Downy mildew is a disease of major importance on the horticultural Brassica species. It is most serious on the flowerhead types (cauliflower, broccoli), less serious on the leaf brassicas (cabbage, Brussels sprouts, and least serious on the root brassicas (turnips, swedes) and oil brassicas (rape). The pathogen persists as oospores in the soil. Attacks are most important in Brassica seedbeds, with infection appearing as yellow speckling of the upper surface of seedling leaves, and white mildew on the lower surface. Severely affected seedlings are stunted or killed. Similar symptoms appear on leaves in the field, causing premature senescence and favouring bacterial spoilage in the cold store after harvest. Affected Brussels sprouts show black spotting, which reduces their market quality. The heads of flowerhead brassicas can be severely affected, and become unmarketable.
Control by fungicide treatment is essential at the seed-bed stage, with reduction of humidity and leaf wetness as fas as possible. Similarly, flowerhead brassicas grown under protected conditions may require fungicide sprays. In the field, control relies on the use of healthy seedlings, adequate crop rotation and avoidance of unfavourable conditions. Fungicide sprays are not very effective. However, fungicides may in any case be used against white rust (Albugo candida), and these have some action against H. brassicae.