Downy mildews are obligate plant parasites that attack a wide range of plants and are responsible for some of the world's most destructive plant diseases, e.g. downy mildew of grapes and blue mould of tobacco. They also cause considerable losses to the nursery production of ornamental and vegetable seedlings. Downy mildew organisms are distinctly different from the powdery mildew fungi. Some common downy mildew hosts include Cissus, lisianthus, poppy, ranunculus, rose, snapdragon, stock, pansy, alyssum, impatiens, sweet pea, cucurbits, helichrysum and brassica seedlings.