Fungal Phytotoxins and Ascochyta Blights of Cool Season Legumes

The fungal pathogens that cause Ascochyta blights of cool season food legumes are host specific with  Ascochyta rabiei specific to chickpea, A. lentis to lentil, A. fabae to faba bean. One of four species pathogenic on pea, A. pisi, is specific to pea, although the other three species (Mycosphaerella pinodes, Phoma koolunga and Phoma medicaginis var. pinodella) may infect other legume hosts.  These pathogens also produce several phytotoxins but they are not host-specific. Nevertheless, these phytotoxins have often been proposed to be virulence/pathogenicity factors without conclusive evidence.  Some of these pathogens produce a unique set of secondary metabolites, which is highly correlated to evolutionary relationships among species. To examine the role of the phytotoxins in pathogenesis, we selected the solanapyrone phytotoxins produced by A. rabiei as a case study. All isolates of Ascochyta rabiei produce solanapyrone toxins, which are unique among Ascochyta blight fungi, but identical to those produced by the potato pathogen Alternaria solani. To determine the potential roles of solanopyrones in pathogenesis, the solanapyrone synthase gene was disrupted in both Ascochyta rabiei and Alternaria solani through targeted gene replacement. The resulting mutants did not produce solanapyrones, but did accumulate the precursor prosolanapyrone that is not toxic to plants. Surprisingly, these solanapyrone-deficient mutants were equally virulent as the wild-type strains, on their respective hosts. Results show that solanapyrone toxins are not required for pathogenicity, but may play important roles in the biology and ecology of these fungi including competitive interactions and survival in nature.