4.3.3.1Detection and identification of plant diseases
AQA GCSE Biology Foundation (8461), Foundation tier · Infection and response › Plant disease
Practise Detection and identification of plant diseases. 3 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The plant pathogens and pests you need (TMV, rose black spot and aphids), and the effects of nitrate and magnesium deficiency. This subtopic is GCSE Biology only (not Combined Science).
Key facts
Examples: TMV (virus), rose black spot (fungus), aphids (insects)
Nitrate ions → protein synthesis; lack → stunted growth
Magnesium ions → chlorophyll; lack → chlorosis
Chlorosis = yellowing of leaves caused by a lack of chlorophyll
Notes
Pathogens and pests
Plants can be infected by viral, bacterial and fungal pathogens, and damaged by insects.
Tobacco mosaic virus (TMV), a virus: a mosaic pattern of discolouration on the leaves.
Rose black spot, a fungus: purple or black spots on the leaves, which turn yellow and drop early.
Aphids, insects: they push their mouthparts into the phloem and feed on the sap, taking sugars the plant needs. They can also carry viruses from plant to plant.
Mineral ion deficiencies
diagram
Plants can also be damaged by a lack of mineral ions in the soil. These ion deficiency conditions are not caused by pathogens.
Nitrate ions are needed for protein synthesis. A lack of nitrate causes stunted growth.
Magnesium ions are needed to make chlorophyll. A lack of magnesium causes chlorosis: the leaves turn yellow. With less chlorophyll there is less photosynthesis, so the plant grows less.
Dashed leaves are yellow (chlorosis).
A deficiency can be put right by adding a fertiliser that contains the missing ion.
Don’t lose marks
Swapping the two ions: magnesium is for chlorophyll, nitrate is for protein synthesis.
Writing 'the plant can't photosynthesise' for magnesium deficiency. Photosynthesis is reduced, not stopped.
Calling an ion deficiency an infection. It is caused by a lack of mineral ions in the soil, not by a pathogen.
Confusing chlorosis (yellowing of leaves) with chlorophyll (the green pigment) or chloroplasts (where photosynthesis happens).
More tips
Memory tricks
Mg for green: magnesium makes chlorophyll, so a lack of it gives yellow leaves (chlorosis).
Nitrate → protein → growth, so a lack of nitrate gives stunted plants.
Exam technique
For deficiency questions, give the full chain: ion → what it is used for → symptom → effect on growth.
Use the evidence in the question to decide between causes, and say which evidence supports your choice.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 5
Give examples of plant pathogens and pestsTobacco mosaic virus (a virus), rose black spot (a fungus) and aphids (insects).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Medium7 marks
Plants need mineral ions from the soil to grow well.
(a) Explain why a plant growing in soil that lacks nitrate ions has stunted growth.[3]
(b) Explain why a plant growing in soil that lacks magnesium ions grows slowly.[3]
(c) Suggest how a gardener could treat plants that lack nitrate ions.[1]
Show the answer and mark scheme
(a)
nitrate ions are needed to make amino acids / proteins
proteins are needed for growth / to make new cells / enzymes
(so) less protein is made and the plant grows less
(b)
magnesium ions are needed to make chlorophyll
(so) less chlorophyll / leaves turn yellow (chlorosis)
(so) less light is absorbed / less photosynthesis, so less glucose for growth
(c)
add a fertiliser containing nitrates (to the soil)