Confirmation of the fire blight QTL of Malus x robusta 5 on linkage group 3

Recently, a major QTL for fire blight resistance was identified in the wild species accession Malus x robusta 5 (R5) based on a single phenotypic evaluation of a progeny of 154 seedlings derived from a cross with 'Idared'. In this paper, we confirm the presence of this QTL in a repeated inoculation of the 'Idared' family in Germany as well as in a M.9 x R5 family in New Zealand. Linkage maps were constructed for each family and susceptibility to Erwinia amylovora was determined by artificial shoot inoculation of scions grafted onto rootstocks in the greenhouse in 2005 and 2006 in Germany and 2006 in New Zealand. In 2005, interval mapping of susceptibility resulted in the identification of a QTL on linkage group 3 (LG3) of apple between microsatellites Ch03E03 and Ch03G07 explaining up to 79% of phenotypic variation. The QTL was confirmed in the replicate inoculation in the 'Idared' x R5 family explaining up to 67% of the phenotypic variation with an average of 75% for both years and 83% in a New Zealand M.9 x R5 family. The collective results of the German and New Zealand populations confirm the presence of a significant QTL for fire blight resistance on LG3 of R5 under different phenotyping conditions. The interpretation of this QTL as a major fire blight resistance locus when regarding resistance as a dominant trait will be discussed.