BSc 2nd Year Introduction to Plant Pathology Notes Study Material
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BSc 2nd Year Introduction to Plant Pathology Notes Study Material
The term pathology etymologically means (Gr. pathos = suffering; logos = the study or to speak or discourse) the “study of the suffering”. Thus plant pathology is the study of the suffering plants.”
Plant pathology has the following four main objectives : (i) etiology, (ii) pathogenesis, (iii) epidemiology, and (iv) control.
1. Etiology (or Aetiology). It concerns the causal organism(s). This is the study of living and non-living entities including environmental conditions that cause diseases in plants. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
2. Pathogenesis. This is the actual mechanism of disease development. It mainly concerns with the processes of infection and colonization of the host by the pathogen. This phase involves complex host-pathogen interactions.
3. Epidemiology. It is mainly concerned with epidemics (more correctly termed epiphytotic in the case of plant diseases). It is infact spread of the pathogen within crop areas. There are various ways in which pathogens multiply and spread through populations of susceptible hosts in the field during the growing season. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
If conditions are favorable for pathogen development, sometimes there are extensive outbreaks of the disease. But the term epidemiology has a wide meaning and also includes most field aspects of the disease. Actually, it is the interaction of crops, pathogens, and the environment, where populations of plants and pathogens rather than individuals are involved. Thus epidemiology in this wider sense includes also the seasonal carryover and subsequent dispersal of pathogens. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
4. Control. This concerns the development of suitable methods of controlling diseases with the objective of reducing the loss in the yield of the crop to its minimum. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Broadly, plant pathology involves two phases: (i) the science of learning and understanding the disease, and (ii) the art of applying the knowledge to real-life problems. Thus plant pathology is both, science as well as art. A plant pathologist is concerned both, with its science i.e. learning and understanding the nature of a disease, as well as its art i.e. diagnosis, treatment, or control of plant diseases as an attempt to solve real problems of life. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
According to Stakman and Harrar (1957) “Diseases of individual plants are relatively unimportant, and plant pathology is, therefore, essentially a community aspect or plants public health science.” Here lies the basic difference between plant pathology and animal pathology, particularly human pathology. In human pathology, the medical doctor’s main target is to cure an individual by treatment, whereas a plant pathologist hardly bothers about the diseased individual plants. He is rather concerned mainly with those plants which are still uninfected and healthy in the field. His main objective is to adopt measures that could prevent the infection of healthy plants.
He is least concerned with the individual diseased plant, except in rare cases such as trees, where individuals can be cured by some curative methods. In plant pathology, most methods of disease control are preventive (prophylactic) whereas in human pathology these are curative. Plant pathology, with such a viewpoint, is thus a community aspect. At the same time, it must be understood that from a scientific viewpoint each suffering plant in the field is important. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Scope and Significance of Plant Pathology
Plant pathology is concerned with all aspects of plant disease and thus has a much wider scope than human pathology which is only one of the many aspects of disease in man (other aspects being hygiene, public health, immunology, therapy, etc.). However, within recent years plant pathologists have begun to specialize in similar individual aspects of disease in plants.
The fields in which notable advances have been made are interactions between host and pathogen at chemical, molecular and genetic levels, plant virology, chemistry of fungitoxicity, and disease forecasting systems. On the practical side, there has been much progress in the development of plant protection chemicals and breeding disease-resistant varieties.
There has been a rapid rise in the world population. To meet the food needs of these people and of the additional millions to come in the next few years, all possible methods to increase the world food supply are being pursued. These include:
(i) expansion of crop areas, (ii) improved methods of cultivation, (iii) increased use of fertilizers, (iv) use of improved varieties, (v) increased irrigation, and (vi) improved crop protection.
Crop protection from diseases and pests reduces a substantial amount of plant products. Thus the welfare of plants through protection from pests and diseases is of particular interest to mankind. This concerns most directly growers and indirectly those who support industries on plant products. In a way, the welfare of plants is of concern to every one of us as growers of food and as consumers of many plant products.
Plant diseases are important as they cause damage to plants and plant products. The kind and amount of losses caused by plant diseases vary with the plant or plant product, the pathogen, locality, environment, control methods, etc. There are several historical and present examples of losses caused by diseases. Wheat, rice, corn, potato, and legumes are sources of staple food in different parts of the world. With advancements in human societies, the need for fiber, shelter, paper, rubber, drugs, etc. increased tremendously. The plants yielding these products suffer from severe losses each year due to pests and diseases.
