Plant disease

Plant disease

Plants, like any other organisms can be infected by pathogens. A pathogen is an organism that causes a disease. The attack by pathogens often causes leaf damage and reduce photosynthesis.

There are four types of pathogen: virus, bacteria, fungi and protists.

Plants have physical and chemical defences against pathogens. Diseases caused by pathogens are called communicable diseases.


The tobacco mosaic virus infects tobacco and other species like tomatoes and peppers.

Virus infects the chloroplasts of plant leaves and changes their colour from green to yellow or white.


Rose black spot is caused by a fungus. It infects leaves and causes black or purple spots on the leaves.

Black spot is a fungal disease that affects roses.

This reduces plant's growth and ability to photosynthesise.


Plants can be attacked and infested by insects.

Aphids are very destructive common sap-sucking insects.

They are found in large numbers and reproduce rather rapidly.

Aphids can be controlled by using natural predators like ladybirds.

Aphids are eaten by ladybirds.

The insects are attracted towards bright coloured petals.

Mineral ion deficiencies

Plants suffer from deficiency diseases when they have less essential vitamins or mineral ions.

Lack of nitrogen gives stunted growth and leaves turn a pale green or yellow colour.

If a plant is deficient in magnesium not enough chlorophyll can be made and the plant suffers from chlorosis.

A plant's natural defences against diseases

Plants defend themselves from pathogens.

Each plant cell has a cellulose cell wall which acts as a barrier against infection.

The leaves and stems of most plants are covered with a waxy cuticle that acts as a barrier.

The waxy cuticle also prevents excessive loss of water from the leaves.

Some plants like mint produce antibacterial chemicals and these limit the spread of bacteria.

Witch hazel produce antibacterial chemicals.

Plants have layers of dead cells around their stems that acts as a physical barrier to pathogens.

Some plants produce toxic chemicals called saponins, that destroy the cell membranes of fungi and pathogens.

Some plants produce chemicals called antiseptics that kill bacteria and fungal pathogens.

Some plants like roses have large thorns to avoid being eaten.

Plants like cacti have thin spines, to prevent animals eating them.

Some plant like the mimosa pudica, has evolved to close its leaves when touched by an animal or insect.

Some plants have evolved the ability to mimic other organisms.

The passion flower vine has evolved small yellow spots on its leaves that look like butterfly eggs, deterring other butterflies from laying their eggs on the leaves.

Some species of plant look like stones and pebbles, so they are less likely to be eaten.

Plants like ivy contain poisons, which can be toxic if ingested by insects.