The following points highlight the two main principles for controlling plant diseases. The principles are: 1. Prevention or Prophylaxis 2. Cure.
Principle # 1. Prevention or Prophylaxis:
It is the protection of the host from infection and exposure to pathogen. Prevention includes a wide variety of control measures.
These are:
1. Immunity:
It means the resistance of the host to infection and disease development. It is most effective and promising method of protecting a crop against disease. It consists in developing and growing disease resistant varieties of crop plants.
It is an ideal, the cheapest and the best method of controlling all plant diseases. In nature it is a very slow and gradual process. Plant breeder can, however, accelerate the process by continued selection and hybridisation.
Some success in this direction has already been achieved. Several disease resistant varieties of crop plants have been produced. Much more, however, has still to be done.
Breeding for disease resistance; however, is a very complicated and arduous task. It is made still more difficult by the occurrence of new physiologic races of pathogens from time to time.
2. Protection of Non-Immune Varieties:
In majority of the crop, plants immune varieties are either not available or have not been so far developed. In such cases the development of disease-free crops requires much attention and forethought.
They are protected by more positive measures. All these methods involve eradication of the pathogen. It is carried out in a wide variety of ways. These methods include sanitation and eradication.
(a) Sanitation:
Many plant pathogens over-winter or over-summer on the remnants of previous year’s crop in the soil, wild perennial hosts or weeds and seeds. They serve as a source of primary inoculum.
The inoculum is transmitted to the hosts by various agencies such as wind, water, insects, manure of livestock and agricultural implements.
To reduce infection the following sanitation measures are put into practice:
1. Destruction of crop debris by burning or burying.
2. Removal of plant parts from the field as they become diseased and their destruction.
3. Soil sterilisation by physical treatment such as flooding, application of heat or steam or hot water.
4. Chemical treatment of soils for controlling soil home diseases is very effective.
5. Disinfection of agricultural implements and pruning tools by the use of fungicides.
(b) Eradication of the Pathogen:
It is carried on in a wide variety of ways.
The most important ones are:
1. Rotation of Crops:
The chief object of rotation of crops is to prevent the increase of inoculum in the soil and consequently to reduce losses from diseases. When the same crop is grown year after year in the same soil the fungus may increase enormously.
Economic production is no longer possible in such soils unless disease resistant varieties are grown or rotation of crops is resorted to. The rotation of crops allows the disease producing organism in the soil to die before the suitable host is again sown there.
2. Elimination of Weed Hosts:
Many weeds particularly biennial and perennial ones are potential sources of infection. They harbour the pathogens which later infect the cultivated crops seeded in the same soil. Elimination of these wild and alternate hosts is a very effective measure of disease control.
3. Prevention of the Disease Carried by the Plant Parts:
Normally the seeds, tubers, rhizomes, bulbs, etc., carry heavy loads of disease causing organisms or their spores. Under favourable conditions they cause disease. In such cases the disinfection of these infected plant organs by using sterilising agents, before they are sown, is quite effective.
The choice of the agent to be employed depends upon whether the causal organism is borne on the surface or has invaded the deeper tissue.
Against surface borne causal organisms, surface disinfection of propagative plant parts by chemical disinfectants is very effective. It will destroy the pathogen. These external plant protectants may be used in the form of gases in fumigation or as solids in the form of dusts.
These plant parts are also disinfected through the use of soaks, dips, washes, etc., in liquids containing toxic compounds. The chief character of these disinfectants is that they must be cheap, effective, easy to apply, persistent and safe.
They should be toxic to the pathogen but not to the host. Chemical compounds in dust form are now considered much more popular as external protectants.
In case the disease is carried inside the propagative plant part the application of moist heat appears to be the only effective control.
In the case of crop plants growing in the field, external protection is provided for foliage and other growing parts by protective sprays and dusts of chemicals.
Principle # 2. Cure:
It involves the destruction of the pathogen after it has gained entry into the host. In fact it is the cure or control of disease in the growing crops. It is done by the application of strong chemicals in the form of sprays. These actually destroy the pathogen within the diseased tissues of the host.
A thorough Mixture spray of lime sulphur, Bordeaux mixture and many inorganic mercury compounds will destroy the spores or the exposed fungus hyphae. The spray is delivered with considerable force and in the form of fine mist.
This conventional method of plant disease control by fungicidal sprays or dusts is cheap and effective against a wide range of pathogens.
However, it suffers from the following disadvantages:
(i) They are bulky and corrosive.
(ii) Bordeaux mixture is somewhat difficult to prepare.
(iii) Lime sulphur is unpleasant to use.
(iv) Spraying and dusting is too expensive against diseases of low cost crops.
Because of the limitations of the above-mentioned conventional method, pathologists were induced to make a search for new compounds having fungicidal potentialities. Consequently various chemicals, organic and inorganic and antibiotics have been tested. Some of them have shown encouraging results.
Antibiotics:
The use of antibiotics is the most recent development in plant chemotherapy. It is receiving increasing attention. The antibiotics are most effective in the control of bacterial plant pathogens. However, their use as disease controlling agents in general is still in the experimental stage.
Brown and Boyle (1945) were the first to use antibiotics for the control of bacterial diseases. They worked on the crown galls of various plants caused by Bacterium tumefaciens. For this they used filtrate of Penicillium notatum.
The fluid was either injected into the galls or applied with cotton wool soaked in it. The result was encouraging. The expansion of galls was inhibited. Identical results were obtained by Hampton (1948) with penicillin and streptomycin.
The mixture of streptomycin with terramycin or the former alone when sprayed on pear blossom suffering from fireblight gave good results (Ark and Alcem, 1956).
Agrimycin which is a mixture of streptomycin and terramycin proved effective in obtaining an economic control of bacterial spot of tomatoes. The causative agent in this case is Xanthomonas vasicatoria.
The results of the above-mentioned experiments and others have shown that the use of antibiotics to combat plant diseases is of more than academic interest. Whether they can be used economically as plant protectants is difficult to say.
An antibiotic which is antifungal has been discovered at the Indian Agricultural Research Institute. It is named as “Bulbiformin”. Experiments are being carried out to test its efficacy in controlling disease.
The antibiotics are considered to yield promising results as:
1. Soil disinfectants for the control of pathogens in the soil.
2. Seed treatment both when the pathogen is surface borne and when it has invaded the deeper tissues.
3. External protectants against many diseases such as rots, mildews, leaf spots, etc.
4. Systemic plant protectants against pathogens causing diseases of vascular tissues.
