Fundamentals of Plant Pathology PAT-101 2019-20 PDF

Extended Syllabus for the Course PAT-101(2+1) Semester: II Year-2019-20 Class : I year B.Sc. (Ag. and Agmaco) Class no. Topic 1 Introduction about course, Definition of plant Pathology, scope and objectives of plant pathology. 2-3 Significance and Economic importance of plant diseases : late blight of potato, coffee rust, brown leaf spot of paddy, stem rust of wheat , bacterial blight of paddy. 4-6 History of plant pathology with special reference to Indian work (Important contributions of Leeuwenhoek, M. Tillet, Prevost, Anton de Bary, Millardet, Flor, Robert Koch, T.J Burrill, E.F.Smith, Needham, N.A.Cobb, Mayer, Ivanowski, Beijerinck, Stanley, Bawden, Doi et al, Diener, E.J.Butler, J.F.Dastur, B.B.Mundkur, K.C.Mehta, L.C.Coleman,Y.L.Nene. 7-10 Terns and concepts of plant pathology. Pathogenesis, causes/factors affecting diseases development: disease triangle and tetrahedron. Classification of plant diseases based on etiology, parts affected, geographical distribution, crops infected, source of inoculum, symptoms, No. of life cycle 11 General symptoms of plant diseases. 12 Important plant pathogenic organisms, different groups. 13-17 Fungi: General characters definition of fungi, somatic structures, types of fungal thalli, fungal tissues, reproduction ( asexual and sexual) , asexual and sexual spores and fruiting bodies. Nomenclature: Binomial system of nomenclature. Classification of fungi with special reference to characters and important examples of Peronosporales, Erysiphales, Uredinales and Ustilaginales. 18-20 Bacteria: General morphological characters and reproduction , out line of classification. General symptoms produced by bacteria. 21-23 Viruses: General properties plant viruses, replication, transmission and general symptoms produced by plant viruses. 24-26 Nematodes: Morphology of plant parasitic nematodes, outline of classification, symptoms and nature of damage caused by plant parasitic nematodes (Heterodera meloidogyne, Anguina, Radopholus) 27-28 General morphology, character of fastidious vesicular bacteria, phytoplasma, Spiraplasma, viroids, algae, Protozoa with examples of diseases caused by them. 29 General morphology of Phanerogamic parasites. 30 Diseases and symptoms due to abiotic causes(Black heart of potato, khaira of rice, blossom end rot of tomato) 30-31 Liberation/ dispersal and survival of plant pathogens. 32 Types of parasitism and variability in fungi. 33-34 Pathogenesis: Role of enzymes,(cutinase,pectinolytic enzymes) toxins(victorin, Tab toxin, T- toxin, Ak-toxin, patho-toxins, vivo-toxins, phyto-toxins) and growth regulators in diseases development. 35-36 Defense mechanism: structural (cork layer, abscission layer, tyloses,) Biochemical (phenolics, phytoalexin and PR-proteins), 37-38 Epidemiology: Elements of an epidemic, Factors affecting development of epidemic. Reference books: 1.Plant Pathology – G.N. Agrios, 2005 (5th Edition) 2.Introduction to principles of Plant Pathology – R.S. Singh. 3. Plant Pathology – P.D. Sharma, 4. Plant Pathology – R. S. Mehrotra Practical Ex.no. Title of the experiment Date Page No. Remarks 1 Study of microscope 2 Collection and preservation of diseased specimens 3 Study of symptoms and Diagnosis of plant diseases 4 Morphological characters of Fungi 5 Preparation of Culture Media 6 Methods of Sterilization and Disinfection 7 Isolation techniques for Fungi and bacteria 8 Methods of Inoculation and proving Koch’s postulates 9 Morphology and staining of Bacteria 10 Morphology and Transmission of Viruses 11 Morphology of plant parasitic nematodes and symptoms 12 Extraction of Nematodes from the soil 13 Study of phanerogamic parasites 14 Study of fungicides and their formulations 15 Calculation of spray concentration and evaluation of fungicides 16 Methods of pesticide application and their safe use Fundamentals of Plant Pathology FUNDAMENTALS OF PLANT PATHOLOGY PAT-101 (2+1) Introduction: Plant Pathology or Phytopathology is a branch of agricultural, botanical or biological science which deals with the cause, etiology, interaction of plant and pathogen resulting losses (both quality and quantity) and management of plant diseases. It is derived from Greek term Pathos= suffering, / ailment, Logos = to study / to discourse / to speak. The term ‘pathology’ etymologically means the “study of the suffering”. Objectives of Plant pathology: It has four main objectives 1) Etiology: To study the cause of disease / disorder (living, non-living including environmental factors) in plants. 2) Pathogenesis: To study the mechanism of disease development by pathogen/s (Host – Pathogen interaction). 3) Epidemiology: To study the development of disease in relation to interaction among plant, pathogen and overall environmental conditions. 4) Control / Management: To develop systems of management of disease/s and reducing the losses caused by diseases. Plant pathology is an integrated science and profession that uses the basic knowledge of botany, mycology, bacteriology, virology, nematology plant anatomy, physiology, genetics, molecular biology, genetic engineering, biochemistry, horticulture, agronomy, tissue culture, soil science, chemistry, physics, meteorology, engineering, nanotechnology, precision agriculture, remote sensing and many other branches of science including applied aspects. It is both science & art. Importance of plant diseases: Crop loss can appear in the field, in transit, in store or any time between sowing to consumption either in quality or quantity or both. Historical examples which affected human activity are mentioned below. Irish famine: Disease: Potato Late blight : Phytophthora infestans Place : Ireland,Year – 1845 In 1845- this destroyed the potato crop of Ireland. Potato was the staple diet of the population. In England free trade and import of food grains was not permitted. When the epidemic of late blight destroyed potato crop in 1845, there was famine in these countries especially in Ireland. Population was reduced from 80 lakhs to 60 lakhs. A large number of people died of hunger and many more became diseased due to physical weakness. There was migration of population to other countries (Canada, Australia, and New Zealand, including the North American continent). These migrated people became pioneers in development of those countries where they settled. Free trade was permitted leading to import of corn (maize). Navy developed for the movement to cross the oceans. Cause was identified by Anton de Bary (1867). UASD/COA/PAT/101/2+1/2020 Page 1 Fundamentals of Plant Pathology Thus late blight epidemic brought plant pathology to limelight along with many social and political changes. Bengal famine: Disease: Brown leaf spot of paddy Helminthosporium leaf spot (sesame leaf spot) Causal agent : Drechslera oryzae (Heleminthosporium oryzae) Year : 1942 Food was not available to common people because paddy crop suffered heavily due to this disease. Ear head and grains became brown and chaffy. In last years of World War II, Bengal had to face a serious famine. One of the reasons was loss in yield of rice due to Brown spot of rice which virtually destroyed the crop. In addition public distribution system also failed at this crucial time. Wheat rusts: Causal agents: Stem rust or Black stem rust of wheat : Puccinia graminis f.sp. tritici Leaf rust or brown rust : Puccinia triticina ( Puccinia recondita f.sp tritici) Yellow rust : Puccinia striiformis They appeared in epiphytotic form in many parts of the world. This forced the farmers in many countries to change the cropping pattern and population to opt for a particular kind of bread. No grain formation and stem becomes black in color. If at all grains are formed they are shriveled and nutritionally very poor for consumption. Ex: Northern and southern Europe wheat bread became more common. Central Europe people preferred to eat rye bread. In recent years Ug-99 (Uganda in Africa) of stem rust race of wheat affected wheat crop in Uganda. Later it spread to other parts of Africa and Asia. This new race was highly virulent and devastated important stem rust resistant genes of wheat Coffee rust: Place : Srilanka (Ceylon) Causal agent: Hemileia vastatrix Year : 1867 During mid 19th century, coffee and tea were being equally consumed in England as these were available in India, Ceylon, and Malaya. Ceylon was producing maximum coffee in the world. In 1867, coffee rust epidemic affected the coffee plantation and by 1871, there was substantial reduction in the yield (228 kg/acre) going down further to 101 kg/acre in 1878. There was great loss to Ceylon and by 1893 coffee export of Ceylon was reduced by 93%. This forced the growers to eradicate coffee plantation and to take tea plantation. Ceylon began to export tea to England, where tea consumption increased with time. When coffee rust was spreading in Sri Lanka (Ceylon) and it was not prevalent in south America, decline in coffee cultivation in Sri Lanka caused a rapid increase of its cultivation in Brazil, now producing maximum coffee in the world. But presently (from 1978 onwards) coffee rust is also reported from Brazil. Southern leaf blight of maize Place : USA Causal agent : Helminthosporium maydis Year : 1970 This disease appeared in severe form in USA due to use of Texas Male Sterile (TMS) line in corn breeding program in 1970. The prevalent race “O” was UASD/COA/PAT/101/2+1/2020 Page 2 Fundamentals of Plant Pathology mutated to race “T” and female parent line showed high susceptibility to race “T” of Helminthosporium maydis Citrus canker : Year : 1910 Place : Florida, USA Causal agent: Xanthomonas axonopodis pv citri (Bacterial disease) Citrus canker appears on all aerial parts of the plant parts viz;, leaves, petiole, stem, thorn including fruits affecting the marketability of fruits. The affected plants and nurseries were uprooted and compensation was given to farmers for the loss of plants and planting material It was considered as an example of Eradication of a disease from USA. However, in 1984, the disease reappeared and now it was decided to accept that Eradication of citrus canker is not possible. It is also an example for failure of Eradication of Plant Disease. Bacterial leaf blight of paddy: Year : 1963 Place : South India Causal agent: Xanthomonas oryzae pv. oryzae It reduced the yield and also quality by broken rice grains. Wavy margin on leaf surface, leaves becoming hard and formation of yellow pearls on leaves are important symptoms. It is seed borne and affected many weeds of paddy crop. At warm temperature it spreads through irrigation water and rain. Bacterial blight of pomegranate : Place : Maharastra, Karnataka, AP Year : 2002 & afterwards Causal agent: Xanthomonas axonopodis pv. punicae This disease was a minor disease in 1952 causing leaf spot. It caused more loss after introduction of Kesar / Bhagawa variety and getting the fruits in Mrig bahar (June pruning) in South India. First Research centre for single fruit of minor fruit crop in the world is