Plant Diseases & Defences

• Plants need mineral ions
• Plants need mineral ions from the soil. If there aren't enough, plants suffer deficiency symptoms
• Nitrates are needed to make proteins and therefore growth. A lack of nitrates causes stunted growth
• Magnesium ions are needed for making chlorophyll, which is needed for photosynthesis. Plants without enough magnesium suffer from chlorosis and have yellow leaves
• Plants can get diseases
• Plants can be infected by viral, bacterial and fungal pathogens. They can also be infested and damaged by insects. E.g. aphids are an insect that can cause huge damage to plants
• It's usually clear that a plant has a disease. The common signs are:

  • Stunted growth
  • Abnormal growths, e.g. lumps
  • Spots on the leaves
  • Malformed stems or leaves
  • Patches of decay
  • Discolouration
• Infestations of pests are easy to spot too - you should be able to see them on the plants
• Different plant diseases have different signs. They can be identified by:

  • Looking up the signs in a gardening manual or on a gardening website
  • Taking the infected plant to a laboratory, where scientists can identify the pathogen
  • Using testing kits that identify the pathogen using monoclonal antibodies
• Plants have physical, chemical and mechanical defences
• Physical defences:

  • Most plant leaves and stems have a waxy cuticle, which provides a barrier to stop pathogens entering
  • Plant cells themselves are surrounded by cell walls made from cellulose. These form a physical barrier against pathogens that make it past the waxy cuticle
  • Plants have layers of dead cells around their stems, e.g. the outer part of the bark on trees. These act as a barrier to stop pathogens entering
• Chemical defences:

  • Some can produce antibacterial chemicals which kill bacteria - e.g. the mint plant and witch hael
  • Other plants produce poisons which can deter herbivores - e.g. tobacco plants, foxgloves and deadly nightshade
• Mechanical defences:

  • Some plants have adapted to have thorns and hairs. These stop animals from touching and eating them
  • Other plants have leaves that droop or curl when something touches them. This means that they can prevent themselves from being eaten by knocking insects off themselves and moving away from things
• Mechanical defences:

  • Some plants can cleverly mimic other organisms. E.g. the passion flower has bright yellow spots on its leaves which look like butterfly eggs. This stops other butterflies laying their eggs there. Several species of plant in the 'ice plant family' in southern Africa look like stones and pebbles. This tricks other organisms into not eating them