The Pathogen: Hyaloperonospora parasitica (Gaum.) Goker [H. brassicae (Gaum.) Goker]

Downy mildew of crucifers is caused by an obligate pathogen, Hyaloperonospora parasitica (Gaum.) Goker synonymy Peronospora parasitica (Pers. ex. Fr.) Fr. The genus Peronospora s. lat. is one of the largest genera of downy mildews (Peronosporales, Peronosporomycetes, phylum Heterokonta). Recently, phylogenetic analyses of DNA sequences have received increasing attention in Peronosporales, opening a new era in the investigation of their phylogenetic relationships. Both genera Hyaloperonospora and Perofascia almost exclusively infect hosts in the Brassicaceae and related families. The genera were segregated from Peronospora due to pronounced morphological and molecular differences. The earliest reference of downy mildew on crucifers is by Persoon (1796) who described the cause of the disease on Capsella bursa-pastoris to the fungus Botrytis parasitica Pers. Recently, researchers presented molecular and morphological evidence to split Peronospora into three separate genera, Peronospora s. str., Hyaloperonospora, and Perofascia, and the latter two genera were found to be entirely restricted to a single host family, Brassicaceae.