Response of apple proliferation-resistant Malus sieboldii hybrids to multiple infections with latent apple viruses

Apple proliferation (AP) is the most important phytoplasma-associated disease affecting apple in Europe. The failure in controlling this disease by standard means strongly increased the importance of adopting resistant genotypes. About 6000 seedlings were obtained from a breeding programme crossing M sieboldii, donor of resistance to AP, with standard apple rootstocks (M9 mainly) as donor of agronomic value. Resistance screening showed that the trait was inherited to the progenies and trials are in progress to test the agronomic value of these genotypes. In an additional trial, the response of AP-resistant genotypes to a superinfection with different latent apple viruses was investigated. For this, M. sieboldii-derived first and second generation hybrids were analysed. In summer, three repetitions for each genotype were inoculated with apple chlorotic leaf spot (ACLSV), apple stem grooving (ASGV) and apple stem pitting (ASPV) virus. The two following springs after infection, the presence of the viruses was assessed by ELISA test and virus-specific symptom recording on young leaves. In parallel, the reaction of the plants to infections with Trentino strains of 'Candidatus Phytoplasma mali' was evaluated. AP-susceptible Malus x domestica genotypes were considered as controls. The results confirmed an incidence of the viral infections on Malus sieboldii as it was reported in the past. However, the M sieboldii hybrids showed a high variability of response ranging from no viral symptoms to severe symptoms. Nevertheless, highly phytoplasma-resistant genotypes which showed no presence of viral superinfections could be identified in these experiments.