It causes downy mildew of species of Brassica, Raphanus, Sinapis and probably other genera within the Brassicaceae.

In the past, the cause of downy mildew in any plant in the family Brassicaceae was considered to be a single species Peronospora parasitica.

Similar symptoms appear on leaves in the field, causing premature senescence and favouring bacterial spoilage in the cold store after harvest.

Control by fungicide treatment is essential at the seed-bed stage, with reduction of humidity and leaf wetness as fas as possible.

In the field, control relies on the use of healthy seedlings, adequate crop rotation and avoidance of unfavourable conditions.