6.3.3 Plant Disease - Detection & Identification

• Animals possess a wide range of defence mechanisms to defend themselves against disease
• The same can be said for plants
• In leaves, plants put up some strong physical defences
• Waxy cuticle
  • This keeps the leaf waterproof from rain and also prevents pathogens from getting in through the leaf surface
  • Many pathogens are water-borne so they roll off the leaf in a raindrop and drop quickly onto the soil
•  The cell wall
  • Its main purpose is to provide strength and support but cellulose cell walls also act as a physical barrier to pathogens within the leaf
•  Callose production
  • The presence of pathogens can cause callose to form which forms a temporary extra cell wall between cells and adds support to cell walls
  • This is particularly important when a piece of plant tissue is wounded or injured, exposing the cell contents to pathogens
•  Production of antimicrobial substances
  • Some plants produce antimicrobial substances to defend against pathogens
  • These may kill the pathogen or prevent it from reproducing
  • For example, basil plants produce essential oils called terpenoids that are active against species of Salmonella bacteria
    • Even fruit sugar fructose, a simple sugar found in cranberries and many other fruit, has antibacterial properties because many pathogenic bacteria cannot metabolise fructose

Defence mechanisms employed by plants include those in the leaves: waxy cuticle, cell wall

Higher Tier Only

• As with human disease, plant disease can be diagnosed by detecting disease antigens and by genetic testing
• This adds a whole extra layer of sophistication to plant medicine because the disease can be detected before it causes major damage to crops or woodland etc
• Monoclonal antibodies are used with samples of the plant tissue
  • Another antibody is then added which can bind to the monoclonal antibody
  • This second antibody has a marker attached, such as a dye or radioactive isotope, which can then indicate that the disease antigen is present
• DNA testing and genome sampling can also reveal the presence of genetic plant diseases
• Less high-tech methods such as observation and microscopy can reveal signs of plant disease out in the field which can then be investigated further using antigen or DNA testing in the laboratory