⏱ Historical Background

Historical development in India and World, Important Terms

World

The term Plant pathology or Phyto-pathology has been derived from three Greek words.

Phytopathology = Phyton (Plant) + Pathos (Ailments) + logus (Knowledge)

Disease is a malfunctioning alteration of one or more ordered processes of energy utilization in a living system, caused by the continued irritation of a primary factor or factors.
Ancient Indian writer Surapal gave a detailed account on plant diseases in his book “Vraksha Ayurveda”. In this book he has classified plant diseases into External and Internal diseases.
Theophrastus the ancient Greek Philosopher in his book “Enquiry into plants” had included some of his observations regarding plant diseases.
Dutch worker Leewenhock invented the microscope in 1675 and through it observed and described bacteria in 1683. This lead to the beginning of a new era in Biology.
Italian botanist Michell (1729) was the first to study fungi and observe fungal spores.
A research paper on Bunt or Stinking smut of wheat was published by French botanist Tillet in 1755. In his paper Tillet proved that wheat seeds with black powdery mass on their surface had greater potential to cause diseases than healthy seeds. Thus it was concluded that Bunt was an infectious disease having some relationship with fungi.
Prevost in 1807 proved that wheat bunt disease was caused by a fungus. He also discovered the life-cycle of the Bunt fungus.
In 1853 Anton de Bary (1831-1888) confirmed the findings of Prevost. De Bary through his extensive studies on the late blight disease of potato conclusively proved that microorganism play an important role in plant diseases. Besides this De Barry also discovered the heteroecious nature of the rust fungus. He also described the life-cycle of Downey mildew fungus and provided information about the role of enzymes in host-pathogen relationships.
Because of his great contributions De Bary was given the title Father of Mycology.
Julius Koon in 1858 wrote the first book on plant pathology in which he provided valuable information regarding the role of fungi in plant disease development. Julius Koon is therefore called the Father of Modern Plant Pathology.
German scientist Brefeld, who was a co-worker of De Bary, developed techniques of Artificial culture of microorganisms in between 1875 and 1912. These techniques made the study of infectious microorganisms easier.
In the latter half of Nineteenth century France had extensive cultivation of grapes for manufacturing wine. Downey mildew of grapes was introduced into Europe from America in 1878. Prof. Millardet of France discovered Bordeaux mixture for the control of this disease. At that time Bordeaux mixture was successfully used to control late blight of Potato and Downey mildew disease of Grapes.
In 1876 Louis Pasteur and Robert Koch reported that Anthrax disease in Cattle was caused by a bacterium.
Term microbiology given by Louis Pasteur.
American scientist Prof. T.J. Burill (1878) discovered that Fire blight disease in apple and pear was of bacterial origin.
His fellow American scientist E.F. Smith till the end of 1895 had conclusively proved the importance of bacteria as plant pathogens.
Swedish scientist Erikson in 1894 reported about the existence of Physiological races in Rust fungus.
Ward (1903) and Salmon (1903, 1904) discover Physiological specialization in Cereal rust and powdery mildew.
According to E.C. Stakman of U.S.A. due to continuous evolution of races & biotypes of the rust fungus their pathogenic capacity also keeps changing and so does the resistance capacity of the host.
