Downy mildew of lettuce, caused by Bremia lactucae, occurs worldwide and is a serious disease of glasshouse and field lettuce (Lactuca sativa) and many wild Asteraceae species (Crute & Dixon, 1981; Lebeda et al., 2002). In natural plant communities Lactuca serriola (prickly lettuce) is the most common wild host of B. lactucae (Lebeda et al., 2001a, 2002).
The relationships between L. sativa, L. serriola and B. lactucae conform to a gene-for-gene interaction (Crute, 1992). More than 40 genes or factors (Dm and R) for resistance of the host, and complementary virulence (v) genes (factors) in the parasite have been identified so far (Lebeda et al., 2001b, 2002; Michelmore et al., 2002; Lebeda & Zinkernagel, 2003a). Detailed analyses of the genetics of the host–parasite interaction made it possible to interpret the variability of virulence in B. lactucae in terms of virulence factors (v-factors) and virulence phenotypes (v-phenotypes) (Lebeda, 1982; Crute, 1987). Bremia lactucae is a highly variable pathogen with a number of forms differing in their virulence to genotypes of L. sativa, L. serriola and other wild Lactuca spp. (Lebeda et al., 2002; Lebeda & Zinkernagel, 2003b). In general the pathogen and its populations could be considered to have a high evolutionary potential sensuMcDonald & Linde (2002). During recent decades, studies of this phenomenon have focused on the population level, and only on lettuce as a crop (L. sativa) (Crute, 1987; Lebeda & Zinkernagel, 2003a). So far there has been no detailed research of B. lactucae in natural plant populations, and only very limited information about their virulence structure has been added (Lebeda, 2002). A study of the wild (natural) plant pathosystem L. serriola–B. lactucae which began in the early 1980s demonstrated the existence of various v-phenotypes among eight isolates of B. lactucae originating from naturally growing L. serriola (Lebeda, 1984). These v-phenotypes differed substantially from those isolated from L. sativa and made it possible to identify new race-specific resistance factors (R-factors) in L. serriola accessions (Bonnier et al., 1994; Lebeda & Zinkernagel, 2003b). Recently, more detailed research has focused on the natural distribution and virulence structure of B. lactucae populations on L. serriola (Lebeda, 2002; Petrželová & Lebeda, 2004).
The aim of the present study was to determine the variation of virulence for individual B. lactucae isolates and the distribution of v-phenotypes in natural populations of L. serriola in the Czech Republic and elsewhere in Europe.