Microbial Populations, Host-Specific Virulence & Plant Resistance Genes

Abstract

Plant pathogens are not found randomly distributed on all plants, but are restricted in host range to one or more host species. A phytopathogenic microbial species may have a wide or narrow host range, and some with a wide host range can attack a large number of plant species in many different plant families. However, individual strains of wide host range species may have a very limited host range, often limited to a single host species. Host-species specificity is considered to be relatively stable, and therefore useful in sub-species classifications. The subspecies ranks recognized on the basis of host-species (range) specificity are “forma specialis” for fungi and “pathovar”, “pathotype” or “biovar” for bacteria. A plant species is included in the host range of a pathogen if even one variety of the species is found to be susceptible to that pathogen. If some plant varieties of a species are susceptible and some resistant to a given pathogen, the forma specialis or pathovar may be further subdivided into pathogenic “types”, or more usually, “races”. Race specificity is controlled by avirulence (avr) genes, and is a relatively unstable characteristic [Gabriel & Rolfe, 1990].