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Plant Diseases
Level 2 – Plant health problems
Introduction
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Organisms causing diseases are called
pathogens and their study is plant pathology.
Diseases may be caused by:
a] Fungi
b] Bacteria,
c] Viruses
Fungi
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Fungal Diseases
Fungi, of which there are over 100,000
species, are plant-like but lack chlorophyll so
they have to obtain, their food and energy from
other dead or living matter. Fungi affect their
hosts in different ways and may be:-
Fungi
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1] Parasitic - these are the main horticultural
fungal diseases. They attack and damage or
kill living plants.
2] Saprophytic - fungi living on dead and
dying tissues and organic matter. Many are
responsible for initiating the decay of dead
plants and breaking them down into organic
matter and eventually humus as in a compost
heap.
Fungi
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3] Symbiotic - fungi living in close and
mutually beneficial association with plants
eg. mycorrhizal fungi which are found on
the, roots of many woody plants in particular.
Fungi
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The majority of fungal diseases are
caused by microfungi. These are
very small but usually visible to the
naked eye eg mildews, grey mould,
rust, fungal leaf spots.
The larger macrofungi are
identified by typical toadstools
many of which are specific to
particular plants. There are no
methods of biological control.
Fungi
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Both types of fungi have similar structures
and life cycles. A fungus is composed in
most species of microscopic tubular threads
called hyphae which form a loose 'rootlike' structure called the mycelium which
ramifies into living or dead matter.
The hyphae of most fungi can produce
spores by which they reproduce, spreading
by wind or water.
They often produce tough resting spores
called sclerotia which can remain dormant
for some years and then infect a new crop.
Bacterial Diseases
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Bacteria are much less significant horticultural
pathogens compared to fungi [only 1,600 species].
Most bacteria are involved in decomposition.
They are structurally simpler than fungi consisting of
a single microscopic cell which reproduces by simple
division into two cells, a very rapid process in warm
conditions like a compost heap.
Symptoms and effects are very similar to fungi. Many
infections occur initially through injured tissues and
may be spread by rain splashes, insects, animals and
pruning tools. They are difficult to control.
Virus Diseases
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Extremely small organisms - sub microscopic.
Viruses enter plant cells and take over the
organization of the cell nucleus and produce
many more virus particles. Infection is
systemic in plants although symptoms may be
local. There is no method of chemical or
biological control.
Viruses can cause substantial loss of yield and
unacceptable appearance.
Virus Transmission
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Transmission methods include:a] Vectors such as peach potato aphid, leaf
hoppers, thrips, white fly.
b] Vegetative propagation methods
c] Mechanical transmission by hand and on
tools eg. tomato mosaic
Virus Symptoms
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Symptoms may include:a] Leaf mosaic, a yellow mottling eg, tomato
mosaic
b] Leaf roll eg. potato leaf roll
c] Stunting_ eg. Chrysanthemum stunt
d] Flower colour break eg. tulip break virus
Virus - Control
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Control is primarily by cultural methods including:a] Using virus free certified plants. eg. potatoes,
strawberries, raspberries
b] Using virus resistant plants eg. Tomatoes resistant
to TMV
c] Rotation to minimise soil borne viruses transmitted
by eelworms
d] Controlling vectors especially aphids
e] Rogueing and burning infected plants