Peronospora is a genus of downy mildews that are obligate plant pathogens. They can cause severe damage to many different cultivated crops, as well as wild and ornamental plants. Peronospora is most closely related to Pseudoperonospora, and together they form the clade of downy mildews with coloured conidia. Peronospora has far more species than any other genus of the downy mildews. However, many species have been moved from this genus to other genera based on phylogenetic evidence. Among these are the species now in Hyaloperonospora, important pathogens of the Brassicaceae. Now, the Peronospora species of most importance is likely Peronospora tabacina. Peronospora tabacina causes blue mold on tobacco plants and can severely reduce yields of this economically important crop to the point where it has been classified as a bioweapon.

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