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
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.
plants with insecticides is a chemical method, but potentially
harmful to harmless organisms including poisoning pollinating
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.
of biology notes on plant diseases
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