Here is an elaborated discussion on principles and economics of plant disease management.
Principles of Plant Disease Management:
Plant diseases cause enormous economic losses, both quantitatively and qualitatively. Conservative estimates proclaim that pre- and post-harvest diseases result in up to 30% losses particularly in the developing countries. Man has always been fighting to control plant diseases with the objective to prevent economic loss and increase the value of the crop.
But the word ‘control’ evokes the notion of finality, of having controlled or permanently settled the problem. In fact, this is not so. The complete destruction of pathogens under field conditions is not only impossible but also economically impractical.
One has to repeat the control operation in each crop season to combat the disease. Therefore, the word ‘management’ is preferred over ‘control’ as the former conveys the concept of continuity. It implies that diseases are inherent component of an agro-ecosystem that must be dealt with on a continuous and knowledgeable basis.
Plant disease management is therefore based not on the principle of only eradication of the pathogen but mainly on the principle of maintaining the damage or loss below the economic threshold level or at least minimizing occurrence of a disease above that level.
Plant disease management mainly bases upon the principles of:
(1) Avoidance of the Pathogen,
(2) Exclusion of the Pathogen,
(3) Reduction or Eradication of Pathogen Inoculum,
(4) Resistance (Immunization) to Pathogen, and
(5) Integrated Approach.
1. Avoidance of the Pathogen:
Avoidance of Pathogen aims to enable the host to avoid contact with the pathogen or to ensure that the susceptible stage of the host and favourable circumstances for the pathogen should not coincide.
This aim can be achieved by preventing the diseases by the following strategies:
i. Proper selection of such a geographic area to grow the crop wherein the pathogen does not occur; alternatively, vectors required for transmission of a pathogen may be absent. For example, seed potatoes are mainly produced in Scotland, the cooler northern region of United Kingdom, where the chances of aphids transmitting viruses are much less than in warmer southern regions.
ii. Selection of field for crop cultivation should be in such a manner that the following requirements be taken into account.
(a) Crop should not be sown in the same field, when in previous year(s) the crop was infected with a pathogen that survives in soil. For instance, Colletotrichum falcatum has ability to persist in the soil and it can result in development of disease if sugarcane is successively cultivated in the same field.
(b) Water drainage conditions are satisfactory. Low lying, waterlogged fields favour many diseases, e.g., downy mildew of bajra, red rot of sugarcane.
(c) Suitable site selection particularly in case of orchards is significant. For example, there would be chances of severe collar rot disease (Rosellinia sp.) in apple trees, if they are planted where oak trees were previously grown.
iii. Choice of timing of sowing should be in such a way that the crop is not infected by the pathogen even during its most susceptible period. The pathogen Rhizoctonia that causes root rot of gram, for example, develops rapidly under high temperature and moisture conditions and the disease is severe if gram is sown following the rains. If the crop is sown slightly delayed, the incidence of the disease is considerably reduced. Similarly, early sowing of black rust susceptible wheat varieties attract less damage because Puccinia graminis f. sp. tritici causes more damage to late sown wheat varieties.
iv. Choice of disease escaping varieties is the strategy of avoidance in which such varieties of crops should be preferred that escape the onslaught of the pathogen and resist the attack due to their inherent growth characteristics. Early maturing varieties of wheat or pea escape the damage due to black rust (Pucinia graminis f. sp. tritici) and powdery mildew (Enysiphe polygoni), respectively.
v. Selection of disease-free seed and planting stock from such areas where the disease has not occurred due to certain environmental factors. For instance, seed potatoes are advised to be transported in the southern states from the Shimla hills in our country to control viral and bacterial diseases of potato. In case of those plants that propagate by vegetative parts, the planting stock from disease-free areas should be preferred.
vi. Cultural practices like distance between plants, mixed-cropping, depth of sowing, timing and amount of irrigation, quantity or quality of fertilizers, etc. should be followed with such necessary alterations that help reduce disease incidence by avoiding the contact between the pathogen inoculum and the plant.
For example, seeds of wheat should be sown shallow to avoid contact between the plants and the pathogen in reference to bunt disease (Tilletia foetida and T. carries) and flag smut disease (Urocystis tritici) because the incidence of these diseases is higher in deep sown wheat. Mixed-cropping practice brings considerable distance between the plants of the same crop therefore it renders effective check to contact-spreading diseases.
2. Exclusion of the Pathogen:
The main objective behind the principle of exclusion of the pathogen is to prevent the entry of a pathogen into a country, state, or area. Quarantine measures, seed certification, plant disease notification, and prevention of sale of diseased plants are the methods, which are regulatory and can be enforced by way of regulations framed by the government.
i. Quarantine measures refer to the legislations (regulations) passed by the governments to check the import and export of diseased plant materials from one country to the other or to check their transport from disease prone areas to disease free areas within the country. Several quarantine measures operate in most countries, backed up by national and international legislation.
