AQA GCSE Biology (8461)

4.3.3 Plant disease (Bio only)

Plant disease (4.3.3) is the Biology-only follow-up to communicable diseases. It covers two big ideas: plants get infected by pathogens (viral, bacterial, fungal) AND suffer when they lack mineral ions; and plants have their own three-layered defence system — physical, chemical and mechanical. This page works through the named examples AQA expects (TMV, rose black spot, crown gall, aphids, nitrate and magnesium deficiency), the symptoms to recognise, and the marking phrases you need for a 4-mark answer on why a virus reduces plant growth or how a plant defends itself.

Why this matters

Plants get sick for two completely different reasons. The first is a pathogen — exactly the same four-type list as for animals: viruses (like tobacco mosaic virus), bacteria (like Agrobacterium causing crown gall), fungi (like rose black spot) and protists. Insects can also cause damage directly and act as vectors that carry pathogens between plants. The second reason is nutritional: plants need mineral ions from the soil to make particular molecules, and if those ions are missing the plant cannot grow normally even when no pathogen is present. The two GCSE-named deficiencies are nitrate (needed for proteins, including the enzymes that build new tissue — without it growth is stunted) and magnesium (needed for chlorophyll — without it the leaves turn yellow, a condition called chlorosis). Plants cannot move and they have no immune cells, so their defences are built into their structure: tough physical barriers like waxy cuticle and bark, chemical weapons like antibacterial compounds and bitter alkaloid poisons, and mechanical features like thorns and hairs. Plants can even communicate — when attacked by insects they release volatile chemicals that warn neighbouring plants and attract predators of the herbivore.

How to learn this topic

Build on what you already know

4.3.1: pathogens are microorganisms that cause disease; four types — bacteria, viruses, fungi, protists.
4.4.1: photosynthesis happens in chloroplasts and produces glucose used for growth.
KS3: plants need mineral ions from the soil — nitrate for proteins, magnesium for chlorophyll.

Why plants get sick: pathogens vs ion deficiency — two distinct causes with overlapping symptoms.
Pathogen examples: TMV (virus), rose black spot (fungus), crown gall (bacterium), aphids (insect damage + vector).
Ion deficiency examples: nitrate → stunted growth (proteins) and magnesium → chlorosis (chlorophyll).
How to tell deficiency from infection: add ions and see if symptoms improve; lab tests; gardening manuals.
How pathogens spread between plants: wind, rain splash, insects, contaminated tools.
Plant defences — physical (waxy cuticle, cellulose cell walls, bark, dead-cell layers).
Plant defences — chemical (antibacterial compounds, alkaloid poisons against herbivores).
Plant defences — mechanical (thorns, hairs, drooping or curling leaves, mimicry).
Plant-to-plant signalling: detecting insect attack and releasing volatile chemicals.

Key terms

tobacco mosaic virus (TMV): A virus that infects tobacco, tomato and many other plants. Produces a mottled mosaic pattern on leaves and reduces growth by damaging chloroplasts. (AQA wants the chain: TMV damages chloroplasts → less photosynthesis → less glucose → less growth.)
rose black spot: A fungal disease of roses that causes purple or black spots on leaves. Affected leaves yellow and fall off early, reducing photosynthesis and growth. (Examples of named fungal disease are heavily marked. Spread by water (rain splash) and wind.)
chlorosis: Yellowing of plant leaves due to a lack of chlorophyll. In GCSE Biology, the named cause is magnesium ion deficiency. (Marking phrase: 'magnesium ions are needed to make chlorophyll'.)
nitrate deficiency: A condition where the plant cannot absorb enough nitrate ions from the soil. Nitrogen is needed for amino acids and proteins, so the plant shows stunted growth. (Linking phrase: 'nitrate needed for protein synthesis → no proteins → no new tissue → stunted growth'.)
vector (plant disease): An organism that carries a pathogen between plants without itself being made ill. Aphids are the classic plant vector and can spread TMV. (Don't confuse with 'host'. The vector carries; the host is infected.)
waxy cuticle: A waterproof, tough layer covering the upper surface of leaves. Acts as a physical barrier that pathogens cannot easily penetrate. (Named on the spec — always usable as a physical defence in an answer.)
alkaloid: A bitter or toxic chemical compound made by some plants that deters or poisons herbivores (e.g. caffeine, nicotine, ricin). (Category: chemical defence. Examiners accept 'poisons that deter herbivores'.)
trichome: A hair-like outgrowth from the plant epidermis. Trichomes deter small insects from feeding and reduce water loss. (Mechanical defence — pair with 'thorns' in extended answers.)
volatile chemical (plant signalling): A small airborne chemical released by a plant when attacked. Detected by neighbouring plants (which start their own defences) and sometimes attracts predators of the herbivore. (Marking-phrase shape: 'plant detects damage → releases chemical into the air → neighbouring plants detect it → neighbours produce their own defensive chemicals'.)

