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Introduction to Plant Pathology
Importance of the Plant Diseases: Globally, enormous losses of the crops are caused by the plant diseases. The loss can occur from the time of seed sowing in the field to harvesting and storage. Important historical evidences of plant disease epidemics are Irish Famine due to late blight of potato (Ireland, 1845), Bengal famine due to brown spot of rice (India, 1942) and Coffee rust (Sri Lanka, 1967). Such epidemics had left their effect on the economy of the affected countries. Plant Pathology or Phytopathology: Consist of three Greek words
- Phyton means plant, - Pathos means suffering / ailments or diseases and - Logus means knowledge /study/ science. Plant Pathology or Phytopathology is that branch of agricultural or biological science which deals with the study of plant diseases, their cause, etiology, resulting losses and management of plant diseases. OBJECTIVES OF PLANT PATHOLOGY: The science of plant pathology has four main objectives: 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. 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 deviation from normal functioning of physiological processes due to continuous irritation caused by some biotic agents or disease is a condition that occurs in consequence of abnormal changes in the form, physiology, integrity or behaviour of the plant or disease is a deviation from normal functioning of physiological processes of sufficient duration or intensity to cause disturbance or cessation of vital activities (APS, 1940). The British Mycological Society in 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. 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. Importance of the Plant Diseases: Plant diseases have been associated with many important events in the history of mankind on earth**. World scenario
- Irish Potato Famine** : Great Irish Famine, or Famine of 1845– 49 , famine that occurred in Ireland in 184 5 – 49 when the potato crop failed in successive years. The crop failures were caused by late blight, a disease that destroys both the leaves and the edible roots, or tubers, of the potato plant. The causative agent of late blight is Phytophthora infestans. The Irish famine was the worst to occur in Europe in the 19th century ( Fig.1 ). The Irish Famine of 1846-50 took as many as one million lives from hunger and disease, and changed the social and cultural structure of Ireland in profound ways. The Famine also spurred new waves of immigration, thus shaping the histories of the United States and Britain as well. The combined forces of famine, disease and emigration depopulated the island; Ireland's population dropped from 8 million before the Famine to 5 million years after. If Irish nationalism was dormant for the first half of the nineteenth-century, the Famine convinced Irish citizens and Irish-Americans of the urgent need for political change. The Famine also changed centuries-old agricultural practices, hastening the end of the division of family estates into tiny lots capable of sustaining life only with a potato crop. 2) Coffee rust in Sri Lanka (1869) took less than three decades to anhilate the entire coffee production of the island. 3) Downy mildew occurred in epiphytotic way during 1878-1882 in France. 4) Powdery mildew of grape - France (1851 and 1878). 5) Panama wilt of banana damaged the crop during 1910 in Panama and Surinam. 6) Sigatoka leaf spot of banana damaged the crop during 1930 in Central and South America. 7) Bunchy top of banana durin 1940 in Sri Lanka Indian scenario
- Bengal famine: In 1943, Bengal had to face a serious famine. One of the reasons to which this famine has been attributed is the loss of yield of rice due to attack of Helminthosporium oryzae
Country France (Europe) Period 1878 Reason/ Cause Downy mildew caused by Plasmopara viticola Consequences 1. Reduced grape yields and quality greatly
- Killed young vines in many vineyreds
- Wine industry affected and Economy ruined Scientific Advancement Discovery of **Bordeaux mixture (1885) by P.A.Millardet
- BENGAL FAMINE** Country India Location Bengal state Period 1943 Cause Brown spot of paddy Pathogen Drechslera oryzae Impact 1. Shortage of food
- Raise in the price of rice
- Death of 2 million people due to hunger
- Migration of people to other states.
