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GCSE level biology exam revision notes on plant diseases Part
1
1A-1B.
Introduction
to plant diseases and harmful pests - examples of types of pathogen, crops attacked by insect
infestations
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INDEX
of Plant Disease Notes
(1A) Introduction
to plant diseases
They will help you answer questions
such as ... How do plants protect themselves from
pathogens? What chemical defences can plants
employ? What physical defences do adapted
plants have? What methods are used to detect plant
diseases?
Plants, like any other living organisms, are susceptible to attack by
pathogens causing diseases.
The study of plant diseases, pests and nutrient
deficiency and their effects are important for two reasons:
(i) Plants are usually the primary producers
in food chains - all part of the world's ecological systems.
(ii) We rely on plants for our food directly
or indirectly and these affect crop yields or destroy crops.
Its not difficult using fertilisers or
compost/manure to correct nutrient deficiency in the soil, but
its often quite difficult to control plant diseases and insect
pests.
The attack by
pathogens often causes leaf damage that reduces photosynthesis on which
plants depend for their own food and energy - so any means of defence is
useful and examples are described on this page.
The pathogens can be viral, bacterial or fungal.
A virus, bacteria or fungus can have harmful effects on a plant with
serious consequences if it is unable to defend itself against such attacks.
Plants can also be attacked and infested by insects
e.g. aphids cause
considerable damage to plants, and are not difficult to spot..
Aphids (greenfly) are very destructive common sap-sucking insects
that reproduce rather rapidly and can cause:
a lack of plant vigour - decreased growth
rate, distorted growth, mottle or yellow eaves (suggests
decreased photosynthesis), and often excrete a
sticky substance (honeydew) on foliage which allows the growth of sooty
moulds leading to infection.
Unfortunately, aphids can also act as
disease vectors conveying pathogens from plant to plant.
Aphids have specially adapted mouthparts
that pierce phloem tubes to feed on sap - in doing so they can
transfer viruses to the plant from their saliva.
The feeding punctures in the phloem cells
has the effect of the lowering turgor pressure in the plant
cells causing wilting.
Aphids can be controlled by using their
natural predators like ladybirds (eat them) and parasitic wasps
(lay eggs in them), these are biological methods.
Spraying
plants with insecticides is a chemical method, but potentially
harmful to harmless organisms including poisoning pollinating
bees.
Plants have also developed physical defences against pathogens and also
to deter animals from eating them.
e.g. sharp prickly
thorns!
Plants have evolved means of fighting against pathogens including the use
of chemical defences and some of these compounds have been of great interest
to pharmaceutical companies developing and producing drugs-medicines.
Plants are the start of most food chains, so they are
of obvious importance to any subsequent source of food for animals.
Therefore, the ability of plants to defend themselves
against infection by pathogens is not only important to the plant species
itself, it is also important for the survival of other organisms,
including ourselves!
Damage to crops lowers yields and endangers the
ability of human populations (or any animal population) to feed themselves
properly with a nutritious diet, both in quantity and quality.
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Plant disease:
1B.
Examples of
plant diseases and how
they are spread
including barley mildew fungus, chalara ash dieback, tobacco
mosaic virus, pathogen bacteria infections in soil, rose black spot fungus, potato blight
viral disease and other pathogens. There are various sections to work through, after 1 they can be read and studied in any order.
INDEX
of Plant Disease Notes
(1B) Examples of
plant diseases and how
they are spread
There several ways in which plant diseases
(pathogens) spread.
Plant pathologists analyse the distribution of diseased plants because it
helps identify the type of pathogen involved e.g.
isolated patches of disused plants suggest the disease is spread
through the soil and entering the plant through the roots,
but, a random distribution of diseased plants suggests an airborne
pathogen - a pathogen moved around at random by the movement of air and
e.g. just settling on plant leaves.
Examples of the ways pathogens can spread
Some pathogens are carried in the air
(airborne)
Erysiphe graminis is
a fungus that causes
barley mildew which produces white fluffy patches to appear on
the leaves of barley plants. The powdery coating of the mildew
reduces photosynthesis by reducing the light intensity and leads to
a decrease in the crop yield. The fungus is spread by spores blown
around from plant to plant by the wind.
Chalara ash dieback disease of ash
trees is caused by the fungus hymenoscyphus fraxineus.
