Plant diseases and defences

What you'll learn

This revision guide covers all AQA GCSE Biology content on plant diseases and defence mechanisms. You'll learn how to identify common plant diseases, understand both physical and chemical defences plants use against pathogens, and apply this knowledge to real-world agricultural contexts. This topic appears in Paper 1 (Biology 1) and is essential for 6-mark extended response questions.

Key terms and definitions

Pathogen — a microorganism that causes disease (including bacteria, viruses, fungi and protists)

Physical defence — a structural barrier that prevents pathogens from entering plant tissues

Chemical defence — substances produced by plants that deter herbivores or kill pathogens

Chlorosis — yellowing of leaves caused by lack of chlorophyll, often a symptom of disease or mineral deficiency

Necrosis — death of plant tissue, appearing as brown or black patches

Ion deficiency — lack of essential mineral ions needed for plant growth and metabolism

Cellulose cell wall — the rigid outer layer of plant cells that provides the first line of defence against pathogens

Nitrate ions — mineral ions absorbed from soil that plants need to synthesize amino acids and proteins

Core concepts

Detecting and identifying plant diseases

Plants can be affected by a range of diseases caused by pathogens or by ion deficiencies. Identifying diseases correctly is crucial for farmers and gardeners to take appropriate action.

Common observable symptoms include:

• Stunted growth (plant fails to reach expected height)
• Spots on leaves (black, brown or yellow patches indicating infection or necrosis)
• Areas of decay or rotting (on stems, leaves, roots or fruits)
• Abnormal growths or galls (lumps caused by certain pathogens)
• Malformed stems or leaves (curling, twisting or distortion)
• Discolouration including chlorosis (yellowing due to loss of chlorophyll)
• Presence of pests (aphids or other insects that may transmit viral diseases)

Methods for identifying diseases:

Using gardening manuals or websites to match symptoms with known diseases. These reference materials contain photographs and descriptions of common plant diseases specific to different species.

Laboratory testing can identify the exact pathogen. Plant pathology laboratories use techniques including microscopy to observe fungal hyphae or bacterial cells, and monoclonal antibody testing to identify specific viruses.

Specific plant diseases you must know

The AQA specification requires knowledge of specific plant diseases and their causes.

Rose black spot is a fungal disease affecting roses:

• Caused by the fungus Diplocarpon rosae
• Symptoms: purple or black spots develop on leaves, which then turn yellow and drop off prematurely
• Effect: reduced photosynthesis due to leaf loss, weakening the plant and reducing growth
• Spread: fungal spores are dispersed by water and wind
• Treatment: removing and destroying affected leaves, applying fungicides

Tobacco mosaic virus (TMV) affects tobacco plants and other members of the Solanaceae family (including tomatoes):

• Caused by a virus that infects plant cells
• Symptoms: distinctive mosaic pattern of discolouration on leaves (light and dark green patches)
• Effect: reduced photosynthesis because chloroplasts are damaged, leading to stunted growth and reduced yield
• Spread: transmitted by contact between infected and healthy plants, or by vectors such as insects
• No cure exists; prevention involves removing infected plants and controlling insect vectors

Ion deficiency diseases result from lack of essential minerals in soil:

Nitrate deficiency:

• Nitrate ions are needed to produce amino acids, which combine to form proteins
• Deficiency symptoms: stunted growth and chlorosis (older leaves yellow first)
• Plants cannot make sufficient proteins for growth without adequate nitrate

Magnesium deficiency:

• Magnesium ions are essential components of chlorophyll molecules
• Deficiency symptoms: chlorosis, particularly in older leaves as magnesium is mobile and translocated to younger leaves
• Reduced photosynthesis due to decreased chlorophyll production

Physical defences in plants

Plants have evolved multiple structural barriers that prevent pathogen entry or make it difficult for herbivores to feed.

Cellulose cell walls form the first barrier. This tough, rigid structure surrounding all plant cells makes it physically difficult for pathogens to penetrate into cells. The cellulose provides mechanical strength and must be breached before pathogens can access the cell contents.

Waxy cuticle covers the epidermis of leaves and stems. This waterproof layer prevents water from collecting on plant surfaces (which would allow pathogens to reproduce) and acts as a barrier to pathogen entry. The waxy surface is difficult for fungal hyphae to penetrate.

