Aphids have already been mentioned in the
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
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
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:
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.
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.
Crown gall disease can be prevented by
dipping the roots of the plant into a suspension of a similar
This is done before the plant is planted in a potentially
The selected bacteria does not infect the plant, but
it does produce an antibiotic that prevents the crown gall pathogen
You can use another organism to control an
insect pest of viral/bacterial pathogen e.g. predatory
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
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
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
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.
methods of cloning plants for more
Be able to describe and understand the control,
reduction and prevention of plant diseases.
Know that methods of controlling plant disease
include using chemicals, destroying affected plants,
biological control using predatory insects, crop rotation and
polyculture methods and planting genetically modified plants.