Notes

Two causes of plant disease

Plants suffer from two completely different kinds of problem, and you need to be able to tell them apart.

Pathogen infection — caused by viruses, bacteria, fungi or insects.
Ion deficiency — the plant lacks a particular mineral ion needed to grow normally.

The trick is that they sometimes produce similar-looking symptoms (yellowing, stunted growth, spots), so the spec wants you to know how scientists work out which is which.

Named pathogen examples

### Tobacco mosaic virus (TMV) — viral

A virus that infects tobacco, tomato and many other plants.
Symptoms: a distinctive mottled mosaic pattern of light and dark patches on the leaves — and the plant grows poorly.
Mechanism: TMV damages the chloroplasts. With fewer working chloroplasts, photosynthesis is reduced, less glucose is made, and so less energy is available for growth.

### Rose black spot — fungal

A fungus that infects roses (and related plants).
Symptoms: purple or black spots appear on leaves; affected leaves often turn yellow and drop off early.
Effect: less leaf area means less photosynthesis, so growth is reduced.
Spread: spores carried in water (rain splash) and on the wind. Controlled by removing infected leaves, fungicide sprays and using resistant varieties.

### Crown gall disease — bacterial

Caused by the bacterium Agrobacterium tumefaciens.
Symptoms: large lumpy growths (galls) appear on the stem near soil level.
The bacterium enters through wounds and transfers genes that make the plant cells divide uncontrollably.

### Aphids — insect

Small sap-sucking insects that pierce plant tissue with mouthparts and feed on the sugary phloem sap.
Symptoms: weakened, wilting plant; sticky honeydew on leaves; sometimes leaf curling.
Aphids also act as vectors that transfer viral pathogens (including TMV) between plants.

Ion deficiency diseases

### Nitrate deficiency — stunted growth

Plants absorb nitrate ions (NO3-) from the soil and use the nitrogen to build amino acids, which join to make proteins.
Without nitrate, the plant cannot make new proteins (including the enzymes needed for growth).
Symptom: stunted growth — the plant stays small.

### Magnesium deficiency — chlorosis

Magnesium ions are a key part of the chlorophyll molecule.
Without magnesium, the plant cannot make enough chlorophyll, so the leaves turn yellow — a condition called chlorosis.
Less chlorophyll means less photosynthesis, so growth slows too.

Telling deficiency from infection

The symptoms can overlap (stunted growth, yellowing) so scientists use several methods:

Add mineral ions — fertiliser or specific ion supplement. If the plant recovers, it was a deficiency; if not, suspect a pathogen.
Look for specific signs — mosaic patterns suggest TMV; black spots suggest rose black spot; whole-leaf yellowing suggests magnesium deficiency; thumb-sized galls suggest crown gall.
Use a testing kit — gardener test kits identify common pathogens or mineral levels in the soil.
Consult a gardening manual or website to match symptoms to known diseases.
Send a sample to a lab — for tricky cases, a plant pathology lab can identify the exact pathogen using microscopy and DNA tests.

How plant pathogens spread

Wind — fungal spores are blown between plants.
Water / rain splash — droplets carry pathogens from infected leaves to healthy ones nearby.
Insect vectors — aphids and other insects pick up pathogens while feeding and transfer them when they feed on the next plant.
Direct contact — touching diseased plants or using contaminated tools (pruning shears) moves pathogens between hosts.
Soil — some pathogens survive in soil and infect roots; stomata and wounds are common entry points.

Plant defences — three categories

### 1. Physical defences

Waxy cuticle — a waterproof, tough layer on the upper surface of leaves; physical barrier that stops pathogens entering.
Cellulose cell walls — every plant cell is surrounded by a tough cell wall that acts as a physical barrier.
Layers of dead cells — outer layers around stems (like bark on trees) form a tough protective skin; dead cells fall off and take pathogens with them.

### 2. Chemical defences

Antibacterial compounds — many plants make chemicals that kill or inhibit bacteria (e.g. mint and witch hazel are used in human medicine for this reason).
Poisons (often alkaloids) — bitter or toxic chemicals that deter or kill herbivores eating the plant (caffeine in coffee, nicotine in tobacco, ricin in castor beans).

