Accelerated introgression of fire blight resistance from Malus xrobusta 5 and other wild germplasm into elite apples

Introgression of fire blight resistances from wild apples to develop high quality apple cultivars is an important breeding strategy of the Agroscope apple breeding programme. In order to obtain parental material suitable for cultivar breeding several pseudo-backcrosses with high quality parents are required. On F2 progeny plants of Malus ×robusta 5 a low-input fast-track breeding approach was applied to accelerate the generation cycle. Seedlings were screened with molecular markers for FB_MR5 fire blight resistance QTL and Rvi6 scab resistance. Selected seedlings were grown on their own roots under optimal growing conditions to enhance flowering, including non-limiting fertilization, regular prohexadione-Ca and ethephon treatments followed by winter simulation. From flowering plants carrying the FB_MR5 QTL, pollen was collected for crosses with high quality Malus ×domestica cultivars and flowers were pollinated with elite apple germplasm. Parents for new introgression cycles carrying the FB_MR5 QTL were selected based on fruit evaluations and on results of artificial shoot inoculation tests with a Swiss strain of Erwinia amylovora. In order to include new sources of fire blight resistance into the breeding programme, additional Malus crabapples, described as fire blight resistant, were inoculated by shoot injection. In this paper we report on the progress of the introgression of the fire blight resistance FB_MR5 using a fast-track breeding approach and present results of fire blight shoot inoculation tests in wild and pre-breeding germplasm.