Disease can be define as a condition of ill-health in  an organism. Disease can also  be referred to as deviation from the normal physiological condition which results in the abnormal function of the body system either in part or as a whole.
A crop disease is a condition in which crop plants suffer from abnormal health condition.

The following conditions must be met before a disease can occur in crop plants.
1.      There must be a Pathogen/Disease causing organism.
2.      There must be a Susceptible Host.
3.      There must be a conducive environment.



                              Crop Plant                                              Favourable Condition

If any of the three conditions is absent, there cannot be a disease infection.


The various stages of  development of diseases include:
i.                    Over-seasoning stage.
ii.                  Innoculation stage.
iii.                Penetration stage.
iv.                Infection stage.
v.                  Incubation stage.
vi.                Invasion stage.
vii.              Dissemination.

i.                    Over-seasoning stage: This is the resting period during which disease organism (pathogen) changes to a particular form to survive unfavourable environmental conditions. At this stage the pathogen is not active on until a conducive period is attained and it becomes active.

ii.                  Innoculation Stage: This is the stage at which the pathogens makes contact with the plant either by wind, water vector, etc .

iii.                Penetration Stage: This is the stage  at which the disease organism gains entrance into the cell of the host plant through leaves, cuts or other opening .

iv.                Infection Stage: This is the stages at which the pathogens causes damages to the cell of the host plant .

v.                  Incubation Stage: This is the stage between the period of contact and when symptoms begin to manifest. It is the stage at which the diseases is fully establish in the cell of the host plant and it shows changing in the appearance of the host plant indicating the presence of diseases.

vi.                Invasion Stage: At this stage the disease is fully established in the host plant multiplies rapidly and spread from one cell to the other in the host plant until it's tissues are complete damage.

vii.              Dissemination stage: This is the stage at which the disease continuous to spread from the host plant to the other plant by wind, water, vector.etc .

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