The Disease: Downy Mildew

The downy mildew disease is caused by the fungus Hyaloperonospora parasitica (Gaum.) Goker. The nomenclature of the disease is based on symptoms on the infected leaves with downy fungal growth. The upper surface of affected young and older leaves has ill-defined, irregular, pale yellow necrotic lesions, whereas the lower surface is covered by white-grey mycelium. The cotyledons and older leaves may be killed prematurely when single lesions coalesce to form large blotches. Diseased pods may be visible with angular brown lesions, and under high humidity, a sparse white-greyish mycelium with numerous sporangia and sporangiophores may be visible. Severe attacks may lead to premature ripening and drying of the siliquae. This pathogen, alone or in combination with Albugo candida (white rust), is responsible for causing severe losses in the yield of several temperate and tropical Brassicaceae crops, particularly rapeseed and mustard.