GCSE · Biology · Edexcel · Spec 1BI0
Detection and identification of plant diseases
Your plant's leaves have gone yellow. Disease? Maybe. Or maybe something else entirely. Before anyone jumps to a conclusion, it's time to play plant detective.
Biology · Plant disease
Which method produced this evidence?
Pick a card, then choose the method that produced the clue. Read the reason before you move on.
Which method gives you this kind of evidence?
Still to sort
Observing symptoms (0)
Look closely at the plant itself, and compare what you see with photographs.
Where the line is: Symptoms are what you can see on the plant. They are clues, but not proof of which disease it is.
Analysing distribution (0)
Look at which plants are affected and how the disease passed between them.
Where the line is: This is about the pattern across plants, not about what one leaf looks like.
Culture on an agar plate (0)
A scientist grows a sample from the plant and studies the colonies.
Where the line is: Culture is about how the colonies look and the conditions they grow in. Looking at single cells is the microscope's job.
Microscope and stains (0)
A scientist looks at the pathogen's cells, using stains.
Where the line is: The microscope shows the shape of the cells. It doesn't need the pathogen to be grown into colonies first.
Antibody test (0)
A sample from the plant gives a positive or negative result for a suspected disease.
Where the line is: An antibody test answers one question about one suspected pathogen. Genetic tests are different: they use DNA.
Genetic tests (0)
DNA is used to show that a pathogen is present, or to read its genome.
Where the line is: Genetic tests work with the pathogen's DNA base sequence, not with its antigen.
Each card is one clue from a plant-disease investigation. Decide which method gave you that clue, then re-sort the same cards by who can gather it.
Biology · Plant disease
One symptom, several possible causes
Yellow leaves. Pick a branch to see what it could mean.
Possible cause → What kind of disease
4 possible explanation.
Start at yellow leaves and follow each branch to see where it ends.
Predict, then check
Think about what the pattern of who is ill and who is healthy could rule out.
In a garden, plants of one species show the symptoms, while plants of other species growing right beside them stay healthy. What does that suggest?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
A symptom is a clue, not a diagnosis. So how do you work out what is really wrong with a plant?
What you need to know
- Finding out what is causing a plant disease matters: it means quick treatment and stops the disease spreading to other plants.
- Symptoms are clues: dark spots on leaves, yellow or wilting leaves (drooping downwards), or leaves discoloured in a mosaic pattern.
- A gardener or farmer can compare their plant with photographs in gardening manuals or on websites to work out what is wrong.
- Yellow leaves can also come from environmental factors such as a mineral deficiency or pollution, so those must be investigated too.
Have a goYour friend looks at a plant with yellow leaves and announces, 'Definitely a disease. Case closed.' Give them one other thing the yellow leaves could be telling you.
A mineral deficiency (or pollution).
Environmental factors can cause yellowing too, so yellow leaves alone can't prove a disease.
- Distribution helps: some plant diseases infect only specific species, so seeing which species are affected can eliminate certain diseases.
- Knowing how a disease spread between plants, by wind, water or infected tools, helps to identify it and stop the spread.
- The same symptoms can often come from more than one type of disease, so scientists may be needed to identify the pathogen.
Have a goTwo different diseases give a plant exactly the same spots. Your friend says, 'Just look harder.' Why won't that solve it, and who might step in?
Looking can't separate them, because the same symptoms can come from more than one disease. A scientist may be needed.
When symptoms are shared, you need evidence from the pathogen itself, which is where a scientist's tests come in.
- Scientists can grow a sample in culture on an agar plate; colony appearance and specific growing conditions help identify the pathogen.
- Under the microscope, bacteria cells can be sphere-shaped (coccus) or rod-shaped (bacillus); shape plus particular stains helps identify them.
Have a goA scientist sees rod-shaped bacteria cells under the microscope. Which name goes with rod-shaped: coccus or bacillus?
Bacillus.
Coccus is the sphere shape and bacillus is the rod shape; the cell shape together with stains helps identify the pathogen.
- An antibody test and a DNA probe each bind only to one specific pathogen, so a positive result shows that pathogen is present.
The big picture
Finding out what is causing a plant disease means gathering evidence. A gardener or farmer can look at symptoms, compare them with photographs, rule out environmental causes and study where the disease is. When the clues are shared, a scientist can culture a sample, use a microscope and stains, and run antibody and genetic tests.
Key points
⚠ Watch out
Thinking you can identify a plant disease just by looking at its symptoms. Yellow leaves or dark patches can be caused by environmental factors, and the same symptoms can come from more than one type of disease. That is why distribution and scientific tests are used as well.
Memory hook
Clue, not verdict. And the two lock-and-key tests: an antibody fits one pathogen's antigen, a DNA probe fits one pathogen's base sequence, and a positive signal means that pathogen is there.
Check yourself
A farmer's crop has yellow leaves. Name one cause that isn't a disease, one thing to look at across the field, and one test that binds to a single specific pathogen.
Flashcards
(15)Why is it important to diagnose what is causing a plant disease?
What does it mean when leaves are 'wilting'?
How can a gardener use manuals and websites to help identify a plant disease?
Besides pollution, name an environmental factor that can make leaves turn yellow.
Why does knowing which species are affected help identify a plant disease?
Name three ways a plant disease can be spread between plants.
Why might a scientist be needed to identify a plant disease?
What does each bacterium grow into on an agar plate?
Why do specific growing conditions help identify a pathogen?
What two things together help identify a pathogen under the microscope?
Why does a positive antibody test show that a particular pathogen is present?
What is a DNA probe?
Which bases pair up when a DNA probe binds?
How does a DNA probe show that a pathogen is present?
What is genome sequencing?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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