Plant pathologists study the diseases caused by fungi, bacteria, mycoplasmas, parasitic higher plants, viruses, viroids, nematodes, and protozoa. They also study plant disorders caused by the excess, imbalance, or lack of certain physical or chemical factors, such as moisture, temperature, and nutrients. Plant damages caused by insects, man, or other animals are not ordinarily included in the study of plant pathology. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
What other areas of science are important to plant pathology? Plant pathology utilizes the basic knowledge of botany, mycology, bacteriology, virology, nematology, plant anatomy, plant physiology, genetics, biochemistry, horticulture, soil science, forestry, chemistry, physics, meteorology, engineering, and many other branches of science. Plant pathology derives many benefits from these sciences.
Although plant pathology as science attempts to increase our knowledge of the causes and development of plant diseases, it is also a science with a more practical goal. The purpose is to develop controls for all plant diseases. The goal is to save the produce which is destroyed by diseases. Control measures are often based on knowing something of the principles of plant pathology and these principles combine knowledge from the entire range of science. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Thus a plant pathologist, whether involved with the art or the science of plant pathology, must acquire and use knowledge from all disciplines of science. As long as man must eat, the plant pathologist will find a job to do – to ensure enough food, fibre, and fuel for all.
Terminology of Plant Pathology
Some of the terms used frequently in plant pathology are explained below:
Disease, pathogen, parasite, pathogenicity, virulence, and aggressiveness. It is difficult to define a disease. This involves harmful physiological changes in the plant. We would consider the nature of the disease in detail later. The disease is “a pathological process caused by continuous irritation.” Generally, abnormal physiological changes due to non-parasitic agents are referred to as disorders. Literally, a pathogen is any agent which causes damage, but the term is generally used to denote living organisms, such as fungi, and bacteria.
Most pathogens are also parasites in that they derive the nutrients for growth from a living plant, the host, or suscept. All parasites may not be pathogens. Parasites may be obligate (biotrophs) if restricted to living tissues or facultative when they also colonize living or dead tissue. Necrotrophs grow on dead tissues which they kill in advance, thus more dangerous than biotrophs.
Pathogenicity is the ability to cause disease. It should not be confused with virulence and aggressiveness, somewhat ambiguous terms. Virulence is used as a measure or degree of pathogenicity in a qualitative sense. Thus a pathogen may have several strains (races) of varying virulence, some of which may be avirulent (non-virulent). Aggressiveness is an extremely ambiguous term used by Gaumann to describe the capacity of a parasite to invade and grow in its host plant and to reproduce on or in it.
Inoculum, inoculum potential. Usually, this is not the whole pathogen, but only a part of it that is important in disease. Inoculum is that portion of a pathogen that is transmitted to or contacts a host and is capable of infecting the host. Thus inoculum in bacteria is cells, in virus particles itself, nematodes itself, eggs or cysts, in fungi, vegetative hyphae, and spores. After the inoculum has come in contact with a susceptible host, whether it will be able to infect the host depends on several factors. One such factor is the inoculum potential of the pathogen. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
The term coined by Horsfall (1932) denotes “the number of infective particles present in the environment of the uninfected host.” Later, Garrett (1956, 1960) and Wood (1967) defined it as “a measure of the biological energy available for the colonization of a host.” It is therefore a function of (i) inoculum density, (ii) nutrients available to the infectious units that allow them to germinate or grow, (iii) the virulence or genetic capacity of the pathogen to cause disease, and (iv) the susceptibility of the host (which is controversial).
Immune, resistant, susceptible, tolerant, symptomless carrier, hypersensitivity. These terms are used to indicate the reactions (response) of plants to pathogens. Immune means exempt from infection. Resistance and susceptibility are the measures of the extent to which the plant is damaged by the pathogen, or more often, the extent to which the plant is able to prevent the entry or subsequent growth of the pathogen within it. High resistance (low susceptibility) approaches immunity, and between it and high susceptibility (low resistance are varying degrees of resistance and susceptibility.
