Fungal and Protist Diseases

This lesson explores fungal diseases and further detail on protist diseases as required by the AQA GCSE Combined Science Trilogy specification (8464). The main examples are rose black spot (fungal) and malaria (protist). You also need to understand how these pathogens differ from bacteria and viruses.

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Fungal Pathogens — An Overview

Fungi are eukaryotic organisms that can be single-celled (e.g. yeast) or multicellular (e.g. mushrooms, moulds). Pathogenic fungi cause disease in plants and animals by:

1. Growing hyphae (thread-like structures) into the host's tissues
2. Releasing enzymes that digest and absorb nutrients from the host
3. Producing spores that spread to new hosts via wind, water, or contact

Feature	Detail
Cell type	Eukaryotic
Cell wall	Made of chitin (not cellulose)
Reproduction	Sexual and asexual; often by producing spores
How they feed	Secrete enzymes externally to digest organic matter, then absorb the nutrients (saprophytic or parasitic)

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Rose Black Spot

Feature	Detail
Pathogen	A fungus (Diplocarpon rosae)
Host	Rose plants
Symptoms	Purple or black spots develop on the leaves; the leaves then turn yellow and drop off early
Effect on the plant	Fewer leaves means less photosynthesis, which reduces growth
Transmission	Spores spread by wind or water; spores can also be carried on hands, tools, or contaminated soil
Treatment	Fungicides (chemical sprays) can be applied; infected leaves should be removed and destroyed (not composted, as spores survive)

How Rose Black Spot Reduces Growth

graph TD
    A[Fungal spores land on rose leaf] --> B[Hyphae grow into leaf tissue]
    B --> C[Purple/black spots appear on leaves]
    C --> D[Leaves turn yellow and drop off]
    D --> E[Fewer leaves on the plant]
    E --> F[Reduced surface area for photosynthesis]
    F --> G[Less glucose produced]
    G --> H[Reduced growth and fewer flowers]

Exam Tip: When explaining the effect of rose black spot, always link the loss of leaves to reduced photosynthesis. The logic chain is: fewer leaves → less light absorbed → less photosynthesis → less glucose for growth.

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Other Fungal Diseases You May Encounter

Although rose black spot is the main named example, you should be aware of other fungal infections:

Disease	Pathogen	Host	Key Features
Athlete's foot	Trichophyton (fungus)	Humans	Itchy, flaky skin between toes; spread by direct contact in warm, moist environments
Ringworm	Dermatophytes (fungi)	Humans / animals	Circular red rash; spread by direct contact or contaminated objects

Exam Tip: Rose black spot is the named plant disease caused by a fungus on the AQA Trilogy spec. You do not need detailed knowledge of athlete's foot or ringworm, but understanding that fungi can infect both plants and animals is useful.

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Protist Pathogens — Further Detail

Protists are a diverse group of eukaryotic organisms, many of which are single-celled. Most protists are harmless, but some are parasites — organisms that live on or inside a host organism and cause harm.

The key protist disease for AQA Trilogy is malaria, covered in detail in the previous lesson. Here we explore protist biology in more depth.

Characteristics of Protist Pathogens

Feature	Detail
Cell type	Eukaryotic (with a nucleus and membrane-bound organelles)
Size	Typically 10–100 $\mu m$μm — larger than bacteria
Lifestyle	Many disease-causing protists are parasites that require a host
Vectors	Protist diseases are often spread by vectors (organisms that carry the pathogen between hosts)

The Role of Vectors

A vector is an organism that carries a pathogen from one host to another without being affected by the disease itself. Vectors are crucial in the spread of protist diseases:

Vector	Disease	Pathogen
Female Anopheles mosquito	Malaria	Plasmodium (protist)

graph LR
    A[Infected host] -->|Mosquito feeds on blood| B[Mosquito picks up Plasmodium]
    B -->|Mosquito bites new host| C[Plasmodium transmitted]
    C --> D[New host infected]
    D -->|Mosquito feeds again| A

Controlling Vectors to Prevent Disease

Strategy	How It Works	Advantages	Disadvantages
Insecticide-treated bed nets	Physical barrier prevents mosquito bites during sleep; insecticide kills mosquitoes on contact	Cheap, effective, protects sleepers	Nets can tear; insecticide wears off
Insecticide spraying	Kills adult mosquitoes in homes and surrounding areas	Reduces mosquito population	Mosquitoes can develop resistance; harms other insects
Draining stagnant water	Removes mosquito breeding sites	Eliminates larvae before they mature	Difficult in areas with heavy rainfall
Biological control	Introducing fish that eat mosquito larvae	Targets larvae specifically	May disrupt ecosystems
Antimalarial drugs	Taken by travellers to prevent infection	Protects individuals	Expensive; does not reduce mosquito numbers

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Comparing Fungal and Protist Diseases

Feature	Rose Black Spot (Fungal)	Malaria (Protist)
Pathogen type	Fungus	Protist
Host	Plants (roses)	Humans
Transmission	Wind/water (spores)	Vector (mosquito)
Key symptom	Black spots on leaves, leaf drop	Recurring fever and chills
Effect	Reduced photosynthesis	Can be fatal
Treatment	Fungicides, remove infected leaves	Antimalarial drugs

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Common Mistakes

Mistake	Correction
"Rose black spot is caused by a bacterium"	It is caused by a fungus
"Malaria is caused by the mosquito"	The mosquito is the vector; the pathogen is the protist Plasmodium
"Fungicides work on all pathogens"	Fungicides target fungi only — they do not kill bacteria or viruses
"Protists are the same as bacteria"	Protists are eukaryotic (with a nucleus); bacteria are prokaryotic

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Summary

• Rose black spot is caused by a fungus that infects rose leaves, causing black spots, yellowing, and leaf drop — reducing photosynthesis
• Rose black spot spreads via wind- or water-carried spores and is treated with fungicides
• Protist diseases like malaria are often spread by vectors — organisms that carry the pathogen between hosts
• Vectors can be controlled using insecticide-treated nets, insecticide spraying, draining stagnant water, and antimalarial drugs
• Fungi are eukaryotic organisms with chitin cell walls that reproduce using spores

Exam Tip: When comparing pathogens, use a table to organise your answer — examiners love clear, structured comparisons. Include pathogen type, size, how it spreads, and how it is treated.

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Extended Worked Examples

Worked Example 1 — Linking Leaf Loss to Yield