B17: Plant Diseases
Plant pathogens, nutrient deficiencies, physical and chemical defences, and disease detection
Key Definitions
Plant disease — Any condition that affects the normal growth and functioning of a plant, caused by pathogens, environmental factors, or nutrient deficiencies.
Chlorosis — The yellowing of leaves due to the loss of chlorophyll; a symptom of several plant diseases and nutrient deficiencies.
Physical defence — Structural features of a plant that prevent pathogens or pests from entering or causing damage.
Chemical defence — Substances produced by plants that deter herbivores or kill pathogens.
Chlorosis — The yellowing of leaves due to the loss of chlorophyll; a symptom of several plant diseases and nutrient deficiencies.
Physical defence — Structural features of a plant that prevent pathogens or pests from entering or causing damage.
Chemical defence — Substances produced by plants that deter herbivores or kill pathogens.
Common Plant Diseases
| Disease | Pathogen Type | Symptoms | Transmission | Treatment/Control |
|---|---|---|---|---|
| Tobacco mosaic virus (TMV) | Virus | Mosaic discolouration on leaves; stunted growth; reduced photosynthesis and yield | Contact between infected and healthy plants; contaminated tools | No cure; remove infected plants; use resistant crop varieties; disinfect tools |
| Ash dieback | Fungus (Chalara fraxinea) | Leaf loss; bark lesions; dieback of shoots and branches; eventually tree death | Spores carried by wind; movement of infected saplings | No cure; remove and destroy infected trees; restrict movement of ash plants; breed resistant trees |
| Bacterial infections (e.g. crown gall) | Bacterium (Agrobacterium) | Tumour-like growths (galls) on stems and roots; stunted growth | Through wounds in the plant surface; contaminated soil; water splashing | Remove infected tissue; avoid wounding plants; use clean tools; destroy severely infected plants |
Nutrient Deficiency Diseases
Plants need mineral ions from the soil for healthy growth. Deficiencies cause specific symptoms:
Nitrate deficiency:
— Nitrates are needed to make amino acids and therefore proteins (for growth and enzymes).
— Without enough nitrate, the plant cannot grow properly — stunted growth.
— The plant cannot synthesise enough chlorophyll, so older leaves may yellow (chlorosis) as the plant moves its limited nitrogen to newer leaves.
Magnesium deficiency:
— Magnesium is needed to make chlorophyll (the magnesium ion is at the centre of the chlorophyll molecule).
— Without enough magnesium, the plant cannot produce chlorophyll — leaves turn yellow (chlorosis).
— Yellowing typically starts in older leaves as magnesium is moved to newer growth.
— Reduced chlorophyll means less photosynthesis, leading to reduced growth and yield.
Nitrate deficiency:
— Nitrates are needed to make amino acids and therefore proteins (for growth and enzymes).
— Without enough nitrate, the plant cannot grow properly — stunted growth.
— The plant cannot synthesise enough chlorophyll, so older leaves may yellow (chlorosis) as the plant moves its limited nitrogen to newer leaves.
Magnesium deficiency:
— Magnesium is needed to make chlorophyll (the magnesium ion is at the centre of the chlorophyll molecule).
— Without enough magnesium, the plant cannot produce chlorophyll — leaves turn yellow (chlorosis).
— Yellowing typically starts in older leaves as magnesium is moved to newer growth.
— Reduced chlorophyll means less photosynthesis, leading to reduced growth and yield.
| Deficiency | Mineral Ion Needed For | Symptom | Why the Symptom Occurs |
|---|---|---|---|
| Nitrate deficiency | Amino acids and proteins for growth | Stunted growth; yellowing of older leaves | Cannot make enough protein for cell growth and new tissue; limited chlorophyll synthesis |
| Magnesium deficiency | Chlorophyll production | Chlorosis (yellow leaves); reduced growth | Cannot make chlorophyll — less light absorption, less photosynthesis, less glucose for growth |
Example 1: Identifying Nutrient Deficiency from Symptoms
A farmer notices that his crop plants are growing very slowly but their leaves are still green. This suggests nitrate deficiency — the lack of nitrates limits protein synthesis and growth, but the leaves are green because magnesium is available for chlorophyll production. If the leaves were yellow but growth was otherwise normal, this would suggest magnesium deficiency — chlorophyll cannot be made, but protein synthesis and growth are not directly affected.
Example 2: How Magnesium Deficiency Reduces Yield
Magnesium is essential for chlorophyll synthesis. Without chlorophyll, the plant cannot absorb light energy for photosynthesis. Less photosynthesis means less glucose is produced, so the plant has less energy for growth and for producing fruits or storage organs (e.g. potatoes). This directly reduces the crop yield. The yellow (chlorotic) leaves are a visible sign of this problem.
Plant Physical Defences
Physical barriers that prevent pathogen entry:
— Cellulose cell wall — A rigid layer surrounding every plant cell; provides a tough physical barrier that most pathogens cannot penetrate directly.
