Exam Tips: Plant Diseases and Defenses

Exam Tips: Plant Diseases and Defenses

Key Command Words

• "Name": Give specific examples (rose black spot, TMV)
• "Describe": Give symptoms or characteristics
• "Explain": Give reasons why defenses work or diseases affect growth
• "Compare": Show similarities AND differences between plant and human defenses

Common Mistakes to Avoid

• Confusing physical and chemical defenses
• Forgetting to explain HOW TMV affects photosynthesis
• Not linking environmental conditions to disease spread
• Giving vague symptoms instead of specific examples
• Saying yellow leaves always mean disease — mineral deficiency (nitrate or magnesium) also causes chlorosis
• Confusing nitrate and magnesium deficiency — nitrate = proteins/stunted growth/older leaves first; magnesium = chlorophyll/all leaves yellow

Required Practical Links

• Know how to design investigations to test disease treatments
• Understand control variables in field studies
• Be able to identify symptoms and suggest causes
• Link field observations to economic and social impacts

Quick Memory Aids

• Defense types: "Physical-Chemical" — two layers, work together
• Physical defenses: "Waxy-Walls-Bark-Thorns"
• TMV effects: "Mosaic then Less chlorophyll then Poor photosynthesis then Stunted growth"
• Rose black spot: "Spots then Yellow then Drop then Dead"

Exam Technique

• Name both diseases and their pathogen type. If asked to give an example, always give the full name (rose black spot) AND state it is caused by a fungus. One-word answers often score zero.
• Explain the full consequence chain for TMV. Virus disrupts chloroplasts → mosaic pattern → less chlorophyll → less light absorbed → less photosynthesis → less glucose → stunted growth. Each arrow represents a mark point.
• Distinguish physical from chemical. Physical defenses prevent entry (cuticle, cell walls, bark, thorns). Chemical defenses kill pathogens that have entered (antimicrobial compounds, toxins). For full marks, give one of each type.
• When comparing plant and animal defenses, be specific. Write: "plants lack mobile immune cells and specific antibodies; instead they rely on physical barriers such as the waxy cuticle and chemical compounds such as antimicrobial proteins".
• Spread of rose black spot — include conditions. Mark schemes award marks for stating that spores spread in moist/wet conditions or via water splash/wind.