Biology Notes on Plant Pathology

The below mentioned article provides notes on plant pathology.

Plant pathology is a branch of botany. It deals with the diseases of plants, helps to main­tain good health of plants, and also take proper steps to increase the productivity. Plant diseases caused disasters like famine in Ireland (1845- 1846) and Bengal (1943) by late blight disease of potato (C.O. Phytophthora infestans) and brown spot of rice (C. O. Helminthosporium oryzae) respectively.

To overcome such problems, it is essential to carry out research on development of disease-tolerant varieties or on production of more effective pesticides in comparatively low cost or in inducing plant’s own defense mecha­nism. Thus, plant pathologists are the plant doc­tors, responsible to maintain the good health of plants.

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.

With the same kind of mistake of others, the activity of your elder brother will be different. He can slap twice the younger bro­ther; he may tell mother what you are doing; but to father, he has nothing to say. So in the above cases your father is resistant, mother is moderate resistant and your younger brother is susceptible to him.

If you consider yourself (Student + enjoying fellowship) as a moderate virulent pathogen, you have nothing to do with both your father and mother, but you can slap once your younger brother. But if you slap him twice then he may report to your father and you may be punished.

Lastly, the avirulent pathogens are not able to produce any disease in any kind of host, like a jobless inefficient uncle in your family who is a human being, yet nobody cares of him (Fig. 5.1).

An agency which incites (induces) a disease is the incitent. This includes both inanimate (unfavourable environmental conditions and similar situation) i.e., non-parasitic or non­infectious agencies and animate (fungi, bacteria, angiospermic plants etc.) i.e., infectious parasitic agencies. Similarly, an organism which generates a disease is known as pathogen or causal orga­nism.

For the development of disease on a host, in addition to pathogen, other environmental factors such as atmospheric humidity, tempe­rature, wind, light, etc., are essential. Pathogen along with other environmental factors are collectively called causal complex (causal com­plex = causal organism + other environmental factors).

Considering a case in your family — father (host) brought raw materials from the market and mother (pathogen) is ready to prepare the food, but gas and/or water is not available; so no food can be prepared.

Casual organisms are generally parasites. Commonly the term parasite is applied to an organism which depends partly or wholly upon the living tissue or cell. On the other hand, a saprophyte is an organism which lives on organic materials or dead organic matters. In some cases, the organisms are parasites but do not play any role or may participate in disease development.

Conversely, the by-products of a parasite are the major causal factors in disease development, so that parasite behaves indirectly like a pathogen. But the term pathogen is practically used for a living entity. The efficiency of a pathogen to produce disease is the pathogenecity and patho­genesis is the process or chain of events of disease development.

For the development of disease caused by fungi, five steps are followed starting from inocu­lum arrival to symptom expression. These are i. Germination ii. Penetration iii. Infection iv. Incubation period, and v. Symptom.

In bacte­ria it consists of four steps except the first one i.e., germination. The development of mycelium by the inoculum like spore is called germination. The initial invasion of the pathogen inside the host tissue is called penetration and its establish­ment inside the host is called infection. The infection may be visible externally on the host surface, called visible infection.

On the other hand, when a pathogen has established itself inside the host tissue but is not visible from out­side, it is invisible infection. After that, the pathogen grows (fungi) or multiplies (bacteria) inside the host tissue and ultimately the manifes­tation of the disease is expressed. The manifesta­tion of disease is called symptom. The interval of time between infection and symptom expression is called incubation period.

The above steps of development of disease can be compared with your steps to come to your college.

When you are coming out from your house, consider it as the germination; your entry inside the bus is penetration and then you occupy a seat in a bus or any other vehicle, is the infection; then the bus or other vehicle is run­ning along with you, is the incubation period and finally you reach the college, and show your performance or efficiency — the symptom.

The manifestation of the disease is called symptom. The symptom may be visible externally, called external symptom, e.g., pinhead to eye- shaped, brown coloured spots on leaf, coleoptile, seeds etc. on rice indicate brown spot disease of rice by Helminthosporium oryzae or it may be internally invisible, called internal symptom, e.g., browning of woody tissue in wilt of pigeon pea, by Fusarium oxysporum f. sp. udum.

When a number of symptoms are available for a particular disease, they are collectively called syndrome, e.g., wilt of pigeon pea, where symptoms like i. lodging of the leaves (external), ii. browning of the woody tissue, and iii. presence of fungal mycelium inside the host tissue (internal) are available.

If the disease is identified by observing (with naked eye) the pathogen externally on the host surface, it is called sign, e.g., powdery mildew of cucurbits by Erysiphe cichoracearum, where disease is identified by seeing the fungal spores on the host surface.