Bark on trees provides tough, dead tissue that forms a protective outer layer. Bark contains no living cells that pathogens could infect and creates a thick barrier preventing entry.

Leaf fall (abscission) allows plants to shed infected leaves before pathogens spread. A layer of cells called the abscission layer forms at the base of the leaf stalk, cutting off nutrients and causing the leaf to fall. This removes infected tissue from the plant.

Physical structures that deter herbivores:

• Thorns, spines and prickles on stems (roses, hawthorn, cacti) make plants difficult and painful to eat
• Hairs on leaves (stinging nettles) inject irritant chemicals when touched
• Tough, fibrous stems and leaves require more energy to digest

Chemical defences in plants

Plants produce a wide range of chemical compounds that kill pathogens or deter animals from feeding.

Antibacterial chemicals kill bacteria that land on or attempt to enter the plant. These compounds may be present constantly or produced in response to infection. Examples include phenolic compounds and defensins (antimicrobial peptides).

Antifungal chemicals prevent fungal pathogens from reproducing. Chitinases are enzymes that break down chitin in fungal cell walls, destroying the fungus.

Poisons and toxins deter herbivores from eating plants:

• Digitalis (foxgloves) contains cardiac glycosides that affect heart function and are toxic to mammals
• Deadly nightshade produces atropine and scopolamine, which affect the nervous system
• Tobacco produces nicotine, which is toxic to many insects

Deterrents make plants unpalatable without being lethal:

• Tannins in tea leaves and many other plants have a bitter taste that deters feeding
• Bitter-tasting compounds in many vegetables (Brussels sprouts contain glucosinolates)

Mechanical chemical responses:

• Plants produce sticky resin that traps insects attempting to feed
• Latex (in dandelions and rubber trees) flows from damaged tissues, sealing wounds and deterring herbivores

These chemicals also have uses for humans. Digitalis is used in heart medications, quinine from cinchona bark treats malaria, and aspirin is derived from salicylic acid found in willow bark. Understanding plant chemical defences has led to pharmaceutical developments and crop protection strategies.

Detection of pathogens by plants

Plants lack an immune system like animals but can detect and respond to pathogen attack through chemical signalling.

When pathogens damage plant cells, the cells release specific chemicals that trigger defence responses. These may include:

• Strengthening cell walls with additional lignin or callose deposits
• Producing antimicrobial chemicals at the site of infection
• Initiating programmed cell death (hypersensitive response) around the infection site to prevent pathogen spread
• Releasing volatile compounds that attract predatory insects which eat herbivores

Some plants can detect herbivore saliva and respond by producing toxins specifically targeted at that type of herbivore.

Environmental and agricultural factors

Understanding plant diseases is essential for food security in the UK and Caribbean regions.

In the Caribbean context:

• High humidity and warm temperatures accelerate fungal disease spread
• Diseases like Panama disease (affecting bananas) and coffee rust have devastated crops
• Integrated pest management strategies reduce pesticide use

In UK agriculture:

• Potato blight (caused by Phytophthora infestans, a protist) remains a significant concern
• Crop rotation helps prevent soil-borne pathogen build-up
• Climate change is enabling Mediterranean pests and diseases to establish in the UK

Control strategies in agriculture:

• Chemical control: fungicides, bactericides (expensive, environmental concerns, resistance development)
• Biological control: using natural predators or pathogens that target crop pests
• Cultural control: crop rotation, removal of infected material, resistant varieties
• Physical control: barriers, traps, removal by hand

Worked examples

Example 1: Extended response question (6 marks)

Question: Describe and explain how plants defend themselves against pathogens and herbivores.

Mark scheme answer:

Plants have physical defences (1 mark):

• Cellulose cell walls provide a barrier preventing pathogen entry (1 mark)
• Waxy cuticle on leaves prevents water collecting and pathogen penetration (1 mark)
• Thorns/spines deter herbivores from feeding (1 mark)

Plants have chemical defences (1 mark):

• Antibacterial/antifungal chemicals kill pathogens (1 mark)
• Poisons/toxins such as digitalis deter or kill herbivores (1 mark)

Examiner note: This question requires both description (stating what defences exist) and explanation (how they work). You need examples from both physical and chemical categories. Aim for 5-6 developed points in approximately 6-8 minutes.

