The genetic relationship between races of Bremiae lactucae and cultivars of Lactuca sativa

The reactions of lettuce cultivars to physiologic races of Bremia lactucae are interpreted in terms of a gene-for-gene relationship between pathogen and host. The hypothesis takes into account the parentage of cultivars and the origins of their resistance, the characteristics of the resistance reactions and data available from detailed genetical analysis of various race/cultivar combinations. Cultivars are classified with respect to ten postulated resistance genes and B. lactucae races are defined by the virulence genes present. The practical significance of these studies is discussed in relation to both future lettuce breeding programmes and to the choice of cultivars available to counteract any given local race situation.