eChapters

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 .

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.

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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 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.

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.

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.

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.

18.1 Introduction

The farmers have limited resources and a number of enterprises/or enterprise combinations of crops and livestock to choose from. So, the question is how much of what to produce and with what technology. In other words, what combination of enterprises should be produced is expressed as.

26.1 Introduction

In a single equation model, the dependent variable Y is expressed as a linear function of one or more explanatory variables (say X1 , X2 , ... XK ). In such models, an implicate assumption is that the cause and effect relationship between (Y, Xs) is unidirectional. Here explanatory variables are the cause and the dependent variable Y is the effect.

However, there are some situations where two way flow of influence among economic variables exists. Thus one economic variable affects other economic variables and in turn is affected by them. Thus, in the regression of money M on the rate of interest r, the single-equation method assumes implicitly that the rate of interest r is fixed and it is tried to find out the response of money demanded when the level of interest rate is changed.

25.1 Introduction

Farmers must be able to take appropriate decisions at appropriate time. Incorrect judgement and decisions will return in failure of execution of farm plan and in turn economic loss. The farm management decisions can be broadly categorised into two ways.