The study of plant disease is called plant pathology. Diseases are defined as disorders or physiological disturbances of the normal function of plants caused by physical, chemical or biological factors. The biological factors are those referred to mainly as Pathjogens.

In other words, diseases are caused mainly by the low forms of plant life such as fungi, bacteria and viruses, but some diseases or disorders in crop plants are caused by other physical or soil factors.

The visible morphological expressions of diseases condition are referred to as symptoms. Decision on the importance of crop disease and the need for control are usually based on economic considerations.

Generally, a plant is diseased when it is continuously disturbed by some casual agents that results in an abnormal physiological process that disrupts the plant’s normal structure, growth, function or other activities.

This interference with one or more plants essential physiological or biochemical systems elicits characteristic pathological conditions or what is known as symptoms.

The malfunctioning processes due to inroads of a foreign factor or due to some other biotic causes should make the plant abnormal on the sense that it is losing its economic value.

However, if there is some malfunctioning or abnormality, even if caused by some biotic or abiotic factor, which does not cause loss of economic value or enhances the beauty or value of the plant it should not be called a disease in general sense.

For example, the “broken tulips’ in Holland which were actually due to viral infection but they fetched very high market price because of their beauty. Variegation on ornamental perennial is another example.

Causes of Plant Diseases

A pathogen is always associated with a disease. A pathogen is any agent or factor that incites “pathos” or disease in an organism. Thus in strict terms, the pathogens do not necessarily belong to living or animate groups.

They may be non – living or in – between the living and non – living (such as viruses).

The plant pathogens are thus grouped under the following categories;

1. Abiotic Factors

These include mainly the deficiencies or excesses of nutrient, light, moisture aeration abnormalities in soil conditions; atmospheric impurities etc. Examples of diseases caused by these factors are described below:

a. Nutrient deficiencies: The deficiency of major elements as nitrogen, potassium, phosphorus and sulphur is generally characterized by yellowing of leaves and poor plant growth.

Magnesium deficiency is characterized by mottling or chlorosis, as well as cupping of leaves and necrotic spotting. Calcium deficiency results in irregular, distorted, brown scorched leaves and necrotic spotting.

b. Insufficient light: This slows down chlorophyll formation and leads to a condition known as etiolation – lean growth with long internodes, pale green leaves and premature abscission of leaves and flowers.

c. Too high temperature: The side of the fruit facing the sun dries up (sun scald disease) below the skin and becomes dicoloured and water – soaked.

d. Low soil moisture: The symptom is generally poor growth. In maize, the leaves turn brown and the crop hardly flowers. When severe, the plant wilts and dies.

e. Air pollution: Dusts settling on leaves could lead to chlorosis and poor growth. Phytotoxicity may result when some dissolve in rain water.

f. High soil moisture: Waster logging of soils causes root decay due to reduced oxygen. The situation if prolonged could lead to the collapse of the root cells and death of the plant.

g. Pesticides toxicity: Leaf burn may result from a pesticide spray n that is too highly concentrated.

2. Mesobiotic Causes

These are disease incitants which are neither living nor non – living. They are considered to be on the threshold of life.

They include;

(i) The viroids which are naked infectious strands of nucleic acid. Spindle tuber of potato and tomato bunchy are some of the examples

(ii) Viruses which are infectious agents made up of one type of nucleic acid (RNA or DNA) enclosed in a protein coat.

Examples of virus diseases of plants are leaf roll of potato, leaf curl of tomato and chili, mosaic diseases of many crops.

3. Biotic Causes

This category includes diseases by animate or living organisms.

Damage Caused By Plant Diseases

Disease in crop plant is usually due to the disruption of form, function and energy usage and is caused mainly by pathogens.

The effect of diseases may range from complete destruction or loss of crop stands (as in tomato wilt or maize stalk rot), to inconspicuous damage which nevertheless results in the depression of yields or loss in quality (as in root rots of sugar cane, maize and other cereals, the leaf spot of legumes cotton and other fibre and oil seed crops, and virus diseases of cassava, sweet potatoes and a wide variety of cultivated crops).

