The disease is caused by the fungus-like organism Bremia lactucae and is found wherever lettuce is grown, with lettuce being the principal host.  The pathogen has also been reported to infect several other hosts, but these pathogenic strains appear to be host-specific and do not infect lettuce.  Regarding the lettuce pathogenic strains, there exist many races of the pathogen and each race is pathogenic on specific cultivars of lettuce.  The pathogen thrives under cool moist conditions and produces an abundance of short-lived spores, which are quickly dispersed during periods of high moisture. In contrast to other species of downy mildew in other cropping systems, Bremia lactucae does not appear to be soilborne.