Blakeslee in 1904 discovered Heterothallism in fungi and informed that in the life-cycle of fungus dissimilar nuclei participate.
Heterokaryosis i.e. coming-together of dissimilar nuclei in a single fungal cell was discovered by Burgett (1912 & 1914).
Hansen and Smith for the first time demonstrated about the development of physiological races as a result of heterokaryosis.
Flor (1955) proposed the Gene for gene hypothesis to explain disease resistance and susceptibility.
Needham in 1743 reported plant parasitic nematodes in Wheat gall.
In 1875 Berkebey and Schacht discovered the root knot nematode and cyst nematode of beet.
Cobb (1913-1932) studied the structure of many plant parasitic nematodes and classified them.
Adolf Meyer for the first time in 1836 discovered the viral disease tobacco mosaic. He proved that sap derived from diseased plant leaves has the potential to cause infection in healthy plants.
F.F. Smith in 1891 through his studies on Peach yellow disease concluded that it was an infectious disease whose transmission from diseased to healthy plants occurs through Grafting and Budding.
Ivanowski in 1892 found that the agent causing tobacco mosaic could not be filtered through a chamberland filter although bacteria got easily filtered through it. Thus viruses were found to be smaller than bacteria.
Beijernick in 1898 proved that tobacco mosaic disease was not caused by a microorganism rather it originated from Contagium Vivum fluidum which he later called virus.
Stanley in 1935 treated the sap obtained from Tobacco mosaic virus infected leaves with (NH₄)₂SO₄ and obtained crystalline protein. For this he was awarded Nobel Prize.
Bowden & Pirie in 1936 proved that the crystalline powder of Bushy stunt virus of tomato contained Protein and nucleic acid.
After the invention of Electron microscope by Knoll & Ruska (1932), Kanschietal in 1939 using this microscope studied the shape and size of the virus particle.
Geirer and Schramm in 1956 proved that nucleic acid of the virus particle was the actual disease inciting agent.
Viroid is only a naked nucleic acid molecule. The term ‘viroid’ was by T.O. Diener. Viroid is devoid of any protein coat and its RNA has a high molecular weight.
Potato Spindle tuber was the first disease reported to have been caused by a viroid.
Virus → Nucleic acid (DNA or RNA) + Protein (outer cover)
Lipo-virus → Nucleic acid + Protein + lipid (e.g. Influenza virus)
Animal virus → DNA + Protein (or Bacteriophage)
Plant Virus → RNA + Protein
Viroid → Nucleic acid only
Plant viroid → RNA only
DNA → Deoxyribose nucleic acid
Plasmid/Episome: Extra-chromosomal fragments found in bacterial cells.
Virus, viroid and plasmids all contain nucleic acid and lack their own metabolic potential.
Japanese scientist Doi et at in 1967 explained that diseases like witches broom of Potato, mulberry dwarf and Aester yellows etc. which were earlier thought to be of viral origin were actually caused by MLOs (Mycoplasma-like Organism’s).
Mycoplasma are larger than viruses but smaller than bacteria. They are devoid of Cell wall and cytoplasm is enveloped by a lipo-protein plasma-membrane. It is highly resistant against the antibiotic Penicillin but is sensitive to Tetracyclin antibiotic. Tetracyclin is therefore used to control mycoplasma.
Mycoplasma was first isolated from sheep infected by Pleuro-pneumonia and therefore called PPLO (Pleuro-pneumonia like organisms). Most of the yellow diseases of plants are caused by Mycoplasma.