- Change in the Agriculture policy. Significance of Plant Disease/ Plant Pathogens Of the 30,000 plant diseases recorded from different countries about 5,000 diseases are present in India. It is estimated that the total annual production for all agricultural crops worldwide is about 12000 - 13 000 billion $. Of this, about 500 billion $ worth of produce is lost annually to diseases, insects and weeds. **Following are the significance of Plant diseases
- Famine:** Irish famine – Late blight of potato, Bengal famine – Brown spot of paddy 2. LOSSES a) Quantity (Yield loss): During Wheat rust epidemic in Bihar (1956-57) yield reduced from 900kg/ha to 50 kg/ha. b) Crop quality losses: Example: Citrus canker, Chiku rough storage, Apple scab, Groundnut – aflatoxin, Wheat – bunt, ear cockle. 3. Human/ animal hazard /food poisoning: Consumption of Ergot infected rye – causes gangrene, loss of fingers, limbs and death. (Known as Saint Antony's fire). Death of one Lakh turkeys in England due to consumption of Peanut meal contaminated with Aflatoxin. Aflatoxin is immune suppressive and causes Liver cancer in Human beings. 4. Cropping pattern changed: In Srilanka Coffee was replaced by Tea plantation due to Coffee Rust (1867). 5. Limit the cultivation of variety: Due to Downy mildew of Bajra, cultivation of GJ 104 variety was discouraged in Gujarat. Due to Wilt & red rot cultivation of co. 671, co. 8603 sugarcane variety was discouraged in south Gujarat.
6. Diet change/ food habit change: In Sri Lanka drinking habit changed from coffee to tea due to coffee rust. In Southern U.S.A food habit changed i.e. from wheat to corn due to wheat rust 7. Agro- industries affected: Due to Wilt and red rot diseases Sugar Industries in Gujarat affected. Wine industry in France was severly affected due to Downy mildew disease. 8. People’s migration: Due to Irish Famine in more than 1.6 million people migrated from Ireland to North American continent. 9. Sociological effect :. Increase in Poverty due to close of Agro industries 10. High/rise market price: Shortage of food due to heavy crop loss leads to Rise in price of Agril. Commodities 11. High cost of production: Extra Expenditure to be incurred on pesticides, labour etc. for control of diseases 12. Political effect: Due to Irish famine, the free trade was allowed between England and Ireland 13. Destroy the beauty of environment: By damaging plants around houses, along street, in gardens & in forest. 14. Ecological implications: Environmental pollution due to fungicidal use or due to disease. **TERMINOLOGIES RELATED TO PLANT PATHOLOGY
- Host :** An organism harbouring a parasite or pathogen. 2. Parasite: An organism living on or in another living organism (host) and obtaining its food from the latter (host). 3. Ectoparasite : a parasite that lives and feeds from the exterior of its host's cells or tissues. 4. Endoparasite : a parasitic organism that lives and feeds within the cells or tissues of its host. 5. Pathogen: An entity, usually a micro-organism, that can incite disease. 6. Abiotic : pertaining to physical and inorganic components. For example, diseases/disorders in plants can be caused by abiotic factors such as extremes of heat, light, moisture, lack of nutrients etc. 7. Biotic : pertaining to life and therefore living organisms. For example, plant diseases of a biotic origin are caused by living organisms such as fungi, bacteria, virus, viroids, algae, insects, nematodes, etc. 8. Disease: Any malfunctioning of host cells and tissues that results from continuous irritation by a pathogenic agent or environmental factor and leads to development of symptoms. 9. Disorder: Non-infectious plant diseases due to abiotic causes such as adverse soil and environmental conditions are termed disorders. 10. Saprophyte: An organism that derive their nutrition from dead organic matter by absorbing soluble organic compounds. Example: Cellulomonas 11. Pathogenicity: The capacity of a pathogen to cause disease. 12. Symptom: The external or internal reactions or alterations of a plant as a result of a disease.
