Definition:
Plant pathology or phytopathology consists of three Greek words (1) phyton = plant, (2) pathos = ailments or diseases and (3) logus = discourse or knowledge. Thus, the term plant pathology or phytopathology means knowledge of plant disease.
Image Source: plantphysiol.org
Scope of Plant Pathology:
Scope and responsibilities of plant pathology is unlimited. Its ultimate goal is to prevent and control plant diseases of economic importance. Responsibilities of the science of plant pathology may be summarized as under;
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1. Survey of fields, orchards and areas in order to find out prevalence of diseases and their incidence.
2. Recording new diseases of economic importance if any, with their identification and extent of incidence.
3. Assessment of losses caused by different diseases of economic importance.
4. Study of etiology, symptoms, predisposing factors and recurrence of such diseases.
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5. Find out suitable and economic methods of management of economically important plant diseases.
6. Assisting in breeding of disease resistant varieties.
7. To train the extension workers and subject matter specialists in order to bridge the gap between pathologists and farmers for better crop production.
Importance of Plant Pathology in Agriculture:
1. Plant pathology has advanced the knowledge to protect the crop from losses due to diseases.
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2. The science of plant pathology has contributed disease free certified seed production.
3. Most of the diseases with known disease cycle can now be avoided by the modification of cultural practices.
4. With the knowledge of mode of disease spread, many diseases of economic importance can now be checked, minimized or controlled.
5. Crop improvement and varietal resistance have been achieved against many diseases through the joint effort of breeder and plant pathologist.
6. Plant pathology has made possible to restrict the spread of plant diseases from one place to other and one country to other through suitable measures and quarantine legislation.
7. With the knowledge of plant pathology various prophylactic measures are adopted for successful management of diseases. These measures are seed treatment in seed borne diseases soil treatment and crop rotations.
8. Diseases can be avoided in cold storage by the application of plant pathological measures as per recommendations made for different diseases for protection of fruits and vegetables in storage.
9. Plant pathology has made possible to recognise, discard or utilize toxic substances by producing toxic substances or by competition or by parasitism. Organism exerting such lethal or damaging effect on the other is called antagonist.
Disease Triangle:
Plant disease are caused by the interaction among susceptible host x abundance and virulence of pathogen x favourable environmental conditions for desirable time. These three factors constitute disease triangle, which are essential for disease incidence and its development. Each side of the triangle represents one of the three components. The size of each side is proportional to the sum total of the characteristics of the each component favouring disease.
Classification of Plant Diseases:
Plant disease have been classified into three groups on the basis of the occurrence of the pathogens.
(i) Endemic diseases:
Endemic diseases is one which is more or less constantly present in an area or locality. Such diseases are observed year to year in moderate to severe form. The causal agent of the diseases remains well established in the field or in the locality. By virtue of its ability of survival in weed, the disease is seen almost every year. Wilt of pigeon pea, red rot of sugar cane are observed as endemic diseases in Eastern U.P., Onion smut in Andhra Pradesh and flag smut of wheat is Punjab are endemic diseases observed over there, every year in moderate or severe form depending upon presence or predisposing factors.
(ii) Epidemic or Epiphytotic diseases:
Word epidemic is popularly used for animal and human diseases occurring in severe form. Therefore, in case of plant diseases the word epiphytotic should be used which means the occurrence of a disease widely and severely but periodically. The casual agent of such diseases may or may not remain present constantly in the locality. It is because the environmental conditions which become favourable occasionally.
A disease may be endemic or sporadic but may take form of epiphytotic at occasions. Late blight of potato-occurred in epiphytotic form in 1913 in the district of Rangpur now in Pakistan.
(iii) Sporadic diseases:
Sporadic diseases occur at very irregular intervals and locations. Green ear disease of bajra and black rust of wheat may be cited as examples of sporadic diseases which may appear in a locality or a district after one or more than one year or every year in sporadic form.
(iv) Pandemic diseases:
Diseases which occur all over the world and cause mass mortality, are called pandemic diseases. Example – late blight of potato.
(v) Classification according to Causal Agents:
According to causal agents the diseases may be classified into two main groups:
1. Parasitic or biotic diseases and
2. Non- parasitic disease
(a) Parasitic or infectious diseases:
These diseases are caused by fungi, bacteria, viruses, nematodes and phanerogamic plant parasites. These diseases are usually contagious in nature and there is easy and efficient dispersal of the pathogens causing these diseases.
(b) Non-parasitic or non-infectious diseases:
These diseases are not infected by biotic pathogens like fungi, bacteria, virus and nematodes etc. but are infected by the nutritional deficiencies or excess, by low and high temperature, by low and extreme light and by air pollutants etc. These diseases are not infectious in nature.
(vi) Classification According to nature of Diseases:
Diseases classified according to the nature may be of following types:
(a) Seed borne diseases:
In such diseases the inoculum is found associated with the seed internally or externally and are called as internally seed borne or externally seed borne diseases.
(b) Soil borne diseases:
These diseases are incited by the inoculum present in the soil in the form of oospores, sclerotia and mycelia etc.
(c) Air borne diseases:
These diseases are incited by the inoculum present in the air in the form of conidia, chlamydospores and sporangia etc.
(vii) Classification According to Symptoms:
Symptoms of the diseases may also be considered as a base of classification and according to the symptoms, the diseases, may be grouped as rust diseases, leaf spot diseases, smut diseases, downy mildew diseases etc.
(viii) Classification According to the Host:
According to the host diseases may be grouped as cereal diseases and plantation diseases etc.
(ix) Classification According to the Nature of Infection:
According to the nature of infections, the diseases may be classified into two groups namely, (1) the systemic diseases such as green ear diseases of bajra and loose smut of wheat and, (2) Localized diseases such as leaf spot diseases.