present in India i.e., NRC on pomegranate in Solapur, Maharastra. Highly susceptible genotype for this disease is Bhagawa or Kesar variety of pomegranate in India, till today there is no resistant genotype is found for this disease. Chestnut blight disease Plant diseases limit the kind of plants or industries that can grow in large geographical area, Ex: American chestnut (the queen of eastern American forest trees) is eliminated from North America as a timber tree by the chestnut blight Cryphonectria parasitica (Endothia parasitica). Dutch elm disease Causal agent: Ophiostoma ulmi Place : USA American elm eliminated as a shade tree by Dutch elm disease. Dutch elm disease is caused by Ophiostoma ulmi, (USA), O. novo ulmi, (Europe) and O. himal ulmi (Himalaya region of India). It is an ascomycetes fungi also known as Ceratocystis ulmi. Spread of the fungus is by the beetle (Scolytus multistriatus). Spores when come in contact with xylem vessels, they reproduce rapidly by budding. The fungus can also spread by natural root grafts. Important symptoms include wilting symptom leading to death of plants in 2 -3 months. It is an example of Pandemic disease. UASD/COA/PAT/101/2+1/2020 Page 3 Fundamentals of Plant Pathology Downy mildew of grapes Place : Europe (France) Year : 1878 Causal agent: Plasmopara viticola Formation of white color mycelial growth on lower surface of leaf with corresponding oily yellow spot on upper surface of leaves is important symptom. Inflorescence and berries are also affected leading to defoliation and immature fruit drop hence no fruit bunches. Downy mildew of grape is responsible for discovery of Bordeaux Mixture by Millardet from by Bordeaux University and it is the first copper based chemical for bacterial and fungal disease management and till date it is used in horticultural crops. Composition of Bordeaux Mixture (BM) is Lime: Copper sulphate : Water (1Kg : 1Kg : 100 liters in CGS OR 5 lbs : 5 lbs : 50 gallons in FPS system) 1 Kg lime in 50 liters of water and 1 Kg copper sulphate in other 50 liters of water and both are poured simultaneously in third vessel to get one per cent BM. Powdery mildew of grapes Place : Europe (France) Year : 1954 Causal agent: Uncinula necator Presence of white colored mycelium observed on upper surface of leaf, tendril, inflorescence and berries. Due to severe attack cracking of berries is seen. It reduced French wine production by 80% in 1954. These instances of plant epidemics are worth mentioning because they left their effect not only in the country concerned but also in other countries and human beings. Plant diseases are of much significance to humans in several ways: 1) Plant diseases limit the kind of plants or industries that can grow in large geographical area, Ex: American chestnut (the queen of eastern American forest trees) is eliminated from North America as a timber tree by the chestnut blight Cryphonectria parasitica (Endothia parasitica). American elm being eliminated as a shade tree by Dutch elm disease (Ophiostoma ulmi) 2) Disease may reduce the quality and quantity of plant produce. Extent of loss varies with plant pathogen and environment and management practices followed. Quality of produce is affected by post harvest disease of vegetables/ fruits, citrus canker, mango anthracnose, chilli fruit rot, apple scab. 3) Some diseases make the plant product unfit for consumption by contaminating them with poisonous substances. Grains infected with fungi will produce mycotoxin Ex: Aspergillus flavus produce Aflatoxin in groundnut, maize, chilli Ergot of sorghum (Claviceps sorghi,), Bajra(C. microcephala.) and rye(C. purpurea) contain toxic alkaloids (Ergotin, Ergotamine). 4) Plant diseases are responsible for direct and indirect financial losses. This includes the cost of plant protection, producing resistant varieties etc. 5) They destroy the beauty of environment by damaging plants around homes, streets, parks etc. 6) Pollution- Toxic pesticides used for managing diseases, pollute the water, soil and environment. UASD/COA/PAT/101/2+1/2020 Page 4 Fundamentals of Plant Pathology Classification of plant diseases: Causes of plant diseases Abiotic factors: They include mainly the deficiency or excess of nutrients, light, moisture, aeration, abnormality in soil condition like pH, atmospheric impurities etc. Mesobiotic: These are diseases which are caused by neither living nor non living things. They are considered to be threshold of life. They are viruses, virusoids and viroids Biotic causes: These category includes the pathogen which are living or cellular organisms. They are phytoplasma, spiroplasma, fungi, protozoa, bacteria, algae, nematode, phanerogamic parasites. Grouping of Plant Diseases in various ways. I. Based on Crops infected/ plant species Ex: Diseases of cereals ( rice, wheat, maize, sorghum, ragi, bajra) Diseases of commercial crops (sugarcane, chilli, cotton, tobacco) Diseases of oilseeds (groundnut, sunflower,linseed, sesamum) Diseases of pulses (greengram, blackgram, bengalgram, cow pea) Diseases of fibre crops (cotton, flax, sunhemp) Diseases of horticultural crops (vegetables, fruits, flowers, medicinal and aromatic plants) Diseases of plantation crops (arecanut, coconut, cocoa, black pepper) Diseases of forage crops (maize, sorghum, lucerne, cowpea) This system is useful to field workers, diagnosticians as well as research workers specializing in the diseases of a plant species. But, this method does not help in understanding the basic principles of plant pathology. II. According to plant parts affected Foliar diseases – Leaf spots, leaf blights, rust, downy mildew, powdery mildew Fruit diseases – Fruit rot, scab, canker, Stem diseases – Stem rot, wilt, stem canker, collar rot, crown gall Root diseases – Root rot, damping off, club root, wilt Many pathogens show tissue specificity- the basis on which this classification is made. Ex: Sclerotium rolfsii on stem and roots But many pathogens don’t show the tissue or organ specificity Ex: Pyricularia grisea- Blast of rice (PS. Magnaporthe grisea and Puccinia graminis f.sp. tritici – Black stem rust of wheat on all aerial parts of plant. III. Based on chief source of inoculum Diseases due to seed borne pathogens Ex: Loose smut of wheat, grain smut of sorghum Diseases due to soil borne pathogens Ex: Wilt, root rot, damping off Diseases due to air borne pathogens Ex: Downy mildew, leaf spot, powdery mildew, rust UASD/COA/PAT/101/2+1/2020 Page 5 Fundamentals of Plant Pathology IV. Based on the extent to which disease are associated. Localised diseases: these diseases are limited to a definite area or only part of the plant Ex: Leaf spot, blast, blight, rust, Systemic diseases: In this pathogen spreads throughout the entire plant to varying extent and is associated with almost every stage of plant’s life cycle. Ex. Crown gall, tobacco mosaic, leaf curl, bud necrosis, wilt of tur. Some time symptoms seen only at ear head stage. (Loose smut of wheat). V. Based on geographic distribution / spread and severity of infection/ occurrence. a. Endemic b. Epidemic c. Sporadic d. Pandemic. a. Endemic diseases: word ‘endemic’ means prevalent in and confined to a particular district or location. When a disease is more or less constantly present from year to year in moderate to severe form in a particular area, it is called endemic to that area. Causal agent is well established in the field, environmental conditions are congenial for survival and multiplication. Ex: Wart of potato is endemic in Darjeeling. Blast of rice in Ponnampet. Late leaf spot of ground nut in Dharwad in Kharif season. b. Epidemic/ Epiphytotic: Derived from the Greek word meaning “among the people” Disease occurs periodically but in a severe state involving major area of the crop. This is because environmental conditions favourable for rapid disease development occurs only periodically. (An epiphytotic disease is one which occurs widely but periodically). Ex: Cereal rust, powdery mildew, blast of paddy c. Sporadic diseases: Occur at very irregular interval and locations and in relatively few instances. Ex. False smut of rice, udabatta disease of rice. d. Pandemic diseases: Diseases assuming epidemic proportion over a very extensive region causing devastating damage in a short period. Spread of disease from one continent to other continent. Ex: Late blight of potato, Stem rust of wheat (Ug-99 race), Dutch elm disease A given disease may be endemic in one region and epidemic in another. VI. Based on symptoms Rust: It appear as relatively small pustules of spores usually breaking through the host epidermis. These pustules may be dusty or compact with red, brown, yellow or black in color. Ex: Wheat rusts, coffee rust. Usually appears on leaf or stem. In case of black stem rust of wheat initially redish brown coloured pustules are seen on leaf and stem. In due course of time all aerial parts are affected including awns. At the final stage of disease the red colour pustule is converted into black due to formation of telium. Hence, the name black stem rust of wheat. In case of yellow rust of wheat (caused by Puccinia striiformis) –yellow pustules are arranged in a straight line i.e., parallel to wheat leaf midrib or veins. UASD/COA/PAT/101/2+1/2020 Page 6 Fundamentals of Plant Pathology Note: white rust of crucifer: it is not true rust because they are taxonomically belong to different group (Albuginaceae of Oomycetes). True rusts belong to uredinales of Basidiomycetes. Recently new name is Pucciniomycetes. Smut: It means sooty or black or charcoal like powder. Usually appears on floral parts. In some cases it appears on stem, leaf and root. The affected parts of the plants show a black dusty mass of spores. Example: Sugarcane whip smut (growing vegetative shoot), loose smut of wheat (grains). In sorghum four smuts are noticed on inflorescence.a. head smut (no grain formation), b. grain smut, c. loose smut and d. long smut. Leaf smut is seen in case of onion, maize. Wilt: In wilt, leaves or shoots lose their turgidity and drooping of leaves with yellowing is seen because of disturbance in the vascular system of root and stem. Drooping of leaves is also called epinasty symptom. The plant when split open longitudinally, xylem vessels (water conducting tissue) changed its color to brown (melanin pigment) as it is attacked by the pathogen. Organisms which cause wilt disease are– Fusariuim, Verticillium both fungal and Ralstonia solanacearum cause bacterial wilt of solanaceous crops. Host plants like brinjal, tobacco, potato are affected. Old name of Ralstonia solanacearum was Pseudomonas solanacearum. Bacterial wilt is identified by oozing test. Fusarium wilt shows continuous browning of vascular bundle while Verticilium wilt shows discontinuous browning of vascular bundles. Based on temperature, Fusarium wilt usually appears at higher temperature (28 to 30 ) 0C while Verticillium appears at lower temperature (16 to 18 0C. Cucumber wilt is caused by bacterial pathogen (Erwinia tracheiphila) Mildews: Here fungal