Key Takeaways: Plant Diseases

1. Types of Plant Pathogens

  • Viruses:
    • Example: Tobacco Mosaic Virus (TMV).
    • Effects: Causes mosaic-like discolouration on leaves, reducing photosynthesis. Survives in soil below 40∘C40∘C.
    • Economic Impact: Costs ~$60 billion/year globally.
  • Bacteria:
    • Example: Pseudomonas syringae (infects snapdragons, celery).
    • Effects: Discoloured leaves, increased frost susceptibility.
  • Fungi:
    • Example: Rose Black Spot (Diplocarpon rosae).
    • Effects: Purple/black spots on leaves → leaf drop. Treated with antifungal sprays.
  • Insects:
    • Example: Aphids (suck sap from stems).
    • Effects: Stunted growth, wilting, curled leaves. Controlled by ladybirds (natural predators).
  • Nematodes:
    • Example: Potato cyst nematodes (Globodera).
    • Effects: Parasitic in roots → reduced crop yields.

2. Symptoms & Effects of Diseases

  • General Symptoms:
    • Stunted growth, spots, decay, discolouration (e.g., chlorosis), pests.
  • Key Examples:
    • TMV: Mosaic patterns → reduced glucose production.
    • Rose Black Spot: Leaf death → defoliation.
    • Aphids: Sap loss → curled leaves.

3. Detection & Identification

  • Methods:
    • Visual inspection (e.g., books, internet).
    • Laboratory testing (e.g., monoclonal antibodies).
  • Tip: Focus on TMV, black spot, and aphids for exams.

4. Deficiency Diseases

  • Nitrate Deficiency:
    • Role: Needed for protein synthesis.
    • Symptoms: Stunted growth.
  • Magnesium Deficiency:
    • Role: Required for chlorophyll (Mg2+Mg2+ in chlorophyll structure).
    • Symptoms: Chlorosis (yellow leaves).
  • Absorption: Active transport (against concentration gradient: ΔCΔC).

5. Plant Defence Mechanisms

  • Physical Defences:
    • Cellulose cell walls: Structural barrier.
    • Waxy cuticle: Reduces water loss + pathogen entry.
    • Bark: Dead outer layer sheds pathogens.
  • Chemical Defences:
    • Antibacterial compounds: Mint, witch hazel.
    • Poisons: Foxglove (digitalis), deadly nightshade.
  • Adaptations:
    • Thorns/spines: Deter herbivores.
    • Mimicry: Passiflora (butterfly egg spots).

6. Experiments & Data Analysis

  • Nutrient Deficiency Experiment:
    • Method: Grow radish seedlings in soils lacking specific nutrients (N, K, Mg, P). Compare growth to nutrient-rich soil.
    • Measurements: Use rulers (height), mass balance (biomass).
  • Spectrophotometer Experiment:
    • Key Concept: Turbidity (cloudiness) indicates bacterial growth. Lower % transmission = more bacteria.
    • Control: Essential for comparison (e.g., no essential oil).

7. Exam Tips & Tricks

  • Mnemonics:
    • “Mag Chloro”: Magnesium → Chlorophyll → Chlorosis.
    • “Nitrate for kNitrogen”: Nitrates → Proteins.
  • Common Mistakes:
    • Confusing viral (mosaic patterns) vs. fungal (black spots) symptoms.
    • Misidentifying chlorosis (deficiency) vs. pathogen-induced discolouration.
  • Key Equations:
    • Active transport: Requires ATP (ATP→ADP+PiATP→ADP+Pi​).

Show You Can…

  • Explain how TMV, rice blast, and black spot affect plants.
  • Design an experiment to test nutrient deficiencies.
  • Compare physical and chemical plant defences.

Revision Checklist:
☑ Know 3 pathogens (virus, bacterium, fungus) + examples.
☑ Understand deficiency symptoms for Mg and nitrate.
☑ Describe plant defences with examples.
☑ Interpret experimental data (e.g., % transmission graphs).

50 GCSE Biology Questions on Plant Diseases

Section 1: Types of Plant Pathogens

  1. Name a viral plant disease and describe its symptoms.
  2. What pathogen causes rose black spot?
  3. How do aphids harm plants?
  4. Name two crops affected by Pseudomonas syringae.
  5. What is the economic impact of potato cyst nematodes?

Section 2: Symptoms & Effects

  1. List three symptoms of plant disease.
  2. Why does TMV reduce photosynthesis?
  3. What happens to leaves infected with rose black spot?
  4. Explain how chlorosis affects plant growth.
  5. Why are plants with magnesium deficiency less healthy?

Section 3: Detection & Identification

  1. How do gardeners identify plant diseases?
  2. What laboratory method is used to diagnose plant diseases?

Section 4: Deficiency Diseases

  1. What causes stunted growth in plants?
  2. Why do plants need nitrate ions?
  3. Describe the appearance of a magnesium-deficient plant.
  4. Explain why plants use active transport to absorb minerals.

Section 5: Plant Defence Mechanisms

  1. Name two physical defences plants use against pathogens.
  2. How does bark protect trees?
  3. What chemical defence does foxglove use?
  4. How do Passiflora plants deter butterflies?
  5. Why do some plants produce poisons?

