to 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
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.
plants with insecticides is a chemical method, but potentially
harmful to harmless organisms including poisoning pollinating
Plants have also developed physical defences against pathogens and also
to deter animals from eating them.
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,
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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(b) A plant's
and mechanical defences
Physical defences rely on barriers preventing
pathogens from getting into undamaged plants.
The leaves and stems of most plants have an outermost
layer of the covering that acts as a barrier to inhibit pathogens entering
from getting through the epidermis and damaging the leaves or stem.
tough waxy cuticle also stops water accumulating on the leaves, so
reducing infection by pathogens that are transferred between plants via
(Note: The waxy cuticle also prevents excessive loss of water
from the leaves - it reduces the rate of evaporation so the plant does
not become dehydrated.)
Unlike animal cells, plant cells have a
stronger cell wall made of
This acts as a physical barrier against intrusion by a
pathogens that get through the waxy cuticle.
If pathogens do get past these physical defences,
their presence can trigger the cell to produce a chemical called
callose. The callose is deposited between the plant cell walls and
the inner cell membranes to reinforce the cell wall.
Around their stems,
plants have layers of dead cells that acts as
a physical barrier to pathogens e.g. the bark on trees is he obvious
Pests cannot get to the healthy living cells
underneath the bark.
Plants show specific
adaptations to deter animals from even
touching, as well as not eating them.
You might call them mechanical defence systems! e.g.
Thorns and hairs
Thorns will prick insects and deter them from
egg laying. The sharp thorns will also deter grazing herbivore
animals from munching the plant and maybe wear down their teeth!
Hairs stop larvae from reaching and feeding on
the outer epidermis layers of leaves.
Some plant leaves droop or curl when touched by an animal.
When touched, some plants release chemicals
that releases water from vacuoles causing cells to collapse and the
leaves to curl.
help prevent being eaten by having the insects knocked off
Certain plants can
mimic other organisms
e.g. adaptations to inhibit animals like insects
from feeding or laying eggs on them.
Mimicry is displayed by the passion flower has bright yellow spots on its leaves that look
like butterfly eggs, deterring other butterflies from laying their eggs
on the leaves.
Some grasses have evolved anthers that look like
aphids and hollyhocks have stem markings that also look like aphids -
these visual signals deter aphids from attacking and feeding off the
Some species of plant in the 'ice plant family' in southern Africa
look like stones and pebbles, and so don't look very tasty to some
predatory animal, so they are far less likely to be eaten!
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(c) A plant's
Plants do not have specialised immune cells or
antibodies like animals do.
However, if a pathogen gets past the physical defences
(described above), the plant can detect it and the response is to produce
antimicrobial molecules that act as a chemical defence.
plants can produce chemicals to help defend themselves against damage.
These chemicals can make the plants unpleasant to
taste to herbivores (grazers!) or even cause death by interfering with a
pathogen's metabolic pathways - poisoning!
However, some of these poisons are useful to
us humans via the pharmaceutical industry.
e.g. The deadly nightshade (great name!)
produces a toxin called atropine which is used to treat cardiac
These chemicals, called
against microbes) which can kill pathogens or inhibit their growth.
Some plants increase production of these
antibacterial chemicals when they are attacked by herbivores or
Some plants produce toxic chemicals called
saponins, which are believed to destroy the cell membranes of fungi
and other pathogens.
Certain plants produce chemicals called
phytoalexins when pathogen infection is detected. Phytoalexins
disrupt the metabolism and cell structure of some species of bacteria
Other plants can produce chemicals called
antiseptics that kill bacteria
and fungal pathogens.
The willow tree produces an antiseptic chemical.
Mint and witch hazel produce
chemicals toxic to insects.
Mint produce menthol - an insect toxin, and witch hazel produce
chemicals which kill bacteria and tannin which is another insect toxin.
Spearmint plants produce an oil that acts as a
natural fungicide to kill fungus.
The concentration of antibacterials increases if
the plant is attacked by herbivores or pathogens.
Plants can produce
chemicals to deter
being eaten e.g. insect pests from feeding on
Chemical defences include plants containing
unpleasant tasting chemicals to deter animals from feeding off them,
some chemicals are so toxic they may even kill the 'grazer'.
Plants like the tobacco plant, foxgloves and deadly nightshade
produce poisons that inhibit organisms that eat plants (herbivores).
Tobacco plants contain nicotine which acts as an
Foxgloves contain a poison called digoxin, which
can be toxic if ingested by insects, but digoxin is also used in some
treatments for cardiovascular disease!
