub-categories:
i. Exclusion:
Method which prevents a disease from entering into a new area. For example – Quarantine and Inspection, embargo, certification etc.
ii. Eradication:
This measure is taken for the elimination of the pathogen after its establishment on the host. It includes crop rotation, sanitation, elimination of alternate and over wintering hosts, elimination of pathogen from the seeds, tubers etc.
iii. Direct Protection:
The principle behind this measure is that noncompliance with it can result into infection. Fungicidal seed treatment methods are either eradicant or protective in nature. Spraying and dusting of fungicide on plant comes under direct protection.
Other examples are – Controlling environmental factors in green houses and warehouses, altering the time of sowing & harvesting in order to make it favourable for the host and unfavourable for the pathogen, protecting plants from cold, frost, heat etc. and maintaining nutrient elements.
2. Immunization:
Immunization deals with development of disease resistance in plants. The host thus becomes capable of defending itself from the pathogen. Such characters are developed through selection and hybridization. Temporary immunization of plants can also be brought about by Chemotherapy. The host plant absorbs the chemotherapeutant and homogeneously circulates it in their body.
On studying the different methods of plant disease control three conclusions are derived:
(i) Direct action against the pathogen or the attack on the pathogen or exclusion.
(ii) Genetic modification of the host to resist disease or strengthening of the host.
(iii) The alteration in the environment to make it unfavourable for disease development or to modify the environment.
Principles to Control Plant Diseases:
1. Avoidance of the Pathogen:
It includes the following measures:
(a) Selection of Geographical Area:
A geographic area is selected on the basis of the favourable temperature and humidity requirement for a particular crop. Most of the species of fungi and bacteria develop & grow more & have more pathogenicity in humid areas in comparison to dry areas. For example – Smut and ergot disease of bajra are more in humid areas where the flowering stage prolongs in the rains for several days.
(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) 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.
2. Exclusion of Inoculum:
It includes the following methods:
(a) Seed Treatment:
The pathogens present on the surface of seeds, tuber & graft etc. can be excluded by chemical treatment. This helps in avoiding entry of the pathogen in new area.
Physical Method:
Jensen for the first time used heat to control internal infection of potato tubers by late blight fungus (Phytophthora infestans). It was observed that heat treatment of potato at 40°C for 4 hours results into death of the internal mycelium.
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.
Solar energy treatment to control loose smut was first developed by Luthra. In this method seeds are first rinsed or soaked in water for 4-5 hrs. Before drying them in scorching sun.
Hot air treatment for the control of virus in propagating stocks was first developed by Kunkal in Peach yellow.
Chemical Methods:
Seed treatment with fungicide before transplanting is done with following objectives:
(i) To control disease caused by seed borne infection
(ii) To protect germinating seeds or seedlings from the attack of soil borne pathogens.
Chemicals used in Seed treatment:
Chemically treated seeds are kept dry during storage. Such seeds should undergo treatment at least one week before sowing. But seeds treated with liquid chemicals are not stored and the treatment is therefore performed just before sowing.
Seed dressing with organomercurials is potent enough to control many diseases except loose smut disease of wheat and barley. Therefore organomercurial is called a broad spectrum seed treating fungicide. Systemic fungicides are suitable to control loose smut of wheat & barley.
Types of Seed Treatment:
On the basis of nature and purpose seed treatment has been divided into three categories:
(i) Seed Disinfection:
This type of seed treatment is for the eradication of such pathogens which are internally seed borne i.e. they are established in the seed coat or deep tissues in the seed. Examples – Loose smut of wheat & barley, strike disease of barley, Rhizoctonia disease of tomato. Earlier only physical measures like hot water or solar energy treatment was used for the eradication of such diseases but now a days systemic fungicides like Plantvax/oxycarboxin, vitavax, carboxin, Benlate/Benomyl, Bavistin etc., are used for the purpose.
(ii) Seed Disinfestation:
Distinction of fungal or bacterial pathogens present on the seed surface is called seed disinfestation. The fungicides used in this process are either in powder or wettable form. Many externally seed borne diseases such as – covered smut of barley or oat, Loose and grain smut of Jowar, Wheat bunt etc. can be controlled by seed disinfestations.
The fungicides used in seed disinfestations are as follows:
Cesesan, Panogen, Copper sulphate, Copper carbonate etc. These fungicides loose their effect when the seeds are sown in soil. Due to this these are not considered good seed protectant.
