Plant defences and diseases

In a nutshell

Despite having good defences, plants can also be infected by viruses, bacteria and fungi. Identifying the visible symptoms is the first step to identifying a plant disease. Analysing the way plants are distributed in the environment and carrying out tests in a laboratory can identify the possible causes of plant disease.

Plant defences

In order to protect themselves from infection, plants have certain physical, chemical and mechanical defence mechanisms.

Physical defence	Explanation
Stem	The stem has a waxy cuticle that prevents pathogens from entering.
Cell walls	Plant cells have cell walls that are made from cellulose and act as a physical barrier to prevent pathogen entry.
Dead cell layers	The stem has a layer of dead cells on the outside (like bark on a tree) and this acts as a barrier to prevent pathogen entry.

Chemical defence	Explanation
Antibacterials	Some plants produce chemicals that kill bacteria. Examples The mint plant and witch hazel produce antibacterials.
Poisons	Some plants produce poisons that deter organisms that may eat them. Examples Tobacco plants and foxgloves produce poisons.

Mechanical defence	Explanation
Thorns or hairs	Some plants will have thorns or hairs to prevent animals from eating them.
Drooping/curling leaves	Plants can protect themselves from insects by curling or drooping their leaves. This causes the insects to fall off.
Mimicry	The passion flower mimics yellow spots on its leaves that look like butterfly eggs to stop other butterflies from laying their eggs there.

Using plant chemicals

Plant produce chemicals that can also be used as medicines to treat humans.

chemical	Plant source	Use
Quinine	Cinchona tree	Treats malaria.
Digoxin	Foxglove	Treats heart disease.
Aspirin	Willow tree	Relieves pain and fever.

Note: The following section is for students studying the Higher tier specification.

Detecting and identifying plant diseases

Plants can become infected by viruses, bacteria and fungal pathogens. They can also be infested and damaged by insects. For example, aphids are an insect that can damage plants. The first step in identifying the disease will be to identify visible symptoms. Then, analysing the way plants are distributed in the environment can identify the possible causes of disease.

Visible symptoms

The visible symptoms of plant diseases may give an indication of the type of disease the plant is infected with.

1.	Stunted growth
2.	Spots on the leaves
3.	Death of parts of the plant
4.	Underdevelopment of the plant
5.	Discolouration
6.	Abnormal growths (e.g., lumps)
7.	Malformed stems and leaves
8.	Pest infestations

Distribution analysis

Analysing the distribution of the affected plants can give an indication of the type of disease the plants are infected with.

Possible questions	REason
How many plants are affected?	If all are affected by the disease then it is probably caused by an environmental factor such as soil pH.
Where are the affected plants found?	If all diseased plants are located in the same place then it may be caused by the location. For example, being next to a polluted road.
How many species are affected?	If only one species is diseased then it may be caused by a species-specific pathogen.
Do the symptoms change over time?	This may help to identify the disease.

Laboratory testing

Laboratory testing can be carried out to identify the cause of the disease.

Test	Description
Microscopic examination	Examining plant material to identify the pathogen causing the disease.
Antibodies	Using antibodies to test for the presence of a pathogen.
Genetic testing	To try and identify any pathogens found.
Soil samples	To test if environmental factors are causing the disease. Examples Soil pH or nutrient deficiencies.
Culturing pathogens	Growing the pathogens identified to get a large sample that can be genetically tested.