Some of these plant produced
natural chemicals, or their derivatives,
be used as drugs to treat human diseases or in medicines to relieve symptoms
The medicinal compound Aspirin is used to relieve pain and fever
- aspirin is an anti-inflammatory drug.
It is synthetically derived from a chemical compound found in the bark
and leaves of the willow tree.
The willow extract was known for
centuries to be a pain reliever - an example of traditional medicine -
now transformed into a 'modern' analgesic drug completely synthesised
from a basic organic compound.
Quinine, an ant-fever drug, is still one of the main treatments
Malaria is caused by a parasitic single celled organism
from which you can be infected from a mosquito bite. Quinine is a very
complex molecule and it is too costly to synthesise.
Therefore it is
still obtained from its original natural source, the bark of the
The pharmaceutical industry and genetic
There are many cases where a naturally
occurring chemical compound in plants (with known 'medical' effects
found) is used as a starter molecule for developing new drugs
Some of these molecules are those produced by
the plant for self-defence.
Pharmaceutical chemists can then synthesise
different forms of the molecules and these derivatives then tested
to see if they are potentially useful drug.
Scientists have identified the plant genes
that are responsible for producing these self-defence molecules.
This genetic knowledge used in genetic engineering to produce insect
resistant and disease resistant crops. In other words, to get plants
that don't normally produce these self-defence molecules to be
genetically modified to produce them! Smart stuff!!!
Know and understand that
plants may be adapted to cope with
specific features of their environment, these specialised features to deter
predators include thorns,
poisons and warning colours to deter predators e.g.
Roses have thorns, hedgehogs
have needle like spikes/spines over the upper side of their body and can
curl up to give all round protection.
Cacti have sharp spines to deter
animals (herbivores) eating them, turtles, armadillos and tortoises have
hard protective shells. These are examples of organisms having a sort of
'armour' for protection!
Plants like ivy contain poisons,
some desert shrubs secrete toxic
compounds into the soil to prevent other plants growing nearby.
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plant diseases and how
they are spread
There several ways in which plant diseases
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
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
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
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
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
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
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.
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control, reduction and prevention of plant diseases
Aphids have already been mentioned in the
Plants are the start of most food chains, so they are
of obvious importance to any subsequent source of food.
Plant crops are the most important source of food
for most of the World's population.
In poorer and developing countries anything that
reduces crop yields affects people and can lead to famine.
The causes are usually the weather, increasingly so by
climate change and disease affected plants.
Plant disease can also affect:
ecosystems, affecting the balance of
biodiversity, some plant species might be
more susceptible to pathogen attack than others, possibly removing a
whole local population of a plant species.
Therefore, it is obviously important to control
plant disease as much as we are able to, but with little if any
The first step would be to identify the
disease-pathogen affecting the plants (described in the next section).
Examples of methods of controlling plant disease
Destroying affected plants:
the source of infection, BUT, wasted crops are costly to the farmer.
Healthy plants to start with
By using healthy plants, free of infection,
you avoid introducing a plant disease to wherever you plant them.
Fungicide sprays can be used to kill fungal
infections. Bulbs and seeds can be coated with an anti-fungal agent
that prevents the attack of the pathogen in the first place.
BUT, evolution is always at work, throwing up
mutations in the DNA of the pathogens. The result is the formation
of pathogen strains which are resistant to the chemical.
Crown gall disease can be prevented by
dipping the roots of the plant into a suspension of a similar
This is done before the plant is planted in a potentially
The selected bacteria does not infect the plant, but
it does produce an antibiotic that prevents the crown gall pathogen
You can use another organism to control an
insect pest of viral/bacterial pathogen.
Ladybirds are very fond of
the aphid insect, so ladybirds can be released to reduce the
population of aphids.
This is fine as long as the introduced
controlling organism doesn't become a pest itself, causing further
Since many pathogens are specific to a
particular plant, changing the plant that grows in a particular
field inhibits the pathogen from becoming permanently established in
However, there is an economic consequence of crop
rotation - extra cost from having to change crop each year.
Controlling the movement of plant material:
The basic idea is to prevent diseased plants
from coming into contact with healthy plants and slow down the
spread of the disease e.g.
To slow down Chalara ash dieback disease
you can remove infected ash trees and replanting them with other
different tree species e.g. birch
Plant nurseries must be careful not to sell
infected plants and must adhere to any sale restriction regulations
e.g. the import and export of plants such as ash trees or any
other potentially disease carrying plant.
Polyculture methods of crop production
Polyculture involves growing different types
of plants in alternately within the same single area at the same
time. The idea is that if a pathogen is present and specific to a
particular plant species, it is less likely to infect neighbouring
plants of a different species. Thus limiting the spread of the
pathogen through the crop.
