Objectives, Scope, Concept and Causes of Plant Diseases

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Objectives of Plant Pathology

Plant Pathology (Phytopathology) deals with the cause, etiology, resulting losses and control or management of the plant diseases. The objectives of the Plant Pathology are the study on:  i. the living entities that cause diseases in plants; ii. the non-living entities and the environmental conditions that cause disorders in plants; iii. the mechanisms by which the disease causing agents produce diseases; iv. the interactions between the disease causing agents and host plant in relation to overall environment; and  v. the method of preventing or management the diseases and reducing the losses/damages caused by diseases. 

Scope of Plant Pathology

Plant pathology comprises with the basic knowledge and technologies of Botany, Plant Anatomy, Plant Physiology, Mycology, Bacteriology, Virology, Nematology, Genetics, Molecular Biology, Genetic Engineering, Biochemistry, Horticulture, Tissue Culture, Soil Science, Forestry, Physics, Chemistry, Meteorology, Statistics and many other branches of applied science.

  

Concept of Plant Disease

The normal physiological functions of plants are disturbed when they are affected by pathogenic living organisms or by some environmental factors. Initially plants react to the disease causing agents, particularly in the site of infection. 

Later, the reaction becomes more widespread and histological changes take place. Such changes are expressed as different types of symptoms of the disease which can be visualized macroscopically. As a result of the disease, plant growth in reduced, deformed or even the plant dies. 

When a plant is suffering, we call it diseased, i.e. it is at ‘dis-ease’. Disease is a condition that occurs in consequence of abnormal changes in the form, physiology, integrity or behaviour of the plant. 

According to American Phytopathological Society (Phytopathology 30:361-368, 1940), disease is a deviation from normal functioning of physiological processes of sufficient duration or intensity to cause disturbance or cessation of vital activities. 

The British Mycological Society (Trans. Brit. Mycol. Soc. 33:154-160, 1950) defined the disease as a harmful deviation from the normal functioning of process. Recently, Encyclopedia Britannica (2002) forwarded a simplified definition of plant disease. A plant is diseased when it is continuously disturbed by some causal agent that results in abnormal physiological process that disrupts the plants normal structure, growth, function or other activities. This interference with one or more plant’s essential physiological or biochemical systems elicites characteristic pathological conditions or symptoms.

 

Causes of Plant Diseases 

Plant diseases are caused by pathogens. Hence a pathogen is always associated with a disease. In other way, disease is a symptom caused by the invasion of a pathogen that is able to survive, perpetuate and spread. Further, the word “pathogen” can be broadly defined as any agent or factor that incites 'pathos or disease in an organism or host. In strict sense, the causes of plant diseases are grouped under following categories:

  

1. Animate or biotic causes: Pathogens of living nature are categorized into the following groups.

(i) Fungi   

(v) Algae

(ii) Bacteria

(vi) Phanerogams

(iii) Phytoplasma  

(vii) Protozoa   

(iv) Rickettsia-like organisms 

(viii) Nematodes

2. Mesobiotic causes :

These disease incitants are neither living or non-living, e.g.  (i) Viruses  (ii) Viroides 

3. Inanimate or abiotic causes: In true sense these factors cause damages (any reduction in the quality or quantity of yield or loss of revenue) to the plants rather than causing disease. The causes are:

  

(i) Deficiencies or excess of nutrients(e.g.‘Khaira’ disease of rice due to Zn deficiency)

(ii) Light

(iii) Moisture

(iv) Temperature  

(v) Air pollutants (e.g. black tip of mango) 

(vi) Lack of oxygen (e.g. hollow and black heart of potato) 

(vii)  Toxicity of pesticides 

(viii) Improper cultural practices 

(ix) Abnormality in soil conditions (acidity, alkalinity)