Resistance may be horizontal (uniform) when a given variety of more or less uniformly (evenly) resistant to all races of a pathogen, or vertical (differential) when resistance is effective only against some races of the pathogen but not against others. Thus a given variety may be most resistant to some and less resistant or not at all to other races of a pathogen. Resistance may be polygenic (determined by many genes), oligogenic (several genes), or monogenic (one gene). Tolerant plants offer little resistance to pathogens.
Although infected, are only slightly affected in the sense that the symptoms i.e. the visible reaction of the plant to the pathogen, are relatively mild. The climax of tolerance is shown by symptomless carriers in which the disease-causing organism (usually a virus) is present but causes no symptoms. Hypersensitive plants develop minute necrotic flecks or spots resulting from the rapid death of the cells in the vicinity of invading pathogen (infection court) so that the progress of the pathogen is halted. Thus sometimes the oversensitive pathogens check their own development. Hypersensitivity thus confers high resistance to the host plant.
Entry, infection, colonization, symptoms. Under favorable conditions, the entry (penetration) of a pathogen into a susceptible plant is followed by infection. Entry should not be confused with infection. Pathogens may fail to infect even after entry. Infection is the establishment of a nutritional relationship between the pathogen and the host. In highly resistant or immune plants, penetration may occur but not an infection. Infection is followed by colonization in which the pathogen advances through the tissues of the host to varying extents.
Colonization may be limited or involve a considerable area. In systemic infections, the pathogen spreads through the plant to varying degrees and is associated with all phases of its life cycle. In diseased plants, the harmful physiological alterations inside the plant have their counterpart as visible symptoms expressed externally on the host. Thus diseased plants show a number of visible symptoms known collectively as the syndrome. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
These are characteristics of a disease by which it can be recognized in the field. The three general categories of symptoms are, (i) necrosis (death) of the infected tissue, (ii) overdevelopment due to increased cell division (hyperplasia) and/or increase in individual cell size (hypertrophy), leading to overgrowths as galls, tumors, witches brooms: or (iii) underdevelopment, due to reduced growth (hypoplasia) or stunting of the infected plant or organ chlorosis, the failure of chlorophyll development, also included by some under this category.
Seasonal carry-over, transmission, spread, epidemiology. During the period, when the susceptible host is absent from the field, the pathogens perennate in different ways. Mode of perennation is thus important in the seasonal carry-over of pathogens from the previous to the next seasons of crop plants. Pathogens perennate in infected plant debris in or on soil, on subsidiary host plants, and in the infected and contaminated planting materials (mainly in seeds and other propagative parts in soil). Transmission (dissemination or dispersal) is what happens between the take-off of the inoculum (from the site of its production) and its deposition on the – surface.
It, therefore, does not include germination of inoculum or infection of the plant. Technically, the causal organism and not the disease is transmitted. In practice, dissemination involves long-distance dispersion (cf. spread that includes focal distribution in the field, from one plant to another). Spread means that the pathogen reaches and shall infect plants. The term is used for the process by which pathogens multiply and infect susceptible plants, spreading in the population of larger areas of crop plants in the field.
Sometimes, the spread causes extensive outbreaks of diseases in that area i.e. the development of epidemics (epiphytotic). Epidemiology is mainly concerned with the development of epiphytotic i.e. spread of pathogens in plant populations. However, in its wider sense, epidemiology includes most field aspects of the disease, which involves the interaction of the population of crop plants, of pathogens, and the environment. Thus besides spread, modes of seasonal carry-over, and transmission of pathogens are also to be included under epidemiology.
Fungitoxic, fungicidal, fungistatic. Fungitoxic (antifungal) substances are harmful to fungi. These include fungicides which kill fungi and fungistats which simply prevent growth as long as fungus and fungistat are in contact. Similar chemicals are bactericides and bacteriostats for bacteria. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Symbiosis, mutualism, antagonism, synergism. Symbiosis is the living together of unlike organisms and includes parasitism if the host remains alive. More often it is used for association where there is a considerable degree of tolerance between the organisms involved (mutualism). In antagonism (antibiosis), one organism is harmed by another. Synergism occurs when the combined effect of two organisms acting is greater than the sum of two separate effects. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
A useful guide to terms in plant pathology is given by the Federation of British Plant Pathologists (1973).
