A collection of high quality essays on ‘Plant Pathology’.
Essays on Plant Pathology
Essay # 1. Introduction to Plant Pathology:
The word “pathology” has been derived from two Greek words, pathos = suffering and logos = discourse or to speak. Plant pathology (also called “Phytopathology”), represents the area of biological sciences which concerns with the sufferings of plants (or diseases of plants). Plant pathology deals with the cause, etiology, resulting losses and prevention and management (control) of the diseases of plants.
The objectives of the plant pathology are the study on:
(i) The living entities that function as causal agents of diseases in plants,
(ii) The non-living entities and the conditions of the environment that cause disorders in plants,
(iii) The mechanisms by which the causal agents produce diseases,
(iv) The interactions between the causal agents of the disease and host plant in relation to overall environment, and
(v) The devices used to prevent or manage (control) the diseases to reduce the losses/damages caused by diseases.
Plant pathology is a biological science inter-disciplinary in nature. It flourishes with the help of the basic knowledge and techniques of different branches of science. Knowledge of the basic physical and biological sciences as well as comprehension of the environmental and social sciences is the foundation stones upon which the science of plant pathology rests. It is, therefore, indispensable for a plant pathologist to have atleast knowledge of basic facts of the disciplines of science.
Plant pathology is perhaps the greatest single contribution of plant diseases which have indeed played an important role in the social development of the world since a very long time. The birth of plant pathology was, in fact, a blessed event. Exactly when plant pathology was born is a moot question; why it was born is no question at all. It was born of necessity. Society needed agriculture and agriculture needed plant pathology. The latter has its genesis in fields and granaries more than in halls of ivy.
Many plant pathologists proclaim that plant pathology is a discipline which encompasses both “art” and “science”. Art is “doing” and science is “understanding”. An artist paints beautiful pictures with pigments and oils. This is doing. He does not usually worry about the chemistry and physics of their action.
A scientist aims to understand the constituents in the oils, their chemistry, the degree of unsaturation in the molecules, and so on. He does not paint the pictures that the artist does. A farmer who first diagnoses a disease on the basis of its symptoms and then treats crop plants to fight the disease, in fact, practices the “art” of plant pathology.
But, a plant pathologist who keeps going with understanding the sick plant and finding out the devices to treat it, practices the “science” of plant pathology. The aspect of art of plant pathology is not new. It has been in practice from the very beginning of civilization as many diseases have been recognized and “unscientifically” treated by man since.
But, the foundation of the scientific aspect of plant pathology though laid in the early part of the 19th century, has struggled strenuously to attain its stature and can be said “young” because it is only within the beginning of the twentieth century or so that plant pathology came to be recognized as a distinct science.
Plant pathology is an applied science concerned primarily with practical solutions to problems of disease in agriculture, horticulture, and forestry. The study of plant disease, and the development of devices for its management, still is the vital elements in crop productivity improvement drives mainly because of continuous explosion in world population growth.
In recent years, however, experimental analysis of the interactions between plants and pathogens has become fertile territory for plant pathologists interested in fundamental aspects of plant recognition and response systems, signal pathways, and stress physiology.
Molecular genetic techniques usage has opened new insights into the way the pathogens cause disease, and the way the plants defend themselves against pathogen attack. In turn, this new understanding is coming forward with novel mechanisms that can help managing plant diseases effectively in days to come.
However, the ultimate aim of plant pathology is to increase our knowledge of the causes and the development of plant diseases, to develop control devices for all plant diseases so that the produce that today is destroyed by plant diseases could be saved and made available to the hungry and ill-clothed millions of our increasingly overpopulated world.
Essay # 2. History of Plant Pathology:
Plant pathology is supposed to have had its beginnings at the dawn of civilization. One can find mention of diseases of plants and their control devices in ancient literature. Unfortunately, most of the books on plant pathology come from the western world and they emphasize the knowledge generated and accumulated in western countries. Since these books completely overlook Indian contributions, particularly made by ancient Indians, the reader is forced to believe reading them that the man’s attention to plant diseases and their controls was drawn first only in countries of western world.
For convenience, books from western countries emphasize that it was Theophrastus (370-286 BC), a Greek philosopher, who first talked about plant diseases. Historically, plant pathology in India is quite ancient as the Indian agriculture, which is nearly 4000 years old. This confirms that the mention of plant diseases was made in our own country much before the time of Theophrastus. There are references to plant diseases and their control devices in Rigveda. Atharva Veda (1500-500 BC), Arthashashtra of Kautilya (321-186 BC), Sushruta Samhita (200-500 AD), Agni Purana (500-700 AD) etc., the holy books written in ancient India.
However, Vraksha Ayurveda, written by Surapal in ancient India, is most probably the first treatise in which he gave detail account on plant diseases and their control. The plant diseases were divided into two groups, internal and external, in Surapal’s book. Internal diseases were thought to be due to disorders in the system of the plant while external diseases were supposed to be due to attack by living agents.
