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Biology · Plant Disease

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Biology · AQA Paper 1

Plant Disease

Detecting and Identifying Plant Diseases (Separate only)

To treat a plant problem you must first detect it, and then work out whether it is a disease, a pest or a missing mineral.

  • Aphid A small insect that feeds on plant sap and can damage plants.
  • Pest An animal that damages plants or crops.
  • Stunted growth Poor growth, so a plant is smaller than normal.
Six signs of plant disease: stunted growth, spots on leaves, rot, growths, malformed leaves and discolouration.
Several different signs can show that a plant has a problem.

Nitrate and magnesium deficiency compared

  • Nitrate deficiency Nitrate is needed to make amino acids and proteins; The plant cannot make enough protein to grow; Symptom: stunted growth
  • Magnesium deficiency Magnesium is needed to make chlorophyll; The leaves cannot make enough chlorophyll; Symptom: yellow leaves (chlorosis)

Plant Defence Responses (Separate only)

Plants have physical, chemical and mechanical defences, built in to their structure.

  • Physical defence A barrier, such as a cell wall or waxy cuticle, that stops pathogens getting into a plant.
  • Chemical defence A chemical, such as an antibacterial substance or poison, made by a plant to protect itself.
  • Mechanical adaptation A feature of a plant's structure, such as a thorn, that deters animals.
Three panels showing physical, chemical and mechanical defences of plants.
Plants have physical, chemical and mechanical defences.

Physical, chemical and mechanical defences compared

  • Physical and chemical Cell walls, waxy cuticle and dead cells resist microorganisms; Antibacterial chemicals kill bacteria; Poisons make animals ill
  • Mechanical Thorns and hairs deter animals physically; Drooping or curling leaves; Mimicry tricks animals