Plant Diseases and Defences — AQA GCSE Biology
Plants can be affected by diseases and ion deficiencies, and they have their own defences against pathogens and pests.
Plant diseases
Plants suffer from communicable diseases (caused by pathogens) and from ion deficiencies.
- Tobacco mosaic virus (TMV) — gives leaves a mosaic discolouration, reducing photosynthesis and growth.
- Rose black spot (fungal) — black/purple spots on leaves; leaves may drop early; reduces photosynthesis. Spread in water and air; treated with fungicides.
Detecting and identifying disease
Signs of disease include stunted growth, spots on leaves, discolouration, decay (rot), abnormal growths, malformed stems/leaves and visible pests. Diseases can be identified by reference to a gardening manual or website, by lab testing of the pathogen, or using monoclonal antibody test kits.
Mineral ion deficiencies
- Nitrate deficiency — reduces protein synthesis, causing stunted growth.
- Magnesium deficiency — needed for chlorophyll; causes chlorosis (yellow leaves).
Plant defence responses
- Physical: waxy cuticle, cellulose cell walls, layers of dead cells (e.g. bark) that fall off.
- Chemical: antibacterial chemicals and poisons to deter pests.
- Mechanical: thorns and hairs, leaves that droop or curl when touched, and mimicry to trick animals.
Exam tips
- Learn TMV (viral) and rose black spot (fungal) with their effects.
- Link nitrate deficiency to stunted growth and magnesium to chlorosis (chlorophyll).
- Give examples of physical, chemical and mechanical plant defences.
- Know the methods used to identify plant disease.