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Since the periods of Hippocrates and Lord Buddha cholera like symptoms was described in human. The first outbreak of cholera was described in India in 1817. Filippo Pacini (1854), an Italian anatomist, observed a gram-negative bacterium associated with human cholera. Robert Koch (1884) first isolated and confirmed the association of ‘comma-shaped bacteria’, later designated as Vibrio cholerae with the disease in Calcutta (India). In 1959, Sambhu Nath De, Professor of Pathology at Nilratan Sircar Medical College in Calcutta discovered the exotoxin produced by V. cholerae and a simple method (rabbit ligated ileal loop assay) to study the in vitro effect of exotoxins. The Ganges delta of India and Bangladesh is considered as the worst affected place with cholera although the infection is widespread throughout the world. The spread of cholera in different continents occurred through six ‘waves’ (‘pandemics’) since 1817. The current seventh pandemic (7P) was originated in Indonesia (Sulawesi) in 1961 and spread throughout Asia, Africa and Latin America. In 1963, the seventh pandemic reached South Asia and circulated for more than 50 years with intermittent transmission to other continents. The recent major 7^th pandemic outbreak was detected in Haiti causing more than 9400 deaths in 2010 which was originated through patients from endemic countries.

In 1886, Escherich observed Campylobacter like organisms in stool samples of children with diarrhoea. In 1913, McFaydean and Stockman first confirmed the bacteria as Campylobacter in foetal tissues of aborted sheep. Confirmatory tests were also carried out by Smith in 1918 when similar bacteria were isolated from aborted bovine foetuses. The bacteria were then known as Vibrio foetus. In 1963, due to certain differentiating characteristics, the organisms were separated from Vibrionaceae family and the new genus Campylobacter (means ‘curved rod’) under Campylobacteraceae family was proposed.

Guillame de Baillou (1673), a physician, described an outbreak of recurrent severe cough that could induce vomition and suffocation in human in Paris. The infection was described as ‘Quintan^tussis which was later confirmed as ‘whooping cough’. The causative agent Bordetella pertussis was discovered by Jules Bordet and Octave Gengou in 1906 in Paris. The Gram-negative bacilli were isolated from sputum of Jules Bordet’s son. Bordetella bronchiseptica was first isolated by Ferry (1911) and M’Gowan (1911) who considered the bacteria as the cause of distemper in dogs. Later B. bronchiseptica was known as primary cause of ‘kennel cough’ infection along with the Canine Adenovirus and Canine Parainfluenza virus. Moreno- Lopez named the bacteria B. bronchiseptica.

The genus Moraxella was named after Morax who first isolated the bacteria from human conjunctivitis in 1896.

Properties

1. Bacteroides, Dichelobacter and Fusobacterium are major gram negative anaerobes isolated from human and animals. Other gram negative anaerobes are listed below (Table-1).

2. The bacteria are tolerant to oxygen in comparison to strict anaerobes. The bacteria do not die after oxygen exposure.

3. The bacteria naturally inhabit the intestine and mucosal surfaces of human and animals. Majority of them are opportunistic pathogens. Often they cause mixed bacterial infection along with aerobes. The aerobes remove oxygen and create an anaerobic condition favourable for their growth. The aerobes may also supply other growth requirements. For e.g. Porphyromonas asaccharolytica requires vitamin K for its growth, supplied by another aerobic bacterium, not the host.

Antony Van Leeuwenhoek (1681) first observed the spirochetes under the microscope and described them. In 1833, Ehrenberg used the name Spirochaeta for free living spiral shaped organisms. Obermeir (1873) first described a spirochete of pathogenic significance isolated from human patients with relapsing fever. Inada (1916) first isolated Leptospira icterohaemorrhagiae from human patients of Weil’s disease. In 1980, Nogochi proposed the name as Leptospira.

In India, Taylor and Goyale (1931) first reported Leptospira from human patients suffering with jaundice in Andaman Islands. Ayyar (1932) first reported Leptospira from dogs in Chennai city.