pathogen is seen as white, grey, brownish or purplish growth on host surface either lower or upper or on both surfaces of leaves, berries and inflorescence. Downy mildew: Downy growth on leaf is observed with corresponding yellow spot on upper surface of leaf. In case of bajra downy mildew half leaf symptom is seen due to downy mildew wherein half of the basal part of lower surface of leaf is covered with white mycelial growth. At the ear head emergence it shows the green ear symptom. In pace of normal grains in the ear head small leaves like projection are seen. The family of causal agent is Peronosporaceae and order is Peronosporales of Oomycetes Class. It is seen in humid condition where relative humidity is more. Powdery mildew: Appears on upper surface of leaf during winter season or cool and dry season. Powdery mildew of mango in winter season appears on young leaves of mango, but prominently observed on inflorescence as white dusty growth. In powdery mildew of chilli, tomato, bengal gram, tur white powdery growth is seen on lower surface of leaves. In severe case on both surfaces may be seen. UASD/COA/PAT/101/2+1/2020 Page 7 Fundamentals of Plant Pathology Rots: The affected tissues die(necrotized), decompose to a great extent and turns brown. The symptom is brought about by fungi and bacterial pathogens. In soft rot of vegetables due to bacteria, where the middle lamellae of the host cell walls is dissolved by means of pectinolytic enzymes. Soft rot of vegetables is caused by bacteria (Pectobacterium carotovora ssp. carotovora). Soft rot and dry rot: Maceration and disintegration of fruits, roots, bulbs, tubers and fleshy leaves. Spots: Regular or irregular but confined to limited area of host. In this the cells are killed in the definite limited area and dead tissues become brown or yellow. The dead areas of leaves often shrink and get separated from surrounding healthy tissue resulting in shot hole symptoms. Blight: the term refers to sudden death of leaves, blossoms or twigs. The dead organs usually turn brown or black and may soon disintegrate. Eg: Early blight of potato / tomato caused by Alternaria solani. Late blight of potato (Phytophthora infestans) Leaf curl: It is due to thickening of the veins on the lower surface of leaf. Later small leaves like projections are seen on lower surface of leaf, most predominantly seen in cotton leaf curl. Usually leaf curl symptom is produced by viruses. Exception case is peach leaf curl which is caused by fungus called as Taphrina defomans. Galls: Localized enlargement of plant parts. These are the small raised structures which are easily identified and it is mainly due to change in growth regulator concentration. Crown gall: Uncontrolled growth of plant tissue, systemic in nature, similar to cancer in human beings. Organism responsible for crown gall is Agrobacterium tumefaciens. Cedar apple rust also cause galls (Gymnosporangium juniperi verginianae) Iatrogenic disorder: Side effects of excess chemical used on plant surface. Teratology: Study of abnormalities of physiological development due to mercury,high temperature, radiation etc. VII. Based on cause: [Aetiological classification] Merits of the classification lie in its simplicity and advantage for beginners and for convenience pathogens are well defined. a. Abiotic b. Mesobiotic c. Biotic a. Abiotic factors include mainly deficiencies or excesses of nutrients, light, moisture, aeration. Abnormalities in soil condition, atmospheric impurities etc. Ex: Khaira disease of rice (Zinc deficiency) UASD/COA/PAT/101/2+1/2020 Page 8 Fundamentals of Plant Pathology Black heart of potato (Low O2 and high temp) Blossom end rot of tomato (Calcium deficiency) Mango black tip or necrosis (smoke (SO 2), Boron deficiency) Whiptail of cabbage, cauliflower (Molybdenum deficiency) b. Mesobiotic factors: these are disease causing agents which are either living or non-living Ex: Viruses: infectious agents made up of one type of nucleic acid (RNA or DNA) either single or double stranded enclosed in a protein coat. Ex: Leaf curl of tomato and chilli, Tobacco mosaic, Groundnut bud necrosis disease Viroids: are naked infectious strands of ribo nucleic acid Ex:Potato spindle tuber viroid disease, Citrus exocortis viroid disease, Tomato bunchy top viroid disease, Coconut cadang cadang disease C. Biotic: Includes living or cellular organisms I. Prokaryotes II. Eukaryotes Prokaryotes: 1. Mollicutes: These are wall less prokaryotes that include MLO or Phytoplasma and Spiroplasma Ex: Phytoplasma diseases: Little leaf of brinjal, Sandal spike, Big bud of tomato, Grassy shoot of sugarcane, Sesamum phyllody Spiroplasma diseases: Citrus stubborn and Corn stunt. 2. Fastidious Phloem limited bacteria (Rickettsia like bacteria): These are very small sometimes submicroscopic, walled bacteria causing diseases like citrus greening and Pierce’s disease of grapevine. 3. True Bacteria : Brown rot/wilt of potato, bacterial leaf blight of rice, citrus canker, pomegranate bacterial blight, black arm of cotton, fire blight of apple Eukaryotes 1. Fungi- Rust of groundnut, blast of paddy, leaf blight of maize, 2. Fungal like organisms –Downy mildew of grape, late blight of potato 3. Protozoa- Hart rot of coconut, phloem necrosis of coffee , club root of cabbage, 4. Algae- Red rust of mango (Cephaleuras virescens) 5. Metazoan animals- (Nematodes) Root knot on vegetables, molya disease of wheat, ear cockle of wheat, citrus decline, burrowing nematode of banana 6. Flowering plant parasites: These are angiosperms (phanerogamic parasites) which are grouped based on habit and place of occurrence. Complete stem parasite: Dodder on niger, onion, Partial stem parasite: Loranthus on mango, Complete root parasite: Orobanche on tobacco, tomato, brinjal Partial root parasite: Striga on sorghum, sugarcane UASD/COA/PAT/101/2+1/2020 Page 9 Fundamentals of Plant Pathology Mc New (1960) has proposed a similar classification based on the affected physiological processes as follows. A. Diseases affecting storage of food Ex: soft rot and seed decay B. Diseases affecting hydrolysis and utilization of stored food. Ex: damping off, seedling blight. C. Diseases affecting absorption and accumulation of water and minerals. Ex: Root rot and stem rot affecting cortex. D. Diseases affecting growth (meristematic) activity. Ex: Gall, Smut, Root knot E. Diseases affecting conduction of water F. Diseases affecting photosynthesis- G. Diseases affecting translocation of elaborated food materials Ex: diseases due to virus and phytoplasma. Mycoplasmas Prior to 1967, many plant diseases such as corn stunt, sugarcane grassy shoot, yellows, etc. were considered as viral diseases. In 1967, Doi et al. and Ishiie et al. (1967) from Japan proposed that these diseases are caused by MLO and they were sensitive to tetracycline and thermolabile. The other name given to this organism was Mollicutes Characteristics of Mycoplasma (PPLO): Pleuro Pneumonia Like Organism) 1. They are very small, unicellular, usually non-motile prokaryotic organisms. (60 -100nm) (Gram-ve) 2. Lack of rigid cell wall. Cells are bounded by a triple layer unit membrane. 3. They can be grown on cell free media producing typical “fried-egg” shaped colonies. 4. They are highly pleomorphic showing small coccoid bodies, ring forms and fine filaments which may be branched. 5. They are filterable through bacterial filters (450 nm membrane) 6. They are highly resistant or insensitive to penicillin antibiotic but are inhibited by tetracycline. Penicillin mode of action is on cell wall synthesis while tetracycline acts on metabolism / cytoplasm of cell content. 7. They usually require sterol for growth, 8. Absence of reversion to bacterial forms. 9. Taxonomically belongs to Domain: Eubacteria, Division: Tenericutes, Class: Mollicutes, Order: Mycoplasmatales Family Mycoplasmataceae – Mycoplasma Family Acholeplasmataceae – Acholeplsma Family Spiroplasmataceae - Spiroplasma UASD/COA/PAT/101/2+1/2020 Page 10 Fundamentals of Plant Pathology PHYTOPLASMA They are similar to mycoplasma except they are not yet cultured on artificial medium. Reproduce by binary fission or budding. They are generally present in phloem sieve tubes (osmotic pressure, pH with isotonic concentration). They are transmitted by leaf hoppers, plant hoppers, psyllids. (Insects that feed on phloem tissue). They don’t have flagella, produce no spores and are Gram-ve They are sensitive to tetracycline and chloramphinicol antibiotics Diseases caused by Phytoplasma: 1. Little leaf of brinjal 5. Aster yellows 2. Sugarcane grassy shoot, 6. Sandal spike 3. Coconut lethal yellowing, 7. Pear decline 4. Peach yellow, 8. Rice yellow dwarf. 9. Apple proliferation Incubation period in vector is 10 to 45 days. Phytoplasma multiply in insect vectors in haemolymph, alimentary canal and salivary glands. Spiroplasmas: are helical mollicutes. They were discovered by Davis et.al.,(1972) 1. Cells are pleomorphic varying in shape from spherical or slightly ovoid to helical in liquid medium and branched non-helical filaments 100 - 240 nm or larger in diameter on solid medium 2. Cells lack true cell wall and are bounded by a single triple layered lipo - protein membrane. 3. Helical forms are motile showing two types of motion- a. a rapid rotary or screw type motion b. slow undulating motion 4. Colonies on solid media exhibit a typical “fried egg” appearance. Some 5. Organism is sensitive to tetracycline and resistant to penicillin. 6. Reproduce by budding and binary fission. Diseases caused by spiroplasma Citrus stubborn and Horse radish brittle root - Spiroplasma citri Corn stunt Spiroplasma. kunkelii Phytoplasma and Spiroplasma are phloem inhabiting prokaryotes Fastidious vascular Bacteria: Xylem- inhabiting Fastidious Bacteria (XLFB)- (1973) Phloem- inhabiting Fastidious Bacteria (PLFB)- (1972) UASD/COA/PAT/101/2+1/2020 Page 11 Fundamentals of Plant Pathology Xylem- inhabiting Fastidious Bacteria- (1973- Pierce’s disease of grapes) Fastidious xylem inhabiting bacteria are generally rod shaped cells, very small (0.2-0.5 x1-4µm), bounded by a rippled layer cell wall and cell membrane. Nutritionally fastidious in nature. They are Gram-ve and don’t have flagella Genus Xylella (X. fastidiosa) is the first plant pathogenic bacterium, whose genome was completely sequenced. Sugarcane ratoon stunting bacteria (Leifsonia xyli) is Gram + ve. Earlier name of this organism was Clavibacter xyli ssp.xyli. Bermuda grass stunting is also Gram + ve. No vector for Gram + ve bacteria. They are (Gram - ve.) transmitted by xylem feeding insects, such as leaf hopper (Cicadellidae) and spittle bug (Cercopidae). In two hours it can transmit the disease but no trans ovarial in nature. Graft transmission is seen Sensitive to both tetracycline and penicillin. Diseases caused by Xylem- inhabiting fastidious bacteria- 1. Pierce’s disease of grape 2. Phony peach disease 3. Almond leaf scorch 4. Plum leaf scald 5. Ratoon stunting of sugarcane. 6. Citrus variegated chlorosis. 