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: ‘Khaira’ disease of rice due to Zn deficiency, Chlorosis in Groundnut due to Ca deficiency. (ii) Light: Damage to potato plants due to lightening (iii) Moisture: High moisture cause ‘edema’ in cabbage, Damping off in vegetables. (iv) Temperature: High temperature cause tip burn in betel vine, Dry root rot, charcoal rot (v) Air pollutants: Black tip of mango, apple scald (vi) Lack of oxygen: Hollow and black heart of potato (vii) Mineral toxicity: Leaf crinkle of cotton due to manganese toxicity, Akiochi in Paddy due to H 2 S (Hydrogen sulphide) toxicity. (viii)Improper cultural practices or Iatrogenic diseases: Zineb controls downy mildew but enhance the incidence of powdery mildew of grapes. Classification of Plant Disease To facilitate the study of plant diseases they are needed to be grouped in some orderly fashion. Plant diseases can be grouped in various ways based on the symptoms or signs (rust, smut, blight etc.), nature of infection (systemic or localized), habitat of the pathogens, mode of perpetuation and spread (soil-, seed- and air-borne etc.), affected parts of the host (aerial, root disease etc.), types of the plants (cereals, pulses, oilseed, ornamental, vegetable, forest diseases etc.). But the most useful classification has been made based on the type of pathogens that cause plant diseases. Since this type of classification indicates not only the cause of the disease, but also the knowledge and information that suggest the probable development and spread of disease along with their possible control measures. I. The classification based on type of pathogen is as follows: II. The classification based on occurrence of disease is as follows: Based on type of pathogen Infectious plant diseases Disease caused by pathogenic organisms. Non-infectious diseases (Disorder) Inanimate or abiotic Based on occurrence of disease Epidemic diseases Occurs periodically over a widespread area. eg. Wheat stem rust Endemic diseases Remain constantly present in moderate to severe form in confined area eg. Wart disease of potato Sporadic diseases Occurs at irregular interval over limited area or locality eg. Grain smut of jowar Pandemic diseases Prevalent throughout the country/continent eg. Late blight of potato
HISTORY OF PLANT PATHOLOGY
LANDMARKS IN THE DEVELOPMENT OF PLANT PATHOLOGY
1.Ancient period: A literature of European and vedic eras will give us some information on the plant diseases and their control measures. Earlier people were aware about plant diseases but they aware unable explain scientifically hence they believed that plant diseases were a manifestation of the wrath of God and, therefore, that avoidance or control of the disease depended on people doing things that would please that same superpower. In the fourth century b.c.; the Romans suffered so much from hunger caused by the repeated destruction of cereal crops by rusts and other diseases that they created a separate god, whom they named Robigus. To please Robigus, the Romans offered prayers and sacrifices in the belief that he would protect them from the dreaded rusts. The Romans even established a special holiday for Robigus, the Robigalia , during which they sacrificed red dogs, foxes, and cows in an attempt to please and pacify Robigus so he would not send the rusts to destroy their crops. The first person to study and write about the plant diseases is the Greek philosopher Theophrastus. He made observations on the plant diseases in his book enquiry into plants. His experiences were mostly based on imagination and observation but not on experimentation. Theophrastus (370 B.C. – 286 B.C) ➢ First botanist to study and write about the diseases of trees, cereals and legumes. ➢ He divided the plant diseases into external and Internal. External diseases are caused by external factors like Temperature, Moisture etc; internal diseases are caused by internal conditions of plant. ➢ He stated that different plants are affected by different types of diseases ➢ Some plants are prone to infection than others. Ex: Cereals are most affected by Rust than pulses. Among cereals Barley is more attacked than any other crop. ➢ First to recognize influence of environment and topography of the land on plant diseases. Example: Plant grown in elevated areas are less affected than plant grown in low land. ➢ He wrote two important books : 1. Historia plantarum (the enquiry into plants)
