Preface This would be a long preface if I was to set down in detail the reasons why I attempted to write this book in present form. Suffice it to say that the text is based on course which I have been teaching to undergraduate students. The purpose is to provide the basic and emerging facts whereby the students may be introduced to the scientific foundation of this noble course called “Plant Pathogens and Principles of Plant Pathology”. This course is offered at undergraduate levels in all Indian Agricultural Universities. There are several books especially on the subject. However, the course requirement of Indian students is not fulfilled by any single source. The prime objective of writing this bookisto fill upthe gap and help Indian students. The mainfeatures of this volume are as follows; The subject matter has been presented systematically with suitable examples. Efforts have been made to provide recent information. The language is simple and easily understandable. It covers, in its 41 chapters, the major part of syllabi offered by Indian universities especiallyin Plant Pathology. The glossary of technical terms has been presented at the end for ready reference. I do not claim originality in the preparation of this book and has taken help from a large number of books, journals, periodicals, bulletins, internet, etc. I humbly thank authors, editors, and publishers of the books, journals, etc. I am also indebted to my wife Dr. (Mrs) Archana Rani for inspiration and help and children Saumya and Adyan for bearing with me during the preparation of this book.

Plants make up the majority of the earth’s living environment as trees, grass, flowers, etc. Plants are the only higher organisms that can convert the energy of sunlight into stored, usable chemical energy in carbohydrates, proteins and fats. Directly or indirectly, plants make up all the food on which humans and all animals depend. Plants however also gets sick, grow and exhibit various types of symptoms and sometimes whole plant die. It is not known whether diseased plant feel pain or discomfort. If a plant is looking different from its community then it is equal to be disease one. Any biotic or abiotic agents which induce the disease in plant is referred as the cause of diseases.The causative agents of disease in plants are pathogenic such as fungi, bacteria, viruses, protozoa and nematodes and environmental conditions such as lack or excess of nutrients, moisture, light, etc to presence of toxic chemicals in air or soil. These biotic constraints can, at times, seriously compromise food security, For example, potato late blight, caused by Phytophthora infestans, struck Europe like ‘a bolt from the blue’ in the 1840s. In Ireland, about a million people died of starvation and more than a million attempted to emigrate. The reasons for this calamity were the arrival in Europe of a virulent strain of the pathogen, the high dependence of much of the Irish population on potato for sustenance, the lack of resistance in the plant to the pathogen, and weather conditions favorable to epidemic development. There have been other disasters caused by plant diseases such as the Great Bengal Famine of 1943 and the southern corn blight epidemic of 1970-71 in the USA, to name but two. In the former, an estimated 2 million people died owing to the high dependence of most of the population on a single crop, rice, which was attacked by the fungus Cochliobolus miyabeanus. In the USA, by contrast, although in some genus, Cochlobolus heterostrophus, alternative sources of nutrition were plentiful so no one died, although the effect on the agricultural economy was severe. The first two of these heartbreaking examples demonstrate with brutal clarity that in areas of the world where a large proportion of the population is dependent on a single crop or a few crops, they are at risk should that crop fail owing to one or more devastating diseases. At the present time, the threat is partially great in developing countries, where populations are growing fastest, poverty is endemic , the population depends upon on locally produced staples, and the infrastructure of extension and R&D is often poorly resourced. The losses due to weeds, disease and insects have been estimated to around 40% in the developing and underdeveloped countries. If the post harvest losses (15-20%) are also added , the situation becomes even more alarming.

DEFINITION The term Plant Pathology is derived from Greek words- pathos (suffering) + Logos (study) i.e. the study of the suffering plant. Plant Pathology has two phases broadly- Science- understanding of the disease i.e. the theoretical consideration of the suffering plants and how do pathogens invade, how do plants defend themselves, what causes symptoms etc. Art- the application of the science to the field problems i.e. Diagnosis and Control. Plant pathology or phytopathology is the branch of agricultural, botanical or biological science which deals with the cause, etiology, resulting in losses and management methods of plant diseases. Plant pathology can also be defined as the study of the nature, cause and prevention of plant diseases. Plant pathology is related to most of the old and new sciences like biology, physics, chemistry, physiology, mathematics, genetics, soil science, biochemistry, biotechnology etc. Study of plant pathology includes the study of sciences viz, microbiology, bacteriology, virology, mycology, nematology, protozology, phycology, unfavorable environmental factors, nutritional deficiencies and flowering plant parasites.

History in general reveals chronological account of important events, contribution of persons who significantly influenced the thinking of their era and the interpretations of the observed facts or phenomenon over the period of time. The progress in plant pathology leading to the major land mark in mycology, plant bacteriology, plant virology and plant disease control has been described in chronological order for easy and better understanding of the student in following sub-headings.

