Plant Pathology

Meaning of Disease

Introduction:

Plants are the only higher organisms that can convert the energy of sunlight into stored, usable chemical energy in carbohydrates, proteins and fats. All animals including humans depends on these plant substances for survival.

Plants whether cultivated or wild grow and produce well as long as the soil provides them with sufficient nutrients, moisture sufficient light and temperature within a normal range.

Plants however also gets sick, grow and exhibit various types of symptoms and sometimes whole plant die. It is not known whether diseased plant feel pain or discomfort. If a plant is looking different from its community then it is equal to be diseases one. Any biotic or abiotic agents which induce the disease in plant is referred as the cause of diseases.

The causative agents of disease in plants are pathogenic such as fungi, bacteria, viruses, protozoa and nematodes and environmental conditions such as lack or excess of nutrients, moisture, light, etc to presence of toxic chemicals in Air or soil.

Disease in plants has been known since ancient time.

Definition of Disease:

Disease is defined as a disturbance in the rhythmical equilibrium in the activities of host in respect of structure or physiology or both, leading to the death of a part or entire host, or reduces the economic value of the products.

Or

Disease is a complex phenomenon; it is an interaction between the host, the pathogen and the environment.

Disease is malfunctioning process caused by continuous irritation, which results in some suffering producing symptoms.

The plant disease is, therefore , a structural abnormality or physiological disorder or both due to an organism or unfavorable conditions that may affect the plant or its parts or products or may reduce their economic value. 

Meaning and Objective of Plant Pathology

Definition of Plant Pathology:

The science that deals with the study of diseases of plants, their development and control is called Plant Pathology.

Plant diseases are caused by biotic agents like fungi, bacteria, actinomycets, Mycoplasma, viruses, nematodes, and flowering parasites or by abiotic like unfavorable environmental conditions or nutritional deficiencies. Study of plant pathology includes the study of sciences viz, Microbiology, Bacteriology, Virology, Mycology, Nematology, protozology, phycology, unfavorable , environmental factors, nutritional deficiencies and flowering plant parasites.

1. Microbiology: Study of Microorganisms
2. Bacteriology: Study of Bacteria
3. Virology: Study of Viruses
4. Mycology: Study of Fungi
5. Nematology: Study of Nematode
6. Protozology: Study of Protozoa
7. Phycology: Study of Algae.

Objectives of Plant Pathology:

Plant Pathology is one among the branches of agricultural science that deals with cases, etilogy, resulting losses and management of plant diseases with four major objectives.

1.  Study the diseases (s) or disorders caused by biotic and abiotic agent.
2. Study of mechanism of disease development by pathogens.
3. Study of interaction between plant and pathogen in relation to the overall environment.
4. Develop suitable management strategies for managing the diseases and losses caused by them.

History of Plant Pathology

By studying the history of science, we get a better perspective of the subject , we come to know the contributions made in that field, the problems that are encountered and the manner in which they are tackled.

The history of plant pathology is divided into different five eras:

1. Ancient era: Ancient to 5 th Century (476 A.D)
2. Dark era: 5 th to 16 th Century (476 A.D. to 1600)
3. Premodern era: 17 th Century to 1853 (1600 to 1853)
4. Modern era: 1853 to 1906
5. Present era: 1906 onwards

1. Ancient Era: Ancient to 5 th Century (476 A.D)

Diseases in plant have been known since ancient times. Rust, blight, mildews, smuts, were familiar to Hebrews, Greeks, Romans, Chinese and Indians. Plant disease was recorded in Vedas (Rugveda, Athavaeda) as early as 1200 B.C. Symptoms and control of disease have been mentioned in “VRIKSHAYURVED” by Surapal in ancient India. Even mention of plant disease has been made in Budhist literature of 500 B.C.

Theophrastus (300 B.C) a great botanist noted occurrence of crop disease and suggested some remedies to control them. He also wrote about plant disease in this era.

Lord Pliny (100 A.D) He descried plant diseases and suggested some remedies. He believed that disease originates from the plants or from the environment.

