Plant Diseases and Defences

Plants are infected by viral, bacterial and fungal pathogens, and by insects. You cannot ask a plant where it hurts, so disease is recognised from visible signs on the plant itself.

The three named plant diseases for AQA 8461 are tobacco mosaic virus (viral), black spot (fungal) and aphids (an insect). You may be asked to recognise signs of these, but the signs below apply generally.

The spec lists a fixed set of signs to look for:

Sign of disease	What you would see
Stunted growth	The plant is smaller than a healthy one of the same age
Spots on leaves	Discrete marks, e.g. the dark spots of black spot
Areas of decay (rot)	Soft, rotting patches on leaves, stems or fruit
Growths	Abnormal lumps or swellings on the plant
Malformed stems or leaves	Stems or leaves that are misshapen or distorted
Discolouration	Patches of abnormal colour, e.g. the mosaic pattern of TMV
Presence of pests	Visible insects such as aphids on the plant

A single sign rarely identifies the exact disease on its own — discolouration could be a virus or an ion deficiency. Recognising the sign is the first step; identifying the cause comes next.

Spotting a sign tells you the plant is unwell; it does not tell you which pathogen is responsible. Identification matches the signs to a known cause.

(Higher Tier only — both the signs of disease above and the identification methods below are assessed only on the Higher Tier Biology papers.)

The spec gives three methods, from simplest to most reliable:

Method	How it works	Reliability
Reference to a gardening manual / website	Match the observed signs against described diseases and photographs	Quick and cheap, but easy to misread
Take the plant to a laboratory	Trained staff and equipment identify the pathogen directly	Reliable, but slower and costlier
Use a testing kit with monoclonal antibodies	The kit's antibodies bind to one specific pathogen antigen, giving a result	Fast and specific to one pathogen

The monoclonal-antibody kit works on the same principle as a pregnancy test: each monoclonal antibody binds to one specific antigen. If the kit's antibodies bind to a molecule from the plant sample, that specific pathogen is present and the kit signals a positive result.

(Extra context — the structure of a lateral-flow antibody kit is covered in the monoclonal antibodies topic and is not required here. For 4.3.3.1 you only need to know that such kits exist as an identification method and that they use monoclonal antibodies.)

Not every sick-looking plant has a pathogen. A plant short of certain mineral ions shows physical symptoms even though nothing has infected it. The spec restricts this to two ions and two symptoms — learn them precisely.

Ion missing	Why the plant needs it	Symptom of deficiency
Nitrate	Needed for protein synthesis (and therefore growth)	Stunted growth
Magnesium	Needed to make chlorophyll	Chlorosis (yellowing leaves)

Work through the cause-and-effect chain rather than memorising the pairing blindly:

• Nitrate ions → proteins → growth. Proteins are needed to build new cells and tissues. Without enough nitrate the plant cannot make enough protein, so it cannot grow normally — the result is stunted growth.
• Magnesium ions → chlorophyll → green colour. Magnesium is part of the chlorophyll molecule. Without magnesium the plant cannot make enough chlorophyll, so the leaves turn yellow. This yellowing is called chlorosis.

Memorise the precise links: nitrate → stunted growth (lack of protein synthesis); magnesium → chlorosis (lack of chlorophyll). Mixing these two up is the single most common slip in this topic.

A real diagnostic skill the exam tests (AQA WS 1.4) is using signs to decide on a likely cause. Here, the same sign can point to different causes, so reason from the full picture.

Scenario. A gardener notices a tomato plant whose lower leaves are turning yellow between the veins, while the plant is otherwise growing at a normal size. There are no spots, no visible insects, and no rotting patches.

Walk through the reasoning:

1. List the signs. Yellowing leaves (discolouration) — but normal size, no pests, no rot, no spots.
2. Rule things out. No visible pests rules out aphids. No leaf spots argues against black spot. No mosaic discolouration pattern across the whole leaf argues against TMV.
3. Match what remains. Yellowing leaves with otherwise normal growth fits chlorosis, the symptom of magnesium deficiency (magnesium is needed to make chlorophyll).

The plant most likely has a magnesium ion deficiency. The growth is not stunted, which argues against nitrate deficiency — that comparison is what secures the marks.

