Plant Pathology: Definition, Objective, Causes of Disease and Important Terminologies

 

Phytopathology or Plant Pathology and Its Importance

Plant protection has been accepted as broad area of research and technology at the national level by the Indian Council Agricultural Research, New Delhi; and Plant Pathology is an important discipline of Plant Protection.

 

Plant Pathology - Definition

“Plant Pathology, also known as Phytopathology is a branch of agricultural, biological or botanical science which deals with the study of diseases in plants - their causes, etiology, epidemiology, resulting losses and management.”

 

 

Father of Modern Plant Pathology: Anton de Bary

In 1863, he studied the epidemics of late blight of potato and renamed as casual organism as Phytophthora infestans.

 

Plant disease:- Plant Disease as abnormal changes in physiological processes which disturb the normal activity of plant organs.

 

“Plant disease is the interaction of host, pathogen and weather. It is a physiological process that affects some or all plant functions.”

For example:-

• Infection of roots may cause roots to rot and make them unable to absorb water and nutrients from the soil;
• Infection of xylem vessels interferes with the translocation of water and minerals to the crown of the plant;
• Infection of the foliage (leaf spots, blights, rusts, mildews etc.), interferes with photosynthesis.

 

Objectives of Plant Pathology

• To study living, non-living and environmental causes of diseases or disorders of the plants.
• To study the mechanism of plant disease development.
• To study interaction between host/susceptible and the pathogens.
• To develop systems of management of plant diseases and reducing losses caused by them.

Causes of Plant Diseases

Plant diseases are caused by a variety of pathogens.

The word pathogen can be broadly defined as any agent or factor that incites “pathos” or disease in an organism. Thus in strict sense, the pathogens do not necessarily belong to living or animate groups.

 

(A). Abiotic (Inanimate) Factors

They include mainly the deficiency or excess of nutrients, light, moisture, aeration, abnormality in soil condition, atmospheric impunities etc.

Examples are:-

• Black tip of mango (due to SO2 toxicity)
• Khaira disease of rice (due to Zn deficiency)
• Whiptail of cauliflower (due to Mo deficiency)
• Hollow and black heart of potato (due to excessive accumulations of CO2 in storage)
• Bitter pit of apple (due to Ca deficiency).

 

(B). Mesobiotic Factors

These are the disease incitants which are neither living nor non-living. They are considered to be on the threshold of life.

Examples are:-

(1). Viruses:- They are infections agents made up of one type of nucleic acid (RNA or DNA) enclosed in a protein coat. Examples of viral diseases of plants are:-

• Potato leaf roll
• Leaf curl of tomato and chillies
• Mosaic disease of many plants.

 

(2). Viroids:- They are naked, infectious strands of nucleic acid. They cause diseases like Potato Spindle Tuber, Citrus Exocortis, Chrysanthemum Stunt, Cadang Cadang of Coconut Palm, Star Crack of Apple etc.

 

(C). Biotic Factors

Important Phytopathogenic organisms which are completely living an cause disease in plants.

1. Fungi:-  Fungi are eukaryotic, spore bearing, achlorophyllous organisms that generally reproduce sexually and asexually and whose filamentous, branched somatic structures are typically surrounded by cell walls consisting chitin or cellulose or both with many organic molecules.

2. Bacteria:- Bacteria are extremely minute, rigid, essentially unicellular organisms free of true chlorophyll and generally devoid of any photosynthetic pigment, most commonly multiplying asexually by simple transverse fission, the resulting cell, being of equal or nearly equal in size.

3. Fastidious vascular bacteria (RLO‘s):- Fastidious vascular bacteria are similar to bacteria in most respects but are obligate parasites or can not be grown on routene bacteriological media.

4. Mollicutes (Phytoplasma and Spiroplasma):-

  (a). Phytoplasma:- Phytoplasmas are pleomorphic, wall less prokaryotic micro organisms, that can infect plants and can not yet to be grown in culture.

  (b). Spiroplasma:- Spiroplasmas are helical, wall less prokaryotic micro organisms that are present in phloem of diseased plants, often helical in culture and are thought to be a kind of mycoplasma and can be cultured on artificial medium.

5. Algae:- Algae are eukaryotic,photosynthetic, uni or multicellular organisms, containing chlorophyll and a few algae mainly green algae cause plant diseases.

