Detection and identification of plant diseases — AQA GCSE Biology
Test yourself on Detection and identification of plant diseases with AQA GCSE practice questions.
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Detection and identification of plant diseases explained
Plants show visible signs when they are diseased, and gardeners and scientists use these signs to detect a problem before identifying the cause.
Read the full explanation
Stunted growth means the plant is smaller than expected for its age and conditions. Spots on leaves may be discoloured patches caused by a pathogen. Areas of decay or rot are soft, broken-down tissue, often on fruit, roots or stems. Growths are abnormal lumps, such as crown galls. Malformed stems or leaves are twisted or misshapen. Discolouration is a change in the normal colour, such as yellowing. The presence of pests, such as insects on the plant, is itself a sign to look for. These signs indicate disease but do not by themselves identify the pathogen.
(HT only) Identification can be made by: • reference to a gardening manual or website • taking infected plants to a laboratory to identify the pathogen • using testing kits that contain monoclonal antibodies.
Once a plant shows disease symptoms, a grower must work out the cause before choosing a treatment. A gardener can compare the visible signs, such as leaf spots or distorted growth, with photographs and descriptions in a gardening manual or a reliable website; this is quick and free but depends on correct matching. If the cause is unclear, a sample of the infected plant can be sent or taken to a laboratory, where specialists culture or examine the pathogen and identify it accurately. Testing kits containing monoclonal antibodies are also used: the antibodies bind only to a specific pathogen antigen, giving a rapid result, often on a dipstick. Each method trades speed, cost and accuracy, so the choice depends on the situation.
Plants can be infected by a range of viral, bacterial and fungal pathogens as well as by insects.
Plants are not attacked by a single type of organism; viruses, bacteria, fungi and insects can all cause disease or damage. Viruses, such as tobacco mosaic virus, are tiny pathogens that reproduce inside plant cells and often cause mosaic patterns or distorted growth. Bacteria are single-celled pathogens that can multiply in plant tissue and cause wilting, spots or soft rot. Fungi, such as black spot, grow on or inside plants and often produce visible spots, moulds or decay. Insects, such as aphids, are not pathogens in the same way but damage plants directly by feeding and can also transmit pathogens. Recognising the broad groups helps a grower predict how a disease spreads and what control method may work.
Knowledge of plant diseases is restricted to tobacco mosaic virus as a viral disease, black spot as a fungal disease and aphids as insects.
For this specification, you only need detailed knowledge of three named examples. Tobacco mosaic virus is a viral disease that causes a mosaic pattern of discoloured patches on leaves, reducing photosynthesis and stunting growth. Black spot is a fungal disease, common on roses, that produces dark spots on leaves and can cause leaf loss. Aphids are insects that feed on plant sap and can weaken plants; they also act as vectors, transmitting viruses between plants. You should be able to name each example, state which group it belongs to, describe its visible effect and explain how it harms the plant. Other diseases may appear in questions as unfamiliar contexts, but the required named examples are these three.
Plants can be damaged by a range of ion deficiency conditions: • stunted growth caused by nitrate deficiency • chlorosis caused by magnesium deficiency.
Not all plant problems are caused by pathogens; a lack of essential mineral ions also damages plants. Nitrate ions are needed to make amino acids and proteins, so a plant short of nitrate cannot build enough protein for new cells and shows stunted growth, with older leaves often affected first. Magnesium ions are needed to make chlorophyll, the green pigment that absorbs light for photosynthesis. A plant short of magnesium cannot make enough chlorophyll, so its leaves become pale or yellow, a condition called chlorosis. The two deficiencies produce different symptoms because the ions have different roles. Recognising these symptoms helps a grower distinguish mineral deficiency from disease and choose the right treatment, such as adding fertiliser.
Knowledge of ions is limited to nitrate ions needed for protein synthesis and therefore growth, and magnesium ions needed to make chlorophyll.
Plants absorb mineral ions from soil water, and two are specified. Nitrate ions supply nitrogen that is built into amino acids, which are joined into proteins; proteins include enzymes and structural components, so without nitrate ions a plant cannot synthesise protein and its growth is stunted, with older leaves often yellowing first. Magnesium ions are incorporated into chlorophyll, the green pigment that absorbs light for photosynthesis; a shortage causes chlorosis, so leaves look pale or yellow and less light energy is captured, reducing glucose production and growth. A useful method is to compare plants given complete nutrient solution with those lacking one ion, then link each deficiency symptom to its biochemical role.