7. Alfalfa dwarf Phloem inhabiting Fastidious Bacteria: They are generally rod shaped cells (0.2-0.5 x 1- 4µm) Bounded by rippled cell wall and cell membrane They are Gram-ve and have no flagella. Leaf stunting, clubbing shoot proliferation and witches broom are important symptoms Diseases caused: 1. Citrus greening- Citrus greening is also called as Huanglongbing (HLB) in China. It is transmitted by Psyllid - Diaphorina citri and Trioza citri Causal agent: Candidatus Liberobacter asiaticus., Ca. Liberobacter africanus, Ca.Liberobacter americanus 2. Yellow vine diseases of watermelon (Transmitted by cucurbit squash bug) Bunchy top disease of papaya. Transmitted by Leaf hopper (Empoasca papayae) 3. Clover club leaf UASD/COA/PAT/101/2+1/2020 Page 12 Fundamentals of Plant Pathology HISTORY OF PLANT PATHOLOGY Greek philosopher –Theophrastus (384-322 BC) recorded some observation on plant diseases about 2400yers ago. His books ‘Historica plantarum’ and ‘De causis plantarum’ in which he elaborately mentioned about (rust, mildew) plant diseases, but expressed the opinion that they were due to bad air and bad nutrition. In India, agriculture is about 4000 years old and mention of plant diseases was made in this county much before Theophrastus. Ancient books four Vedas (Rigveda, Samveda, Yajurveda, Atharvarveda), Artha shastra of Kautilya, Sushrut samhita by Sushruta are ancient books from India where plant diseases and other enemies of plants have been mentioned. The oldest text in Indian Agriculture ‘Krishi Parashar’ was probably written by Parashar before the Arthashastra. ‘Vrikshayurveda’ – a book written by Surapal in ancient India is the first book exclusively devoted to diseases of plants. (Vriksha- plant, ayurveda- knowledge of medicine) Rubigalia is the festival celebrated by Romans to please God Rubigus and Rubigo by sacrificing red dog to manage stem rust of wheat. Discovery of role of fungi: In 1675, Dutchman Anton van Leeuwenhoek developed the first microscope and described the bacteria seen with this. Zakaria Jenson : Developed microscope and worked for physics of microscope Micheli, P.A. (1729): Italian botanist studied the fungi and saw their spores under microscope. If spores are placed on a piece of fruit (watermelon), they grow into a new thallus. Written a book entitled “Nova Plantarum genera”. He erected genus Puccinia in his friend name Puccini who helped for his research activities. He is considered as Father of Mycology. Tillet (1775), French botanist -. Wheat seeds (stinking smut) which contained a black powder on their surface produced more diseased plants than clean seeds. Proved contagious nature of bunt or stinking smut of wheat Observed that bunt occurrence can be reduced by seed treatment. He believed that some toxin produced by black powder caused the disease but not the fungus. Benedict Prevost in 1807 proved that diseases are caused by microorganisms. Like Tillet he was working with bunt. He discovered the life cycle of the bunt fungus. CuSO4 could be used as chemical treatment for control of disease. Anton de Bary (1831-1888) German scientist. He laid the foundation of modern experimental plant pathology. In 1861, he experimentally proved that Phytophthora infestans was the cause of late blight He also studied rust, smut, downy mildew and rots. UASD/COA/PAT/101/2+1/2020 Page 13 Fundamentals of Plant Pathology Discovered the heteroecious nature of rust fungi in 1885 He was the first to study the physiology of plant diseases He reported the role of enzymes and toxins in tissue degradation Coined the term symbiosis and first to report lichen consists of a fungus and an alga. Considered as “Father of plant pathology” Orton and Biffen – Pioneers in the field of resistance breeding. In 1905 Biffen described inheritance of resistance to yellow rust of wheat. In 1909 Orton while working with wilt of cotton, watermelon and cowpea, developed varieties resistant to disease and distinguished disease resistance from disease escape. Real science of history of plant pathology Eriksson (1894): Swedish scientist studied the variability in fungi for the first time in 1894 when he reported the existence of physiologic specialization in rust fungi. Puccinia graminis var tritici E.C. Stakman (1922): Physiological races in Puccinia graminis f.sp. tritici after testing the performance of Pgt isolates on 12 International Differentials selected from different species of wheat and related species. Julius Khun (1878): German scientist and has given emphasis to management of disease along with study of disease hence Americans accepted him as FATHER OF PLANT PATHOLOGY. In 1878 he wrote a book called “Diseases of cultivated crops their causes and control”. Flor (1946) proposed the gene for gene concept of disease resistance and susceptibility in linseed rust. (For every gene controlling resistance or susceptibility in the host there must be matching genes for avirulence or virulence in the pathogen). Linseed rust is caused by the fungus Melampsora lini. Van der Plank (1963) suggested that there are two kinds of resistance: one controlled by few major genes (oligo genes) and is race specific - vertical resistance and other determined by minor genes (poly genes) but non race specific horizontal resistance. He wrote a book entitled “Disease resistance in plants” He is called father of plant disease epidemiology Gaumann (1946) – Reported the phenomenon of resistance through hypersensitivity. He worked on toxin production by disease causing pathogens. Further, he worked on taxonomy of fungi i.e., classification of fungi. UASD/COA/PAT/101/2+1/2020 Page 14 Fundamentals of Plant Pathology Bacteria: The discovery that bacteria can act as specific infectious agents of disease was first made in animals through the study of anthrax by Robert Koch in 1876. Rod shaped bacteria were seen in the blood stream of diseased animals and he also worked on tuberculosis disease of animals He gave the famous Koch’s postulates for proving that a particular organism is the cause of a particular disease. He also demonstrated the biological specificity of disease agents. Postulates: 1. Continuous association of organism with the corresponding host producing the symptom. 2. Isolation of the organisms in pure culture on artificial media 3. When inoculated to the healthy susceptible host it should show same symptom as it was earlier. 4. After re-isolation of the organism from the diseased specimen, it must be same as that of the first organism. The fourth postulate was given by Erwin F. Smith Limitations: Obligates cannot be cultured in artificial media. Eg: Virus, Viroids, Plant parasitic nematodes, Phytoplasma, Protozoa, fungal pathogens causing the disease like rusts, downy mildews, powdery mildews. T.J. Burrill (1882) (USA): For the first time, reported that plant disease (Fire blight of pear) was caused by a bacterium known as Erwinia amylovora. E.F. Smith (1901) (USA): He discovered many bacterial plant diseases like bacterial wilt of cucurbits and solanaceous crops. He was also the first to notice and study the crown gall disease caused by Agrobacterium tumefaciens (1893-94). He considered crown gall similar to cancerous tumours of humans and animals. He is considered as “Father of Phytobacteriology” for his discoveries and methodologies he introduced to study bacterial plant diseases. Christain Gram: Proposed Gram reaction in bacteria to group bacteria into two major groups (1884) Gram + ve and Gram – ve In the staining procedure, if the bacteria retains the colour of Primary stain till the end i.e. crystal violet color it is Gram positive Eg: Bacillus, Streptomyces, Clavibacter. If it retains the secondary stain after losing the primary stain i.e. safranin (pink color) it is Gram –ve. Eg: Xanthomonas, Erwinia, Pseudomonas, Ralstonia Agrobacterium. Yoshii (1953) showed bacterial leaf blight of paddy can be managed by using bacteriophage. Bacteriophage in a medium can be easily identified by transparent zone in bacterial colony. Waksman S.A. (1951) Received Nobel prize for the discovery of streptomycin. UASD/COA/PAT/101/2+1/2020 Page 15 Fundamentals of Plant Pathology Dye et al. (1980) Introduced the pathovar in the taxonomy of plant pathogenic bacteria. January 1, 1980 is the base line for accepting pathovars in Plant Pathogenic Bacteria. Phytoplasma: Doi et al (1967) in Japan observed that Mycoplasma like bodies were constantly present in the phloem of plants suffering from yellows. Ishiie et al (1967) reported that temporarily MLO’s were disappeared when the plants were treated with tetracycline antibiotic. Spiroplasma: Davis et al (1972) observed, motile, helical, wall-less micro organism associated with corn stunt disease. They called it spiroplasma and this organism could be cultured and characterized. RLB: Windsor and Black (1972) observed a new kind of phloem inhabiting bacteria causing club leaf disease of clover. Next year, similar organisms were observed in grapes infected with Pierce’s disease. UASD/COA/PAT/101/2+1/2020 Page 16 Fundamentals of Plant Pathology VIROLOGY The year 1882 – may be considered as the beginning of the era of plant virology when scientific studies were initiated by Adolf Eduard Mayer, German scientist, working in Netherland (1882-1886). He reported that tobacco mosaic disease was neither due to a micro-organism nor due to nutritional imbalance. He demonstrated the contagious nature of causal agent by artificial inoculation. (Sap transmission or Mechanical transmission) Showed that boiling of the sap of infected leaves destroyed infectivity of causal agent. Dimitri Ivanowskii (Russian botanist) - In 1892, he confirmed Mayer’s findings. He reported the filterability of this through bacterial filters. Martinus Wilhem Beijerinck – Dutch scientist – In 1898 confirmed the findings of both of Mayer and Ivanowskii Causal agent of tobacco mosaic was something other than microbe Agent could pass through porcelain filters and diffuse through agar gel. Called it as “contagium vivum fluidum” (contagious living fluid). Considered as “Founder of Virology” W.M Stanley (USA) - was able to obtain a crystalline protein by treating juice of TMV infected leaves with ammonium sulphate. Crystalline substance remained infective. He concluded that virus was an autoclavable protein that could multiply within living cells. Bawden F.E and Pirrie N.M (1936) showed that TMV is a nucleoprotein and contained phosphorus. H.O Holmes- In 1929 provided a method to quantify the virus in tissues by local lesion host. He showed that amount of virus present in a plant sap preparative is proportional to the nunber of local lesions produced on an appropriate host. Viroids: Diener T. O. (1971) reported the potato spindle tuber disease caused by a small, naked single stranded circular molecule of infectious RNA, which he called viroid. It is written as PSTVd. These are smallest infectious agents in plants and multiply in plant cells. The amount of nucleic acid is insufficient to code for an aminoacid. Other examples are: Coconut cadang cadang disease Citrus