The word ‘Mycoplasma’ is originated from the Greek word mykes (fungus) and plasma (formed). It was first used by Albert Bernhard Frank (1889), a German biologist. He thought it was a fungus. An older name for Mycoplasma was pleuropneumonia-like organisms (PPLO), referring to organisms similar to the causative agent of contagious bovine pleuropneumonia (CBPP).It was later found that the fungus-like growth pattern of Mycoplasma mycoides is unique to the species. It was first isolated in 1898 by Edmond Nocard and E. Roux from bovine lungs. Mycoplasma pneumoniae was considered as a virus previously. Eaton and his co-workers cultured the causative agent of human primary atypical pneumonia (PAP) or ‘walking pneumonia’. It was known as ‘Eaton’s agent’. In the early 1960s, Leonard Hayflick, in Philadelphia, was first to isolate and cultivate the causative agent of primary atypical pneumonia and identified the agent as a Mycoplasma pneumoniae.

Properties of genus

1 Morphology: Mycoplasma have no cell wall and therefore are resistant to the antibiotics used to inhibit the cell wall synthesis like penicillin, cephalosporin and vancomycin. The bacteria are the smallest prokaryote (0.3-0.8µm). They are pleomorphic due to absence of the rigid cell wall. The different shapes include ring, globules, filaments and elementary bodies. Due to their plasticity in shape and size, they can be passed through the membrane filter with pore size of 0.45µm. The cell membrane is constituted of trilaminar structure enriched with phosphoprotein, lipoprotein, glycolipid, phospholipid and sterol. So Mycoplasma require sterol for their growth.

History of rickettsial infections went back to 16^th century and it has long been associated with famine and war. Epidemic typhus had major impact to decide over the outcome of several wars. The epidemic alone killed or caused great sufferings to over 1,00,000 people during World Wars. However, the causative agent was not determined until the early part of the 20^th century. Rocky Mountain spotted fever (RMSF) was the first detected rickettsial infection in 1896 in Idaho Valley, USA. It was originally known as ‘black measles’ due to its clinical appearance in the form of characteristic spotted rash throughout the body. Howard Taylor Ricketts, an American pathologist for the first time described the causative agent of Rocky Mountain spotted fever and was able to isolate it in the laboratory animals during 1902-1909. He was a dedicated scientist and on several occasions injected himself with the pathogen for experiment. The causative pathogen of Rocky Mountain spotted fever, Rickettsia rickettsii was named after him. Later, the family Rickettsiaceae as well as the order Rickettsiales was named after him. Ernest William Goodpasture, an American pathologist and physician, invented the methods of isolation and culturing the rickettsial pathogens in chicken embryos and fertilized chicken eggs.

The word ‘chlamydia’ comes from the Greek word ‘chlamys’ means ‘cloak draped around the shoulder’. It describes how the intracytoplasmic inclusions produced by the bacterium appear under the microscope(‘draped’ around the host cell nucleus). Chlamydia was first discovered by a Czech zoologist and parasitologist, Stanislaus von Prowazek in 1907 in Berlin. Originally because of its structure it was thought to be a virus. In 1963 it was recognized as a bacterium rather than a virus.

Properties of genus

1. Morphology: Chlamydia are Gram-negative, non-motile, pleomorphic and obligate intracellular bacteria. The Gram-negative cell wall consists of cytoplasmic and outer membrane. However, they lack peptidoglycan layer. Moreover, they also encode major outer membrane protein (MOMP), the key antigenic determinant used for serological differentiation of chlamydiae.

They are poorly stained by Gram’s stain. Chlamydiae are readily stained by Castaneda (blue in appearance), Giemnez and Macchiavello (red in appearance) methods. However, Giemsa staining is preferable to stain the inclusions (chlamydial micro colony) in the cell culture. Inclusion bodies of the Chlamydiae are basophilic in nature. The staining of the inclusion bodies may differ depending on the species. In C. trachomatis, the inclusion bodies contain glycogen matrix and can be easily stained with iodine solution.

Agriculture has been the base of human livelihood since ancient time when the man started planting of seeds of some edible plants now called Agricultural crops. Domestication of food plants and animals was started with the development of human civilization. Thus agriculture can be explained as science and technology of growing food crops and rearing of animals. Still in several countries of the world, most of population is based on agriculture for their livelihood particularly in rural areas of that country. According to Williams and Haq (2002) there are over 7000 species of plants that can be used for food. The edible plants species grown for food are called agricultural crops. The crops grown in garden are known as garden cops or Horticulture crops. Horticulture is a branch of agriculture, which deals with intensive cultivation of garden crops such as fruits, vegetables, f lowers, ornamental plants, plantation crops, spices, medicinal and aromatic plants, and mushroom. It also deals with post-harvest technology of horticultural crops. Processing and Preservation of fruits and vegetables is a part of Post-harvest Technology of horticultural crops which deals with methods of processing and preservation of these crops to extend the shelf life of fruits and vegetables after harvesting.

