Marker assisted selection for PL1 powdery mildew resistance in apple - old markers for a new resistance gene?

The major gene Pl-1 derived from Malus robusta has been used widely in breeding resistance to powdery mildew (Podosphaera leucotricha) in cultivated apple. New AFLP markers for Pl-1 have been identified by a BSA approach. A SCAR/CAPs marker was developed that is more closely linked to the resistance phenotype than the hitherto known marker for Pl-1, the SCAR marker AT20-450. Clear segregation patterns for resistance/susceptibility and a good correlation between phenotypical and marker data over three years under high mildew infection pressure is in agree-ment with the hypothesis that Pl-1 is controlled by a single major gene. Furthermore, we present data from analyses evaluating our Pl-1 markers within the framework of the apple breeding programme at HortResearch. In a population derived from a cross between X3191 x ‘Novosibirski Sweet’ OP, a linkage to a strong mildew resis-tance phenotype indicated that the “old” Pl-1 marker AT20-450 was linked to a strong mildew resistance gene segregating from ‘Novosibirski Sweet’ OP. It is still unknown if this “new” gene is identical to Pl-1. In a second breeding population, re-sulting from the cross ‘Royal Gala’ x ‘Robusta 5’ the Pl-1 markers did not correlate with the powdery mildew infection data. This suggests that the ‘Robusta 5’ accession used as resistance donor in New Zealand also has a novel mildew resistance gene. An-other explanation is discussed concerning the genetic situation at a putative second locus that might be acting as a precursor gene. A genetic two-gene-model is presented that point out the relationship between the genetic situation regarding the two loci in segregating progenies and an applicability of MAS.