Self-incompatibility alleles of 104 apple cultivars grown in northern Europe

Gametic self-incompatibility in apple is determined by a set of S-locus alleles, which can be identified using PCR with allele-specific DNA primers. Allele composition has previously been determined for a large number of apple cultivars grown in North America, central and southern Europe, and Asia. In the present study, 103 not previously studied apple cultivars that are grown mainly in northern Europe were scored for their S-allele composition in order to provide apple growers and breeders with information on incompatibility among cultivars. In addition, S-allele frequencies were determined in some control cultivars and compared with data reported previously. A different genotype was found for 'Discovery', which was therefore added to our list. The most common S-allele in the resulting set of 104 cultivars investigated at Balsgard was S-7 (18%) followed by S-3 (17%), S-5 (14%), and S-1 and S-2 (both at 11%). Comparisons of allele frequencies obtained from previous compilations showed that the frequency of S-9, in particular, deviated considerably; 15-16 % was noted in two other studies, but only 1 % in the Balsgard study. This discrepancy is most likely due to the importance of 'Red Delicious' (S9S28) in almost all areas of the World except northern Europe. A preponderance (30%) of the S-3 allele was seen in a collection of 23 scab-resistant cultivars, suggesting that crosscompatibility may become a problem, especially in organic orchards.