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Except a few cases, unlike the animal doctors, they try to look after the plant population as a whole —taking care mostly for their protection rather than cure.
During growth, plants are subjected to environmental conditions which may or may not be favourable. In or with the onset of one or more unfavourable factors of the environment, certain structural and or physiological changes may take place.
On the other hand, in contact with certain organisms, the normal metabolism and/or some structure may alter and ultimately the normal growth of the plant is affected, thus the plants with unusual growth are called diseased plants. Several definitions of diseases of plants have been proposed by different scientists from time to time.
These are:
1. Anon (1950) defined disease as “harmful deviation from normal functioning of physiological process”.
2. Stakman and Harrar (1957) defined plant disease as “a physiological disorder or structural abnormality that is deleterious to the plant or to any of its parts or products or that reduces their economic value”.
3. Agrios (1997) defined plant disease as “a series of invisible and visible responses of plant cells and tissues to a pathogenic microorganism or environmental factors that result in adverse changes in the form, function, or integrity of the plant and may lead to partial impairment or death of the plant or its parts”.
4. According to the modern concept, disease is an interaction among the host, parasite, and environment.
Plant pathology (Gr. pathos — suffering; logos— knowledge) is a branch of botany which deals with the study of the nature, development and control of plant diseases or the study of the suffering plants.
It is very difficult to distinguish between a healthy and a diseased plant with some abnormality. The disease of a plant may be caused by environmental factor or factors, called nonparasitic disease (Freezing injury of potato, water core of apple – due to high temperature) or it may be caused by microorganisms such as fungi (Helminthosporium oryzae — brown spot of rice), bacteria (Xanthomonas campestris pv. oryzae- Bacterial leaf blight of rice), plasmodiophorales (Plasmodiophora brassicae — club root of crucifer), nematodes (Heterodera rostochiensis — golden-nematode disease of potato) etc. — called parasitic disease.
For a parasitic disease two organisms are required, one is host, on which disease takes place and the other one is pathogen, which causes disease.
Roughly, we can classify the hosts into three categories:
i. Resistant,
ii. Moderate resistant, and
iii. Susceptible.
Normally a disease takes place in a susceptible host with maximum intensity. Resistant hosts are those which are able to withstand, resist or overcome the attack of a pathogen completely or at a maximum degree. Moderate resistant hosts are able to resist the pathogen attack at a moderate level and, lastly, the susceptible plants, which are not able to resist the attack of the pathogen, produce maximum disease.
The tendency of plants to escape from disease is called klenducity. In some cases, a variety of a plant may escape from disease, not for their resistance but for early- maturity, thus they escape from natural disease incidence, called disease escape and the variety is called disease escaping variety.
Similarly, one can classify the pathogen into three categories:
i. Virulent,
ii. Moderate virulent, and
iii. Avirulent.
The efficiency of a pathogen to produce disease is called virulence and the pathogen with maximum efficiency is called virulent pathogen. Moderate virulent pathogen has the efficiency at moderate level and the avirulent pathogen is inefficient in the production of disease.
The above situation can be compared with the activity of you and your elder brother in your family to other family members. Consider your elder brother (who is in service) as a virulent pathogen and father (resistant), mother (moderate resistant) and younger brother (susceptible) as different hosts.