Often, the signs of infection are hidden underground and there may be a tendency to ignore the diseases (as in nematodes attack).

In many other cases the effect if disease is in reducing the quality and market value of the crop or crop products. This is particularly important in perishable farm products, such as vegetables, fruits, seeds and tubers.

The blemish caused by bacteria or fungi on tomatoes, bananas, oranges, etc. and the rots on tubers largely contribute to the lowering of the market value of such products.

Categorization of Plant Diseases

A disease may be localized if it affects only specific or parts of the plant. It may be systemic if it affects the entire plant. The diseases are called soil – borne or seed – borne when the causal agent inciting the disease perpetuate through the agency of the soil or seed (or any propagating material).

However, a “disease” is never soil – borne. It is causal agent that is soil borne. Similarly, a “disease” is seed – borne only when the seed carrying its causal agent is infected and the pathogen is seed – borne, not the disease.

The pathogens may be air – borne if they are disseminated by wind. Often, in this case also, a disease is called air – borne which is technically incorrect.

The symptoms or signs which appear on the affected parts or the entire plant also form a basis for grouping of plant diseases. Thus, we fund diseases known as rusts. Smuts, root rot, wilt, blight, canker, mildew, and fruit rot, leaf spots etc.

In all these examples, the name of the diseases is derived from the most conspicuous symptoms of the disease appearing on the host surface.

According to the host plants, the disease can be grouped as cereal, forage crop, flax, millet, root crop and plantation crop diseases.

Diseases caused by animate and virus pathogens are often classified in relation to their occurrences under the following groups:

1. Endemic Diseases

The word “endemic” means prevalent in, and confined to, a particular country, district, or location. 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 severe form in a particular geographic region, it is classified as endemic to that area.

The causal agent is well established in the fields or in the locality by virtue of its ability to survive through soil or other means for long durations and environmental conditions are not adverse to its survival all those disease which become persistent through their survival on alternate or wild hosts from one crop season to the next are also included in this group.

2. Epidemic or Epiphytotic Diseases

The term “epidemic” is derived from a Greek word meaning “among the people” and in the true sense was applied to those diseases of humans which appear very violently among a large section of the population.

To carry the same sense in the case of plant diseases the term “epiphytotic” was coined. However, in general usage, the term epidemic is used for plant diseases also. An epiphytotic disease is one which occurs widely but periodically.

It may occur in the locality every year but assumes severe form only on occasions. This may be because the environments or conditions favourable for severe occurrence occur only periodically.

It is also possible that the environments are favourable but the pathogen is irregular in its occurrence or its inoculums concentration has not reached the desired level to cause the disease in the plant population.

When epiphytotics become prevalent throughout a country, continent or the world, the disease wanes, and, unless the host species has been completely wiped out, the disease subsides to a low level of incidence and becomes endemic.

This balance may change dramatically by conditions that favour as renewed epiphytotic. These conditions are weather (mainly temperature, moisture) very favourable for multiplication, spread and infection by the pathogen, introduction of a new and more susceptible host, development of a very aggressive race of the pathogen, and change in cultural [practices favouring disease.

3. Sporadic Diseases

Sporadic diseases occur at very irregular intervals and locations and in relatively few instances. A given disease may be endemic in one region and endemic in another.

4. Infectious and non–infectious diseases

Plant diseases may be infectious or non – infectious. All diseases caused by animate and virus and viroid pathogens under a set of suitable environments are infectious. Association of a definite pathogen is essential with such disease.

In non – infectious diseases, no animate, virus or viroid pathogen is associated and, therefore, they remain non – infectious and cannot be transmitted from a diseased plant to a healthy plant.

These disorders are due to disturbances in the body caused by lack of proper inherent qualities, by improper environmental conditions of soil and air, and by injurious mechanical influences.

General Symptoms of Pant Diseases

Sign or evidence of disease or disorder as shown by plant or any objective evidence or bodily is called symptoms of the disease.

Symptoms are seen on the plant either due to character and appearance of the visible pathogen or its structure or organs, or due to some effect upon or change in the host due to interaction between the host and the pathogen.

