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.
-
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.