Plant diseases are classified on the basis of type of pathogenic or non- pathogenic causes of the disease. The classification is based on the plant pathogenic organisms as follows. PARASITES They include both biotic and mesobiotic agents. The diseases are incited by parasites under a set of suitable environment. Association of definite pathogen is essential with each disease. i. Biotic agents: They are also called as animate causes. They are living organisms. Biotic agents include:

GENERAL CHARACTERISTICS OF FUNGI 1. Thallus The body of the fungus is called as Thallus, which is without stem root and leaves. It maybe Plasmodial, Pseudoplasmodial , Pseudomycelial or Mycelial. A single thread like filament is called as hypha. A hypha is made up of a thin, transparent tabular wall filled or lined with a layer of protoplasm. A group of hypha constituting the body of fungus is called as mycelium may be septate or aseptate. i.e coenocytic. a) Coenocytic or Nonseptate or Aseptate Mycelium When mycelium is not divided by cross walls called as Cornocytic mycelium. Depending upon the nature of parasitism with the host plant, mycelium is either ecotophytic or endophytic. b) Septate Mycelium When mycelium is divided by cross walls or septa called as septate mycelium.

Taxonomy Taxonomy is the science that deals with the identification nomenclature and classification of organisms. Nomenclature It is the system of assigning names to the taxonomic groups or organism according to international rules. Systematics It is scientific study of organisms with the ultimate object of characterizing and arranging them in an orderly manner.

GENERAL CHARACTERS The Mastigomycotina includes all eumycota fungi which produce flagellated cells during their life cycle. Majority of them are with filamentous hyaline coenocytic mycelium. Cell wall contains cellulose. They show centric nuclear divison. The mode of nutrition is typically absorptive, because a majority of mastigomycotina contains some or other type of haustoria. They produce asexual spores called zoospores. Oospore is the sexual spores. Members of the class oomycetes are mostly aquatic but some are facultative or obligate parasites of vascular plants.

GENERAL CHARACTERS The majority of the members are saprobic. A few Zygomycetes are weak parasites, attacking plants and animals. Most Zygomycetes produce a well developed and branched mycelium, consisting of coenocytic hyphae. Cell wall is mainly composed of chitin. Motile cells or zoospores are absent. Asexual reproduction takes place by non motile sporangiospores called aplanospores. Sexual reproduction takes place by gametangial fusion. Gametangial fusion results in the production of a thick walled resting spore, called zygospore.

GENERAL CHARACTERS Ascomycotina includes only such fungi in which the zygospores are absent and the perfect state spores are the ascospores. The Ascomycetes and Basidiomycetes are sometimes combindly called ‘higher fungi’. Cell wall is made up of chitin. Mycelium is well developed branched and septate. Asexual spores are non-motile conidia. Sexual spores are ascospores. Ascospores are usually 8 in an ascus. They are produced endogenously inside the ascus. The asci are usually grouped to form a definite type of multicellular fruiting body called ascocarp. The ascocarps are either cup or saucer shaped ( apothecium), flask shaped (perithecium), or closed, spherical and indehiscent (cleistothecium). The characteristic ascospores are present in sac- like body, called ascus and therefore these fungi are also commonly called ‘sac fungi’ Yeast is single celled organism.

GENERAL CHARACTERS The members are terrestrial, and saprophytic or parasitic. The mycelium is well developed, branched and septate. The mycelium is of primary, secondary and tertiary type. Dolipore septum is present except rusts and smuts. Clamp connections present. Cell wall consists of chitin and glucans. Basidiomycetes reproduce asexually by conidia, arthrospores, oidia, fragmentation or budding. No specialized sex organs develop in Basidiomycetes. Plasmogamy takes place by somatogamy or spermatization. Sexual spores are basidiospores. They are exogenously produced on basidium. Usually four basidiospores are develop on basidium. The Basidiomycotina includes rusts, smuts, mushrooms, jelly fungi, puffballs, shelf fungi, toadstools, bird’s nest fungi, bracket fungi and earth stars.

GENERAL CHARACTERS Deuteromycotina includes the fungi in which the ‘perfect stage’(zygote, ascus, basidium) is either lacking or has not been discovered so far. Because of the apparent absence of any perfect stage or sexual phase, these fungi are commonly called ‘imperfect fungi’ or technically ‘Fungi imperfecti’. The characteristic feature of Deuteromycotina is the absence of sexual reproduction. The members reproduce only be asexual methods, and that too also chiefly by conidia, which develop on conidiophores. The conidia are produced either directly on the conidiophores or in some special types of fruiting bodies such as synnemata, acervuli, sporodochia or pycnidia. Fungi possess branched, septate and multinucleate mycelium except the unicellular yeast like members of Blastomycetes. Sexual reproduction is completely absent. Parasexuality is shown by some Deuteromycotina. Under this phenomenon, the process of plasmogamy, karyogamy and haplodization take place, but not at specified time or specified points in the life cycle of the fungus.

