Malus sieboldii-based rootstocks mediate apple proliferation resistance to grafted trees

Over a period of 24 years, three trials were carried out in which open pollinated apomictic seedlings were examined for apple proliferation (AP) resistance in comparison with several clonal rootstocks. The screening was performed under experimental inoculation and natural infection conditions. Criteria for the resistance rating were occurrence of AP symptoms, percentage of affected trees, incidence of the small fruit symptom and the effect of the disease on vigor. In all trials, similar results were obtained. Satisfactory resistance was only identified in trees grown on progenies of M. sieboldii and M. sieboldii-derived experimental rootstocks. The screening also showed that rootstocks with M. sieboldii parentage are often highly susceptible to latent apple viruses.