Genetically modified plants for agriculture
(the controversial GM crops)
Plant scientists are working on the
development of disease-resistant crops by gene modification.
One particular problem are fungal rusts that
cause decreased crop yields in things like apples, coffee, oats and
pears, amongst many other types of crop.
It is hoped that GM plant seed will maintain
or even increase crop yields.
Disease resistant crops can be cloned and
plant cells and plants can be made in large numbers quickly.
methods of cloning plants for more
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detection and analysis of
plant diseases or nutritional deficiencies
No matter how much care a farmer, horticulturalist
or the humble gardener working in the garden, it is practically
impossible to stop all plant diseases.
Pathogens or insect of one kind or another
will find their way past a plants defences causing damage.
You therefore need methods to diagnose what
has actually caused the plant damage.
If you are a keen gardener you can look up your observations of an
apparently unhealthy plant in your gardening manual or gardening website -
the Royal Horticultural Society website has lots of information.
At some cost, really only for larger organisations like a farm, you can
send samples of the plant to be tested in a laboratory.
However, it is possible to do some advanced analysis for yourself using
testing kits that can identify the pathogen using monoclonal antibodies.
There is a need to detect plant diseases from field observations -
direct observation of plants in their natural habitat - often the symptoms
are quite plain to see.
Plant scientist, or even the amateur gardener!, can
recognise the symptoms of specific plant diseases.
Common signs of plant disease or mineral
1. stunted growth,
2. abnormal growths (e.g. lumps
- tumor galls, burrs),
3. spots on leaves,
4. rot - patches of decay,
5. discolouration - often yellowing or brown patches rather than a
healthy green tissue,
6. malformed stems or leaves,
Gardening manuals or website will
describe all these symptoms and possible remedies.
Experts in plant diseases, called plant pathologists (sounds
dramatic!), are able to recognise the symptoms of particular plant diseases
Abnormal growths, called galls, can indicate crown gall disease
(caused by a bacterial pathogen) in several different types of plants
e.g. apple trees and other fruit trees.
The crown gall pathogen enters the plant
through wounds in roots, stems and branches stimulates the plant tissues to grow in a disorganised
way, producing swollen galls (tumor growths).
The 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.
Tar spot (sycamore leaf picture on the
right) is a very conspicuous fungal leaf spot disease (rhytisma
acerinum) of sycamore and some others of the acer tree family like
maple. Although the large leaf spots are unsightly and sometimes cause
gardeners concern, they actually do very little damage to the tree, but
no photosynthesis can take place below the black spots. The disease can
cause slightly premature leaf fall, but fortunately it has no long-term
effect on the vigour of affected trees.
The tobacco mosaic virus causes the leaves to become discoloured and mottled which
You can control this virus by removing weeds
that may have this virus.
Remove plants infected with the virus.
Disinfect your gardening tools - sterilisation
Thoroughly washing hands after handling
These methods apply to try to control
other plant diseases.
Yellow leaves or stunted growth can be a symptom of
disease, but from some environmental cause e.g. a nutrient deficiency.
Some important nutrients are mineral ions from the soil
Without these essential mineral ions the plant cannot grow and
develop into healthy state and will display symptoms related to a
particular deficiency. If the soil is deficient in any essential
mineral ion, characteristic symptoms will show up!
(you will come across these ions in your GCSE chemistry
Nitrates provide the nitrate ion (NO3-),
a source of nitrogen for protein synthesis. Proteins are needed for
e.g. in tissue structure and enzymes, so nitrogen deficiency leads
to stunted growth.
The green chlorophyll molecule, essential for photosynthesis,
contains a magnesium ion (Mg2+). If a plant is
deficient in magnesium not enough chlorophyll can be made and
the plant suffers from chlorosis - a yellowing of the leaves,
and photosynthesis is much reduced - as is the supply of food and
energy for the plant.
You can also get chlorosis in plants from an
iron(II) ion (Fe2+)
If you change the environmental conditions e.g. by adding
nutrients to the soil (general fertiliser or specific nutrient
chemical like an iron or magnesium compound) you can then look for
any changes in the observed symptoms.
The treatment may work or not. Either way you learn something. If
the plant's health improves, problem solved, if not, then you must
look for other causes of the plant's poor health e.g. a disease
rather than a nutrient deficiency.
For more on this topic see the Appendix:
deficiency in plants and its consequences
We also need to be able to analyse plants for diseases in the
laboratory and to conduct research on prevention, if possible.