Your focus
- List the visible signs that can indicate plant disease.
- Describe each sign accurately, including stunted growth, spots, decay, growths, malformation, discolouration and pests.
- Explain that these signs detect disease but do not by themselves identify the pathogen.
Show all 18 objectives
- Describe how a gardening manual or website can be used to identify a plant disease from symptoms.
- Explain how laboratory testing and monoclonal antibody test kits identify plant pathogens.
- Compare the three identification methods in terms of speed, cost and accuracy.
- Identify viruses, bacteria, fungi and insects as causes of plant disease or damage.
- Describe one characteristic feature of each group in relation to plants.
- Explain how insects can both damage plants and transmit pathogens.
- Name tobacco mosaic virus, black spot and aphids as the required plant disease examples.
- State the group to which each named example belongs.
- Describe the visible effect of each example on a plant.
- State the symptoms caused by nitrate deficiency and magnesium deficiency in plants.
- Explain why nitrate deficiency causes stunted growth and magnesium deficiency causes chlorosis.
- Distinguish mineral ion deficiency symptoms from those caused by pathogens.
- State that nitrate ions are needed for protein synthesis and therefore growth.
- State that magnesium ions are needed to make chlorophyll.
- Relate deficiency symptoms such as stunted growth or chlorosis to the missing ion.
Detection and identification of plant diseases exam tips
Marking Points
- Stunted growth is when a plant is smaller than expected for its age and growing conditions.
- Spots on leaves are discoloured patches that can indicate infection by a pathogen.
- Areas of decay or rot are regions of soft, broken-down plant tissue.
- Growths are abnormal lumps on the plant, and malformed stems or leaves are twisted or misshapen.
- Discolouration is a change from the normal colour, such as yellowing of leaves.
- The presence of pests, such as insects, is a visible sign that can be recorded when detecting disease.
- Reference sources such as a gardening manual or website allow a grower to match visible symptoms to a likely cause without specialist equipment.
- Laboratory identification involves sending or taking infected plant material so that a specialist can identify the pathogen directly.
- Testing kits use monoclonal antibodies that bind specifically to antigens of the target pathogen, giving a rapid and specific result.
- Monoclonal antibodies are produced from a single clone of cells, so they are identical and bind to one specific antigen.
- The three methods differ in speed, cost and reliability, so the most suitable method depends on the context.
- Identification is needed before appropriate control measures can be chosen, because different pathogens need different treatments.
- Plants can be infected by viral pathogens, which reproduce inside plant cells and cause symptoms such as mosaic patterns.
- Plants can be infected by bacterial pathogens, which are single-celled and can cause wilting, spots or rotting tissue.
- Plants can be infected by fungal pathogens, which grow on or in plant tissue and often produce visible spots or decay.
- Insects can damage plants directly by feeding and can also act as vectors that transmit pathogens.
- Different pathogen groups have different structures and reproduction methods, which affects how disease spreads and is controlled.
- A single plant can be affected by more than one type of pathogen or pest at the same time.
- Tobacco mosaic virus is the named viral disease and causes mosaic discolouration of leaves.
- Black spot is the named fungal disease and causes dark spots on leaves, often leading to leaf loss.
- Aphids are the named insect pest and feed on plant sap, weakening the plant.
- Aphids can act as vectors, transmitting viral pathogens from plant to plant.
- Tobacco mosaic virus reduces photosynthesis because discoloured areas of leaf contain less chlorophyll.
- Each example belongs to a different group, so the three together illustrate viral, fungal and insect causes of plant problems.
- Nitrate deficiency causes stunted growth because nitrate is needed to make amino acids and proteins for growth.
- Magnesium deficiency causes chlorosis because magnesium is needed to make chlorophyll.
- Chlorosis is the yellowing or paling of leaves due to reduced chlorophyll.
- Stunted growth means the plant is smaller or grows less than expected for its age and conditions.
- Mineral ion deficiency is a nutritional problem, not an infection by a pathogen.
- The symptoms differ because nitrate is used for proteins while magnesium is used for chlorophyll, so each deficiency has a distinct effect.
- Nitrate ions are absorbed from the soil and their nitrogen is used to make amino acids, which are assembled into proteins.
- Because proteins include enzymes and growth materials, a shortage of nitrate ions reduces protein synthesis and therefore restricts growth.