— Waxy cuticle — A waterproof layer on the upper surface of leaves; prevents water from collecting on the leaf surface, reducing the opportunity for fungal spores to germinate and enter the leaf.
— Bark — A thick, tough outer layer on stems and roots of woody plants; contains antimicrobial chemicals and is difficult for pathogens to penetrate.
— Thorns and hairs — Deter herbivores from eating the plant, reducing damage that could create entry points for pathogens.
— Stomatal closure — Guard cells can close stomata when under attack, reducing entry points for pathogens.
— Falling leaves — Plants can shed infected leaves to prevent the spread of disease to the rest of the plant.
— Cellulose cell wall — A rigid layer surrounding every plant cell; provides a tough physical barrier that most pathogens cannot penetrate directly.
— Waxy cuticle — A waterproof layer on the upper surface of leaves; prevents water from collecting on the leaf surface, reducing the opportunity for fungal spores to germinate and enter the leaf.
— Bark — A thick, tough outer layer on stems and roots of woody plants; contains antimicrobial chemicals and is difficult for pathogens to penetrate.
— Thorns and hairs — Deter herbivores from eating the plant, reducing damage that could create entry points for pathogens.
— Stomatal closure — Guard cells can close stomata when under attack, reducing entry points for pathogens.
— Falling leaves — Plants can shed infected leaves to prevent the spread of disease to the rest of the plant.
Plant Chemical Defences
Chemical substances that deter pests or kill pathogens:
— Antimicrobial substances — Many plants produce chemicals that kill bacteria and fungi (e.g. mint produces menthol, which has antimicrobial properties; garlic produces allicin).
— Toxins — Some plants produce poisonous chemicals that deter herbivores from eating them (e.g. foxglove produces digitalis, which is toxic to animals; deadly nightshade produces atropine).
— Tannins — Bitter-tasting chemicals in leaves that make the plant unpalatable to herbivores; also interfere with insect digestion.
— Nicotine — Produced by tobacco plants; toxic to insects and deters herbivory.
— Phytoalexins — Chemicals produced rapidly by plant cells when they are attacked by a pathogen; these have antimicrobial properties and help limit the spread of infection.
— Antimicrobial substances — Many plants produce chemicals that kill bacteria and fungi (e.g. mint produces menthol, which has antimicrobial properties; garlic produces allicin).
— Toxins — Some plants produce poisonous chemicals that deter herbivores from eating them (e.g. foxglove produces digitalis, which is toxic to animals; deadly nightshade produces atropine).
— Tannins — Bitter-tasting chemicals in leaves that make the plant unpalatable to herbivores; also interfere with insect digestion.
— Nicotine — Produced by tobacco plants; toxic to insects and deters herbivory.
— Phytoalexins — Chemicals produced rapidly by plant cells when they are attacked by a pathogen; these have antimicrobial properties and help limit the spread of infection.
Example 3: How Plant Defences Work Together
A rose plant has multiple layers of defence. The waxy cuticle prevents water and fungal spores from settling on the leaf surface. The cellulose cell wall makes it hard for pathogens to penetrate individual cells. Thorns on the stem deter herbivores from eating the plant, reducing wounds where pathogens could enter. If a pathogen does penetrate, the plant produces phytoalexins (antimicrobial chemicals) to limit the spread. If a leaf is severely infected, the plant can shed it to prevent the disease from spreading further.
Detecting and Identifying Plant Disease
Methods for detecting and identifying plant disease:
— Observation of symptoms — Visible signs such as yellowing leaves, spots, stunted growth, or lesions can indicate the type of disease or deficiency.
— Testing kits — Commercial kits can identify specific pathogens (e.g. using monoclonal antibodies to detect viral antigens).
— Gardening manuals and websites — Reference guides that allow identification of diseases from symptoms and photographs.
— Laboratory analysis — Sending samples to a lab for microscopic examination, culture, or DNA testing to identify the pathogen precisely.
— Monitoring — Regular inspection of crops for early signs of disease, allowing prompt treatment before the disease spreads.
— Observation of symptoms — Visible signs such as yellowing leaves, spots, stunted growth, or lesions can indicate the type of disease or deficiency.
— Testing kits — Commercial kits can identify specific pathogens (e.g. using monoclonal antibodies to detect viral antigens).
— Gardening manuals and websites — Reference guides that allow identification of diseases from symptoms and photographs.
— Laboratory analysis — Sending samples to a lab for microscopic examination, culture, or DNA testing to identify the pathogen precisely.
— Monitoring — Regular inspection of crops for early signs of disease, allowing prompt treatment before the disease spreads.
Example 4: Diagnosing a Plant Problem
A gardener notices that the leaves on their rose bush have purple-black spots. By consulting a gardening manual or website, they can identify this as rose black spot (a fungal disease). They then know to remove and burn the affected leaves and apply a fungicide. If the symptoms were instead uniform yellowing of older leaves with normal growth, the gardener might suspect magnesium deficiency and add a magnesium-containing fertiliser. Correct identification is essential for choosing the right treatment.
