Downy mildews are a large group of plant diseases caused by microscopic fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.
The downy mildew pathogen produces resting spores in plant debris which carry it through winter. These can infect via the roots, sometimes causing a systemic infection (one that spreads throughout the plant). However, since there will be brassicas (or their wild relatives) of some type in leaf throughout the year, the pathogen is always present on green tissue and these infections provide a more important source of infectious spores whenever suitably wet conditions occur. Spores are released from the fuzzy fungal growth on the lower leaf surface and are spread by wind and water splash. Infections in mature leaves are vein-limited, giving rise to an angular-shaped lesion.
Infections are favoured by warm, wet conditions. These also favour infection by Brassica white blister (Albugo candida), another foliar pathogen of brassicas and the two often therefore occur together.
The downy mildews are described as biotrophic, meaning that they can only live and grow in association with living plant tissues, during which time they disperse infective spores. When the weakened plant tissues die the pathogen also dies, or forms a dormant resting spore.