The virulence characteristics of Bremia lactucae populations present in Sweden from 1971 to 1976

One hundred and four Swedish isolates of Bremia lactucae were tested between 1971 and 1976 for their virulence phenotypes. Eighty of these isolates showed patterns of virulence previously recorded in other countries while the remainder conformed to seven previously unreported virulence factor combinations. The simplest resistance factor combination that could give resistance to all isolates collected was R2 plus Rl 1. This combination now exists in some recently bred lettuce cultivars. Most of the possible virulence factor combinations occurred in the population at the expected frequency. However, several combinations, which had not been directly selected (by the growing of cultivars carrying the comparable resistance factor combinations) were present at higher or lower than expected frequencies.