exocortis disease Till now viroids are not reported in animals and humans in causing disease. Pruisner (1972) reported PRIONS. These are protein molecule of approximately 55,000 dalton, encoded by chromosomal gene of the host produced in brain cells. UASD/COA/PAT/101/2+1/2020 Page 17 Fundamentals of Plant Pathology Eg: Mad cow disease, Scarpie disease in sheep. Till now no prions are reported to cause disease in plants. NEMATOLOGY Association of a nematode in plant disease was first reported by Needham in England in 1743. He described the wheat seed gall nematode association with wheat seeds, now known as Anguina tritici. Berkeley (1855) reported the root knot nematode Meloidogyne sp. Kuhn- (1857) reported the Bulb or stem nematode Ditylenchus dipsaci. Schacht H.: reported cyst nematode of sugar beet Heterodera schachtii Cobb N.A. Studied the structure of many plant parasitic nematodes and classified them. Coined the term Nematology Developed the techniques for extraction of nematodes. He is called Father of Plant Nematology. Atkinson- Reported the association between Fungus + Nematode. Fusarium wilt of cotton was more severe in presence of nematode. Hunger (1901) Showed that bacterial wilt of tomato was facilitated by root knot nematodes. Hewitt et al (1958) - Virus was transmitted by a nematode. This started studies on nematodes as vectors of viruses. Protozoan diseases: Woronin (1878), Russian scientist studied club root of cabbage caused by Plasmodiophora brassicae. Plasmodium is naked mass of protoplasm covered by a thin membrane hyaloplasm. They do not have cell wall. They produce sporogenous and cystogenous spores. Now they are placed in flagellate Protozoa. Lafonte (1909): Protozoa (flagellates) are found in latex bearing plants – Euphorbiaceae. They thought to be the causal agent of disease. Stahel (1931) found flagellate protozoans infecting the phloem of coffee trees (non laticiferous plant) and causing abnormal phloem formation and wilting of the tree. In recent years myxomycetes and plasmodiophoromycetes members are placed in Protozoa Kingdom. Taxonomy of Protozoa Kingdom : Protozoa Phylum : Euglenozoa Order : Kinetoplastidae Family : Trypanosomatidae No Kochs postulates were proved due to obligate nature of the protozoans. UASD/COA/PAT/101/2+1/2020 Page 18 Fundamentals of Plant Pathology In human beings in Africa, sleeping sickness is caused by Trypanosoma and transmitted by tsetse fly Chemical control: Prevost (1807): Recommended CuSo4 for seed treatment against bunt of wheat. Robertson (1824): Recommended the sulphur for management of Powdery mildew of peach. P.M.A. Millardet (1882 -85) :He was Professor of botany at Bordeaux University, France. He discovered Bordeaux mixture in 1885, (CuSo4 + hydrated lime) for management of downy mildew of grapes. Mathews (1919): Controlled the root knot nematode by chloropicrin or teargas. Tisdale and Williams (1934): Development of dithiocarbamate fungicides from dithiocarbamic acid which is highly unstable in rubber vulcanization process. (Example Ferbam, Nabam, Mancozeb, Maneb, etc) Carter (1943): Reported the nematicidal properties of DD (Dichloropropene dichloropropane). Von Schmeling and Kulka 1966 - Introduced first systemic fungicide, oxathiin derivatives effective against higher fungi. Plantvax (Oxy Carboxin) for rust disease and Vitavax (Carboxin) for smut disease management. Plant Pathology in India Cunningham and Barclay 1850-1875 started identification of fungi in India. Cunningham made a special study of rusts and smuts. Kirtikar, K.R.: was the first Indian scientist who collected and identified the fungi in India. Edwin J. Butler: He made a scientific study of most of the fungal diseases in India. He worked on wilts of cotton, pigeon pea and smuts of cereals. Written a book ‘Fungi and diseases in plants’ and a monograph ‘Pythiaceous and allied fungi’. He was the First Director of Imperial Mycological Institute (1920) (later it was called as Commonwealth Mycological Institute. At present it is called International Mycological Institute situated at Kew, Surrey, London).He is called Father of Plant Pathology in India. Dastur, J.F.: (1886-1971) colleague of Butler. He studied Phytophthora parasitica on castor. He is internationally known for the establishment of Phytophthora parasitica from castor. Kulkarni, G.S.: Worked on downy mildew and smuts of pearl millet and sugarcane. Ajrekar, S.L.: Studied wilt of cotton, smut of sugar cane and ergot of sorghum. UASD/COA/PAT/101/2+1/2020 Page 19 Fundamentals of Plant Pathology B.B Mundkur: Book “Fungi and plant diseases” Control of cotton wilt through vertical resistance. Started Indian Phytopathological Society (IPS) 1947. Journal- Indian phytopathology in 1948. Mehta K.C.: (Agra University): Worked on wheat rust epidemiology in India. Showed that primary source of inocula for wheat rusts are two. One is foot hills of Himalaya and second one is in South India (Neelgiri and Pulney hills). Barbery has no role in Indian subcontinent in relation to black stem rust of wheat i.e., in Puccinia graminace pv tritici. Through wind current uredospore travel and cause the damage to wheat crop. Dasgupta, S.N.:: He carried out study on black tip of mango which is caused by abiotic agent i.e., it mainly related to the Sulphur dioxide injury coming from brick kilns chimney and to some extent boron deficiency. These are responsible to cause the disease. Luthra, J.C. (Punjab): He described the Solar heat treatment for loose smut of wheat , causal agent is Ustilago nuda tritci. It is internally seed borne disease and the mycelium is present in seed as knotted mycelium. Sadasivan T.S.: He is from Madras University, who developed the concept of vivo toxins and worked out the mechanism of wilting in cotton due to Fusarium species. Pavagi, M.S.: From Banaras Hindu University- Varanasi, He worked on Synchytrium, Protomyces, Taphrina. Coleman, L.C.: He is a Canadian botanist who worked on koleroga of areca nut in India which is caused by Phytopthora palmivora or Phytopthora araceae.. He was a first Director of Agriculture in the then Mysore state. Deccan wingless grasshopper is named after him (Colemanea sphaneroides). He encouraged arecanut farmers by giving prizes for keeping the orchard free from koleroga of arecanut. Nene, Y.L.: He worked on pulse or legumes virology in ICRISAT, Hyderabad. He is the 1st Indian to occupy the position of Deputy Director in ICRISAT (Hyderabad). Zinc deficiency in paddy was identified by him in Tarai region of Up i.e., Khaira disease of paddy. Use of sago as solidifying agent in culture medium was given by him. Rayachaudhuri, S.P.: worked on plant virology and he also known as Virology Bhishma of Indian subcontinent. Patel M.K. (1948), From Pune he studied about 40 species of plant pathogenic bacteria and proposed that all plant pathogenic bacteria are to be placed under single family called as Phytobacteriaceae. He suggested seed treatment to manage the seed borne bacterial disease. Bhide: V.P. Worked on bacterial diseases in plant pathology. UASD/COA/PAT/101/2+1/2020 Page 20 Fundamentals of Plant Pathology Rangaswami G. Wrote a book called Diseases of crop plants in india. Thirumalachar M.J.: He conducted studies of smuts and rusts. His association with the Hindustan Antibiotics Limited at Pune resulted in discovery of several antifungal antibiotics. Eg: , aureofungin. Hingorani: Worked on stalk rot of maize. In 1952 described about bacterial blight of pomegranate and recorded about this disease but its affect was seen during 2000’s. A. N. Srivastava: He worked on bacterial leaf blight of paddy SOCIETIES Indian Phytopathological Society -1947 Indian Society of Mycology and Plant Pathology- 1971 (Journal of Mycology and Plant Pathology) Nematological Society of India- 1971 (Indian Journal of Nematology) UASD/COA/PAT/101/2+1/2020 Page 21 Fundamentals of Plant Pathology Disease (DIS - EASE) Components of disease 1) Host: All conditions in host that favour susceptibility 2) Pathogen: Virulence, abundance etc 3) Environment: Favourable for the pathogen and predispose the plant. These three components constitute the traditional disease triangle. Pathogens will not be able to cause a disease unless environmental conditions and suitability of the host are favorable for survival, multiplication and entry of the pathogen into the plant and further development of the disease. This interaction of Host, Pathogen and Environmental constitute the disease triangle. Recently, Stevens (1960), Zadoks (1972) and Van der plank (1975), have recognized the fourth factor time which is an important dimension of plant disease. Time is important in many ways. viz, time of year when host and pathogen are exposed (placed close to each other), time and duration of leaf wetness while temperature is favorable. The effect of time on disease development becomes apparent when we consider the importance of time of year, duration and frequency of favorable temperature and rains, time of appearance of vector etc, Adding time to the disease triangle as a fourth dimension gives disease- pyramid. Disease development in cultivated plants is also greatly influenced by a fifth component, human. Human activity affect disease development in various ways. They affect the kind of plants grown in a given area, their level of resistance, time of planting, density of plants, by cultural practices, chemical and biological control etc. They affect amount of primary and secondary inoculum available to attack plants. They also modify environment of disease development by manipulating time of planting etc. MYCOLOGY Sanskrit name of fungus is KAVAK Totally five kingdoms are there in Eukaryotes. They are 1.Animalia 2. Plantae 3. Protozoa 4. Chromista 5. Fungi In Prokaryotes two kingdoms 1. Archaea 2. Bacteria The largest mushroom known is having 170 cm diameter and weighs 284 Kgs Armillaria ostoyae – Honey mushroom is underground and 1 meter below the ground level and occupies 1500 acres. This is present in Mount Adams of Washington State of USA. Importance of fungi As food, vitamins: Agaricus, yeast Antibiotics : Penicillin Fermentation, Source of enzymes ; Yeasts Association with other organisms: Lichens – Fungi + Algae, Mycorrhizae : Fungi + Roots of higher plants Toxins : Afla toxin (Aspergillus flavus), Ergotins (Claviceps sp.), Fuminosens (Fusarium sp.), UASD/COA/PAT/101/2+1/2020 Page 22 Fundamentals of Plant Pathology Diseases in plants: Late blight of potato – Phytophthora infestans Black stem rust of wheat – Puccinia graminis f.sp. tritici Apple scab - Venturia inaequalis Powdery mildew of grapes – Uncinula necator Loose smut of wheat – Ustilago nuda tritici Diseases in humans and animals: Dendruf, Fungus Fungi are highly evolved forms of microbes which are included in the traditional classification in Thallophyta. (Thallus is a plant body that is undifferentiated into root like, stem like and leaf like structure. It may be a single cell, filament of cells or complicated multicellular structure). Fungi can be described