Causes of plant diseases

Describe about different agents in causing the plant diseases with simple example so that it will be easy to understand for under graduate students especially

1. Causes of p lant diseases N . H. SHANKAR REDDY 1 st , 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 ) The organism must always be present, in every case (lesion) of the diseases. The organism must be isolated from a host containing the diseases and grown in pure culture. The culture must be able to create characteristic symptom of the same disease, when it is inoculate into healthy, susceptible plants in the laboratory 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 . T he 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.

6. Plant pathogenic organisms Protozoa Chromista Fungi Bacteria Virus Viroids Phytoplasma Spiroplasma Nematodes Phanerogamic plant parasites

7. Protozo a E ukaryotes , either free-living or parasitic plant pathogens which causes diseases in plants Eg – Club root of cabbage – Plasmodiophora brassicae

8. Chromista Oomycota Group of plant pathogens which cause disease in plants Eg - Late blight of potato ( Phytophthora infestans ),

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 closed, circular, 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, motile prokaryotic organism bounded by a triple layer 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 1 st 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