PROKARYOTES Prokaryotic organisms are that in which nucleus is primitive type and nuclear material is not enclosed within the nuclear membrane. Bacteria are placed in the kingdom “Prokaryotae” because of the prokaryotic cellular organization of the members. However, extremely diverse groups of microorganisms differing in morphological, physiological and ecological properties are found within this kingdom. In the beginning, description and information of bacterial systematics or classification was being published in the comprehensive volumes of “Bergey’s Manual of Determinative Bacteriology” (1923, first edition). The 9th edition of Bergey’s Manual, published in 1984 was the last of such comprehensive manual and from 1984 onwards it was renamed as “Bergey’s Manual of Systematic Bacteriology”. It is the most widely accepted and used reference document or book for classification and identification of bacteria. In the 9th edition, the kingdom Prokaryota (Monera) is divided into 4 divisons based on nature of the cell wall.These are : Gracilicultes; Firmicutes; Tenericutes and Mendosicutes .

DEFINITION Bacteria belong to prokaryota which encompasses organisms with a primitive type of nucleus lacking a clearly defined membrane .The bacteria are smaller than fungi and measure about 0.5 to 1.0 x 2.0 to 5.0µ. Structure of Bacteria A bacterial cell consists of a cellwall and a compound membrane enclosing protoplasm. Inside the protoplasm, nucleus, vacuoles, mesosomes, polysaccharides, lipids, mitochondric granules and spores are found. Externally bacterial cells may show capsules, pili, fimbrae and flagella. The bacterial cell contains a characteristics cell wall. The cell wall of bacteria is composed of a peptidoglycan. It is composed of acetyle-glucosamine and cetyle-muramic acid. The rigid peptidoglycan layer is located between the cytoplasmic membrane and an outer multiple tract layer. The latter layer is composed of lipoprotein . It is common in gram negative bacteria. Many bacteria possess other intracellular membrane systems such as mesosomes and chondrioids. The mesosome structure is formed by an invagination of the cytoplasmic membrane.

DEFINITION OF VIRUS Viruses are very small (submicroscopic) infectious particles (virions) composed of a protein coat and a nucleic acid core. They carry genetic information encoded in their nucleic acid, which typically specifies two or more proteins. Translation of the genome (to produce proteins) or transcription and replication (to produce more nucleic acid) takes place within the host cell and uses some of the host’s biochemical “machinery”. Viruses do not capture or store free energy and are not functionally active outside their host. They are therefore parasites (and usually pathogens) but are not usually regarded as genuine microorganisms. Mathwas (1981) considers a virus as a set of one or more template molecules normally encased in a protective coat or coats of protein or lipoprotein, which is able to organize its own replication only within suitable host cells where its production is: Dependent on hosts protein synthesizing machinery (ribosomes). Organised from pools of required material rather than binary fission and Located at sites which are not separated from the host cell contents by a lipoprotein bilayer membrane. Bos (1983) defines virus as an infectious agent often causing disease, invisible with the light microscope (Sub-microscopic), small enough to pass through a bacterial filter, lacking a metabolism of its own and depending on a living host cell for multiplication. Viruses are small packages of host alien genetic information of one type. (RNA or DNA), either in one strand or in a few segment. Encapsulated together or separately and enclosed in a coat of one or more types of protein, some time with an extra coat (envelope) and some other constituents.

PHYTOPLASMA OR MYCOPLASMA Definition Phytoplasmas are unicellular, ultramicroscopic, walless, prokaryotic, self- replicating, highly pleomorphic, filterable organisms or entities. Mycoplasma lack rigid cell wall, being surrounded only by single triple unit membrane which allows them to be highly plemorphic. They assume vast array of shapes and size. They require sterols (Lipoproteins) for growth. Mycoplasma cannot be grown on artificial media and they reproduce by budding and binary fissions.

VIROIDS Defnition Viroid is circular, encapsulated, low molecular weight (1.1-1.3x105), self replicating, highly infectious ssRNA molecules . General Characters The viroid exists in in vivo as unencapsulated RNA. They never contain any protein coat (capsid). Genome of the viriod naked, single stranded ( RNA) with 250-400 nucleotide, either linear or mostly circular. Viroids are smaller in size than viruses i.e 50 nm or 1.1 to 1.3 X 103 molecular weight. Despite its small size, the infectious RNA is replicated autonomously in susceptible cells; that is, no helper virus is required for multiplication. The infectious RNA consists of one molecular species only. Viroids are notable to synthesize protein and replicase enzyme required for replication. Viroids replicate by direct RNA, copying in which all components required for viroid multiplication including RNA polymerase are provided by the host. They cause diseases only in plants. Viroids concentration and translocation is higher in growing parts of the plants (up to 0.2 mm from the apex) Most of the types of symptoms observed with viral diseases also occurs viroids like epinasty, leaf distortions, vein clearing, localized chlorotic or necrotic spots, mottling of leaves, necrosis of leaves, and death of the whole plants.