- Decauses Plantarum ➢ Regarded as Father of Botany In India, the information on plant diseases is available in ancient literature such as rigveda, atharveda (1500-500BC), arthasashtra of Kautilya (321-186 BC), Sushruta sanhita (200-500AD),
➢ Beleived that the toxin present in the black powder caused the disease rather than the fungus. ➢ He showed contagious (infectious) nature of “wheat bunt” disease. (5) Persoon, C.H. (1801): ➢ First to gave real systematic of fungi and laid the foundation for taxonomy of fungi ➢ He published a Book – “ Synopsis Methodica Fungorum”. ➢ He classified many fungi using binomial nomenclature as Uredinales, Ustilaginales and Gasteromycetes. (6) Fries Elias Magnus (E.M. Fries) (1821-1832): ➢ He published “Systema Mycologicum”. ➢ He regarded as “Linnaeus” of Mycology. “Father of Systematic mycology” ➢ He considered rust and smut fungi as a products of the diseased plants. (7) Prevost (1807) : ➢ He proved that diseases are caused by microorganisms. ➢ He was first man to show microorganism as disease causing agent. ➢ He discovered the life cycle of “Bunt fungus”. ➢ He observed the spore germination in bunt fungus. ➢ He was the first man to suggest or use of chemical copper sulphate for seed treatment to control seed borne disease. (8) Anton De Bary (1831-1888): ➢ German Scientist born in 1831 ➢ Experimentally proved that the Phytophthora infestans was the cause of late blight. ➢ He studied late blight of potato fungus, its nomenclature and renamed it as Phytophthora infestans (infectious plant destroyer) ➢ Studied the other diseases also such as rust, smut, downy mildews. ➢ First to study the physiology of plant disease and reported the role of enzymes and toxin in tissue degradation caused by Sclerotinia sclerotiorum in carrot. ➢ Reported heteroecious nature of Rust fungi. ➢ Introduced the terms Biotrophs, Facultative parasite and Facultative saprophytes ➢ Reported the plasmodial stage of Myxomycetes ➢ Authored two books:
- Morphology and Physiology of Fungi, lichens and Myxomycetes (1867)
- Comparative Mophology and biology of the Fungi, Mycetozoa and Bacteria (1884) ➢ For all his contributions regarded as
- Founder of “ Modern Experimental Plant Pathology”.
- Father of “ Plant Pathology” ➢ Guided the no. students among them Woronin (Russia), Brefeld (Germany), Millardet (France), Ward (England), Farlow (USA) (9) M. J. Berkeley (1855) : ➢ Observed root knot nematodes in galls of cucumber. ➢ Founder of Mycology in Bristish - described 6000 species of fungi ➢ Studied the Late blight of potato pathogen, Rust of coffee and Grape downy mildew. Authored books: 1. Outlines of British Fungology
- Introdcution to cryptogamic botany (10) Julius Gotthelf Kuhn (1858) : ➢ Published a text book in Plant Pathology entitled “ The Diseases of Cultivated Crops, Their Causes and Their Control”. ➢ Demonstrated the penetration of Bunt fungus into wheat and its development in the plant. ➢ Copper sulphate seed treatment was standardized and brought into usage at farmers level. ➢ He studied the Rusts, Smuts of cereals, ergot sooty mold and diseases of Mustard. (11) Robert Harting (1935-1901): ➢ Worked on Forest Pathology ➢ He wrote two important books in German on : (1) Important Diseases of Forest Trees (1874) (2) Text Book of Tree Diseases (1882) ➢ He is called the “ Father of Forest Pathology”. (12) Brefeld & its colleague (1875-1912) : ➢ Discovered the methods of artificial culture of micro-organisms i.e. gave “ Pure culture Techniques”. ➢ Given the Single spore isolation technique ➢ Studied the life cycle of Cereal smut (13) Pierre Marie Alexis Millardet (P.M.A. Millardet) (1882) : ➢ He discovered the Bordeaux mixture in 1882 for the control of downy mildew of grape caused by Plasmopara viticola. ➢ studied the root aphid phylloxera of the grape vine and hybridization experiments in order to have resistant plants. (14) M.S.Woronin (Russia)
- Blackslee : Gave concept of Heterothallism in Fungi.