These diseases were controlled employing a number of treatments. Hygiene, tree surgery, protective covering with pastes, and special culture of plants were the control practices which are still recommended in many diseases. Honey, ghee, milk, barley flour, pastes made from herbs, and so on was the chemical devices of control in those days. Symptoms of plant diseases are sited in other ancient Indian literatures viz. Jataka of Budhism, Reghuvamsha of Kalidas etc.
However, almost all the knowledge in connection with plant diseases and their control developed in ancient times was based on speculations and observations rather than scientific experiments. To one’s surprise, unfortunately, little was added to the knowledge of plant pathology of ancient times in the next thousand years (almost 2000 years after the time of Theophrastus) inspite of the fact that plant diseases continued to harm the crops.
It was the discovery of the microbial world by Antony van Leeuwenhoek around the middle of the 17th century that opened a new era in the area of life sciences. Antony van Leeuwenhoek developed his own microscope and first discovered microorganisms (bacteria, etc.) in 1675 with its aid.
Fungi as Cause of Diseases:
The Italian botanist P.A. Micheli (1679-1737) was the scientist who first described many new genera of fungi and illustrated their reproductive structures in his book ‘Nova Plantarum Genera’, the first authentic literature on fungi published in 1729. Micheli, considered as the founder of the science of mycology, placed the spores of fungi on pieces of melon, quince and pears, and found the development of mycelium together with sporangia and spores characteristic to the parent fungus. He thus proved that fungi arose from their own spores. Although this was a successful experiment, it did not attract the favour of most of the scientists of that time who believed that life originated spontaneously.
Tillet, a French botanist, published a paper on stinking smut (bunt) disease of wheat in 1755. He described in his paper that when he added the “black dust” taken from bunted wheat seeds (he did not know that “black dust” represented spores of the fungus) to healthy wheat seeds, the bunt appeared in plants produced from such seeds than from non-dusted ones. Tillet thus showed that wheat smut (bunt) is an infectious (contagious) disease. He, however, believed that it was a poisonous substance contained in the smut dust, rather than living microorganisms, that caused the disease.
Tessier (1783), a prominent French agriculturist, repeated some of Tillet’s experiments on bunt of wheat and conducted others of his own. He also studied other diseases of cereals and confirmed Tillet’s results on the contagious nature of bunt.
Germ Theory of Diseases:
The foundation of the “science” of plant pathology was laid in the early part of the 19th century when Benedict Prevost (1807), a French Scientist working on wheat, first demonstrated the germ theory of disease; life arises from pre-existing life only. He studied the spores, their production and germination and proved conclusively that the bunt disease is caused by a fungus which penetrated the young wheat plant. Prevost’s experiment provided the first proof and interpretation of the role of a microorganism in the causation of disease, that is, he clearly established the principle of pathogenesis.
Prevost controlled the disease by dipping the seeds in a copper sulphate solution. However, Prevost’s findings were ahead of his time and his contemporaries did not accept his views until 40 years later when the devastating epiphytotic (epidemic) of potato blight attracted the attention of workers to plant diseases.
L. Pasteur (1860) strongly favoured the germ theory given by Prevost. He ‘disproved’ the Aristotelean belief in “spontaneous generation” by publishing evidence that microorganisms arise from pre-existing ones and gave the contention “Life begets life”. He also proved that fermentation’ is a biological phenomenon rather than a purely chemical one.
Robert Koch (1876-1882) working on bacterial diseases postulated certain fundamental laws known as Koch’s postulates. These laws still hold good in testing the pathogenic nature of a microorganism. In addition, he discovered the causal organisms of anthrax, tuberculosis and cholera diseases, and established “germ theory”.
Essay # 3. The Beginning of Modern Plant Pathology:
A mysterious and terrible potato murrain appeared in Western Europe in the mid of nineteenth century. In 1845, this potato murrain, or late blight, as it is known today, destroyed the Irish potato crop to the extent that one million people died of starvation and malnutrition and a similar number of people emigrated from Ireland to the United States. This epiphytotic (epidemic) of late blight of potato tragically dramatized the importance of plant diseases and greatly stimulated the interest of scientists to explore their causes.
Several investigators, described various aspects of the disease and of the pathogen, but it was Heinrich Anton DeBary who finally proved experimentally in 1857 that the causal agent of late blight of potato has been a fungus, namely, Phytophthora infestans. DeBary (1831-1888) was an outstanding biologist of his time who made many new and important discoveries, and is rightly considered as the “Father of Experimental (Modern) Plant Pathology”. Heteroecism in Uredinales was made clear by his experiments and he gave account of the development and sex in a number of fungi. He was the first to indicate the nature of obligate and facultative forms of life.
J.G. Kuhn (1825-1910) contributed significantly to the studies of infection and development of smut in wheat plants, and promoted the development and application of control measures, particularly seed treatment for cereals. He published numerous papers on diseases of cereals, vegetables, and other plants, and also wrote his famous text-book in 1858 entitled, “The Diseases of Cultivated Crops – Their Causes and Their Control” which is considered the first textbook of plant pathology dealing with the climatic and soil conditions, insects, parasitic higher plants, and microorganisms as causes of plant diseases.
