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Friday, January 04, 2013

Introduction and History of Plant Pathology

Posted by Shakil Shaukat On Friday, January 04, 2013

Plant Pathology is a practical discipline which focuses on the study of the diseases of plants, disease causing organisms, the mechanisms by which they cause disease and ways to manage or control these diseases and minimize the damage caused to plants.
Every year around 40 percent of world food is lost due to disease, post-harvest losses, insects and weeds. And out of these around 14.1 percent losses are due to plant diseases. This ratio may seem small but the losses account about 220 billion US dollars annually (according to a 2003 survey).
Root galls due to nematode infection
Plant Disease is defined as the deviation from normal physiological functioning of the plants. Plants perform poorly in term of both growth and production when diseased. Symptoms are the visible manifestations of the disease. Diseases are caused in plant due to a number of living organisms and environmental factors.
We will discuss about the types of diseases and disease causing organism later, here will try to summarize the history of plant pathology.

History of Plant Pathology

In the prehistoric times, man lived in caves, collected seeds, leaves and fruit to consume and hunted the animals for meat. Thus plant disease was seen as wilts and blights to the plant which meant that there will be little or no food and herbivorous animals were also reduced. As the man began to settle, he selected fewer plants to be grown and cultivated in his own lands. In those days, disease on plants came either due to organisms or environmental factors and destroyed a part or all of the produce.
Hence the citing of such conditions of disease on plants can be obtained from old books and manuscripts available and were feared as much as human diseases or wars. In older civilizations, plant disease was considered as the “wrath of god” and the people used to celebrate festival, sacrifice and pray for keeping their crops healthy and free of disease.

Timeline of Major developments in Plant Pathology

The important events in the development of plant pathology as a major science are being highlighted as under;
1000 BC – Sulfur was reported to control plant diseases.
470 BC – Democritus recommended using olive ground left after extraction of olive oil for controlling blights on plants.
1200 AD – Mistletoe (a higher plant) established as a ‘parasite’ of plants, causing sickness to its host. Hence is control came along this observation, the branches having mistletoe were pruned. It became first known plant pathogen.
Mid 1600s AD – French farmers noticed that rust was more common in fields where barberry plant was near the wheat fields.
1667 – Spores of rust fungi were observed by Hooke.
1670 – French physician Thoullier observed that ergotism (a serious disease) of humans was caused due to consumption of grains of rye which had ergot.
1676 – Antone Van Leeuwenhoek reported the discovery of micro-organisms.
1683 – Bacteria were first visualized by Leeuwenhoek.
After the discovery of microscope and improvement of structure of microscope; many improvements were made in structure of microscope by Leeuwenhoek who discovered many microorganisms (fungi, bacteria, protozoa and algae.
1735 – Carl Von Linne published his work in “Systema Nature” in which he described diagnosis of plants and nomenclature of plants.
1729 – Italian botanist Pier Antonio Micheli described many new genera of fungi and illustrated their reproductive structures. He used melon as medium to grow fungi.
1743 – Needham (father of nematology) observed nematodes for the first time in abnormally round wheat kernels.
1755 – Tillet worked on smut spores but failed to conclude the causal organism instead he reported that some poisonous agent is causing smut of wheat and smut is the reaction and not the cause of disease.
1807 – Prevost, worked again on smut of wheat and discovered the causal organisms and reported that smut spores are the cause of diseae.
1845 – Irish potato famine due to late blight disease of potatoes.
1855 – Another nematode was discovered from the galls of cucumber roots. In the next 4 years two other plant parasitic nematodes, the bulb and stem nematode and the sugarbeet cyst nematode, were reported from infected plant parts.
1861-1863 – deBary proved that late blight of potato is caused by fungi. Louis Pasteur proved that new microorganisms are produced only by preexisting microorganisms. And most of the infectious diseases are caused by germs and gave rise to ‘germ theory of disease’. This theory set a new path for working on microorganisms.
In the same era, Robert Petri discovered artificial media for growing microorganisms which is used in Petri plates.
1859 – Charles Darwin published his book on the evolution of species titled ‘The Origin of Species by Means of Natural Selection’ and argued that species have evolved in through process of natural selection.
1870s - Kühn also wrote the first book on plant pathology, “Diseases of Cultivated Crops, Their Causes and Their Control” in which he recognized that plant diseases are caused by an unfavorable environment but can also be caused by parasitic organisms such as insects, fungi, and parasitic plants. He also worked on smut of wheat and many other diseases himself.
1878 – Burril proved that fire blight disease of pear and apple is caused by bacterial pathogen. It was the first time bacteria were shown to cause plant diseases.
1886 – Mayer wrongly concluded that tobacco mosaic disease is caused by some bacteria because he could not isolate fungi from infected plants.
1887 – Robert Koch gave his theory about working on infectious diseases aka ‘Koch’s postulates’.
Early 1890s – Smith showed that crown gall disease of plants was caused by bacteria. The discovery of mechanism of infection of this bacterium Agrobacterium tumefaciens led to evolution in the field of genetic engineering of plants.
1892 – Ivanowsky concluded that tobacco mosaic was caused either by some toxin or some unusually small bacteria which could pass through filters.
1898 – Beijerinck finally concluded that tobacco mosaic was caused by some ‘contagious living fluid’ and he named it as virus.
1909 – Lafont observed flagellate trypanosomatid protozoa in the latex-bearing cells of laticiferous plants of the family Euphorbiaceae.
1935 – Stanely crystallized the tobacco mosaic virus and wrongly concluded that it is an autocatalytic protein.
1936 – Bawden and colleagues discovered that the crystals of tobacco mosaic virus contained both proteins and RNA.
1939 – Kausche and colleagues for the first time obtain electron micrographs of tobacco mosaic virus particles.
1931 – Stahel found flagellates infecting the phloem of coffee trees, causing abnormal phloem formation and wilting of the trees.
1963 – Vermeulen presented convincing evidence of the pathogenicity of flagellates to coffee trees.
1976 – Flagellates were reported to be associated with several diseases of coconut and oil palm trees in South America and in Africa.
1967 – Doi and colleagues in Japan observed mollicutes, i.e., wall-less mycoplasma-like bodies in the phloem of plants exhibiting yellows and witches’ broom symptoms. That same year the same group showed that the mycoplasma-like bodies and symptoms disappeared temporarily when the plants were treated with tetracycline antibiotics. Since then, mycoplasma-like organisms (MLOs) that infect plants have been reclassified as phytoplasmas, and some of them that have helical bodies and can be found in other environments besides plants are known as spiroplasmas.
1971 – Potato spindle tuber was discovered to be caused by naked, small, single-stranded circular molecule of RNA which was named viroid later on.