Plant diseases and plant defences

Introduction to Plant Diseases and Defences

Just like humans, plants can be attacked by pathogens like bacteria, viruses, and fungi. Because plants can’t move around to escape danger, they have developed amazing "armor" and "chemical weapons" to protect themselves. This chapter explores how plants stay healthy and how scientists can figure out what is making a plant sick.

Note: This content is for the Biology-only (1BI0) course. If you are taking Combined Science, you do not need to study this specific chapter.

Physical Defences (Biology Only)

Physical defences are the plant's version of a "fortress wall." These are structural barriers that make it physically difficult for pathogens to enter the plant's cells.

1. The Waxy Cuticle

The surface of most leaves is covered in a waxy cuticle. Think of this as a waterproof raincoat. It prevents water from collecting on the leaf. Since many pathogens (like fungal spores) need a film of water to survive and infect the plant, the cuticle acts as a first line of defence.

2. Cellulose Cell Walls

Every plant cell is surrounded by a cell wall made of cellulose. This is a very tough, rigid layer. If a pathogen manages to get past the waxy cuticle, it still has to find a way to "break into" the individual cells through these strong walls.

Key Takeaway: Physical defences are all about building barriers to keep pathogens out of the plant tissues.

Chemical Defences (Biology Only)

If a pathogen manages to breach the physical barriers, the plant can fight back using "chemical warfare."

Antiseptics and Antimicrobials

Many plants produce antiseptic chemicals that kill or inhibit the growth of bacteria and fungi. Humans have actually learned to use some of these chemicals for our own medicine! For example:

- Mint plants produce menthol, which has antimicrobial properties.
- Witch hazel produces chemicals that can be used as a skin antiseptic.
- Some plants produce poisons to stop pests (like insects) from eating them, which also helps prevent the spread of diseases carried by those pests.

Did you know? In your Core Practical 5.18B, you might investigate how effective these plant extracts are at killing bacteria by measuring the "clear zone" on an agar plate using the formula \( \text{area} = \pi r^2 \).

Detecting and Identifying Plant Diseases (Higher Tier Only)

When a plant looks unwell—perhaps it has yellow leaves, spots, or is wilting—scientists and farmers need to identify the cause so they can stop it from spreading. This is done in two main stages.

1. Field Identification

In the "field" (the garden or farm), experts look for symptoms and patterns:

- Observation: Looking for specific signs like "dieback" (seen in Chalara ash dieback) or leaf lesions.
- Distribution: If the diseased plants are all in one area, it might be spread by the wind or soil. If the disease is spread randomly, it might be carried by insects (vectors).

2. Lab Identification

Sometimes, symptoms look very similar for different diseases. To be 100% sure, samples are sent to a laboratory:

- Diagnostic Testing: Scientists look for the specific antigens (unique proteins) of a pathogen or use DNA testing to identify the specific genetic code of the virus, bacteria, or fungus causing the trouble.
- Growing Pathogens: They may also take a sample and try to grow the pathogen in a lab to see exactly what it is.

Key Takeaway: Identifying plant diseases starts with looking at symptoms in the field and finishes with high-tech testing in the lab.

Quick Review: Summary Table

Physical Defences: Waxy cuticle (waterproof barrier) and Cellulose cell walls (tough structure).
Chemical Defences: Production of antiseptics and antimicrobial chemicals to kill pathogens.
Identification (HT): Symptoms/distribution (Field) followed by DNA/antigen testing (Lab).

Common Mistakes to Avoid

- Mixing up Human and Plant Defences: Don't talk about white blood cells or antibodies when discussing plants! Plants do not have an immune system like humans; they rely on their physical barriers and chemical extracts.
- Forgetting the "B": Remember that this entire section on plant defences is for Biology-only students. If you see "5.9B" in your syllabus, the "B" stands for Biology!
- Thinking Physical = Chemical: Always categorize your answer. If a question asks for a physical defence, talk about the cell wall or cuticle. If it asks for a chemical defence, talk about antiseptics.

Note: For more information on how pathogens spread, see the chapter "Pathogens and the spread of infection". To learn about how we use plant extracts to make medicine, see "Developing new medicines and monoclonal antibodies".