1. Disease is a malfunctioning process that is caused by continuous irritation which results in some suffering producing symptoms (American Phytopathological society & British Mycological society).

2. Disease is an alteration in one or more of the ordered sequential series of physiological processes culminating in a loss of coordination of energy utilization in a plant as a result of continuous irritation from the presence or absence of some agent or factor.

3. Disease: Any malfunctioning of host cells and tissues that result from continuous irritation by a pathogenic agent or environmental factor and leads to development of symptoms (G.N.Agrios, 1997).

Pathogens bring about these irritating processes through different but inter-related pathways

1. by utilizing the host cell contents,

2. by causing death of cells or by interfering with their metabolic activities through their enzymes, toxins and growth regulators,

3. by weakening of tissues due to continuous loss of nutrients, and

4. by interfering with translocation of food, minerals and water.