8.4 Plant Diseases
Level 2 -Plant health problems
PLANT DISEASES
Organisms causing diseases are called pathogens and theirstudy 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 butlack 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. Theyattack and damage or kill living plants.
2] Saprophytic - fungi living on dead and dying tissues and organicmatter. Many are responsible for initiating the decay of dead plants andbreaking 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. Theseare very small but usually visible to the naked eye eg mildews, greymould, rust, fungal leaf spots. The larger macrofungi are identified bytypical toadstools many of which are specific to particular plants. Thereare no methods of biological control.
Both types of fungi have similar structures and life cycles. A fungusis composed in most species of microscopic tubular threads calledhyphae which form a loose 'root-like' structure called the myceliumwhich ramifies into living or dead matter. The hyphae of most fungi canproduce spores by which they reproduce, spreading by wind or water.
They often produce tough resting spores called sclerotia which canremain dormant for some years and then infect a new crop.
Bacterial Diseases
Bacteria are much less significant horticultural pathogenscompared to fungi [only 1,600 species]. Most bacteria are involved indecomposition.
They are structurally simpler than fungi consisting of a singlemicroscopic cell which reproduces by simple division into two cells, a veryrapid process in warm conditions like a compost heap. Symptoms andeffects are very similar to fungi. Many infections occur initially throughinjured tissues and may be spread by rain splashes, insects, animals andpruning tools. They are difficult to control.
Virus Diseases
Extremely small organisms - sub microscopic. Viruses enter plantcells and take over the organization of the cell nucleus and produce manymore virus particles. Infection is systemic in plants although symptomsmay be local. There is no method of chemical or biological control.
Viruses can cause substantial loss of yield and unacceptableappearance.
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:-
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 byeelworms
d] Controlling vectors especially aphids
e] Rogueing and burning infected plants
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