22. J.C.Walker: ✓ Reported the role of Phenolic compound in plant defense mechanism ✓ Reported the resistance of coloured onion bulbs to smudge fungus ( Colletotrichum circinans ) was due to catechol and Protochatecuic acid present in the dry scale ✓ He wrote two books on ▪ Plant Pathology (1968) ▪ Diseases of Vegetable Crops (1952). 23. Muller and Cruckshank: ✓ Involvement of Phytoallexin in plant disease resistance. **Bacteriology:
- Anton Van Leeuwenhoek:** ✓ He was a "cloth merchant" living in Delft Holland. ✓ Developed magnifying lenses (300- 500 times magnification). ✓ He developed simple microscope ✓ First to discover the micro-organism like – Bacteria, Protozoa, alage and named them as “Little animalcules”. ✓ He is regarded **as Father of ‘bacteriological world’.
- Louis Pasteur:** ➢ Proved the germ theory ➢ coined the terms Aerobic, anaerobic organisms ➢ Proved fermentation is biological process ➢ Developed Pasteurization process ➢ Developed vaccine for Rabies disease ➢ Regarded as “ **Father of Microbiology”
- Robert Koch (1876) :** ➢ He proved that the anthrax disease of cattle is caused by a bacterium ( Bacillus anthracis ). ➢ He gave Koch’s rules or Koch’s postulates or proof of pathogenicity. ➢ Use of Gelatin as a solidifying agent ➢ Found causal organism of Tuberculosis disease. ➢ He considered as “ Father of Medical bacteriology”. ➢ Regarded as father of bacteriological techniques Koch’s postulates: 1. The suspected micro-organism must be present in every diseased host. 2. The suspected causal agent must isolated from the diseased host and grown in pure culture.
3. The specific disease must be produced when a pure culture of the suspected causal agent is inoculated into a healthy susceptible host. 4. The same causal agent must be recovered again from the experimentally inoculated and infected host. 4. T.J. Burrill (1878) : ➢ He first time proved that the fireblight of apple & pear was caused by a bacterium ( Erwinia amylovora ). ➢ First to Show **Bacteria can cause plant disease
- E. F. Smith (1900)** ➢ Final proof for bacteria can cause plant disease ➢ Studied bacterial wilt of cucumber, Crown gall of apple ➢ Gave Fourth Step of Koch’s postulates ➢ Wrote books : ‘Bacterial diseases of plants’, ‘Bacteria in relation to plant diseases’ ➢ Father of “ Phyto bacteriology” ➢ He first time demonstrated that budding or grafting could be another method of transmission of plant viruses. 6. Walther and Frau Hessey (1881) ➢ **Used Agar – Agar as a solidifying agent in media.
- R.J.Petri** ➢ **Developed the petriplates (two circular plates holding the media)
- Doi and Ishi (1967):** ➢ Discovered Mycoplasma like organisms (MLO’s) causing yellows type diseases (Mulberry dwarf, Aster yellows) in plants. ➢ Ishi et al Reported Tetracycline can be used for Management 9. Davis and Worley, J.E. (1972) : ➢ He first time discovered “ Spiroplasma ” in corn stunt disease. 10. Windsor, I.M. and Black, L.M. (1972) : ➢ He observed first occurrence of Rickettsia like organisms (RLO’s) in clover club leaf disease. **Virology:
- Carolus Clusius** ➢ Recorded breaking of tulips due to viral infection (2) Adolph Mayer (1886): ➢ First time discovered the tobacco mosaic virus (TMV) disease.
NEMATOLOGY:
(1) Nathan Augustus Cobb (N.A. Cobb) (1913-1932) : ➢ He contributed a lot to nematode morphology, taxonomy and methodology. ➢ Given the term "Nematology" ➢ Given techniques for seperation nematodes from soil - Cobbe Seive technique ➢ He is considered as “Father of Plant Nematology”. (2) T. Needham (1743): ➢ He reported first plant parasitic nematodes in wheat-galls-Ear cockle of wheat ( Anguina tritici ).
