3.3 Plant disease

• What causes plant disease?

  1. Pathogens - viruses eg. TMV / bacteria / fungi eg. rose black spot
  2. Insects - such as aphids
• How do aphids cause plant disease?

  ● Feed on phloem sap and remove sugars → not enough glucose for respiration to release energy, or to make proteins → stunted growth

  ● Act as vectors → transfer pathogens between plants
• Nitrate ions

  Role: Required to make proteins which are needed for growth.

  Symptoms of deficiency: Stunted growth.
• Magnesium ions

  Role:

  ● Required to make chlorophyll

  ● Which absorbs light for photosynthesis

  ● To make glucose which is needed to make proteins and for respiration (to release energy) for growth

  Symptoms of deficiency:

  ● Chlorosis (lack of chlorophyll → yellow leaves)

  ● This may lead to stunted growth
• How can plant diseases be identified? (HT only)

  1. Compare to (pictures / descriptions in) a gardening manual or website

  2. Taking infected plants to a laboratory (for DNA analysis) to identify the pathogen

  3. Using testing kits that contain monoclonal antibodies (bind to antigens present on certain pathogens)
• why is this statement wrong?

  “All plant diseases are caused by pathogens.”

  Insects such as aphids can cause plant disease, too.
• Why is this statement wrong?

  “Chloroplasts, chlorophyll and chlorosis are all the same thing.”

  Chloroplasts are where photosynthesis happens in plant cells. They contain chlorophyll which absorbs light for this to happen. Chlorosis is caused by lack of chlorophyll.
• Plants infected with aphids have stunted growth. Explain one way the removal of dissolved sugars from the stem of the plant causes stunted growth. (2)

  Less sugars for respiration

  So less energy released

  OR less amino acids made

  So less protein produced

  OR less cellulose made

  So weaker cell walls
• An aphid feeds by inserting its mouthpiece into the stem of a plant. Explain why the mouthpiece of an aphid contains a high conc. of dissolved sugars after feeding. (1)

  ● Mouthpiece has pierced / entered phloem / aphid has been feeding from the phloem
• Plants infected with aphids may show symptoms of magnesium deficiency. Magnesium deficiency symptoms include yellow leaves and stunted growth. Explain how a deficiency of magnesium could cause these symptoms. (4)

  ● Yellow leaves due to lack of chlorophyll

  ● So less / no light absorbed (by chlorophyll)

  ● So lower rate of / no photosynthesis

  ● So plant makes less / no glucose

  ● So plant converts less / no glucose into protein (for growth, so growth is stunted)
• effects of deficiency of magnesium ions on plants

  1. Lack of magnesium (ions)

  2. (so) not enough chlorophyll for photosynthesis

  3. (so) not enough glucose to make proteins for growth / or not enough glucose to release energy for growth
• effects of infection

  1) infection by pathogen / bacteria / virus / fungus / rose black spot / TMV

  2. (so) leaves become discoloured so less photosynthesis

  3. (so) not enough glucose to make proteins for growth or not enough glucose to release energy for growth
• Give a reason why a farmer cannot rely on only visible symptoms to identify a disease. (1)

  ● Different diseases may cause the same symptoms