Plant diseases & defences

• Main causes of plant diseases:

  • Infection by microorganisms such as:
  • fungi
  • bacteria
  • viruses
  • Attack by larger organisms, such as insects.
  • Deficiency diseases = Lack of essential minerals:
  • nitrates ions
  • magnesium ions

  
• The primary mode of transmission for tobacco mosaic virus (TMV) is Direct contact with infected plants.
• • Plants require nitrates To make proteins for growth.
  • Nitrates contain the element nitrogen.
  • It's essential for making amino acids.
  • These amino acids can then be combined to make proteins.

  
• A symptom of nitrate deficiency (lack of nitrates) in a plant is stunted growth:

  • Lack of nitrates means the plant can't produce proteins.
  • Proteins are for growth.
  • The plant won't grow properly.

  
• Plants needs magnesium to make chlorophyll for photosynthesis:


  • Chlorophyll is the green pigment in chloroplasts that makes leaves green.
  • Lack of magnesium mean that leaves lose their green colour.

  
• The main symptom of magnesium deficiency (lack of magnesium) for a plant:


  Chlorosis - yellow leaves due to lack of chlorophyll.

  
• Physical changes that could indicate that a plant has a disease:

  • Stunted growth
  • Spots of leaves
  • Abnormal growths / lumps
  • Patches of decay (rot)
  • Malformed stems or leaves
  • Discolouration on leaves

  
• You can sometimes see the organism responsible for the plant diseases like:

  • Aphids
  • spider mites

  
• Ways in which plant diseases can be identified:

  • Compare the symptoms to a gardening manual or website
  • Take the infected plants to a laboratory (plant pathologist) to identify the pathogens
  • Use testing kits that contain monoclonal antibodies
  • Test the dna of the pathogens
  • Use trial and error to test different treatments

  
• When trying to diagnose plant disease:

  • Send a sample of the diseased plant to a plant pathologist (someone who specialises in plant disease).
  • To do more detailed testing.

  
• Detailed testing - For example:

  • Take a tissue sample
  • Look at it under a microscope
  • To see the problems in more detail

  
• Detailed testing - For example:

  • Look for unique antigens that come from particular pathogens by:
  • Using monoclonal antibodies

  or

  • Run DNA tests that look for pathogen's DNA

  
• Using trial and error to test different treatments:

  • If you think a plant has a magnesium deficiency.
  • Then give it some magnesium to see if that fixes it.

  Or

  • If you think it’s a fungal disease, then spray it with antifungal chemicals and see what happens. 

  
• The benefit of the technique of Using trial and error to test different treatments.


  You can just try one thing and if it doesn’t work, you try something else.
• Plants' defences against pathogens:

  • physical defences
  • chemical defences
  • mechanical defences

  
• Examples of plant physical defences:

  • Strong cellulose cell walls around every plant cell - physical barrier.
  • A waxy cuticle on the leaves and stems - stops entry of microorganisms.
  • Layers of dead cells around their stem (e.g. outer part of bark of tree).

  
• Examples of plant chemical defences:

  • Production of antimicrobial substances to kill bacteria or fungi.
  • Production of poisons that deter or kill insects / herbivores from eating / living on them. e.g. foxgloves, deadly nightshade.

  Usually the poisons are specific to herbivores / pests that target them.

  
• • Plants make so many different chemicals, that a huge number of our drugs comes from plant chemicals.

  E.g. The painkiller aspirin, which comes from the bark and leaves of willow trees.

  
• Mechanical defences:


  Structures that reduce the chance of being attacked.

  • More of a function rather than just acting as a barrier.

  
• Examples of plant mechanical defences:

  • Mimicking other organisms to trick animals - e.g. plants looking like pebbles to stop them from being eaten.
  • Hairs and thorns to stop animals from eating / touching them.
  • Curling or drooping leaves when touched to prevent being eaten - when insects land on them.

  
• Plants need mineral ions from the soil in order to grow and thrive like:

  • Nitrates
  • magnesium

  If there isn't enough, plants suffer deficiency symptoms.
• Plant diseases:

  • Tobacco mosaic virus
  • Rose black spots (fungal)
  • Crown galls (bacterial)
• Ways plants Mimic other organisms to trick animals:

  • Ice plant family look like pebbles to stop from being eaten.
  • Some plants Patterns that mimic other organisms or objects.
  • e.g. The passion flower has yellow spots, which look like butterfly eggs to stop butterflies laying eggs on them.

  
• Non-specific Plant defence mechanisms, Do not target individual species:

  • a mechanism to prevent pathogen entry such as thick waxy cuticle or bark.

  Specific defence mechanisms, Targets specific species:

  • antibacterial compounds kill specific bacteria poison.
• In winter, leaves fall off of plants to prevent disease, if any pathogen infects the leaves.
• Both plants & humans have non-specific defence mechanisms.


  As both have barriers to prevent pathogen entry:

  • skin in humans
  • waxy cuticle / bark in plants
• Both plants & humans have Specific defence.


  As they produce chemicals to kill pathogens:

  • hydrochloric acid / antimicrobial secretions in humans
  • antibacterial chemicals in plants
• Plant defence mechanisms may be exploited to benefit humans like:

  Mint / witch hazel as they produce mildly antimicrobial chemicals & have antibiotic properties.