Breeding apple proliferation-resistant rootstocks: durability of resistance and pomological evaluation

To develop apple proliferation (AP) resistant rootstocks, a breeding program was initiated in 2001 employing apomictic Malus sieboldii and M. sieboldii-derived hybrids as donors of the resistance trait and mainly standard apple rootstock Malus x domestica cv. M9 as donor of agronomic values. Examination of the experimentally inoculated progeny of seven crossings made in 2001 and 2002 over a period of seven to eight years showed that inheritance of resistance differs considerably among the parental lines by yielding between 10 to 55% of resistant offspring. Resistant rootstocks were characterized by poor host properties for the AP phytoplasma. This is evidenced by lower phytoplasma titers in resistant genotypes than in standard stock M9 and by preventing detectable phytoplasma development in the top grafted susceptible cv. Golden Delicious. At the end of the observation period, 80% of the root samples collected from resistant rootstocks tested PCR-negative. Size and productivity of trees grown on resistant rootstocks varied over a wide range. A preliminary pomological evaluation was done on the cv. Golden Delicious which has been grafted as infected scion on the progeny genotypes to test. Comparisons with trees grown on M9 rootstocks indicated that there are genotypes among the offspring examined that fulfill the requirements of commercial apple growing.