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).
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| 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.
