4.3.3 Plant disease (biology only)

4.3.3.1 Detection and identification of plant diseases

Identifying Plant Disease

1. Stunted growth: it can be caused by many factors such as Mealybugs and other pets. It is often caused by a reduction in photosynthesis.

2. Spots on leaves: infection can lead to black spots on leaves

3. Areas of decay: leaves can often decay and rot in certain areas

4. Growths: there may be unusual bumps and growths from the plant

5. Malformed stems or leaves: infection can lead to poor development of stems and leaves

6. Presence of pests: near a plant crop are an indicator that infection has occurred

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Identifying Disease by Naked Eye

• Stunted growth: it can be caused by many factors such as Mealybugs and other pests
  • It is often caused by a reduction in photosynthesis
• Spots on leaves: infection can lead to black spots on leaves
• Areas of decay: leaves can often decay and rot in certain areas
• Growths: there may be unusual bumps and growths from the plant
• Malformed stems or leaves: infection can lead to poor development of stems and leaves
• Presence of pests: near a plant crop are an indicator that infection has occurred

Further Investigation

Gardening manuals: to look up specific symptoms and check common diseases

Laboratory tests: professionals will identify the ailment or use specific testing kits

Monoclonal antibodies: testing kits often contain monoclonal antibodies

Specific Case Studies

• Tobacco mosaic virus: this virus can cause a viral disease
• Black spot: this is a fungal disease which can be identified by presence of black spots on leaves
• Aphids: these are examples of insects which can cause infection

Nitrate Deficiency

• Nitrates are the most important element in the formation of protein
• They are taken in from the soil in order to allow proper growth
• A deficiency of nitrates will lead to stunted growth

Magnesium Deficiency

• Magnesium ions are taken in to synthesise chlorophyll
• A deficiency of magnesium = chlorosis (the discolouration of the plant)
• Magnesium deficiency may be present if plants have yellow leaves

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4.3.3.2 Plant defence responses

Plant Defences

• Plants have developed natural defence systems to fight against infection
• The adaptations may develop over many generations through natural selection

Physical Defences

1. Cellulose Cell Wall – form a physical barrier between the cells and pathogens, thus protect against infection
2. Waxy Cuticles – prevent entry of pathogens into the leaf and prevent water loss
3. Bark – protects the stems, to protect against infection as it stops the passage of pathogens

Chemical Defences

1. Antibacterial chemicals: some plants secrete antibacterial chemicals to kill bacterial pathogens. Different plant species may secrete different types of chemical
2. Poisons: some plants produce poisons to protect against herbivores

Mechanical Defences

1. Thorns and hairs: some plants protect themselves with thorns and hairs. For example. cacti have thorns which deters herbivores
2. Drooping leaves: some plants have leaves that droop or curl when touched, which can push insects off the plant
3. Mimicry: a plant may scare predators by mimicking the features of a dangerous species