Disease Causing Organisms.

Disease Causing Organisms.

Definition: Disease causing organisms are known as organisms that cause diseases in both plants and animals. They are also known as pathogens.

Diseases can be recognized by signs which are also know as symptoms.

Factors That Can Cause Diseases

Disease can be caused by living organisms which are known as pathogen such as;

  1. Fungi
  2. Bacteria
  3. Virus
  4. Nematodes
  5. Protozoa

Disease can also be caused by non living organisms such as;

  1. Poor nutrition
  2. Inadequate space/ over crowding
  3. Inadequate water

Fungal Diseases

Features/description of fungi as pathogens

  1. They are small plants
  2. They have no leaves, flowers and chlorophyll.
  3. They reproduce by means of a structure called SPORES.
  4. They are parasitic (they survive on other organisms) or saprophytic( they feed on organic matter)
  5. They obtain ready made food from the host on which they grow.
  6. They can be controlled chemically by fungicides.

         Mode Of Transmission Of Fungi

  1. By wind (air)
  2. By rain splash
  3. By insects
  4. By contaminated seed.

       Examples Of Fungal Diseases Of Animals.

  1. Ringworm
  2. Aspergillosis

     Examples Of Fungal Diseases Of Plants

  1. Black pod disease of cocoa
  2. Maize smut
  3. Rice blight
  4. Damping-off of tomato, okro, cowpea etc
  5. Onion twister
  6. Tikka disease of groundnut

             VIRAL DISEASES

Feature/ description of virus as a Pathogen;

  1. They are very small organisms.
  2. They can only be seen through electron-microscope.
  3. They can also live outside the body the living cell.
  4. They cannot be controlled by chemicals.

         Mode Of Transmission Of Viral Diseases

  1. By insect vectors (i.e piercing and sucking insects such as aphids)
  2. By nematodes
  3. By infected plant material e.g stem cutting
  4. By infected tools.

       Examples Of Viral Disease Of Animals

  1. Foot and mouth disease.
  2. Rinder- pest disease
  3. Newcastle disease.

       Examples of Viral Disease Of Plants

  1. Mosaic disease of cassava/ cassava mosaic disease
  2. Cocoa swollen shoot disease.
  3. Rosette disease of groundnut.
  4. Tristeza of orange.
  5. Leaf curl of cotton.


Description/features of bacteria as a pathogen.

  1. They are very small organisms.
  2. They can only be seen with a microscope.
  3. They are parasitic.
  4. They can also be saprophytic.

       Mode Of Transmission

  1. By wind
  2. By insects
  3. By rain splash.

Examples Of Bacterial Diseases Of Animals

  1. Anthrax
  2. Brucellosis
  3. Tuberculosis.

         Examples of Bacterial Diseases of Plants

  1. Yam rot
  2. Leaf blight of cassava
  3. Leaf spot of cotton
  4. Bacterial wilt of tomato
  5. Galadima disease of oil palm.


Description/Features of nematode as a pathogen.

  1. They are found in the soil.
  2. They are microscopic worms
  3. The have long, cylindrical and elongated bodies.
  4. They affect only plants.
  5. They affect the root and tubers of crops.
  6. They cause root knot or galls.
  7. They may also be parasitic or saprophytic.

                Mode Of Transmission

  1. By rain splash
  2. By soil movement during the the cultivation such as ridging in contaminated soil.

      Examples of Nematodal Diseases

  1. Root knot of tomato, cowpea etc.
  2. Die-back disease of citrus.
  3. Yam dry rot.

                 PROTOZOAN DISEASES

Description/ features of protozoan Diseases.

  1. They affect only animals.
  2. They are transmitted by insect vectors e.g blue tick, tse-tse fly etc.
  3. They can be controlled by control of its vectors and use of drugs.

             Examples of Protozoan Diseases

  1. Trypanosmiais (sleeping sickness)
  2. Coccidiosis
  3. Red water fever.


  1. What are pathogens
  2. Name any two groups of pathogens you know.
  3. Give two examples of living organisms that are pathogens of crops.
  4. Give 3 examples of non living organisms that cause diseases.