Plant Disease

Detection and identification of plant diseases

Signs that a plant may be diseased include:

• stunted growth,
• spots on the leaves,
• patches of decay or rot,
• unusual growths,
• malformed stems or leaves,
• discolouration,
• visible pests such as insects.

To identify the cause, a gardener or scientist can consult a gardening manual or website, send an infected sample to a laboratory, or use a testing kit that contains monoclonal antibodies.

Plants can be affected by pathogens (viruses, bacteria and fungi), by insect pests such as aphids, and by mineral ion deficiencies:

• Nitrate deficiency causes stunted growth, because nitrate ions are needed to make proteins.
• Magnesium deficiency causes chlorosis — yellowing of the leaves — because magnesium ions are needed to make chlorophyll.

Plant defence responses

Plants have three kinds of defence.

Physical defences stop pathogens getting in:

• cellulose cell walls,
• a tough, waxy cuticle on the leaves,
• layers of dead cells on the outside of stems, such as bark, which fall away and take pathogens with them.

Chemical defences:

• antibacterial chemicals that kill bacteria,
• poisons that deter herbivores from eating the plant.

Mechanical adaptations:

• thorns and hairs that put animals off,
• leaves that droop or curl when touched, dislodging insects,
• mimicry that tricks animals, for example leaf markings that look like insect eggs, so butterflies avoid laying their own eggs there.

In questions, match each named feature to the right category — a waxy cuticle is a physical defence, while thorns are a mechanical adaptation.