All horticultural crops have their own importance in our society. Fruits, vegetables and spices are major ingredients of our meal. Spices add taste and flavor to our food and also have high medicinal values. Flowers and ornamental plants are used for decoration of environment and landscape. Aromatic and plantation crops have become industrial crops. Mushroom growing is a great enterprise. Among these crops fruits and vegetables are major crops and have been considered presently as highly economical and important food crops. There are 1200 crop species are used for vegetable purpose in the world (Hazra and Som, 2006). Fruits and vegetables are richest and cheapest source of all essential nutrients (carbohydrates, fat, protein, vitamins and minerals) required by our body for proper functioning of body organs and maintaining good health. Besides these nutrients they also supply water and fibers to our body which play vital roles in our body. Fruits and vegetables have important place in our daily diet and based on survey made by experts FAO WHO (2003) has recommended 400 g/capita/day of fruit and vegetables (fruits -100 g/capita/day and vegetables -300 g/capita/day) to be consumed for good health and prevent chronic diseases. Therefore, the demand of these food commodities has increased in the modern society due to increased awareness of the nutritional importance of fruits and vegetables. So that people must be aware that what part of fruit and vegetable is to be consumed?

World Scenario

The world climatic conditions are divided into Tropical; Sub-Tropical and Temperate climates. Fruits and Vegetables suited to these climatic conditions are grown successfully. Considering the nutritional and economic importance of fruits and vegetables, there is tremendous increase in area and production of fruits and vegetables throughout the world. The present global scenario of fruit and vegetable crops (areas harvested, production and productivity (yield) has been given in Table 1. It shows that the fruits 865.88 million tones has been harvested from 65.24 million Ha land with productivity 13.27 t/ha (Table 1) where as vegetables produced 1075.20 million tonnes from area 57.00 million ha with yield 18.86 tonnes/ha (Table 1) during 2016.

The food we eat contains nourishing substances called nutrients which are required by human body for proper functioning and maintenance of body health. They are classified into five groups such as:

Carbohydrates,

Protein,

Fats,

Minerals

Vitamins

Fruits and Vegetables are highly perishable in nature due to which huge spoilage (damage) takes place during the period between harvesting and consumption. Fruits and Vegetables after harvesting are often poorly handled and in many cases neither cleaned nor washed. Improper handling may cause physical damage due to tissue breakdown which results mechanical damages and physiological changes in harvested fruits and vegetables. Mechanical losses include bruising, cracking, cuts, microbial spoilage by fungi and bacteria and physiological losses include changes in respiration, transpiration, pigments, organic acids and flavour.

Processed fruit and vegetable are considered as staple food products throughout the world, and are becoming more popular with the increase of the global urban population. As a result, demand for processed fruit and vegetable products tends to increase with population growth in the world. There is strong demand for processed fruits and vegetables in the developing world over the past few years. In poor countries, people consume less fruits and vegetables due to the relatively high prices. All forms of preserved fruits and vegetables are available only to the urban elite people. But rural mosses producing more than 90 % of these fruits and vegetables are usually deprived of their uses.

Demand of Processed Fruits and Vegetables

Demand of processed fruits and vegetables is increasing in the developing as well as under developing countries of world. The following reasons are responsible for day by day increasing demand of processed fruits and vegetables.

After harvesting the quality of fruits and vegetables begins to diminish and products is edible for only a limited time then products begin to spoil or rot. Losses occur during harvesting and handling of fruits and vegetables can be minimized by proper techniques but processed food spoilage is bit difficult to control and stop after infection and contamination of microorganism.

Spoilage

Spoilage is deteriorative process which results reduction in quality of food (fruits and vegetables) which become inedible. The changes in food that cause it to lose its desired quality and eventually become inedible is called food spoilage or rotting. After harvesting, the quality of fruits and vegetables begins to diminish and product is edible for only a limited time. The deterioration and spoilage of fruits and vegetables may occur at different ways.

