Plant disease — AQA GCSE Biology
How plant diseases are detected and identified, ion deficiency conditions, and the physical, chemical and mechanical defences of plants.
Revision notes
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Plant diseases are detected by signs such as stunted growth, spots on leaves, areas of decay, growths, malformed stems or leaves, discolouration and pests. They are identified using a gardening manual or website, laboratory tests, or testing kits containing monoclonal antibodies. Nitrate deficiency causes stunted growth, and magnesium deficiency causes chlorosis, which makes leaves yellow.
Plants defend themselves with physical, chemical and mechanical adaptations. Physical defences include cellulose cell walls, a tough waxy cuticle on leaves and layers of dead cells such as bark, which fall off. Chemical defences include antibacterial chemicals and poisons to deter herbivores. Mechanical adaptations include thorns and hairs, leaves that droop or curl when touched, and mimicry.
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Quick check questions
Which ion deficiency causes stunted growth, and why?
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Nitrate. Nitrate ions are needed for protein synthesis and therefore growth.
Which ion deficiency causes chlorosis?
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Magnesium, because magnesium ions are needed to make chlorophyll.
Name three physical plant defences.
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Cellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells around stems.
Give two mechanical adaptations of plants.
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Any two of: thorns and hairs, leaves that droop or curl when touched, mimicry.
Frequently asked questions
How are plant diseases identified?
Higher tier Plant diseases can be identified by referring to a gardening manual or website, by taking infected plants to a laboratory to identify the pathogen, or by using testing kits that contain monoclonal antibodies. A testing kit gives a reliable result because the monoclonal antibody binds only to an antigen from the specific pathogen.
What are the signs of disease in plants?
Higher tier Signs of plant disease include stunted growth, spots on leaves, areas of decay (rot), growths, malformed stems or leaves, discolouration and the presence of pests. Once a disease is detected, it can be identified using a gardening manual or website, laboratory testing to find the pathogen, or monoclonal antibody testing kits.
Why do plants need nitrate and magnesium ions?
Plants need nitrate ions for protein synthesis, and therefore for growth, so a nitrate deficiency causes stunted growth. They need magnesium ions to make chlorophyll, so a magnesium deficiency causes chlorosis, in which the leaves turn yellow. Knowing about ion deficiencies allows horticulturists to provide the optimum conditions for plants.
How do plants defend themselves against pathogens and animals?
Plants resist pathogens with physical defences: cellulose cell walls, a tough waxy cuticle on leaves, and layers of dead cells around stems, such as bark, which fall off. They also make antibacterial chemicals. To deter animals, plants use poisons, thorns and hairs, leaves that droop or curl when touched, and mimicry to trick animals.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026