India

K.R. Kirtikar was pioneer worker on plant pathology in India. Kirtikar was the first Indian scientist who collected many Fungi and identified them.
E.J. Butler of Imperial Research Institute, Pusa (Bihar) before 1910, did detailed studies of Fungi and diseases caused by them. He wrote a book Fungi and Diseases in Plants. He is therefore called the Father of Modern Plant Pathology in India.
J.F. Dastur (1886-1971) was the first Indian plant pathologist to study in detail on fungi and plant diseases.
B.B. Mundkur developed resistant varieties to control cotton wilt disease. The credit for identifying and classifying the smut fungi found in India also goes to Mundker. He established the Indian Phytopathological Institute and started the publication of Indian Phytopathology in 1948.
Dr. Karam Chand Mehta (KC Mehta) of Agra College discovered diseases cycle of cereal rust in India.
Prof. Jaichand Luthra and Sattar developed Solar heat treatment technique of seeds to control loose smut disease in wheat.
M.J. Thirumalachar performed extensive studies on **rusts and smuts **in India. Thirumalachar on joining ‘Hindustan Antibiotics’ developed antibiotics like Oreofungin and Streptocyclin which lead to their successful use in plant-disease control in later years.

Important Terms

Blight: A non-restricted tissue disintegrating symptom characterized by general and rapid killing of leaves flowers & stem.
Hyperplasia: Excessive development due to increase in the number of cells.
Hypertrophy: Excessive growth due to increase in size of cells.

Hypersensitivity: Excessive sensitivity of plant tissues to certain pathogens. Affected cells are killed quickly, blocking the advance of obligate parasites.
Parthenogenesis: Formation of embryo without fertilization.
Rogueing: Removing of unwanted (virus infected) plants from a field of crop.
Obligate Parasite: A parasite that in nature can grow and multiply only on living organisms.
Facultative parasite: An organism that is usually saprophyte under certain conditions may become parasite.
Facultative saprophyte: An organism that is usually parasite but may also live as a saprophyte.
Damping off: Destruction of seedling near the soil surface, resulting in the falling of seedling on the ground.
Mildew: A plant disease caused by fungus in which the mycelium and spores are seen as a whitish growth on the host surface.
Virulent: Strongly pathogenic.
Incubation period: Period between infection and appearance of symptoms induced by parasitic organisms.
Ination: Deformity caused by viral infection.
Eradication: Control of plant diseases by eliminating the plants that carry the pathogen.
Exclusion: control of plant disease by excluding the pathogen or infected plant material from disease free areas.
Necrosis: The death of cells or of tissues.

Alternate host: One of the two kinds of plants on which a parasitic fungus (e.g. black rust of wheat caused by Puccinia graminis tritic) must develop to complete life cycle.
Autaceious fungus: A parasitic fungus which completes its entire life cycle on the same host (e.g. Melampara lini).
Hetoroecious fungus: Passing different stages of life history in different hosts.
Agar: A gelatin-like substance obtained from sea weed (red algae Gracilaria, Gelidium etc.) and used to prepare culture media on which microorganisms are grown for study.
Smut: A disease caused by Ustilaginace characterized by masses of dark, Powdery spores.

Sooty mould: The sooty envelope formed by the fungal mycelia on the surface of leaves and fruits.
Witches broom: Broom like growth.
Mycorrhiza: Symbiotic relationship between roots of higher plants and fungal mycelia which is essential for the growth of these plants.
Fungistatic: A compound which prevents fungal growth without killing the fungus.
Rust: A disease of grasses and other plants giving a rusty appearance to the plant and caused by uredinales (rust fungi).
Canker: A necrotic or sunken lesion on a stem, branch or fruit of a plant (e.g. citrus canker caused by Xanthomonas citri).
Plasmogamy: Fusion of cytoplasms of two cells.
Physiologic race: One of a group of microorganisms like in morphology but unlike in certain cultural, physiological, pathological or other characters.
Physiological specialization: The existence of a number of physiologic races or forms within a species or a pathogen.
Biotype: A subgroup within a species usually characterized by the possession of a single or a few characters in common.
Race: A genetically distinct mating group within a species; also a group of pathogens with distinct pathological or physiological characteristics.
Latent virus: A virus that does not induce symptoms in its host.
Latent infection: The stage in which a host is infected with a pathogen but does not show any symptoms.
Gummosis: Production of gum by plant tissue.
Masked symptoms: Virus induced symptoms which are not visible under certain environment conditions but get to be expressed under certain conditions of temperature and pressure.
Spot: Disease symptom in which certain restricted tissue disintegrating areas are produced on leaves, stem and fruit.
Hustorium: A projection of hyphae into host cells which acts as a penetration and absorbing organ.
Rhizoids: A short, thin hypha produced by a thallus that grows towards the substrate.
Toxin: A compound produced by a microorganisms and being toxic to a plant or animal.
Bacteriostatic: A chemical or physical agent that prevents multiplication of bacteria without killing them.
Bacteriophage: A virus which infects specific bacteria and kills them.
Scorch: Burning of leaf margins as a result of infection or unfavourable environmental conditions.
Blotch: A disease characterized by large and irregular spot or lesions on leaves, shoots and stems.
Exudate: Liquid discharge from plant tissue.
Heterotrophic: An organism depending on an outside source for organic nutrients.
Host: A plant that is invaded by a parasite and from which the parasite obtains its nutrients.
Scab: A rough, crust like diseased area on the surface of a plant organ. A disease in which such areas are formed.
Gall: A swelling produced on a plant as a result of infection by certain pathogens.
Yellows: Yellowing and stunting of host plant.
Antigen: A substance (usually a protein lipid or carbohydrate) which after entering into a body activates the production of antibody.
Antibiosis: the phenomenon in which a substance produced by one microorganism is harmful to another organism.
Antibody: A protein produced by specific stimulation when a foreign antigen enters into the blood of an organism. Antibodies get attached with the antigens and makes them ineffective or harmless.
Phytoalexin: A substance that inhibits the development of a fungus on hypersensitive tissue, formed only when host plant cells come in contact with the parasite.
Etiolation: yellowing of the plant due to deficiency of light.
Chlorosis: Yellowing of green tissue due to chlorophyll destruction.
Mottle: A mottle is a pattern or blotches of different colors and shades that cover the surface of a plant. The colored spots are due to anthocyanine pigments.

