PLANT DISEASES and defences against pathogens and pests

including nutrient deficiency, the spread and analysis of diseases and the reduction and prevention of plant diseases

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page 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?

Introduction 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, 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.

A plant's physical defences against diseases

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 water.

(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 cellulose.

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 example.

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 automatically!

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 against disease

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 and fungi.

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 antibacterial chemicals which kill bacteria.

Plants can produce chemicals to deter e.g. insect pests from feeding on their leaves.

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 e.g.

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 cinchona tree.

The pharmaceutical industry and genetic engineering applications

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 ad medicines.

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 (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. 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.

The 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 populations,

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 environmental costs!

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.

Chemical control:

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.

Biological control:

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 from reproducing.

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 problems.

Crop rotation:

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.


Field observations

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 e.g.

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 photosynthesis.

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+) deficiency.


Laboratory testing

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 specific pathogens.

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

Detecting antigens - 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 monoclonal antibodies.

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.


DNA analysis - 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 tissue.

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

Photosynthesis, importance 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

See also 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

See also Adaptations, lots explained including plant examples  gcse biology revision notes

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