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Plant disease (biology only)

3.3.1 Detection and identification of plant diseases

Plants infected by viral, bacterial, fungal & insects (eg aphids)

Aphids

  • Sharp mouthparts – penetrate into phloem vessels for phloem sap
  • Attack in huge no. – deprive plant cells – damage & weaken plant
  • Act as vectors – transfer pathogens

Prevention – pesticides, enclosed spaces eg greenhouses, aphid-eating insects eg ladybirds

Mineral ion deficiency (non-communicable disease)

Nitrate deficiency – stunt growth

  • Nitrate ions from soil needed for proteins synthesis for growth

Magnesium deficiency – yellow leaves (chlorosis)

  • Magnesium ions from soil needed to make chlorophyll

Treatment – fertilizers (repair damage, plant recovers)

Symptoms of plant disease

  • Stunted growth – coz nitrate deficiency
  • Spots on leaves eg rose black spot (fungi)
  • Areas of decay / rotting eg rose black spot / blights
  • Abnormal growths eg crown galls (bacterial)
  • Malformed stems & leaves – coz aphid infestation
  • Discoloration eg chlorosis coz magnesium deficiency / mosaic patterns coz tobacco mosaic virus
  • Presence of visible pests eg aphids

Treatment / Prevention – pesticides / antifungal treatments, remove diseased plants prevent spreading

Identifying diseases

  • Using disease descriptions in gardening manual / online & compare them
  • Taking infected plants to lab to identify pathogen using DNA analysis
  • Use testing kits containing monoclonal antibodies to identify pathogens

3.2.2 Plant defence responses

Physical – act as barrier to prevent pathogens entering

  1. Cellulose cell walls
  2. Waxy cuticle on leaves
  3. Layers of dead cells around stems – fall off with pathogens

Chemical

  1. Antibacterial chemicals – kill bacteria eg mint
  2. Poisons – deter predators / herbivores (organisms that eat plants) eg foxgloves

Mechanical

  1. Thorns & hairs – difficult / painful for herbivores to eat eg roses
  2. Leaves drop / curl when touched – knock insects off / frighten animals eg mimosa pudica
  3. Mimicry – mimic unhealthy plants – deter herbivores, mimic butterfly eggs – butterflies don’t lay eggs eg variegation