Micropropagation of Malus sieboldii hybrids resistant to apple proliferation disease

Natural resistance to apple proliferation (AP) disease was found in apomictic Malus sieboldii-derived genotypes which can be used as rootstocks for apple. Whereas their agronomic value is currently improved in ongoing breeding programs they are recalcitrant to propagation by standard procedures. Therefore, a micropropagation protocol was developed for in vitro establishment, multiplication and rooting of eleven interesting AP-resistant genotypes. Four different macro and micro element formulations were tested: MS, QL, WPM and DKW. Phytohormones (0.25 µM IBA, 4.44 µM BAP and 0.28 µM GA3) and vitamins (MS modified for thiamine at 2.96 µM), established for the propagation of M. × domestica, were also suitable for the propagation of M. sieboldii genotypes. The MS medium yielded in general the highest proliferation rates and the best shoot growth. A significant improvement of the growth was obtained by replacing Fe-EDTA by Fe-EDDHA as iron source. By comparing four different rooting treatments a significantly higher percentage of rooting was observed when the induction was carried out in the dark with 25 µM IBA either in liquid or agar-solidified medium. Three classes of genotypes with low, medium and high rooting efficiency were found. The acclimatisation method used yielded survival rates between 90-100% for most of the genotypes