Introduction :

• Plant are essential for maintenance of life.
• It not only sustain man and animals , they are also food for micro-organisms living in the ecosystem.
• It is in this context that the need for fighting the competing micro-organisms and other agencies that lack loss of productivity has been felt.
• The attack on plants has not only changed the appearance but has caused loss in the productivity of the crop , and this observed change is called disease.
• We call a plant healthy only if it continues to perform all its normal physiological activities and give the expected yield according to its genetic potentiality.

Physiological activities of a healthy plant.

1. Normal cell division, differentiation and development.
2. Uptake of water and nutrients from the soil.
3. Synthesis of food from sunlight by photosynthesis.
4. Translocation of water and food to the sites of necessity through xylem and
5.  
6. Metabolism of synthesized material
7. Reproduction

                       A diseased plant fails to perform one or more of these functions. The effect of a disease on functioning of an organ depends on which cells or tissues were first attacked by the pathogen. Thus, disease can be defined as malfunctioning process that is caused by continuous irritation by a pathogen (Dimond, 1959).