Plant Diseases and Defences

This lesson covers:

  1. How diseases are transmitted between plants 
  2. Examples of plant diseases 
  3. How plants defend themselves against pathogens 

Modes of transmission

Like in animals, communicable diseases can be passed from one plant to another.


This can occur via direct or indirect transmission:

  1. Direct transmission - This involves the direct contact of a healthy plant with any part of an infected plant.
  2. Indirect transmission - This can take place via soil contamination when infected plants leave pathogens or spores in the soil ready to infect other plants. It also takes place using vectors. 


Examples of vectors include: 

  • Wind - Bacterial, viral, or fungal spores are carried by the wind to uninfected plants.
  • Water - Spores can travel on the surface of water to reach uninfected plants.
  • Animals - Insects and birds can carry pathogens or spores from one plant to another.
  • Humans - Pathogens and spores can be transmitted by handling plants, clothing, farming tools and practices.

The risk of communicable disease is increased by certain factors

  • Crop variety - Some crops are more susceptible to disease than others. 
  • Overcrowding - This increases the likelihood of direct contact. 
  • Mineral nutrition - Poor nutrition reduces resistance of plants.
  • Climate change - Increased rainfall and wind increase the spread of disease. 

Examples of plant diseases

You need to be able to describe the pathogen and effect of the following plant diseases:

Table showing examples of plant diseases including ring rot, tobacco mosaic virus, black sigatoka, and potato tomato blight. It lists the pathogens, plants affected, and effects on the plants.

Plant defences

Plants have a number of physical and chemical defences mechanisms to protect themselves from pathogens.  


Physical defences include: 

  • Waxy cuticles - Leaves and stems are covered in a waxy cuticle which provides a physical barrier against pathogens. 
  • Cell walls - Plant cells are surrounded by cell walls that form a physical barrier against pathogens. 
  • Production of callose - When plants are attacked by pathogens they produce a polysaccharide called callose. This is deposited between the cell wall and the cell-surface membrane to make it harder for pathogens to enter cells.


Chemical defences involve the production of chemicals including: 

  • Insect repellents - These reduce the number of insects feeding on plants to prevent them from transmitting pathogens.
  • Insecticides - These kill insects to prevent them from transmitting pathogens. 
  • Antibacterial substances - Chemicals such as antibiotics are produced to kill bacteria or inhibit their growth.
  • Toxins - Some plants produce chemicals that break down into cyanide (a toxic chemical) when the plant cells are attacked.