6. Flagellated Protozoans:- Protozoa are microscopic, non photosynthetic, eukaryotic, flagellate motile, single celled organisms which causes disease.

7. Phanerogamic Plants:- Some plants which affects physiological and metabolic processes of other plants are called Phanerogamic Plants. Examples: Cuscuta, Loranthus, Striga etc.

8. Nematodes

 

Different Terminologies related to Plant Pathology

 

1. Pathogen: A pathogen is an agent that incites (ailment,suffering) disease.
2. Symptoms: Any detectable change in color, shape and/or functions of the plant in response to a pathogen or disease-causing agent is a symptom.
3. Signs: These are physical evidence of the pathogen, for example, fungal fruiting bodies, bacterial ooze, or nematode cysts. Signs can also help with plant disease identification.
4. Disease cycle: The chain of events in disease development is known as the disease cycle.
5. Disease: According to Horsfall and Diamond (1959), disease may be defined as a malfunctioning process that is caused by continuous irritation by a pathogen and/or environmental factor resulting in some suffering producing symptoms.
6. Disorder: The diseases caused by the deficiency of nutrients or unfavourable environmental are sometimes termed as disorders or physiological disorders.
7. Pathogen: It is the agent responsible for inciting “pathos” i.e. ailment or damage.
8. Parasite: These are the organisms which derive the food materials needed for their growth from other living organism (the host). All the pathogens are parasites but all the parasites are not pathogens. As some of the parasites live on their hosts without causing any damage to them as symbiotic relationships, e.g., Rhizobium bacterium in legume roots, mycorrhizae and lichens.
9. Biotrophs: are the organisms which regardless of the ease with which they can be cultivated on artificial media obtain their food from living tissues only in nature in which they complete their life cycle). They were earlier also called obligate parasites, e.g., rusts, smuts, powdery mildews etc.
10.  Saprophytes/saprobes: are the organisms which derive their nutrition from the dead organic matter. Some parasites and saprophytes may have the faculty or (ability) to change their mode of nutrition.
11. Facultative saprophytes: are ordinarily parasites which can grow and reproduce on dead organic matter under certain circumstances. They are also called hemibiotrophs which attack the living tissues in such a way as biotrophs but continue to grow and reproduce after the tissues is dead.
12. Necrotroph: A parasite is called necrotroph when it kills the host tissue in advance of penetration and then lives saprophytically, e.g. Sclerotium rolfsii and Pythium species. Similar to necrotrophs are facultative parasites which live as saprophytes but under favourable conditions they can attack living plants and become parasites. The necrotrophs are also known as perthotrophs or perthophytes.
13. Pathogenicity: is the ability of a pathogen to cause disease under a given set of environmental conditions. Whereas, pathogenesis is the chain of events that leads to development of a disease in the host.
14. Parasitism: is a phenomenon by which a plant parasite becomes intimately associated with the plant; it draws nutrition and multiplies and grows at the expense of the plant host.
15. Virulence: is a measure or degree of pathogenicity of an isolate or race of the pathogen. The term aggressiveness is often used to describe the capacity of a pathogen to invade and grow  in the host plant and to reproduce on or in it. This term like virulence is used as measure of pathogenicity.
16. Immunity: of a plant against a disease is absolute quality. It denotes the freedom of plant from disease, when the pathogen cannot establish parasitic relationship with the host. High resistance and low susceptibility approach immunity.
17. Disease resistance: is the ability of an organism to overcome completely or in some degree the effect of a pathogen or other damaging factor; whereas susceptibility in the inability of the plant to resist the effect of the pathogen or other damaging factor.
18. Hypersensitivity: is the extreme degree of susceptibility in which there is rapid death of the cells in the vicinity of the invading pathogen. It halts the further progress of the pathogen. Thus, hypersensitivity is a sign of very high resistance approaching immunity.
19. Infection: is the establishment of the parasitic relationship between the pathogen and host following entry or penetration. 
20. Incubation period: is the time elapsing between penetration and completion of infection i.e. development of the disease symptoms.
21. Invasion and Colonization: is the growth and multiplication of the pathogen through the tissue of the host varying extent.

 

Thank You

Vikas Kashyap

Join Us

Telegram