to plant diseases
Plants, like any other living organisms, are susceptible to attack by
pathogens causing diseases.
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. Aphids are very common sap-sucking insects
that can cause a lack of plant vigour, distorted growth and often excrete a
sticky substance (honeydew) on foliage which allows the growth of sooty
moulds leading to infection.
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.
A plant's physical defences
The leaves and stems of most plants have an outermost waxy cuticle
layer covering that acts as a barrier to inhibit pathogens entering or pests
from damaging them.
The waxy cuticle 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
Plants show specific adaptations to deter animals from even
touching, as well as not eating them.
e.g. thorns and hairs
Some plant leaves droop or curl when touched by an animal. This can
help prevent being eaten by having the insects knocked off
Certain plants can mimic other organisms e.g.
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 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!
A plant's chemical defences
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.
SO, plants can produce chemicals to help defend themselves against damage.
These chemicals, called antimicrobials (act
against microbes) which can kill pathogens or inhibit their growth.
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
They can produce chemicals called antiseptics that kill bacteria
and fungal pathogens.
The willow tree produces an antiseptic chemical.
Other plants like mint and witch hazel produce
chemicals which kill bacteria.
Plants can produce chemicals to deter e.g. insect pests from feeding on
Plants like the tobacco plant, foxgloves and deadly nightshade
produce poisons that inhibit organisms that eat plants (herbivores).
Some of these plant produced natural chemicals, or their derivatives, can
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.
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
for malaria. 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.
The spread of plant diseases
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. It is
transferred at random from infected trees to healthy trees through
the air by the movement of air - the wind.
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 attacks many species
of plants including the tobacco, tomato, cucumber, pepper and some
ornamental flower plant. The disease is caused by the tobacco mosaic
virus which causes the infected leaves to become discoloured and
mottled (hence described as mosaic). The discolouration causes a
decrease in photosynthesis, this inhibits growth and reduces the
crop yield or quality of flower. The tobacco mosaic virus is spread
by infected leaves brushing against healthy leaves.
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.
control, reduction and prevention of plant diseases
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: This removes
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
bacterium. This is done before the plant is planted in a potentially
infected soil. 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
that location. 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.
Plant nurseries must be careful not to sell
infected plants and must adhere to any sale restriction regulations.
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.
The detection and analysis of
plant diseases or nutritional deficiencies
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.
Plant scientist, or even the amateur gardener!, can
recognise the symptoms of specific plant diseases.
Common signs of plant disease include
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,
malformed stems or leaves,
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
or stems and 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 affects
Yellow leaves or stunted growth can be a symptom of
disease, but from some environmental cause e.g. a nutrient deficiency.
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.
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) e.g.
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+)
We also need to be able to analyse plants for diseases in the
laboratory and to conduct research on prevention, if possible.
Its much more convenient in the laboratory to do accurate and
detailed diagnostic testing of plant samples for the presence of
Apart from visually examining the plant with the naked
eye to look for obvious symptoms (see section above) a microscope may be
needed to sort out more finer structural details e.g. to distinguish between
different strains of fungi that may look similar to the naked eye.
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
Reminder - antibodies are proteins
that bind to a specific antigen.
You do this by testing the plant tissue using monoclonal
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.
General PLANT BIOLOGY revision notes
See also cell biology section
explained, limiting factors affecting rate, leaf adaptations
gcse biology revision notes
Transport and gas exchange in plants,
transpiration, absorption of nutrients, leaf and root structure gcse biology revision notes
Diffusion, osmosis, active transport, exchange of
substances - examples fully explained
Respiration - aerobic and anaerobic in plants gcse
biology revision notes
Hormone control in plants 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