Race specific resistance to lettuce downy mildew (Bremia lactucae) in the lettuce cultivar Bourguignonne Grosse Blonde d'Hiver

A stock of lettuce cv. Bourguignonne Grosse Blonde d'Hiver used in previous studies was shown to carry the race specific resistance factor R1 and not R9 as originally thought. Investigations into a commercial stock of cv. Bourguignonne which exhibited a heterogeneous reaction to some B. lactucae isolates showed that it was a mixture of two lines. Each line carried one of two R-factors (referred to as R9A and R9B) in a homozygous condition, although their identification was made difficult by the presence of R5/8. No plants were found which carried both R9A and R9B.