2. Dark Era (476 A.D to 1600):

Plant pathology made very little progress during this era some Arabians like Ibnal-awan described symptoms and control measures for some plant disease.

1440: Printing was introduced in Europe and this reflected interest in learning science.

3. Premodern Era 18 th century to 1853 (1600 to 1853)

Robert Hooke 1665:

The father of cell theory. He had developed or invented first compound microscope. He reported that plant tissues are made up minute units called as cells.

Anton Van Leeuwenhoek 1676:

A Dutch worker from Holland. He invented first simple microscope with home ground lenses between two metal plates. He described different types of protozoa and bacteria as “ Little animalcules”. All unicellular microorganisms ( Protozoa , algae, and bacteria) were firstly recorded by him.

P.A Micheli 1729:

An Italian Botanist studied several fungi and described their morphology for first time. He studied that fungi originates from spores. Father of Founder of Mycology.

John Needham 1743:

Reported plant parasitic nematodes in wheat galls.

Carlous Linnaeus 1753:

Established Latin Binomial system of Nomenclature of Plants and animals in his book “Species plantarum”.

Tillet 1755:

Proved that Bunt of wheat is contagious or infectious and can be controlled by seed treatment.

Prevost 1807:

A French Botanist suggested CuSO4 seed treatment for bunt of wheat.

This is known as autogenic or physiologic period, since plant disease were distinctly physiologic with tendency towards the mycology. At the end of the period it was clear that fungi were very closely associated with diseases.

In 1845, late blight of potato was appeared in Ireland, over one million people get died and one and half million get migrated and the history entered the next era.

Modern Era 1853 to 1906:

This known as pathogenic period which was devoted the study of role of fungi causing plant diseases.

Anton de Bary 1853:

He proved that late blight of potato was caused by phytophorn infestans. Founder or father of plant pathology.

T.J. Burril 1873:

American plant pathologist. He proved, Bacterial Nature of Fire Blight of Apple and pear.

Robert Koch 1876:

Bacterial nature of Anthrax disease in animals ( 1881) Gelatin is used as solidifying agent in culture media which is replaced by Agar-Agar. He described the theory called “KOCH’S POSTULATES”.

P.A. Millardet 1882-85:

Use of Bordeaux mixture (CuSO4 + Lime) for control of Downey Mildew of grapes.

 Adolf Mayer 1886:

Described TMV and proved that TMV should be transmitted from diseased plant to healthy plants.

Jenson 1887:

Hot water treatment for loose smut of wheat.

E.F Smith 1890:

Father of phytobacterilogy. He worked on bacterial wilt of cucurbits and crown gall diseases.

Iwanowski 1892:

Demonstrated that Tobacco Mosaic Virus (TMV) can pass through bacteria proof filters and proved filterable nature of viruses.

Cragie 1827:

Showed function of Puccinia in rust fungi.
Biffen 1905:

Pioneers in Genetic of Plant diseases resistance.

4. Present Era 1906 Onwards:

The present or current era commencing from 1906 has since remarkable discoveries.

J.C Luthra 1931:

Solar heat treatment for loose smut of wheat.

W.M. Stanley 1935:

He proved crystalline nature of virus. He got Nobel Prize.

F.C Bowden and Pierie 1936:

Nuckeoprotenous nature of virus.

G.H. Flor 1955:

Gene for gene theory hypothesis.

Doi and Asuyama:

Discovered Mycoplasma like organism (MLO) responsible for yells type of disease.

Important Contribution of Indian Psychopathologist

E.J Butler:

Imperical Mycologist at IARI, New Delhi since 1905 to 1921 and trained many workers in Mycological and Plant Pathological Research. He wrote a Book- “Fungi and Disease in Plants” in 1918. Appointed as first Director of CWMI – in Britain. He is called as the “Father of Modern Plant Pathology in India.

K.C.Mehta (1892-1950):

Physiology and Epidemiology of cereals rusts in country. Monograph on further studies on cereals rusts in India, in 1940.