The fungus infects ash trees - the symptoms include black blotches on
leaves and leaf loss (both reducing photosynthesis) and bark
lesions leading to fluid bleeding.
In the majority of cases the tree dies,
either directly or weakening so much it that the tree cannot
defend itself against another pathogen.
The spores are
transferred at random from infected trees to healthy trees by the movement of air - the wind.
It can also be spread when diseased ash trees are moved from one
area to another - not a good move!
Direct contact between plant and pathogen
A plant can become diseased if it comes into
contact with a surface contaminated with a pathogen.
The tobacco mosaic virus
(TMV) attacks many species
of plants including the tobacco, tomato, cucumber, pepper and some
ornamental flower plant.
The tobacco mosaic virus gets into the
plant through a lesion in the skin or epidermis of the plant.
The disease is caused by the tobacco mosaic
virus which causes the infected leaves to become discoloured and
mottled (hence described as a mosaic pattern).
The discolouration causes a
decrease in photosynthesis, this inhibits growth and reduces the
crop yield or quality of flower.
Other effects include curled leaves,
stunted growth and yellow streaks or spots on leaves.
The tobacco mosaic virus is spread
by infected leaves brushing against healthy leaves.
To control TMV
you need to remove infected plants, sterilising tools at high
temperatures, washing hands after handling infected material,
possibly use pest control too.
Pathogens in the soil
Certain pathogens can live and thrive in soil
and therefore plants can easily infected from the contaminated soil.
The bacteria Agrobacterium tumefaciens causes
crown gall disease and spreads freely in soil and can actually grow
on the roots of plants. The crown gall pathogen enters the plant through wounds in roots
or stems and stimulates the plant tissues to grow in a disorganised
way, producing swollen galls (tumor growths). The galls damage the
plant tissue, restricting the flow of water in that part of the
plant, weakening the plant as a whole and can cause it to die.
Rose black spot
fungus
Rose black spot is a serious parasitic fungal disease that
causes purple or black spots to develop on the stem and upper surface of leaves of rose
and other plants
and eventually the leaves turn yellow and drop off.
Rose black spot likes warm wet conditions to
grow.
With less green upper surface leaf area, photosynthesis is
reduced and thus reducing the growth of the plant.
The black spot fungus grows in the
mesophyll and penetrates into the intercellular space and the
plant cells themselves - damages cell membranes, causing
nutrients to leak out into the intercellular spaces - this
prevents photosynthesis, interrupting the food supply the plant
needs.
Rose black spot fungus produces spores (act
like seeds) and can spread through the
environment by the movement of wind or water.
Gardeners treat this kind of disease in
various ways:
(i) using
fungicides to kill fungus cells
(ii) stripping the affected leaves
from the rose plant - the stripped leaves should be then destroyed
to reduce the spread of the rose black spot fungus,
(iii) pruning shoots in the spring and
burning all cut stems,
(iv) don't use infected leaves and stems
in compost heaps,
(v) placing manure or mulch around the
plants in spring to stop fungal spores getting to the stems.
Potato
blight
Potato blight is another viral pathogen that attacks potato and
tomato crops.
The potato blight virus causes the potato plants to rot and turn
brown, the leaves shrivel, and the tubers decay.
Any infected potato plant material should be deeply buried, well
below the depth of cultivation, removed to the local council green
waste collection or burned, so best not to compost it.
In cultivation, earthing up potatoes well provides some protection
to the tubers from blight spores washed down into the soil from
lesions on the leaves or stems.
No fungicide is available at the moment to try and control potato
blight.
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Key points on
plant diseases caused attacks of pathogens and insects and spreading
Source of information is
based on textbooks and syllabus-specifications for students taking the AQA
GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)
Biology notes on
Introduction to Plant Diseases, including types of
pathogens, insect infestations, and
examples of common diseases and how they are spread:
Introduction to Plant
Diseases
Plant diseases hinder
growth, reduce crop yield, and can devastate entire ecosystems or
economies.
They are caused by
pathogens - microorganisms that infect plants - and by
insect infestations, which can weaken plants directly or act as
vectors for disease.
Types of Pathogens That
Infect Plants
1.
Viruses
-
Example:
Tobacco Mosaic Virus (TMV)
-
Effect:
Causes a mosaic pattern of light and dark green on leaves, stunted
growth, reduces photosynthesis.
