Preface The ultimate aim of crop husbandry is to cultivate crops and obtain maximum returns according to their genetic potential. Pathogenic microorganisms stand in the way of crops attaining their full growth and maturity. Some plant pathogens, as a result of infection of the host plant may cause death of a part or the entire plant and cause total loss, while some others affect some parts of the plant resulting in reduction of yield. Some pathogens may affect the stored produce and cause spoilage. So, it becomes imminent that plant pathogens have to be controlled by adopting appropriate control measures so as to minimize the losses and maximize the yield. The causative agents of diseases in plants include pathogenic microorganisms, such as fungi, bacteria, viruses, mycoplasma, protozoa and nematodes; phanerogamic parasites, algae and lichens of the Plant Kingdom; abiotic agents or unfavorable environmental factors, such as deficiency due to lack of certain macro or micro nutrients or toxicity due to excess of certain macro and micro nutrients; moisture, light or heat stress, as well as the presence of some toxic chemicals in the air, soil or water. Plant pathology deals with the study of plant diseases caused by various pathogenic agents and environmental factors; the mechanisms by which these agents or factors induce diseases in plants; occurrence and mode of spread of the diseases and the methods of preventing or controlling them, thereby reduce the damage and consequent loss caused by them. As a separate field of Biological Science, Plant Pathology helps to enhance our knowledge about plant diseases, at the same time it tends to develop methods, equipments and materials through which plant diseases can be avoided or controlled. In an ecological community, the majority of living organisms are plants. Plants have the ability to convert solar energy into chemical energy through the process of photosynthesis and this chemical energy is stored in the form of carbohydrates, proteins and fats in the plant cells. All vertebrates including humans, animals etc. and invertebrates, such as insects, nematodes etc. depend entirely on these plant materials for their survival and existence. It has been established that microorganisms were present on this earth several thousands of years before humans and plants came into existence and had intruded into the world of microorganisms. These microorganisms, which had been living entirely on the organic matter for their sustenance, had gradually changed their mode of life and had become parasites on plants. These parasites, which cause various types of plant diseases, are known as plant pathogens. Infection by plant pathogens invariably exhibits certain specific abnormalities in the host plants called disease symptoms. Each and every pathogenic microorganism produces quite different and distinct symptoms. These symptoms may be visible to the naked eye or in some cases may not be visible externally. From such characteristic symptoms, most of the diseases, such as leaf spots, blights, rusts, smuts, wilts, rots etc. can be identified. Further, in the case of several diseases, the spores, spore bodies, sporophores or such other structures produced by the pathogens can also be seen and the diseases can be identified. So, a basic knowledge about the disease symptoms, the causal organisms responsible for causing the diseases, their life cycle, reproductive capacity, mode of survival and spread of the pathogens etc. goes a long way in adopting appropriate control measures to combat the diseases. Diseases may spread through air, water, soil, seed or vegetative propagating materials. Many virus and mycoplsma diseases are spread through insect and non-insect vectors actively or passively. Control measures and plant protection chemicals to be used for the control of different diseases differ depending upon the causal organisms and other characteristics of the pathogens. Various measures under evasion, exclusion, eradication and protection help to keep in check many plant diseases to a large extent. However, under certain circumstances, especially when the disease intensity crosses the economic threshold level, direct control measures by the use of plant protection chemicals have to be resorted to so as to control the disease and avoid economic losses. In this respect, knowledge about plant protection chemicals is also of vital importance. To attain self-sufficiency in food production, plant protection is an essential and powerful tool. To use this tool effectively, it is quite important that we should acquaint ourselves with the various aspects concerning pathogenic microorganisms. This knowledge will serve as a basis for adopting suitable control measures at the appropriate time, thereby pave the way to maximize production. In this context, we hope that this book, which has been brought out as per the syllabus of B.Sc.(Ag.) Degree course of the Agricultural Universities will be of immense help and guidance to the students of Agriculture, people working in the Department of Agriculture, people involved in Scientific Agriculture and the General Public. Numerous illustrations have been given to enable the reader to understand the text easily and to make the study more interesting’ The authors are highly thankful to Dr.G.Kuppuswamy, Dean, Faculty of Agriculture, Annamalai University, Chidambaram, Tamil Nadu for having given the foreword to the book. Acknowledgements are also due to friends and colleagues who have inspired, helped and encouraged the authors in their endeavor to complete the book successfully. The authors wish to express their sincere thanks to M/S. New India Publishing Agency, 101, Vikas Surya Plaza, Pitam Pura, New Delhi - 110 088 for the excellent manner in which this book has been brought out.