(iii) Seed Protection:
Protection of seeds from soil borne pathogens during the process of seed germination is called seed protection. Pre-emergence Damping off disease in many vegetables can be controlled by seed protection. Organo-mercuric fungicides act as seed protectants. Other fungicides used as seed protectants are – Captan, thiram, Arasan, Scmasan, Agrosan GN, Agallol, Aretan, Folpet etc.
(a) Systemic Fungicides are used for seed disinfection, seed Disinfestation and seed protection.
(b) Inspection and Certification:
Crops grown for seed production are regularly inspected so that seed borne disease could be effectively controlled. Such disease free seeds are certified.
(c) Plant Quarantine:
A plant quarantine can be defined as a legal restriction on the movement of agricultural commodities for the purpose of exclusion, prevention or delay in the establishment of plant pests and diseases in areas where they are not known to occur.
(d) 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:
3. Eradication of the Pathogen:
It includes the following principles:
(a) Biological Control of Plant Pathogens:
Decomposition of organic matter in the soil leads to intense microbial activity. Among them certain microorganisms have the potential to kill pathogens. In our country Root-knot nematodes and Black scurf of potato have been successfully controlled by organic soil amendments. Organic soil amendments come under biological control methods. This technique is helpful in controlling many soil borne diseases.
(b) Crop Rotation:
Crop rotation is essential for controlling soil borne diseases and pathogens.
(c) Removal and Destruction of Diseased Plant Organs:
The following methods are brought into use:
(i) Roguing.
(ii) Eradication of Alternate and Collateral hosts.
(iii) Sanitation of Fields.
(d) Heat and Chemical Treatment in Diseased Plants:
This method is used to kill the resting structures of pathogen present on the host surface. This method is very effective in controlling diseases of fruit trees.
(e) Soil Treatment:
The main objective of this method is to inactivate pathogens and nematodes present in the soil. For this, different chemicals, heat, flooding and fallowing etc., like physical and chemical methods are used. For controlling nematodes volatile soil fumigants are mainly used.
Chemical treatment is performed by any one of the following methods:
(i) Drenching of soil with solution or suspension.
(ii) Broadcasting of dusts, powders or granules.
(iii) Furrow application.
(iv) Soil fumigation.
4. Protective Measures:
(a) Chemical Treatment:
The main objective of chemical spray, dusting and seed treatment is to form a protective poisonous layer on the host surface or to kill the parasites already present on the host surface.
(b) Chemical Control of Insect Vectors:
Certain viral diseases are transmitted only through insect vectors. Therefore it becomes essential to kill these insect vectors.
(c) Modification of Environments:
Hot and humid conditions are favourable for diseases and pathogens. Ploughing, frequency and amount of irrigation etc. change the environment upto a certain extent.
(d) Modification of Host Nutrition:
Host nutrition also affects diseases. Excessive nitrogen in plant promotes leaf diseases whereas excess of Ca and K makes the plant disease resistant. Balanced nutrient management in crops can help in reducing the intensity of many diseases.
5. Development of Resistance in Hosts:
Development of resistance in hosts is brought about by following methods:
(a) Selection and Hybridization -for disease resistance
(b) Through chemotherapy – when systemic fungicides and antibiotics are applied to plants in form of foliar spray or through roots, their toxic effects persists on crops.
(c) Through host nutrition.
6. Therapy of Diseased Plants:
Diseased plants are cured through following methods:
(a) Chemotherapy.
(b) Heat therapy.
(c) Tree-surgery.
The diseased branches of trees are removed or the disease parts scrapped and fungicidal paste is applied on the wounded areas. This helps in checking infection. Important diseases of apple – Black stem, Brown stem and Pink disease etc. are controlled through surgery.
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.
Seed Treating Chemicals:
It is used for controlling seed borne diseases.
Seed borne diseases are of two types:
(a) Externally Seed Borne Diseases:
For controlling these diseases, Chemicals such as Formalin, TMTD, Copper carbonate, Captan, Organo-mercurials (Agrosan GN and Ceresan) are used for seed treatment.
(b) Internally Seed Borne Diseases:
For controlling internally seed borne disease, hot water treatment and solar treatment are used.
Systemic Organic Compounds are effective chemicals for controlling both externally and internally seed borne diseases.
Examples of Systemic Fungicides:
Oxanthin derivatives (Plantvax and Vitavax), Benlate, Bavistin, Demosan
For controlling air borne diseases. Foliar application of chemicals is more effective.
The common copper fungicides are:
Perenox, Perelan, Blitox, Cuprokyt, Cuprosan, Fytolan. Its use is comparatively better than that of Bordeaux mixture.
Use of Organo-sulphur compounds such as Thiram and Dithiocarbamates (Zineb, Ziram) is a safer alternative for tender and sensitive foliage.