B5.6 Diseases Caused by Fungi and Protists

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: rose black spot, malaria and how its mosquito vector is controlled. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the symptoms of rose black spot and explain how it affects plant growth
• Describe how rose black spot spreads and how it is treated
• Describe the life cycle of the malaria protist, including the mosquito vector
• Explain and evaluate ways of controlling the spread of malaria using data

Key words

• Fungus: A eukaryotic organism, from single cells such as yeast to large networks of threads (hyphae). Some fungi are pathogens. Plural: fungi.
• Spores: Tiny reproductive cells released by fungi. They spread the fungus to new hosts.
• Fungicide: A chemical that kills fungi.
• Protist: A eukaryotic microorganism, usually single-celled. Some protists are pathogens, e.g. the one that causes malaria.
• Malaria: A disease caused by a protist and spread by mosquitoes. It causes recurrent fevers and can be fatal.
• Insecticide: A chemical that kills insects.

Fungal diseases: rose black spot

Fungi are eukaryotic, like plants and animals. Pathogenic fungi grow as threads that spread through tissues and release spores that are carried to new hosts. In humans, athlete's foot is a fungal disease of the skin. In plants, rose black spot is a fungal disease of roses:

• Symptoms; Rose black spot Purple or black spots develop on the leaves. The leaves often turn yellow and fall off early.
• Effect on plant; Rose black spot Growth is reduced because there is less leaf area for photosynthesis, so less glucose is made.
• How it spreads; Rose black spot Spores are carried by water (rain splash) or by the wind.
• Treatment; Rose black spot Spraying with fungicides, and/or removing and destroying affected leaves (burning, not composting).

Malaria: a disease caused by a protist

Malaria is caused by a protist. Its life cycle includes mosquitoes, which are the vectors. Malaria causes recurrent episodes of fever (the fevers keep coming back), and can be fatal. Hundreds of thousands of people die from it each year.

Controlling malaria

There is no simple cure, so the aim is to stop the vectors breeding and stop people being bitten.

• Drain or fill in standing water (ponds, puddles, marshes); How it helps Mosquitoes lay their eggs in still water, so fewer mosquitoes breed.
• Spray insecticide; How it helps Kills adult mosquitoes and their larvae.
• Sleep under mosquito nets (often treated with insecticide); How it helps Mosquitoes often bite at night. Nets stop them reaching people.
• Insect repellent; antimalarial drugs for travellers; How it helps Fewer bites; drugs kill the protist if infection happens.

Worked example: comparing rates

In one season, a town using treated nets had 42 malaria cases among 2800 people. A nearby town without nets had 117 cases among 1800 people.
With nets: 42 ÷ 2800 × 100 = 1.5%. Without nets: 117 ÷ 1800 × 100 = 6.5%.
Reduction = (6.5 − 1.5) ÷ 6.5 × 100 = 76.9%.

Answer: The rate was about 77% lower in the town with nets

Exam tip: Other factors could also differ between the towns, such as how close each is to standing water. A fair comparison controls these.

Common mistakes

• Calling malaria a bacterial or viral disease. It is caused by a protist.
• Saying the mosquito causes malaria. The mosquito is the vector; the protist is the pathogen.
• Saying rose black spot is spread by insects. It is spread by water and wind.
• Putting infected rose leaves on the compost heap. They should be destroyed, or the spores spread again.

Revision questions

Try these, then check your answers below each one.

Question 1. What type of pathogen causes malaria? (Choose from: Virus, Bacterium, Fungus, Protist.)

Answer: Protist

Working: Malaria is caused by a protist. Mosquitoes carry it from person to person.

Question 2. The diagram shows a healthy rose leaf and a leaf infected with rose black spot. How does this disease spread from plant to plant? (Choose from: By insects feeding on sap, By spores carried in water or by the wind, Through the soil only, By birds eating the leaves.)

Answer: By spores carried in water or by the wind

Working: Rose black spot is a fungus. Its spores are spread by water (such as rain splash) and by the wind.

Question 3. True or false? The mosquito is the pathogen that causes malaria.

Answer: False

Working: False. The protist is the pathogen. The mosquito is the vector: it carries the protist from one person to another.

Related lessons

• Previous lesson: B5.5 Bacterial Diseases
• Next lesson: B5.7 Human Defence Responses
• In this chapter: B5.1 Health and Disease
• In this chapter: B5.2 Pathogens and Disease
• In this chapter: B5.3 Preventing Infections

Continue in the full course

This lesson is section B5.6 (AGCSC0506) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.