B.B.Mundkar (1896-1892) :

Worked on cotton wilt in Bombay state, published ustilaginales in India. Pioneer in establishment of Indian Psychopathological Society ( IPS) in 1947 and its organs Indian Phytopathology in 1948. He worked on Smut Fungi. Author of Fungi and plant diseases.

J.F Dastur:

Anthracnose of cotton. Cotton wilt, pink disease of citrus , foot rot of betelvine. He published 36 original papers of 4 books.

B.N. Uppal:

He worked on Downey mildew of Maize, Bajara and showed physiologic specialization in Sclerospora graminicola. He worked on several fungal and bacterial diseases.  

G.S. Kulkarni:

Downey mildew of Sorghum and Pearl millet, sorghum Smut.

V.P.Bhide:

Bacterial diseases of plant.

M.J. Tirmalachar:

500 research papers, 20 genara and 300 species of fungi. He discovered antibiotic Aureofungin.

G.Rangaswami:

Nematode, bacterial and other diseases. Published 5 books of micro biology and plant pathology and over 300 research papers.

P.N.Patel:

Bacterial disease of plant.

Concepts of Plant Disease

Old:

Plant diseases were considered to be a curse and punishment to the people by god for wrongs and since they had committed (religious belief and suspertition).

The greak philosopher Theophrastus (300, B.C) was the first to study and write about diseases of trees, cereals and legumes. He wrote a book named “Enquiry into Plants”. In this book he mentioned his experience about plant diseases. His experience was not based on experimentation. He being unable to explain diseases. He beloved that god controlled the weather that brought diseases. Plant diseases were a manifestation of the worth of God. It is due to religious belief, occulation, superstitions or it is the effect of star moon and bad wind. E.g. Romans actually created a special rust God called Robigo for rust diseases of grain crops. They offered sacrifice of red dogs and sheeps.

This was continued for almost 2000 years after Theophrastus. After invention compound microscope in the mid 1600 scientist enable to see many microorganisms associated with diseased plants and they come to believe that the mildews, rust, and other symptoms observed on plants and microorganisms found on diseased plant. Plant parts were the natural product of diseases than the cause and effect of disease.

Louis Pasteur (1860 -63) provided irrefusable evidence that microorganisms arises only from pre-existing microorganisms and fermentation is a biological phenomenon not just a chemical one. It is accepted that a plant is healthy or normal when it can carry out its physiological functions to the best of its genetic potentials.

1. Cell division, differentiation and development.
2. Absorption of water and nutrients.
3. Translocation of nutrients and water synthesis.
4. Photosynthesis.
5. Translocation and Metabolism or store the photosynthetic products.
6. Production of seed or reproductive organ or survival and multiplication.

Whenever the ability of the cells of a plant or plant part to carryout one or more of these essential functions is interfered either by a pathogenic microorganism or adverse environmental factor. The activities of the cells are altered or inhibited, the cells die or plant become diseased.

Pathogen may Cause Disease in Plant by:

i) Weakening the host by continuously absorbing food from the host cells for their own use.
ii) Killing or disturbing metabolism of host cells through toxins enzymes or growth regulating substances, they secrete.
iii) Blocking the transportation of food, mineral, nutrients and water through the conductive tissues.
iv) Consuming the contents of the host cells upon contact.

Disease Caused by Viruses

                                                                
Viruses causes disease in human, animals and plants, disease in human: small pox, AIDS (HIV), hepatitis, poliomyelitis, diseases in animals foot and mouth diseases (FMD), plants: Yellowing, Mosaic, Yellow mosaic, Yellow vein mosaic, Mottling Chlorosis, bunchy top, sterility mosaic etc.   
  Viroid:

1. T.O diener 1971 coined the term viriod.

2. Viroids are sub-microscopic, a cellular, infectious, self replicating, low molecular weight , naked ribonucleic acid (RNA).

