Notes on Important Plant Pathogenic Organisms

Notes on Important Plant Pathogenic Organisms

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Source: TNAU Notes

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Plant Pathogenic Organisms:

Parasites:

• Include both biotic and mesobiotic agents.
• Diseases incited by parasites under suitable environment.
• Association of definite pathogen is essential with each disease.

Biotic Agents:

• Also known as animate causes.
• Living organisms.
• Include Prokaryotes and Eukaryotes.

Prokaryotes:

• True bacteria or bacteria: Facultative parasites, e.g., Citrus canker.
• Rickettsia-like bacteria (RLB): e.g., Citrus greening, Pierce's disease of grape.
• Mollicutes or wall-less prokaryotes.
• Mycoplasma-like organism (MLO): e.g., Sesame phyllody, eggplant little leaf.
• Spiroplasma: e.g., Corn stunt, Citrus stubborn.

Eukaryotes:

• Protists: Unicellular, coenocytic, or multicellular, little/no cell differentiation.
• Fungi: e.g., wilt of cotton.
• Protozoa: e.g., heart rot of coconut.
• Algae: e.g., red rust of mango.
• Parasitic flowering plants (phanerogamic parasites): e.g., Broomrape of tobacco.
• Animals: Extensive cell differentiation, e.g., Nematodes - Root knot nematode.

Mesobiotic Agents:

• Include viruses and viroids.
• Infectious agents; can be crystallized.
• Considered non-living but multiply in living plants.
• Viruses: e.g., yellow mosaic of blackgram.
• Viroids: e.g., spindle tuber of potato.

Non-parasites or Abiotic Agents:

• Also known as non-infectious or physiological disorders.
• Caused by non-living or environmental factors.
• Occur due to disturbances in plant system by improper environmental conditions.

Examples of Abiotic Agents:

• Too low or too high temperature.
• Lack or excess of soil moisture.
• Lack or excess of light.
• Lack of oxygen.
• Air pollution (Toxic gases, etc.).
• Mineral deficiencies or toxicities.
• Soil acidity or alkalinity.
• Toxicity of pesticides.
• Improper agricultural practices.

Important Plant Pathogenic Organisms:

Parasites:

• Parasites are both biotic and mesobiotic agents.
• Diseases are incited by parasites under suitable environments.
• Each disease requires a definite pathogen association.

Biotic Agents:

• Also known as animate causes.
• These are living organisms.

Prokaryotes:

• True bacteria or bacteria: Examples include Citrus canker.
• Rickettsia-like bacteria (RLB): Examples include Citrus greening, Pierce's disease of grape.
• Mollicutes or wall-less prokaryotes.
• Mycoplasma-like organism (MLO): Examples include Sesame phyllody, eggplant little leaf.
• Spiroplasma: Examples include Corn stunt, Citrus stubborn.

Eukaryotes:

• Protists: These are unicellular, coenocytic, or multicellular with little or no cell differentiation.
• Fungi: An example is the wilt of cotton.
• Protozoa: An example is heart rot of coconut.
• Algae: An example is red rust of mango.
• Parasitic flowering plants (phanerogamic parasites): An example is Broomrape of tobacco.
• Animals: These have extensive cell differentiation, e.g., Nematodes - Root knot nematode.

Mesobiotic Agents:

• Include viruses and viroids.
• Infectious agents that can be crystallized.
• Multiply in living plants but considered non-living.
• Viruses: An example is yellow mosaic of blackgram.
• Viroids: An example is spindle tuber of potato.

Non-parasites or Abiotic Agents:

• Also known as non-infectious or physiological disorders.
• Caused by environmental factors when no pathogen is found.
• Result from disturbances in the plant system due to improper environmental conditions.

Examples of Abiotic Agents:

• Extreme temperatures (too low or high).
• Imbalanced soil moisture (lack or excess).
• Inadequate or excessive light.
• Oxygen deficiency.
• Air pollution including toxic gases.
• Mineral imbalances (deficiencies or toxicities).
• Soil pH extremes (acidity or alkalinity).
• Pesticide toxicity.
• Improper agricultural practices.

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