In this article we will discuss about the study of plant diseases.
One of the important aspects of study of plant diseases is how to either cure the disease or prevent its recurrence. Before any cure can be recommended, the cause or in other words, the causal agency of the disease should be known—whether parasitic or non-parasitic.
Plant diseases can be studied in extremely variable methods which depend on the nature of disease, the causal agency, and the host involved. But the initial study of symptoms, periodicity, incidence, and extent of damage done to the host—all these aspects are to be studied irrespective of the nature of disease, causal agency, and host infected.
This initial study often produces information to identify the disease under study either as a parasitic or a non-parasitic disease. If the disease is a parasitic one, the next step will be to determine the nature of causal agency.
This area of study extends:
(i) A closer examination of disease infected host tissue in the laboratory in naked eye and under microscope,
(ii) Isolation of pathogen from the disease infected tissue, and
(iii) Growing of pathogen in pure culture.
The presence of a pathogen can frequently be determined by naked-eye examinations of lesions.
But in many cases the pathogen is too small to be seen without magnification, or may be internal within the tissues programs. Microscopic examination of surface scrapped mounts or dissected mounts of lesions without staining in water or staining in lactophenol cotton blue or potassium hydroxide and phloxine may also reveal much information about the pathogen.
Often microscopic examination of thin free hand or serial microtome sections stained in suitable dye combinations is necessary to work out the details of the pathogen.
The microscopic examination is expected to reveal the nature of pathogen, at least, the group to which it belongs and the details of mode of growth in the host tissue, nutrition, and damage done to the host cells by the pathogen.
Isolation is a technique by which a pathogen is transferred from its natural habitat (infected host tissue) to a suitable artificial medium. Isolation of pathogen should be done from vigorously infected areas. Small beats of infected tissue should be placed on sterilized nutritive agar medium in Petri dishes or culture tubes.
Earlier to of is the selected infected host tissue bits should be cleaned from dirt with water and surface sterilized either by applying mercuric chloride or silver nitrate and sodium chloride technique. For isolation work two per cent, potato dextrose agar medium has been found very suitable.
The entire isolation process should be done aseptically in an isolation room or chamber especially built for the purpose. The Petri dishes or culture tubes, whatever used, should then be put in an incubator having incubation temperature range of 20°C. to 25°C. In a few days-time the pathogen grows out on the agar surface from the infected host tissue.
The pathogen is then transferred to fresh nutritive agar medium and grown in pure culture under the same cultural condition. The pathogen should then be accurately described. The procedures described above is, however, very suitable for the study of disease caused by fungi.
But just because a fungus is present in a disease infected tissue does not really prove that this particular fungus is responsible for inducing this particular disease. The pathogenicity or causal relationship of the fungus concerned has to be established. The pathogenicity tests of a pathogen are also designated as Koch’s postulates or Koch’s Rules of Proof.
They comprise of certain postulates or conditions that are to be fulfilled, only when the causal relationship of a pathogen is established.
They are as follows:
1. The causal organism must be constantly associated with the disease in question.
2. The causal organism must be isolated from the diseased plant and grown in pure culture and accurately described.
3. Inoculations with inocula from pure culture of the causal organism must reproduce the disease in question in the same species or variety of plant from which the causal organism was isolated.
4. The causal organism must be re-isolated from the plant in which disease has been produced by inoculating with inocula from pure culture.
These postulates cannot, however, be fulfilled with organisms which cannot be grown on artificial media. In that case constant accompaniment of the organism with the diseased plant may also be taken as proof of causal relationship.
Procedures for the first and second postulates have been discussed above. As to the third and fourth postulates, the inoculum should be a suspension of spores or mycelium of the fungus isolated from the infected host tissue.
Suspension of spores or mycelium should be prepared by following standard technique. The suspension so prepared should be sprayed with an atomizer on the surface of the host plant which has been grown in a pot in the greenhouse.
The variety of host plant must be the same as the one on which the disease had first appeared. The sprayed plant should then be covered with a bell-jar or polythene covering. The soil of the pot in which the host plant is grown should be kept moist. This enables to maintain humid condition around the plant.
The sprayed potted plant with covering is then placed for twenty-four to forty-eight hours in a place where the temperature and humidity both are favourable for the appearance of the disease. The sprayed plant should not be exposed to direct sunlight, as the spore germination and mycelial growth may be hampered. Usually lesions start appearing after twenty-four to forty-eight hours.
Ultimately typical disease symptoms that have been recorded earlier will be visible. The fungus is then re-isolated from the lesions produced by artificial inoculation. Its character is then studied and compared with what has been recorded earlier. When all the stages produce expected results, then only the pathogenicity of a particular fungus is established.
The procedure described above is, however, suitable for diseases where the infection is not soil borne. But in soil borne plant diseases the isolation of pathogen should be done from soil around the diseased plant.
These isolations will form the inocula and they are aseptically mixed with sterilized potted soil in which the seeds of the exactly same variety of plant should be sown after surface sterilization by suitable fungicides.
Under suitable temperature and humidity the seeds develop into seedlings and receive infection through roots from the pathogen that has been growing in the soil and ultimately the disease appears as expected. In all the above experiments good number of un-inoculated controls should be kept side by side to make a comparative study.
While growing in pure culture, the fungus concerned often produces spores.
The study of sporulation and spores in details is also essential for identifying the fungus. The mode of formation of spores and their nature and function, all these aspects besides being useful in the identification of the fungus also yield clue about the mode of dissemination of the disease and perpetuation of its inoculum. All these data may also be utilized in connection with recommending control measures of the disease.