Plant pathogens and pests
Technical and Teaching Notes
Teaching Notes
This practical is designed to allow an open ended investigation into how pathogens and
pests infect plants and the consequences this has on World food production. It is relevant to
many aspects of A-level biology, such as leaf structure during the teaching of
photosynthesis, productivity of crop species and reasons for using pest control. The
investigations will start with preparing microscope slides of leaf pathogens and asking pupils
to describe what they see.
Some diseases will change the colour of infected plant cells dramatically (e.g. to a bright
pink), allowing cells, stomata, guard cells and trichomes to be much more clearly seen.
In view of the current publicity over ash dieback and similar plant diseases, this may also
make an intriguing activity for a science club.
Aims of the practical
1.
2.
3.
4.
To be able to recognise plant diseases in the environment
To investigate how plant pests infect a plant
To suggest ways in which a pathogen might infect a plant
To suggest how plants might defend themselves from pathogens
Safety and biosecurity
Until further guidance is provided by the relevant authorities, we would advise against using
ash in this practical, to help minimise the spread of ash dieback (Challara fraxinea).
Guidance from the Society for General Microbiology should be followed when carrying out
microbiology work in schools: http://www.microbiologyonline.org.uk/teachers/safetyinformation.
To minimise the spread of plant diseases, all plant material should be disposed of in your
domestic waste route (i.e. ‘black bins’), rather than composted.
We recommend using plant material sourced as locally as possible to your site to minimise
the spread of disease.
The plant diseases looked at here are common in parks and gardens and are not
understood to pose a risk to human health. However, some plant diseases can be harmful to
immune-compromised humans (e.g. the bacterium Pantoea agglomerans, as shown in the
picture of the tea plantation on the SAPS website).
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Plant pathogens and pests: p. 1
This document may be photocopied for educational use in any institution taking part in the SAPS programme. Revised 2012.
Since students will be working in close proximity to the diseased leaves, you may wish to
place the leaves in small clear plastic bags and ask students to keep them within the bags
while looking at them. Leaf diseases can still be seen clearly under a microscope from within
a clear bag.
Students should wash their hands thoroughly after each activity.
Method
The practical will begin by looking at leaves collected from the local environment. These
should be brought into school to save time, especially if the school does not have a wildlife
area or is in a very urban setting. If this investigation was run as a project, perhaps in an
extracurricular club, there might be opportunity to go and investigate what can be found
around the school.
Pupils can investigate the leaves in a number of ways:

Powdery mildew samples should be removed using a piece of sticky tape which can
be stuck onto a microscope slide. This can then be viewed under a microscope, and
fungal hyphae should be observed.

Images of the surface of the leaves can be taken using a hand held digital
microscope, and spores and branching hyphae of the fungus can be viewed, see
Figures 1-3. Some diseases will change the colour of infected plant cells dramatically
(e.g. to a bright pink), allowing cells, stomata, guard cells and trichomes to be clearly
seen.

Rose black spot can be investigated by taking a leaf peel using clear nail varnish (as
in a stomatal peel) to paint a section of the leaf. When dry, peel the nail varnish off
and stick it to a microscope slide. View the slide under a microscope.

Take a Sycamore leaf with Tar Spot fungus on it and take a sample using a piece of
sticky tape.
Fig.1. The surface of a leaf
covered in branching hyphae
Fig. 3. A spore on the surface of
ScienceFig.
& Plants
2. A spore
for Schools:
on thewww.saps.org.uk
leaf
the leaf.
Plant pathogens and pests: p. 2
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Looking out for infected leaves in the local environment
There are many good examples that are easy to spot, even for the non-specialist. You could
pick leaves that have a powdery mildew (many species of fungi cause this) across their
leaves. I found examples of Oak and Sycamore leaves in my garden (see Fig 4 and 5.)
Fig. 4.
Powdery
mildew on an
Oak leaf.
Fig. 5. Powdery mildew on a Sycamore leaf
Rose black spot is also abundant (Fig 6.) and if you paint clear nail varnish across the
surface of the spot and then do a peel, you should be able to see signs of the fungus.
Fig. 6. Rose black spot
Another example would be to collect Sycamore leaves with red galls on them, which have
developed as a result of feeding by the mite Aceria macrorhynchus. The female mites
survive the winter in crevices in the bark, emerging in spring to feed on the new leaves. The
galls on the underside of the leaves are fringed with hairs, and the mites use these galls to
lay eggs in, around May time. The developing larvae feed on the tissues that line the galls.
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Fig. 7. The red galls caused by mites. This leaf
also has Tar Spot fungus evident.
There may also be some Tar Spot fungus present on Sycamore leaves, which can be
investigated (Fig. 8).
Fig. 8. Tar Spot Fungus on Sycamore leaves.
The Horse Chestnut trees were infected with leaf miner everywhere I went this year. The leaf
miner is a moth , Cameraria ohridella, that infect the leaves and feeds on the sap in the
veins of the leaf (Fig. 9). It eventually breaks out of the leaf.
Fig. 9. The results of the Chestnut leaf miner
Alternatively if no examples of pathogens or pests can be found in the wild, downy or
powdery mildews may be found on the surfaces of grapes that have been left out. Leeks
bought, from a farm shop, with their roots attached and potted at home will show start to
show signs of rust fungus. If you buy leeks with the root base still attached and put them in
water, roots should quickly develop and them they can be potted.
Apparatus
Per group


Microscope
Hand held digital microscope (if available)
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Plant pathogens and pests: p. 4
This document may be photocopied for educational use in any institution taking part in the SAPS programme. Revised 2012.




