48. Plant Disease and Defences

Plants, like humans, can be affected by various pathogens and nutrient deficiencies. Identifying these issues and understanding how plants protect themselves is crucial for plant health.

1. Causes of Plant Disease

• Pathogens: Microorganisms such as fungi, bacteria, and viruses, as well as larger organisms like insects (e.g., aphids).

• Deficiency Diseases: Caused by a lack of essential minerals in the soil:

  • Nitrates: Needed for protein synthesis and growth. Deficiency leads to stunted growth.

  • Magnesium Ions: Needed to make chlorophyll for photosynthesis. Deficiency leads to chlorosis (yellow leaves).

2. Identifying Plant Disease

Common symptoms include:

• Abnormal growths or lumps

• Malformed stems or leaves

• Patches of decay or spots of discoloration

• Visible pests like aphids or spider mites

Diagnosis Methods:

• Observation: Matching symptoms to descriptions in gardening manuals or websites.

• Trial and Error: Applying a treatment (e.g., adding magnesium or using fungicide) to see if the plant recovers.

• Professional Lab Testing: A plant pathologist can perform detailed analysis:

  • Microscopic examination of tissue samples.

  • Identifying unique pathogen antigens using monoclonal antibodies.

  • Running DNA tests to detect pathogen genetic material.

3. Plant Defences

Plants have evolved three main types of defense mechanisms:

Physical Defences

These act as barriers to prevent pathogens from entering:

• Waxy Cuticle: A waterproof layer on leaves and stems.

• Cellulose Cell Walls: Provide a tough barrier around individual cells.

• Layers of Dead Cells: For example, bark on trees acts as a protective shield.

Chemical Defences

Plants can produce and secrete specialized chemicals:

• Antimicrobial Substances: These kill bacteria and fungi.

• Poisons: Designed to deter or kill herbivores like insects.

  • Note: Many human drugs, such as aspirin (from willow trees), are derived from these plant chemicals.

Mechanical Defences

These have a specific function beyond being a simple barrier:

• Thorns and Hairs: Deter animals from eating or touching the plant.

• Drooping/Curling Leaves: A rapid response to touch that can dislodge insects or deter animals.

• Mimicry: Some plants mimic the appearance of unhealthy plants or dangerous insects to avoid being eaten.

Summary Table: Defense Mechanisms