Symptoms due to character and appearance of the visible pathogen:

A parasite is present in all parasitic host tissues, but in most cases the growing vegetative portion of the parasite is within the host tissues and invisible. However, they usually form reproductive or resting structures either outside the plant organs or partly emerging from the host tissues.

In either case they become visible provided they are of sufficient size or in sufficient mass. In some diseases, almost the entire body of the parasite including both vegetative and reproductive portions is external to the host and is, then readily seen, partly on account of its mass.

In some diseases, large structures of the pathogen constitute the most prominent symptom. Several of these symptoms are described below:

Mildew: Mildews are plant diseases in which the pathogen is seen as a growth (mildew) on green surfaces of the most. These growths appear as white, gray, brownish or purplish patches of varying size. In down mildew the superficial growth is a tangled, cottony or downy layer.

In powdery mildews enormous numbers of spores are formed on the superficial growth of the fungus giving a dusty or powdery appearance. Black minute fruiting bodies of the fungus may also develop in the powdered mass.

Rust: the rust disease appear as relatively small pustules of spores, usually breaking through the host epidermis. The pustules may be either dusty or compact and red. Brown, yellow, orange, or black in colour.

Smut: The word “smut” means a sooty or charcoal – like powder. In plant diseases known as smut the affected part of the plant shows a black dusty mass composed of the fungus spores. These symptoms appear in floral organs, particularly the ovary part (ovariculous smuts). The pustules are usually considerably larger than those of the rust. Smut symptoms may also be found on leaves stems and even roots (culmiculous smuts).

White blisters: On leaves of crucifers and many other plants there may be found numerous white, blisters – like pustules which break open and expose white powdery mass of spores. These pustules resemble rust pustules in texture but are white. Such diseases have, therefore, been often called white rust.

Scab: the term scabs refer to a roughened or crust – like lesion or to a freckled appearance of the diseased organ. In some diseases of this type the parasite appears at a certain stage, in others it is never seen. Thus, this term may be listed in both major groups of symptoms.

Sclerotia: a sclerotium is a compact, often hard, mass of dormal fungus myceluium. In some cases, as in ergot of masses, the sclerotium assumes a characteristic shape; in others the shape may be variable. Sclerotia are most often black, or they may be buff or dark brown of purplish in colour.

Blotch: This symptom consists of a superficial growth giving the fruit a blotched appearance as in sooty blotch and fly – speck disease of apple fruits.

Fruitring bodies: the wood rotting fungi develop relatively large spore bearing structures (sporophores) which are either fleshy or woody. The parasite can be identified by means of the characteristics of these sporophores.

Erudations: In bacterial, such as in bacterial blight of rice and fibre blight of pomes fruits, mass of bacterial cells oozes out to the surface of the affected organ where it may be seen as drops of various size or as a thin smear over the surface.

Tar spots: There are somewhat raised, black coated fungus bodies with the appearance of a flattened out drop of tar on leaves.

Symptoms Due To Some Effect On, Or Change in the Host Plant

As a result of disease, there may be a marked change in the form, size, colour, texture, attitude or habit of the plant of some of its organs. Such changes are usually readily observed and often constitute the most prominent symptom of the disease.

Two or more of these changes may occur in the same host organ as effects of the same disease. In most diseases, these changes are brought about by the presence and activity or life processes of the pathogen and reaction of the host tissue to such activity.

The pathogen may be found within the affected tissue or upon the surface or in some cases it may develop certain structures internally and other structures externally. Fruiting bodies or other structures of the pathogen may thus accompany the more striking changes in the host organs.

Colour changes: change of colour from normal, mostly green, is one of the most common symptoms due to the effect of a disease in plant. The green pigment may disappear entirely and its place may be taken by a yellow pigment.

When the loss of green colour is due to prolonged exposure to darkness the condition is called etilation. A similar condition may be brought about by virus disease or from disturbances caused by fungal or bacterial pathogens.

In these cases the yellowing is known as chlorosis when the green pigment is replaced by red, purple or orange pigment the condition is known as chromosis. In some diseases, the leaves are devoid of any pigment. This condition is known as albinism.