History of Plant Pathology in India:
D. D. Cunnighham (1850-1875) ➢ collection and Identification of Fungi of India ➢ Identified the causal agent of Red rust of tea in Assam ( Cephaleurous virescens ). ➢ Studied the Rust and Smuts K.R. Kirthikar ➢ First Indian Scientist to collect fungi and Indentify in India E.J. Butler (1905): ➢ First cryptogamist to India ➢ worked on red rot of Sugarcane, Wilt of Red gram, Rice diseases ➢ studied the life cycle of smut diseases. ➢ He wrote book on “ Fungi and Disease in Plants” in 1918. ➢ Monograph "Pythiaceous and Allied Fungi" ➢ Established Herbarium At IARI ➢ He considered as “Father of Indian Plant Pathology”. ➢ He became First Director of Imperial Mycological Institute (1929) J.F. Dastur (1886-1971) : ➢ First Indian plant Pathologist credited for Detailed study of Fungi and Plant diseases. ➢ He was studied the genus Phytophthora and diseases of castor. ➢ He is internationally known for establishing the species of Phytophthora parasitica from castor. Mehta, K.C. (1929): ➢ Worked on Epidemiology of Wheat rust ➢ He also proved Barbery has no role in perpetuation of rust fungi in India.
➢ Discovering the disease cycle of cereal rusts in India. ➢ Wrote Monograph : Further studies on Cereal Rust In India (1940) ➢ Regarded as Father of cereal Rusts in India B.B. Mundkur (1948) : ➢ He worked on Managment of cotton wilt through Resistance breeding ➢ Worked on Smut fungi, wilt diseases. ➢ He published a textbook “Fungi and Plant Diseases”, ➢ Wrote Monograph: Ustilaginales in India ➢ He started single handed “Indian Phytopathological Society” with its journal “Indian Phytopathology” in 1948. M.J. Thirumalachar ➢ He successfully introduced the use of antibiotics in control of plant diseases eg. Aureofungin and Streptocycline. ➢ Extensively studied the Smuts and Rusts ➢ Wrote Monograph: Uredinales of world, Cercosporae J.C. Luthra and his associates: ➢ He developed the solar treatments of wheat seed for control of loose smut (internally seed borne disease). T.S.Sadasivan: ➢ Worked on biochemistry of Host- Parasite Relation ➢ Developed the concept of Vivotoxin ➢ Worked on the mechanism of Cotton wilt (due to Fusaric acid) S. N. Dasgupta: ➢ Worked on biochemistry of Host- Parasite Relation ➢ Studied the Black tip of Mango M.K. Patel (1948-51): ➢ Studied the bacterial plant diseases in cotton, paddy. ➢ Proposed new family Phytobacteriaceae to include all the phytopathogenic bacteria ➢ Devised a synthetic medium known as PATEL’s MEDIUM to cultivate bacterium. ➢ Discovered several Xanthomonas and Psuedomonas species causing plant diseases. ➢ Regarded as Father of Phytobacteriology. Nene Y.L. ➢ worked on pigeonpea sterility mosaic disease ➢ Reported Khaira Disease of Paddy due to Zn deficiency ➢ published books on: 1. Fungicides in plant disease control
- Viral diseases of pulse crops in U.P. G.S. Kulkarni: a student of Butler, generated detail information on downy mildew and smut of jowar and bajra.