Disease: Concept, Components, Causes
The nature and concept of disease are very much varied. A number of definitions have been proposed for a diseased plant by Hartig (1894), Ward (1901), Morse and Lewis (1910), Owens (1928), Heald (1933), Dodge and Rickett (1943), Anon (1950), Walker (1957), Stakman and Harrar (1957), Horsfall and Dimond (1959) and Agrios (1969). Of these, we will consider only a few which emphasize the dynamic nature of disease as a reaction between the plant and the causal agent.
Anon (1950) defined the disease as a “harmful deviation from the normal functioning of physiological processes.” According to Stakman and Harrar (1957), plant disease is “a physiological disorder or structural abnormality that is deleterious to the plant or to any of its parts or products, or that reduces their economic value.”
All aspects of the disease, as expressed by different authors in their independent definitions are considered in detail by Horsfall and Dimond (1959). A suffering plant is diseased. What then is disease? The disease is very often confused with condition (symptoms), pathogen (cause), or injury. The disease is very often considered a condition, but a disease is not a condition. A plant or animal may be in a diseased condition. The condition is not a disease, the condition actually results from the disease. The resulting diseased condition has a symptoms complex syndrome. Sometimes the disease is confused with the pathogen (cause). It is usually said that loose smut of wheat is Ustilago Nuda tritici.
Actually, it is the cause of disease. Generally, it is said that disease is catching. But, only the pathogen be caught, not the disease. Sometimes we very often consider injury, as synonymous with disease. If you cut your finger with a knife, you are injured, not diseased. In such cases, the knife is in contact with the flesh for only a fraction of a second. The knife comes in contact and goes suddenly. In general, we are diseased only if the action of the causal agent is prolonged, and injured if it comes and goes suddenly.
Thus it may be concluded from above, that disease results from continuous irritation (cf. injury which results from transient irritation). Moreover, if something is functioning poorly in the body we say that we are sick. For instance, it is common to say “My stomach is acting up”, “My head aches” or “My Vision is blurred”. Here there is a verb full of action. Hence we may conclude that disease is a malfunctioning process that is caused by continuous irritation. This process results in suffering. Hence disease can be defined as a pathological process.
Components of disease – Disease pyramid
It is common to recognize three important components of disease = a susceptible host, a virulent and aggressive pathogen, and a favourable environment. These three components constitute the traditional disease triangle. Very recently, Stevens (1960), Zadoks (1972), and Van der Plank (1975) have however recognized the fourth factor, time, as an equally important dimension of plant disease. Time is important in many ways, viz. time of year when host and pathogen are juxtaposed, time and duration of leaf wetness while the temperature is favourable, time of the next period favourable for spore dispersal and germination, and for the establishment of infection, etc.
The effect of time on disease development becomes apparent when we consider the importance of the time of year (that is, climatic conditions, and stage of growth when host and pathogen may exist), the duration and frequency of favourable temperature and rains, the time of appearance of the vector, the duration of the cycle of a particular disease, the earliness, and lateness of maturity of the host, etc. Adding time to the disease triangle as a fourth dimension gives a disease pyramid.
If the four components of the disease pyramid could be quantified, its volume would be proportional to the amount of disease in a plant or in a plant population, and the area under a disease progress curve. Reducing any component’ of the disease pyramid through management (the host susceptibility, the virulence of the pathogen, degree of favourableness of time or of the environment) will decrease the amount of disease in a plant or the intensity of the epidemic in a population of plant, and hence the volume of the pyramid. Plant pathologists thus can manage disease components in many ways. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Disease development in cultivated plants is also greatly influenced by a fifth component, humans. Humans affect disease development in various ways. They affect the kind of plants grown in a given area, their level of resistance, the number of plants planted, the time of planting, and the density of us. Through their cultural practices, chemical, and biological control, etc. humans affect the amount of primary and secondary inoculum available to attack plants. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
They also modify the environment on disease development in several ways (manipulating the time of planting and harvesting etc.) The human component has sometimes been used in place of the “time” component in the disease pyramid. But it should be considered a distinct fifth component that directly and indirectly affects disease development.
In Fig. host, pathogen and environment are each represented by one of the sides of the triangle, time is represented as the perpendicular line arising from the center of the triangle, and humans as the peak of the pyramid whose base is the triangle and height is the length of time. In this way, humans interact with and influence each of the other four components of the disease.
