PLANT PATHOLOGY- AN OVERVIEW
The term Plant Pathology or Phytopathology is derived from three Greek words.
Phyton= plant, pathos= suffering/disease/ailments and logus= study.
Definition
Plant Pathology is the branch of science deals with study of causes, etiology, resulting losses and management of plant disease.
What is Healthy Plant?
It refers to ability of plants to carry out normal physiological functions to the best of its genetic potential. Normal physiological functions are:
- Cell division, differentiation and development.
- Absorption of water and minerals from the soil and their translocation.
- Photosynthesis and translocation of photosynthates and utilization and storage of photosynthates.
- Metabolism of metabolites and synthates.
- Storage of reserves for overwintering or reproduction.
What is Plant Disease?
It is malfunctioning alteration in plant body caused by continuous irritation by some agent which results in plant suffering and/or produce symptoms.
The deviation or alteration that results in disease are-
- Cells and tissues become affected which determine the type of physiological functions results in disruptive functions. For example, infection of roots may cause root rot make them unable to absorb water and minerals from the soil.
- Infection of xylem vessels interferes with the translocation of water and minerals to the crown of the plant.
- Infection of foliage like leaf spots, blights, rusts, mildews, mosaics etc. interferes with photosynthesis.
What is Plant pathogen?
Any agent or entity that can induce disease in plants is called pathogen, e.g., fungi, bacteria, viruses, viroids, mollicutes, phytoplasma, spiroplasma etc.
How pathogen affects the Plant?
- By utilizing host cell contents.
- By killing hosts or by interfering with its metabolic processes through their enzymes, toxins etc.
- By weakening the hosts due to continuous loss of the nutrients.
- By interfering with the translocation of the food, minerals and water.
- Thus, they can suppress the chlorophyll content, can reduce the leaf area, they can curb the movement of solutes and water through the stems. They sometimes reduce the water absorbing capacity of roots. They suppress the translocation of photosynthates away from the leaves and they sometimes promote wasteful use of the products of photosynthesis in the formation of galls.
Estimated annual losses worldwide
Disease- 14.1%
Insects- 10.2%
Weeds- 12.2%
Total average losses- 36.5%
Important work/event/discovery in Plant Pathology
- Theophrastus, a Greek philosopher wrote book “Enquiry into plants” regarding plant disease.
- Dutch worker Leeuwenhoek invented the microscope in 1675 and with the help of this he observed and described bacteria in 1683 which will become milestone in Biology.
- Micheli, an Italian botanist (1729) was the first to study fungi and observe fungal spores.
- Needham (1743) reported plant parasitic nematode in wheat gall. In 1875, Berkeby and Schacht discovered the root knot nematode and cyst nematode of beet.
- Prevost in 1807 proved that wheat bunt disease was caused by a fungus and also discovered life-cycle of bunt fungus.
- De Barry confirmed the findings of Prevost. He discovered the heteroecious nature of the rust fungus. He also described the life-cycle of Downey mildew fungus and found role of enzymes in host-pathogen relationship. He is known as father of Mycology.
- Julius Koon wrote the first book on plant pathology (1858).
- Brefeld, a German scientist developed techniques of Artificial culture of microorganisms.
- Downey mildew of grapes was introduced into Europe from America in 1878. Milardet (France) discovered Bordeaux mixture for the control of the same disease.
- Anthrax disease in cattle caused by bacteria was reported by Louis Pasteur and Robert Koch (1876). Later in 1878, fire blight in apple and pear was discovered by J. Burill.
- Erikson, a Swedish scientist reported the existence of physiological races in Rust fungus in 1894.
- Heterothallism in fungi was discovered by Blakeslee in 1904. He informed that in the life-cycle of fungus dissimilar nuclei participate. Coming together of dissimilar nuclei in a single fungal cell is called Heterokaryosis was discovered by Burgeff. Development of physiological races was result of Heterokaryosis.
- Flor (1955) proposed gene to gene hypothesis to explain disease resistance and susceptibility.
- Tobacco mosaic, a viral disease was discovered by Adolf Meyer in 1836. However, Beijernick in 1898 proved that tobacco mosaic disease is not caused by a microorganism rather caused by Virus.
- Knoll & Ruska discovered the electron microscope in 1932 that led to study the shape and size of the virus particles.
- O. Diener coined the term Viroid. Viroid is devoid of any protein coat and its RNA has a high molecular weight. Viroid has nucleic acid only. Plant viroid = RNA only. Potato spindle tuber was the first disease reported to have been caused by viroid.
- Virus consists of Nucleic acid (DNA or RNA) + Protein (outer cover). Animal virus = DNA + Protein while plant virus = RNA + Protein.
- Virus, viroid and plasmids all contain nucleic acid and lack their own metabolic potential.
- Mycoplasma are devoid of cell wall and cytoplasm is enveloped by a lipo-protein plasma membrane. Tetracycline is used to control mycoplasma. Most of the yellow disease of plants are caused by mycoplasma.
- In India, K.R. Kiritikar was the first who collected many fungi and identify them.
- J. Butler wrote the book “Fungi and disease in plants”. He is called as father of modern plant pathology in India.
- B. Mundkur developed resistant varieties to control cotton wilt disease. He also identifies and classify the smut fungi found in India. He established the Indian phytopathological institute.
- Karan Chan Mehta discovered disease cycle of cereal rust in India.
- Jaichand Luthra and Sattar developed solar treatment to control loose smut disease in wheat.
- Thirumalachar developed antibiotics like aureofungin and streptocyclin.
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