b) Meristem tip culture c) Hypovirulence d) Horizontal and vertical resistance e) Use of Antibiotics f) Hot water treatment g) Cross protection h) Bordeaux mixture i) RNA interference Important Plant Pathogenic micro-organisms Nematodes: ✓ Microscopic in nature, the length of the nematode may vary from 0.2 mm ( Paratylenchus ) to about 11.0mm ( Paralongidorus maximus ). ✓ Inhabit marine, freshwater and terrestrial environments as free – livers and parasites. ✓ Vermiform Bilateraly symmetrical, triploblastic, unsegmented and pseudocoelomates. ✓ Posses stylet and oesophagus is tripartite ✓ Growth accompanied by molting of juvenile stages, usually four juvenile stages. ✓ Oral opening surrounded by 6 lips and 16 sensory structures. ✓ Four systems are present - Digestive, Nervous, excretory and reproductive systems. ✓ Circulatory and respiratory systems are absent. Importance: ✓ Cause disease in Plants, animals and human beings. ✓ Used as biocontrol agents for the management of Insect pest and diseases. ✓ Used in biological studies Eg: Cnerorhabditis elegans in Gene silencing. ✓ Important plant diseases caused by nematodes: Wheat gall ( Anguina tritici ), Molya disease of wheat and barley ( Heterodera avenae ). Bacteria: ✓ They are prokaryotic, unicellular organism. ✓ The cell wall is made up of N-acetyl glusoamine and N-acetyl muramic acid. ✓ Most of them are aerobic, few are facultative anaerobe ( Erwinia ). ✓ They may be rod, cocci, cylindrical, spiral and vibrios shape. ✓ Some bacteria are motile with flagella ✓ It can be grown artificially on media in laboratory. ✓ They reproduce asexually by binary fission. ✓ They may be Gram positive or Gram negative. ✓ Some having extra chromosomal DNA called ‘Plasmid’ in their cell. Importance: ✓ They cause diseases in plants Ex: Brown rot of potato by Ralstonia, Bacterial blight of rice by Xanthomonas oryzae pv. oryzae. ✓ They play important role in natural cycling of elements (Nitrogen, sulphur etc;). ✓ Play important role in organic matter decomposition. ✓ Used in industry for the preparation of valuable compounds. ✓ Used as biocontrol agents. Eg: Pseudomonas fluoroscens, Bacillus subtilis.
Phytoplasmas: Discovered by Doi et al and Ishii et al in 1967. ✓ They are prokaryotic organisms lacking cell wall but covered by cytoplasmic membrane. ✓ They contain both DNA and RNA. ✓ They are pleomorphic and smaller in size… ✓ They are obligate parasites and cause disease only in plants. ✓ They are present in the phloem of the plants ✓ They are resistant to penicillin and sensitive to tetracycline. ✓ They transmitted by Leaf hoppers and Psyllids. ✓ They can be cultivated in laboratory conditions. ✓ Reproduce by binary fission. Diseases caused by phytoplasmas : Little leaf of Brinjal, Sesamum phyllody, Aster yellows, Spiroplasma : ✓ Discovered by Davis et al. ✓ They are prokaryotes, Lack cell wall, bounded by a single triple layered membrane. ✓ They are helical shape and move by screw motion of the helix. ✓ They contain both DNA and RNA. ✓ They are present in the phloem of the plants. ✓ They are resistant to penicillin and sensitive to tetracycline. ✓ They can be cultivated in laboratory conditions. ✓ The colonies look Fried egg shape on solid media. ✓ Reproduce by binary fission. ✓ They cause disease only in plant and Transmitted by Leaf hoppers. Diseases caused by Spiroplasmas : Citrus stubborn, Corn stunt. Rickettsia : Obligate parasitic bacteria of Arthropods. It causes diseases in animals and Human beings. They have cell wall. They very small, non-motile, rod shaped or coccoid shaped. It contains nucleic acid DNA and RNA. Multiplication by binary fission. They can be cultured on egg yolk artificially. Sensitive to antibiotic penicillin. They are transmitted by as arthropod vector like Lice, mites, fleas. Human Diseases : Rocky mountain fever (Ticks), Typhus fever (lice), Murin typhus fever (Fleas) Fastidious vascular bacteria: Fastidious vascular bacteria earlier called as Rickettsia Like organisms (RLO’s). They are prokaryotes, bounded by cell wall and cell membrane. They have no flagella and most of them are gram negative. They are present either in the phloem or xylem of the plant. They are transmitted by leaf hoppers. They are sensitive to antibiotics such as penicillin, tetracyclines. They are two types: Xylem Limited FVB Phloem Limited FVB Can be cultivated artificially Cannot be cultivated artificially They are limited to Xylem of the Plant They are Limited to phloem of the plant