Causes of plant disease
It is very important primarily to know about the cause of the disease. Any agent that causes the plant to suffer is called a cause. Very often, this cause is a living entity, designated as a pathogen. Some use the term pathogen in a wider sense and include such non-living causes of disease as mineral deficiencies or excesses in soil, improper light, moisture, temperature, aeration, etc. of atmosphere also as a pathogen. However, very often the term pathogen is used only for living entities causing the disease.
Some authors do not agree with the viewpoint that pathogens are the only cause of diseases. According to them, the environment plays an equally important role in disease development. This relationship between the environment in disease development has been mainly advocated by Wisconsin plant pathology established by Jones.
They introduced a new term to incite, and a pathogen no longer causes a disease, it incites it. According to Walker, it is incorrect in the strict sense to say that a given microorganism only causes a given disease, since this implies that it is the sole cause. It should better be referred to as the incitant or causal organism since it implies that the organism is a part of the causal complex. Thus a pathogen is an agency that incites the disease. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
But most of the authors including Horsfall and Dimond, Agrios, and Luttrell agree that the verb to incite is unnecessary and that the old-fashioned word, the cause is sufficient. The cause is a larger word than incite. If plant pathology is a science, then diseases are caused and not fomented, encouraged, aroused, goaded, or incited. The pathogen is the basic cause. It must be there. Pathogen characterizes the disease. The weather does not. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
The causes of disease may be of two kinds:
1. Living or animate. The pathogens vary in their form, structure, and precise nature of pathogenesis (mechanism of actual disease development). Pathogens are of the following types :
(i) Viruses and viroids, (ii) Mycoplasmas (PPLO), Rickettsias and related organisms, (iii) Bacteria, (iv) Fungi, (v) Algae; (vi) Phanerogams, (vii) Nematodes, (viii) Insects. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
2. Non-living or inanimate. These include mainly the deficiencies or excesses of nutrients, continuous use of plant protection chemicals (i.e. pesticides) causing iatrogenic diseases, light, moisture, aeration, abnormalities in soil, etc.
Of course, the pathogens will be able to cause disease only if suitable hosts and favourable environmental conditions prevail there.
Classification of Diseases
Plant diseases may be grouped in various ways. On the basis of the extent to which diseases are associated with plants, diseases may be:
1. Localised diseases. Those diseases which are limited to a definite area of varying extents of an organ, or only to a particular part of the plant, and
2. Systemic diseases. The diseases in which pathogen spreads throughout the entire plant to varying extents and is associated with almost every stage of a plant’s life cycle. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
On the basis of their method of perpetuation and mode of primary infection, diseases are grouped as:
Soil-borne diseases. In pathogens causing diseases like pre-emergence killing and damping off of seedlings, seedling blight (Pythium, Phytophthora, Fusarium, Corticium), root-rots (Verticillium, Colletotrichum, Macrophomina), vascular wilts (Fusarium, spp. Verticillium) and other root diseases caused by Gauemannomyces graminis, Phymatotrichum omnivore and Armillaria mellea the primary infection takes place from the soil. These soil-borne pathogens survive in soil or on infested plant debris lying in soil either as their resting spores or as mycelial strands and rhizomorphs. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
They all attack the root systems of host plants. Under favorable conditions, the spores germinate and infect roots either at the seeding stage or in an adult plant. Mycelial strands, rhizomorphs, and even individual hyphae (Gauemannomyces) can grow actively from one point to another in the soil, and when coming in contact with the root of a susceptible host, they infect it.
2. Air-borne diseases. Some pathogens, however, infect the host plant through the air and bring primary as well as secondary infection. Fungi as rusts and mildews develop their thick-walled resting structures (teliospores in rust, cleistothecia in powdery mildews, oospores in downy mildews). Under favorable conditions, these germinate to produce thin-walled spores. The spores are disseminated by wind and infect the shoot of the plant. Loose smuts bring about secondary infection through the air.
3. Seed-borne diseases. Some pathogens survive as dormant mycelium in the seeds or other propagative structures of host plants. Many smuts and other fungi belong to this category. In most smuts, the dormant mycelium becomes active at the time of seed germination. The mycelium grows with the growing apex of the seedling. Later, the mycelium spreads throughout the entire plant including the floral parts. Such diseases are mostly systemic. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Diseases are sometimes classified on the basis of the kind of symptoms:
(i) rusts (ii) smuts (iii) mildews (iv) root-rots (v) blights (vi) leaf spots (vii) wilts (viii) cankers (ix) fruit rots etc. In such cases, it may be seen that the disease derives its name from the type of characteristic symptoms it produces in plants.
