.      Bacteria
2.      Fungi
3.      Virus
4.      Nematode

1.      Bacteria: These are micro-organisms which can not be seen with naked eye unless we use a powerful microscope. Bacteria are found every when even under a condition where other living organism cannot exist such as in water in soil in the air in plant in animal, etc
Bacteria: These  are useful to man in the following ways:
i.                    The help in the decomposition of compost  and organic materials.
ii.                  They are useful in the fermentation process e.g. production of drinks such as beer, wine, etc.
iii.                They are also useful in the making of cheese.
iv.                They also help to fix atmospheric nitrogen into the nodules of legumes..e.g Azotobacter, Nitrosomonas.

On the other hands, bacteria causes crop disease which are transmitted  from one plant to another by wind, rain splash and by insects. Examples, of such disease include the following Bacteria Blight of Cotton, Bacteria Blight of Guinea Corn,
Bacteria Wilt of tomatoes.

2.      Fungi: These are also microscope organism and they are regarded as plant but they do not process chlorophl root, stem and leaver. But they leave as parasite on plant and transmitted of disease or diseases e.g. maize, smut, leave rust or leave spot, black pour, of cocoa, damping off, can be transmitted by wind , rain splash , by insect, etc.

3.      Virus: These are microscopic  organism, smaller than Bacteria and Fungi.They can only be seen under very powerful microscopic or electronic microscopic. They cause serious and civil diseases in crop plant. Virus diseases are transmitted by piercing and sucking insects. Disease cause by viruses include Coca Swollen Shoot, Groundnut Rosette,  Cassava and Yam Mosaic, etc.
4.      Nematode: These are soil born organism. They are worm like animals. Many of them live on dead organic matter while others live as parasites on plants. Some are microscopic while others are  big enough to be seen with naked eye. The parasitic nematode  attack plant hosts and caused diseases such as  Tuber rot, Root knot, Die back, etc.
1.      Yellowing and wilting of plants leaves
2.      Curling, twisting and winkling of leaves.
3.      Stunted Growth.
4.      Rottening  of leaves, stems and roots.
5.      Defoliation of plants
6.      Brown patches  with spots showing dead areas on leaves.
7.      Mouldy grains.
8.      Death of Plants
9.      Reduced product quality.
1.  Low Yield.
1.      Poor yield
2.      Low quality and loss of market value.
3.      Delayed maturity.
4.      Death of crops

Prevention is any practice used to prevent pathogens from attacking a plant, Control involves those practices used to stop and prevent further damage after  the invasion  by a pathogen.

i.                    To increase the  life span of crops.
ii.                  To increase the quality and quantity of crops.
iii.                To minimize the spread of diseases.
iv.                To increased  the income of farmers.

i.                    Cultural Method
ii.                  Biological Method
iii.                Chemical Method
iv.                Use of Resistant Varieties
v.                  Prohibition
vi.                Quarantine.

i.                    Cultural Method
Good cultural practices may  reduce or destroy insect pest population. Crops may equally escape the attack by such pests. Such cultural practices include
Crop rotation
Ploughing to destroy the source of  innoculum.
Adequate spacing and correct seedrate/ha –to provide a micro-climate not suitable for the development of pests.
Adequate weeding to destroy weed hosts of pests.
Correct timing of operation( i.e. planting and harvesting) when pests are less rampant.

ii.                  Biological Method
This is the use of living organism to control diseases. A natural enemy such as a parasite , predator or disease organism is introduced into  the environment  of the disease, if already present, it is encouraged to multiply and become more effective in reducing the disease organism.e.g. a fungus Trichoderma virida is antagonistic to another fungus , Rhizoctonia solanii (Potato Blight), the African Marigold has been known toreducenematode population   .
This includes the introduction, multiplication and dispersed of natural enemies of pests .e.g. the ladybird beetle(Rodolia cardinalis) has been used to control the cotton Cushion scale(Iceria purchasi), Chalcid wasp to control Citrophilus mealybug, Japanese beetle controlled with bacterium Bacillus popilliae.
iii.                Chemical Method
iv.                Use of Resistant Varieties
This involves the breeding of crops which are resistant to the disease. It requires  cooperation between plant breeders, pathologists and farmers.The use of disease resistant varieties (if available) remains the last hope when other methods of control of diseases are ineffective particularly viral diseases .e.g. cassava mosaic resistant varieties   

v.                  Prohibition/Embargo/Banning
This is the practice of avoiding the introduction of certain diseases into an area. It is done on a nation wide basis  by legislation . it is an effective method when strictly enforced.

vi.                Quarantine.
The introduction of crops varieties is often accompanied with associated diseases. This is avoided by imposing a period of isolation and observation on imported seeds and crop plants,  crop products , unsterilized soil and any material mixed with unsterilized soil. Plant  materials are kept  in sealed compartments  for a period long enough for any disease symptom or to be detected. If symptoms appear, the materials are destroyed or re-exported at the expense of the importer.

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