Introduction

• Toxin can be defined as a microbial metabolite excreted (exotoxin) or released by lysed cells (endotoxin) which in very low concentration is directly toxic to the cells of the suscept (host).
• Toxins are different from enzymes in that they do not attack structural integrity of host tissues but affect the metabolism of the host because the toxins will act on protoplast of the cell.

Toxin hypothesis (Luke and Wheeler, 1955):

1. A toxin should produce all symptoms characteristic of the disease
2. Sensitivity to toxin will be correlated with susceptibility to pathogen
3. Toxin production by the pathogen will be directly related to its ability to cause disease.

Except, victorin, the toxic metabolite of Cochliobolus victoriae, the vast majority of toxins associated with plant diseases fail to exhibit all the above characters.