Plant Disease

This lesson covers plant disease as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how plant diseases can be detected, the effects of mineral deficiencies on plants, and the defence responses plants use to protect themselves against pathogens and herbivores.

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How Do Plants Get Diseases?

Plants, like animals, can be infected by pathogens (bacteria, viruses, fungi and protists). Plants can also suffer from mineral deficiencies caused by a lack of essential minerals in the soil.

Plant diseases are a major concern in agriculture because they can reduce crop yields, leading to food shortages and economic losses.

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Detecting Plant Disease

Plant diseases can be detected in several ways:

Method	Description
Visual inspection	Looking for visible symptoms such as spots, discolouration, wilting, stunted growth, abnormal growths, or decay
Distribution analysis	Mapping where affected plants are located to identify patterns (e.g. disease spreading from one area)
Diagnostic testing	Using laboratory tests such as antibody-based tests (similar to pregnancy tests) that detect specific pathogen proteins
DNA analysis	Using molecular techniques (e.g. PCR) to identify pathogen DNA in plant tissue
Plant identification guides	Using reference materials or databases to match symptoms to known diseases
Expert consultation	Sending samples to a laboratory or consulting a plant pathologist

Common Symptoms of Plant Disease

Symptom	Possible cause
Spots on leaves (e.g. purple/black)	Fungal infection (e.g. rose black spot)
Mosaic pattern on leaves (patches of discolouration)	Viral infection (e.g. TMV)
Yellowing of leaves (chlorosis)	Mineral deficiency (e.g. nitrate or magnesium)
Stunted growth	Mineral deficiency or pathogen infection
Wilting	Bacterial infection or lack of water
Rotting	Bacterial or fungal infection
Abnormal growths / galls	Bacterial infection or insect damage

Exam Tip: In the exam, you may be given a photograph or description of a diseased plant and asked to suggest a possible cause. Use the table above to link symptoms to likely causes.

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Mineral Deficiencies in Plants

Plants absorb mineral ions from the soil through their roots (by active transport and diffusion). If the soil lacks certain minerals, the plant shows specific deficiency symptoms.

Key Mineral Deficiencies

Mineral	Why the plant needs it	Deficiency symptoms
Nitrate (NO₃⁻)	Needed to make amino acids and proteins (for growth)	Stunted growth; older leaves turn yellow because the plant cannot make enough chlorophyll
Magnesium (Mg²⁺)	Needed to make chlorophyll (the green pigment essential for photosynthesis)	Chlorosis — leaves turn yellow (especially between the veins), starting with older leaves; reduced photosynthesis

graph TD
    A["Plant absorbs minerals<br/>from soil via roots"] --> B{"Sufficient minerals<br/>available?"}
    B -->|Yes| C["Healthy growth<br/>Green leaves<br/>Normal photosynthesis"]
    B -->|No - Nitrate deficiency| D["Stunted growth<br/>Yellow leaves<br/>Poor protein synthesis"]
    B -->|No - Magnesium deficiency| E["Chlorosis<br/>Yellow leaves<br/>Reduced photosynthesis"]

Why Deficiencies Matter

• Nitrate is needed to make amino acids, which are used to build proteins. Without proteins, cells cannot grow or divide properly, so the whole plant grows poorly.
• Magnesium is a key component of the chlorophyll molecule. Without enough chlorophyll, the plant cannot photosynthesise efficiently, so it produces less glucose for energy and growth.

Exam Tip: Both nitrate and magnesium deficiency can cause yellow leaves, but for different reasons. Nitrate deficiency causes yellowing because of poor overall growth and protein shortage. Magnesium deficiency causes yellowing specifically because the plant cannot make enough chlorophyll. Make sure you state the correct reason for each.

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Plant Defence Responses

Plants cannot run away from threats, so they have evolved a range of physical, chemical and mechanical defences to protect themselves against pathogens and herbivores.

Physical Defences

Defence	How it works
Cellulose cell wall	Every plant cell has a rigid cell wall that acts as a barrier, making it harder for pathogens to enter
Waxy cuticle	The upper surface of leaves has a waxy layer that prevents water collecting on the surface (reducing fungal growth) and acts as a barrier to pathogen entry
Bark	Trees have a thick layer of bark that protects the inner tissues from infection and physical damage
Leaf drop	Some plants shed infected leaves to prevent the disease from spreading to the rest of the plant (e.g. rose black spot)

Chemical Defences

Defence	How it works
Antimicrobial chemicals	Some plants produce chemicals that kill or inhibit pathogens. For example, mint and witch hazel produce antimicrobial compounds
Poisons	Some plants produce toxins that deter herbivores from eating them. For example, deadly nightshade produces atropine; tobacco produces nicotine

Mechanical Defences

Defence	How it works
Thorns	Sharp structures on stems (e.g. roses, brambles) that deter herbivores from eating or damaging the plant
Spines	Modified leaves that reduce water loss and deter herbivores (e.g. cacti)
Hairs	Some plants have tiny hairs on leaves and stems. Stinging nettles have hollow hairs that inject irritating chemicals into animals that touch them
Curling leaves	Some leaves curl up when touched (e.g. mimosa), which may deter insects
Tough leaves	Thick, leathery leaves are harder for insects to eat

graph TD
    A["Plant Defence<br/>Responses"] --> B[Physical]
    A --> C[Chemical]
    A --> D[Mechanical]
    B --> B1[Cell wall]
    B --> B2[Waxy cuticle]
    B --> B3[Bark]
    B --> B4[Leaf drop]
    C --> C1["Antimicrobial<br/>chemicals"]
    C --> C2["Poisons /<br/>toxins"]
    D --> D1[Thorns]
    D --> D2[Spines]
    D --> D3[Stinging hairs]
    D --> D4["Tough / curling<br/>leaves"]

Exam Tip: When describing plant defences, give a specific example for each type. For instance, do not just say "plants have chemical defences" — say "some plants produce antimicrobial chemicals, such as mint, which inhibit the growth of pathogens."

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Summary