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A paradigm shift in the higher agricultural education system has been found to be necessary in order to comply with the various provisions of the National Education Policy 2020. ICAR continues to work on the reforms needed to ensure the quality of agricultural education in India. The council appointed a national core group and BSMA committee to review and restructure the post graduate curriculum. The MSc(Ag)Plant Pathology syllabus has been revised and new courses have been introduced. The curriculum has been revised to enable students to acquire knowledge, approach entrepreneurship and improve their employability and skills to prepare for international competitiveness. This ‘Principles of Plant Pathology’ book has been carefully written for the PG class and is fully compliant with the latest syllabus.

Prologue: The Issues Farmers lose a significant portion of their income when their crops and products become infested with various insects, diseases and weeds, resulting in much lower crop productivity and lower actual yields than the attainable yield of crops. Yield losses due to pests, pathogens and weeds are responsible for 20-40% losses in global agricultural productivity. India reports crop losses of 20-25% annually due to pest infestations. Without the use of crop protection solutions, there could be a 78% loss in fruit production, 54% loss in vegetable production and 32% loss in cereal production. According to the Ministry of Agriculture, India loses INR 1.48 billion worth of agricultural production annually due to pests, weeds and plant diseases.

Disease is one of those terms that are very difficult to define. It is realized that disease (literally dis-ease) implies lack of ‘comfort’and therefore, involves deviation from normal functioning. From time to time several definitions which have been proposed, in fact descriptive but not simultaneously exclusive. The definitions for the term disease are.

History of plant pathology records chronological account of important events, contributions of persons who significantly influenced the thinking of their era and interpretation of the observed facts or phenomenon over a period of time. Historical writings help us to understand systematic development in plant pathology, and there problems faced by certain outstanding workers whose view points were unacceptable to majority of opinion makers, simply because it was new and against the established norms of the day. History of plant pathology is a must read for the students of this discipline. The historical accounts will unfold the stories that may motivate the new generation to think and interpret the findings in their own way, without fear of established theories and dogmas carried from the past.

Plant Pathogen Pathogen is an inducer of disease. A pathogen can therefore be defined as any factor that induces unresolved impairment or disruption of one or more important plant functions. Some plant functions such as nutrient uptake, are highly localized. Others, such as respiration, occur in all living tissues. Still others, such as photosynthesis and photorespiration, are extensively coordinated. Pathogen effects therefore range from very local to very general.

1. Growth of plant pathogens Various plant pathogens, namely fungi, bacteria, nematodes, viruses, viroids, mollicutes, fastidious bacteria, and protozoa, grow in different ways. 1.1 Fungi and parasitic higher plants Fungal pathogens and parasitic higher plants usually invade and infect tissues by growing on or in them from the first point of inoculation. Most of these pathogens, whether they cause small lesions or large infected areas on the plant, grow, branch, and spread to more and more plant tissues until the infection stops spreading or the plant dies.

Plant diseases are a recurring threat. Everywhere they appear in a certain cycle. Chains of infection (pathogenesis) initiated by primary inoculums comes spontaneously at the end of the harvest season or at host death due to the production of various types of pathogenic reproductive organs/propagules. These propagules must find ways to bridge the gap between harvest times to ensure continuity of infection. Therefore, plant pathogen survival is a critical step in a successful pathogen disease cycle. Pathogen spread is another important phenomenon in plant disease recurrence and epidemiology. Effective spread/diffusion of virulent propagules is critical for initiation of chains of infection and further progression of the disease in time and space.

Disease development depends on three factors: host, pathogen, and environment. Host must be susceptible. The pathogen must be virulent to cause disease. The environment must be conducive to disease development. Role of environment on disease development  Environment    

Host -Pathogen Interaction Most often, plants become diseased when attacked by living (biotic) or affected by non-living (abiotic) pathogens. In the first case, at least two components viz., host and pathogen must come into contact and interact for plant disease to occur. Then, for disease to develop, a third factor, a favorable range of environmental conditions, must also occur. The interaction of the three disease factors was often visualized as a triangle called the “Disease Triangle". Each side of the triangle represents one of three components.

