Plant Disease

Healthy plants grow well, have green leaves, intact stems and produce flowers/fruit on schedule. When a plant is diseased, gardeners and farmers look for visible symptoms that something is wrong.

Visible symptoms AQA expects you to know:

• Stunted growth — plant is smaller than expected.
• Spots on leaves — circular, often discoloured patches.
• Areas of decay (rot) — soft, brown, dying tissue on stems, roots or fruit.
• Growths — abnormal swellings or galls.
• Malformed stems or leaves — twisted, curled or unusually shaped.
• Discolouration — yellowing (chlorosis), mosaic patterns or unusual purple/black areas.
• Presence of pests — visible insects (aphids, caterpillars) on the plant.

How is a plant disease IDENTIFIED? AQA lists three methods:

1. Reference to a gardening manual or website — symptoms are matched to a list of known diseases. Cheap and fast but easily mistaken.
2. Taking infected plants to a laboratory — scientists can identify the pathogen using microscopes and culture techniques.
3. Using testing kits that contain monoclonal antibodies — the antibody binds only to the antigen of one specific pathogen, giving a colour change. Highly specific, like a pregnancy test for plants.

AQA names three specific plant diseases/pests:

1. Tobacco mosaic virus (TMV)

• Type: virus.
• Hosts: tobacco plants AND about 150 other species (tomatoes, peppers).
• Symptoms: distinctive 'mosaic' pattern of light and dark green/yellow patches on leaves. Affected areas have less chlorophyll, so the plant photosynthesises less and growth is stunted.
• Spread: by contact between infected and healthy plants — on hands, tools, even on smokers' clothing.
• Treatment: there is no cure; affected plants are destroyed and tools sterilised.

2. Rose black spot

• Type: fungus.
• Hosts: roses.
• Symptoms: purple or black spots develop on leaves. Leaves then turn yellow and drop early. Less leaf area → less photosynthesis → weaker plant, poor flowering.
• Spread: spores blown in wind and splashed in water from leaf to leaf.
• Treatment: fungicides, and removing/burning affected leaves to prevent spread.

3. Aphids

• Type: insect pest (not a pathogen itself, but causes disease-like damage).
• Symptoms: visible greenfly/blackfly on stems and undersides of leaves. Aphids pierce the phloem with sharp mouthparts and remove sugars/amino acids, depriving the plant of nutrients and stunting growth.
• Spread: flying adults move between plants; reproduce very rapidly.
• Treatment: insecticides, or biological control (e.g. introducing ladybirds, which eat aphids).

These three examples are commonly examined in 4-6 mark questions — make sure you can match each cause (virus / fungus / insect) to its symptoms and one method of treatment.

Not all plant 'diseases' are caused by pathogens. Plants also become unhealthy when they lack key mineral ions absorbed from soil through their roots.

Mineral ion	Used to make…	Deficiency symptom	Why
Nitrate ions (NO₃⁻)	Amino acids → proteins	Stunted growth	Without nitrate, the plant cannot synthesise proteins for new cells and enzymes, so growth slows
Magnesium ions (Mg²⁺)	Chlorophyll molecule	Chlorosis (yellow leaves)	Magnesium is at the centre of every chlorophyll molecule — without it, leaves cannot make chlorophyll and turn yellow

Linking back to earlier topics. Mineral ions are absorbed by active transport into root hair cells (spec 4.1.3.3), against a concentration gradient. Healthy soil → healthy uptake → healthy plant. Fertilisers (especially NPK fertilisers — nitrogen, phosphorus, potassium) replace ions used up by previous crops.

Exam tip. A 'yellow leaves' question could be magnesium deficiency OR a viral disease (TMV mosaic) OR rose black spot causing early yellowing. Read carefully: 'uniform yellowing' = magnesium; 'mosaic pattern' = TMV; 'with dark spots' = rose black spot.

Plants don't have an immune system like ours — they cannot move away from danger and they cannot produce antibodies. But they have evolved physical barriers that work as a first line of defence, similar in role to our skin.

1. Cellulose cell walls. Every plant cell is surrounded by a rigid wall made of cellulose. This wall is too strong for most pathogens to break through. Even if a pathogen lands on a leaf, it would need to digest the wall before it could reach the contents of a cell.

2. Tough waxy cuticle. The upper surface of most leaves is coated with a layer of wax (the cuticle) secreted by the epidermal cells. This cuticle:

• Is waterproof — pathogens travel in water films, so a dry waxy surface limits spread.
• Is physically tough — fungal hyphae and bacteria struggle to penetrate.
• Reduces water loss (also helps with osmoregulation, but that's a bonus).

3. Layers of dead cells (bark). Tree trunks are covered in bark — many layers of dead cork cells. If pathogens manage to land on bark, they cannot reach living tissue. When the outer bark falls off naturally, pathogens fall off with it. Leaf fall in deciduous trees works the same way: leaves carrying fungi or pests drop in autumn and the tree starts fresh in spring.

Plants cannot run, so they fight back chemically. AQA names two types of chemical defence:

1. Antibacterial chemicals. Some plants produce substances that kill or stop the growth of bacteria. Examples include:

• Mint (and other herbs in the same family) — produces oils with antibacterial activity.
• Witch hazel — extracts contain compounds that kill bacteria and have been used in skin treatments.

These chemicals are part of why some plant extracts are used in toothpastes, mouthwashes and skin lotions.

2. Poisons to deter herbivores. Many plants make toxic chemicals so animals that eat them get sick (or worse). Examples:

• Foxglove — produces digitalis, a powerful heart toxin. (In tiny, controlled doses it has been turned into a heart-disease drug — but eating raw foxglove can kill.)
• Deadly nightshade — produces atropine and related toxins; lethal even in small doses.
• Stinging nettles — combine a mechanical sting with a chemical irritant injected into anything that touches them.

Herbivores quickly learn (or evolve to avoid) plants that taste bitter or make them ill, so the chemical defence works even without killing every attacker.

A neat cross-over. Several powerful human medicines started as plant chemical defences:

• Aspirin from willow bark (anti-inflammatory, painkiller).
• Digitalis from foxglove (heart drug).
• Quinine from cinchona bark (anti-malarial).

This links to spec 4.3.2.3 'Discovery and development of drugs'.

Mechanical defences directly stop animals from eating or landing on the plant.

1. Thorns and hairs.

• Thorns (e.g. on roses, blackthorn, hawthorn): sharp, woody outgrowths that physically prevent large herbivores from grazing.
• Hairs (trichomes): tiny needle-like structures on the surface of stems and leaves. They can irritate insects' mouthparts or trap small herbivores.

2. Drooping or curling when touched.

• Mimosa pudica (sensitive plant): leaves droop and fold within seconds when something touches them. This sudden movement dislodges insects that have landed to feed.
• Some leaves curl up when chewed, hiding inner tissue.

3. Mimicry to trick animals.

• Bee orchids mimic the appearance of a female bee on their flower — male bees try to mate with the flower and accidentally transfer pollen. (This is mimicry helping reproduction rather than direct defence — but AQA lists mimicry under 'mechanical' defences.)
• Passion flower vines have markings that look like butterfly eggs. Butterflies avoid laying eggs on plants that 'already' have eggs (to reduce competition for their own larvae) — so the plant avoids being chewed by caterpillars.

Defence categories overlap. Stinging nettles use both mechanical (the hairs/stings) AND chemical (the histamine and formic acid injected). When you describe a defence, name the category AND the mechanism.