Over growth or hypertrophy: The most apparent effect in some diseases is the abnormally increase in size of one or more organs of the plant or of the plant or of certain portions of them. This is usually the result of stimulation of the host tissues to excessive growth. It may be brought about by either or both of two processes, hyperplasia and hypertrophy.

Hyperplasia is the abnormal increase in symptoms. Spots, streaks, strips, canker, blight, damping – off, burns, scald or scorch and rot are as a result of necrosis of tissues size of a plant organs due to an increase in number of cells of the organ.

In hypertrophy the increase in size of the organ is due to increase in size of the cells. Both these conditions may be simultaneously present. In some disease the increase in size s of the plant organ is due to increase in size of the cells and also due to presence of fungus structures.

Galls, knots, leaf curl, pockets and bladders, witches’ broom, and hairy root are all the result of some form of overgrowth.

Atrophy, hyperplasia or dwarfing: in many diseases one of the results is inhibition of growth in stunting or dwarfing. The whole plant may be dwarfed or only certain organs may be so affected. Sometimes hypertrophy and atrophy both are present in the same organ.

Water – soaking is water – soaked, translucent condition of tissues caused by water moving from host cells into intercellular spaces due to damage to cell walls by enzymes and toxins of the pathogen.

Necrosis: This term is used to indicate the condition in which the death of cells, tissues, or organs has occurred as a result of the parasitic activity.

The characteristic appearance of the dead area differs with different hosts and host organs and with different parasites so that there are different types of necrotic symptoms, spots, streaks, stripes, and canker blight, damping – off, burns, scald or scorch and rot are result of necrosis of tissues.

Anthracnose: This term is derived from Greek word meaning ulcer. Ulcer – like lesions, on twigs, stems, pods, and fruits constitute anthracnose disease caused by a specific group of fungi. Anthracnose is also a type of necrosis.

Die – back: Die – back is also result of necrosis of terminal tissues of twigs in which the twigs and branches start dying from the tip backwards.

Wilt: In many diseases the most striking effect of the disease is drying or wilting of the entire plant. The leaves and other green or succulent parts lose turgidity, become flaccid, and droop. This effect is usually seen first in some leaves.

Later, the young growing tip or the whole plant suddenly or gradually dries. Wilting may be the result of injury to the root system, to partial plugging of water conducting vessels or to toxic substances secreted by the pathogen and carried with water to delicate tissues.

Humidification: stage in certain fruit rots in which the dried, shriveled and wrinkled fruit is called a “mummy”. The stage is brought about by loss of moisture due to permeation of the flesh by fungus hyphae.

Miscellaneous Symptoms of Plant Diseases

Alteration in habitat and symmetry can occur under the influence of some pathogens. Plants which, under normal conditions, are prostrate or creeping become ascending or even erect.

Leaves become lobed from being simple. Inflorescence is changed from a head to a spike Premature dropping of leaves, blossoms, and fruits twigs occurs in many parasites. In smut the entire inflorescence or individuals flowers are completely destroyed.

Organs are transformed or replaced by new structures. The floral organs can be transformed into a mass of leafy structures. Ovaries may be transformed into sclerotia.

Diagnosis of Plant Disease

Rapid and accurate diagnosis of diseases is necessary before proper control measures can be suggested. It is the first step in the study of any disease. Diagnosis of plant disease is a field science and practice is the soundest method of identifying a disease in the field.

Many illustrated guides and charts are available which can help in tentative identification. It is easy to identify such disease as ruts, smuts, downy mildews and powdery mildews because the structures of the pathogen are prominently visible to the naked eye.

However, in many diseases for example, chlorosis or yellowing of leaves is a symptom which can be caused by a virus, fungus or a bacterium or even nutritional deficiency without association of a parasite.

Similarly, plants may wilt due to fungal infection or bacterial infection. For effective and economic management of any disease, accuracy in diagnosis is important because some diseases such as damping off may be caused by many fungi and same treatment may not work against all the fungi.