Definition of Plant Pathology ‘Plant pathology’ may be defined as ‘the study of microorganisms and of the environmental factors that cause disease in plants; of the mechanisms by which these factors induce disease in plants; and of the methods of preventing or controlling disease and reducing the damage it causes’. Plant Pathology or ‘Phytopathology’ is a branch of Biological Science, which exclusively deals with plant diseases and various aspects connected with plant diseases. The term Phytopathology had been derived from the Latin language. The Latin word ‘PHYTON‘ means plants; ‘PATHOS‘ means suffering and ‘LOGOS‘ means Science. So, Phytopathology means the Science dealing with suffering plants. Trees, shrubs, grasses etc., whether cultivated or wild are termed as plants and they form the majority of the earth’s living environment. These plants, which are so vital for humans, animals and insects are vulnerable to attack by numerous microorganisms, called ‘plant pathogens’. Some environmental factors are also detrimental to the normal growth of plants, resulting in diseases. Diseased plants exhibit some type of abnormalities and these abnormalities are termed as ‘disease symptoms’. Depending upon the pathogens attacking the plants or adverse environmental factors the disease symptoms vary markedly. The Science of Plant Pathology has four main objectives: To acquire knowledge about biotic plant pathogens and abiotic environmental factors, which are responsible for causing diseases. To know more about the occurrence and mode of spread of the pathogens on a large scale. To study the interaction between the host and the pathogen with respect to environmental factors. To adopt suitable methods of preventing or controlling the diseases, thereby reducing the damage caused by the pathogens. Systematic knowledge about Plant Pathology will be of much help in protecting the plants from attack by the pathogens, thereby allowing the plants to grow and attain maturity according to their genetic potential.

‘Plant disease management’ involves the integration of various measures adopted to protect the plants by preventing them from contact with the pathogenic organisms, preventing the growth of the pathogens causing diseases, preventing sporulation of the pathogens and spread of diseases, destroying insect vectors responsible for transmitting diseases and such other prophylactic and curative measures. The ultimate aim of these measures is to avoid large-scale occurrence of diseases, reduce the severity of diseases and minimizing the losses caused by diseases. In disease management, importance should be given to control all the diseases attacking a particular crop at a particular time, especially those which occur in severe form in all the seasons and cause extensive damage to the crop and yield loss. Each and every crop is subjected to attack by one or more than one diseases. At the same time the control measures for controlling each of the disease may differ. In this respect a knowledge of the pathogen causing the disease, the symptoms of the disease, mode of infection and spread of the disease, the growth stage at which the crop is attacked, interaction between the host (crop) and the pathogen and weather conditions favorable for the occurrence and spread of the disease is absolutely necessary, so as to adopt suitable control measures to combat the disease and to minimize the damage to the crop, and reduce the yield loss. The occurrence and spread of diseases are very much influenced by the environment, host and the pathogen. When the environmental conditions are suitable for the pathogen and not quite suitable for the host, the incidence of the disease is liable to be more. Further, if the host plants are not maintained properly and are weak, then they are more vulnerable to attack by the pathogen and the disease incidence may be severe. When the inoculum of the pathogen is present abundantly, the chances of attack by the pathogen are also more. Taking these facts into consideration, plant disease management is depicted as ‘manipulation of the disease triangle’ (Fig. 60) All control measures should be aimed at protecting the plants from becoming diseased and not at curing the plants after they have become diseased. The principal methods adopted for controlling plant diseases are broadly categorized under five different heads viz. :

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