INTRODUCTION Nematodes belong to the animal kingdom, and phylum Nematoda. The body of nematodes is elongate ( thread like; nema in Greek means thread) without any segment. It is cylindrical, tapering at each end especially towards the tail. Most of the important parasitic genera belong to the order Tylenchida and few under Dorylaimida.

Fungi, nematodes, bacteria, and viruses are probably the first things that come to mind when thinking of plant pathogens. These organisms certainly do cause damage to plants of economic importance, but it may surprise you to know that parasitic flowering plants are also important pathogens.There are few seed plants, which are parasitic on living plants and are called parasitic higher plants or Phanerogamic parasites. These parasitic higher plants attack some valuable crops and trees causing considerable losses. They produce flowers and seeds and belongs to several widely separated botanical families they differ to each other on their dependency on host plants. These parasites have haustoria as absorbing organ, which sent deep into the vascular bundle of the host to draw water and nutrients. More than 2500 species of higher plants are known to live parasitically on other plants.

DEFINITION Plant disease in which no pathogens or parasites is associated with the cause is known as Non-parasitic disease. They are also called as non-infectious or physiological disorders. When no pathogen is found, cultured from or transmitted from a diseased plant, then the disease is said to be caused by a non-living or environmental factor. These diseases occur because of disturbances in the plant system by the improper environmental conditions in the air or soil or by mechanical influences.

For the accurate identification and diagnosis of plant disease and plant problems a foundational knowledge of terms and definitions is vital for developing concepts, doing research, discussing and communicating issues and providing clarity to your work. The following terms and definitions are basic to the study of plant pathology. They are, however, just a brief introduction to the vocabulary of the science. If you have limited or no background in the subject and you are just getting started, the concepts and terminology of plant problems can seem somewhat daunting. However, your vocabulary and skill will develop through exposure to diagnostics, experience and correct use of the appropriate terms. Plant pathology is the study of plant diseases and the abnormal conditions that constitute plant disorders. Etiology is the determination and study of the cause of disease. A pathogen can be living or non-living, but usually refers to a live agent. A pathogen is an organism which causes a disease. Pathological is a condition of being diseased. Pathogenic is having the characteristics of a pathogen and pathogenicity is the capability of a pathogen to cause a disease. A plant disease is an abnormality in the structure and/or function of the host plant cells and/or tissue as a result of a continuous irritation caused by a pathogenic agent or an environmental factor. A disease is not static; it is a series of changes in the plant. All plants, to some extent, are subject to disease. Plant disease is the result of an infectious, or biotic agent or a noninfectious, abiotic factor. Plant injury is an abrupt alteration of form or function caused by a discontinuous irritant. Plant injury includes insect, animal, physical, chemical or environmental agents. A causal agent is a general term used to describe an animate or inanimate factor which incites and governs disease and injury. A causal organism is a pathogen of biotic origin. When a pathogenic agent is virulent it can cause disease and if the agent is avirulent it is a variant of a pathogen that does not cause severe disease.

There are thousands of diseases, which attack crop plants. Classification can be made based on several criteria. The various ways of classifying diseases of plants are given below. A. Based on Type of Infection 1. Localized disease- affecting only a part of the plant; leaf spots and anthracnoses caused by different fungi. 2. Systemic disease- affecting the entire plant. B. Based on Symptoms Rusts, smuts, wilts, blights, cankers, mildews, rots, damping-off, die-back, scab etc.

INTRODUCTION Plant pathogens induce diverse reactions in the body of their hosts. This result in creation of abnormalities which become visible on the plants. A visible or otherwise detectable expression of abnormal physiology, development, or behaviour in a plant resulting from disease is called symptoms. Normally, a distinction is made between symptoms and signs A symptom of disease is expressed as a reaction of the host to a causal agent, where as, a sign is evidence of disease other than that expressed by the host. Signs are usually the structures of the pathogen.A disease is first noticed by the presence of symptoms and /or signs, and recognition of specific type of symptoms or sign aid in the eventual diagnosis of the disease.