The losses in harvested products begin immediately after detachment of products from their parental plants. After some time, theses products begin to spoil or rot. Spoilage can be defined as the deteriorative process which renders the food inedible or results in reduction in quality. Spoilage of fresh fruits and vegetables usually occurs during storage, transportation and during processing. They also get contaminated with spoilage organisms either from each other or from the containers. Mechanical injuries during harvesting and transportation further aggravates the deterioration process.

The fruits and vegetables are highly perishable in nature. The losses in harvested fruits and vegetables start immediately after harvesting and the resulting spoilage caused by biochemical changes (enzymatic) and activity of microorganisms such as bacteria, moulds and yeast. The losses caused by the microorganism in fruits and vegetables products can be prevented by controlling or destroying these organisms in order to preserve fruits and vegetables by processing and preservation means. The processing and preservation is one of the most important activities among various types of activities of Horticultural Science.

Processing: The processing can be defined as an activity, which is performed to maintain or improve the quality or to change the form or characteristics of the product. Processing operations are undertaken to add value to horticultural produce after their production. The main purpose of processing is to minimize the qualitative and quantitative deterioration of produce after harvest.

Preservation: Preservation can be defined as the science which deals with the methods of prevention of decay or spoilage of produce and allow it to be stored in a fit condition for future use.

Fruits and vegetables can be preserved for short and long term. For short term preservation can be achieved by proper handling of produce and low temperature storage. Long term preservation can be achieved by canning, drying and dehydration, fermentation and by using chemical preservatives.

The basic processing and preservation techniques such as canning, freezing, dehydration, salting, pickling and freeze drying are designed to change perishable foods into more usable form and prevent undesirable changes caused by microorganisms, or by chemical, physical and biochemical reactions in the food itself. Hence an ideal processing technology would involve prevention of growth of microorganisms and at the same time retention of f lavor, texture, nutritive value and palatability of the products.

Processing and Preservation

Processing and preservation is the best way of utilizing surplus production of fruits during seasonal gluts.

In the physical methods of preservation, fruits and vegetables are preserved by (i) adding heat, (ii) removing moisture content of produce to prevent spoilage and (iii) irradiated by Gama rays which kill the microorganisms responsible for spoilage of foods. These methods are described below.

Processing and Preservation by Adding Heat or Heating

In this method, heat treatment is given to destroy the microorganisms which are responsible for spoilage of the produce. Heat treatment is given by many methods.

In the previous chapter the physical methods of fruit and vegetables processing and preservation have been discussed. The present chapter deals with chemical method of fruit and vegetable preservation. In this method of preservation of fruits and vegetables, chemicals as preservatives are used to prevent the spoilage of fruit and vegetable products. These preservatives react with food (produce) constituents or microorganisms and create unfavourable conditions which hinder growth, activities and multiplication of microbes responsible for spoilage of food products. These chemical agents have maximum permissible limit of their use as law. They can be classified into two classes class I and class II.

Class I: Sugar, Salt and Oils

Class II: Sodium benzoate, Potassium metabisulphite (K M S) and Sorbic acid

Sugar also acts as preservative at high concentration 70% or more in food products (fruit and vegetable products). High concentration of sugar in food materials create unfavourable conditions for microbial survival and prevent spoilage. Based on this principle of preservation, sugar based products are prepared from fruits and vegetables so that they can be preserved for long time. This method of preservation with high sugar concentration is principally based on the reduction in moisture content to arrest or stop microbial spoilage. Preparation of sugar based products is one of the most important aspects of home scale as well as industrial level processing of fruits and vegetables. Sugar based value added products include.

Fermentation

The decomposition of carbohydrates by microorganisms and enzymes is known as fermentation. During fermentation process of food organic acids and alcohol etc are produced which act as preservatives and also provide fine flavor and aroma that increase the value of raw materials. Most fermented foods contain a complex mixture of carbohydrates, proteins and fats etc. Fermentation occurs when microorganisms consume susceptible organic substances as a part of their own metabolic processes. Natural fermentation is probably oldest form of food preservation.

