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
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| 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
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| Antibacterials | Some plants produce chemicals that kill bacteria.
Examples The mint plant and witch hazel produce antibacterials.
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| Poisons
| Some plants produce poisons that deter organisms that may eat them.
Examples Tobacco plants and foxgloves produce poisons.
|
Mechanical defence | Explanation
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| 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.
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| How many species are affected? | If only one species is diseased then it may be caused by a species-specific pathogen.
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| Do the symptoms change over time?
| This may help to identify the disease.
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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.
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| Genetic testing
| To try and identify any pathogens found.
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| Soil samples | To test if environmental factors are causing the disease.
Examples Soil pH or nutrient deficiencies.
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| Culturing pathogens
| Growing the pathogens identified to get a large sample that can be genetically tested.
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