2.6 Plant Diseases and Defences

Question 1

What are the three different categories that plants can catch diseases from?

Answer 1

• Microorganisms: fungi, viruses, bacteria
• Larger organisms: insects
• Deficiency diseases: nitrates, magnesium ions

Question 2

How can plants develop diseases from deficiency of something? (2)

Answer 2

• Nitrates are needed for making proteins and therefore for growth. Plants without sufficient nitrates often get stunted growth where they don’t grow as much as they should
• Magnesium ions are needed for making chlorophyll which is needed for photosynthesis. Plants with a magnesium deficiency often suffer from chlorosis from lack of chlorophyll (yellow leaves)

Question 3

What are some common symptoms for plants that have diseases? (6)

Answer 3

• Abnormal growths/lumps
• Malformed stems/leaves
• Patches of decay
• Yellow leaves
• Stunted growth
• Spots of discolouration on leaves

Question 4

In some cases you can see the organism responsible for giving a plant a disease. What are 2 examples of these?

Answer 4

• Aphids
• Spider mites

Question 5

What are the different methods of finding out which disease infected a plant? (4)

Answer 5

• Take basic observations and try to match the disease that you see to a disease listed in a gardening manual or website
• Send a sample of the diseased plant to a plant pathologist (someone who specialises in plant disease). They will be able to perform more detailed testing, e.g. taking a tissue sample where they can look at it under a microscope and see the situation in more detail. They can also look for unique antigens that come from particular pathogens or run a DNA test that looks for pathogen DNA
• Trial and error: if your plant has yellow leaves so you think it has magnesium deficiency, you could give it some magnesium and see if the problem is solved then OR if you think it is a fungal disease you could spray it with antifungal chemicals and see what happens
• Use testing kits that contain monoclonal antibodies

Question 6

What are the 3 categories that plant defences are split by?

Answer 6

• Physical
• Chemical
• Mechanical

Question 7

What are the physical defences that plants have against pathogens? (3)

Answer 7

• Waxy cuticle: acts as a waterproof barrier. Stops pathogens entering the plant. It also helps prevent water collecting on the leaf too, reducing the risk of infection by pathogens that are transferred in water
• Cellulose cell walls: form a physical barrier against pathogens that make it past the waxy cuticle
• Layers of dead cells some plants have around their stem, e.g. bark

Question 8

What are the chemical defences plants use against pathogens? (2)

Answer 8

These are chemicals that the plants can secrete
- If a pathogen manages to cross their physical defences, they can detect it and respond by producing anti microbial substances as a form of chemical defence. e.g. phytoalexins may be produced in response to infection by a pathogen - these are thought to disrupt the metabolism and cell structure of some species of fungi and bacteria
- Poisons that deter or kill insects

Question 9

What are the mechanical defences plants use against pathogens? - similar to physical (2)

Answer 9

They act as more of a function rather than just acting as a physical barrier
- Thorns/hairs that stop animals eating/touching them
- Leaves that curl or droop if insects land on them

Question 10

How does the pathogen that causes crown gall disease enter the plant?

Answer 10

Through wounds on the plant