Pyramiding fire blight resistances Fb_E and FB_MR5 in Malus × domestica using a "Fast Track" breeding approach

Fire blight, caused by the bacterium Erwinia amylovora, is the most destructive bacterial disease in apple production. Although strong resistances in wild apples are known, there are still only a few partially resistant varieties on the market. In 2008, a new breeding approach introducing major fire blight resistances originating from Malus × robusta 5 (MR5) and 'Evereste', and partial resistances originating from modern cultivars was established under glasshouse conditions, called “Fast Track”. This method allows for reducing the long generation cycle of 4 to 5 years in apple under field conditions to approximately 2 years without using GMO techniques. Marker assisted selection, artificial winter simulation, and the use of growth regulators are part of the method. Several pseudo-backcrosses (pBC) with high fruit quality cultivars are required to achieve commercial fruit quality following the introgression of wild material. After ten years, the most advanced generations carrying the resistance locus FB_MR5 (MR5, pBC'4) or Fb_E (Evereste, pBC'3), including results of artificial shoot and flower inoculation tests, population studies, breeding achievements, and fruit quality development are presented. Recently, several crosses were performed to combine the resistance genes FB_MR5 and Fb_E in the most advanced generations in order to increase durability of resistance.