Plant Diseases and Defences
Build confidence in Plant Diseases and Defences for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) biology topic covering how plants show symptoms of disease and defend themselves using physical, chemical and mechanical adaptations.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Plants can be infected by a range of pathogens and are also damaged by ion deficiencies. This is a Triple-only topic.
Plant diseases can be detected by stunted growth, spots on leaves, patches of decay, abnormal growths, malformed stems or leaves, and discolouration. Identification methods include using a gardening manual, taking the plant to a laboratory to identify the pathogen, or using testing kits containing monoclonal antibodies.
Deficiencies produce their own symptoms. Stunted growth indicates a nitrate deficiency, because nitrates are needed for protein synthesis and therefore growth. Chlorosis — yellow leaves — indicates a magnesium deficiency, because magnesium is needed to make chlorophyll.
Revision notes
Detecting plant disease
Signs include stunted growth, spots on leaves, areas of decay, growths, malformed stems or leaves, and discolouration.
Identification: a gardening manual or website, laboratory identification of the pathogen, or testing kits containing monoclonal antibodies.
Ion deficiencies
Stunted growth is caused by nitrate deficiency, because nitrate ions are needed for protein synthesis and therefore for growth.
Chlorosis, meaning yellow leaves, is caused by magnesium deficiency, because magnesium is needed to make chlorophyll for photosynthesis.
Plant defence responses
Physical: a waxy cuticle, cell walls, and layers of dead cells such as bark that fall off.
Chemical: antibacterial chemicals and poisons that deter herbivores. Mechanical: thorns and hairs, leaves that droop or curl on touch, and mimicry to trick animals.
Key points
- Plant disease signs include stunted growth and discolouration.
- Monoclonal antibody test kits can identify pathogens.
- Nitrate deficiency causes stunted growth.
- Magnesium deficiency causes chlorosis.
- Physical defences include a waxy cuticle and bark.
- Mechanical defences include thorns and drooping leaves.
Worked examples
Example 1
A plant has yellow leaves. Name the ion likely to be deficient and explain why this causes yellowing. [3 marks]
Model answer
Magnesium (1 mark). Magnesium ions are needed to make chlorophyll (1 mark), so without enough magnesium the plant cannot produce sufficient chlorophyll and the leaves appear yellow rather than green (1 mark).
Example 2
Explain why a nitrate deficiency causes stunted growth. [2 marks]
Model answer
Nitrate ions are needed for protein synthesis (1 mark), and proteins are required for growth, so without enough nitrate the plant cannot grow properly (1 mark).
Example 3
Describe two mechanical defences plants use against herbivores. [2 marks]
Model answer
Thorns or hairs which deter animals from eating the plant (1 mark), and leaves that droop or curl when touched, or mimicry to trick animals into not eating them (1 mark).
Common mistakes
Confusing nitrate and magnesium deficiency symptoms.
Nitrate deficiency stunts growth; magnesium deficiency causes yellow leaves.
Not explaining why the symptom occurs.
Link magnesium to chlorophyll and nitrate to protein synthesis.
Calling chlorosis a disease.
It is a deficiency symptom, not an infection.
Mixing up the three defence types.
Physical are barriers, chemical are substances, mechanical involve movement or structures like thorns.
Exam tips
- Learn the two deficiencies and their specific causes.
- Always explain the biological reason behind a symptom.
- Group plant defences as physical, chemical and mechanical.
- Remember this is a Triple-only topic.
Key terms
- Chlorosis
- Yellowing of leaves caused by lack of chlorophyll.
- Nitrate ion
- An ion needed by plants for protein synthesis.
- Magnesium ion
- An ion needed by plants to make chlorophyll.
- Mimicry
- A defence in which a plant resembles something else to deter animals.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.