3. Viroids are naked nucleic acid without coat protein. It is a smallest known agents of infectious disease with low molecular weight ribonucleic acid ( RNA) that can infect plant cell, relicate themselves and cause disease.

4. Genome of the viriod naked, single stranded ( RNA) with 250-400 nucleotide, either linear or mostly circular. Viroids are smaller in size than viruses i.e 50 nm or 1.1 to 1.3 X 103 molecular weight.

5. Viroids are notable to synthesis protein and replicase enzyme required for replication.

6. Viroids replicate by direct RNA, copying in which all components required for viroid multiplication including RNA polymerase are provided by the host.

7. Viroids are transmitted by sap transmitted by vegetative propagation, through pollen and seeds.

Disease Caused for Viroids:

 Potato spindle tuber, Citrus exocortis, Coconut cadang, tomato bunchy top.

Prokaryotes

Prokaryotic organisms are that in which nucleus is primitive type and nuclear material is not enclosed within the nuclear membrane.

Bacteria are placed in the kingdom “Prokaryotae” because of the prokaryotic cellular organization of the members. However, extremely diverse groups of microorganisms differing in morphological, physiological and ecological properties are found within this kingdom. In the beginning, description and information of bacterial systematics or classification was being published in the comprehensive volumes of “Bergey’s Manual of Determinative Bacteriology” (1923, first edition). The 8 th edition of Bergey’s Manual, published in 1974 was the last of such comprehensive manual and from 1984 onwards it was renamed as “Bergey’s Manual of Systematic Bacteriology”. It is the most widely accepted and used reference document or book for classification and identification of bacteria. Begey’s manual consists of four separate volumes and each volume is further divided into different sections ( Ist Volume : 1-11 sections, IInd volume : 12-17 sections, III rd volume : 18-25 sections and Iv th volume: 26-33 sections. )

Classification of Bacteria or Prokaryotes – Volume I

The classification of bacteria or prokaryotes (of significance in agriculture and allied fields) given in “Bergey’s Manual of Systematic Bacteriology” is listed as follows.

Section-I:

The spirochetes, the two important families are i.e. i. Leptospiracear, important genus : Leptospira and ii. Spirochaetaceae, important genus is Spirochaets.
    
Section -2:

Aerobic or microacerophilic, motile, helical or viborid gram negative bacteria. The important genera (of academic interest) in this section are: Azospirillium, Campylobacter and Badellovibrio. Azospirillium cells are vibrioid with single polar flagellum, present or found within the roots of grasses, wheat, corn and other plants or as free-living soil organisms. They are either aerobic or microerophilic and fix N2 within plant roots. Important species are A.limpoferum and A.brasilense.
     
Section-3:

Non – motile, Gram negative curved bacteria: Family- Spirosomaceae, Genera: Spirosoma, Runella and Flectobacillus.

Section-4:

Gram –negative, aerobic rods and cocci: It is on the largest section containing most diverse group of bacteria important in agriculture, such as:

i) Family: Pseudomonadaceae, Three Important genera are………

a. Pseudomonas:

Several species are pathogenic to humans, animals and plants; cause spoilage of meat and other foods, species like P. syringae, cause diseases like leaf spots, leaf stripe, wilt and necrosis, P. fluorescens, is common soil saprophyte that produce a fluorescent pigment.

b. Xanthomonas:

Produce characteristics yellow pigment Xanthomonadin, all species are pathogenic to plants causing diseases such as spots, streaks, cankers, wilts and rots. Xanthomonads, produce exocellular polysaccharides i.e. xantham gums useful in industrial application.

c. Zoogloea:

Cells are embedded in gelatinous matrix to from slimy masses with finger like morphology. Species are saprophytic, commonly found on tricking filter beds, in sewage treatment plants, where they oxidize the organic matter of the sewage.

