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 disease is spread throughout the summer by spores produced on the infected leaves. These spores are splashed by rain, and are also carried for long distances on the wind. Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are only likely to occur during wet summers.
The airborne spores remain viable for just a short time, but the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for a long time.
The fungus can remain dormant within affected bulbs, producing spores in spring that spread the disease to newly-sown plants.