Plant disease and plant defences – GCSE Combined Science Biology

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Plants get ill too, and they can be eaten by pests. A pest is an animal that damages plants, such as an aphid. A plant cannot run away or see a doctor, so it has its own ways to cope.

This matters because we depend on crops for food. A diseased crop gives less food for everyone.

By the end you will be able to spot the signs of plant disease and say how it is identified. You will explain what happens when a plant lacks nitrate or magnesium. You will also describe how plants defend themselves.

Signs of plant disease

Plants can be infected by viruses, bacteria and fungi. They can also be damaged by pests such as aphids.

Signs of disease include stunted growth, which means growth that is smaller than normal. Look for spots on the leaves, areas of decay and growths on the stem or leaves. Stems or leaves may be malformed, which means the wrong shape. Leaves may change colour. You might also see the pests themselves.

Damaged leaves cannot photosynthesise as well. So the plant grows less and a crop gives a lower yield.

Finding the cause

Once you see the signs, you need the cause. The cause tells you what treatment to use. A fungus needs a fungicide, for example.

A gardener can look in a gardening manual or on a website. A sample can be taken to a laboratory, where scientists compare it with known pathogens.

Testing kits can also be used. They contain monoclonal antibodies. These are antibodies that bind to the antigens of one specific pathogen. If the kit changes, the pathogen is there.

Missing nitrate

Plants take in mineral ions from the soil through their roots. Mineral ions are charged particles from dissolved minerals. They are needed in small amounts.

Nitrate ions are needed to make amino acids. Amino acids are the building blocks of proteins. Think of proteins as the bricks used to build a new wall.

A plant with too little nitrate cannot make enough protein for new cells. So its growth is stunted. Farmers add fertilisers to the soil to supply nitrate.

Missing magnesium

Magnesium ions are needed to make chlorophyll. Chlorophyll is the green pigment that absorbs light for photosynthesis. Think of chlorophyll as the solar panels on a roof.

A plant without enough magnesium cannot make enough chlorophyll. Its leaves turn yellow. This yellowing is called chlorosis.

Fertilisers can supply magnesium too. With less chlorophyll, the plant absorbs less light. Less photosynthesis happens, so the plant grows poorly. The table shows both mineral shortages side by side.

Physical and chemical defences

Plants have physical barriers, like the walls of a castle. The cellulose cell wall is hard to break through. The waxy cuticle on leaves stops water and pathogens getting in. Bark has layers of dead cells that fall off, taking pathogens with them.

Plants also make chemicals. Some are antibacterial, which means they kill bacteria. Others are poisons that stop animals eating them. Some of these chemicals are used as drugs for people.

Mechanical adaptations

Some plants have thorns and hairs. These stop animals eating them. A mechanical adaptation is a body feature that works by its shape or movement.

Some leaves, such as the mimosa, droop or curl when touched. This can scare off or knock off insects.

Some plants use mimicry. They look like something harmful, such as a leaf that looks like a caterpillar. Animals then avoid them. These adaptations help the plant survive, but they are not a response to one specific infection.

Recap

Signs of disease are stunted growth, spots, decay, growths, malformed parts, colour change and pests.

Identify the cause with a manual, a laboratory or a testing kit.

Nitrate makes proteins, so a lack gives stunted growth. Magnesium makes chlorophyll, so a lack gives yellow leaves.

Defences are physical, chemical or mechanical. Always link damage to less photosynthesis.