Revisiting Stagonosporopsis species associated with chrysanthemum and pyrethrum ray blight

Ray blight is a destructive disease of Asteraceae affecting chrysanthemum and pyrethrum industries worldwide. Three morphologically similar but phylogenetically distinct species of the family Didymellaceae; Stagonosporopsis chrysanthemi, S. inoxydabilis and S. tanaceti are associated with the disease. Despite their close evolutionary relationship and cross host pathogenicity, these species have marked differences in their biology and epidemiology. Stagonosporopsis chrysanthemi and S. inoxydabilis are both homothallic with a MAT locus that carries both mating type genes. Ascomata play a major role in survival and dispersal of S. chrysanthemi, contributing to the onset of ray blight epidemics on chrysanthemum. However, S. tanaceti is either asexual or heterothallic due to the presence of only MAT1-2 idiomorph in its genome. Morphological similarity of the species causing ray blight on various Asteraceae, multiple changes in their taxonomy and historical confusion with other Phoma-like species have resulted in lack of clarity in their taxonomy, host range and global distribution. Host specificity studies and reports on global distribution of S. chrysanthemi published before its separation from S. inoxydabilis should be treated with caution. A recently developed species-specific multiplex PCR assay provides a rapid and robust tool to study the global distribution of these important quarantine pathogens. When global populations from cultivated and/or wild Asteraceae are available, population genetic analyses may aid in understanding the origin, evolutionary history and global migration patterns of the Stagonosporopsis spp. associated with ray blight of Asteraceae.