To make a healthy plant sick or diseased, primary requirement of a pathogen is spread of its inoculums from the source of survival to the susceptible host. The spread of a plant pathogen within the common area in which it is already established is called dispersal or dissemination. Moving the inoculum only a few inches and transporting it for hundreds of kilometers both represent its dispersal. Nevertheless, the pathogen dispersal is not necessarily only for spread of diseases but also for continuity of the life cycle and development of the pathogen. A detail knowledge of pathogen dispersal is necessary to find out effective control measures for diseases because the possibilities of preventing dispersal and thereby breaking the infection chain always exist. Plant pathogen dispersal generally occurs through two key groups:

The availability of a plant pathogens in given area presupposes its ability to survive not only during its parasitic relations with hosts but also during those seasons in which the hosts are not growing. The latter part of continued existence of plant pathogens in which they remain in dormant condition to overcome the unfavourable condition of non availability of hosts is called “survival”. Pathogens may survive between crop seasons by means of specialized resting structures, by functioning as saprophyte in or on the soil or in diseased plant debris, or by living insome intimate association withlivingplants or otherorganism. Modes of Survival: The modes of survival of plant pathogens are described below: 1. Parasitic Survival Organisms, parasitic on perennial plants (fruits, plantation and forest trees) survive in or on their hosts actively or passively depending on the environmental conditions. Certain bacterial pathogens viz, X. campestris pv citri, X. campestris pv jugalandis, X. campestris pv. Pruni and Erwinia amylovora are known to be carried over perennially in holdover cankers or blighted twigs, which may produce bacterial ooze in favourable weather to fresh inoculums. Similarly several fungal plant pathogens like Colletotrichum gloeosporioides (anthracnose of mango), Oidium mangiferae (powdery mildew of mango), Podosphaera leucotricha (powdery mildew of apple), etc survive on infected organs of the plants. Along with cultivated crops, several undesirable plant (weeds, etc) both annual and perennial, grow independently in the nature. Such plants which belong to the same botanical family to which cultivated hosts belong , are known as collateral hosts. Some important examples include the survival of three Puccinia spp. causing wheat rusts in Indian subcontinent . These fungi survive in active sporulating stage on wild species of Triticum, Aegilops, Horedeum and Agropyron repens. Similarly, Rice tungro virus survive actively on Oryza spp., Echinocloa spp. and Leersia hexandra .

A parasitic must establish an intimate relationship with the host tissues to absorb the desired nutrients. Successful establishment of this relationship is called infection. The chain of events that takes place in the causation of disease is called pathogenesis or infection process. It encompasses the germination or multiplication of an infective propagule in or on a potential host through to the establishment of a parasitic relationship between the pathogen and the host. The process of infection is influenced by properties of the pathogen, the host and the external environment. If any of the stages of the infection process is inhibited by any of these factors, the pathogen will not cause disease in the host. While some parasites colonise the outside of the plant (ectoparasites), pathogens may also enter the host plant by penetration, through a natural opening (like a stomatal pore) or via a wound. The symptoms of the diseases produced by these pathogens result from the disruption of respiration, photosynthesis, translocation of nutrients, transpiration, and other aspects of growth and development. The “infection process”can be divided into three phases:

Pathogens are different from parasites, Parasites penetrate into the host and obtain nutrients from the host. They live inside the host without causing any other damage. They may be at times symbionts also i.e. they may be beneficial to the host. The examples for parasites are Rhizobium spp. which form nodules in leguminous plants and mycorrhizal fungi. Pathogens not only take nutrients from the host but also damage the host severely. They induce numerous symptoms of the disease and even kill the plants. Pathogens in their life cycle enter into two distinct phases. One is biotrophic and the other is necrotrophic phage. In the biotrophic phase they invade the host cells inter and intracellularly and establish themselves in the living cells. In the second phase, they kill the cells and obtain nutrition from the dying cells. This phase is called necrotrophic phase. In case of obligate pathogens the biotrophic phase may be much longer while in case of facultative saprophytes, long biotrophic phase may be followed by necrotrophic phase. In case of facultative parasites necrotrophic phase will be much longer with short biotrophic phase. However in case of all pathogens, both the phases may exist either as a short or long spell. In the biotrophic phase the pathogens establish themselves in the host cells. The first barrier between host plant and the attacking pathogen is the cuticle. Host cells have cell wall, structure surrounding the protoplast. Cell wall is divided into three regions: middle lamella, primary wall and secondary wall. Below the cell wall membranes are found. The membrane provides a semi permeable barrier between the cell and its external environment. The membrane consists of lipids and proteins. During the biotrophic phase, the pathogen invade the cells by degrading cuticle, middle lamella,cell walls and cell membranes, by producing a series of enzymes.After the biotrophic phase, the pathogens produce toxins which kill the cell and enter into the necrotrophic phase. Thus, enzymes, toxins, growth regulators are important tools of the pathogens for their pathogenesis.

