Molecular markers applied to apple breeding: analysis of oligogenic and single gene resistances

The use of molecular markers can dramatically improve apple breeding by increasing selection efficiency. We show two examples of how DNA markers have been successfully applied to prove that the resistance gene present in the cultivar 'Nova Easygro' (said to be Vr), is tightly linked to V f and is most probably Vf itself. We were able to detect homozygous resistant plants and showed that those plants are on average more resistant than the heterozygous ones. In a population obtained by crossing a powdery mildew resistant selection (A679-2) and a susceptible one ('Iduna') we studied the resistance to powdery mildew introgressed from Malus zumi (Pl 2). Genetic linkage maps of the two parental plants were constructed. By analysing Quantitative Trait Loci (QTL) we were able to detect two major genomic regions containing genes controlling the resistance. Several other minor regions have been detected both in the resistant and in the susceptible parents. The analysis of a larger sample and repeated observations over several years will enable a more precise localisation of the QTLs and evaluation of the importance of the genomic regions containing putative loci controlling powdery mildew resistance.