Slides
Sticky tape
Clear nail varnish
Examples of pathogens, as detailed above
Answers to some discussion questions
How do fungi infect leaves?
Fungi such as rust fungi have spores that can be blown for vast distances. If a spore lands on a leaf
and there is a water source, the hyphae will begin to grow. The spore contains all the nutrients the
fungus needs until it is able to penetrate the leaf surface. The spores are touch sensitive and can feel
their way across the leaf surface until they reach a stoma they can enter. The fungus will produce a
specialised structure, an Appressorium, that provides a hold fast platform to allow the fungus to
penetrate the stoma. A thin fungal hypha can then grow into the plant (an infection peg) and
eventually into a cell where it forms a haustorium, a specialised nutrient absorbing structure.
What modes of transmission do pathogens use to get to the leaves?
Fungi may spread from water droplets or splashes onto the leaves. They may have spores that lie
dormant in soil or in dead leaves. Viruses may be transmitted by vectors such as insects which may
pierce the stem of plants. Aphids, for example use their stylets to suck sap from the phloem of plants
and may spread viruses this way.
Why do fungi infect plants? What advantage do they gain?
Fungi infect plants to gain nutrients from the cells or from the phloem. If they are biotrophic, as many
of the examples looked at are, they invade host tissue but do not kill the plant.
How might this infection affect crop productivity?
As crop productivity is closely linked to the rate at which a plant can photosynthesise, any damage to
the photosynthetic organs would lead to a drop in productivity. If sugars and other nutrients are also
being absorbed by pathogens and pests, then this will allow less biomass to be made.
What effects would this have on World hunger and poverty?
A loss of crop productivity would have a huge effect on World hunger, if it affected a major crop such
as rice or wheat. As these are staple food sources in the developing world, a loss in productivity
would mean that there would not be enough food. As many people in the developing world also farm
crops such as rice, a loss in productivity would also increase poverty levels.
What defences do you think a plant might have against pathogen attack?
Protection from a pathogen's initial invasion is achieved via physical and chemical barriers.
Physical barriers
The structure of the surface of many leaves includes the epidermis, a waxy cuticle made from a
polymer called cutin, and stomata. The cell walls of plant cells can also create a barrier for pathogens.
Pathogens produce a range of cutin-degrading enzymes, which may cause penetration through the
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cuticle. The thickness of the cuticle, the presence of secondary cell wall, and the size of stomatal
pores can all affect the success with which a pathogen invades a host. Leaves that lie in a vertical
position also help to improve plant resistance, by stopping moisture films forming on the leaf surfaces,
reducing infection by pathogens that rely on water for movement.
Chemical barriers
Plants can metabolise a great number of different antimicrobial chemicals. Phenols and quinones are
two classes of antimicrobial compounds produced by some plants. Lactones, cyanogenic glucosides,
saponins, terpenoids, stilbenes and tannins are also plant-produced compounds associated with
pathogen resistance. Saponins destroy membranes in saponin-sensitive parasites, and are stored in
an inactive form in the vacuoles of the plant cell, becoming active when plant cells are wounded or
infected. Defensins are a group of compounds which interfere with pathogen nutrition and halt their
development
How might defences of plants compare to animal defences?
Discuss the differences in terms of an immune system. This would be a good aspect for pupils to do
further research into, perhaps as a lead into an immunity topic.
Why do crops need passports?
There are no border checks for plants and plant products travelling between EC member states.
Serious pests to important crop plants could have devastating effect on the populations of crops and
native wild species. Some plant material hosts serious 'quarantine' pests and diseases and requires a
plant passport to allow its movement. Where required, a passport is needed both for movements
within and between member states, and additional requirements apply for movements into and within
EC Protected Zones.
Plant passports may only be issued by Growers who are registered and authorised for the purpose.
When marketed within the UK plant material that is ornamental, fruit and vegetable must satisfy
minimum quality standards. Such material must be accompanied by a supplier's document when
marketed. With some exceptions those supplying material must be registered and, in the case of fruit
and vegetables, accredited, by the Fera Plant Health and Seeds Inspectors (PHSI).
References
Deacon, J. (2000) Modern Mycology. Blackwell Science, Oxford.
http://bugs.bio.usyd.edu.au/learning/resources/PlantPathology/introduction.html Plant Pathology,
University of Sydney.
Dr Charles Lane and Dr Paul Beales. Practical leaders of Plant Pathology Practical, Gatsby Plants
Summer School.
Dr Robert Zeigler, Gatsby Plants Summer School.
This resource was developed by Sarah Bayliss after attending the Gatsby Plants Summer School
2011, funded by an IMPACT award from the National Science Learning Centre.
The British Society of Plant Pathologists has a useful website, including photo gallery, from which the
image on the SAPS website is taken. http://www.bspp.org.uk/imagegallery/index.php
Science & Plants for Schools: www.saps.org.uk
Plant pathogens and pests: p. 6
This document may be photocopied for educational use in any institution taking part in the SAPS programme. Revised 2012.