The 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.
Like other downy mildews, Bremia lactucae is described as a biotroph; a pathogen that penetrates into host plant tissues over an extended period without killing them, while it extracts nutrients from the living host cells. During this period it releases airborne spores from the fuzzy fungus-like growth on the underside of leaves, which disperse to initiate new infections. These spores land on other lettuce leaves, germinate in water on the leaf surface and penetrate directly into the tissues. This disease is therefore worse under wet conditions.
When the leaves finally die, the fungus produces resting spores (known as oospores). These can remain in the soil and may infect the roots of a succeeding lettuce crop. However, most infection occurs from wind-blown spores.
Infected tissues are often colonised by grey mould (Botrytis cinerea) and this can lead to further rotting.