Example 2: Application question (4 marks)

Question: A gardener notices that the leaves of their rose bushes have purple-black spots and many leaves are turning yellow and falling off. Suggest what disease this might be and explain the effect on the plant.

Answer:

• Disease is rose black spot (1 mark)
• Caused by a fungus/Diplocarpon rosae (1 mark)
• Leaves fall off/leaf loss reduces leaf area (1 mark)
• Less photosynthesis occurs so less glucose is made for growth/respiration (1 mark)

Examiner note: Application questions require you to use your knowledge in unfamiliar contexts. Here you must recognize the symptoms, name the disease, and link leaf loss to reduced photosynthesis.

Example 3: Ion deficiency question (3 marks)

Question: A plant is showing stunted growth and yellowing of older leaves. Explain what is likely causing these symptoms.

Answer:

• Nitrate/mineral ion deficiency (1 mark)
• Nitrates are needed to make amino acids/proteins (1 mark)
• Without sufficient proteins, growth is reduced/stunted (1 mark)

Alternative for yellowing only:

• Magnesium deficiency (1 mark)
• Magnesium is needed to make chlorophyll (1 mark)
• Without chlorophyll, leaves cannot photosynthesise effectively and turn yellow (1 mark)

Common mistakes and how to avoid them

• Confusing physical and chemical defences. Remember: physical defences are structural barriers (walls, thorns, cuticle), chemical defences are substances produced by the plant (poisons, antibacterials). The waxy cuticle is physical even though it's a chemical substance, because it acts as a structural barrier.

• Saying plants have an immune system. Plants do not have immune systems with white blood cells like animals. They have defence mechanisms including barriers and chemical responses but cannot produce antibodies.

• Mixing up symptoms of different diseases. Learn the specific symptoms: rose black spot causes spots then yellowing and leaf drop; TMV causes a mosaic pattern of discolouration. Nitrate deficiency causes stunted growth and chlorosis; magnesium deficiency causes chlorosis especially in older leaves.

• Not explaining the consequence of leaf loss in rose black spot. Don't just say "leaves fall off" — explain that reduced leaf area means less photosynthesis, so less glucose for respiration and growth, weakening the plant.

• Forgetting why specific ions are needed. You must know: nitrates make amino acids which form proteins (for growth); magnesium is part of chlorophyll molecules (needed for photosynthesis). Don't just memorize the deficiency symptoms.

• Writing vague exam answers. Avoid saying plants have "protection" or "defences against disease" without specifying the type. Name specific mechanisms: cellulose cell walls, waxy cuticle, antibacterial chemicals, thorns. Use precise scientific terminology.

Exam technique for "Plant diseases and defences"

• Command words matter. "Describe" requires you to state features or characteristics. "Explain" requires reasons or mechanisms (use "because," "so that," "this causes"). "Suggest" means apply your knowledge to unfamiliar situations and indicates there may be multiple acceptable answers.

• In 6-mark questions, write logically structured answers with clear paragraphs. Cover multiple defence types. Include specific examples (named diseases, named plant species, named chemical compounds). Quality of written communication is assessed, so use correct spelling of scientific terms.

• Link structure to function. Don't just list defences. Explain how the structure enables the defence mechanism: "The cellulose cell wall is tough/rigid which prevents pathogens from penetrating into the cell." This demonstrates understanding, not just memorization.

• Use data interpretation skills. Questions may provide photographs of diseased plants, graphs showing disease progression, or tables comparing treatments. Practice identifying patterns and relating them to your biological knowledge.

Quick revision summary

Plants detect and respond to pathogens using defence mechanisms. Physical defences include cellulose cell walls, waxy cuticles, bark, and thorns that prevent pathogen entry or herbivore feeding. Chemical defences include antibacterial and antifungal compounds, plus poisons like digitalis that deter herbivores. Rose black spot is a fungal disease causing spotted, yellowing leaves that fall prematurely, reducing photosynthesis. Tobacco mosaic virus creates a mosaic discolouration pattern and stunts growth. Ion deficiencies (nitrate and magnesium) cause stunted growth and chlorosis. Disease identification uses reference materials and laboratory analysis.