Plant Pathogens and Pests

Plant pathogens and pests

Students’ Sheet

Killer diseases?

Plant disease – it doesn’t sound very serious, does it?

But each year just one killer fungus, rice blast, destroys enough rice to feed 60 million people. That’s almost the whole of the UK population. When an epidemic of rice blast sweeps through a country, harvests fail, food prices soar and people starve. Plant diseases are just as deadly to people as a human disease can be.

Plant pathologists around the world are working to understand the science behind these killer diseases, and are collaborating with farmers to solve the problem.

Around 70% of major crop diseases are caused by fungi, which leads to an economic cost of billions of dollars each year. Fungi have unique adaptations which allow them to exploit plants and gain essential nutrients. Conversely, plants also exhibit a range of defence mechanisms to combat attack from pathogens.

Your investigation

Your task is to carry out an investigation to find out how plants can be infected by pathogens, and damaged by other pests. You will be able to consider the impact of these plant diseases on crop productivity and world hunger and poverty. You will also carry out your own research into the defences exhibited by plants and compare them to animal defences.

Although some plant diseases, like rice blast, are very serious, others can be safely worked with in the school lab. In this investigation, you will use common plant diseases as models for more serious organisms.

You will prepare microscope slides of diseased leaves and use a microscope to find out how the pathogens are able to infect leaves.

Safety: wash your hands thoroughly after handling the plant material.

Instructions

1. Powdery Mildew

Collect a leaf with powdery mildew growing on it. Take a small piece of sticky tape, place it across the surface of the leaf, so that it sticks to the powdery mildew and then remove. You should have an impression of the leaf on the tape.

Stick the tape to a glass slide and view at the lowest magnification. Focus the image and look for hyphae of the fungus. Describe what you see.

2. Phytophora

Phytophthora may be found on Rhododendron leaves. Choose a suitably infected area and place a piece of sticky tape firmly on to it. Peel it off gently and place it on a microscope slide. Look under a microscope for signs of fungal infection.

3. Rose Black Spot

Collect a rose leaf and paint some clear nail varnish onto the surface- try both sides for now. Wait for it to dry and then peel off.

Place the peel on a microscope slide and have a look using a microscope. Describe what you can see.

4. Tar Spot fungus

Place a piece of sticky tape firmly on the infected part of the leaf and then place on a slide. Look for signs of leaf invasion.

5. Mite infection on Sycamore leaves

Place a piece of sticky tape to the underside of the leaves and press firmly. Look for evidence of mites, or other pathogens or pests that may be present.

6. Chestnut leaves

Break open a leaf that looks infected. You may be able to see the larva of a moth within the leaf.

Discussion Questions

1. What modes of transmission do pathogens use to get to the leaves?

2. How do fungi grow on leaves?

3. How do you think fungi infect the plant? (Think about the structure of leaves.)

4. Why do fungi infect plants? What advantage do they gain?

5. How might this infection affect crop productivity?

6. What effects would this have on World hunger and poverty?

7. What defences do you think a plant might have against pathogen attack?

8. How might defences of plants compare to animal defences?

9. Why do crops need passports?

Plant pathogens and pests: p. 1

Revised 2012.