Enzymes, toxins, growth regulators, and polysaccharides are much more common and perhaps more important in the development of plant diseases. The pathogen produce these substances when grown on certain substrates (inducible). Let’s talk carefully one by one Enzymes in plant diseases. Toxins in plant diseases. Growth regulator in plant diseases. Polysaccharides in plant diseases.

Philosophy of defence in plants Plants cannot move and must resist or tolerate co-located pathogens. Plants have evolved very powerful preformed defense systems in the form of structural and toxic barriers. Plants cannot keep intruders out, they can only limit (contain) them. Plants can tolerate more nutrient runoff. They often sacrifice a piece of tissue to contain pathogens.

Introduction Plant pathogens include fungi, bacteria, viruses, viroids, phytoplasma, and nematodes. They have evolved different strategies for invading, feeding on, and reproducing inside plants. Biotrophic pathogens require living tissue to grow and reproduce. Necrotic pathogens kill host tissue at the onset of infection and feed on the dead tissue. Viruses generally require living tissue for survival.

1. Genes and Disease When various plants such as rice, wheat, tomato and apple are infected with pathogens and cause disease, pathogens are generally different for each host plant type. Venturia inaequalis, the fungus that causes apple scab, only affects apples. Fusarium oxysporum f. sp. lycopersici which causes tomato wilting, only affects tomatoes and so on. It is the presence in the pathogen of one or more genes of pathogenicty, specificity, and virulence to the respective host that enables disease development in the host.  

Genetic variations Genetic variation is the presence of differences in gene sequences between individual organisms of a species. Enables natural selection, one of the major forces driving the evolution of life. When an individual’s offspring exhibit mutations in the traits of their parents, such offspring are called variants.

Introduction Most common improvement of host resistance to almost any pathogen is brought about by improving the genetic resistance of the host, that is by breeding and using resistance varieties. Genetic engineering technology made possible the isolation of individual resistance (R) genes from resistant plants and the transfer of such genes into susceptible plants in which they induce the hypersensitive resistant response. Thus, genetic engineering technology combined with Conventional plant breeding, will provide one of the most effective tools for controlling plant diseases.

1. Markers: Easily identifiable features are called markers or marker features. 2. Types of marker These markers are three types 2.1.Morphological markers: These relate to the size, shape, color and surface area of different parts of the plant. 2.2.Biochemical markers: Such markers refer to changes in protein structure. For example, isozymes and storage proteins.

Introduction Genetic engineering is the process of using molecular techniques to add, modify and delete genes of interest to the DNA strands of recipient cells or recipient plants. Genetic engineering is also called genetic technology, transgenic technology, transformation technology and recombinant DNA technology. Genetic engineering consists of tools and techniques used to study, identify, or modify the genes of living organisms. It is also defined as a technique for developing transgenic organisms. The key points of genetic engineering are briefly listed below.

Disease Control vs. Disease Management The word control evokes the notion of finality, the final elimination of problems . It is impossible to exclude , eliminate or prevent diseases. Control is ecologically and practically not possible. Although disease control is an established and widely understood concept, there are compelling reasons to replace control with management .

Changes in philosophy about plant disease management To achieve reasonable and sustainable results crop disease management philosophy should shift focus from Management of pathogens (or insect vectors) to managing host plants. A single goal of high productivity to multiple goals of high yield, efficiency, quality and safety.

Introduction It is becoming increasingly difficult to find workers to work in agriculture. Younger generations do not want to work in plant protection. We can see that the average age of farmers is getting older. This leaves the fields unprotected from disease and insects and greatly reduces yields. Fortunately, this gloomy situation has changed since 2015 thanks to modern technology. Digital tools are among the most innovative crop protection tools in modern agriculture.

India’s agricultural accomplishment is a global success story Post Green revolution; Some of the most pertinent questions that humanity faces Can we sustain our food production ecosystem, while feeding 9 billon people? Can we improve farm economics without reversal of soil biodiversity loss and carbon content? Can agriculture play an important role in reducing carbon footprints and mitigating the current climate crisis? Answers to abovementioned questions do not lie in the currently dominating chemical intensive farming practices, which rely too heavily on

A Abiotic Pathogens 26 Adaptation 157 Agriculture revolution 3.0 214 Application of molecular markers in genome analysis of plant pathogens 170 Artificial intelligence 209 A solution in biological agriculture 214 Attenuation 160 Avirulence (avr) genes 132