Forest Pathology:
Robert Hartig (1839-1901) contributed greatly from 1866-1878 to forest pathology and was the author of important text-books, namely Wichtige Krankheiten der Waldaume in 1874 and Lehrbuch der Baumkrankheiten in 1882 on diseases of forest trees and is aptly called the “Father of Forest Pathology”.
Phytopathogenic Nature of Bacteria:
T.J. Burrill of the University of Illinois, USA for the first time in 1878 showed that fire blight disease of pear and apple was caused by a bacterium (now known as Erwinia amylovora).
In fact, it was the discovery of phytopathogenic nature of bacteria. Shortly thereafter, several other plant diseases were known to be caused by bacteria. Phytopathogenic nature of bacteria was established beyond any doubt with numerous and excellent contributions on the study of bacterial diseases of plants by E.F. Smith from 1895 onward. Smith is considered the father of phytobacteriology for his discoveries and methodologies he introduced for study of bacterial plant diseases during 1905-1920.
Discovery of Viruses:
The tobacco crop in Holland was struck by a severe disease around 1870. Adolf Mayer, Director of Agricultural Experimental Station, Wageningen, began his studies on this disease in about 1880 and published his results in 1886. Mayer christened the disease as “Mosaikkrankheit”, mosaic-like, from the mosaic-like pattern on leaves of diseased plants and succeeded in reproducing the disease by infecting juice extracted from infected tobacco plants into healthy ones but he could not succeed in identifying the real agent that caused the disease.
However, Mayer’s contribution will always be remembered as he was the person who put first step forward in the development of a new discipline of plant pathology which, later on, came to the recognized as “Virology”. Similarly, Smith (1891) demonstrated that the peach yellows disease was contagious but he also could not succeed in determining its real cause.
It was Ivanowski (1892) who first successfully experimentally demonstrated that the tobacco mosaic disease has been caused by agents which successfully passed the Chamberland filter that retains even the smallest bacteria. It was an important clue but, contrary to his experimental results and despite his inability to isolate any bacterium, Ivanowski still maintained that either the pathogenic bacterium somehow passed through the filter or a toxin secreted by it passed through the filter and made filtrate infectious.
Beijerinck (1898) tested Ivonowski’s hypothesis and concluded that the tobacco mosaic disease was caused not by any pathogenic bacterium or toxin secreted by it but by “contagium vivum fluidum” (i.e., living infectious fluid) which he referred to a “virus” (i.e., poison) and said that the virus multiplies on in vivo. For this contribution, Beijerinck is honoured as the Father of Virology.
Fungal Variability:
The important phenomenon of variability among fungal pathogens was first discovered by Erikson in 1894. He found the existence of physiologic races in the rust fungus and held them responsible for availability of short-lived resistance among many of the crop varieties.
Plant pathology well matured as a science only during 20th century. Many hundreds of plant pathogens have been successfully identified, various diseases described at length and considerable number of control measures have been explored. The study of genetics and of the physiology of plant diseases has expanded greatly, and new chemical compounds have been developed to control diseases.
Simultaneously, plant pathologists of this century successfully developed many disease resistant varieties of plants and studied the nature of host-parasite-interactions. However, some of the most outstanding contributions in the field of plant pathology during twentieth century are being given in chronological form as under.
Essay # 4. Chemical Treatment of Plant Diseases:
i. Use of Sulphur:
Sulphur has been in use to treat plant diseases since ancient times. The first recorded mention of use of sulphur in plant disease control is in the writings of Greek poet, Homer (1000 B.C.). Also the Roman Patriot, Cato (200 B.C.) mentioned the fumigation of trees with sulphur.
In 1803, Forsyth came up with a tobacco-sulphur-quicklime-elder bud spray for application to fruit as a protective measure. In 1821, sulphur-soap spray was recommended for use against peach mildew and in 1848, Duchartre recommended sulphur as the remedy for vine powdery mildew (Uncinula necator), which had caused heavy losses in European vineyards.
ii. Discovery of Bordeaux Mixture:
Very little advance was made in the treatment of plant diseases upto the end of 1880’s when a fungicide, namely, Bouillie Bordeaux or Bordeaux Mixture was discovered by Professor P.M.A. Millardet of University of Bordeaux (France).
This fungicide was discovered to prevent the downy mildew of grapes (caused by Plasmopara viticola) which endangered grape cultivation in Europe and threatened the wine industry in France. Millardet noticed the grape vines on the roadside at Medoc in Gironde (France) escaped the disease while those away from it were heavily diseased. It was because the farmers had protected the plants by sprinkling verdigris, a mixture of copper sulphate and lime, to discourage pilfering by the school going children.
This clue prompted Millardet to proceed ahead and, as a result, he performed extensive spraying experiments with many preparations of copper, iron, and calcium salts used singly and in various combinations. Millardet Finally concluded his work and published it in May, 1885, disclosing that a mixture of copper sulphate and hydrated lime could effectively control the downy mildew of grape. This mixture became popular as ‘Bordeaux Mixture’. The latter is the most widely used fungicide all over the world even today to effectively control a variety of plant disease.
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