### 3. Mechanical defences

Thorns and hairs (trichomes) — physically injure or discourage large herbivores from feeding.
Leaves that droop or curl when touched — sudden movement startles insects off the leaf; the Mimosa plant is the classic example.
Mimicry — some plants have markings that look like butterfly eggs, fooling butterflies into not laying their own (no overcrowding for the caterpillars).

Plant-to-plant chemical signalling

Plants can detect changes in their environment, including damage from insects. When attacked, a plant releases volatile chemicals into the air. These chemicals are detected by neighbouring plants, which then start producing their own defensive chemicals before they are even attacked. Some of these volatiles also attract predators of the herbivore (e.g. parasitic wasps that target caterpillars).

This is the closest thing plants have to a 'communication system' — and AQA does expect you to know about it.

Exam tips

Always split causes of plant disease into TWO categories: pathogens (virus, bacterium, fungus, insect) AND ion deficiency. Mention both even in short answers.
Named example trio: TMV (viral), rose black spot (fungal), crown gall (bacterial). Examiners love a named example for each kingdom.
Magnesium → chlorophyll → green colour. Nitrate → proteins → growth. Lock that pairing in.
For TMV growth questions, write the four-step chain: chloroplasts damaged, photosynthesis reduced, less glucose, less energy for growth.
When asked to distinguish deficiency from pathogen, the gold answer is: ADD mineral ions — if symptoms improve it was deficiency; if not, send a sample to a lab or use a testing kit.
Plant defences come in three categories: physical, chemical, mechanical. Try to give one example from EACH in extended-answer questions.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What is the difference between an ion deficiency and a pathogen infection in plants?

Ion deficiency means a vital mineral is missing from the soil — the plant cannot build the molecules it needs. Without nitrate, no proteins, so growth slows. Without magnesium, no chlorophyll, so leaves turn yellow. A pathogen infection means a virus, bacterium or fungus has invaded the plant and is damaging tissue (TMV destroys chloroplasts; rose black spot fungus kills leaf cells in patches). Symptoms can look similar — both can cause yellowing and stunted growth — but the cure is different: top up the missing ion vs treat or remove the infected plant.

How exactly does TMV reduce plant growth?

Tobacco mosaic virus attacks the chloroplasts of leaf cells. Chloroplasts are where photosynthesis happens, so when they are damaged the plant cannot capture as much light energy and makes less glucose. Glucose is the energy currency that drives growth, including making new cell walls, proteins and tissue. Less glucose means less energy available for growth, so the plant ends up smaller, weaker, and produces less fruit. The mottled mosaic appearance of the leaves is the visible sign that chloroplasts are damaged unevenly across the leaf.

What are the three types of plant defence?

PHYSICAL defences are built-in structural barriers: a waterproof waxy cuticle on leaves, tough cellulose cell walls in every cell, and outer layers of dead cells like bark on trees. CHEMICAL defences are toxic or repellent compounds the plant produces: antibacterial chemicals (some used in human medicine), and bitter or poisonous alkaloids (like nicotine and caffeine) that deter herbivores. MECHANICAL defences are physical features that injure or scare off attackers: thorns, leaf hairs (trichomes), leaves that droop or curl when touched (like the Mimosa plant), and even mimicry — markings that look like butterfly eggs, fooling butterflies into laying elsewhere.

How do plant pathogens get from one plant to another?

Several routes. Wind blows fungal spores between plants. Rain splash flicks pathogen-containing water droplets from infected leaves to healthy ones. Insects (especially aphids) pick up viruses while feeding on one plant and transfer them when feeding on the next — acting as vectors. Contaminated tools such as pruning shears can carry bacteria and fungi from plant to plant. Pathogens usually enter the new host through wounds, stomata, or insect bite sites. Gardeners reduce spread by removing infected plants quickly, controlling insect pests, sterilising tools, and choosing disease-resistant varieties.

Do plants really communicate with each other?

In a limited but real way, yes. When a plant is attacked by insects, it releases small airborne chemicals called volatiles. Neighbouring plants detect these chemicals and respond by starting to produce their own defensive compounds — getting ready before they are attacked themselves. Some of the same volatiles also attract predators of the herbivore (such as parasitic wasps that hunt caterpillars). It is not communication in the conscious animal sense, but it is a genuine chemical signalling system that the spec asks you to know.