Resistance to disease in a host plants is a condition in which the plant conquer the pathogen’s attack and thus suffers small or no injury. Resistant hosts prevent or slow the development and reproduction of the majority of pathogen propagules that they come into contact with. Different plants defend themselves in different ways. Each kind of plant, probably, employ different defense mechanism against each of the pathogens that attack it. The defense barriers erected by plants are a co-ordinated system of molecular, cellular and tissue-based responses to pathogen attack. There are various mechanisms of defense used by plants. For convenience, such mechanisms can be considered under two broads heads; 1. Pre existing or passive defense (Defense present on or inside the plant since beginning) a. Pre existing structural defense b. Pre existing biochemical defense 2. Post infection or active defense (Defense in response to attack by the pathogen) a. Post infection structural defense b. Post infection biochemical defense

INTRODUCTION Variability in different organisms is common feature and is an important process for the continuity of life under stress conditions. Plant pathogens especially fungi, bacteria, virus and nematode show variations in their pathogenic potential (virulence) and physiological functions that support their survival and perpetuation in different environmental conditions. These variations may be largely due to genetic factors that may be conditioned by environmental factors. Various studies on genetic variability of microbial plant pathogens have shown that variations may occur in characters which affect their ability to infect host plants and also other characters that do not alter the pathogenic potential, but they may be concerned with their survival. Knowledge of variation in pathogenic characteristics is valuable in planning effective measures for disease management. In addition, development of resistant to fungicides has been a serious concern to growers, researchers as well as the administrators, since the decision to continue application or withdrawal of a fungicide can affect both the growers and industry.

CONCEPT The word “epidemic” is derived from the greek word, epi= (on) and demons=(people) and in true sense applies to those disease of human being which appear very virulently among a large section of population. To carry the same sense in the case of plant diseases the term epiphytotic has been coined. an epiphytotic disease is one which occur widely but periodically. it may be present constantly in a locality but assumes severe form only on occasions. Of late, the term “epidemiology” has come to have a broad meaning within plant pathology. The term has been variously defined as the study of disease in populations; the study of environmental factors that influence the amount and distribution of diseases in population, and the study of either increase or decrease in the amount of disease in time, in space, or both. Elements of An Epidemic Generally, the elements of an epidemic are referred to as the “disease triangle”: a susceptible host, pathogen, and favorable environment. For disease to occur all three of these must be present. Where all three items get together there is disease. As long as all three of these elements are present disease can commence, an epidemic will only arise if all three continue to be present. Sometimes a fourth factor of time is added as the time at which a particular infection occurs, and the length of time conditions remain viable for that infection, can also play an important role in epidemics If all of the criteria are not met, such as a susceptible host and pathogen are present but the environment is not conducive to the pathogen infecting and causing disease, disease cannot occur. Factors that affect the development of epidemics

DEFINITION Fore casting involves all the activities in ascertaining and notifying the grower of community that conditions are sufficiently favourable for certain diseases, that application of control measures will result in economic gain or on the other hand, and just as important that the amount expected is unlikely to be enough to justify the expenditure of time , energy and money for control. The above statement made explicit distinction between positive forecast and negative forecast and both have value for growers as well as society in general. Positive forecast: Employs need based chemical sprays, provides adequate protection to crop and reduces damage to environment. Negative forecast: Avoids unnecessary chemical sprays, no risk to the crop health and no disruption of environment.

INTRODUCTION Plant diseases have caused severe losses to humans in several ways. Starvation and uprooting of families resulted from the Irish famine caused by potato late blight . A valued resource was lost with the virtual elimination of the American chestnut by chestnut blight. And direct economic loss such as the estimated one billion dollars lost in one year to American corn growers from southern corn leaf blight . Many plant diseases cause less dramatic losses annually throughout the world but collectively constitute sizable losses to farmers and can reduce the aesthetic values of landscape plants and home gardens. The goal of plant disease management is to reduce the economic and aesthetic damage caused by plant diseases. Conventionally, this has been called plant disease control, but current social and environmental values believe “control” as being absolute and the term too rigid. More multifaceted approaches to disease management, and integrated disease management, have resulted from this shift in attitude. However, single, often severe, measures, such as pesticide applications, soil fumigation or burning are no longer in common use. Further, disease management measures are often determined by disease forecasting or disease modeling rather than on either a calendar or prescription basis. Disease management might be viewed as proactive whereas disease control is reactive, although it is often difficult to distinguish between the two concepts, especially in the application of specific measures. Plant disease management practices rely on anticipating occurrence of disease and attacking vulnerable points in the disease cycle (i.e., weak links in the infection chain). Therefore, correct diagnosis of a disease is necessary to identify the pathogen, which is the real target of any disease management program. A thorough understanding of the disease cycle, including climatic and other environmental factors that influence the cycle, and cultural requirements of the host plant, are essential to effective management of any disease.

PRINCIPLES The principles involved in thermotherapy is that the pathogens present in seed material are inactivated or eliminated at temperatures nonlethal for the host tissues.The exact mechanism by which heat inactivates the pathogen is not fully understood. However, it is universally accepted that heat causes inactivation and not immobilization of the pathogen by heat. Thrate at which the pathogen is inactivated is determined by temperature, the higher the temperature, the faster is the inactivation.