2. Family: Azoobacteriacea

Two important genera are Azotobacter and Azomonas. The species are saprophytes found in soil, water and plant rhizosphere, fix N2 under aerobic conditions and forms desiccation resistant spores called “Cysts”.
   3. Family: Rhizobiaceae:

Three important genera are Rhizobium, Brandyrhizobium and Agrobacterium. Bacteria of the genus Rhizobium and Brandyrhizobium fix atmospheric. N2 symbiotically in legumes by including root nodules. Whereas, species of the genus Agrobacterium, do not fix N2 but they are plant pathogenic inducing tumors in crown, roots and stems of dicotyledons.  
  4. Family: Methylococcaceae:

 Two important genera are Methyloccus and Methylomonas. Bacteria are obligate methane- Oxides, use methane gas as a sole carbon and energy source under aerobic and microacerophilic conditions.

5. Family: Acetobacteriaceae:

Two important genera are Acetobacter and Glucanobacter Member of these two genera are saprophytes, found in sugar or alcohol, enriched acidic environments such as flower, fruits , bear , wine , vinegar, honey etc. They are industrially important. Acetobacter peritrichous are used to make vinegar and Glucanobacters (Polar Flagella) are involved in the manufacture of chemical like dihydroxyacetone, sorbose etc.

Section -5:       

Facultatively anaerobic Gram-negative rods-

Two important families and important genera in respective families are:

1. Family: Enterobacteriaceae, Important genera are…………

a. Escherichia:

E.g. E.coli, inhabitant of lower portion of the intestine of human and warm-blooded animals, caused gastrogenteritis and urinary tract infections.

b. Shigella:

Species are pathogenic, causing bacillary dysenter in human called Shigellosis.
     c. Salmonella:

All species are pathogenic in humans causing enteric fevers typhoid and paratyphoid fevers gastroenteritis and septicaemia.

d. Enterobacter:

Species occur in water, sewage, soil, meat plants and vegetables. Some species are opportunistic human pathogens.

e. Erwinia:

Species mainly associated with plants, causing disease such as blights, cankers, die-back, leaf spot, wilts, discolouration of plant tissues and soft rots.

f. Yersinia:

These are parasites of animals but can also cause infections in humans such as plague. (Y.pestis).

Family:

Vibrionaceae

Important Genera are: Vobrio, Aeromonas

Most Vibrio species are harmless saprophytes, but some species are pathogenic in human. E.g. V. cholerae, causing cholera in humans.

Section-7: Dissimilatory Sulfate or Sulphur reducing Bacteria:

The bacteria or organism belonging to this section are obligate anaerobes using sulfate, sulphur or other oxidized sulphur compounds as election acceptors and reducing them to H2S. These are gram-negative found in mud and marine environments and in intestinal tract of humans and animals. The important genera are Desulfovibrio (Vibrioid and helical cells), Desulfococcus (Spherical cells) and Desulfosarcina.

Section-8: The Rickettsias and Chlamydias:

These are tiny, non motile, Gram- negative bacteria. They are obligate parasites, able to grow only within host cells. The two important genera are Rickettisia and Coxiella. The species of Rickettisai caused diseases like Rocky Mountain spotted fever, classical typhus fever, scrub typhus and the single of species of genus Coxiella causes Q fever a type of pneumaonia (Coxiella burnetii).

Section-9: The Mycoplasma:

These are very small organisms devoid of cell wall. Because of lack of cell wall, mycoplasmas are not inhibited by penicillin antibiotic; however they can be inhibited by antibiotics that affect protein synthesis (E.g. tetracycline or Chloramphenicol). They can be cultivated invitro (In laboratory) on synthetic media as facultative anaerobes or obligate anaerobes. Mycoplasmas are placed in the Division –Teericutes , class –Mollicutes and Order- Mycoplasmatales, containing three families, viz.

i) Mycoplasmataceae, genera Mycoplasma and Ureplasma, 
ii) Acholeplasmatoceae, genus Acholeplasma
iii) Spiroplasmataceae, genera, Spiroplasma, Anaeroplasma and Thermoplasma

Species of the genus, Mycoplasma are pathogenic to human and animals, E.g. M. pneumoniae, causing primary atypical pneumonia in humans, members of the genus Ureplasma cause arthritis in humans, pneumonia and urogential disease in cattle. Citrus stubborn is one of the important disease caused by Spiroplasma (S. citri) in citrus.

