Nectria haematococca MPVI can be found in many different biological habitats but has been most studied as a pathogen of pea (Pisum sativum). Genetic analyses of isolates obtained from a variety of biological sources has indicated that a number of genes control pathogenicity on pea but that one important PEa Pathogenicity (PEP) gene isPDA, which confers the ability to detoxify the pea phytoalexin pisatin. In these studies, all naturally occurring isolates that lackedPDA (i.e. Pda^– isolates) and all Pda^– progeny were essentially non-pathogenic on pea. However, we have demonstrated recently that Pda^– mutants created by transformation-mediated gene disruptions, while having a modest reduction in virulence, and more virulent than any naturally occurring Pda^– isolates. In addition we know thatPDA genes are on dispensable (DS) chromosomes in this fungus. We believed that the gene disruption mutants have allowed the detection of otherPEP genes that are present on the DS chomosomes along withPDA and that naturally occuring Pda^– isolates usually lack this DS chromosome. This would explain why naturally occurring Pda^– isolates are always low in virulence. We propose that the DS chromosomes in fungi are analogous to bacterial plasmids which allow those microorganisms to colonise different habitats, i.e. the DS chromosomes ofNectria haematococca contain genes that allow individual isolates of this broad host range pathogen to occupy different biological niches.