-
Spread:
Via contact between plants, contaminated tools, insect vectors.
2.
Bacteria
-
Example:
Agrobacterium tumefaciens
-
Effect:
Causes crown galls (tumour-like growths), damaging plant tissues.
-
Spread:
Through wounds in plants, contaminated soil or water.
3.
Fungi
-
Example:
Rose Black Spot
-
Effect:
Purple/black spots on leaves, leaf drop, weakens plant and reduces
flower production.
-
Spread:
Spores carried by wind or water, thrives in damp conditions.
4.
Protists
-
Example:
Late blight (caused by Phytophthora infestans)
-
Effect:
Affects potatoes and tomatoes; blackened leaves and tuber rot.
-
Spread:
Via water, rain splash, or contaminated equipment.
Insect Infestations
Insects can:
Common Examples:
-
Aphids:
-
Whiteflies:
-
Caterpillars:
Examples of Common Plant
Diseases and Their Spread
|
Disease |
Caused by |
Symptoms |
Spread By |
|
Tobacco Mosaic Virus
(TMV) |
Virus |
Mottled leaves,
stunted growth |
Contact with infected
plants, tools, insects |
|
Rose Black Spot |
Fungus |
Purple/black leaf
spots, early leaf drop |
Spores spread by
rain/wind |
|
Late Blight (Potato) |
Protist |
Blackened leaves,
soft, rotten tubers |
Waterborne spores,
contaminated soil/equipment |
|
Crown Gall |
Bacteria |
Tumour-like growths on
stems and roots |
Wounds, soil or plant
material contamination |
Summary: How Plant Diseases
Spread
-
Direct contact
between infected and healthy plants
-
Insect vectors
carrying pathogens
-
Soil, water, or
wind transporting spores
or bacteria
-
Contaminated
tools, hands or machinery
-
Seeds or planting
material already infected
More on how each main
type of plant pathogen affects plant health, with a focus on the
underlying damage they cause and how that influences growth,
reproduction, and survival:
1.
Viruses
✦
How they impact:
-
Hijack plant cells
to replicate themselves, interfering with normal cell function.
-
Cause mosaic
patterns on leaves, often reducing chlorophyll.
-
Stunt growth due to poor
photosynthesis and damaged tissues.
Example:
Tobacco Mosaic Virus (TMV)
-
Affects tobacco, tomatoes,
and peppers.
-
Leads to yellowing and
distorted leaves, less energy for the plant = smaller crops.
2.
Bacteria
✦ How they impact:
-
Invade through
wounds or stomata, then multiply rapidly in tissues.
-
Release toxins and
enzymes that destroy cell walls.
-
Cause galls,
rotting, and wilting depending on the species.
Example: Agrobacterium
tumefaciens (Crown Gall)
-
Induces tumorous growths
by inserting its DNA into plant cells.
-
Disrupts transport of
water and nutrients, making plants weak and vulnerable.
3.
Fungi
✦ How they impact:
-
Grow hyphae
that penetrate plant surfaces and absorb nutrients.
-
Block vascular tissues,
interrupting water and nutrient transport.
-
Reproduce via
spores that spread quickly in moist conditions.
Example:
Rose Black Spot
-
Spots reduce
photosynthetic area.
-
Leads to leaf loss,
weakening the plant and reducing flower production.
4.
Protists
✦ How they impact:
-
Typically waterborne, they
invade and live inside plant cells.
-
Cause cell
destruction, leading to rotting or necrotic lesions.
Example: Phytophthora
infestans (Late blight)
Each of these pathogens
disrupts a different system - whether it’s photosynthesis, water
transport, or cell structure—and leads to weaker plants, reduced
yield, and poor survival in the long run.
Keywords, phrases and learning objectives for this part on
plant diseases and nutritional
deficiency
Know the various ways in which plants become diseased
e.g. types of pathogen
like viruses, bacteria, fungi and attacks from insect infestation.
Be able to describe and understand examples of plant
diseases and how they are spread including fungus that causes barley
mildew, chalara ash dieback disease, tobacco mosaic virus, other
pathogens of bacterial infections in soil, rose black spot fungus
and potato blight, a viral disease.
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igcse/gcse biology notes on plant diseases & harmful pests, Edexcel gcse
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