Plant Diseases

• Symptoms of plant disease

  Stunted growth.

  Spots on leaves.

  Area decaying/rotting.

  Visible pests.

  Discolouration of leaves.

  Growths.

  Malformed stems and leaves.
• Aphids

  Plants can be infested with insects such as aphids.

  Aphids have long sharp mouthpieces that pierce the surface of the stem and into the phloem so they can feed on the sap.

  They can infest a plant in large numbers and remove lots of sap, causing weakness.

  Aphids can also act as a vector for diseases.

  Biological pest control using ladybirds which feed on aphids, or using chemical pesticides are the best ways to control infestation with aphids.
• Methods of identifying plant diseases

  By comparing symptoms to those described in gardening manuals or online.

  Using testing kits that contain monoclonal antibodies.

  Taking infected plants to a laboratory to identify the pathogen.
• Mineral deficiencies

  Plants need nitrate ions for production of amino acids to make proteins. Nitrate deficiencies can therefore cause stunted growth.

  Plants need magnesium ions for production of chlorophyll, which is needed to absorb light for photosynthesis. Magnesium deficiencies therefore cause yellow leaves (chlorosis).
• Physical defences against infection

  Cellulose cell wall protects cells from microorganisms.

  Waxy cuticle on leaves is a barrier to microbes entering the plant.

  Bark provides a tough layer around the stem of the plant to block pathogens from entering.
• Chemical defences against infection

  Plants produce antimicrobial chemicals.
• Adaptations against herbivores

  Producing poisons; deters herbivores from eating them.

  Thorns; deters herbivores from eating them.

  Hairy stems; deters herbivores and insects from eating them/laying eggs.

  Leaves that droop or curl when touched; dislodges insects.

  Mimicry to trick animals; resemble larger insects to scare potential threats, droop to look diseased and trick animals into not eating them.