Symptoms	Likely deficiency
Yellow leaves + normal size	magnesium deficiency (chlorosis)
Stunted growth + pale plant	nitrate deficiency (low protein synthesis)

Understanding ion deficiencies lets horticulturists give plants the optimum conditions — adding the missing mineral (e.g. a magnesium feed) to correct the symptom.

Plants cannot run away, so they rely on built-in defences. The spec groups these into three types: physical, chemical and mechanical. This slide covers the first two.

Physical defences are structural barriers that stop pathogens getting in:

Physical defence	How it protects the plant
Cellulose cell walls	A tough wall around each cell resists pathogens entering
Tough waxy cuticle	A waterproof layer on leaves blocks pathogen entry
Layers of dead cells that fall off (e.g. bark)	Pathogens are shed when the dead outer cells drop away

Chemical defences are substances the plant produces to attack or deter attackers:

Chemical defence	How it protects the plant
Antibacterial chemicals	Kill or stop the growth of invading bacteria
Poisons to deter herbivores	Make the plant toxic or unpleasant, so animals avoid eating it

Keep the two categories separate: physical = a barrier or structure (wall, cuticle, bark); chemical = a substance produced (antibacterial chemical, poison). The test is what kind of thing the defence is.

Mechanical defences are physical features or movements that deter or trick an attacker — usually a herbivore rather than a microbe.

Mechanical defence	How it protects the plant
Thorns and hairs	Physically deter animals from eating or landing on the plant
Leaves that droop or curl when touched	Movement startles or dislodges insects (e.g. Mimosa)
Mimicry	The plant tricks animals — e.g. looking like an unhealthy plant, or having markings that mimic insect eggs so insects do not lay there

Worked example — classify and justify. A question gives three features of a rose plant and asks you to classify each defence and explain it.

1. A waxy cuticle on the leaves → physical. It is a structural barrier that stops pathogens entering the leaf.
2. Sharp thorns on the stem → mechanical. The thorns physically deter animals from eating the plant.
3. An antibacterial substance in the sap → chemical. It is a substance the plant produces that kills bacteria.

The justification is where the marks are: name the type (physical / chemical / mechanical) and say why — what the defence is and what it does. "Thorns are mechanical because they physically deter herbivores" scores; "thorns are mechanical" alone is weaker.

The boundary between physical and mechanical can feel blurry — both are structural. A useful rule of thumb: defences that block pathogens getting in (walls, cuticle, bark) are treated as physical, while features that deter or fend off animals (thorns, hairs, touch movements, mimicry) are mechanical.

1. Swapping the two ion deficiencies

Nitrate deficiency causes stunted growth (less protein synthesis); magnesium deficiency causes chlorosis (less chlorophyll). Reversing these is the most frequent error in this topic. Link each ion to its job first — nitrate → protein → growth, magnesium → chlorophyll → colour — and the symptom follows.

2. Forgetting that detection and identification are Higher Tier only

The signs of disease and the three identification methods (gardening manual, laboratory, monoclonal-antibody kit) are assessed only on the Higher Tier papers. Ion deficiencies and plant defences are on both tiers. Target your revision to the right paper.

3. Confusing chlorosis with the disease itself

Chlorosis is a symptom (yellow leaves from a lack of chlorophyll), not a disease or a pathogen. It can be caused by magnesium deficiency, but yellowing can also have other causes — use the full set of signs before concluding a cause.

4. Mixing up physical and chemical defences

A physical defence is a structure or barrier (cellulose cell wall, waxy cuticle, bark). A chemical defence is a substance the plant makes (antibacterial chemical, poison). Ask whether the defence is a thing the plant is built from or a substance it produces.

5. Naming a defence without saying what it does

Mark schemes for "describe how the plant defends itself" questions reward the mechanism, not just the label. Pair every named defence with its effect — "a waxy cuticle stops pathogens entering", not just "a waxy cuticle".

6. Assuming every unhealthy plant has a pathogen

Stunted growth and discolouration are signs of disease and symptoms of ion deficiency. Check for pests, spots and rot before assuming infection — a plant short of nitrate or magnesium has no pathogen at all.