Classification of Bacteria or Prokaryotes – Volume II

The classification of bacteria or prokaryotes (of significance in agriculture and allied fields) given in “Bergey’s Manual of Systematic Bacteriology” is listed as follows.

Volume II:

Gram Positive Cocci:

In this section, 15 diverse genera of bacteria are placed together only because they are non-spore forming, chemo-organotrophic, Gram – positive cocci. The two important families are i) Deinoccaceae, ( Genus Deinococcus ) ii) Micrococcaceae , ( Genera: Micrococcus, Planococcus and Staphylococcus).  
Micrococci are non-motile, aerobic, oxidative harmless saprophytes occurring in soil and fresh water. Planococci are also harmless saprophytes that occur in marine environments. Staphylococci are non-motile facultatively anaerobic parasites. Major pathogenic species is S. aureus, causing wound infections, postoperative infections, food poisoning (Stapholococcai) in humans and mastitis in cattle.  
 
Other genera are Streptococcus, aerotolarant, homofermentative (end product of sugar fermentation is lactic acid only), most are pathogenic to humans and animals. E.g. S. pyogenes causes sore throat, scarlet fever and other human infections, S.mutans, inhabit in human oral cavity and cause dental caries, S. facalis, inhabit in the intestinal tracts humans and animals causing urinary tract infections, S.lactis and S. cremoris are harmless contaminants of milk and dairy products and therefore, widely used as “Starter cultures” in the manufacture of buttermilk and cheeses, S. Pnemoniae, also called Pneumoccus causing lobar pneumonia in humans.

Leuconostocs are harmless saprophytes occurring in grass, silage, grape leaves and spoiled food. They are hetero fermentative, producing CO2 and ethanol or acetic acid in addition to lactic acid and are often used as starter cultures for manufacture of butter, buttermilk, and cheese.

Section 13: Endospore Forming Gram: Positive Rods and Cocci

The important genera under this section are:

i) Bacillus:

Species such as, B. subtilis and B. cereus are mesophilic saprophytes producing exoenzymes that hydrolyze starch and casein, later species can cause a type of food poisoning. B. stenothermophilus, is thermophilic species associated with spoilage of canned foods, B.Polymyxa, has ability of N2 fixation under anaerobic conditions, B.turingiensis and B. popillae are pathogenic to insects, E.g. b.propillae, cause milky disease of Japanese better grub , B.anthracis is the only species of Bacillus that is highly pathogenic to humans and animals causing anthrax disease.

ii) Sporosarcina:

This genus contains cocci that are arranged in tetrads or cubical packets of eight cells. They are widely distributed in fertile soil where they decompose urea.

iii) Clostridium:

Members of the genus are fermentative and same species are important in agriculture. i.e. C. botulinum, causes food poisoning known as botulism, C.tetani, cause tetanus in humans, C.perfringes, cause wound infections (gas gangrene) and food poisoning; C. pasteurianum, mesophile inhabiting in soil and having ability to fix N2.

iv) Desulfatomaculum: Sulfate Reducing Bacteria:
Section 14: Non Spore forming Gram-Positive Irregular Shaped Rods:

This section includes heterogenous and variety of bacteria with aerobic or facultative anaerobic nature and, filamentous or non filamentous rods. Some of the important genera are:

a. Corynebacterium:

These are saprophytes occurring in soil and water, animal and human parasites and pathogens, e.g. C.diphtheriae cause diphtheria in humans, also causes diseases in plants.

b. Arthrobacter:

Soil saprophytes with characteristics rod-cocous cycle. i.e. cells in log phase are irregular rods and cell in stationary phase are coccoid.

c. Microbacterium:  
These are saprophytes occurring in milk, dairy products and diary equipments.

d. Cellulomonas:

The species are important cellulose decomposer or degrading bacteria.