Preservation of food (fruits and vegetables) in common salt sand vinegar is called pickling. It is one of the most ancient method of preservation of fruits and vegetables. Pickling is the result of fermentation by Lactic acid forming bacteria which are found in large number on the surface of fruits and vegetables. They can grow in acidic medium and in the presence of 8-10 percent salt solution. The growth of majority of undesirable bacteria is inhibited. Lactic acid most active at 30 °C temperature. This temperature should be maintained during the process of pickling.

Preparation of pickles

Pickles are prepared with salt, vinegar and oil or combination of these ingredients. Spices are also used for pickle preparation.

Process of preservation of food stuffs such as fruits and vegetables whole or in pieces in sugar syrup or brine by heating them in hermetically sealed containers for long storage is known as canning. In other words canning is the process of sealing food stuffs hermetically in containers and sterilizing them by heat for long storage.

Several products of fruits and vegetables are prepared on commercial and home scale. Products development from fruits and vegetables is the best way to utilize excess produces and save them from post harvest losses..

Value Addition

This is the process of improving the value of a commodity through processing by changing its form, increasing its shelf life or better presentation like packaging. Value addition can be done through the ways.

Packaging

Packaging is a mean or system by which a fresh or processed product will reach from the production center to consumer in safe and sound condition at an affordable price.

Objectives of packaging and packaging materials

The primary objective or packaging is to protect the contents during storage, transportation and distribution against deterioration. It may be physical, chemical or biological. However, followings are objectives of packaging and packaging materials.

Several fruits and vegetables are unexploited for proper utilization and product development. Due to lack of utilization, there is a lot of wastage and ultimately the great loss to the people as well as nation. Some unexploited fruits and vegetables have potential for introducing new products. Therefore unexploited fruits and vegetables have to be exploited for new product development especially from underutilized sub-tropical and tropical fruits and vegetables. Research and development should be undertaken to utilize these fruits and vegetables.

Unexploited Fruits

Unripe Green Mango: Although pickles are prepared but droped green mango fruits are not preferred for pickle preparation. They can be dried and utilized for preparation of dried slices or amchur. Amchur is used to develop flavor and taste in Ice cream. These products are used to develop flavor and taste in meal or food. It is also used to prepare chatney. But utilization of Amchur is not known to common people.

Avocado: It is found in tropical and sub-tropical climatic regions having more humidity and rainfall. In Ethiopia it is found throughout the country but not utilized properly by people. They prefer to eat ripe fruits with salt and lime juice. It is very healthy fruit for chronic disease like cardiac and diabeted diseases. This is seasonal use but no product is available for off season. Therefore researches should be carried out for different products development of Avocado products.

Of the 7 billion global population, about 800 million do not have enough food to eat and majority of the them live in developing countries. By 2050, the world population is estimated to reach 9 billion. Plant pests and diseases are responsible for losses of 20 to 40% of global food production. Due to increasing global trade and a changing climate, plant diseases pose a major threat to food security today than ever before. Agriculture will have to cope with weather fluctuations that can affect the survival, distribution and multiplication of plant pathogens apart from shift in pathogenic microbial flora. Emergence of new races of pathogen potentially threaten the production of food. To cite an example, migration of 13 -A2 clonal lineage of Phytophthora infestans from Europe, during 2013, devastating tomato and potato in India. Plant pathogens also impact on environment sustainability, forest health, recreational horticulture, human and animal health.

The science of plant pathology can play a much needed role in alleviating hunger and malnutrition through development of affordable solutions for devastating plant diseases leading to increase in food production. It hold promise to ensure quality produce by containing microbial invasions and toxin contamination of food crops. Plant Pathologists have an important role to play in meeting the food security need of the nation by developing technologies for ensuring proper crop health. There is an urgent need to produce more food per unit area for feeding rapidly increasing human population. We need to act ahead of pathogens in our efforts to ensure normal crop yields. Cutting edge biotechnologies such as CRISPR gene editing systems and RNA interference are being exploited to rapidly developed crop varieties having resistance to specific pathogen races.

Crop protection has come a long way from predominantly chemical based approaches to more environment friendly IDM technologies including green chemistry. There is a need to develop bio-intensive modules for management of diseases. Decision support system need to be developed for important diseases for their timely, more efficiently management. There is need to adopt information technology at a larger scale to facilitate quick dissemination of new crop protection solutions among the farmers and other stakeholders.

