Abstract
True leaves of young lettuce plants (3 to 4 weeks old) were more susceptible than their cotyledons to isolates of Bremia lactucae (BL) collected from Ibaraki Prefecture. In the three commercial cultivars described as resistant ones in the seed company catalogs, cv. Haru P was consistently resistant to three isolates of BL, however, the other two showed susceptible reaction and rich conidial formation was observed on the inoculated true leaves of young lettuce plants.
