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Introduction 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 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.

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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A plant's natural physical and mechanical defences against diseases

Physical defences rely on barriers preventing pathogens from getting into undamaged plants.

The leaves and stems of most plants have an outermost waxy cuticle layer of the covering that acts as a barrier to inhibit pathogens entering or pests from getting through the epidermis and damaging the leaves or stem.

The tough waxy cuticle also 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.

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.

This can help prevent being eaten by having the insects knocked off automatically!

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 plant.

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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A plant's natural 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 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 arrest.

These chemicals, called antimicrobials (act 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 pathogens.

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.

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 antibacterials and chemicals toxic to insects.

Mint produce menthol - an insect toxin, and witch hazel produce antibacterial 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 their leaves.

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 insecticide.

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, 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 - 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 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.

 

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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.

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The control, reduction and prevention of plant diseases

Aphids have already been mentioned in the introduction!

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 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.

See methods of cloning plants for more details

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The 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.

 

Field observations

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 deficiency 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,

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 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, 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 slows down photosynthesis.

You can control this virus by removing weeds that may have this virus.

Remove plants infected with the virus.

Disinfect your gardening tools - sterilisation procedure.

Thoroughly washing hands after handling infected plants.

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 course) 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.

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: Mineral deficiency in plants and its consequences

Laboratory testing

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 grower.

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 naked 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

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.

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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.

 

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.

See 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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Appendix: Mineral deficiency 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 growth.

The magnesium ion (Mg²⁺) is required by the chlorophyll molecule to function in photosynthesis and is also essential for the metabolism of carbohydrates.

 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 in respiration.

Phosphorus, from 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!

 

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See also action and uses plant hormones,   communicable diseases    and    non-communicable diseases

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 General PLANT BIOLOGY revision notes

Photosynthesis, importance explained, limiting factors affecting rate, leaf adaptations  gcse biology revision notes

Plant cells, transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure

See also Diffusion, osmosis, active transport, exchange of substances - examples fully explained

Respiration - aerobic and anaerobic in plants (and animals)  gcse 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

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

and a section on Stem cells and uses - meristems in plants (at the end of the page!)

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Doc Brown's School Biology Revision Notes

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