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.
Potato
blight
Potato blight is another viral pathogen that attacks potato and
tomato crops.
The potato blight virus causes the potato plants to rot and turn
brown, the leaves shrivel, and the tubers decay.
Any infected potato plant material should be deeply buried, well
below the depth of cultivation, removed to the local council green
waste collection or burned, so best not to compost it.
In cultivation, earthing up potatoes well provides some protection
to the tubers from blight spores washed down into the soil from
lesions on the leaves or stems.
No fungicide is available at the moment to try and control potato
blight.
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