Plant disease — detection, defence and deficiency symptoms
Plants get sick too. The GCSE biology Higher-tier specification expects you to identify, diagnose and explain plant diseases — both pathogenic and nutritional.
Detecting and identifying plant diseases
A diseased plant might show:
- Stunted growth — overall poor size
- Spots on leaves — rose black spot, blights
- Areas of decay — rotting fruit
- Mosaic patches of yellow / pale green — TMV
- Discolouration — leaves yellowing (chlorosis)
- Visible pests — aphids, caterpillars
- Malformed stems / leaves — stunted, twisted growth
Identification methods:
- Reference to a gardener's manual / website — match symptoms to known diseases.
- Take infected plant to a lab for DNA / antibody test — definitive identification.
- Use testing kits containing monoclonal antibodies specific to a particular pathogen — quick, in-the-field diagnosis.
Plant defence responses
Plants can't make antibodies, but they have impressive defences in three categories:
Physical:
- Cellulose cell walls — hard for pathogens to penetrate.
- Tough waxy cuticle — physical barrier on leaves.
- Bark on trees — layers of dead cells; falls off, taking pathogens with it.
Chemical:
- Antibacterial chemicals — e.g. mint produces menthol; marigolds release pyrethrins.
- Poisons to deter herbivores — e.g. tobacco plants make nicotine; foxgloves contain digitalis.
Mechanical:
- Thorns and hairs — deter feeding herbivores.
- Leaves that droop or curl when touched — Mimosa pudica responds to insect activity.
- Mimicry — looking like an unhealthy or already-eaten plant; passion flowers have egg-shaped marks that fool butterflies.
Mineral ion deficiencies
Plants need certain ions to make essential molecules. If they don't get them from the soil, you see specific deficiency symptoms:
| Ion | Used to make | Deficiency symptom |
|---|---|---|
| Nitrate (NO₃⁻) | Amino acids → proteins | Stunted growth, pale yellow older leaves (chlorosis) |
| Magnesium (Mg²⁺) | Chlorophyll | Chlorosis (yellow leaves) — limited photosynthesis |
Why these particular symptoms? A plant short of nitrate cannot make new proteins, so growth stops; older leaves are stripped of nitrogen first. A plant short of magnesium cannot make chlorophyll, so leaves turn yellow and photosynthesis falls.
⚠Common mistakes— Common mistakes / exam traps
- "Plants make antibodies" — they don't; they use chemical / physical / mechanical defences instead.
- Confusing TMV (virus) symptoms with magnesium deficiency — both involve discolouration but TMV gives a patchy mosaic; Mg deficiency tends to be uniform yellowing.
- "Plants get diseases from the soil only" — they can also be infected by airborne fungal spores, sap-feeding insects (vectors) or contaminated tools.
- "Bark is just dead skin" — it actively protects, and shedding helps remove pathogens.
Links
Connects to B3.1 (TMV, rose black spot), B3.3 (monoclonal antibody diagnosis) and B7 (mineral cycling and plant nutrition).
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