Section 6: Experiments & Data Analysis

  1. Design an experiment to test the effect of nitrate deficiency.
  2. In Figure 8.6, what was the disease incidence at 6 days?
  3. Explain why turbidity increases in bacterial cultures.
  4. Which essential oil had an anomaly in Table 8.1?
  5. Calculate the % reduction in light transmission for rosemary oil over 300 minutes.

Section 7: General Review

  1. What is Dutch elm disease caused by?
  2. How is mistletoe a plant parasite?
  3. Why is rice blast a significant global issue?
  4. What happens to leaves of Mimosa pudica when touched?
  5. Name a plant with antibacterial compounds.
  6. How does the waxy cuticle defend plants?
  7. What is a lesion in plant disease?
  8. Why is burning infected leaves effective against rose black spot?
  9. What is chlorosis?

Section 8: Application & Analysis

  1. Compare TMV and rose black spot symptoms.
  2. Why might farmers prefer ladybirds over insecticides?
  3. Explain how nutrient deficiencies impact food security.
  4. Analyse the data in Figure 8.6: describe trends from day 3–5.
  5. Why is a control essential in the spectrophotometer experiment?

Section 9: Calculations

  1. Calculate the difference in light transmission between control and rosemary oil at 300 minutes.
  2. If a plant loses 30% biomass due to TMV, what is its new mass if original was 50g?

Section 10: Extended Response

  1. Explain the evolutionary battle between plants and pathogens.
  2. Evaluate the effectiveness of chemical vs. biological pest control.
  3. Discuss how plant diseases impact ecosystems.

Section 11: Definitions

  1. Define ‘active transport’.
  2. What is a pathogen?
  3. Define ‘chlorosis’.
  4. What are monoclonal antibodies?
  5. Define ‘turbidity’.

Detailed Answers

  1. Tobacco Mosaic Virus (TMV) causes mosaic-like discolouration on leaves, reducing photosynthesis. Survives in soil below 40∘C40∘C.
  2. Fungus (Diplocarpon rosae).
  3. Aphids suck sap, causing stunted growth, wilting, and curled leaves.
  4. Celery and cucumbers (also snapdragons, plums).
  5. Up to $300 million/year lost in potato yields.
  6. Stunted growth, spots, decay, discolouration, pests.
  7. Discoloured leaves reduce chlorophyll → less glucose production.
  8. Leaves develop black/purple spots → die and drop.
  9. Yellow leaves (lack of chlorophyll) → reduced photosynthesis.
  10. Magnesium is needed for chlorophyll → deficiency causes chlorosis.
  11. Visual inspection (books, internet) or laboratory testing.
  12. Monoclonal antibodies.
  13. Nitrate deficiency (needed for protein synthesis).
  14. To make proteins and amino acids.
  15. Yellow leaves (chlorosis).
  16. Minerals are absorbed against concentration gradient (ΔCΔC) using ATP.
  17. Cellulose cell walls, waxy cuticle, bark.
  18. Dead outer layer sheds pathogens.
  19. Digitalis (treats heart conditions).
  20. Spots mimic butterfly eggs → deter egg-laying.
  21. To deter herbivores from eating them.
  22. Grow plants in nitrate-deficient soil vs. control; measure growth over 4 weeks.
  23. ~60% incidence at 6 days (Figure 8.6).
  24. Bacterial growth increases turbidity → less light transmitted.
  25. Grapeseed oil at 240 minutes (86% → anomaly).
  26. From 100% to 52%: 100−52100×100=48%100100−52​×100=48% reduction.
  27. Fungus (Ophiostoma novo-ulmi).
  28. Mistletoe steals water/nutrients via root-like structures in host branches.
  29. Destroys rice crops → affects 60 million people/year.
  30. Leaves droop/fold → insects fall off.
  31. Mint and witch hazel.
  32. Acts as a waterproof barrier against pathogens.
  33. A diseased area (e.g., wound, ulcer).
  34. Destroys fungal spores to prevent spread.
  35. Yellowing due to lack of chlorophyll.
  36. TMV: mosaic patterns; black spot: dark spots + leaf drop.
  37. Ladybirds are natural predators → eco-friendly.
  38. Deficiencies reduce crop yields → food shortages.
  39. Infection rises sharply from 30% (day 3) to 60% (day 5).
  40. To compare results and ensure validity.
  41. Control: 9% vs. rosemary: 52% → difference = 43%.
  42. New mass = 50g−(0.3×50)=35g50g−(0.3×50)=35g.
  43. Plants evolve defences (e.g., thorns), pathogens evolve to bypass them.
  44. Chemical: fast but harmful; biological: sustainable but slower.
  45. Loss of host plants disrupts food chains (e.g., Dutch elm disease).
  46. Movement of molecules against gradient using ATP.
  47. Disease-causing organism (virus, bacterium, fungus).
  48. Yellowing due to chlorophyll deficiency.
  49. Lab-produced antibodies to identify pathogens.
  50. Cloudiness in liquid due to suspended particles (e.g., bacteria).