The different kinds of symptoms are described in detail later in this post. According to the type of host plants, diseases are classified as:
(i) field crop diseases, (ii) vegetable diseases, (iii) fruit diseases, (iv) forest diseases, (v) ornamental plants diseases.
On the basis of their extent of occurrence and geographic distribution, diseased are classified into:
1. Endemic diseases. These diseases are natural to one country or part of the earth. When a disease is more or less constantly present from year to year in a moderate to a severe form, in a particular country or part of the earth, it is classed as endemic.
2. Epidemic or epiphytotic diseases. Such a disease occurs periodically but in a severe state involving a major area of the crop. It may be constantly present in the locality but assumes severe form only on occasion. This is because the environmental conditions favourable for rapid disease development occur only periodically. Thus, the major determining factor is the environmental conditions.
3. Sporadic diseases. Those diseases occur at very irregular intervals and locations and in relatively few instances.
A given disease may be endemic in one region and epidemic in another.
Diseases may also be classified according to the plant organ they affect. Thus there are root diseases, stem diseases, foliage diseases, fruit diseases, etc.
Till 1850 or so, diseases were classified mainly on the basis of the effects they bring about in plants, or other products. However, after 1850, the diseases were classified on the basis of their cause rather than their effect.
The most satisfactory way to classify diseases is on the basis of their cause. Thus according to to cause the diseases are classified as (i) Infectious (parasitic) diseases and, (ii) Non-infectious (Non-parasitic) disorders (as many prefer to call them).
[I] Infectious plant diseases
They are caused by living agents, pathogens. They may spread by contact with a diseased plant with healthy plants. All pathogens are parasitic on plants. On basis of the kind of pathogen, these are further classified as follows.
(1) Diseases caused by fungi
(2) Diseases caused by bacteria
(3) Diseases caused by mycoplasmas
(4) Diseases caused by viruses and viroids
(5) Diseases caused by nematodes
(6) Diseases caused by protozoa
(7) Diseases caused by parasitic higher plants.
[II] Non-infectious or physiological disorders
These do not spread from plant to plant (non-infectious). These are caused due to improper conditions of soil air or mechanical injury. Common examples are tip rot or necrosis of mango fruits, tip burn of paddy, black heart of potato, etc. On the basis of the nature of the cause, these are classified as follows:
(1) Too low or too high temperature
(2) Lack or excess of soil moisture
(3) Lack or excess of light
(4) Lack of oxygen
(5) Air pollution
(6) Nutrient deficiencies
(7) Mineral toxicities
(8) Soil acidity or alkalinity (pH)
(9) Toxicity of pesticides
(10) Improper agricultural practices.
The above type of classification has the advantage that it indicates, the cause of the disease, knowledge of which suggests the probable development and spread of the disease and also possible methods of its control.
Iatrogenic Diseases
The rapid rise of organic pesticides in the late 1940s was accompanied by a rapid rise in iatrogenic diseases. The importance of iatrogenic disease is well recognized in human medicine but its significance in plants as a consequence of crop production practices has received little attention. It is a disease produced by appropriate therapy for another disease, usually occurring as a side effect of pharmacological agents (Chambers Scientific and Technical Dictionary, 1971). (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Indeed the first use of the term in plant disease literature appears to have been by James G. Horsfall (1972) who used it in a broad sense to include the effects of all crop protection chemicals which resulted in either the appearance of a new disease or, much more commonly, an increase in the disease already present. As far as we know Johnson (1946) was the first to report an iatrogenic disease due to pesticides.
He found that DDT converted rust-resistant wheat to a susceptible one. He however did not use the term. As defined by Horsfall (1979) “An iatrogenic disease of a crop is one that is induced or worsened by a plant pathologist’s prescription for that crop It is a disease generated by the “doctor” so to speak latro is the Greek word for doctor. Details of this interesting class of diseases are given by Yarwood (1970) Cowling (1978), Griffiths and Berrie (1978), Horsfall (1979), and Griffiths (1981).
In 1956 in Switzerland, Horsfall saw a chemically induced iatrogenic disease for the first time. Dr. S. Blumer of the Wadenswil Experiment Station showed him how zineb was controlling Plasmopara viticola on the grapes but was worsening powdery mildew (Uncinula necator) on the same vine and Botrytis on the grapes they bore. This stirred Horsfall’s interest in the subject.
