Breeding of apple rootstocks resistant to ‘Candidatus Phytoplasma mali’

To obtain apple rootstocks resistant to apple proliferation and suitable to modern fruit growing, 24 cross combinations were performed over a 5-years period using M. sieboldii and its hybrids as donors of the resistance trait and standard apple rootstock M. domestica genotypes as donors of agronomic values. As the resistance donors had different degrees of polyploidy, not all cross combinations were compatible and produced seeds. Out of 17 cross combinations more than 3,000 individuals were obtained which were analysed by co-dominant SSR DNA markers in order to distinguish non-recombinant, apomictic progeny from recombinant progeny. In total, 13% of all progenies showed a complete recombination of the maternal and paternal genotype and in 25% the unreduced apomictic genotype was recombined with one haploid M. domestica genotype. Thus, breeding with the apomictic genotypes could be achieved. All recombinant progeny seedlings were graft-inoculated with ‘Candidatus Phytoplasma mali’ to evaluate the resistance behaviour. The inoculated seedlings were maintained in the nursery and apple proliferation symptoms were recorded every year in autumn. Two years after inoculation the roots of the inoculated plants were analysed by PCR for the presence of ‘Ca. P. mali’. A quantitative real-time PCR assay was employed to quantify the phytoplasmas in the different genotypes. The results indicate that the resistance trait can be inherited to the progeny. An agronomic evaluation of the resistant genotypes has now to follow.