Accurate diagnosis requires systematic field observations (symptoms, structures of the pathogen if any pattern of occurrence) and some laboratory studies.

In those cases where symptoms are such that may be caused by a variety of living or non – living disease incidents, the first step is to determine whether the incitant is infectious or non – infectious.

This can be done by observing the pattern of development of the disease in the plant population and the possible spread of symptoms on other plants. If the disease is spreading in the plant population it is infectious. An infectious disease may be caused by fungi, bacteria, viruses, or nematodes.

These can be determined by visual observation of the affected parts for presence of fungal structures, bacterial exudates, or nematodes cysts or females and then by laboratory studies. If present on the host and examined under the microscope, the fungal structure may reveal presence of a particular fungus.

If the fungus is not a biotroph and can be cultured, isolation on an artificial medium and tests for Koch’s postulates can pinpoint the actual cause of the disease. Bacteria can also be detected in similar manner.

Examination of cut pieces of the affected part in water under microscope reveals streaming of bacterial cell masses in water. The bacteria can be isolated on suitable media and Koch’s postulates proved.

Nematodes, if present can be seen on the host or in the tissues examined under the microscope. By separating them from the host and multiplying them, under aseptic conditions, such as chlorosis.

However, nutritional deficiency is non – infectious, while virus disease symptoms spread in the plant population. If in artificial cultures no pathogen is obtained and in tissues examination, no fungal structure of a biotroph or structures of nematodes are seen but the disease is infectious, it can be expected that the disease is caused by a virus or mycoplasma – like organism (phytoplsma).

Transmission tests by grafting sap inoculation or use of insect vectors can help in final diagnosis. The differentiation between virus and MLO can be made by electron microscopy and by spraying tetracycline antibiotics which mask symptoms of MLO disease but not of virus disease.

Developments in microscopy, serology and immunology, molecular biology, and laboratory instrumentation have resulted in many new and sophistication laboratory procedures for the identification of plant pathogen, particularly bacteria, viruses and vitroids

General Control Methods of Plant Diseases

Crop diseases can be controlled using the cultural, biological and chemical methods.

Cultural method: The agronomic measures a farmer may adopt include the adoption of crop rotation, proper and timely land tillage, changes in the time of planting, destruction and burning of crops residues, regular and timely weeding, seed dressing before planting, use of diseases free seeds for planting, ;planting of resistant crop varieties and harvesting at the right time.

Biological control: This involves the use of natural enemies to control the diseases.

Chemical control: This is the most effective method of crop diseases control. Although certain side effects are associated with this method, it remains the most effective means of reducing plant pathogens.

The type of chemicals used depend on the pathogens responsible for the disease. As a result, the following are used to control plant diseases:

Fungicides are used for the control of fungal diseases e.g. copper, Bordeaux mixture, lime, captan etc.

Viral diseases are controlled with viricides. It should be noted that viral diseases are difficult to eradicate, but the vectors transmitting the diseases can be controlled using insecticides.

Bacterial diseases are controlled with antibiotics like cuprous oxide, Agrosan etc

Nematicides are used to control nematodes. Examples include Nemagon, Vapan D.D and Methyl bromide.

In summary, crop losses due to diseases cannot be overlooked, hence the need for a detailed study of the nature of plant diseases and their causative agents. This will help prevent any form of outbreak that will result in economic loss to the farmer.

A disease is a departure from normal state of health presenting marked symptoms or outward visible signs. Some effects of plant diseases include malformation of plants and sometimes death of the plant, low yield and reduction in crop quality.

Diseases symptoms include mildew, rust, smut, white blisters, exudation, scab, sclerotia, blotch, overgrowth or hypertrophy, atrophy, chlorosis (colour change), necrosis etc. crop diseases are caused by abiotic (nutrient deficiencies, insufficient light, too high temperature, low soil moisture, air pollution, high soil moisture and chemical toxicity), mesobiotic (viroids) and biotic factors ( bacteria, fungai, virus and nematodes).

Rapid and accurate diagnosis off diseases is necessary before proper control measures can be suggested. Diseases of crops are controlled through cultural, biological and chemical methods.