Foliocellosis/Frenching: A disease caused due to deficiency of Zn in fruit trees (specially belonging to citrus family) in which new leaves develop inter-veinal chlorosis, get reduced in size and branches are also not properly developed. The plant has a bushy appearance and the branches show dieback symptoms. It is also called leaf mottle disease.
Pleomorphism/polymorphism: Having various forms in a life cycle - The rust fungus is allomorphic as it produces five different types of spores in its life-cycle.
Epidemic disease: A wide spread & severe outbreak of a disease.
Endemic disease: A disease which regularly occurs on a particular area of earth or country.
Saprophyte: In organism which lives on dead and decaying organic matter.
Downey mildew: A plant disease in which the mycelium & spores of the fungus appear as a downey growth on the host surface.
Mosaic: Symptom of certain viral diseases of plants characterized by intermingled patches of normal and light green or yellowish colour.
Wilt: Loss of rigidity and dropping of plant parts wholly or partially.
Rugose: Rough and crinkled leaves produced as a result of viral infection e.g. Rugose mosaic of Potato.
Russetting: Brownish roughened areas on fruit skin produced as a result of excessive cork formation.
Rickettsia like Organisms: RLOs a prokaryotic microorganism having a cell wall and obligate infra-cellular parasite.
Susceptibility: The inability of a plant to resist the effect of a pathogen.
Susceptible: A plant or species which is incapable of resisting the effect of a pathogen.
Pathogen: An disease causing agent in plant.
Resistance: The ability of an organism to overcome, completely or partially the effect of a pathogen.
Vector: An insect able to transmit a pathogen.
Disinfectant: A physical or chemical agent that frees a plant or organ from infection.
Immunity: The state of being exempted from infection by a given pathogen.
Ring spot: A circular chlorotic area with a green centre symptom of many viral diseases.
Virion: A complete virus particle.
Carrier: A plant or an organism which carries an infectious agent but does not show symptoms of disease produced by the agent.
Lesion: A localized area of discoloured, diseased tissue.
Rot: The softening, discolouration and disintegration of a succulent plant tissue as a result of fungal or bacterial infection.
Disinfectant: An agent that kills or inactivates pathogens in the environment or on the surface of the plant, prior to infection.
Antagonistic symbiosis: Parasitic symbiosis in which one organism benefits from another.
Shot hole: A symptom in which small diseased fragments of leaves fall off and leave small holes in their place.
Vein clearing: Distruction of chlorophyll in the vein tissue, as a result of infection by a virus or other pathogen.
Vein banding: Bands of green tissue along the veins while the tissue between the veins become chlorotic.
Die back: Progressive death of shoots and roots generally starting at the tip.
Anthracnose: A leaf spot or fruit spot type of disease caused by fungi that produce their sexual spores in an acervulus.