The quarantine regulation was first enacted in the USA in 1912 under the title ‘Federal Quarantine Acts’ to exclude the epidemics that appeared in the country as a result of transportation of diseased plant materials from other countries. In India, it was in 1914 when the first quarantine regulation was passed by the government under the title ‘Destructive Insects and Pests Act (DIP ACT)’ to prevent the introduction of exotic pests and diseases into the country from abroad.
ii. Seed certification is a voluntary programme subject to regulation by national, regional, or provincial (state) agencies. The growers of seeds or propagating materials, who desire their stock to be certified, contact the regulatory agencies whose plant pathologists inspect the fields and orchards and certify them or refuse depending upon the disease incidence therein.
iii. Plant disease notification is such a measure in which the grower informs the concerned authority about the occurrence of a disease of potential danger, the pathogen of which might penetrate and spread in the area in near future. The authority then notifies other growers to take necessary precautions against the disease. In this connection therefore the growers require to be educated time to time.
iv. Prevention of sale of diseased plants refers to the principle of pathogen exclusion in which the authorities have legislative power to ban the sale of any diseased plant or planting material to prevent spread or multiplication of a pathogen in an area hither to free from the disease.
3. Reduction or Eradication of Pathogen Inoculum:
The aim of the principle of reduction or eradication of pathogen inoculum is to reduce or eradicate the number of pathogen propagules present in the field before crop is planted, or develop on or in the crop plants after the establishment of the disease. To achieve this aim, various cultural practices, physical methods, chemical treatments, and biological control devices are employed.
i. Cultural practices in which variety of manipulations with respect to host and the environment are made by man, without involving any of the biological or chemical agents, that help reducing or eradicating pathogen inoculum. Such cultural practices include eradication of diseased host, eradication of alternate hosts, wild host and weeds, drying and ageing of seeds, seed cleaning, crop-rotation, sanitation, etc.
ii. Physical methods are those that depend on physical factors like heat and cold, light wavelengths, and radiations to reduce or eradicate pathogen inoculum. These methods include heat treatment, refrigeration and radiations.
iii. Chemical treatments are highly effective measures to reduce or eradicate the pathogen inoculum. Traditionally chemical treatments were best applied to protect the crop before significant exposure to a pathogen, but the newer generation fungicides have curative properties and thus may be worth applying even after infection is established in a crop.
iv. Biological control refers to partial or complete destruction of pathogen populations by other organisms that occur routinely in nature. There are, for instance, many a number of diseases in which the pathogen cannot develop in certain areas either because the soil, called suppressive soil, is rich of microorganisms antagonistic to pathogen or because the plants have been naturally inoculated with antagonistic microorganisms during pre-or post-infection stages.
4. Resistance (Immunity) to Pathogen:
Genetic resistance of host to pathogen is one of the most important and effective principles of achieving successful management of plant diseases. Plant breeders either search or develop such resistant varieties of plants in which the pathogen and the host are incompatible to each other, that is, the host defends itself against the pathogen by means of various defence mechanisms either pre-existing or developed in response to pathogen attack.
5. Integrated Approach:
The principle of integrated approach is the use of a combination of different disease management strategies with overall aim to develop sustainable system of disease management based on a sound understanding of the whole crop ecosystem. This approach, as is considered, will not only maintain the efficacy of host resistance and agrochemicals, but also bring other benefits such as reduced environmental impacts and lower control costs.
The Economics of Plant Disease Management:
Almost all disease management measures require significant expenditure in terms of man power and material costs and these costs are directly related to the benefits gained by the grower in terms of produce. The grower therefore has options of depending not only upon the type of crop and the nature of the pathogen concerned, but also upon the economic returns.
In such a condition, one has no point to recommend such a management measure, however effective, to the grower if the expenditure costs more than the likely return in crop yield or quality. The only exception may be the cases where some wider strategic significance comes forward, for example, the case where eradication of a pathogen removes a long-term threat to crop production in a given area.
Fig. 14.2 shows a simplified presentation of the amount of capital input and the importance of quality of product in reference to different fanning systems. We know that the latter differ widely and disease management options appreciable for one farming system may not be economic in another.
In subsistence fanning, which is in common practice over large areas of the developing countries, the crop return is insufficient to sustain expensive inputs of fertilizers or chemicals. In this farming system therefore the management measures must be cheap and easy to apply.
Contrary to it, production of horticultural crops of high-value in protected glasshouses are high input oriented because this farming system involves sophisticated devices for regulation of the crop environment, including the use of biological agents and chemicals for disease management. Most farming systems fall between these two extremes, and the level of inputs varies depending upon conditions of the market.
Certain governments subsidize certain crops, which helps maintain the market value of the product above its normal base line. The grower, in such cases, may be able to justify the use of extra expenditure for disease management. But the market conditions remain variable and usually change every year, the grower is bound to adjust accordingly the options for disease management.
A generalized relationship between cost input in disease management and the crop income can be plotted as shown in Fig. 14.3. The simplified model shows that increase in expenditure is necessary to improve the degree of disease management for better yield.
In the light of this, therefore, it becomes necessary for a grower to compromise with the disease to a level in which some damage is tolerated. It is so because the point at which the cost of management is balanced by the income (called economic threshold) is usually an intermediate where some disease is still present.
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