as eukaryotic, achlorophyllous, nucleated, unicellular or multicellular, filamentous thallus which reproduces by division of vegetative cells, well defined asexual /sexual spores and whose cell wall is made up of chitin and small amount of glucan. Thallus of the fungi may be plasmodial, unicellular, pseudomycelial or mycelial. In lowest form (myxomycetes or slime mold) thallus is an amoeboid plasmodium lacking a true cell wall. Plasmodium is naked slimy mass of protoplasm containing several nuclei and enveloped by thin membrane hyaloplasm. Tubular structure consisting of nuclei and cytoplasm is called hypha (Pl. hyphae) and entire vegetative body is called mycelium. Unit of mycelium is hypha. In simple type of thalli the whole body is converted into reproductive cells and is called Holocarpic. At a given time the thallus is in vegetative or reproductive stage. Eucarpic- Part of thallus is converted into reproductive structure and part is in vegetative structure. At a given time both reproductive and vegetative structures are seen. Mycelium may be septate (presence of cross wall) or aseptate (coenocytic) i.e. absence of cross wall. Mycelium may be Intercellular (between the cells) or Intracellular (within the cells). Branching of Mycelium Verticilliate: A whorl of three or more branches is produced from the same point. Sympdodial : Branching at each successive node leading apex becomes restricted in growth and is overtaken by a lateral branch below. Monopodial-The apex of the leading hypha is not suppressed but keeps pace in growth with the most active of the lateral branching from below. Fungus cell: Chemical composition of cell wall is different from that of plant and bacteria and is variable among fungi. Chief Components- Protein and polycarbohydrates or polycarbohydrates mixtures. Cellulose predominate in lower fungi ex: Oomycetes and pseudo fungi. UASD/COA/PAT/101/2+1/2020 Page 23 Fundamentals of Plant Pathology Chitin in higher fungi and majority of fungi. Calcium carbonate and others salts may also be deposited. Function of cell wall Protects the cells from osmotic shock, lytic enzymes of micro organisms. Gives the cell its morphology Prevents desiccation Helps in adhesion of the cell to host surface Protoplasm: Fluid in nucleus is nucleoplasm, outside nucleus is cytoplasm. Cells are described according to their nuclear component and behaviour. 1. Monokaryotic cells: contain single haploid nucleus (n) 2. Dikaryotic cells: contain pair of nuclei which differ in character (n+n) 3. Synkaryotic cells: contain one nucleus with diploid number of chromosomes after fusion of two haploid nuclei of the dikaryotic cells. 4. Heterokaryotic cells: contain more than one nuclei which are genetically different. KINGDOM- FUNGI (TRUE FUNGI) Characters: These are eukaryotes identified by cell wall made up of chitin+glucan. Achlorophyllous, nucleated and spore bearing organisms. These may be saprophytic, parasitic or pathogenic. Reproduce asexually and sexually and unicellular (yeast)or multicellular (filamentous) True fungi has 4 phyla: 1. Chytridiomycota 2. Zygomycota 3. Ascomycota 4. Basidiomycota 1. Phylum Chytridiomycota: Class: Chytridiomycetes Cell wall has chitin with the aseptate mycelium. Zoospores with single whiplash posterior position flagellum.(CPW) Unequal or equal motile gametes fuse to form meiosporangium. Ex: Physoderma maydis –Brown spot of maize. Synchytrium endobioticum-Potato wart Olpidium brassicae- Parasitic on roots of alfalfa and vector for plant viruses. 2. Phylum Zygomycota Class: Zygomycetes (Bread mold) Asexual spores in sporangia Non motile spores i.e, aplanospores and no zoospores Aseptate mycelium. Male and female are not distinguished morphologically hence represented by + and – strain (self incompatibility). Sexual spores are zygospores by gametangial copulation. Ex: Mucor : Bread mold Rhizopus stolonifer: Soft rot of fruits. Glomus and Gigaspora: Endomycorrhiza UASD/COA/PAT/101/2+1/2020 Page 24 Fundamentals of Plant Pathology 3. Phylum Ascomycota Characters: Sexual(telomorph) after meiosis produce ascospore Ascospores (8) generally produced in a sac like structure called ascus (spores are produced in or endogenosly) Asexual spores production has reached the zenith and these are produced in asexual fruiting body (pycnidium ,acervulus , sporodochium and synnema) and externally on conidiophores. Septate mycelium and no zoospores production. Male is antheridium and female is ascogonium. Ascospores are haploid in nature (n). 1. Class: Archiascomycetes: These are primitive fungi having naked ascus and no zoospore production. They have intercellular mycelium. No special fruiting body. Ex:Taphrina deformans Peach leaf curl Taphrina is the only fungi which cause leaf curl disease. 2. Class :Saccharomycetes: These are unicellular , sugar loving fungi. Ascus is naked and no ascocarp. No special fruiting body. Nuclear condition may be haplontic, diplontic and haplo-diplontic type of life cycle. They reproduce by budding or by binary fission Ex: yeast Saccharomyces cerevsiae : Bread yeast Galactomyces sp.: Citrus sour rot 3. Class :Filamentous ascomycetes Sexual fruiting body (Ascocarp) is Cleistothecium (cleisto-closed) They cause powdery mildew disease. These are obligate parasites. Closed ascocarp with appandages on surface. Ex: Powdery mildews Erysiphe polygoni : Powdery mildew of many crops (IS: Oidium) Uncinula necator(PS): Powdery mildew of grapes (IS: Oidium tuckeri). Sphaerotheca pannosa: Powdey mildew of rose Subclasses: A. Pyrenonycetes: Perithecium is sexual fruiting body which is flask shaped with asci inside and in between sterile structures like paraphyses and periphyses (at ostiolar or neck region). Ex: Ergots, wilts, Hypocrea: Perfect stage of Trichoderma Claviceps spp.: Ergot of cereals Gibberella fujikuroi (PS) (IS: Fusarium sp.) (Bakane / Foolish disease of rice). UASD/COA/PAT/101/2+1/2020 Page 25 Fundamentals of Plant Pathology Ceratocystis paradoxa: Pineapple disease of sugarcane. Ophiostoma novo ulmi: Dutch elm disease. B. Loculoascomycetes: Fruiting body is Ascostroma. Asci within locules preformed in ascostroma and asci double wall (bitunicate) Ex: Apple scab. Sigatoka of banana, Mycosphaerella musicola- Sigatoka of banana Venturia inaequalis- Apple scab. Capnodium species- Sooty mold C. Discomycetes: Fruiting body is apothecia (saucer shaped). Paraphyses are seen in between asci. Ex. Blast, white mold of vegetables Sclerotinia sclerotiarum: White mold of vegetables Magnaporthe grisea : Blast of paddy Diplocarpon rosae: Black spot of rose D. Deuteromycetes: These are Fungi Imperfectii and Mitosporic fungi. Sexual reproduction is lacking Well developed mycelium Asexual spores are singly, in groups or in asexual fruiting body, may be hyaline or coloured Ex: Leaf spots and wilts Alternaria solani: Early blight of potato Fusarium spp.: Wilt of many crops Cercospora spp: Leaf spot of many crops Imperfect stage: Perfect stage: Alternaria Lewia Fusarium Ggibberella Cercospora Mycosphaerella 4. Phylum : Basidiomycota Class : Basidiomycetes (Club fungi) Club shaped basidium on which four basidio spores are produced. Sexual fruiting body is basidiocarp. Septate mycelium and non motile spores. Mycelium is differentiated into three a). Primary mycelium (n) –After germination of basidiospore b). Secondary mycelium (n+n) – After somatogamy or spermatisation c). Tertiary mycelium –Sterile for fruiting body (Basidiocarp) Clamp connection seen to maintain dikaryotic condition. Basidium is club shaped Dolipore septum is seen Fertile layer is called Hymenium and sterile layer is called Trama Basidium may be Holobasidium (Entire) or Heterobasidium (Septate) Ex. Rusts (Uredinales), and Smuts and bunts (Ustilaginales), Ganoderma Amanita phalloides - Poisnous mushroom UASD/COA/PAT/101/2+1/2020 Page 26 Fundamentals of Plant Pathology Black stem rust of wheat (Puccinia graminis f.sp.trtici) (Life cycle) Fruiting body Spores Nuclear condition Stage Host Pycnium Pycniospores n 0 Barberry Aecium Aeciospores (n+n) I Uredium Uredospore (n+n) II Wheat Telium Teliospore (n+ n) 2n III Basidium Basidiospore n IV - Other rusts Hemileia vastatrix Coffee rust Puccinia helianthi Sunflower rust Puccinia arachidis Groundnut rust Melampsora lini Linseed rust Gymnosporangium juniper virginianae Cedar apple rust On Primary host telial stage is seen and secondary host is the other host. Autoecious rust: All five stages of rust are seen on single host – Sunflower rust. Heteroecious rust : Part of life cycle is seen on one host and remaining on another host. Two genetically different hosts are required to complete the life cycle. Primary and secondary hosts of heterocious rust fungi Name of the Primary Secondary Causal agent disease host host Wheat stem rust Wheat Barberry Puccinia graminis f.sp.trtici Bajra rust Bajra Brinjal Puccinia penniseti Cedar apple rust Cedar wood Apple Gymnosporangium juniperi virginianae Sorghum rust Sorghum Oxalis Puccinia purpurea. Smut (Ustilaginales) Black mass of spores are usually produced in inflorescence but in sugarcane it is produced in vegetative bud and in maize in leaves, stem and cob. No basidiocarp and no trace of sex organs. Only two kinds of spores are produced (teliospore and basidiospores) Teliospores are also called brand spores, chlamydospores, winter spores, smut spores Basisidiospores are also called sporidia. Dikaryotic mycelium (n+n) is infectious Ex: Smuts and Bunts Ustlago nuda tritici Loose smut of wheat Ustilago maydis Maize smut Urocystis cepulae Onion smut Tilletia caries Hill bunt of wheat Neovassia indica Karnal bunt of wheat Loose smut of wheat is internally seed borne with knotted mycelium in the embryo. Grain smut of sorghum is externally seed borne. UASD/COA/PAT/101/2+1/2020 Page 27 Fundamentals of Plant Pathology BACTERIOLOGY Bacteria are single celled prokaryotes, whose genetic material (DNA) is not bound by a membrane and therefore it is not organized into a nucleus. Bacterial cell is composed of protoplasm which includes cytoplasm, chromatin, vacuoles and whole mass is surrounded by a well defined membrane- cell wall Colony characters: Shapes, sizes, margin and colour etc., are characters taken into consideration. Size: ordinary bacterial cell is about 1-5 µm long and o.5 µm wide. Mycoplasma cell is much smaller than true bacteria. I. Shape of bacteria: Bacteria exist in five different shapes. a. Rod : One axis is considerably longer than the other, eg: Bacillus, Xanthomonas b. Vibrio : One curve only along the length eg: Vibrio cholera. c. Spherical : Any two axis of similar length eg: Coccus. d. Spiral : More than one curves along the length eg: Spirillum. e. L form : Variable shape eg: Clavibacter. II. Size: Bacteria show great variation in size. Spherical form is measured by its diameter, a rod or spiral form by its length and breadth. Most commonly the bacteria measure about 0.5-2 m in diameter as in the case of spherical ones and 0.5 x 2-3 m in case of rods. Mycoplasma cell is much smaller than true bacteria. III. Cell grouping: It is based on 1. Plane of the cell division Behavior of daughter cells following the division of spherical cells are designated as a. Micrococcus: One cell, separately seen. b. Diplococcus: Pair of cells or twins (Diplococcus) c. Streptococcus : Daughter cells arranged in a row in chains.