Plant Diseases and Defences

Question 1

What are the 3 main causes of plant diseases?

Answer 1

1. Infection by microorganisms such as fungi, bacteria, or viruses.
2. Attack by larger organisms, such as insects.
3. Lack of essential minerals, such as nitrates or magnesium ions (or too much of something, e.g. water).

Question 2

What are 7 symptoms of plant disease?

Answer 2

Stunted growth.
Spots on leaves.
Areas of decay.
Growths.
Malformed stems or leaves.
Discolouration.
In some cases the pests themselves may be present (e.g. aphids).

Question 3

What are 5 ways in which plant diseases can be identified?

Answer 3

Compare the symptoms to a gardening manual or website.
Take the infected plants to a laboratory (plant pathologist) to identify the pathogen.
Use testing kits that contain monoclonal antibodies.
Test the DNA of the pathogens.
Use trial and error to test different treatments.

Question 4

Explain what will happen to plant growth if plants lack nitrates.

Answer 4

Nitrates contain the element nitrogen, which is essential for making amino acids. These amino acids can then be combined to make proteins.

Nitrates ➔ nitrogen ➔ amino acids ➔ proteins ➔ growth

As proteins are essential for growth, a plant that lacks nitrates won’t grow properly, so it will have stunted growth.

Question 5

Explain what will happen if plants lack magnesium.

Answer 5

Magnesium is required to produce chlorophyll, which is the green pigment in chloroplasts that makes leaves green.

A lack of magnesium will mean that leaves lose their green colour and turn yellow, which we call ‘chlorosis’.

Question 6

With reference to plant disease, what are physical defences?

Answer 6

Defences that physically prevent the entry of pathogens.

Question 7

With reference to plant disease, give 3 examples of physical defences.

Answer 7

1. Strong cellulose cell walls.
2. Layers of dead cells around the trunk (bark).
3. A waxy cuticle on the leaves and stems.

Question 8

With reference to plant disease, what are chemical defences?

Answer 8

Chemicals that plants can secrete in order to destroy pathogens.

Question 9

With reference to plant disease, give 2 examples of chemical defences.

Answer 9

1. Production of antimicrobial substances.
2. Production of poisons that kill insects.

Question 10

With reference to plant disease, what are mechanical defences?

Answer 10

Structures that reduce the chance of being attacked.

Question 11

With reference to plant disease, give 3 examples of mechanical defences.

Answer 11

1. Mimicking other organisms in order to trick animals.
2. Leaves that curl or droop when insects land on them.
3. Hairs and thorns.