Inheritance of virulence in Bremia lactucae to match several resistance factors in lettuce

Abstract

A 1∶1 mixture of spores of the physiologic races NL5 and NL6 ofBremia lactucae was sprayed on leaf discs of the lettuce cultivar Olof. The resulting oospores were allowed to germinate on seedlings after a two-month ripening period. Monospore lines of 49 of the 79 recovered isolates were tested on cotyledons and leaf discs of a differential series for virulence to 11 resistance factors. All monospore lines were also tested for the mating type factor. A backcross of a progeny line, line H, with NL5 was made and 41 isolates tested on the differential series.