DEVELOPMENT OF RESISTANT ROOTSTOCKS FOR THE CONTROL OF APPLE PROLIFERATION DISEASE

As apple proliferation disease is difficult to control by classical means the strategy of development of resistant plant material was adopted. This strategy is based on the observation that Candidatus Phytoplasma mali is eliminated each year in the upper parts of the tree during renewal of the phloem. Therefore, a successful resistance strategy can be based solely on resistant rootstocks. Natural resistance has been found in the apomictic Malus species M. sieboldii and in crosses made between M. sieboldii × M. domestica and related species; however, these genotypes are too vigorous for the use as rootstocks and they induce alternate cropping. Therefore, a new breeding program was started in 2001 to introduce resistance into rootstock material of agronomic value. In the years 2001–2004, 14 crosses were made between different resistant genotypes and M. domestica-rootstock genotypes like ‘M9’. Fruit set and seed production varied considerably between the different crosses, depending on the compatibility of the parental genotypes and the degree of apomixis. Microsatellite analysis was employed to screen the progeny for recombinant genotypes. All recombinant progeny seedlings were graft-inoculated with Candidatus Phytoplasma mali in order to evaluate the resistance behaviour. Two years after inoculation the roots of the inoculated young plants were analysed by PCR for the presence of Candidatus Phytoplasma mali. A quantitative real-time PCR assay was employed to quantify the phytoplasmas in the different genotypes. The preliminary results indicate that the resistance trait can be inherited to the progeny. An agronomic evaluation of the resistant genotypes has now to follow.