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 and plant virology will be discussed in chronological order for easy and better understanding.

There is a very impressive and extensive number of terms and definitions used in plant pathology, many of which you will not come in contact with. With interest, study and practice, terms and names will come to light and become familiar to you. When you start out, don’t worry too much about the terms; but also don’t be afraid of them. The more exposure you have to the subject the more comfortable you will be when dealing with your peers and the public. With these you are going to be exposed to some terms and definitions that will be a good ice breaker and prepares you for the information to follow

Definition

Pathogenesis can be defined as the chain of events leading to the development of a disease. In another words it is also called disease cycle.

It tells us about

The Disease Triangle and Disease Pyramid

The disease triangle is one of the first concepts encountered by students in an introductory plant pathology course and often may be encountered in higher level classes. The interactions of the three component namely (1) susceptible host (2) virulent pathogen and (3) favorable environments have been visualized as a triangle, generally referred to as the “DISEASE TRAINGLE”. Each side of the traingle represent one of the three components. The length of each side is proportional to the sum total of the characteristic of each component that favor disease. For example, the more virulent and abundant the pathogen, the longer the pathogen side would be and the greater the potential amount of disease. Similarly if the plant are resistant, the widely spaced or wrong age, the amount of disease would be small or zero. Where as if the plants are susceptible, at a susceptible stage of growth, or densely planted, the host side would be long and the potential amount of disease could be great. Also, the more favourable the environmental conditions that help the pathogen, or that reduce host resistance, that longer the envirnmental side would be and greater the potential amount of diseases. If the three component of the disease traingle could be quantified, the area of the traingle would represent the amount of disease in a plant or in plant population. If any of three component in zero, these can be no disease.

There are number of diseases which attacks crop plants. Therefore It is mandatory to classify plant disease into more manageable and more specific group based on several criteria for easy and better understanding. Let us discuss one by one.

1. On the basis of type of infection

On the basis of type of infection disease can be classified into two types

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.

A. Parasites

The diseases are incited by parasites under a set of suitable environment. Association of definite pathogen is essential with each disease. They include both biotic and mesobiotic agents.

Biotic agents: They are also called as animate causes. They are living organisms.

Fungi

Definition

Fungi are eukaryotic, spore bearing, achlorophyllus, heterotrophic and thallophytic plants with varied forms and habitats, representing heterothallic or homothallic types of sexuality with characteristic sexual and /or asexual means of reproduction without tissue differentiation.

Definition

Plant disease in which no pathogens or parasitic is associated with the cause is known as Non-parasitic disease. They are also called as noninfectious or physiological disorders.

General Characteristics

Occurs in the absence of pathogen and therefore, cannot be cultured or transmitted.

Caused by the lack or excess of something that supports life from diseased to healthy plants.

Effect plant in all stages of their lives, such as seeds, seedlings, mature plants or fruits.

Symptoms may range from slight to severe & affected plants may even die.

These disease cause damage in field, in storage or at the market.

a) Definition

Fungus is Latin word meaning ‘mushroom’

Alexopoulus and Mims 1979 defined fungus as eukaryotic, achlorophyllous organism generally produced by sexual or asexual method and whose filamentous branched somatic structure is typically covered by cell wall.

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, whereas, 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.

Taxonomy

Taxonomy is the science that deals with the identification, nomenclature and classification of organisms.

Systematics

It is scientific study of organisms with the ultimate object of characterizing and arranging them in an orderly manner.

Classification

Classification is the ordering of organisms into groups and systems, based on their morphological, phylogenetic and physiological relationships.

Nomenclature

It is the system of assigning names to the taxonomic groups or organism according to international rules.

Bacteria

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.

Definition of Virus

According to Luria and Darnell (1968), Viruses are entities whose genome is a nucleic acid, either DNA or RNA, which reproduce inside living cells and use their synthetic machinery to direct the synthesis of specialized particles- the virions, which contain the viral genome and transfer it to other cells.

