ABSTRACT
The ability of a fungal or oomycete pathogen to cause disease on a plant species defines that species as a host and others as non-hosts. The presence within the fungus of ‘pathogenicity factors’, i.e. the necessary penetration mechanisms, cellwall-degrading enzymes, toxins, signalling molecules, biosynthetic genes, nutrient transport mechanisms, etc., will all determine which plant species are hosts. However, whilst the whole population of a particular fungus or oomycete will contain certain morphological and phenotypic features that define it as a species, within that species there will be many genetic variants with more specific characteristics. It is the nature of this genetic variation, how it occurs and how it affects fungal pathogenicity that we focus on in this chapter.
