Detecting plant disease: the signs
A sign (symptom) is something we can see that tells us the plant is not well. Look for:
- stunted growth: the plant stays short and small
- spots on leaves
- areas of decay (rotting)
- growths or lumps
- malformed (wrongly shaped) stems or leaves
- discolouration: yellow, brown or pale patches
- pests you can see, such as tiny insects
Causes: pathogens, insects and mineral deficiency
Pathogens
A pathogen is a germ that causes disease. Example virus: tobacco mosaic virus makes a yellow-and-green mosaic on leaves, so the plant does less photosynthesis and grows less. Example fungus: rose black spot makes purple-black spots; leaves turn yellow and drop early.
Insects
Aphids are tiny insects that suck sap (sugar-rich liquid) from the phloem. The plant loses food, and aphids can carry viruses from plant to plant.
Lack of mineral ions
Plants take mineral ions from the soil.
- Nitrate is needed to make proteins. Too little = stunted growth.
- Magnesium is needed to make chlorophyll. Too little = yellow leaves, called chlorosis.
These are not caused by germs, so medicine will not help; fertiliser will.
Identifying the disease
- Compare the signs with a gardening manual or a trusted website.
- Send a sample of the plant to a laboratory to find the pathogen.
- Use a testing kit. Many kits contain monoclonal antibodies that stick only to one pathogen and show a colour if it is there.
Plant defence responses
Physical defences (barriers to germs)
- A waxy cuticle on leaves stops germs getting in.
- Cellulose cell walls are tough to break.
- Layers of dead cells on stems, like bark, act as a shield and fall off with germs on them.
Chemical defences
- Antibacterial chemicals kill bacteria (neem and many herbs make them).
- Poisons stop animals eating the plant (for example tobacco, foxglove).
Mechanical defences (against animals)
- Thorns and hairs stop animals touching or eating the plant.
- Leaves that droop or curl when touched shake off insects (touch-me-not, Mimosa).
- Mimicry: looking like something else to trick animals, such as spots that look like insect eggs.
Try it: gently touch a touch-me-not plant and time how long the leaves take to open again.
Key formulas and definitions
- Pathogen: a microorganism (virus, bacterium, fungus or protist) that causes disease
- Nitrate ions → proteins → growth; lack = stunted growth
- Magnesium ions → chlorophyll; lack = chlorosis (yellow leaves)
- Physical defence = barrier; chemical defence = substance; mechanical defence = structure or movement that stops animals
- Aphid: a sap-sucking insect that weakens plants and spreads viruses
Worked examples
1. A tomato plant has pale yellow leaves, but its height is normal and no pests are seen. Suggest a cause and a fix.
Probably magnesium deficiency: magnesium is needed for chlorophyll, so leaves turn yellow (chlorosis). Add a fertiliser containing magnesium.
2. Leaves of a plant show a yellow-and-green mosaic and the plant grows poorly. Explain why growth is poor.
This looks like tobacco mosaic virus. The discoloured parts have less chlorophyll, so the plant does less photosynthesis, makes less glucose, and so grows less.
3. Classify: (a) waxy cuticle (b) thorns (c) poison in leaves (d) leaves fold when touched.
(a) physical, (b) mechanical, (c) chemical, (d) mechanical.
Common mistakes
- Thinking all plant disease is caused by germs. Mineral deficiency (nitrate, magnesium) also causes disease signs.
- Mixing up the two ions: nitrate deficiency = stunted growth; magnesium deficiency = yellow leaves.
- Calling thorns a physical defence. In this topic, thorns are a mechanical defence; physical defences are barriers like the cuticle and cell walls.
- Saying aphids only damage by eating leaves. They suck sap from the phloem and can carry viruses.