Nature of Virus

The physical and chemical nature of viruses eluded scientists until the 1930s. Stanley (1935) announced crystallization of Tobacco mosaic viruses, a feat for which he got a Nobel Prize. Stanley thought the crystals to be pure proteins. It was the pioneer work of F.C. Bawden and N.W. Pirie of England which showed that the viruses consists of proetein and nucleic acid. The chemical nature of viruses was settled. It consisted of nucleic acid and protein. The viruses chemically were proved to be nucleic acid. The mature particle of a plant virus is generally called virion and the whole infective particle is called as Nuclcocapsid.

The 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

Neamtode may be defined as triploblastic, bilaterally symmetrical, unsegmented, pseudocoelomate individual invertebrate, with 4 main hypodermal chords, a triradiate oesophagus, circum oesophageal nerve ring, and lacking specialized organs for respiration and circulation.They have tubular gonads which open separately in females but join alimentary canal in males to open through a cloacal aperture.

Definition

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.

Definition

The availability of a plant pathogens in given area depends upon 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 in some intimate association with living plants or other organism.

A. Types of parasitism

An organism that lives within or upon another living organism from which it derives nourishment and in which it may cause various degrees of injury is called as parasite. The removal of food by a parasite from its host is called parasitism. Among the parasite one can distinguish different degree of parasitism

Pathogenesis: Role of enzymes

The chain of metabolic events that bring about the disease is pathogenesis. It involves the virulence of the pathogen, susceptibility of the plant and impact of the environment. It is called disease triangle. Penetration infection, invasion, colonization and reproduction are the events that occur during pathogenesis, and lead to development of disease.

The establishment of pathogen in the host and consequent development of disease involves a chemical interaction between the host and the pathogen. The pathogen employs offensive chemical weapons to break host barriers, which the host resists with all its might. Parasitism and disease resistance run parallel and are inseparable, A biochemical ‘tug of war’ ensures between the host and the pathogen, in which one tries to outwit the other. Enzyme, toxins and growth regulators are the important chemical weapons of the pathogen. Viruses, however, because of their basic nature, do not produce any of these chemicals.

Pathogens produce these substances either in normal course of their activities (constitutively) or when, they grow on certain substrates such as their host plants (Inducible). Plant pathogenic enzymes disintegrate the structural components of host cells, breakdown inert food materials in the cell, or affect components of the protoplast directly and interfare with its functioning system. Toxins seem to act directly on protoplast component and interfare with the permeability of its membrane and with its function. Growth regulators exert a hormonal effect on the cells and either increase or decrease their ability to divide and enlarge. Polysaccharides seem to play a role only in the vascular diseases, in which they passively interfere with the translocation of water in the plants or they may also be toxic.

Introduction

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, employs 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.

Epidemiology

Vanderplank has called epidemiology the science of disease in population. Epidemiology puts its emphasis on population phenomena. The important populations are those of the host, on the one hand, and of the pathogen, on the other. But further, epidemiology is concerned with host-pathogen interactions that lead to disease and crop losses. These interactions are often environmentally dependent. So epidemiology deals with effects of the biotic and abiotic environments. Finally , man has had important effects on hosts, pathogens and the environments, effect that today are increasing in magnitude and scale of importance. Epidemiology therefore also deals with the effects of man on disease and sometimes with the effects of plant disease on mankind. All of this can be symbolized by the disease tetrahedron.

Disease Forecasting

Definition

Forecasting 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.

The techniques used in disease management mainly based on two principles: Prevention or prophylaxis and cure. The first principle prevention includes disease management tactics applied before infection i.e., the plant is protected from disease. It includes: avoidance, exclusion, eradication, resistant & protection. Second principle curative action: functions with any measure applied after the plant is infected i.e., the plant is treated for the disease. It includes therapy.

Defintion

Management of plant diseases using physical means is called physical management.

Also called physical therapy or thermotherapy.

It is mostly curative in nature.

Introduction

Cultural management is defined as manipulation /or adjustment of crop production techniques for management of plant diseases. Cultural practices have been as old as possibly agriculture itself. In early stage 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. As a matter of fact, in the present day agriculture, cultural practices are being considered as essential backup methods for management of plant diseases. Adequate adjustment in crop production techniques can modify the environment in such a manner that it becomes unfavorable for the pathogen and pathogenesis. Based on this the disease control affected by cultural practices are preventive, these practices aim at reducing the activity and density of inoculum. It offer the opportunity to alter the environment, the condition of the host, behavior of the causal agent to achieve economic management of disease.