(Streptococcus) d. Staphylococcus : Daughter cells arranged in irregular fashion in clusters (Staphylococcus) e.Sarcina:Successive planes of divisions are at right angles to each other, the daughter cells tend to form packet of eight cells. f. Tetrad : Packet of four cells All these are possible in Bacillus also except Staphylobacillus. Tetrad type is represented by palisade (Four bacilli one above the other in palisade layer) IV. Flagellum: Bacterial locomotion is due to presence of structures known as flagella. They are thin, long hair like appendages protruding through the cell wall with blunt ends directed backwards. They are made up of protein molecule flagellin. Flagellin is sensitive to temperature, mechanical damage. Hence extra care is taken in staining flagella. Flagella differ from pili which also are straight hair like structures originating from protoplast and passing through the cell wall. During forward motion of bacteria cell these organs are bent back words at an angle of 45º Flagella is made up of three parts. i) Base ii) Hook iii) Filament The number and pattern of arrangement of flagella vary according to the species. On this basis number and position of flagella the following groups are recognized. a. Atrichous : Flagellum absent. All cocci are atrichous. (Diplococcus) b. Monotrichous : Only one flagellum at polar or lateral. (Ex.Xanthomonas - Polar position) UASD/COA/PAT/101/2+1/2020 Page 28 Fundamentals of Plant Pathology c. Amphitrichous : One flagellum at each polar end. (Ex. Pseudomonas) d. Lophotrichous : A tuft of flagella at one or both ends.(Spirillum) e. Peritrichous : Flagella surrounding the entire cell. (Ex.Erwinia) Typical bacterial cell Bacterial cell is composed of protoplasm which includes cytoplasm, chromatin, vacuoles, whole mass is surrounded by a well defined membrane- cell wall. Cell wall: Protective device, gives the cell its shape. Mollicutes and L-form bacteria have only a membrane around protoplasm, most prokaryotes have both cell wall and membrane. Cell wall is made up of peptidoglycon or murein. It is a complex polymer of N-acetyl muramic acid and N-acetyl glucosamine linked by amino acid chain. Its chief functions are (i) Mechanical protection, (ii) Acting as selectively permeable membrane, (iii) As an antigen to toxins, (iv) As a site of many enzymatic actions, (v) For determining gram reaction. Capsule is an extracellular, slimy or gelatinous polysaccharide like material forming a loose envelope around the cell. Slime layer: is a modified outer layer of the cell wall. Generally relatively thin layers of exopolymers which are present in almost all bacteria are called slime layer when it is larger and firmly adheres to cell surface- called as capsule. Function : Survival, dissemination and parasitism. Capsules consist of polysaccharides- EPS- Exopolysaccharide. Some bacteria (Ralstonia) use slime as source of (carbon) for growth. EPS is virulence determinant in many bacteria. Ex: Erwinia amylovora. EPS- also protects cells against desiccation and helps in survival of cell in its air-borne stage. Cytoplasmic membrane (Plasma membrane) comprises about 10% of the total dry weight of the cell and contains up to 75 percent protein and 2% carbohydrates. It is a semi-permeable membrane and is principally responsible for the gram and acid fast reaction. It is typically a double track membrane made up of phospholipid and protein. Mesosome is infolding of cytoplasmic membrane. It may appear in two forms (i) Lamellar type and (ii) Vesicular type. It helps in division of bacterial cells. Ribosomes are located in the cytoplasm and act as sites of protein synthesis. Fimbriae or pili In addition to flagella certain structures known as pili are also found. Pili are shorter, thicker threads and more numerous than flagella. The term fimbria is used for common pili, while the term pili have now been reserved for sex pilus (involved in conjugation). Nucleus (Nucleoid) don’t have membrane bound nucleus as in eukaryotes and hence called as nucleoid (genome). It is not easily seen when cells are stained. Bacteria have dispersed DNA. Presence of genetic material (DNA) is confirmed by their ability to stain with Feulgen reagent which is specific for DNA. UASD/COA/PAT/101/2+1/2020 Page 29 Fundamentals of Plant Pathology Endospores: Some bacteria are capable of transforming themselves into small oval or spherical resistant bodies known as endospores. It is characterized by the localized concentration of cytoplasm of the cell finally surrounded by a wall or membrane. None of the plant pathogenic bacteria produce endospores. Bacillus spp. and Clostridium spp. are endospore formers. Capsule: Certain bacterial cells are surrounded by a gelatinous slimy layer known as capsule. It forms a protective covering and acts as a reservoir of stored food. For pathogenic bacteria capsules help in their survival under unfavorable conditions. Plasmid: Extrachromosomal, self replicating, co-valently closed circular double stranded DNA. They play important role in conjugation and tumour inducing capacity in Agrobacterium. Multiplication of plasmid is independent of nuclear division. Vacuoles: Identified in young cells they appear as cavities in protoplasm and contain a fluid known as cell sap. Their number is fewer in old cells. Endospores: When bacteria are gradually subjected to increasingly unfavorable conditions-including lack of water, depletion of available food, many species will die. However, certain rod shaped bacterial cells exhibit striking tolerance to adverse conditions, by forming endospores (spores within cell). Endospores are formed singly, one in each cell and are actually dormant cells. The state of total dormancy is known as Cryptobiosis.The spores are not formed during active and division of cell. The mature spore is released by lysis of mother cell. These spores are highly resistant to UV light, chemicals and heat. Characteristics of plant pathogenic Bacteria 1) Most plant pathogenic bacteria (PPB) are rod shaped. exception Streptomyces which is filamentous. 2) Most plant pathogenic bacteria have delicate, thread like flagella considerably larger than the cells on which they are produced. 3) Most PPB are Gram-ve except Corynebacterium, Rathayibacter, Clavibacter, Curtobacterium, Bacillus, Clostridium and Streptomyces. 4) Cell walls are enveloped by a viscous, gummy material- slime layer (thin and diffuse) capsule (thick and forming definite mass around the cell) 5) Have thin relatively tough, rigid cell walls and an inner cytoplasmic membrane. 6) They are mainly aerobic in nature except Clostridium 7) They are mesophyllic in nature (15- 30 0C) Gram and Acid fast reactions: Differences in cell wall differentiate bacteria in to two groups (Gram + and Gram _ve ; Acid fast and non acid fast) respectively. Staining of bacteria Bacteria are semi transparent and difficult to see in the unstained stage hence, stains are used: To know microscopic and semi-transparent objects visible for their shape and size. To show the presence of various external and internal structures and UASD/COA/PAT/101/2+1/2020 Page 30 Fundamentals of Plant Pathology To produce specific physical or chemical reactions. The term stains and dyes are commonly used by the Biologists but are not the same. A coloring agent that is to be used for general purpose called a dye. A coloring agent that is to be used for biological reaction is called a stain. Biological coloring agents are manufactured with a great care under more rigid specifications. Textile colouring agents with poor characteristics are called dyes. Stains are derived from coal tar, which is a derivative of benzene; some elements or radical may replace one or more hydrogen atoms. Stain is an organic compound containing both chromophore and auxochorme groups linked to benzene rings. Theories of staining: Physical: It is the reaction between two substances without formation of a new compound. Chemical: Some parts of a cell are acidic in reaction whereas other parts are basic. The stains are either anionic (acidic) or cationic (basic). It is proposed that the theory stated that the acidic constituents of the cell (nucleus) reacts with basic stain and the basic constituents (cytoplasm) reacts with acidic stains. Staining solutions: Preparations employed for staining bacteria are largely aqueous solutions with organic solvent like alcohol. Staining solutions generally contain low concentrations (< 1.0%) of stains. Very dilute staining solution acting for a relatively long period of time will produce much better results than a more concentrated solution acting for a shorter interval. Mordant: A mordant is any substance that forms an insoluble compound with a stain and serves to fix the colour to the bacterial cell. Compounds, which function as mordants, are tannic acid and salts of aluminum, iron, tin, zinc, copper and chromium. There are two types of staining solutions: 1. Simple stains 2. Differential stains A simple staining solution contains only one stain dissolved in solvent. It is applied to the bacteria in one application. The purpose of simple stain is to colour the bacteria so that they may be more easily seen and to reveal their shapes and size. The simple staining solutions employed are, a. Carbon fuchsin solution, b. Crystal violet solution, c. Methylene blue solution. Methylene blue is the most widely used stain in bacteriological techniques. Preparation of stain: Methylene blue 0.30 g Alcohol 30.00 ml Distilled water 100.00 ml Dissolve methylene blue in alcohol, add distilled water and make the volume and filter the solution through filter paper. UASD/COA/PAT/101/2+1/2020 Page 31 Fundamentals of Plant Pathology VIROLOGY M.W Beijerinck: used the term ‘Virus’ for Tobacco mosaic agent. Viruses are ultramicroscopic, noncellular entities made up of nucleoprotein which multiplies only in living host cells. NA of virus replicates inside living cells using cellular synthetic machinery (host ribosome) and synthesize Virion or virus particle. 1. Viruses contain one or more pieces of a single type of nucleic acid- either RNA/DNA (never both) and protein coat. 2. Nucleic acid is the genome of the virus which differs among viruses. Viruses rely on living host cells for most of enzymes necessary for their replication. 3. Proportion of NA and protein vary with each virus NA making up 5-40% of the virus and protein from 95to 60%. Normally elongated viruses contain higher protein and lower NA. 4. Genome in NA strand directs the synthesis of specific protein, for the protein coat must be present in all viruses throughout their active phase except at the time of replication. 5. They don’t produce spores. Morphology: Mature particle of plant virus is called virion. Plant viruses differ in size and shape. 