- Magnesium ions are needed to make chlorophyll, the pigment that absorbs light energy in photosynthesis.
- A shortage of magnesium ions causes chlorosis, so less light is absorbed and less glucose is made, which also limits growth.
- A valid comparison is a plant given all mineral ions against one deprived of nitrate or magnesium, with leaf colour and growth measured.
- The specification limits required ion knowledge to nitrate ions and magnesium ions, so other named ions are not needed.
Examiner Tips
- 💡Learn the seven signs as a list and be ready to match each to a description.
- 💡When asked to detect disease, name the sign you observe before suggesting a cause.
- 💡Use precise wording such as stunted growth, discolouration and areas of decay.
- 💡Link each method to a practical advantage or limitation, such as speed, cost or accuracy, rather than listing methods alone.
- 💡When describing test kits, name monoclonal antibodies and explain specific binding to pathogen antigens.
- 💡Apply the methods to a scenario, for example advising a gardener who finds spotted leaves on a tomato plant.
- 💡Name the group and give one example or symptom for each type of pathogen or pest.
- 💡Use the term vector correctly when describing insects that transmit pathogens.
- 💡If asked to compare groups, focus on one clear difference such as cell structure or method of reproduction.
- 💡Learn the group for each named example so you can classify it quickly in a multiple-choice or short-answer question.
- 💡Link each example to its effect on the plant, such as reduced photosynthesis or leaf loss.
- 💡Use the term vector when explaining how aphids spread viruses.
- 💡Link each ion to its role in the plant before stating the symptom, so the answer explains why the symptom occurs.
- 💡Use the exact terms stunted growth and chlorosis when they apply.
- 💡If a question gives a fertiliser or soil test result, use it to decide which ion is deficient.
- 💡Name the ion and its product in the same sentence, for example nitrate ions for protein synthesis and magnesium ions for chlorophyll.
- 💡When explaining poor growth, trace the chain: missing ion, missing molecule, reduced process, reduced growth.
- 💡Use deficiency symptoms as evidence, such as yellow leaves indicating chlorosis from insufficient magnesium.
Common Mistakes
- Treating a visible sign as proof of a named pathogen: the sign shows disease is present, and further identification is needed to name the cause.
- Confusing stunted growth with malformed growth: stunted means smaller than expected, while malformed means misshapen.
- Forgetting that pests count as a detectable sign: the presence of pests is included in the list of signs to look for.
- Thinking any antibody will detect any pathogen; correction: monoclonal antibodies are specific, so a kit detects only the pathogen whose antigen matches the antibody.
- Assuming laboratory testing is always the first step; correction: a manual or website is often used first because it is quick and cheap, with laboratory testing reserved for uncertain cases.
- Confusing identification with treatment; correction: identification establishes the cause, and treatment is a separate later decision.
- Treating insects as a type of pathogen; correction: insects are animals that damage plants and can carry pathogens, but they are not themselves viral, bacterial or fungal pathogens.
- Assuming all plant diseases are caused by fungi; correction: viruses, bacteria and insects also cause disease or damage.
- Believing bacteria and viruses are the same size and treated the same way; correction: viruses are much smaller and reproduce inside cells, while bacteria are cells that can often be treated with antibiotics in other contexts, though plant control differs.
- Calling tobacco mosaic virus a fungus; correction: it is a virus, and black spot is the fungal example.
- Describing aphids as a disease; correction: aphids are insects that damage plants and can transmit disease.
- Confusing the visible symptoms of black spot and tobacco mosaic virus; correction: black spot gives dark spots, while tobacco mosaic virus gives a mosaic pattern of discolouration.
- Mixing up the two deficiencies; correction: nitrate deficiency causes stunted growth, while magnesium deficiency causes chlorosis.
- Thinking chlorosis is caused by lack of nitrate; correction: chlorosis is linked to magnesium deficiency and reduced chlorophyll.
- Assuming all yellowing leaves mean disease; correction: yellowing can be a mineral deficiency, so other symptoms and context must be considered.
- Saying magnesium is needed for protein synthesis: correct this by linking magnesium to chlorophyll and nitrate to protein.
- Saying nitrate ions are used directly as an energy source: correct this by stating that nitrate supplies nitrogen for amino acids and proteins.
- Confusing chlorosis with wilting: correct this by describing chlorosis as pale or yellow leaves caused by too little chlorophyll, not loss of water support.