INTRODUCTION The Cultural practices which includes manipulation /or adjustment of crop production techniques have been as old as possibly agriculture itself. In early stages of agriculture development, the growers through their experiences and observations had known that repeated cultivation of a particular crop species or variety on a piece of land often resulted in crop sickness. By proper crop rotations they had been avoiding such sickness. As a matter of fact, in the present day agriculture, cultural practices are being considered as essential backup methods for plant disease management. Cultural practices often offer the opportunity to alter the environment, the condition of the host, and/or the behavior of the causal agent, to achieve economic management of disease. Most cultural practices used to control plant disease are preventive in nature. Integration of cultural practices, host resistance and pesticides or biocontrol agents may be necessary to provide options for controlling economically important plant diseases

Plant diseases need to be controlled to maintain the quality and abundance of food, feed, and fiber produced by growers around the world. Different approaches may be used to prevent, mitigate or control plant diseases. Beyond good agronomic and horticultural practices, growers often rely heavily on chemical fertilizers and pesticides. Such inputs to agriculture have contributed significantly to the spectacular improvements in crop productivity and quality over the past 100 years. However, the environmental pollution caused by excessive use and misuse of agrochemicals, as well as fear-mongering by some opponents of pesticides, has led to considerable changes in people’s attitudes towards the use of pesticides in agriculture. Today, there are strict regulations on chemical pesticide use, and there is political pressure to remove the most hazardous chemicals from the market. Additionally, the spread of plant diseases in natural ecosystems may preclude successful application of chemicals, because of the scale to which such applications might have to be applied. Consequently, some pest management researchers have focused their efforts on developing alternative inputs to synthetic chemicals for controlling pests and diseases. Among these alternatives are those referred to as biological controls.

INTRODUCTION Plant pathogens can spread rapidly over great distances, vectored by water, wind, insects, and humans. Across large regions and many crop species, it is estimated that diseases typically reduce plant yields by 10% every year in more developed nations or agricultural systems, but yield loss to diseases often exceeds 20% in less developed settings, an estimated 15% of global crop production. Disease control is reasonably successful for most crops. Disease control is achieved by use of plants that have been bred for good resistance to many diseases, and by plant cultivation approaches such as crop rotation, pathogen- free seed, appropriate planting date and plant density, control of field moisture and pesticide use. However, the most attractive form of control is provided by disease resistance, for as long as it remains effective, it provides protection at no cost to the farmers or the community. So plant resistance is a highly useful strategy that can be applied in the control of diseases. It does not require any special action from growers and constitute cheap and practical input in the integrated disease management system. The use of resistant varieties cannot only ensure protection against diseases but also save the time, energy and money spent on other measures of control. In addition to these advantages, resistant varieties, if evolved, can be the only practical method of control of such diseases as viruses, phytoplasmas wilts, and rusts etc. in which chemical control is very expensive and impractical. In crops of low cash value, chemical and other methods of control are often too expensive to be applied. In such crops development of varieties resistant to important diseases can be an acceptable recommendation for the farmer.

INTRODUCTION The current food crisis has put the agriculture production on a tight leash, not allowing the producers any lassitude whatsoever. Even though we have outgrown from our ‘ship to mouth existence’; our least negligence can torpedo our efforts to maintain the steady state of progress achieved in food grain production. Around 35% of what is produced is lost due to pest and diseases. In this situation, plant protection assumes significance. Unfortunately our plant protection regime begins and ends with pesticides. It is the default option in the plant protection menu and all the other options more or less remain defunct and unused. Even though, our major thrust areas of plant protection are promotion of Integrated Pest Management: ensuring availability of safe and quality pesticides for sustaining food production from the ravages of pests and diseases, streamlining the quarantine measures for accelerating the introduction of new high yielding crop varieties, besides eliminating the chances of entry of exotic pests; we remain largely insulated from these aspects in IPM. With the Globolization and liberalization in International trade of plants and plant material in the wake of Sanitary and Phytosanitory (SPS) Agreement under WTO, the plant quarantine measures have become more imperative. Liberalization of trade policies have helped the world agriculture trade to prosper which is key sector for many countries like India whose economies are dependent on agrarian industry. But the likehood of the presence of disease/pest inherent in the import material can very well geopardize the trade. Activities are meant to help agricultural development and Plant quarantine service are charged with the responsibility of preventing the entry of hazardous pests, pathogens and weeds, but to deny entry of valuable genetic re-sources would be against national interest. These activities are meant to help agricultural development and they are complementary to each other and therefore are necessary, and we on our parts need to be fairly vigilant. As has been very truly said-An ounce of prevention is worth a pound of cure.

