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8.4 Plant Diseases
Level 2 -Plant health problems
PLANT DISEASES
Organisms causing diseases are called pathogens and their study is plant
pathology. Diseases may be caused by:
a] Fungi
b] Bacteria,
c] Viruses
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: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.
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.
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.
Both types of fungi have similar structures and life cycles. A fungus is
composed in most species of microscopic tubular threads called hyphae
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8.4 Plant Diseases
Level 2 -Plant health problems
which form a loose 'root-like' 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
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
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.
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
Symptoms may include:Source:
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8.4 Plant Diseases
Level 2 -Plant health problems
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
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
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