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3.15 Detecting and identifying plant diseases

The signs that a plant is diseased (stunted growth, spots, decay, growths, malformed parts, discolouration, pests), mineral deficiencies such as nitrate and magnesium, and how plant diseases are identified in the field and in the laboratory.

Separate only AQA Edexcel OCR 2 min read

What can harm a plant

Plants can be infected by viruses (such as TMV), bacteria (such as Agrobacterium) and fungi (such as rose black spot). They can also be damaged by pests, such as aphids, which suck sap from the phloem and can carry viruses, and by a lack of mineral ions in the soil.

Key terms

  • Symptom: a sign that something is wrong, such as spots or yellow leaves.
  • Aphid: a small insect pest that feeds on phloem sap and can spread plant viruses.
  • Chlorosis: yellowing of leaves because they cannot make enough chlorophyll.

Signs of disease

A plant showing signs of disease: discoloured leaves, decay, galls, malformed parts, spots, aphids and stunted growth
A plant showing signs of disease: discoloured leaves, decay, galls, malformed parts, spots, aphids and stunted growth
  • stunted growth
  • spots on leaves
  • areas of decay (rot)
  • growths, such as galls
  • malformed stems or leaves
  • discolouration, such as yellow leaves
  • the presence of visible pests.

Mineral ion deficiencies

Plants need mineral ions, taken up from the soil by active transport, to make many substances.

A healthy plant, a stunted plant lacking nitrate, and a plant with yellow leaves lacking magnesium
A healthy plant, a stunted plant lacking nitrate, and a plant with yellow leaves lacking magnesium
Deficiency diseases
Mineral ionNeeded forSymptom when lacking
NitrateMaking proteins, so the plant can growStunted growth
MagnesiumMaking chlorophyllChlorosis: yellow leaves, less photosynthesis

Identifying the disease

  • In the field (garden or farm): look closely at the symptoms and where they appear, and compare them with a gardening manual or website. See whether the pattern of affected plants suggests a cause: a disease spreading from plant to plant, or a whole area affected, which suggests a problem with the soil.
  • In the laboratory: send the plant to a lab, where scientists can grow the pathogen and look at it under a microscope, or test its DNA.
  • Testing kits: kits containing monoclonal antibodies that bind to a particular pathogen's antigens can identify it quickly, even in the field.

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