# Plant Disease and Defences
Plants can be affected by diseases caused by pathogens and by mineral deficiencies. They also have physical and chemical defences against attack.
1. Plant Diseases
| Disease | Pathogen | Effect |
|---|---|---|
| Rose black spot | Fungus | Black spots on leaves; reduced photosynthesis |
| TMV (tobacco mosaic virus) | Virus | Mosaic pattern on leaves; reduced photosynthesis |
| Crown gall | Bacterium | Abnormal growths (tumours) on plants |
Detection
- Visual symptoms (spots, discolouration, stunted growth)
- Gardening guides or laboratory testing
2. Mineral Deficiencies
| Mineral | Needed For | Deficiency Symptom |
|---|---|---|
| Nitrate | Amino acids/proteins | Stunted growth; yellow leaves |
| Magnesium | Chlorophyll | Yellow leaves (chlorosis) |
| Phosphate | DNA, cell membranes | Poor root growth; purple leaves |
| Potassium | Enzyme function | Yellow/brown leaf edges; poor growth |
3. Plant Defences
Physical
- Cellulose cell walls — barrier against pathogens
- Waxy cuticle — prevents water loss and pathogen entry
- Bark — tough outer layer of trees
- Thorns/spines — deter herbivores
- Leaf drop — removes infected leaves
Chemical
- Antibacterial chemicals — e.g. mint, garlic
- Poisons — deter herbivores (e.g. foxglove produces digitalis)
Mechanical
- Leaves that droop/curl when touched (e.g. Mimosa)
- Sticky hairs that trap insects
4. Identifying Plant Diseases
Required Practical
- Observe plant leaves for disease symptoms
- Compare with reference images
- Use identification keys
- Laboratory testing for pathogen identification
5. Practice Questions
- Describe the symptoms of TMV.
- What mineral deficiency causes yellow leaves?
- Name two physical and two chemical defences of plants.
- A plant has stunted growth and yellow leaves. What might be the cause?
- How does rose black spot affect the plant's ability to photosynthesise?
Want to check your answers and get step-by-step solutions?
Summary
- Plant pathogens: fungi (rose black spot), viruses (TMV), bacteria
- Mineral deficiencies: nitrate → stunted; magnesium → yellow leaves
- Defences: physical (cell wall, bark, thorns), chemical (poisons, antibacterials), mechanical
- Disease identification: visual symptoms, lab testing