UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 21/05/2023 [SEARCH]

Plant disease: 6. The detection and analysis of plant diseases or nutritional deficiencies

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(6) 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 - symptoms

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 see Part 7. 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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