Genes determining leucine aminopeptidase and mildew resistance from the ornamental apple, 'White Angel'.

Mildew resistance in the ornamental apple 'White Angel' was found to be determined by complementary genes. The gene R w was found to be necessary for the expression of resistance controlled by the resistance gene Pl w . The close linkage between the isoenzyme gene, Lap-2, for leucine aminopeptidase and P1 w was confirmed. The efficiency of Lap-2 as a marker in screening for mildew resistance is limited, as it cannot account for susceptible plants with the r w r w P1 w p1 w genotype. It has, however, an important role to play in combining resistance genes from different sources. The genotypes of 'White Angel' (R w r w , Pl w pl w , Lap-2an), 'Jester' (R w r w , p1 w p w , Lap-2an) 'Katja' (R w r w ,p1 w p1 w , Lap-2an) and 'Gloster 69' (r w r w , p1 w p1 w , Lap-2an) were determined. It also appeared that R w might influence Lap-2 activity in young seedlings.