Signs of plant disease
- Signs include stunted growth, spots on leaves, areas of decay (rot), growths, malformed stems or leaves, discolouration and the presence of pests.
- Diseases can be identified using a gardening manual or website, by sending plants to a laboratory, or with testing kits that use monoclonal antibodies.
Plant pathogens and pests
- Plants can be infected by viruses (such as tobacco mosaic virus), bacteria and fungi (such as rose black spot).
- Insects such as aphids are pests: they feed on sap from the phloem and can spread pathogens.
Mineral ion deficiencies
- Nitrate ions are needed to make proteins. A lack of nitrate causes stunted growth.
- Magnesium ions are needed to make chlorophyll. A lack of magnesium causes chlorosis: yellow leaves, which reduces photosynthesis.
Physical defences
- Cellulose cell walls are tough and hard for pathogens to break through.
- A tough waxy cuticle on leaves stops pathogens getting in.
- Layers of dead cells on the surface, such as bark, fall off and take pathogens with them.
Chemical and mechanical defences
- Chemical: antibacterial chemicals, for example in mint and witch hazel, and poisons that deter herbivores, for example in foxgloves and tobacco.
- Mechanical: thorns and hairs deter animals; leaves that droop or curl when touched; mimicry that tricks animals, for example looking as if eggs have already been laid.
Key terms
- Chlorosis
- Yellowing of leaves caused by a lack of chlorophyll, often due to magnesium deficiency.
- Nitrate ions
- Mineral ions needed by plants to make proteins for growth.
- Magnesium ions
- Mineral ions needed by plants to make chlorophyll.
- Aphid
- A sap-sucking insect pest that feeds from the phloem.
- Physical defence
- A structural barrier that stops pathogens entering, such as a waxy cuticle or bark.
- Chemical defence
- A chemical made by a plant to kill pathogens or deter animals.
- Mechanical defence
- A feature, such as thorns or hairs, that deters animals from eating a plant.
- Mimicry
- Looking like something else in order to trick animals.