Disease Cycle

Infection: Establishment of the pathogen in the host.
Infectious disease: A disease caused by a pathogen which can spread from a diseased to a healthy plant.
Quarantine: Control of export and import of plant to prevent spread of diseases or pests.
Syndrome: A set of symptoms which characterize a disease.
Transmission: Transfer of pathogen such as viruses from one plant to other.
Conjugation: A type of sexual reproduction in which morphologically similar gametes fuse.
Culture: Growing microorganisms on a prepared nutrient medium.
Homothallic fungus: Fungi producing compatible male and female gametes on the same mycelium.
Heterothallic fungi: Fungi producing compatible male and female gametes on the physiologically different mycelia.
Pustule: Small blister like elevation of epidermis.

Rust in India

In our country, out of the major two rusts infesting crop wheat, leaf rust and stripe rust, the former is more harmful.
The three types of host related with this disease are as follows:
- Alternate host: This host is required to complete the life-cycle. e.g. Barberis is the alternate host of stem rust whereas Thalictrum is the alternate host of leaf rust. In India however alternate hosts have no importance in the life-cycle of rust fungus. Yellow rust has no alternate host in India.
- Collateral host: Besides agricultural crops the uredial and conidial stages of the rust pathogen survive on Grass hosts e.g. Bromus sp and Agropyron are the collateral host of yellow rust. Brame grasses serve as collateral host for leaf rust and similarly Bromur sp (In Northern India) and Brija minor (in Southern India) are collateral hosts for Black rust.
- Primary host: The host on which the rust pathogen produces its telial & resting stages is the Primary host. Dr. K.C. Mehta and his team have identified three hosts on which uredial spores are produced.
  - Continued available host
  - Self-growing wheat plants
  - Grass host e.g. Bromus etc.
Mehta and his associates undertook research surveys from the foothills to the higher altitudes of Himalayas and concluded that Rust fungus perpetuates in hills of Northern India. L.M. Joshi, a student of Dr. Mehta reported that although the rust pathogen perpetuates in Northern India but its main source was the coastal areas of Indian Ocean and Bay of Bengal.
According to Dr. K.C. Mehta wheat crop occupies maximum time of the year in hilly areas of Sindh. The short period of 2-3 months when the wheat crop is not available in the fields, the pathogen survives through its uredial stages on wild wheat and other grassy hosts.

Predisposing factors

Nearness of hills.
Speed and direction of wind
Amount of moisture and sunlight
Suitable hosts
There are four stages in the life cycle of rust organism. These have been represented by roman digits in the given table. The functions of these four stages had been well understood before 1927.

Craigie in 1927 discovered the function of Pycnial stage and reported that plasmogamy occurs in this stage.
Craigie for the first time reported Heterothallism in Rust fungus. Among these five stages only uredial (II) and telial (III) are found on wheat.
Pycnial (0) and Aecial (I) stages are present or Berberis and Mahonia whereas Besidial stage is produced on the inactive substratum on which teleutospores are produced.

Physioligical Specialization:

Erikson (1893) reported that Puccinia graminis was not a composite pathogen. He classified it into five formae specialis or varieties.
These varieties differed from each other with respect to the shape of uredospore and other morphological characters such as colour and roughness.
The term f. sp. has been placed between gaminis and tritici which indicates that triticum is a spetial form of Triticum allies. This very property of the organism is called Physiological specialization.
Later Stakman (1915) reported that even puccinia graminis f. sp. tritici was not a composite organism. He concluded that the special form of Puccinits graminis tritici was again made up of several forms which he called ‘Biotypes’. He again observed that even a Biotype was not a composite organism and was composed of several ‘races’.
Biotypes: represented by Arabic numerals i.e. 1, 2, 3, 4
Races: represented by roman letter a, b, c, d
Example: the complete name of fungus causing Black rust/stem rust is as follows:

Puccinia graminis f. sp. tritici 16d

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