You can take infected plants to a laboratory
to identify the pathogen, but is costly, ok for a big commercial
You can get testing kits of monoclonal
antibodies - plants do not produce antibodies.
A rabbit can be injected with the plant
virus or an antigen of the virus, and the antibodies are
obtained from the animal plasma
Its much more convenient in the laboratory to do accurate and
detailed diagnostic testing of plant samples for the presence of specific pathogens.
Apart from visually examining the plant with the
eye to look for obvious symptoms (see section above) a microscope may be
needed to sort out more finer structural details e.g.
(i) to distinguish between
different strains of fungi that may look similar to the naked eye,
(ii) the microscopic detail of the results of
some infection from a virus or bacterium pathogen.
Some of the advanced techniques used by plant scientists
- the ELISA test
Most cells of plants (and animals) have unique molecules on their
surface called antigens.
You can detect the presence of these antigens, which will be
specific to a particular pathogen infecting the plant using antibodies.
Reminder - antibodies are proteins
that bind to a specific antigen.
You do this by testing the plant tissue using
Antigens from the pathogen will be present
in the infected plant.
With the ELISA test, antibodies that match
the pathogen's antigens are used.
These antibodies have enzymes attached to
them which can react with a substrate causing a colour change.
The antibodies are added to the plant
tissue sample being tested and washed off.
BUT, if the antibodies bind to the
antigens, they will remain on the plant sample.
If there is a colour change when the
substrate is added, it shows that the antigen was present i.e.
the pathogen was present.
The detection and identification of the pathogen gives you the
correct diagnosis of the plant disease.
- the Polymerase Chain Reaction (PCR) technique
If a plant is infected with a disease caused by a pathogen, the
pathogen's DNA will be in the plant's tissues.
It is now possible with advanced analytical techniques to detect
very small quantities of the pathogen's DNA in a sample of plant
Parts of the DNA strand complementary to that
of the pathogen are used as the primary template.
Any DNA that matches is repeatedly copied to
give a big enough sample to analyse.
Since all organisms have a characteristic pattern of DNA, its
possible to match the pathogen DNA trace with a database and accurately diagnose the
identity of the specific pathogen.
Isolation and reinfection
In the laboratory, you take a section of a
diseased plant tissue and add it to a growth medium.
This promotes the growth of the pathogen in
the infected plant.
You then isolate the suspect microorganism and
infect healthy plants with it.
If the healthy plants develop the same
symptoms of the disease you know that was the microorganism that
caused the disease in the first place.
Culturing microorganisms like bacteria for
more details of the aseptic techniques - to avoid contamination by
other microorganisms, therefore avoid identifying the wrong pathogen.
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in plants and its consequences
Nitrogen for amino acids
and protein synthesis (including enzymes) is obtained from the
nitrate ion (NO3-).
Lack of nitrogen gives stunted
The magnesium ion (Mg2+)
is required by the chlorophyll molecule to function in
photosynthesis and is also essential for the metabolism of
If a plant is deficient in
magnesium not enough chlorophyll can be made and the plant
suffers from chlorosis - a yellowing of the leaves, and
photosynthesis is much reduced - as is the supply of food and energy
for the plant.
The potassium ion (K+)
is involved in the mechanism for opening and closing stomata, the
activation of some enzymes, involved in photosynthesis, and the production of ATP
phosphate ions. A phosphate structure is part of the ATP
molecule from respiration - energy source, and part of the structure of
cellular DNA and RNA.
Iron ions (e.g. Fe2+
or Fe3+) are important in the synthesis of
chlorophyll and some enzyme functions.
You can also get chlorosis in plants
from iron deficiency. (see magnesium above)
Soil that is deficient in any of
these minerals can be improved by using synthetic artificial
fertilisers (NPK types for nitrogen, phosphorus and potassium) often
supplemented with lots of additives to supply other trace elements like
iron or magnesium that plants need. You must compost or manure if you
are a true organic gardener!
action and uses
communicable diseases and
General PLANT BIOLOGY revision notes
explained, limiting factors affecting rate, leaf adaptations
gcse biology revision
Plant cells, transport and gas exchange in plants,
transpiration, absorption of nutrients, leaf and root structure
Diffusion, osmosis, active transport, exchange of
substances - examples fully explained
Respiration - aerobic and anaerobic in plants (and
biology revision notes
Hormone control of plant growth and uses of plant hormones
gcse biology revision notes
Plant diseases and defences against pathogens and pests
gcse biology revision notes
Adaptations, lots explained including
gcse biology revision notes
and a section on
Stem cells and uses - meristems in plants (at the end of the