Section 16: Mycobacteria:

Contain a single Genus Mycobacterium, these are aerobic, slightly curved or straight rods, cell wall contains 90% mycolic acid and are acid-fast in staining reaction. Many species are pathogenic to humans. E.g. M.tuberculosis cause tuberculosis and M.leprae, causes leprosy disease in human.

Classification of Bacteria or Prokaryotes – Volume III

The classification of bacteria or prokaryotes (of significance in agriculture and allied fields) given in “Bergey’s Manual of Systematic Bacteriology” is listed as follows.

Volume: III

Section 18: Anoxygenic Phototrophic Bacteria:

These bacteria are Gram-negative, photolithographic or photoautotrophic containing bacteriochlorophyll and various water soluble carotenoid pigments. These bacteria grow phototrophically only under anaerobic conditions and are incapable of forming O2 (hence called Anoxygenic). Anoxygenic phototrophic bacteria are divided into two major groups, on the basis of their pigmentation i.e a) Purple bacteria and b) Green bacteria.

a) Purple Bactria:

Have two important families are i) Chromatiaceae (Purple sulphur bacteria) and ii) Rhodospirillaceae (Purple non-sulphur bacteria)

i) Family, Chromatiaceae:

In this have four important genera viz. Chromatium (cells ovoid to rod shaped), Thiocystis (coccoid), Thiospirillum (helical), Thiosarcina. All genera are photolithographic H2S or elemental sulphur as the electron donor for CO2 fixation. Most species are anaerobic and cannot grow in the dark even under microaerophilic conditions.

ii) Family, Rhodospirillaceae:

It contains purple, non-sulphur bacteria with variable shapes: helical (Rhodospirillum), ovoid or spherical (Rhodopseudomonas) or spherical (Rhodopseudomonas) or ovoid (Rhodomicrobium). They are chemoorganotrophs for which organic substances serve both as source of carbon and electrons for the reduction of CO2. Photosynthesis occurs only under anaerobic conditions in the presence of light.

b) Green Bacteria: Two families and Important Genera are

i) Family: Chlorobiaceae, genera: Chlorobium (Green sulphur bacteria) Prosthecochloris.

ii) Family: Chloroflexaceae, genus: Chloroflexus (Green non-sulphur bacteria)           

Section 19: Oxygenic Phototrophic Bacteria:

The typical bacteria included in this section are Cyanobacteria ( Blue green algae). These are widespread in soil, freshwater, and marine habitats. Cyanobacteria are useful in N2 fixation, adding organic matter in soil and preventing incipient erosion. Some Cyanobacteria grow in symbiosis with other organisms. E.g. Lichen. They are also associated with certain protozoa, called as “Cyanellae”. Cyanobacteria contain chlorophyll ‘a’ rather than bacteriochlorophyll and other pigments such as water-soluble cartoenoids and phycobillins. BGA plays important role in N2 fixation in paddy crop. E.g. Anabaena, Nostoc, Chlorela, etc.     

Section 20: Aerobic, Chemolithotrophic bacteria and associated organisms:

This section includes chemolithotrops (E.g. nitrifying bacteria) , colourless sulphur bacteria ( Thiobacillus, Thiospira), obligately chemolithotrophic hydrogen bacteria ( Hyderogenobacter),iron and manganese oxidizing and or depositing bacteria siderocapsa and magnetotatic bacteria Aquaspirillum.  The families and important genera under this section are as follows.

Family:

Nitrobacteriaceae, Nitrifying bacteria are morphologically either, rods, cocci or helical. They are aerobic autotrops, incapable of chemolithotrophic growth with the exception of one species, Nitrobacter wingoradskyi. They nitrifying bacteria comprises two distinct metabolic groups viz. i) ammonia oxidizers ( oxidize ammonia to nitrate) E.g. Nitrosomonas, Nitrosolobus, Nitrosococus and Nitrosospira, and ii) Nitrate oxidiers ( oxidize nitrate to nitrate) e.g. Nitrobacter, Nitrococcus, Nitrospina. Nitrifying bacteria are commonly present in soil, where they play in important role in the nitrogen cycle, N2 fixation and in maintaining the fertility of soil.

