1. Causes of plant diseases
N. H. SHANKAR REDDY
1st , Ph.D Plant Pathology
Annamalai University
2. Koch postulates
Koch postulates are considered as the fundamental aspects of
plant pathology for further studies with more authenticity and
confidence.
The Four Koch postulates are (Otherwise called as
pathogenicity rules)
1. The organism must always be present, in every case (lesion)
of the diseases.
2. The organism must be isolated from a host containing the
diseases and grown in pure culture.
3. The culture must be able to create characteristic symptom
of the same disease, when it is inoculate into healthy,
susceptible plants in the laboratory
4. The pathogen must be re-isolated and on comparison with
original culture it must be found to be identical)
3. Causes of plant diseases
Biotic agents
• Plant diseases are caused by a number of organisms which ultimately
lead to the loss in crop yield both quantitatively and qualitatively.
• The major group of plant pathogens that infect plants are
(i)Fungi
(ii)Bacteria
(iii)Virus
(iv) Protozoa
(v)Phytoplasma
(vi)Spiroplasma
(vii)Algae
(viii)Nematodes
(ix) Phanerogamic higher plants
4. In animate or abiotic disease
• In true sense these factors cause damages (any reduction in the
quality or quantity of yield or loss of revenue) to the plants rather
than causing disease. The causes are:
(i) Deficiencies or excess of nutrients
(ii)Light
(iii)Moisture
(iv) Temperature
(v)Air pollutants
(vi)Lack of oxygen
(vii) Toxicity of pesticides
5. Losses due to plant diseases/ Importance of
the Plant Diseases:
• Globally, enormous losses of the crops are caused by the plant
diseases. The loss can occur from the time of seed sowing in the field
to harvesting and storage.
• Important historical evidences of plant disease epidemics are Irish
Famine due to late blight of potato (Ireland, 1845), Bengal famine
due to brown spot of rice (India, 1942-43) and Coffee rust (Sri Lanka,
1967) etc. Such epidemics had left their effect on the economy of the
affected countries.
7. Protozoa
• Eukaryotes, either free-living or parasitic plant pathogens which
causes diseases in plants
• Eg – Club root of cabbage – Plasmodiophora brassicae
9. Fungi
• Fungi - Fungi are eukaryotic, spore bearing,
achlrophyllous, heterokaryotic and thallophytic
organims that generally reproduce by asexually
and sexually. Fungi having primarly chitin and
little glucans in their cell wall except Oomycetes,
contain cellulose.
• Study of fungi and fungi like organisms is called –
Mycology or Fungology
• Eg –Wheat stem rust (Puccinia graminis tritici)
10. Bacteria
• Bacteria are prokaryotic, unicellular,
microscopic, achlorophyllous microorganisms in
which whole body is surrounded by cell wall,
reproduced chiefly by binary fission.
• (Prokaryotic: - Contains a primitive type of
nucleus and lacking of well-defined membrane
like fungi, except blue green algae.
• Morphologically the bacteria are rod shaped
(bacilli), spherical (cocci), spiral (spirilli), comma
shaped (vibrios) or thread like (filamentous).
• Study of bacteria is called “Bacteriology”
• Eg – Fire blight of apple (Erwinia amylovora)
11. Virus
• Plant viruses are sub-microscopic, infectious,
obligate intracellular parasites, which do
not replicate without a living host.
• ‘A virus is a virus’, it is neither a living organism nor
a non-living chemical, but something between and
betwixt
• Study of viruses is called “ Plant virology”
• Eg – Tobacco mosaic virus (TMV)
12. Viroids
• Viroids are small, covalently
circular,
closed,
single standard RNA (ssRNA)
molecule without protein coat (nucleic acid
devoid of protein coat). That is
approximately ten times smaller than that
of DNA and RNA of viral genomes
• (All viroid’s contains RNA as genetic
material, no other)
13. Virusoid
• Virusoids are circular single-stranded RNAs dependent on
plant viruses for replication and encapsidation
14. Phytoplasma and spiroplsma
• Phytoplasma
Phytoplasma are wall
less (no cell wall), bilayered,
round to elongate, prokaryotic
organism, non-motile,
pleomorphic in shape (usually
spheroidal to ovoid or tubular to
filamentous). Instead of cell
wall ithaving lipoprotein
membrane of 10 nm thickness.
• They belonging to the
order Acholeplasmatales,
and genus Phytoplasma, is
at Candidatus stage
Eg – Little leaf of brinjal
Spiroplasma
Spiroplasma are cell wall less,
helical,
bounded
motile prokaryotic
by a triple layer
organism
membrane,
movement is through rapid rotary
motion.
They belong to the order
Entomoplasmatales, genus Spoiroplasma
Eg – Corn stunt
15. Nematodes
• The typical nematode shape is a long and
slender worm-like animals which causes
diseases in plants and produce galls and
tumours in pants
• Root knot nematode – Meloidogyne
1st nematode discovered is – Anguina tritici
(by Needham)
16. Phanerogamic plant parasites
• Phanerogamic plant parasites depend upon
another vascular plant for food and water.
• Eg- Cuscuta, Loranthus, Orobanche, Striga