INTRODUCTION The use of chemical for protecting plants from the ravages of the pathogens is not an innovation of the 20th century. Chemicals have been used for over 200 years to protect plants against fungal diseases. At present about 150 chemicals belonging to different classes are used as fungicides in world agriculture. Most of the recommended treatments generally provide 90% or greater control of target diseses. Even when a crop has been grown from pathogen free soil and the crop seems superficially healthy, the use of fungicides in some circumstances become unavoidable. Resistant varieties of potato are attacked by late blight if rain occurs during tuber formation. To control this disease use of fungicides become vital to avoid any loss of the crop. Thus, even in the case of resistant varieties, judicious use of chemical protection will extend the life of resistance in the variety. For killing different groups of pathogens, different types of chemicals are required. The chemicals mainly used for controlling diseases are: fungicides for killing fungus, bactericide for killing barteria, nematicides for killing nematodes, and viricides for killing viruses.

Proper selection of a fungicide and its application at the correct dose and the proper time are highly essential for the management of plant diseases. The basic requirement of an application method is that it delivers the fungicide to the site where the active compound will prevent the fungus damaging the plant. The fungicidal application varies according to the nature of the host part diseased and nature of survival and spread of the pathogen. The method which are commonly adopted in the application of the fungicides are discussed Foliar or vegetative application Soil application

Plant diseases are a threat to world agriculture. Significant yield losses due to the attack of pathogen occur in most of the agricultural and horticultural crop species. More than 70% of all major crop diseases are caused by fungi . Plant diseases are usually handled with applications of chemicals. For some diseases, chemical control is very effective; but it is often non-specific in its effects, killing beneficial organisms as well as pathogens. Chemical control may have undesirable effects on health, safety and cause environmental risks. A promising method for protecting plants against diseases is constructing and employing pathogen-resistant cultivars. Although a number of resistant cultivars have been developed through breeding programs, these cultivars become obsolete in a short time due to the rapid evolution of the phytopathogens and the emergence of virulent forms capable to overcome the plant resistance. Breeders are often confronted with the issue of using a limited number of plants in their breeding programs, undesirable traits transferred together with the valuable resistance genes, and, in recent years, also with the depletion of potential gene sources. Control of diseases is a subject of great interest for biotechnologists. The most significant development in the area of varietal development for disease resistance is the use of the techniques of gene isolation and genetic transformation to develop transgenic resistance to fungal, bacterial and viral diseases. Improvements in genetic transformation technology have allowed the genetic modification of almost all important food crops like rice, wheat, maize, mustard, pulses and fruits. Genetic engineering technology has proved to be beneficial in managing viral and bacterial diseases in plants. The advances in gene engineering technologies and the understanding of the molecular nature of plant protection mechanisms have provided means for developing principally new strategies of plant disease control, in addition to the traditional approaches based on employing chemicals or classical breeding schemes. Biotechnology will enhance our understanding of the mechanisms that control plant’s ability to recognize and defend itself against disease caused by fungi. The integration of biotechnology with traditional agricultural practices will be the backbone for sustainable agriculture.

The over emphasis on the use of chemical pesticides and their improper use by the farmers has led to several complications such as residue in food, feed, fodder, development of resistance, resurgence, secondary outbreak, etc. and above all, environmental pollution, leading to a condition known as Pesticide Treadmill characterized by the need to spray ever greater quantities of chemical pesticides. In 1962, Rachel Carson in her book ‘Silent Spring’ aroused worldwide concern about the excessive use of pesticides, which eventually led to the concept of Integrated Pest/Disease Management as an environmentally sound alternative to the sole use of chemicals. The IDM Concept In disease management, all the alternate control measures like cultural, biological, mechanical, resistant varieties, physical etc, are utilized in anintegrated manner in order to maintain the disease below economic injury level. If the disease increases and reaches the economic threshold level, pesticides are used to bring the disease down. This is concept of an Integrated Disease Management in which minimum pesticides are used keeping in view of agro- ecosystem. Actually, we do not go for 100 percent control in disease management rather the disease is allowed below which can be tolerated i.e economic threshold. This also provides opportunity for bioagent to survive, managing upon the diseases keeping them at certain equilibrium level. Thus, the Integrated Disease Management concept aims at managing the disease through all the available tactics with favorable economic, social and ecological consequences.

A Acervulus (pl. Acervuli)—A saucer-shaped, spore-producing body of a fungus embedded in host tissue. Actinomycetes—Filamentous bacteria that produce several antibiotics and give soil its earthy smell. Active ingredient: In pesticides, the chemical responsible for the desired effect. Aggressiveness: Virulent forms of pathogen cause differing degrees of symptom severity. Alternate Host—One of two kinds of plants on which a parasitic fungus (e.g., rust) must develop to complete its life cycle. Anamorph—Asexual stage of a fungus. Antagonism: The counteraction between organisms or groups of organisms.

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