Sulphur Bacteria:

The cultivable genera of sulphur metabolizing bacteria are: Thiobacillus ( Gram –negative , short rods) and Thiomicrospira ( helical). Both genera are widely distributed in soil, fresh water, coal-mines, and marine environments. They derive energy from oxidation of reduced sulphur compounds (E.g. Sulfides, elemental sulphur, thiosulfate, sulfite etc.) and the final oxidation product is sulphate. Some species are obligate autotrophs (E.g. Thiooxidans, T. thioparatus and Thiomicrospira pelophila) and others are facultative autotrophs (E.g. Thiobacillus novellas).

Section 25: Archaeobacteria or Archaebbacteria:

The Archaebacteria differs from eubacteria in at least two bio-chemical traits. i.e. i) they lack peptidoglycan and nuranic acid in their cell walls and ii) Membrane lipids or fatty acids are either – linked to glycerol in Archaebacteria and esterlinked in eubacteria. At present, at least three main categories of Archaebacteria are recognized. a) Methane producers, b) the red extreme halophiles and c) the thermoacidophiles.

a) Methenogenic Bacteria:

These are stringent anaerobics that obtain energy through oxidation of H2 or format and reduce CO2 with the formation of methane gas (CH4). The important genera of methane –producing or Methenogenic bacteria are: Methanobcterium ( Gram-negative , short rods), Methanosarcina ( Gram –positive Cocci) , Methanococcus ( Gram-positive , pleomorphic cooci), methanomicrobrium ( Gram-negative , short rods) , Methanogenium ( Gram-negative , pleomorphic cocci), Methanomicrobium ( Gram-negative, short rods), Methanogenium ( Gram – negative , pleomorphic cocci) and Methanospirillum ( Gram-negative , curved rods or filaments).

b) Extreme Halophiles:

These are chemoorganotrophic, Gram – negative, aerobic rods (E.g. Holobacterium) or cocci (Helococcus).

c) Thermoacidophiles:

These are aerobic, Gram-negative Archaebacteria with ability to grow under highly acidic conditions at high temperatures. Two important genera are: Thermoplasmas and Sulfolobus.

Classification of Bacteria or Prokaryotes – Volume IV

The classification of bacteria or prokaryotes (of significance in agriculture and allied fields) given in “Bergey’s Manual of Systematic Bacteriology” is listed as follows.

Volume IV:

Gram-positive filamentous bacteria of complex morphology.

There are soil organisms which are harmless and few are pathogenic to humans, animals and plants. In soil, they play important role in degradation of plant and animal residues and some are best producers of antibiotics. Bacteria included in this volume Iv are categorized as follows, under different sections.

a) Nocardiform Actinomycetes (sect.26), the important genera are Nocardia Rhodoccus, Pseudonocardia.

b) Actinomycetes with multicolor sporangia (Sect.27) important genera are: Geodermatophilus, Determatophilus and Frankia, like Rhizobium are highly efficient microacerophilic N2 –fixes that induce stem nodules in woody plants (E.g. Alders) .

c) Streptomycetes and related genera (Sect.29), Streptomycetes are actinomyceous, harmless bacteria occurring in neutral soil at saprophytes. They degrade polymeric organic substances in soil (Starch, pection, and chitin), Streptomyces have the ability to produce great number of variety of antibiotics. Some of the important species of the genus Streptomyces producing antibiotics are listed below:

Species	Antibiotics
S. venezulela	Choloramphenicol
S. dureofaciens	Chlorotetracycline (Auremycin)
S.erythraeus	Erthoromycin
S. fradiae	Neonycin
S.noursei	Nystatin
S. rimosus	Oxyteracycline (Terramycin)
S.griseus	Streptomycin
S.viridifaciens	Tetracycline