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Exam questions · Biology · Plant Defences and Disease

Detecting Plant Diseases

  • 6 exam questions
  • 20 marks
  • 9 quick checks
  1. 1 State [2 marks]

    State two symptoms that could show that a plant has a disease.

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    Model answer

    Any two from: spots on the leaves, yellowing, wilting, mould or fungal growth, lesions on bark or stems, stunted growth, galls.

    Mark scheme

    • First symptom — 1 mark
    • Second symptom — 1 mark
  2. 2 State [2 marks]

    State two environmental causes that could make the leaves of a plant turn yellow.

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    Model answer

    Any two from: lack of magnesium, lack of nitrate, too little light, too little or too much water, frost, chemical damage.

    Mark scheme

    • First — 1 mark
    • Second — 1 mark
  3. 3 Explain [3 marks]

    Explain how the pattern of affected plants in a field can help to show whether a disease is caused by a pathogen.

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    Model answer

    If a pathogen is the cause, the affected plants will often be clustered around a point and spread outwards, as the pathogen is passed from plant to plant, and may follow the direction of the wind. If the whole field is affected evenly, the cause is more likely to be an environmental factor such as the soil or the weather.

    Mark scheme

    • Pathogen: clustered and spreading — 1 mark
    • May follow wind or water — 1 mark
    • Even pattern suggests environment — 1 mark
  4. 4 Calculate [3 marks]

    A random sample of 80 plants from a field of 12 000 plants shows that 20 have symptoms of disease. Calculate the percentage of the sample with symptoms, and estimate the number of diseased plants in the field.

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    Model answer

    20 ÷ 80 × 100 = 25%. 25% of 12 000 = 3000 plants.

    Mark scheme

    • 25% — 1 mark
    • 0.25 × 12 000 — 1 mark
    • 3000 plants — 1 mark
  5. 5 Describe [4 marks]

    Describe four ways of identifying the pathogen that is causing a disease in a plant.

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    Model answer

    A sample of infected tissue can be looked at under a microscope to find fungal threads or bacteria. The pathogen can be grown on agar and identified from how it grows. DNA tests can be used to compare the pathogen with known ones. A test kit using monoclonal antibodies can identify a specific pathogen from the colour change or line.

    Mark scheme

    • Microscopy — 1 mark
    • Culturing — 1 mark
    • DNA testing — 1 mark
    • Antibody test kit — 1 mark
  6. 6 Evaluate [6 marks]

    A gardener finds that her tomato plants have yellow leaves and spots. Describe a series of tests that could be used to find out if a pathogen is the cause, and explain the possible results.

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    Model answer

    She should first look at the symptoms and compare them with a guide to find a likely disease. Then she should rule out the environment by checking the soil for minerals such as magnesium and nitrate, checking the watering and the temperature, and she could add fertiliser to see if the plants recover. She should look at where the affected plants are. If they are clustered and spreading, this suggests a pathogen, but if all are equally affected, then the cause is probably the environment. A sample could be taken to a laboratory, to be looked at under a microscope, grown on agar, or tested using DNA or an antibody test, which would identify any pathogen.

    Mark scheme

    • Observe symptoms and compare — 1 mark
    • Check the soil, water and temperature — 1 mark
    • Test by adding fertiliser — 1 mark
    • Look at the distribution — 1 mark
    • Cluster suggests a pathogen, even suggests environment — 1 mark
    • Laboratory tests — 1 mark

Quick check

  1. 1

    What is the first step in detecting a plant disease in the field?

    1. AObserve the symptoms
    2. BDo a DNA test
    3. CSpray with fungicide
    4. DDig up the plant
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    A: Observe the symptoms

    The first step is to observe the symptoms of the plant.

  2. 2

    Which of these could make leaves turn yellow without a pathogen?

    1. AA virus only
    2. BA bacterium only
    3. CA shortage of minerals
    4. DA fungus only
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    C: A shortage of minerals

    A lack of magnesium or nitrate causes yellowing, which is an environmental cause.

  3. 3

    What does a cluster of diseased plants spreading outward suggest?

    1. AA lack of nitrate
    2. BA pathogen
    3. CFrost
    4. DA pH problem
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    B: A pathogen

    A pathogen that spreads from one place to others would give this pattern.

  4. 4

    Which test uses monoclonal antibodies?

    1. AMicroscopy
    2. BSoil analysis
    3. CCulturing on agar
    4. DA test kit that shows a line or colour change
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    D: A test kit that shows a line or colour change

    Antibody test kits use monoclonal antibodies that bind to a specific pathogen.

  5. 5

    Why is a random sample taken from a large field?

    1. ATo kill the disease
    2. BTo make the field smaller
    3. CBecause not every plant can be checked
    4. DTo make the plants grow
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    C: Because not every plant can be checked

    It is not possible to check all of the plants, so a sample is used to estimate the amount of disease.

  6. 6

    In a sample of 40 plants, 8 have symptoms. What is the percentage?

    1. A20%
    2. B8%
    3. C32%
    4. D5%
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    A: 20%

    \(8 \div 40 \times 100 = 20\%\).

  7. 7

    Why is DNA testing a useful way to identify a plant pathogen?

    1. AIt is cheap and requires no equipment
    2. BIt is accurate, and can identify viruses
    3. CIt cures the disease
    4. DIt makes the plant grow
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    B: It is accurate, and can identify viruses

    It is accurate, and can identify viruses that cannot be cultured or seen easily.

  8. 8

    What would evenly distributed damage across a whole field suggest?

    1. AA spreading pathogen
    2. BA single infected plant
    3. CA pest in one corner
    4. DAn environmental factor
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    D: An environmental factor

    An even pattern suggests that the soil, weather or a chemical is the cause.

  9. 9

    Adding fertiliser cures yellowing in the plants. What does this show?

    1. AThe cause was a mineral deficiency
    2. BThe cause was a virus
    3. CThe cause was a fungus
    4. DThe plants were not yellow
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    A: The cause was a mineral deficiency

    It shows that a mineral deficiency was the cause, so a pathogen is not needed to explain the symptoms.