Exam tip: When asked about plant defences, clearly distinguish between physical defences (structural barriers: cell wall, cuticle, bark, thorns) and chemical defences (antimicrobial substances, toxins, tannins, phytoalexins). Use named examples where possible. For nutrient deficiencies, remember: nitrate → stunted growth; magnesium → chlorosis (yellow leaves).
Practice Questions
1. Foundation Describe the symptoms of nitrate deficiency and magnesium deficiency in plants and explain why each symptom occurs.
Nitrate deficiency causes stunted growth because nitrates are needed to make amino acids and proteins for cell growth. Magnesium deficiency causes chlorosis (yellowing of leaves) because magnesium is needed to make chlorophyll; without it, the plant cannot produce chlorophyll, so leaves lose their green colour and cannot photosynthesise efficiently.
2. Higher Describe three physical defences that plants have against pathogens and explain how each works.
1) Cellulose cell wall — a rigid barrier around each cell that pathogens cannot easily penetrate. 2) Waxy cuticle — a waterproof layer on leaves that prevents water from collecting on the surface, reducing the opportunity for fungal spores to germinate. 3) Bark — a thick, tough outer layer on woody stems that is difficult for pathogens to penetrate and may contain antimicrobial chemicals. Also acceptable: thorns (deter herbivores, reducing wounds for pathogen entry), stomatal closure (reduces entry points).
3. Foundation Explain how a farmer could identify whether a crop is affected by TMV or a nutrient deficiency.
TMV causes a distinctive mosaic pattern of discolouration on leaves (patches of light and dark green/yellow), whereas nutrient deficiencies cause more uniform symptoms: nitrate deficiency causes stunted growth, and magnesium deficiency causes overall yellowing. The farmer could also use a testing kit (e.g. monoclonal antibody test for TMV) or send samples to a laboratory for analysis. Gardening manuals can help match symptoms to specific diseases.
4. Higher Describe two chemical defences that plants produce and explain how each helps protect the plant.
1) Antimicrobial substances — e.g. mint produces menthol, which kills bacteria and fungi, preventing infection. 2) Toxins — e.g. foxglove produces digitalis, which is poisonous to herbivores, deterring them from eating the plant and reducing damage that could allow pathogen entry. Also acceptable: tannins (bitter taste deters herbivores, interfere with insect digestion), phytoalexins (produced rapidly after infection, have antimicrobial properties).
5. Higher Explain why ash dieback is difficult to control once it has infected a tree.
Ash dieback is caused by a fungus whose spores are easily spread by wind over long distances, making it hard to contain. There is no cure once a tree is infected. The fungus grows inside the tree's vascular system, making it impossible to treat with surface-applied fungicides. The disease can persist in leaf litter and soil, reinfecting new growth. Control relies on removing and destroying infected trees, but this is difficult on a large scale. Breeding resistant trees is a long-term solution but takes many years.
🔢 Maths Skills
Mathematical Skills
Interpreting nutrient deficiency data: read tables showing mineral ion concentrations in soil and corresponding plant growth measurements. Calculate percentage differences in growth between deficient and control groups.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Plant diseases are always caused by pathogens Correct: Plant diseases can also be caused by nutrient deficiencies (e.g. nitrate or magnesium deficiency) and environmental factors (e.g. lack of water, extreme temperatures)
✍️ 6-Mark Question
Extended Answer
6 marks: Explain how plants defend against pests and pathogens.
Plants have both physical and chemical defences. Physical defences include: cellulose cell walls that form a tough barrier preventing pathogen entry; a waxy cuticle on leaves that stops water collecting and reduces fungal spore germination; bark on woody stems that is hard to penetrate; thorns and hairs that deter herbivores, reducing wounds where pathogens could enter; and stomatal closure to block pathogen entry. Chemical defences include: antimicrobial substances (e.g. menthol from mint) that kill bacteria and fungi; toxins (e.g. digitalis from foxglove) that poison herbivores; tannins that make leaves unpalatable; and phytoalexins produced rapidly after infection to limit spread.
Mark scheme: 1 mark for three physical defences with explanation; 1 mark for two chemical defences with explanation; 1 mark for named examples; 1 mark for explaining how defences reduce pathogen entry; 1 mark for explaining how defences deter herbivores; 1 mark for QWC
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A gardener observes the following symptoms on three plants: Plant A has purple-black spots on leaves; Plant B has overall yellowing of older leaves with normal growth; Plant C has stunted growth with pale green leaves.
1. Diagnose each plant with a specific disease or deficiency, explaining your reasoning. 2. Suggest appropriate treatments for each plant. 3. The gardener uses a monoclonal antibody test kit on Plant A and it confirms TMV. Evaluate whether the symptoms alone were sufficient for diagnosis, or whether testing kits provide essential additional information.
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