Introduction

Within the huge number of microorganisms that exist, few are capable of causing disease in human beings, other animals and plants. External barriers to invasion include the skin and outer surfaces of animals, and the cuticle and bark in plants. Furthermore, in humans and many other higher vertebrates, internal mechanisms of defence such as antibodies may be produced as a response to microorganisms that breach structural defences. In plants, internal deterrents of microbial growth exist in the form of substances present in cell wall and intercellular spaces, and antimicrobial compounds may be released from vacuoles following cell damage. External stimuli may trigger the production and release of antimicrobial compounds from plants.

Evolution has led to the development of some intimate relationships between hosts and biotrophic pathogens. The latter are often host-specific, affecting only one species, and include many of the world's most important pathogens. The relationship between host and pathogen here occurs at gene level, and the interaction of gene products of host and pathogen determines whether the plant is resistant or susceptible. Currently most plant breeding programmes to produce cultivars resistant to plant diseases are associated with these specific host–pathogen systems.