🐛 Aphids
Aphids are insects that damage plants. Seeing the aphids on a plant is one way to detect the problem, because the presence of pests is a sign.
Plant Disease » Plant Disease and Defence
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.3.3.1, 4.3.3.2
- How plant diseases are spotted and identified
- Which pathogens, pests and mineral ion shortages damage plants
- How plants defend themselves with physical, chemical and mechanical defences
This lesson is for Biology only (not Combined Science). The parts on detecting and identifying plant diseases are Higher tier only.
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Unlock This CoursePlants get diseases too
Plants can be damaged by pathogens, by pests and by a lack of the minerals they need. Gardeners and farmers need to notice the damage early, work out the cause and then act. Otherwise a whole crop can be lost.
Plants can be infected by viral, bacterial and fungal pathogens, and also by insects. For your exam you only need to know three named examples: tobacco mosaic virus (a viral disease), black spot (a fungal disease) and aphids (insects). You met the first two in the earlier lessons on viral and fungal diseases, so here we focus on how plant disease is detected and how plants fight back.
Key terms:
- Aphid: an insect that can damage plants.
- Chlorosis: yellow leaves caused by a lack of chlorophyll.
- Ion deficiency: a shortage of a mineral ion that the plant needs to grow healthily.
Detecting plant disease
Higher tier only
You can detect a plant disease by looking for signs that something is wrong. Look for these signs:
- Stunted growth: the plant is much smaller than it should be.
- Spots on leaves: marks on the leaf surface.
- Areas of decay (rot): soft, damaged parts of the plant.
- Growths: lumps or swellings that should not be there.
- Malformed stems or leaves: parts that are twisted or the wrong shape.
- Discolouration: parts that have turned the wrong colour.
- The presence of pests: you can see the insects themselves.
Several of these signs can have more than one cause. Yellow leaves might come from a pathogen or from a shortage of a mineral ion. Stunted growth might be a disease or poor soil. That is why spotting the sign is only the first step. Next you have to identify the cause, because the right treatment depends on it.
Identifying plant disease
Higher tier only
There are three ways to identify a plant disease:
- A gardening manual or website: compare your plant with the pictures and descriptions of known problems.
- A laboratory: take infected plants to a laboratory to identify the pathogen.
- Testing kits: kits that contain monoclonal antibodies. The antibodies bind to one particular pathogen, so a positive result tells you which pathogen is present.
Testing kits work because a monoclonal antibody binds to one particular pathogen only.
Pests and pathogens
🌿 Pathogens
Remember that tobacco mosaic virus is a virus and black spot is a fungus. Both are covered in earlier lessons.
Mineral ion deficiencies
Plants take in mineral ions from the soil. If a plant does not get enough of an ion, it becomes damaged even though no pathogen is involved. You need to know two.
🌱 Nitrate
Plants use nitrate ions for protein synthesis, and proteins are needed for growth. Without enough nitrate, the plant has stunted growth.
🍃 Magnesium
Plants use magnesium ions to make chlorophyll. Without enough magnesium, the leaves cannot make enough chlorophyll and turn yellow. This is called chlorosis.
Understanding ion deficiencies helps gardeners and farmers. If they know which ion is missing, they can give the plants what they need and provide the best conditions for growth.
Worked example
A farmer sees that a field of young plants is much shorter than the field next door, but the leaves are a healthy green. Which ion is probably lacking? Stunted growth points to nitrate, because nitrate is needed for protein synthesis. If the leaves had been yellow instead, the likely cause would be magnesium, because magnesium is needed to make chlorophyll.
Common mistakes
Students often say that a lack of magnesium makes leaves yellow "because there is no sunlight". The real reason is that magnesium is needed to make chlorophyll. Another mistake is to say that nitrate deficiency causes chlorosis. Nitrate deficiency causes stunted growth. Chlorosis is magnesium.
Plant defence responses
Plants cannot run away, so they have built-in defences. Some stop microorganisms getting in, and some stop animals eating them. You should be able to describe the physical and chemical defences, and there are also mechanical adaptations.
🧱 Physical
These resist invasion by microorganisms:
- Cellulose cell walls around every plant cell.
- A tough waxy cuticle on leaves.
- Layers of dead cells around stems, such as bark on trees, which fall off.
⚗ Chemical
These are substances the plant makes:
- Antibacterial chemicals that act against bacteria.
- Poisons that deter herbivores from eating the plant.
🌵 Mechanical
These are adaptations that work against animals:
- Thorns and hairs deter animals.
- Leaves which droop or curl when touched.
- Mimicry to trick animals.
Mimicry means looking like something else. A plant that tricks an animal into leaving it alone has used mimicry. A physical defence such as the waxy cuticle and a layer of bark gives a barrier. A chemical defence gives the plant poison or antibacterial substances to use against attackers.
Common mistakes
Do not mix up the types. A waxy cuticle is physical because it forms a barrier against microorganisms. A poison is chemical because it is a substance the plant makes. A thorn is a mechanical adaptation because it works by its shape. Also remember that the first group resists microorganisms, while poisons, thorns and hairs deter animals.
Exam-style question
A gardener finds that the leaves of a tomato plant are turning yellow because the soil lacks one mineral ion needed to make chlorophyll.
(a) Name the condition shown by the yellow leaves, and name the mineral ion that is lacking. [2 marks]
(b) Explain why a lack of this ion causes the yellow leaves. [2 marks]
(c) (Higher tier only) Give one way the gardener could identify a plant disease. [1 mark]
(d) Describe two ways a plant defends itself against microorganisms. [2 marks]
Model answer
(a) Chlorosis (1); magnesium (ions) (1).
(b) Magnesium ions are needed to make chlorophyll (1); without enough chlorophyll the leaves turn yellow (1).
(c) Any one from: use a gardening manual or website (1); take the plant to a laboratory to identify the pathogen (1); use a testing kit containing monoclonal antibodies (1).
(d) Any two from: cellulose cell walls (1); tough waxy cuticle on leaves (1); layers of dead cells around stems (bark) that fall off (1); antibacterial chemicals (1).
Exam tip
In (b), give the whole chain: magnesium is needed to make chlorophyll, and no chlorophyll means yellow leaves. Just saying "it lacks magnesium" earns only one mark.