Apple (Malus domestica), like most Rosaceae fruit crops has a gametophytically determined self-incompatibility (SI) system. SI prevents inbreeding and promotes outcrossing. The outcome of pollination in SI plants is determined by a single polymorphic gene locus, called the S locus. If the pollen carries the same alleles as the pistil, SI will occur, whereas if one or both alleles are different, fertilization will occur. Two of the commercial pollenizers of 'Topred' in Israel are 'Golden Delicious' and 'Jonathan'. We determined the fertilization potency of 'Jonathan' as pollenizer for 'Topred' was significantly lower than that of 'Golden Delicious' in the years 1997 and 1998 (non-optimal conditions for fertilization). PCR analysis of the S-alleles demonstrated that 'Jonathan' and 'Topred' share the allele, S9. PCR-Sallele analysis of 'Topred' progeny seedlings revealed in the case of 'Jonathan' only S7, the non shared allele, took part in fertilization, whereas both alleles of 'Golden Delicious' (S2, S3) participated at equal rates. Thus, the lower potency of 'Jonathan' as a pollenizer of 'Topred' could be related, at least in part, to semi-compatibility between the two cultivars. Preliminary data from fully compatible pollenizers of 'Topred': 'Golden Delicious', 'Royal Gala', and 'Granny Smith', determined that in non-optimal conditions (1998) 'Royal Gala' conferred significantly lower fruit-set to 'Topred' than the other two pollenizers. However, in 1999, when pollination and fertilization conditions were optimal, no significant differences in fruit-set of 'Topred' were found with any of the fully compatible cultivars analyzed. These findings emphasize that, under extreme conditions for apple cultivation fertilizer, potency should not be concluded experiments performed under optimal growing conditions, but rather should be evaluated specifically under the nonoptimal or severe conditions.