The development of downy mildew resistance has been a focus of lettuce breeding activities for many years.  There are resistant cultivars available, each resistant to specific races of Bremia lactucae.  Some commercial varieties have resistance to many of the races that commonly occur in the West.  However, the fungus is genetically variable and new races continue to emerge that can overcome resistance in current varieties.  As such, lettuce breeders will need to be vigilant for the emergence of new pathogen races and be prepared to move new resistance genes into commercial cultivars currently in use.