1. Elongate – Rigid rod- TMV and Flexuous rod- PVX. 2. Spherical (Isometric / polyhedral) - Tobacco necrosis virus, (TNV), Rice tungro spherical virus (RTSV) 3. Cylindrical / bacillus like- Rice tungro bacilli virus (RTBV). 4. Geminate- arranged in pairs- Bean golden mosaic virus (Begomo) Many plant viruses have split genome- consist of two or more distinct NA strands. Some viruses have all their genome in One particle - Monopartite , Two particles - Bipartite Three particles - Tripartite, Many particles - Multipartite Taxonomy: ICTV International Committee on Taxonomy of Viruses Order : virales, Family : viridae , Subfamily : virinae, Genus : Virus Characters taken for taxonomy of viruses are Morphology: Particle shape is Rod / Filamentous / Isometric / Bacilliform Type of nucleic acid, RNA / DNA Nucleic acid is single stranded (ss) / double stranded (ds) Replication via polymerase enzyme +ve sense RNA or DNA type OR via Reverse transcriptase ( -ve sense RNA or DNA) Whether it exists as as a single unit or divided (genome in one particle (monopartite) similarly bi , tri, multi partite Other characters include symmetry of helix in the helical viruses, or number of arrangements of protein sub units in the isometric viruses, size of the virus and other important physical, chemical and biological properties. UASD/COA/PAT/101/2+1/2020 Page 32 Fundamentals of Plant Pathology Kingdom: Viruses RNA viruses Single-stranded positive RNA [(+) ssRNA] Rod-shaped Genus Type species Remarks 1 ssRNA Tobamovirus Tobacco mosaic virus Contact transmission 2 ssRNAs Tobravirus Tobacco rattle virus Nematode transmission 3 ssRNAs Hordeivirus Barley stripe mosaic virus Seed transmission Filamentous particles 1 ssRNA Potexvirus Potato virus X By contact only. Isometric particles 1 ssRNA Sobemovirus Southern bean mosaic virus Seed, beetles, myrids Filamentous viruses 1 ssRNA Potyviridae Potyvirus Potato virus Y Aphids, Isometric viruses 1 ss(+)RNA Tombusviridae Necrovirus Tobacco necrosis virus Fungal transmission 2 ss(+)RNAs Comoviridae Comovirus Cowpea mosaic virus Chrysomelid beetle 3 ss(+)RNAs Bromoviridae Cucumovirus Cucumber mosaic virus Aphids Bromoviridae Ilarvirus Tobacco streak virus Pollen, seed dsRNA Reoviridae Phytoreovirus Wound tumor virus Leafhoppers (-) ssRNA Bacilliform particles Rhabdoviridae Cytorhabdovirus Lettuce necrosis yellows virus Leafhoppers, Membranous circular particles Bunyaviridae Tospovirus Tomato spotted wilt virus Thrips Thin flexuous multipartite viruses Tenuivirus Rice stripe virus Planthoppers dsDNA Isometric Caulimoviridae Caulimovirus Cauliflower mosaic virus Aphids Caulimoviridae Badnavirus Rice tungro bacilliform virus- Leafhoppers (+)ssDNA Geminiviridae Begomovirus Bean golden mosaic virus, Whiteflies (2 DNAs) UASD/COA/PAT/101/2+1/2020 Page 33 Fundamentals of Plant Pathology 1. Linear ss RNA- monopartite- 2. Linear ssRNA in more than one piece in the same particle Ex: Turnip yellow virus, Potato leaf roll virus 3. Liner ss RNA in > 1 piece, each piece in a separate particle- Ex: Cucumber mosaic virus, cowpea mosaic virus, 4. Liner ds RNA in many pieces but in one particle 5. Circular ss DNA- Gemini virus. 6. Circular ds DNA- Cauliflower mosaic virus Rhabdo viruses are provided with an outer lipoprotein envelope or membrane NA: Each nucleotide consists of base + carbon sugar + phosphoric acid (phosphate) Base- Purine = A, G (Adenine, Guanine) Pyrimidine= C, U, T (Cytosine, Uracil, Thymine) Linear ss RNA = TMV, PVX. Linear ds RNA = Wound tumour virus. Circular ss DNA= Gemini virus, Maize streak virus. ds DNA= Caulimovirus. RNA Viruses –single stranded - TMV, PVX -double stranded - Wound tumour virus DNA Viruses - Gemini Virus -Caulimo virus ICTV- International committee on taxonomy of viruses. Viroids: Viroids are small low molecular weight RNA that can infect plantcells, replicate themselves and cause disease. Ex: Citrus exocortis viroid Potato spindle tuber viroid Chrysanthemum stunt viroid Australian grapevine viroid. Viroids are covalently closed circular RNA molecule. These are the smallest known infectious agents. Structure of viroids is different from rRNA, tRNA and mRNA. Viroids were the first circular RNA s to be discovered in nature. Virusoids: Viruses that contain a viroid like satellite RNA in addition to a linear- single stranded molecule of genomic RNA. Virion: Complete and infectious nucleoprotein particle of the virus. UASD/COA/PAT/101/2+1/2020 Page 34 Fundamentals of Plant Pathology List of plant disease epidemics and their significance Year Name of the Causal agent Place Damage/ disease Significance 1845 Potato late blight Phytophthora infestans Ireland Irish famine 1868 Coffee rust Hemileia vastatrix Ceylon Started Tea (Now plantation in place Srilanka) of coffee 1878 Downy mildew of Plasmapara viticola Europe - Affected wine grape France industry Development of Bordeaux Mixture 1909 Citrus canker Xanthomonas Florida USA Disease eradication axonopodis pv. citri (introduced programme from Japan) 1910 Dutch elm disease Ophiostoma ulmi USA By 2010 around 60 million tees lost 1942 Brown leaf spot of Helminthosporium Bengal Bengal Famine paddy oryzae province, Drechslera oryzae India 1954 Powdery mildew of Uncinula necator Europe Affected wine grapes industry 1963 Bacterial leaf Xanthomonas oryzae pv. South India Affected quality of blight of rice oryzae grain (broken grains) 1970 Southern corn leaf Helminthosporium USA Texas Male Sterile blight maydis line as female parent 1999 Wheat rusts Introduction of Rye Stem rust Puccinia graminis f.sp Central bread tritici Europe and Race Ug 99 Leaf rust Puccinia triticina Uganda Yellow rust Puccinia striiformis 2002 Bacterial blight of Xanthomonas axonpodis Maharastra, Kesar/Bhagwa pomegranate pv. punicae Karnataka variety. highly susceptible 2011 Chestnut blight Cryphonectria parasitica USA Devastated large (Endothia parasitica) area of sweet chest nut trees. UASD/COA/PAT/101/2+1/2020 Page 35 Fundamentals of Plant Pathology Fungal-Like Organisms I. Kingdom: Protozoa — Unicellular, plasmodial, very simple multicells, or phagotrophic, i.e., feeding by engulfing their food. Phylum: Plasmodiophoromycota Plasmodiophoromycetes — Endoparasitic slime molds Order: Plasmodiophorales — Plasmodium is thallus. They produce zoospores that have two flagella of whiplash and they are unequal and Obligate parasites Ex: Plasmodiophora brassicae - Clubroot of crucifers Polymyxa graminis Parasitic on wheat. Transmit plant viruses Spongospora subterranea - Powdery scab of potato tubers 5) Kingdom: Chromista (Stramenopiles) / Chromalveolata Unicellular or multicellular, filamentous Phylum: Oomycota Class: Oomycetes (water molds, white rusts, and downy mildews) Aseptate (coenocytic) mycelium. Produce zoospores in zoosporangia. Zoospores have two flagella (Anterior Tinsel and Whiplash backwards). Sexual resting spores (oospores) produced by the union of morphologically different gametangia called antheridia (male) and oogonia (female), Gametangial contact produces thick-walled sexual oospore. Diploid thallus. Cell wall composed of glucans and small amounts of hydroxyproline and cellulose Order: Peronosporales Family : 1. Pythiaceae 2. Peronosporaceae 3. Albuginaceae Order Peronosporales includes important genera of plant pathogens. They are Pythium and Phytophthora, Albugo, Downy mildew genera. Pythium sp., : seed rot, seedling damping-off, and root rot, soft rots of fleshy fruits. Phytophthora sp., late blight of potato, root rots, fruit rots, blights and diebacks of trees. Downy mildew causing genera are Basidiophora, Bremia, Bremiella, Peronospora, Plasmopara, Peronosclerospora Pseudoperonospora, Sclerospora, and Sclerophthora UASD/COA/PAT/101/2+1/2020 Page 37 Fundamentals of Plant Pathology Examples: Albugo candida - white rust of crucifers Pythium aphanidermatum: Sporangium is finger type (branched sporangium) but produce vesicle in which zoospore differentiation takes place. Pythium debaryanum: Globose type of sporangium and produce vesicle in which zoospore differentiation takes place. If temperature is 10 – 17 0 C sporangia germinates by producing zoospores (indirect). If temperature is > 18 0C sporangia germinates by germ tube (direct germination) In Phytophthora sporangium is papillate and zoospores are produced in the sporangium Phytophthora infestans Late blight of potato Phytophthora aracae Koleroga of arecanut Phytophthora citrophthora Gummosis of citrus DOWNY MILDEW Bremia lactucae Downy mildew of lettuce Hyaloperonospora parasitica, Downy mildew of crucifers Peronospora destructor, Downy mildew of onion Peronospora tabacina Downy mildew of tobacco / blue mold Peronosclerospora sorghi Downy mildew of sorghum and corn Plasmopara viticola Downy mildew of grape Pseudoperonospora cubensis Downy mildew of cucurbits Sclerospora graminicola Downy mildew of bajra Genus Sporangiophore Sporangial Figure germination Basidiophora Club shaped with sterigmata Indirect on which sporangia borne Bremia Dichotomous, with saucer Direct shaped apophyses with sterigmata Bremiella Dichotomous branching with raised apophyses with sterigmata Sclerospora Strong , stout with Indirect branching at the tip Peronospora Dichotomous branching at Direct acute angle and pointed tip on which sporangia are seen Peronosclerospora Strong , stout with Direct branching at the tip Pseudoperonospora Dichotomous branching at Indirect acute angle and pointed tip on which sporangia are seen Plasmopara Branching at perpendicular Indirect with sterigmata UASD/COA/PAT/101/2+1/2020 Page 38 Fundamentals of Plant Pathology Albuginaceae Sporangia in chain Older sporangia is at the top (basipetal) Between two sporangia sterile disjunctor cell is seen. Produce zoospores in sporangia Obligate parasite Albugo candida White rust of crucifers Powdery mildews Phylum : Ascomycota (Ascomycetes, sac fungi) Teleomorph: Sexual stage Anamorph : Asexual stage Ascospores, Eight within an ascus. Class : Filamentous Ascomycetes Order : Erysiphales Family: Erysiphaceae Sexual fruiting body: Cleistothecium / Chasmothecium Characters used in genera identification Type of appendage Number of ascus / asci (one / many) Type of asxual spores produced Characters used in genera identification 7) Type of appendage: Myceloid, coiled at the tip, dichotomously branched, bulbous base with pointed tip Number of ascus / asci (one / many) J. Asexual spores Conidia in chain (Oidium) Conidia in chain but basal (foot cell) is enlarged Conidiophore branched coming from stomata with single conidium at the tip (flame shaped) K. Mycelium is Ecto / semi endophytic Types of appendages on cleistothecia in Erysiphaceae UASD/COA/PAT/101/2+1/2020 Page 39 Fundamentals of Plant Pathology Order: Erysiphales (the powdery mildew fungi) UASD/COA/PAT/101/2+1/2020 Page 40 Fundamentals of Plant Pathology Levillula – Mycelium semi endophytic, conidiophores comes out from stomata and it is branched and conidia singly produced singly and it

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