Molecular diversity of ‘Candidatus Phytoplasma mali’ and ‘Ca. P. prunorum’ in orchards in Slovenia

Phytoplasmas from the 16Sr-X apple proliferation (AP) group are quarantine species in Europe and causal agents of the most important diseases of fruit trees within the family Rosaceae, namely apple proliferation, European stone fruit yellows and pear decline. In this study, a detailed insight into the molecular diversity of isolates of two phytoplasmas from the AP group, i. e. ‘Candidatus Phytoplasma mali’ and ‘Ca. P. prunorum’ obtained from different orchards in Slovenia, was estimated by a multilocus sequence typing, based on analysis of the genomic regions of aceF, pnp, secY and imp. With seven and five genotypes defined for ‘Ca. P. mali’ and ‘Ca. P. prunorum’ isolates, respectively, imp was the most variable among the applied markers. On the other hand, pnp was the least variable with three genotypes defined for ‘Ca. P. mali’ isolates and only one for ‘Ca. P. prunorum’ isolates. The presented results complete the survey of the AP group phytoplasma diversity in Slovenia, which has started with the recent analysis of the ‘Ca. P. pyri’. The comparison of results with those from several European countries shows an important genetic diversity of the Slovenian genotypes with some previously unknown. The genotype distribution reflects the geographic position of Slovenia. Additional grafting experiments with apricot trees tolerant to ‘Ca. P. prunorum’ demonstrated that the tolerance status is transmissible. Some possible mechanisms involved in the process are discussed.