Pests and diseases
"You are looking well, and how you have grown!" Have these words been said to you? They were probably said by an older relative or family friend. But how could they tell? You can tell if someone is well by the way that they look. They might have bright, shining eyes and be lively. If they were ill they may be grumpy, have a look of pain on their face and be tired. Growth is also easy to see ? a person just looks bigger. While it is easy to see if a person is well is it just as easy to spot a healthy plant?

In many ways it is just as easy. A healthy plant should have well formed leaves that are bright green. The leaves should stand out from the plant and not droop and the flowers should be bright and attractive. If you were to examine plants' roots, they would be fully formed. An ill plant may have brown leaves, a drooping stem and a lack of flowers. There are many causes of illness in plants but most of them are caused by an attack from other living things.

The smallest living things to attack a plant are microbes. Some kinds of microbes attack us too. We call them germs. The microbes that attack plants can rob the leaves of their food and stop them working. They can cause the plant to grow in unusual ways and make odd-shaped leaves and stems. These new growths weaken the plant. Some kinds of microbes are really very small fungi. They are related to mushrooms and toadstools. These large fungi feed by sending out threads into the soil. The threads find dead plants and animal remains in the soil and digest them. The microscopic fungi, called moulds and rust, send out threads into living plants and feed on the insides. As the fungi feed they grow and breed. This weakens the plant and may kill it. Although the fungi dies with the plant, it also makes sure that other plants can be attacked. Fungi make spores. These are like microscopic suitcases, which contain a piece of the fungus. A mould or rust produces huge numbers of them. For example, a square centimetre of rust on a leaf can produce half a million spores. The spores escape into the air and travel to other plants. When they land on leaves they begin their attack. The spores open and a feeding thread grows out. There are holes on the underside of a leaf which help it take in air to make food. The threads aim for these holes and grow inside. Once they begin feeding inside the leaf the plant may be doomed.

Plants are attacked by animals too. Many of the animals have jaws, which bite a tiny hole in the plant. Once the hole is made the animal sucks out sap. This may not seem particularly harmful but once the animal has fed it may breed rapidly and soon there may be hundreds of tiny animals biting holes in the plant so they can feed. When this happens, large amounts of sap are lost and the plant may die. Some of the small sap-sucking animals are red spider mites, greenfly, white fly and mealy bugs.

While a plant can be suffering from sapsuckers it can also be suffering from nibblers. The main nibblers of a plant are caterpillars. When a caterpillar nibbles it does it in a different way from us. We nibble by moving our jaws up and down. A caterpillar nibbles by moving its jaws from side to side. This helps the caterpillar as it clambers along the edge of the leaf. If it ate as we do it would have to turn its head to one side to get a bite. Some tiny caterpillars burrow into the leaf and make tunnels. These caterpillars are called leaf miners and the tunnels show up on the surface of the leaf as pale wiggly lines.

While caterpillars nibble away at the leaves, wireworms bite away at the plant's root. A wireworm is the grub stage of a beetle. It can be present in large numbers in the soil. There are even larger numbers of a worm called the eelworm. These are very tiny worms, which suck sap from the root. Some are not content with feeding outside the plant but burrow into it and live and feed inside. One badly affected onion had two million eelworms in it.

With all these living things ready to attack a plant, it may seem odd that so many plants survive. The key to survival is good health. If the plant has all the water, nourishment and light it needs it will grow strong enough to prevent successful attacks from pests and disease.

Can a plant be attacked by more than one living thing at a time?
Yes, it can. You may see trees with bracket fungi growing out of them. These fungi are feeding on the wood inside the trunk. As the fungi feed, they make the wood soft. This makes it easier for wood-burrowing beetles to attack the tree. At the same time as the trunk is being attacked by fungi and beetles, the leaves may be being attacked by greenfly, caterpillars and leaf miners.

Do any plants attack other plants?
A few plants feed on other plants. The broomrape is an example of a plant which can grow on other plants and cause disease. It grows on the root of a bush called the broom. The broomrape does not have green leaves because it gets all the food it needs from the broom plant it is attacking. When the broomrape is full grown it produces flowers and seeds. Broomrape can grow on bean plants and kill them.

Can plant pests destroy the place where plants live?
Yes, they can. Some woodlands become so infested with caterpillars that if you are quiet it is said that you can hear the chomping of their jaws and the patter of their droppings on the woodland floor. This kind of infestation can kill the trees in a woodland. When this happens, the dead trees are a fire hazard. If the woodland then caught fire it would be completely destroyed.

Can plants be helped to fight pests and diseases?
Yes. The best help is to make sure that the plants are healthy. They should be checked regularly for signs of disease. If pests such as fungi or insects are found, the plants can be treated with chemicals to kill them. A fungicide is a chemical, which kills fungi, and an insecticide is a chemical, which kills insects.

Are chemicals harmful to other living things?
They can be if they are not used properly. In the past people used very large amounts of insecticide and other living things were poisoned too. It was found that the insecticides did not just kill insects but they killed birds, which fed on insects. Today people take more care with the chemicals they use so only just enough is used to treat the plants. Some chemicals only last for a short time before they change into harmless substances. This also helps to stop other living things being harmed.