Yarwood (1970) and Cowling (1978) included all agricultural and forest practices that cause such diseases, whereas Horsfall (1979) restricted to agrochemicals. There are so many reports of such diseases. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Iatrogenic diseases can be conveniently divided into three groups according to whether the disease is induced through the action of agrochemicals on (i) the host plant (ii) the pathogen or (iii) the ecosystem in which the host and pathogen co-exist. Thus a chemical can (i) reduce the resistance of the host (ii) increase the inoculum potential of the pathogen, or (iii) alter the microclimate and microhabitat so that it is now more favourable to the disease. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Host resistance can be reduced through (i) structural changes in it, (ii) changes in its chemical composition as in sugar levels, uptake of nitrogen and other mineral nutrients, or (iii) changes in the level of exudates from its surface. The inoculum potential of the pathogen may increase through (i) changes in its nutrition (ii) its stimulation (iii) encouragement of synergistic organisms, (iv) elimination of its predators or parasites, or (v) conversion of saprophyte to the parasite (e.g., a saprophytic Trichoderma viride may develop the ability to attack live tissues). (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Microclimate and microhabitat involve the change in factors (physical as well as chemical) of the site where both host and pathogen co-exist in a way that favour disease development. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
To cite just an example, changes in sugar levels are relevant to such situations. Horsfall and Dimond (1957) infact generalized the sugar story by suggesting that some diseases are high sugar diseases as rusts, whereas others as leaf spots, blights, rots, etc, are caused by species of Alternaria, Helminthosporium, Drechslera, Botrytis, etc. are low sugar diseases.
Chemicals like DDT and maleic hydrazide (MH) increase whereas 2,4-D decreases the sugar content of leaves. DDT and MH inhibit the outflow of sugars from the leaf (increasing its sugar content) and thereby worsen the high-sugar pathogens as rusts 2,4,-D accelerates the outflow of sugar from the leaf (decreasing its sugar content) and thus worsens the low-sugar diseases. Similar changes are brought about by the chemicals in the pathogen as well as in the ecosystem of the host-pathogen system.
General Symptoms of Plant Diseases
As a result of the successful infection of the host plant by the pathogen, a number of physiological changes occur in the plant. Respiration, photosynthesis, nitrogen metabolism, and transpiration are affected. In most cases, host respiration increases after infection. There is a reduced rate of photosynthesis in many diseases. Metabolites like carbohydrates, amino acids, and proteins tend to accumulate in green islands. Normally, there is an increase in transpiration rates.
The physiological changes occurring in the diseased plant have their counterparts in the anatomical and morphological changes—morbid anatomy. Anatomical and morphological changes are brought about by affecting individual cells and the growth of an entire plant or of an organ, vegetative as well as floral. Thus, changes in the physiology of plants due to infection result in several anatomical and morphological changes which are expressed externally in the form of visible symptoms. Symptoms are characteristic of a disease by which it can be recognized in the field. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
In some cases, however, two different pathogens can produce identical symptoms and a single pathogen can develop more than one kind of symptom i.e. disease syndrome. Kinds of symptoms are as varied as the instance different groups of pathogens cause a variety of symptoms as follows:
Myxomycetes and Phycomycetes. Galls, scabs, warts, rots, blights, damping off, white rusts, downy mildew.
Ascomycetes and Fungi Imperfecti. Leaf curls, powdery mildews, cankers, leaf spots, anthracnoses, root and stem rots, vascular wilts, scabs, fruit and vegetable rots.
Basidiomycetes. Rusts, smuts, root and stem rots, wood rots.
Bacteria. Galls, rots, blights, spots, cankers, scabs, vascular wilts.
Mycoplasma. Aster yellows, proliferation, general yellowing, stunting, phloem neurosis.
Viruses. Mosaics and mottles, ring spots, dwarfing, rolls, yellows, galls, tumors, pitting, etc.
Nematodes. Root knots (galls), root lesions, excessive root branching, root rots. Galls, lesions, and rots on a leaf, distortion of leaf, stem, galls in grains.
This enormous variety of symptoms basically belongs to any of the three main categories: (i) Necrosis, (ii) Hypertrophy and Hyperplasia, and (iii) Hypoplasia.