3.3 Plant disease

Question 1

Aphids

Answer 1

Aphids have long sharp mouthpieces that pierce the surface of the stem and into the phloem so that they can feed on the sugar filled sap
They can infest a plant in large numbers and remove large quantities of the phloem sap. This can weaken the plant
Aphids can also act as a vector for the transfer of diseases
Biological pest control (using ladybirds, a natural predator of the aphid) to feed on them or using chemical pesticides are the best ways to control infestation with aphids

Question 2

Detecting plant diseases

Answer 2

Stunted growth - rose black spot, TMV and mineral deficiency.
Spots on leaves - rose black spots
Rotting - rose black spot and potato blight
Visible pests - aphids and caterpillars
Discolouration of the leaves - TMV and magnesium deficiency
Growth - crown gall bacteria infection.
Malformed stems and leaves - aphids

Question 3

Mineral deficiency

Answer 3

Nitrates - used for production of amino acids for protein. Symptoms include stunted growth, upper leaves pale green and lower leaves yellow.

Magnesium - used for synthesis of chlorophyll needed for photosynthesis. Symptoms include yellow leaves (chlorosis) and stunted growth.

Potassium - used for production of fruits and flowers. Symptoms include poor growth of fruits and flowers, yellow leaves and dead spots.

Question 4

Identifying plant diseases

Answer 4

Diseases in the garden can be identified by comparing symptoms to those described in gardening manuals or online

Crop plants or forest trees, experts may visit the field/woodland to take samples which can then be taken to a lab to undergo DNA analysis. This would help to identify the specific pathogen causing the problem

Plant scientists, foresters, farmers and market gardeners can use testing kits which contain monoclonal antibodies to identify the presence of pathogens quickly and easily

Question 5

Fertilisers

Answer 5

Plants like all organisms need the correct amount of nutrients to function properly.
If plants are deficient in a mineral these can be replaced by fertilisers added to the soil.
These fertilisers are often shown as NPK ratios depending on the need of the plant that is being grown.

Question 6

Physical defences of plants

Answer 6

The cellulose cell wall not only provides support for the plant but also protection from microorganisms
The waxy cuticle of the leaf is a barrier to microbes from entering the plant. The only place that they can enter in the leaf is through the stomata
Bark provides a tough layer around the stem of the plant to prevent pathogens from entering
As deciduous trees lose leaves in the winter the infection can be taken with them

Question 7

Chemical defences of plants

Answer 7

Many plants produce chemicals that have antimicrobial properties.
Some plants produce poisons to deter herbivores from eating them. Animals will quickly learn to avoid eating plants that make them feel unwell. Examples include foxglove and yew.

Question 8

Mechanical defence of plants

Answer 8

Thorns make it unpleasant or painful for large herbivores to eat such as cactus and rose.
Drooping or curling when touched can dislodge insects and frighten of larger species such as mimosa.
Mimicry so that plants can look as though they have a disease and trick animals to not eating them. For example passion flowers.