Plant Disease

• Tobacco mosaic virus (TMV)


  A very widespread plant infection that affects a number of different plant species including tomatoes
• Rose black spot


  A fungal plant disease
• Treating rose black spot

  1. Spray plants with fungicides to kill the fungus

  2. Remove and destroy infected leaves
• You will find plenty of questions on infectious plant diseases in the revision workbook
• We can diagnose diseases in plants by looking for insects such as aphids, discoloration spots on leaves, stunted growth, decay or abnormal growths, and malformed stems or leaves
• Ways to identify plant diseases

  1. Use a garden manual or website

  2. Take the infected plant to a lab to identify the pathogen

  3. Use testing kits containing monoclonal antibodies
• Plant ion deficiency diseases

  Plant diseases not caused by pathogens
• A lack of the nitrate ion causes stunted growth because nitrate is needed for protein synthesis and therefore growth
• A lack of the magnesium ion causes chlorosis because magnesium is required to make chlorophyll, causing the leaves to lose their green coloration
• Plant defense systems


  • Physical responses
  • Chemical responses
  • Mechanical responses
• Physical responses


  • Cellulose cell wall difficult for microorganisms to penetrate
  • Waxy cuticle on leaves difficult for microorganisms to penetrate
  • Layers of dead cells (bark) around stems as a barrier to entry by microorganisms
• Chemical responses


  • Release of antibacterial chemicals to kill bacteria and prevent attack
  • Release of poisons to deter herbivores from grazing
• Mechanical defense system


  • Thorns or hairs to directly protect from herbivores
  • Leaves that droop or curl when touched to scare herbivores
  • Mimicry (e.g. white dead nettle looking like stinging nettle)
• Plants can be attacked by microorganisms such as viruses and fungi (e.g. tobacco mosaic virus, rose black spot), and by insects and herbivores
• Plants have three main defense systems against attack: physical responses, chemical responses, and mechanical responses